ABSTRACT 
 
 
Title of Dissertation: The Hydrogel Reimagined: Gel-Derived Sponges and Sheets as 
Absorbents for Water, Blood, and Oil 
 
    Hema Choudhary, Doctor of Philosophy, 2022 
 
Directed By:  Professor Srinivasa R. Raghavan,  
Department of Chemical and Biomolecular Engineering 
 
 
Polymer hydrogels, i.e., crosslinked networks of polymer chains swollen in water, are 
well-studied materials. Superabsorbent polymer (SAP) gels that can absorb more than 100? their 
dry weight in water are widely used in personal hygiene products ? but only in the form of 
microscale beads. If dry SAP gels were larger, they would either take too long to swell or would 
be brittle solids. This dissertation seeks to reimagine polymer gels in very different physical 
forms: as soft sponges or foldable, fabric-like sheets. We want these macroscale dry materials to 
retain the ability to absorb large amounts of liquid, either aqueous or organic. In short, we would 
like to make polymer gels in convenient, usable forms similar to everyday absorbents like towels 
and sponges.   
 
The key to making gels as macroscale absorbents is to make the gels porous. In our first 
study, we devised a way to create porous gels by foam-templating. The approach involves in situ 
foaming of a monomer solution followed by fast polymerization. We generate the foams using a 
double-barrelled syringe that has acid and base in its two barrels. Gas (CO2) is formed at the 
mixing tip of the syringe by the acid-base reaction, and gas bubbles are stabilized by an 
amphiphilic polymer in one of the barrels. The monomers are then polymerized by ultraviolet 
(UV) light to form the gel around the bubbles, and the material is dried under ambient conditions 
to give a porous solid. We show that this dry, porous gel absorbs water at a rate of 20g/s until 
equilibrium is reached at ~ 300? of its weight. This is the fastest swelling and expansion ever 
  
achieved by a hydrogel. We convert the chemical potential energy from gel expansion into 
mechanical work: the gel is able to lift weights against gravity, with a power-density of 260 
mW/kg. 
 
Next, we synthesize porous gels in the form of large sheets that resemble cloth or paper 
towels. For this, we polymerize thin films of the foams and ambient-dry the films after 
plasticization. Our gel sheets are flexible, foldable, and can be cut with scissors like fabrics. At 
the same time, the sheets absorb more than 30? of their dry weight in various aqueous fluids 
(water, blood, polymer solutions). Remarkably, these gel sheets expand as they absorb water, 
unlike any commercial towels. The expanded sheets retain absorbed fluid when lifted upright 
whereas fluid drips out of commercial absorbent sheets. Because of these superior properties, our 
gel sheets could be used to absorb aqueous liquids in various settings such as homes, labs and 
hospitals. 
  
Lastly, we design oleo-sheets, which are counterparts to the above that can absorb oils, 
i.e., non-polar liquids. We synthesize oleo-sheets by templating foams in which the continuous 
phase is non-aqueous and contains hydrophobic monomers. The oleo-sheets are hydrophobic and 
can selectively absorb oil from water. They show a high absorption capacity (> 50 g/g) for a 
range of organic solvents. The sheets can also be made magnetically responsive and an oil-
soaked oleo-sheet can be lifted up by a magnet. We also fabricate a ?Janus omni-absorbent sheet? 
that has two sides: one side selectively absorbs water while the other side absorbs oil/solvents. 
Our oleo-sheets and omni-absorbent sheets could both be used in homes, hospitals, and various 
industries for cleaning up different spilled liquids.   
  
 
 
 
The Hydrogel Reimagined: Gel-Derived Sponges and Sheets 
as Absorbents for Water, Blood, and Oil 
 
 
 
 
by 
 
 
Hema Choudhary 
 
 
 
 
 
 
Dissertation submitted to the Faculty of the Graduate School of the 
University of Maryland, College Park, in partial fulfillment 
of the requirements for the degree of 
Doctor of Philosophy 
2022 
 
 
 
 
 
 
Advisory Committee: 
 
Professor Srinivasa R. Raghavan, Dept. of Chemical and Biomolecular Engineering, Chair  
Professor Chen Zhang, Dept. of Chemical and Biomolecular Engineering 
Professor Gregg Duncan, Dept. of Bioengineering 
Professor Robert M. Briber, Dept. of Material Science and Engineering, Dean?s Representative 
Professor Taylor J. Woehl, Dept. of Chemical and Biomolecular Engineering 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
? Copyright by 
Hema Choudhary 
2022 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
This dissertation is dedicated to my parents for 
 their constant support, motivation, and unconditional love. 
 
  
 ii 
 
 
Acknowledgements 
First and foremost, I would like to express my deepest gratitude to my advisor, Dr. 
Raghavan for his continuous guidance, support and mentorship during the last five years. His 
encouragement to come up with my own ideas pushed me to use my creativity in solving 
research problems. He has always provided me the freedom to explore curiosity which made it 
possible to investigate various research problems. His teaching tactics and mentoring style is 
exemplary which I wish to follow in my scientific career. His invaluable lessons on research 
approach, presentation skills and art of telling a good story has built a strong foundation to 
advance my scientific career and for all this I am immensely grateful.  
 
I would like to thank my thesis committee Dr. Robert Briber, Dr. Taylor Woehl, Dr. 
Gregg Duncan and Dr. Chen Zhang for serving on my defense committee and providing 
insightful feedback on my research projects. I really appreciate the insights and 
recommendations from Dr. Briber, Dr. Kofinas and Dr. Po-Yen Chen during the polymer group 
meetings. I would also like to express my sincere thanks to the Medcura team: Dr. Matthew 
Dowling, Dr. Jaeho Lee and Brandon Wallace for providing materials for my experiments and 
testing my samples. I am very grateful to Dr. Dowling for performing all the animal studies and 
providing valuable advice on my projects.  
 
I am appreciative to my friends: Gaurav Sharma, Rashi Tandon, Mrinalgouda Patil, 
Koushik Marepally and Shashank Maurya for making my last five years amazing. I really 
enjoyed game nights, trips and playing badminton with them.  I am especially thankful to Gaurav 
for cooking delicious food and providing me moral support for the past five years. Additionally, 
 iii 
 
 
I would like to sincerely thank my past and present colleagues in Dr. Ragahvan?s lab for helping 
in my research projects. Specifically, Dr. Nikhil Subraveti and Leah Borden for making lab work 
full of fun, exciting discussions and lending emotional support when experiments failed. I am 
really glad that we started our PhD together. Many thanks to Dr. Niti Agarwal for teaching me 
rheology. I would also like to thank Dr. Sohyun Ahn, Faraz Burni, Medha rath, Wenhao Xu, 
Sairam Ganesh, Futoon Al-Jirafi. and Mahima Srivastava. I also want to extend my appreciation 
to my undergraduate student Christine Zhou for her help with experiments.  
 
Finally, I would like to acknowledge the most important people in my life, my family. 
Without their support, unconditional love and countless sacrifices, I would not have made this 
far. My parents taught me to work hard for my dreams from the very beginning and today, here I 
am because of that support and motivation. I am highly indebted to my siblings who took care of 
my mother and family so that I could focus on my studies. I cannot express my gratitude in 
words for my brothers-in-law who stood by my brother's side when I could not. I deeply thank 
my husband, Sandeep Reddy for his support and care and for standing by me through thick and 
thin. I am very excited to start our adventurous journey together after PhD defense. 
 
 iv 
 
 
Table of Contents 
 
 
Acknowledgements ........................................................................................................................ iii 
Table of Contents ............................................................................................................................ v 
List of Figures ............................................................................................................................... vii 
List of Tables ................................................................................................................................ xv 
List of Abbreviations ................................................................................................................... xvi 
 
Chapter 1 Introduction and Overview ............................................................................................. 1 
1.1 Problem Description and Motivation .................................................................................... 1 
1.2 Proposed Approach ............................................................................................................... 2 
1.2.1 Porous Hydrogels that Rapidly Swell and Expand ........................................................ 3 
1.2.2 Porous Gel-Sheets for Absorbing Aqueous Liquids ...................................................... 4 
1.2.3 Porous Oleo-Sheets for Absorbing Organic Solvents .................................................... 5 
1.3 Significance of This Work .................................................................................................... 5 
 
Chapter 2 Background .................................................................................................................... 8 
2.1 Polymer Gels ......................................................................................................................... 8 
2.2 Polymer-Based Porous Absorbents ..................................................................................... 12 
2.3 Foam Templating ................................................................................................................ 14 
2.4 Foam Stability ..................................................................................................................... 15 
2.5 Chitosan and Hydrophobically Modified Chitosan ............................................................ 18 
 
Chapter 3 Porous Hydrogels that Rapidly Swell and Expand ...................................................... 20 
3.1 Introduction ......................................................................................................................... 20 
3.2 Experimental Section .......................................................................................................... 24 
3.3 Results and Discussion ....................................................................................................... 27 
3.3.1 Synthesis of Porous Gels ............................................................................................. 27 
3.3.2 Microstructure of the Porous Gels ............................................................................... 29 
3.3.3 Swelling/Expansion of Porous Gels in Water .............................................................. 30 
3.3.4 Tuning the Swelling/Expansion of Porous Gels .......................................................... 35 
3.3.6 pH-Induced Expansion/Contraction of Porous Gels .................................................... 40 
3.3.6 Mechanical Work from Rapid Gel Expansion ............................................................. 43 
3.3.6 Other Applications ....................................................................................................... 46 
3.4 Conclusions ......................................................................................................................... 49 
 
Chapter 4 Porous Gel-Sheets for Absorbing Aqueous Liquids .................................................... 50 
4.1 Introduction ......................................................................................................................... 50 
4.2 Experimental Section .......................................................................................................... 53 
4.3 Results and Discussion ....................................................................................................... 57 
4.3.1 Synthesis of Gel Sheet ................................................................................................. 57 
4.3.2 Microstructure of the Gel Sheet. .................................................................................. 59 
4.3.3 Gel Sheets: Tactile and Mechanical Properties. .......................................................... 60 
4.3.4 Tuning the Properties of Gel Sheets. ........................................................................... 66 
 v 
 
 
4.3.5 Gel Sheets: Absorbent Properties. ............................................................................... 69 
4.4 Conclusions ......................................................................................................................... 77 
 
Chapter 5 Porous Oleo-Sheets for Absorbing Organic Solvents .................................................. 79 
5.1 Introduction ......................................................................................................................... 79 
5.2 Experimental Section .......................................................................................................... 83 
5.3 Results and Discussion ....................................................................................................... 87 
5.3.1 Synthesis of Oleophilic Absorbent. ............................................................................. 87 
5.3.2 Microstructure of the Oleosponge. .............................................................................. 88 
5.3.3 Mechanical Properties of Oleophilic Absorbent .......................................................... 92 
5.3.4 Hydrophobicity of Oleosponge .................................................................................... 94 
5.3.2 Oil-Water Separation and Oil Absorption Capacity .................................................... 96 
5.4 Conclusions ....................................................................................................................... 106 
 
Chapter 6 Recommendations and Future Work .......................................................................... 107 
6.1 Project Summary ............................................................................................................... 107 
6.2 Recommendations for Future Work.................................................................................. 110 
6.2.1 Porous Gels for Wound Care ..................................................................................... 110 
6.2.2. Hybrid Porous Gels That Change Shapes ................................................................. 114 
 
References ................................................................................................................................... 115 
 
 
 
 
 
 vi 
 
 
List of Figures 
 
Figure 1.1. Rapidly swelling porous hydrogels synthesized in Chapter 3. The porous gel (~ 1 
cm) when placed in water, rapidly swells within 20 s. The gel expands by 4? and absorbs water ~ 
300? its dry weight. ........................................................................................................................ 3 
 
Figure 1.2. Porous gel-sheets that are flexible and robust after drying (Chapter 4). (A) A 
schematic of gel sheet structure. (B) The gel sheet is foldable and contains connected open pores 
(micrograph). .................................................................................................................................. 4 
 
Figure 1.3. Porous oleo-sheets for absorbing oils from water (Chapter 5). When added to an 
immiscible oil-water mixture, the oleo-sheet selectively absorbs the oil. ...................................... 5 
 
Figure 2.1. Schematic of the structure in a gel. A 3D network of polymer chains is shown. The 
chains are connected at crosslink or junction points. Solvent (such as water) is entrapped in the 
network. .......................................................................................................................................... 8 
 
Figure 2.2. Polymerization reactions in acrylamide-co-acrylic acid hydrogel formation. 
Upon UV irradiation, initiator molecules break down to form radicals. Then radicals begin 
reacting with the vinyl groups to form long chains, which eventually get crosslinked due to the 
two vinyl sites present on BIS crosslinkers. Crosslinked polymer network structure is adapted 
from the paper by Hibbins et.al 47 ................................................................................................. 10 
 
Figure 2.3. Examples of a high-swelling ionic hydrogel and an ionic organogel. (A) An ionic 
hydrogel of DMAA-sodium acrylate is shown in its shrunken and swollen states. (B) An ionic 
organogel containing bulky groups of tetra-alkylammonium tetraphenylborate is shown swollen 
to almost 4 times its initial size in tetrahydrofuran (THF).  Image adapted from Reference 53. . 12 
 
Figure 2.4. Microstructure of a paper towel. SEM micrograph showing that a standard paper 
towel (Bounty? brand) consists of cellulose fibers and is a porous structure. Image adapted from 
Reference 56. ................................................................................................................................ 13 
 
Figure 2.5. Foam templating technique. A foam (dispersion of gas bubbles in liquid) is 
generated in a monomer solution and stabilized by surfactants. Upon solution polymerization, the 
bubbles get trapped in the polymer network (gel). When the gel is dried, the bubbles become 
pores in the final material. ............................................................................................................ 15 
 
Figure 2.6. Amphiphilic molecules. (A) Short amphiphilic molecules with hydrophilic head 
groups and hydrophobic tails. (B) Amphiphilic polymers, with hydrophobic groups attached 
along the hydrophilic backbone of the polymer. .......................................................................... 16 
 
Figure 2.7. Making foams using a double barrelled syringe (DBS). The DBS contains acetic 
acid in one barrel and sodium bicarbonate in the other barrel. The acid and base react to produce 
CO2 gas bubbles, which get stabilized by the amphiphilic polymer (hmC) present in the acid. .. 17 
 vii 
 
 
Figure 2.8. Molecular structure of chitosan and hydrophobically modified chitosan (hmC). 
The hydrophobes are palmitic (C16) groups. ............................................................................... 18 
 
Figure 3.1. Gel-swelling dynamics at different length scales. Dry gels are placed in water at t 
= 0 and allowed to swell (expand) to their final size. Swelling occurs by diffusion of water into 
the dry gel. (A) Microscale gel beads (?10 ?m size) swell in seconds to their final size. (B) A 
solid macroscale gel (?1 cm size) takes ?24 h to expand to its final swollen size. (C) A 
macroscale gel with microscale pores expands much more rapidly compared to (B). In this study, 
one such porous gel is shown to expand to 4? its original size within 15s. ................................. 21 
 
Figure 3.2. A Schematic of the procedure used to synthesize porous gels. (A) A foam of the 
monomers is prepared using a double-barreled syringe (DBS). One barrel of the DBS is an acidic 
solution of monomers, cross-linkers, and the hmC stabilizer, while the other barrel is a basic 
solution with the UV initiator. At the mixing tip of the DBS, CO2 gas is produced, and bubbles 
of the gas are stabilized by hmC chains. (B) The foam is polymerized by UV light for 2 min. (C) 
The bubbles in the foam are retained during the polymerization while a polymer gel network is 
formed around the bubbles. (D) The gel is dried under ambient conditions to give the porous gel. 
The photo (inset) reveals that the material is a robust solid with a sponge-like texture. .............. 27 
 
Figure 3.3. Microstructure of the foam and the porous gel made using the foam as a 
template. (A) A representative optical micrograph of the foam reveals close-packed small 
bubbles, most of which are spherical. (B) Representative SEM images of the dried porous gel at 
two different magnifications. The images show a highly porous structure with interconnected 
pores. ............................................................................................................................................. 30 
 
Figure 3.4. Typical swelling/expansion of a porous gel in water. (A) At time t = 0, a dried gel 
is placed in water. Snapshots of the swelling gel at various time intervals are shown (B) Swelling 
ratio R and size increase (?L/L0) (%) are plotted against time. The gel absorbs more than 300 
times its dry weight within 15 s, and in the process, its size increases by 300% in 15 s. (C) After 
the swelling is complete, the swollen gel (4-fold larger than the original) is robust enough to be 
picked up and held by hand. ......................................................................................................... 32 
 
Figure 3.5. Comparing the swelling rates of porous gels in this study with past ones. The 
swelling rate in this study is 20 g/g?s, whereas those in previous studies were below 5 g/g?s. See 
also Table 1. .................................................................................................................................. 33 
 
Figure 3.6. Effect of ionic monomer content on gel-swelling extent and kinetics. (A) 
Swelling ratios R at equilibrium of porous gels with different proportions of ionic monomer 
(acrylic acid, AAc) to nonionic monomer (acrylamide, AAm). Note: R = mass of swollen 
gel/mass of dry gel. During synthesis, the total monomer (AAc+AAm) was maintained at 25 
wt% while the weight ratio of AAc:AAm was changed. (B) For visualization of the data in (A), 
photos of the various gels are shown in the dry and swollen states. All the ionic gels swell 
significantly. (C) Kinetics of gel-swelling for each of the gels in (A). (D) Zoomed-in plot of the 
initial data in (C), showing that all the ionic gels swell at roughly the same rate (i.e., the initial 
slopes are similar). ........................................................................................................................ 36 
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Figure 3.7. Effect of crosslinker concentration on gel-swelling extent. (A) Swelling ratios R 
of porous gels as a function of the crosslinker concentration. The gels were synthesized with a 
total monomer (AAc+AAm) content of 25 wt% and with the AAc:AAm ratio at 3:1 by weight. 
Only the concentration of the N,N?-methylene(bis)acrylamide (BIS) crosslinker (mol% with 
respect to the total monomer) was varied. (B) For visualization of the data in (A), photos of the 
gels are shown in the dry and swollen states. The less crosslinked the gel, the more it swells. ... 37 
 
Figure 3.8. Effect of stabilizer (hmC) concentration on precursor foams and the 
corresponding porous gels. A foam stabilized by a given concentration of the amphiphilic 
polymer hmC (hydrophobically modified chitosan) is injected into a vial and the time for the 
foam to dissipate to half its fresh height (t1/2) is used as an indicator of foam stability. The 
bubbles in the foam are also analyzed by optical microscopy and the average bubble diameter 
Davg is determined from the images. (A) Plot of t1/2 and Davg vs. hmC concentration. As hmC is 
increased, the bubbles become smaller and the foam stability increases. (B) Foams with different 
hmC content were used to synthesize porous gels, with other compositional variables held 
constant: the total monomer was 25 wt%, the AAc:AAm ratio was 3:1, and the BIS was at 0.375 
wt%. SEM micrographs of the dried gels are shown. Interconnected pores are seen when the 
hmC is 0.1 wt% (B2) or 0.5 wt% (B3). (C) Swelling ratios at equilibrium of the above porous 
gels. ............................................................................................................................................... 39 
 
Figure 3.9. Porous gels in different shapes, before and after swelling. (A) Porous gels with 
the same composition are synthesized in different shapes: with circular, triangular, rectangular, 
and square cross-sections. The image shows the gels in their initial (dry) state. (B) The same gels 
after swelling in water. All gels swell isotropically by 300? their initial weight. Each dimension 
of the initial shapes in (A) is increased by ~ 3?. .......................................................................... 40 
 
Figure 3.10. Response of porous gels to pH. The gel swells (expands) at ambient and higher 
pH and shrinks (contracts) at low pH. Repeated cycling between pH 3 and 10 is done, and the 
swelling ratio R during these cycles is plotted. Both swelling and shrinking occur rapidly. A full 
cycle is completed in ?60 s for the first cycle and ?90 s for subsequent cycles. ........................ 41 
 
Figure 3.11. Extracting mechanical work from the expansion of a porous gel. (A) A 
cylindrical porous gel is placed in a syringe and on top of this cylinder, a load of mass m is 
placed (Photo A1). As soon as water is added, the gel swells and expands, thereby lifting the 
weight by a height h (Photo A2) (B) The height to which the load is lifted by the gel is plotted vs 
the mass m of the load. (C) The work done by the gel in lifting the load m by a height h (W = 
mgh) is plotted against the mass m. .............................................................................................. 44 
 
Figure 3.12. Reversible lifting and lowering of a load by the expansion and contraction of a 
porous gel. (A) A cylindrical porous gel is placed in a syringe, and a load is placed on it (Photo 
A1). When water is added, the gel absorbs water and expands, thereby lifting the load by a height 
h (Photo A2). Next, when ethanol is added, the gel contracts (by expelling solvent), and thereby, 
the load is lowered to its initial position (Photo A3). (B) Repeated cycling is done in water and 
ethanol, and the position (h) of the load is plotted across three such cycles. A full cycle is 
completed in ?70 s. ...................................................................................................................... 45 
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Figure 3.13. Gel-expansion as a way to block the flow of water. A comparison is done 
between a macroscopic porous gel and commercial gel-beads (both of the same weight of 40 
mg). The setup involves a syringe with an open bottom that is covered by a wire mesh (see inset) 
and then a small piece of a paper towel. (A) A cylindrical porous gel is placed on the paper towel 
at t = 0 (1). When water is added from the top, initially it flows out through the wire mesh at the 
bottom (2), but by 15 s, the gel is expanded and fills the syringe, thus blocking the flow (3), and 
no further flow is observed even after 5 min (4). (B) Gel-beads are placed on the paper towel at t 
= 0 (1). When water is added from the top, the beads swell and thicken the water column (2, 3), 
but water continues to flow out through the bottom (2, 3, 4). ...................................................... 48 
 
Figure 4.1. Comparison between current water-absorbents and the gel sheets developed in 
this study. Current absorbents fall into two categories: (A) Pads or towels made from cloth or 
paper, which are soft and flexible, but have low absorption capacity. (B) Superabsorbent polymer 
(SAP) gels, which absorb much water, but are hard and brittle. (C) Our gel sheets combine the 
desirable properties of both the above while avoiding their drawbacks: they are soft, foldable and 
flexible, while also exhibiting high water absorption. Note that the sheet expands as it absorbs 
water. Scale bars: 1 cm. ................................................................................................................ 52 
 
Figure 4.2. Schematic of the procedure used to synthesize gel sheets. (A) A polymerizable 
foam is injected into a Ziploc bag using a DBS. In the foam, bubbles of CO2 are stabilized by the 
polymeric stabilizer hmC. Glass slabs are used to compress the foam into a thin layer. (B) The 
foam is polymerized by UV light for 2 min. The bubbles remain intact and a polymer network is 
formed around the bubbles. (C) The water in the gel sheet is solvent-exchanged with a 15/85 
glycerol-ethanol solution, followed by ambient drying. The dry gel-sheet is soft and flexible. .. 58 
Figure 4.3. Microstructure of gel sheets. Representative optical (A) and SEM (B) images of 
the dry gel sheet show a highly porous structure with interconnected pores. ............................... 59 
 
Figure 4.4. Fabric-like nature of gel sheets. (A) A gel sheet (10 ? 8 ? 0.4 cm) can be folded 
and unfolded several times, without showing cracks or tears. (B) A gel sheet is cut cleanly and 
smoothly like a fabric using a pair of scissors. Scale bars: 1 cm. ................................................. 61 
 
Figure 4.5. Texture of a thick gel sheet. A 15-mm thick gel sheet in cube form (2.5 ? 2.5 cm) is 
compared side-by-side with a cotton ball of similar dimensions. Both materials can be squeezed 
between fingers several times (10 cycles) without any lasting changes in size or structure. Scale 
bars: 1 cm. ..................................................................................................................................... 62 
 
Figure 4.6. Mechanical properties of gel sheets. (A) Tensile stress vs. strain. The tensile 
(Young?s) modulus is 4.8 kPa and the gel sheet can be stretched by 45% until failure. (B) 
Compressive stress vs. strain. The gel sheet is a soft, spongy material that can sustain more than 
85% compression without damage. The photos show that the compressed gel returns instantly to 
its initial size upon removing the load. ......................................................................................... 63 
 
Figure 4.7. Identifying the optimal plasticizer concentration and type. (A) Photos showing 
that a gel sheet prepared without glycerol as the plasticizer in the solvent-exchange step is brittle. 
This shows the need for glycerol as a plasticizer. (B) Tensile (Young?s) modulus of gel sheets 
prepared with various glycerol concentrations in the solvent-exchange step. The optimal content 
 x 
 
 
of glycerol is 15% (circled), i.e., glycerol: ethanol = 15:85. If more glycerol is used, the tensile 
modulus becomes too low. (C) Gel sheets plasticized by propylene glycol (PG), ethylene glycol 
(EG), glycerol, PEG-200 and PEG-400 are compared. In all cases, the sheet is solvent-exchanged 
with a 15:85 plasticizer: ethanol solution. The sheets are then heated in an oven at 70?C, and the 
weight drop over time is the plasticizer lost by evaporation. PG and EG are completely removed 
within 3 h whereas only ~ 30% of the other plasticizers are removed. This data again show 
glycerol to be an optimal plasticizer. ............................................................................................ 64 
 
Figure 4.8. Identifying the optimal crosslinker concentration and type. (A) Gel sheets with 
different concentrations of PEGDA crosslinker are prepared and their porosities are calculated 
using eq 4.2. If the PEGDA content is too low, the porosity is low, indicating that most of the 
pores collapse during drying. This is consistent with the SEMs in Figure 4.3. For this reason, the 
optimal PEGDA is 2.5 mol% (circled), which is used in the rest of the studies. (B) Gel sheets 
crosslinked with three different crosslinkers: BIS, TEGDA, and PEGDA (all at the same 
concentration of 2.5 mol% of total monomers). BIS and TEGDA are small molecules whereas 
the PEGDA has an MW of 575. The bar graph shows the ratio of  gel sheet volume after ambient 
drying (Vdry) to the volume before drying (Vwet). When crosslinked by PEGDA, the dry gel-sheet 
retains 90% of its volume, indicating that the pores are mostly intact (not collapsed).  (C) Photos 
of the gel sheets before and after drying, corresponding to the data in (B). The PEGDA gel-sheet 
shrinks the least, consistent with the data shown in (B).  This indicates that most of the pores in 
the material are intact. Scale bars: 1 cm. These observations indicate that a long-chain crosslinker 
like PEGDA is optimal for the gel sheets. .................................................................................... 68 
 
Figure 4.9. Effect of PEGDA content on gel-sheet porosity. SEMs at two magnifications are 
shown for three different PEGDA concentrations: (A) 1.5 mol%, (B) 2.5 mol%, and (C) 5 mol%. 
Pores are collapsed in (A), whereas both (B) and (C) show open, interconnected pores. This is 
consistent with the data in Figure 4.8A. PEGDA of 2.5 mol% (highlighted) is identified as the 
optimum, and this is used in the rest of the studies. ..................................................................... 69 
 
Figure 4.10. Water mopping by gel sheet compared to controls. At t = 0, a gel sheet (A) or a 
commercial cloth pad (Sungbo Corp.) (B) of identical size (10 ?  8 ?  0.4 cm) are placed over a 
spill of 25 mL water. Snapshots at various stages are shown. The gel sheet absorbs all the water 
and the swollen sheet does not drip when held vertically. The commercial pad only absorbs 60% 
of the water, and moreover, the water drips out when held vertically. Scale bars: 2 cm. ............ 70 
 
Figure 4.11. Water mopping by Bounty? paper towel. At t = 0, a folded Bounty? paper 
towel of 10 ? 8 ? 0.4 cm size is placed over a spill of 25 mL water. Snapshots at various stages 
are shown. The towel only absorbs 48% of the water, and moreover, the water drips out when 
held vertically. Scale bars: 1 cm. .................................................................................................. 71 
 
Figure 4.12. Water absorption limit for gel sheets. The absorption limit (or ?dripping limit?) is 
the amount of water that can be held by a sheet at saturation ? before it starts to drip. (A) This 
quantity is plotted vs. sheet size for gel sheets as well as a commercial cloth pad (Sungbo Corp.). 
(B) This quantity is compared for various sheets, all having a size of 10 ? 8 ? 0.4 cm. The gel 
sheet exhibits 3? the absorption limit of the others. ..................................................................... 71 
 xi 
 
 
Figure 4.13. Expansion of gel sheets upon absorbing water. 3-cm discs of a gel sheet and a 
paper towel (Bounty?) are compared after adding given amounts of water. (A1-A3) Photos at 
different times. (B) Plot of diameter vs. time. The gel sheet expands by 80% whereas the paper 
towel remains at the same size. Scale bars:1 cm. ......................................................................... 73 
 
Figure 4.14. Blood mopping by gel sheet compared to controls. At t = 0, a gel sheet (A) or a 
gauze wound dressing (McKesson?) (B) of identical size (10 ? 8 ? 0.4 cm) are placed over a 
pool of 40 mL blood. Snapshots at various stages are shown. The gel sheet absorbs 99% of the 
blood and the swollen sheet does not drip when held vertically. The commercial gauze only 
absorbs 55% of the blood, and moreover, the blood drips out when held vertically. Scale bars: 2 
cm. ................................................................................................................................................. 74 
 
Figure 4.15. Blood absorption limit for gel sheet and commercial products.  This quantity is 
the amount of blood that can be held by a sheet at saturation and it is compared for a gel sheet, a 
gauze dressing (McKesson?), a polyurethane (PU) sponge, and an Always? sanitary pad. All 
have a size of 2 ? 2 ? 0.4 cm. The gel sheet absorbs about 3? the blood compared to the others.
....................................................................................................................................................... 76 
 
Figure 4.16. Absorption limit for viscoelastic solutions. Solutions of xanthan gum (XG) with 
varying zero-shear viscosities (Pa.s) were tested. The absorption limit is the amount of liquid that 
can be absorbed by a sheet at saturation (without dripping). It is compared for a gel sheet and a 
cloth pad (Sungbo), both of size of 2 ? 2 ? 0.4 cm. ...................................................................... 77 
 
Figure 5.1. Foam stability in aqueous phase vs non aqueous phase. (A) Foams rapidly get 
produced when acidic solution is added to water containing sodium bicarbonate and surfactant 
Tween (TW) 80. In presence of TW80, aqueous foams show very high foamability. (B) When 
water is replaced with oil, TW80 fails to stabilize non-aqueous foams and thus show low 
foamability. ................................................................................................................................... 81 
 
Figure 5.2. Schematics of the method used to create oleophilic absorbent. (A) A foam is 
generated in the oil phase by mixing acidic aqueous solution with base. The oils phase contains 
monomer, crosslinker, thermal initiator, silicone surfactant and dispersed sodium bicarbonate 
(NaHCO3). An accelerator is also added to speed up the polymerization process. (B) The acid 
and base react to produce CO2 gas bubbles. Simultaneously, oil in water emulsions are formed 
due to mixing of two immiscible phases. Both foam and emulsion are stabilized by surfactant. 
Foams are quickly transferred into either a sheet or cylindrical mold and polymerized in 3 min.  
Photos of porous solid in the form of a sheet and cylinder reveals that the material is flexible and 
has a sponge-like texture. .............................................................................................................. 87 
 
Figure 5.3. Microstructure and schematic of foamed-emulsion system and oleosponge (A) 
The monomer solution and water are immiscible and so the monomer foams are expected to have 
both bubbles and oil in water (O/w) emulsions (1). A schematic of this foam solution is shown 
(2). Representative optical micrograph of the foam confirms the presence of emulsion around 
bubbles (3). (B) When these monomer foams are polymerized, the monomer droplets polymerize 
to give a continuous network around bubbles (1&2). Optical micrograph reveals numerous pores 
surrounded by continuous polymer network (3). .......................................................................... 90 
 xii 
 
 
Figure 5.4. Micrographs and size distribution of monomer foam and oleosponge (A) An 
optical micrograph of monomer foam, prior to polymerization, shows nearly closely packed 
bubbles. Five optical micrographs of such monomer foam were analyzed to get bubble size 
distribution which is shown as histogram. (B) The SEM micrograph of oleosponge shows porous 
structure with interconnected pores. The corresponding pore size distribution is obtained by 
analyzing pores in three samples. ................................................................................................. 92 
 
Figure 5.5. Mechanical characterization. An oleosheet of dimensions 10  ? 7 cm and thickness 
4 mm folded/unfolded and rolled several times (1-5). The photos show no visible cracks and 
tearing on the final sheet. (B) Mechanical strength of the oleophilic absorbent is characterized 
using tensile and compression tests. The sheet can be stretched by 35% strain before tearing and 
has a very high tensile modulus (1). It can sustain more than 70% compression strain for several 
cycles and yet shows no structural damages (2). 1 cm scale bar. ................................................. 93 
 
Figure 5.6. Contact angle of oleosponge and PU foam. Droplets of various liquids (acidic, 
basic, water, ethanol, and toluene are placed on a flat (A) oleosponge and (B) PU foam surface. 
Aqueous droplet remains spherical on oleosponge surface whereas droplet slightly settle down 
on PU foam surface. In both foam, solvent droplets get absorbed. The oleosponge and PU foam 
show a contact angle of 130? and 75? with water respectively. The corresponding digital photos 
are shown underneath. .................................................................................................................. 95 
 
Figure 5.7. Solvent absorption. (A) An oleosponge of dimensions 2 ? 2 ? 0.6 cm is added to 
toluene-water mixture (1). The oleosponge selectively absorbs toluene (dyed with oil redo) 
completely and underneath water turns clear. The oleosponge expands as it absorbs solvent 
(2&3). (B) When PU foam of the same size is added to toluene-water mixture, it fails to absorb 
toluene completely indicated by the toluene layer (dyed with oil redo) on the water surface. .... 97 
 
Figure 5.8. Oleosponge and polyurethane (PU) foam after absorbing water and toluene. 
Initially both PU foam and oleosponge are of the same size with dimensions 2 ?  2 ? 0.6 cm. 
When both are added to water which is dyed with methylene blue, PU foam absorbs water and 
becomes bluish in color. Contrarily, oleosponge did not absorb water and thus appeared white. 
Upon absorbing toluene (dyed with oil redo), PU foam expanded slightly whereas oleosponge 
size increased a lot. This difference was clearly visible in the photo. The scale bar is 0.5 cm. ... 98 
 
Figure 5.9. Swelling kinetics and absorption capacity in solvents and oil. (A) An absorbent 
(dimensions 2 ? 2 ? 0.6 cm in all cases) was added to toluene and linear dimension expansion 
was measured. Then this expansion was extrapolated to get volume in a swollen state at any time 
and volume expansion (ratio of final to initial volume) vs. time is plotted for our oleosponge and 
commercial PU foam. The inset shows photos of initial and fully swollen materials. (B) 
Equilibrium volume expansion in various solvents is plotted for both materials. (C) The 
absorption capacity of oleosponge (weight of fully saturated to initial dry material) is also 
measured for a wide range of solvents and oil. (D) For material reusability efficiency test, 
solvent was squeezed out of fully saturated oleosponge and reused. This process was repeated for 
100 cycles in chloroform, toluene and decane and corresponding absorption capacity is plotted.
..................................................................................................................................................... 100 
 xiii 
 
 
Figure 5.10. Oleopsonge and polyurethane (PU) foam expansion (A) To measure material 
expansion as solvent is absorbed, toluene was added dropwise directly to the material and 
corresponding linear size was recorded. A plot of linear size increase % which is defined as 
change in linear dimensions/initial lengthx100 is plotted as a function of added toluene amount. 
The oleopsonge attained equilibrium expansion at ~ 10 ml even though it got saturated at a 
higher amount. Contrarily, PU foam got saturated in less than 5 ml. (B) Photos of both 
absorbents are shown at different states during this process. Scale bar: 1 cm. ........................... 101 
 
Figure 5.11. Large oil spill clean-up using oleosheet. (A) An oleosheet of dimensions 10 cm ? 
7 cm and thickness 4 mm was spread over a 40 ml toluene spill. The sheet absorbs the toluene 
quickly and expands to a size of dimensions ~ 14 ? 10 ? 8 cm. The sheet is robust enough to be 
picked by hand. It holds the solvent within the polymer network when held upright and no 
dripping occurs. Scale bar: 1cm. ................................................................................................. 102 
 
Figure 5.12. Magnetic responsive oleosponge. (A) The oleosponge was synthesized by 
incorporating iron (III) oxide particles in the monomer solution while keeping all the 
composition and mixing steps the same. This oleosponge was added to a bath containing toluene 
(dyed with oil redo) and water. It sticks to magnetic very strongly and swells in toluene as 
expected. The swollen oleopsonge is picked up by magnet. Scale bar: 1 cm. ............................ 103 
 
Figure 5.13. ?Janus omni-absorbent sheet? for absorbing both oil and water selectively. (A) 
A hybrid sheet that has two layers with opposite wettability for water is used to clean up both 
solvent and water spill. The sheet is prepared by gluing the oleosheet to a gel sheet from chapter 
4. In the initial photo 1 both layers are clearly visible. This sheet is first spread on a 30 ml 
toluene spill. It absorbs the solvent and expands while the hydrophilic side remains adhered and 
does not absorb any toluene. (B) Then the sheet is spread on water (dyed with methylene blue) 
(1-4). The hydrophilic side also swells upon absorbing water. No solvent and water drips back 
from this sheet when held vertically. The sheet photo in final state is intact one piece with both 
layers swollen (5). Scale bar: 2 cm. ............................................................................................ 105 
 
Figure 6.1 Blood absorption by our porous gel compared to a commercial blood-absorbent 
(?Gelfoam??). The blood absorption capacity vs. time is plotted for the two materials when 
brought in contact with bovine whole blood. Photos before and after swelling are also shown. 111 
 
Figure 6.2. Porous materials for stopping bleeding from a severe liver hemorrhage. (A) A 
severe injury was created on a swine liver and a gel sheet piece was compressed against injury 
for two minutes. The gel piece absorbed blood and no further bleeding occurred upon removing 
compression. (B) Contrarily on a similar severe injury, Gelfoam failed to absorb blood and got 
pushed away by flowing blood from the wound. No hemostasis was achieved in this case. ..... 112 
 
Figure 6.3. TachoSil structure. Image of Tachosil,(Baxter?) a commercial hemostatic patch 
with two layers: collagen and blood clotting proteins. ............................................................... 113 
 
Figure 6.4. Hybrid porous gel with two layers. (A) Schematic showing a hybrid sheet with two 
porous gel layers. (B) A photo of hybrid gel: top layer based on acrylic acid and bottom layer is 
NIPA. .......................................................................................................................................... 114 
 xiv 
 
 
 
 
List of Tables 
 
Table 3.1. Porous hydrogels synthesized in past that swell fast to a high extent .................. 34 
 
Table 3.2. Porous hydrogels synthesized in past that swell very rapidly ............................... 34 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
 xv 
 
 
List of Abbreviations 
 
 
AAm Acrylamide 
AAc Acrylic acid 
PEGDA Polyethylene(glycol) diacrylate 
BIS Methylenebis(acrylamide) 
lithium phenyl-2,4,6-trimethyl-
LAP 
benzoylphosphinate   
DI Deionized 
DBS Double barreled syringe 
HmC Hydrophobically modified chitosan  
HmA Hydrophobically modified alginate 
UV Ultraviolet 
SDS Sodium dodecyl sulfate 
TW80 Tween 80 
SAP Superabsorbent polymer 
LCST Lower critical solution temperature 
NIPA N-isopropyl acrylamide 
DMAA N, N dimethylacrylamide  
TEGDA Tetraethylene glycol diacrylate 
NaOH Sodium hydroxide 
XG Xanthan gum 
EG Ethylene glycol 
 xvi 
 
 
PG Propylene glycol 
THF Tetrahydrofuran 
DCM Dichloromethane 
VOC Volatile organic compound 
DMPT N, N dimethyl-para-toluidine 
BP Benzoyl peroxide 
DDA Dodecyl acrylate 
ASTM American society for testing and materials 
NaCl Sodium Chloride 
PVA Polyvinyl alcohol 
 xvii 
 
 
Chapter 1 
Introduction and Overview 
 
 
1.1 Problem Description and Motivation  
Materials that can absorb liquids have been in use throughout the course of human 
history.1-7 The first absorbent materials included animal furs, grass, and cloth woven from natural 
fibers including cotton and wool. Disposable absorbents were first developed by nurses for 
wound care on the battlefield in the 1800s.1,2 Since then, much effort has been put into 
developing cheap and effective absorbents for absorbing both aqueous fluids3-6,8 and oils.9-11 
Examples of macroscale absorbents that are commonly used in everyday life include cloth 
towels, paper towels, and personal hygiene products such as diapers and sanitary napkins. 
Generally, these materials all have a porous structure, which facilitates fast liquid imbibition 
through capillary action.12,13 Ideally, absorbent materials should be able to absorb significant 
amounts of liquids at a fast rate. They should also have good mechanical integrity in both their 
dry state as well as when completely soaked in liquid. Cloth and paper towels also have the 
advantage of convenience: they can be folded and rolled up so that they can be stored in a 
compact form until use.  
 
Another class of absorbents are superabsorbent polymer hydrogels (SAPs).4,6,14-19 These 
have been widely studied for the past fifty years, and are known for their ability to absorb more 
than 100? their dry weight in water.14-17 A gel is a three-dimensional (3-D) network of polymer 
chains crosslinked by covalent or physical bonds, and a hydrogel is a polymer network that is 
swollen in water.20-23 Commercial absorbent pads such as diapers typically have SAPs in the 
1 
  
 
form of dry microbeads sandwiched between sheets of fabric or paper. This raises an interesting 
question: why are polymer gels used in the form of microbeads, i.e., why not as macroscale 
materials that can be folded and rolled up? That is, even though SAPs have much higher 
absorption capacity than cloth or paper, they are physically found in very different forms. The 
reason is that when a bulk (centimeter-scale) SAP gel is dried, it usually becomes a hard and 
fragile solid.24,25 Many studies have tried to make gels stronger in the wet state,26-35 but such gels 
remain robust only when they contain water. Therefore, for dry SAPs to be used as water-
absorbents (such as in diapers), their size is limited to the microscale.  
 
To summarize the current state-of-the-art, absorbents based on cloth and paper are used 
as large sheets. Absorbents based on gels are found only as microbeads. This project attempts to 
bridge the gap: can we reimagine a very different kind of polymer gel: one that can be a 
macroscale thin sheet? Or a soft, spongy object? Thus, the motivation for our work is to make 
absorbents that bridge the properties of traditional absorbents (like cloth towels or sponges) and 
absorbents based on polymer gels.  
 
1.2 Proposed Approach 
The broad goal of this work is to engineer new classes of gel-based absorbents with 
improved properties. The properties we are interested in obtaining are: (1) high absorption of 
solvent (water or other liquids), (2) fast expansion/swelling and (3) robust mechanical properties, 
both in the wet and dry states. Three classes of new absorbents are described in the Chapters of 
this dissertation:  
2 
  
 
1.2.1 Porous Hydrogels that Rapidly Swell and Expand 
In Chapter 3, we report the synthesis of porous hydrogels that swell and expand at 
unprecedented rates. We realized that the key to enhancing the swelling rate was to introduce 
macropores into the gels. To do this, we employ a foam-templating technique in which a 
monomer solution is foamed, followed by fast polymerization of the monomers by ultraviolet 
(UV) light. Thus, we form the gel around the bubbles in the foam. The gel is then dried under 
ambient conditions, converting the bubbles into pores. We show that this dry, porous gel absorbs 
water at a rate of 20g/s until equilibrium is reached at ~ 300? its weight. The gel expands by ~ 
4? as it swells (Figure 1.1). To our knowledge, this is the fastest swelling and expansion ever 
achieved by a hydrogel. We convert the chemical potential energy from gel expansion into 
mechanical work: the gel is able to lift weights against gravity, with a power-density of 260 
mW/kg. 
 
 
Figure 1.1. Rapidly swelling porous hydrogels synthesized in Chapter 3. The porous gel (~ 1 
cm) when placed in water, rapidly swells within 20 s. The gel expands by 4? and absorbs water ~ 
300? its dry weight. 
3 
  
 
1.2.2 Porous Gel-Sheets for Absorbing Aqueous Liquids    
 In Chapter 4, we synthesize porous gels in the form of large sheets that resemble cloth or 
paper towels. We again use a foam-templating technique, but first the foam is introduced into a 
Ziploc bag and pressed into a thin film, followed by polymerization. The resulting gel is then 
plasticized by glycerol before ambient drying, which again converts the bubbles into pores. This 
gives gel sheets that are flexible and foldable, like fabrics (Figure 1.2). We show that these gel 
sheets absorb more than 30? their dry weight in various aqueous fluids (water, blood, 
viscoelastic polymer solutions). Remarkably, the sheets expand as they absorb water, unlike any 
commercial towels. The absorption capacity of the sheets exceeds that of commercial materials, 
which suggests that they could be attractive for mopping up spills in homes, labs and hospitals. 
 
 
Figure 1.2. Porous gel-sheets that are flexible and robust after drying (Chapter 4). (A) A 
schematic of gel sheet structure. (B) The gel sheet is foldable and contains connected open pores 
(micrograph).  
  
  
4 
  
 
1.2.3 Porous Oleo-Sheets for Absorbing Organic Solvents 
 In Chapter 5, we extend the absorption capabilities to non-aqueous liquids such as oils 
and organic solvents. For this, we synthesize oleo-sheets by templating foams in which the 
continuous phase is non-aqueous and contains hydrophobic monomers.  The oleo-sheets are 
hydrophobic and can selectively absorb oil from water (Figure 1.3). They show a high absorption 
capacity (> 50 g/g) for a range of organic solvents. We also fabricate a ?Janus omni-absorbent 
sheet? that has two sides: one side selectively absorbs water while the other side absorbs 
oil/solvents. Our oleo-sheets and omni-absorbent sheets could both be used in homes, hospitals, 
and various industries for cleaning up different spilled liquids. They could even be used to clean 
up large oil spills. 
 
 
 
Figure 1.3. Porous oleo-sheets for absorbing oils from water (Chapter 5). When added to an 
immiscible oil-water mixture, the oleo-sheet selectively absorbs the oil. 
 
1.3 Significance of This Work 
 The studies described in this dissertation are significant in many respects. First, we use a 
simple, eco-friendly strategy to make gels that are porous. In this technique, gas bubbles are used 
as the template for pores, and drying is done under ambient conditions. Thus, the technique can 
5 
  
 
be easily scaled up for industrial use. Second, our work seeks to overcome two major limitations 
of current hydrogels, i.e., weak mechanical properties in dry and swollen states and low swelling 
rates. Our porous gels offer an unprecedented combination of fast swelling rate, high absorption 
capacity and good mechanical integrity. Additional aspects regarding specific studies from 
Chapters 3-5 are mentioned below: 
 
? In Chapter 3, we developed a double barrel syringe-based foam templating strategy, which 
could be used to create porous gels in any shape and size. Our porous gels swell and expand 
very rapidly in water and revert to their initial unswollen state upon reduction of pH or 
addition of ethanol. We were able to exploit this fast expansion/shrinking of our gels to 
perform mechanical work. This opens new avenues for our gels in mechano-chemical 
engines, soft actuators, and artificial muscles. Previously, the slow response of gels prevented 
their use in such devices. Thus, our fast-responding gel could be a game-changer for such 
systems. 
 
? In Chapter 4, we created gel-sheets that are flexible, foldable, and robust in their dried form. 
This is the first time, to our knowledge, that gels have been synthesized as such large, fabric-
like sheets. Our gel sheets outperform many commercially available absorbent sheets 
including cloth and paper towels in terms of absorption capacity. Thus, they could prove 
useful in clean-up of spilled liquids in a variety of scenarios, including countertops at home 
or in a lab or in hospitals. Additionally, these could be used for absorbing biological fluids 
during surgeries. We are also evaluating them as hemostatic materials that can help in 
stopping bleeding from serious injuries. 
6 
  
 
 
? In Chapter 5, we created oleo-sheets that absorb oils and organic solvents. While the strategy 
of making porous hydrogels by templating aqueous foams is known, this is the first time the 
same strategy has been used with non-aqueous foams. The oleo-sheets are inherently 
hydrophobic and can absorb a range of solvents/oils very quickly. Such large sheets could be 
used to selectively remove oil/solvents from water bodies, e.g., during oil spills or incidents 
of industrial solvent leakages. Lastly, our omni-absorbent sheets offer the ability to absorb 
both oil and water using two sides of the same material. Such a material is also novel, to our 
knowledge, and could prove useful in various applications.  
  
7 
  
 
Chapter 2 
 
Background 
 
 
In this Chapter, we will discuss the basics of gels, porous materials and different 
strategies for making the latter. Then we will review basics of foam stability and the foam 
templating method that will be used to synthesize porous gels in Chapters 3-5. 
 
2.1 Polymer Gels 
 
 
Figure 2.1. Schematic of the structure in a gel. A 3D network of polymer chains is shown. The 
chains are connected at crosslink or junction points. Solvent (such as water) is entrapped in the 
network.  
 
 
 Gels are three dimensional crosslinked networks of polymer chains swollen in a 
solvent.20-23 The polymer chains in the gel can be crosslinked by chemical (covalent) bonds or 
physical bonds (electrostatic, hydrogen bonding or hydrophobic interactions).23,36-38 The ?Jell-O? 
we eat as dessert is a physically crosslinked gel whereas contact lenses are an example of 
chemically crosslinked gels.39 The structure of the crosslinked network in a gel is shown 
schematically in Figure 2.1.  Gels containing water as a solvent are called hydrogels and these 
8 
  
 
are made from hydrophilic polymers.23,36,37 If the solvent is an oil or organic solvent, the gels are 
known as organogels.40-42 Gels are used in many applications, including drug delivery, tissue 
engineering, regenerative medicine, and as biosensors.43-46 On the other hand, organogels are 
mostly used in food technology, cosmetics, and oil recovery.40,41 
 
 Gels with chemical crosslinks are typically synthesized by free-radical polymerization. In 
this method, three main components, i.e., monomers, crosslinkers and initiators interact 
simultaneously to form a polymer network. The reactions in a typical hydrogel synthesis are 
shown in Figure 2.2. Here, monomers (acrylamide, acrylic acid), a crosslinker (N,N?-Methylene 
bisacrylamide, BIS) and an initiator (lithium acylphosphinate, LAP) are dissolved in water. 
When the solution is exposed to ultraviolet (UV) light, the initiator gets cleaved to generate free 
radicals (this is the initiation step). Free radicals are highly reactive species and begin to interact 
with the vinyl groups (i.e., the carbon-carbon double bonds) present on monomers and 
crosslinkers in the propagation step. The vinyl groups then connect with other monomers or 
crosslinkers to propagate, i.e., grow into chains. The monomers have one vinyl group each and 
so they can grow only linearly whereas crosslinkers generally have at least two vinyl groups. 
Therefore when linear chains encounter BIS, they get crosslinked randomly into a network. This 
process will continue to occur until no vinyl sites are left for reaction or all free radicals are 
terminated due to chain combinations. Note that of the two monomers in this example, 
acrylamide is nonionic whereas acrylic acid is anionic. The monomers will copolymerize 
randomly, leading to a net anionic character to the chains and thereby to the gel.   
9 
  
 
 
 
Figure 2.2. Polymerization reactions in acrylamide-co-acrylic acid hydrogel formation. 
Upon UV irradiation, initiator molecules break down to form radicals. Then radicals begin 
reacting with the vinyl groups to form long chains, which eventually get crosslinked due to the 
two vinyl sites present on BIS crosslinkers. Crosslinked polymer network structure is adapted 
from the paper by Hibbins et.al 47    
10 
  
 
Hydrogels responsive to various stimuli can be synthesized by appropriate choice of 
monomers.48 For example, gels of N-isopropylacrylamide (NIPA) respond to temperature.49 
When these gels are heated above their lower critical solution temperature (LCST), which is ~ 
32?C, they shrink, whereas when cooled below the LCST, they swell back to their initial state. 
This is because, above LCST, the isopropyl groups on the NIPA backbone become more 
hydrophobic and aggregate, causing water to be expelled.48,50 Another response is to solvent 
quality: for example, in solvents like ethanol and acetone, gels of acrylamide (AAm) shrink due 
to incompatibility between the solvent and the polymer.51  
 
Ionic gels, where the polymer chains have ionizable functional groups such as 
carboxylate or amines, are responsive to pH: they swell when the chains are ionized and shrink 
when the chains lose their charge.14-17 In fact, the swelling of ionic gels (with ionized groups) in 
water is particularly high due to the electrostatic repulsions between the polymer chains and the 
high osmotic pressure caused by the counterions.14-17 Photos of an ionic gel in initial and swollen 
states are shown in Figure 2.3A. These gels can be engineered to absorb more than 100 times of 
their dry weight and are known as superabsorbent polymers (SAPs) in the literature. Previously 
in our lab, an ionic hydrogel based on sodium acrylate and N,N?-dimethyl acrylamide was shown 
to absorb water up to ~3000 times its dry weight.16 Ionic gels can also be made to collapse by 
adding organic solvents.52  
 
For a gel to swell in organic solvents, the ionic functional groups should be lipophilic and 
bulky in contrast to small groups like carboxylates and amines.53,54 An organogel containing 
groups of tetra-alkylammonium tetraphenylborate has been shown to absorb tetrahydrofuran 
11 
  
 
(THF) to more than 100 times its initial dry weight (Figure 2.3B)53 Note that this organogel 
required special synthesis and such high-swelling organogels are rare in the literature. Unlike 
ionic gels, nonionic gels swell less in water, and their degree of swelling is determined by many 
factors including the affinity of the polymer towards water and the elastic repulsion when chain 
segments between crosslinks are stretched.55 
 
 
Figure 2.3. Examples of a high-swelling ionic hydrogel and an ionic organogel. (A) An ionic 
hydrogel of DMAA-sodium acrylate is shown in its shrunken and swollen states. (B) An ionic 
organogel containing bulky groups of tetra-alkylammonium tetraphenylborate is shown swollen 
to almost 4 times its initial size in tetrahydrofuran (THF).  Image adapted from Reference 53. 
 
2.2 Polymer-Based Porous Absorbents 
Porous absorbent materials are ubiquitous from paper towels to wound dressings. These 
materials are made from fibers of naturally occurring or synthetic polymers. For instance, paper 
towels are made of cellulose, which is derived from plants. Cotton, obtained from flowers of the 
Gossypium herbaceum plant, is again based on cellulose. Most cloth towels are made from 
cotton, linen, and polyester, as are gauze dressings used to absorb blood from wounds. Absorbent 
12 
  
 
materials, including cloth and paper towels, have a porous structure, which allows water to 
diffuse rapidly through capillary action. Figure 2.4 shows the microstructure of a paper 
towel.56 Conventional absorbents based on cellulose have limited water-absorption capacity. For 
this reason, cellulose is either modified with hydrophilic functional groups or combined with 
synthetic ionic polymers.5,57   
 
 
Figure 2.4. Microstructure of a paper towel. SEM micrograph showing that a standard paper 
towel (Bounty? brand) consists of cellulose fibers and is a porous structure. Image adapted from 
Reference 56. 
 
 
Superabsorbent polymers (SAPs) such as polyacrylic acid show very high water-
absorption capacity and are used in sanitary pads, diapers, and absorbent mats.5,17 SAPs are 
generally used in the form of microbeads in these materials (see Chapter 1), which facilitates 
rapid absorption. For a bulk SAP hydrogel to absorb water fast, pores have to be created in the 
continuous polymer network.24,25 Various strategies have been pursued to create pores in gels. 
These include porogen leaching,58 ice-templating,28,58 and foam templating.59,60 In porogen 
leaching, particles of salt, sugar or polymers are dispersed in the monomer solution and 
13 
  
 
polymerized. The particles are later dissolved to create pores. This method provides control over 
pores size and porosity but leaching particles out takes days. Ice-templating involves partial 
freezing of a prepolymer solution to create ice crystals, followed by polymerization. The ice 
crystals are then removed by evaporation. Gels synthesized using this method are referred as 
cryogels in the literature. The major limitation of cryogelation is poor control over porosity. 
Foam templating is our technique of choice and is discussed in detail in the next section.  
 
2.3 Foam Templating 
In foam templating, gas bubbles are dispersed in a monomer solution prior to 
polymerization.59,60 These bubbles can be generated by various means. A simple way is to stir 
the monomer solution vigorously to trap air bubbles.61 This requires the solution to be viscous; if 
not, the bubbles will quickly escape. Another way to introduce bubbles is using microfluidics in 
which discrete gas bubbles of uniform size are generated in the monomer solution.62 A third 
approach is to create gas bubbles through a chemical reaction,60 such as by mixing acetic acid 
and sodium bicarbonate. The acid and base react to generate CO2 gas bubbles in situ. These 
bubbles then have to be stabilized by surfactants. Upon polymerizing this foam, the bubbles are 
trapped in the polymer gel, and when this gel is dried, the bubbles turn into pores (Figure 2.5). 
The bubble size and density will determine the pore size and porosity respectively. Thus, to 
achieve high porosity, a high volume of gas bubbles need to be generated and these should 
remain stable during polymerization. This strategy is simple and easily and is being used 
industrially to make polyurethane foams for cushions and mattresses.63 
 
14 
  
 
 
Figure 2.5. Foam templating technique. A foam (dispersion of gas bubbles in liquid) is 
generated in a monomer solution and stabilized by surfactants. Upon solution polymerization, the 
bubbles get trapped in the polymer network (gel). When the gel is dried, the bubbles become 
pores in the final material.  
 
 
2.4 Foam Stability 
 Foams are colloidal dispersions of gas bubbles in a continuous phase.64-66 In foams, the 
bubble size is governed by a balance between two counteracting forces: surface tension and the 
pressure difference ?P between the interior and the exterior of the bubbles. The latter is given by 
the Young?Laplace equation: 
2?
 ?P =                                                  (2.1) 
r
where, ? represents the surface tension and R is the radius of the bubbles. The surface tension 
tries to minimize the surface area, which implies the bubbles will coalesce. However, the ?P 
counteracts this and an equilibrium between these two forces will dictate the bubble size.  
 
15 
  
 
 
 
Figure 2.6. Amphiphilic molecules. (A) Short amphiphilic molecules with hydrophilic head 
groups and hydrophobic tails. (B) Amphiphilic polymers, with hydrophobic groups attached 
along the hydrophilic backbone of the polymer.  
 
 
The bubbles in a foam have to be stabilized by surfactants.64 Surfactants are amphiphilic 
molecules: they have both hydrophobic and hydrophilic domains. Examples of small-molecule 
and polymeric amphiphiles are shown in Figure 2.6. Surfactants reversibly adsorb at the air-
water interface by orienting their hydrophobic part toward the air and their hydrophilic domain 
toward water. In doing so, they reduce ? and stabilize bubbles against coalescence through 
colloidal repulsion.64,65 Foam stability can also be enhanced by increasing the solution viscosity. 
Viscous solutions flow slowly and thus hinder liquid drainage in foam, which prevents bubbles 
from coming close and coalescing. Foams stabilized by small surfactants such as sodium dodecyl 
sulfate (SDS) and Tween 80 are stable only for a few minutes whereas foams stabilized by 
amphiphilic polymers can be stable for longer times.67-69  
 
16 
  
 
 
 
Figure 2.7. Making foams using a double barrelled syringe (DBS). The DBS contains acetic 
acid in one barrel and sodium bicarbonate in the other barrel. The acid and base react to produce 
CO2 gas bubbles, which get stabilized by the amphiphilic polymer (hmC) present in the acid.  
 
Our lab has been particularly interested in amphiphilic polymers such as hydrophobically 
modified chitosan (hmC) and hydrophobically modified alginate (hmA). Recently we 
synthesized robust, elastic foams by stabilizing bubbles using hmC and hmA.68 These foams are 
generated in situ through a double barrelled syringe (DBS). As shown in Figure 2.7, one side of 
the DBS contains acetic acid and hmC while the other side contains. sodium bicarbonate. 
Bubbles of CO2 are produced at the mixing tip, and these are stabilized by hmC. These foams are 
stable for more than 3 hours. We will employ this technique in Chapters 3 and 4. Aqueous foams 
can also be stabilized by proteins such as casein and whey protein.70 Whipped cream is a well-
known example of such foams. 
 
17 
  
 
While foams in water are well-studied, foams with a nonaqueous continuous phase are 
much less studied.71-73 These foams cannot be stabilized by regular surfactants because 
nonaqueous liquids have very low surface tensions, due to which surfactants do not preferentially 
move to interfaces. Special surfactants based on fluoroalkyls, and siloxanes are needed to 
stabilize such foams, as we will discuss in Chapter 5.  
 
2.5 Chitosan and Hydrophobically Modified Chitosan 
 
 
 
 
Figure 2.8. Molecular structure of chitosan and hydrophobically modified chitosan (hmC). 
The hydrophobes are palmitic (C16) groups.  
 
Chitosan (Figure 2.8) is a linear polysaccharide derived from chitin (poly-N-acetyl-
glucosamine) by deacetylation.74 Chitin is the second most abundant naturally occurring polymer 
next to cellulose. It is present in the shells of crabs, shrimps, crustaceans, and in the cell walls of 
fungi. Chitosan is obtained by hydrolysis of the N-acetyl groups present on the chitin backbone 
under basic conditions. The extent of hydrolysis is termed as the degree of deacetylation and 
usually it ranges from 70% to 99% in commercially available chitosans. Chitin is insoluble in 
water whereas chitosan can be dissolved in acidic solutions below a pH of 6.5. Under acidic 
conditions, the amines get protonated and chitosan becomes a cationic biopolymer. Due to its 
18 
  
 
biocompatibility and its antimicrobial nature, chitosan is being explored for many biomedical 
applications including wound care, drug delivery, scaffold for tissue engineering and 
antimicrobial coatings.75-78 Our lab has been interested in chitosan particularly for its ability to 
stop bleeding, post hydrophobic modification.79,80 
 
 Hydrophobically modified chitosan (hmC) can be easily synthesized by reacting alkyl 
anhydrides with the amines on chitosan via nucleophilic substitution reaction. In this process, 
one amine hydrogen (NH2) is substituted with an acyl group to form amide bonds.79 The 
structure of an hmC modified with palmitic anhydride (C16 hydropbobes) is shown in Figure 
2.8. The hydrophobicity of hmC varies with alkyl tail length and modification percentage, which 
can both be controlled by the reaction stoichiometry. When chitosan is converted to hmC, the 
solution viscosity significantly increases due to hydrophobic interactions. Generally short (C6-
C10) hydrophobes can be attached up to 10% of the free amines. But with longer hydrophobes 
(C14-C18), the modification % needs to be low (< 2%); otherwise the hmC solution will become 
too viscous or the hmC could even become insoluble. For our studies in Chapters 3 and 4, we use 
an hmC that has C10, C12 and C16 hydrophobes together in varying proportions.  
  
19 
  
 
Chapter 3  
Porous Hydrogels that Rapidly Swell and Expand 
 
The results presented in this chapter have been published in the following journal article:  
Choudhary, H.; Raghavan, S. R. ?Superfast-Expanding Porous Hydrogels: Pushing New 
Frontiers in Converting Chemical Potential into Useful Mechanical Work.? ACS Appl. Mater. 
Interfaces14, 13733-13742 (2022). 
 
 
3.1 Introduction 
Polymer hydrogels are cross-linked networks of polymer chains that are swollen in 
water.20-22 Gels can be engineered to absorb significant extents of water (more than 100 times 
their weight).14-17 Such superabsorbent polymer gels (SAPs) find applications in diapers14 and as 
additives that keep the soil moist for plant growth.81 Recently, our lab has reported a class of 
SAPs that absorb 3000 times their weight in water, which is the highest swelling ratio reported in 
the literature.16 SAPs used in applications such as diapers are typically in the form of microscale 
beads,17 which facilitates their rapid swelling, as illustrated in Figure 3.1A. The same gels are 
commonly made in labs as macroscopic solids (e.g., centimeter-scale cubes or cylinders). 
However, a macroscale dry gel will take a long time (?24 h) to swell to its equilibrium size in 
water (Figure 3.1B). The reason for this slow swelling is that it occurs by diffusion of water (of 
diffusivity D) into the gel, and the timescale ? for this diffusion (? = l2/6D) depends on the length 
scale l of the sample. Thus, for a macroscale gel (l ? 1 cm), this timescale will be in hours, 
whereas for microbeads (l ? 10 ?m, i.e., a 1000-fold smaller size), this timescale is reduced to 
seconds. For a large piece of gel to swell rapidly, it is necessary to make it porous.24,25,58,60,82 The 
length scale relevant for diffusion will then be the pore diameter rather than the overall gel size 
(Figure 3.1C). If the pores are microscale and are interconnected, then porous gels can swell at 
rates that are 100?1000-fold higher than those of nonporous gels. 
20 
  
 
 
 
Figure 3.1. Gel-swelling dynamics at different length scales. Dry gels are placed in water at t 
= 0 and allowed to swell (expand) to their final size. Swelling occurs by diffusion of water into 
the dry gel. (A) Microscale gel beads (?10 ?m size) swell in seconds to their final size. (B) A 
solid macroscale gel (?1 cm size) takes ?24 h to expand to its final swollen size. (C) A 
macroscale gel with microscale pores expands much more rapidly compared to (B). In this study, 
one such porous gel is shown to expand to 4? its original size within 15s.   
 
Research on fast-swelling porous gels over the past two decades has focused on 
increasing the swelling rate by enhancing the porosity while simultaneously ensuring that the gel 
is mechanically robust.26-35,83,84 Various strategies have been pursued to introduce pores into gels, 
including porogen leaching,58 lyophilization,82 ice templating,58,82 cryogelation,28 and foam 
templating. The latter is the most popular strategy and involves making foam, i.e., a dispersion of 
gas bubbles in a monomer solution, prior to polymerization.59,60 To ensure the high porosity of 
the final gel (>90%), it is necessary to have a high density of bubbles in the foam and to keep the 
bubbles stable during polymerization. The bubbles then have to be removed, leaving behind a 
porous gel. Various surfactants or amphiphiles have been used to stabilize the above foams. The 
best examples of porous gels so far are those that can swell to equilibrium within a minute; 
however, their swelling extents are low (?50 times) and so the gels do not expand much.85-89 
Gels that swell more seem to expand slower and also appear to be mechanically weak.24,25  
 
If a gel can expand rapidly, its expansion could be exploited for doing work, i.e., the 
chemical energy associated with gel expansion could be converted into mechanical energy. The 
21 
  
 
most striking examples of natural ?mechano-chemical engines? are the muscles in our body. 
Indeed, a long-standing goal for polymer scientists, which can be traced back to de Gennes, has 
been to use polymer gels as ?artificial muscles?.90 Artificial muscles are devices that can be 
reversibly actuated to perform muscle-like motion (expansion, contraction, and rotation) in 
response to external stimuli.91-93 Such motion can be harvested to perform mechanical work: for 
instance, a cycle of gel expansion and contraction (in response to light,94 temperature,95 or salt96) 
can be coupled to the lifting and lowering of a weight. While gels are the ideal candidate for 
mimicking muscles (due to their soft and wet nature, which mimics living tissues), the timescale 
for actuating a macroscopic gel is currently too slow for muscle-like actuation. Thus, a desire for 
faster response times has led researchers to consider alternative materials for such actuators (like 
liquid crystal elastomers or shape-memory polymers) despite these systems being mostly 
unsuitable in a biomedical context.91,97 If gel expansion rates could be increased, then gels could 
be reconsidered for use in mechano-chemical engines. 
 
 Here, we present a new approach that yields porous gels with an unprecedented 
combination of rapid swelling/expansion rates and high swelling extents. Our approach involves 
foaming of a monomer solution by injecting it out of a double-barreled syringe (DBS).68 As 
shown in Figure 3.2, the foam is generated in situ via the reaction of an acid and a base in the 
two barrels of the DBS, which combine to produce CO2 gas in the form of bubbles. The bubbles 
are stabilized by an amphiphilic biopolymer, hydrophobically modified chitosan (hmC), present 
in one of the barrels.69 Monomers (acrylamide and acrylic acid, with cross-linkers) in the foam 
are then polymerized to form a gel around the bubbles. Subsequently, this gel is dried under 
ambient conditions to give a porous solid with a porosity >90% and a pore size around 200 ?m. 
22 
  
 
When this dry gel is added to water, it absorbs water at a rate of 20 g/g?s until an equilibrium is 
achieved in 15 s at about 300? its weight. In the process, each gel dimension increases by 
?20%/s until its final sizes are 4? the original ones (i.e., there is a 3? increase in size). Such 
rapid and appreciable expansion can be easily observed by the eye, and this expansion rate is the 
highest reported thus far to our knowledge. Moreover, the swollen gel is robust enough to be 
picked up by hand. The gels are responsive to pH and solvent quality, and a full cycle of 
expansion and contraction can be completed within about 60 s. We use gel expansion to lift 
weights against gravity, and the power density (260 mW/kg) achieved is better than in any 
previous gel-based actuators to our knowledge. Thus, rapid gel expansion allows the chemical 
potential energy from the gel to be captured in new ways, and this could enable many new 
applications.    
 
  
23 
  
 
3.2 Experimental Section 
 
Materials. Chitosan (medium molecular weight, 250?400 kDa, 99% deacetylated, product code 
43020) was obtained from Primex Corp. (Iceland). Palmitic (C16), decanoic (C10), and lauric 
(C12) anhydrides were purchased from TCI America. All other chemicals were from Sigma-
Aldrich, including the monomers acrylamide (AAm) and acrylic acid (AAc), the cross-linker 
N,N?-methylene(bis)acrylamide (BIS), the photoinitiator lithium acylphosphinate (LAP) (precise 
name: lithium phenyl-2,4,6-trimethylbenzoyl phosphinate), acetic acid, sodium bicarbonate 
(NaHCO3), and sodium hydroxide (NaOH). 
 
Synthesis of hmC. The following procedure, adapted from our previous works,79,80 was used. 1 
wt % chitosan was first dissolved in 0.2 M acetic acid, and an equal volume of ethanol was 
added. The solution was then heated to 65 ?C. C16, C12, and C10 anhydrides were dissolved in 
ethanol in separate beakers and heated to 65 ?C. The anhydride solutions were then added to the 
chitosan solution such that the stoichiometry (with respect to the amines on the chitosan) 
corresponded to the following: C16 anhydride at 2 mol %, C12 anhydride at 5 mol %, and C10 
anhydride at 10 mol %. In total, 17% of the amines on chitosan were functionalized with 
hydrophobic (alkyl) chains by reacting with anhydrides (the reaction is known to follow the 
stoichiometry79,80). The reaction was allowed to proceed overnight, whereupon the chitosan was 
converted into hmC. To precipitate the hmC from this solution, the pH was increased by adding 
NaOH. The precipitate was washed with ethanol several times, dried, and ground into a powder.    
 
Double-Barreled Syringe Preparation. Syringes were obtained from J Dedoes, Inc. The 
dimensions of the barrel and plunger of the DBS were 3 mL ? 3 mL, while its mixing tip was a 3 
24 
  
 
mm ? 16 element blunt tip. Typically, 3 mL of solution was loaded into each barrel. In one 
barrel, a solution of monomers, cross-linker, and hmC in acetic acid was loaded. In the other 
barrel, a solution of 0.1 wt % LAP and NaHCO3 (dissolved to its saturation concentration at 25 
?C, ?1.4 M) was loaded. This composition of the base was chosen to maximize the foam volume 
(note that the foam was limited by the base because the acid was in excess). The DBS was then 
covered with aluminum foil to avoid degradation of the photoinitiator prior to polymerization. 
 
Synthesis of Porous Gels. The foam containing all the reaction components was injected out of 
the DBS into a container, as shown in Figure 3.2. The geometry of the container was varied 
depending on the experimental needs. In many cases, the container was a Ziploc bag (6? ? 4?). 
The foam was spread uniformly in the container to a thickness of 0.5?1 cm and exposed to UV 
light for 2 min to polymerize the monomers. In the process, the foam is converted into a porous 
gel, with the bubbles constituting the pores. This porous gel was placed in water to remove any 
unreacted monomers. The water in the gel was then exchanged by placing in ethanol for 2 h 
followed by ambient drying overnight to give a solvent-free porous gel. Unless otherwise stated, 
the following composition was used in preparing the gels: a total of 25 wt % monomer, with 
AAc and AAm in a 3:1 weight ratio, and with BIS at 0.7 mol % with respect to the total 
monomer. The hmC concentration was 0.5 wt % unless otherwise stated. 
 
Gel Swelling Kinetics. For swelling studies, such as those in Figure 3.4, a porous gel 
(dimensions of ?10 mm ? 10 mm ? 5 mm) was placed in DI water and allowed to absorb water 
for a specific duration. The swollen gel was then removed from the water and excess liquid was 
wiped from the gel before measuring its weight. Afterward, the gel was placed back into the 
25 
  
 
water for another span and the weight was again measured. This procedure was repeated until the 
weight of the gel became constant. All swelling kinetics studies were done in replicates of at 
least three for each gel, and the average swelling ratios are reported.  
 
Optical Microscopy. A small amount of a given foam was injected onto a glass slide and 
allowed to sit for a few minutes. Images were then captured on a Zeiss Axiovert 135 TV inverted 
microscope at 100? magnification. Bubble size distributions were analyzed using the ImageJ 
program. For each foam, at least five images were analyzed to obtain the average bubble 
diameter. 
 
Scanning Electron Microscopy (SEM). The ambient-dried porous gels were further dried in 
vacuum for 4 h to completely remove any residual moisture. The dried materials were cut with a 
sharp blade to expose the internal pore structure and then sputter-coated with gold. A Tescan 
GAIA FEG SEM was used to obtain images of the gels at magnifications from 100 to 500?. 
 
 
26 
  
 
3.3 Results and Discussion 
3.3.1 Synthesis of Porous Gels 
 
 
Figure 3.2. A Schematic of the procedure used to synthesize porous gels. (A) A foam of the 
monomers is prepared using a double-barreled syringe (DBS). One barrel of the DBS is an acidic 
solution of monomers, cross-linkers, and the hmC stabilizer, while the other barrel is a basic 
solution with the UV initiator. At the mixing tip of the DBS, CO2 gas is produced, and bubbles 
of the gas are stabilized by hmC chains. (B) The foam is polymerized by UV light for 2 min. (C) 
The bubbles in the foam are retained during the polymerization while a polymer gel network is 
formed around the bubbles. (D) The gel is dried under ambient conditions to give the porous gel. 
The photo (inset) reveals that the material is a robust solid with a sponge-like texture.   
 
 
Our procedure for synthesizing porous gels is shown schematically in Figure 3.2. In one 
barrel of the DBS, we load a solution in acetic acid of the monomers acrylic acid (AAc) and 
acrylamide (AAm), the cross-linker N,N?-methylene-bis-acrylamide (BIS), and the stabilizer 
hmC. In the other barrel, we load a solution of the photoinitiator lithium acylphosphinate (LAP) 
in sodium bicarbonate. The acidic and basic solutions come into contact at the mixing tip of the 
DBS, whereupon the following reaction occurs:  
 
R-COOH + NaHCO3 ? R-COONa + CO2 (g) + H2O           (3.1) 
 
27 
  
 
The net outcome is the release of carbon dioxide (CO2) gas in the form of bubbles. These 
bubbles get stabilized by the hmC present in the acidic barrel of the DBS, thus resulting in a 
stable foam (Figure 3.2A). The foam is then spread uniformly in a container and exposed to UV 
light at ambient temperature for 2 min (Figure 3.2B). The UV initiates the photopolymerization 
of the monomers, which results in a cross-linked polymer network around the gas bubbles 
(Figure 3.2C). At this stage, the synthesis of the porous gel is complete. For most experiments, 
we work with dried gels, where we completely remove the water from the sample. For this, we 
first do a solvent exchange with ethanol and then dry the gel under ambient conditions (Figure 
3.2D). The final dried material is a white sponge-like solid, as shown by the photo in Figure 
3.2D. We will continue to refer to the dried material as a ?porous gel? for simplicity as has been 
done in the literature.24,25,58,60,82 
 
 Our approach to making porous gels is simple yet unique in many ways. There are 
several key differences compared to previous approaches reported in the literature, and these 
differences will be important in analyzing the microstructure and performance of our gels. First 
of all, we use a DBS and the acid?base reaction to generate the foam in situ, whereas most 
researchers have simply added NaHCO3 (base) to an acidic monomer solution containing a 
surfactant and agitated the sample to produce a foam.26-35,83,84 The DBS gives rise to smaller 
bubbles and a more homogeneous foam compared to simple agitation.68 Second, we use a 
polymeric stabilizer (hmC) rather than a small-molecule surfactant to stabilize the foam.69 The 
hmC here has hydrophobic n-alkyl tails attached to more than 10% of the amines along the 
chitosan backbone. We expect the hmC chains to adsorb on the gas bubbles, with the 
hydrophobic tails directed toward the gas phase (see schematic in Figure 3.2C). The presence of 
28 
  
 
hmC at the gas?liquid interface ensures that the bubbles remain intact during the polymerization. 
Lastly, we use LAP, which is a well-known UV initiator that is highly efficient at producing free 
radicals upon irradiation. Thereby, we are able to complete the UV polymerization in just 2 min. 
By doing the polymerization quickly, we lock in the porosity in the final gel without the bubbles 
dissipating away. For comparison, we have tried other water-soluble UV initiators like Irgacure 
2959, and in that case, more than 30 min was needed to complete the polymerization, during 
which time the foam largely dissipated, leaving a gel with few pores.    
 
3.3.2 Microstructure of the Porous Gels 
The microstructures of the initial foam and the corresponding dried gel are presented in 
Figure 3.3. The foam was prepared with a monomer composition of 18.75 wt % AAc, 6.25 wt % 
AAm, and 0.375 wt % BIS. Then, 0.5 wt % hmC was used as the foam stabilizer. Optical 
micrographs of the foam (Figure 3.3A) reveal close-packed gas bubbles with an average 
diameter of 400 ?m. These bubbles will form the pores in the gel once the monomers are 
polymerized around the bubbles. SEM micrographs of the gel after ambient drying (Figure 3.3B) 
show interconnected pores and thereby an extensive network of open microchannels. The 
average pore diameter from ImageJ analysis is 211 ?m with a standard deviation of 95 ?m. 
Comparing the SEM and optical micrographs, it appears that the majority of bubbles in the foam 
are retained during polymerization and thereby manifested as pores in the dried gel. The porosity 
?gel of the dry gel can be estimated by eq. 3.2 from density measurements: 
?gel
 ?gel =1?   (3.2) 
?bulk
29 
  
 
where ?gel is the density of the dry gel and ?bulk is the density of the bulk, nonporous solid. From 
the measured values in our case for ? 3gel (0.109 g/cm ) and ?bulk (1.187 g/cm3), we find ?gel to be 
91%, indicating a highly porous material. 
 
 
 
Figure 3.3. Microstructure of the foam and the porous gel made using the foam as a 
template. (A) A representative optical micrograph of the foam reveals close-packed small 
bubbles, most of which are spherical. (B) Representative SEM images of the dried porous gel at 
two different magnifications. The images show a highly porous structure with interconnected 
pores.  
  
 
3.3.3 Swelling/Expansion of Porous Gels in Water 
 We now focus on the swelling of the dried porous gels in water (Figure 3.4). A movie of 
this process was recorded and snapshots at specific time intervals are shown in Figure 3.4A. At t 
= 0, a dried gel of dimensions 10 ? 10 ? 5 mm (Photo A1) is placed in water and allowed to 
swell (Photo A2). Within 10 s, the gel expands appreciably and becomes transparent (Photo A3). 
The gel reaches its equilibrium size in just 20 s (Photo A4), beyond which the size remains 
constant. The above experiment was quantified in terms of two parameters, the first being the 
swelling ratio R = mass of swollen gel/mass of dry gel. A plot of R vs t is shown in Figure 4B. 
We note that R increases linearly to more than 100 in the first 5 s and to 300 in 15 s; beyond 20 
30 
  
 
s, R plateaus at 300. This implies a swelling rate of 20 g/g?s from the initial to the final size. 
Correspondingly, the gel size increase is quantified as ?L/L0, where L0 is the initial length (10 
mm) and L the expanded length at time t. The gel dimensions double (i.e., increase by 100%) in 
just 5 s, and the dimensions plateau at 4? their original values (implying a 300% increase) in 15 
s. These numbers translate to an expansion rate of 20%/s. After the swelling is complete, the 
swollen gel with 300? its weight in water (and 4? its original size) is still robust enough to be 
picked up by hand out of the container, as shown in Figure 3.4C.  
 
 From Figure 3.4, it is evident that our porous gels swell and expand rapidly and to an 
appreciable extent (R = 300; 4? the original size). Such rapid and appreciable swelling can be 
easily observed by the eye in real time. Comparing with the literature, we believe ours is the 
fastest swelling rate reported for any superabsorbent gel. A comparison with the best swelling 
rates reported for other (macroscale) porous gels is shown in Figure 3.5. The porous gels of Chen 
et al.,60 Kabiri and Zohuriaan-Mehr,31 Huh et al.,27 and Kuang et al.30 all reached swelling ratios 
R in the 200?300 range but did so over timescales of a minute or more. Thus, the swelling rates 
calculated from these studies are 1?3 g/g?s, whereas it is nearly 10 times higher at 20 g/g?s in our 
case. The same data are also provided in Table 1, along with details of the gel chemistry in each 
study. Table 2 shows data for a second set of porous gels made by cryogelation.85-89 These have 
higher swelling rates (up to 10 g/g?s), but their swelling ratios R are below 40. A further 
comparison to make is regarding the expansion rate. Although numerous papers have been 
published on porous gels, to our knowledge, none have explicitly reported expansion rates. Even 
reliable movies or photos of gels substantially expanding over time are not available. We are 
confident that the expansion rate of ?20%/s for our gel is the highest reported to date. 
31 
  
 
Importantly, this expansion is fast enough (and appreciable enough) to allow work to be 
extracted from the expanding gel, as will be demonstrated later in the paper.  
 
 
Figure 3.4. Typical swelling/expansion of a porous gel in water. (A) At time t = 0, a dried gel 
is placed in water. Snapshots of the swelling gel at various time intervals are shown (B) Swelling 
ratio R and size increase (?L/L0) (%) are plotted against time. The gel absorbs more than 300 
times its dry weight within 15 s, and in the process, its size increases by 300% in 15 s. (C) After 
the swelling is complete, the swollen gel (4-fold larger than the original) is robust enough to be 
picked up and held by hand.  
 
 
 The higher swelling rate of our gels is attributed to differences in the porous 
microstructure (Figure 3.3). SEM micrographs of porous gels made in previous studies show 
well-separated rather than interconnected pores, and the porosities reported in these studies are 
32 
  
 
generally lower than ours.24,29,83 The higher porosity and the interconnected pores, in turn, arise 
due to the different synthesis method used here, which we summarized above in terms of three 
factors: (a) use of a DBS to create an in situ foam, (b) use of an amphiphilic polymer rather than 
a small-molecule surfactant as the foam stabilizer, and (c) rapid UV polymerization around the 
bubbles. Thus, the novel aspects of our synthesis revolve around both colloid science (foam 
generation and stabilization) as well as polymer science (fast polymerization around a template). 
 
 
Figure 3.5. Comparing the swelling rates of porous gels in this study with past ones. The 
swelling rate in this study is 20 g/g?s, whereas those in previous studies were below 5 g/g?s. See 
also Table 1. 
  
33 
  
 
Table 3.1. Porous hydrogels synthesized in past that swell fast to a high extent 
Equilibrium Equilibrium 
swelling swelling Swelling 
Gel type Method ratio(g/g) time (s) rate(g/g.s) Author/year/ref. 
Acrylic acid-Acrylamide (Ionic) Foaming 310 15 20.6 Our work, 2022 
Acrylic acid-Acrylamide Foaming 368 168 2.19 Chen et al.(1999)60 
Kabiri et al. 
Acrylic acid (Ionic) Foaming 350 200 1.75 (2004)31 
PEG-grafted-Acrylic acid-
Acrylamide (Ionic) Foaming 178 120 1.48 Huh et al. (2005)27 
Acryloyloxystarch sulfate-Acrylic Kuang et al. 
acid (Ionic) Foaming 215 60 3.58 (2011)30 
 
 
Table 3.2. Porous hydrogels synthesized in past that swell very rapidly      
Equilibrium Equilibrium 
swelling swelling Swelling 
Method ratio(g/g) time (s) rate(g/g.s) Author/year/ref. 
Gel type 
Dinu et al. 
Acrylamide (Non-ionic) Cryogel 40 4 10.0 (2007)28 
Wu et al. 
PEGDA (Non-ionic) Cryogel 14 10 1.40 (2012)35 
Dinu et al. 
Chitosan (Ionic) Cryogel+porogen 33.5 90 0.372 (2013)85 
N,N-dimethylaminoethyl 
methacrylate-acrylamide (Weak Dragan et al. 
ionic) Cryogel 22.5 120 0.19 (2016)86 
N-isopropylacrylamide-sodium Strachota et al. 
methacrylate (Ionic) Cryogel 35 40 0.875 (2019)87 
 
34 
  
 
3.3.4 Tuning the Swelling/Expansion of Porous Gels 
 We now discuss how the composition of the polymerizing mixture affects the swelling of 
the porous gels. The first variable studied is the ratio of ionic (AAc) to nonionic (AAm) 
monomer. We kept the total monomer at 25 wt % and varied the AAc:AAm weight ratio (0:1, 
1:3, 1:1, 3:1, and 1:0). The BIS cross-linker was fixed at 0.375 wt %, and the hmC was fixed at 
0.5 wt % across all these samples. Ionic gels are generally expected to swell much more than 
nonionic gels, and there are two reasons for this: first, the charged polymer chains will 
electrostatically repel each other, and second, the osmotic pressure will be higher in ionic gels 
due to the counterions.14,16,17 
 
 The data for the different gels are shown in Figure 3.6, and as expected, we find that the 
higher the fraction of ionic monomer, the greater the swelling ratio R for the gels (Figure 3.6A). 
Specifically, the pure nonionic gel (0:1) absorbs very little water (R = 35), whereas the pure ionic 
gel (1:0) swells 10 times as much (R = 350). These differences are also evident from the photos 
in Figure 3.6B comparing the dry and swollen gels. We did note that the pure ionic gel swelled 
so much that it was difficult to handle. In comparison, the gel with the 3:1 AAc:AAm ratio 
swelled up to R = 310 and still had adequate mechanical integrity in its expanded state. So, this 
was the composition of choice, which is employed in Figures 3.3 and 3.4. We also examined the 
swelling rate of these gels, and plots of R vs t are shown in Figure 3.6C,D. All the ionic gels, 
regardless of the ionic monomer content, swelled at a high rate (? 20 g/g?s) and were fully 
swollen within 20 s. This result suggests that all the ionic gels have similar porosity and pore-
connectivity. 
35 
  
 
 
Figure 3.6. Effect of ionic monomer content on gel-swelling extent and kinetics. (A) 
Swelling ratios R at equilibrium of porous gels with different proportions of ionic monomer 
(acrylic acid, AAc) to nonionic monomer (acrylamide, AAm). Note: R = mass of swollen 
gel/mass of dry gel. During synthesis, the total monomer (AAc+AAm) was maintained at 25 
wt% while the weight ratio of AAc:AAm was changed. (B) For visualization of the data in (A), 
photos of the various gels are shown in the dry and swollen states. All the ionic gels swell 
significantly. (C) Kinetics of gel-swelling for each of the gels in (A). (D) Zoomed-in plot of the 
initial data in (C), showing that all the ionic gels swell at roughly the same rate (i.e., the initial 
slopes are similar).    
 
 Next, we studied the effect of cross-link density on gel swelling. For this, porous gels 
were prepared at various concentrations of the cross-linker BIS. In the typical porous gels 
(Figures 3.3 and 3.4), the BIS was fixed at 0.375 wt %, which corresponds to 0.7 mol % relative 
to the total monomer (AAc + AAm). We now varied the BIS fraction from 0.2 to 7.5 mol % 
while keeping the total monomer at 25 wt %, the AAc:AAm ratio at 3:1, and the hmC at 0.5 wt 
%. Figure 3.7 shows the swelling ratio R for these gels as a function of BIS mol %. As expected, 
36 
  
 
R decreases as BIS increases (Figure 3.7A). This is because when the cross-link density is 
higher, chain segments between cross-links will be stretched more as the gel expands and 
thereby pay a higher entropic penalty for swelling.20-22  Differences in R are also shown by the 
photos in Figure 3.7B comparing the dry and swollen gel sizes. The highest R = 450 is for the 
lowest BIS (0.2 mol %), and this gel expands the most, whereas R reduced to 90 for the highest 
BIS studied (7.5 mol %). The gel with 0.2 mol % BIS lacked mechanical integrity, which is why 
we fixed BIS at 0.7 mol % (i.e., 0.375 wt %) for the rest of our studies. 
 
 
Figure 3.7. Effect of crosslinker concentration on gel-swelling extent. (A) Swelling ratios R 
of porous gels as a function of the crosslinker concentration. The gels were synthesized with a 
total monomer (AAc+AAm) content of 25 wt% and with the AAc:AAm ratio at 3:1 by weight. 
Only the concentration of the N,N?-methylene(bis)acrylamide (BIS) crosslinker (mol% with 
respect to the total monomer) was varied. (B) For visualization of the data in (A), photos of the 
gels are shown in the dry and swollen states. The less crosslinked the gel, the more it swells. 
 
 We then proceeded to vary the concentration of the hmC stabilizer. This has an effect on 
the stability of the foams and thereby on the synthesis of the resultant porous gels. For these 
studies, the total monomer was 25 wt %, the AAc:AAm ratio was at 3:1, and the BIS was at 
0.375 wt %. The hmC was varied between 0.01 and 0.5 wt %, and first, the foams were analyzed 
for their extent of stability as well as their microstructure. Foams were injected into vials, and the 
37 
  
 
foam height was recorded vs time. The half-life t1/2, which is the time when the foam dissipates 
to half its initial height, was used as an indicator of foam stability.68,69 Next, optical micrographs 
of the foams were analyzed by ImageJ and the average bubble diameter Davg was measured. 
These two parameters (t1/2 and Davg) are plotted in Figure 3.8A. As the hmC is raised, t1/2 
increases, indicating that the foams become more stable, likely because the bubbles are more 
densely coated with hmC chains. For instance, the t1/2 for 0.5 wt % hmC is around 25 min, 
whereas for 0.01 wt % hmC, it is just 2 min. Correspondingly, the bubble diameter Davg is 
smaller in the more stable foams?there is a halving of Davg as hmC is increased from 0.01 to 0.5 
wt %. This is because hmC promotes smaller bubbles (i.e., higher gas?liquid interfacial area) 
since hmC adsorption reduces the interfacial tension.68,69 
 
 Porous gels produced by templating the above foams were analyzed by SEM. At the 
lowest hmC tested (0.01 wt %), the t1/2 for the foam (2 min) is comparable to the time it takes to 
generate and polymerize the foam (2?3 min). Thus, a large fraction of the bubbles in this foam 
would have vanished or coalesced into larger bubbles before they could be entrapped by 
polymerization. This is reflected in the SEM images of the resulting porous gel (Figure 3.8B), 
where only a few large pores are seen, and these do not seem to be interconnected with all their 
neighbors. In the case of 0.1 wt % hmC foam, the t1/2 for the foam is increased to 5 min, and the 
SEM of the resulting porous gel reveals numerous interconnected pores. This indicates that the 
bubbles in this foam do remain intact sufficiently long to get locked in by polymerization. 
Interestingly, the swelling ratios for these porous gels (Figure 3.8C) indicate that the swelling is 
the highest for the 0.1 wt % hmC foam (R = 330), whereas upon raising the hmC to 0.5 wt % 
(the typical value), there is a slight decrease in R to ?300. We emphasize that at 0.5 wt % hmC, 
38 
  
 
the t1/2 of 25 min is ample for completing the polymerization (which typically takes just 2 min). 
In other words, the bubbles in the foam largely stay intact as we conduct the UV polymerization. 
 
 
 
Figure 3.8. Effect of stabilizer (hmC) concentration on precursor foams and the 
corresponding porous gels. A foam stabilized by a given concentration of the amphiphilic 
polymer hmC (hydrophobically modified chitosan) is injected into a vial and the time for the 
foam to dissipate to half its fresh height (t1/2) is used as an indicator of foam stability. The 
bubbles in the foam are also analyzed by optical microscopy and the average bubble diameter 
Davg is determined from the images. (A) Plot of t1/2 and Davg vs. hmC concentration. As hmC is 
increased, the bubbles become smaller and the foam stability increases. (B) Foams with different 
hmC content were used to synthesize porous gels, with other compositional variables held 
constant: the total monomer was 25 wt%, the AAc:AAm ratio was 3:1, and the BIS was at 0.375 
wt%. SEM micrographs of the dried gels are shown. Interconnected pores are seen when the 
hmC is 0.1 wt% (B2) or 0.5 wt% (B3). (C) Swelling ratios at equilibrium of the above porous 
gels. 
 
39 
  
 
 Our technique for synthesizing porous gels is versatile and allows gels to be made in 
various shapes. This can be done by simply taking a mold of a desired shape and injecting the 
foam into the mold followed by polymerization. Porous gels with circular, triangular, and 
rectangular cross sections are shown in Figure 3.9. Upon swelling in water, all these gels retain 
their shape while swelling by R ? 300. That is, the expansion is isotropic, and each dimension of 
the dry gel pieces increases by ?3? in the swollen state. Such isotropic expansion implies that 
the pores in the dry gel are also isotropic, i.e., oriented along random directions. 
 
 
Figure 3.9. Porous gels in different shapes, before and after swelling. (A) Porous gels with 
the same composition are synthesized in different shapes: with circular, triangular, rectangular, 
and square cross-sections. The image shows the gels in their initial (dry) state. (B) The same gels 
after swelling in water. All gels swell isotropically by 300? their initial weight. Each dimension 
of the initial shapes in (A) is increased by ~ 3?. 
 
3.3.6 pH-Induced Expansion/Contraction of Porous Gels 
 
 The polymer chains in our porous gels are anionic due to the ionic monomer (AAc) used, 
which means that there will be carboxylate (COO?) groups along the chains. These groups will 
40 
  
 
remain ionized (deprotonated) as long as the pH is above the pKa of AAc (4.2). However, when 
the pH is reduced below the pKa, the groups will be protonated (to COOH) and the gel will then 
behave like a nonionic gel. As seen in Figure 3.6, nonionic gels swell and expand much less than 
ionic ones. So, a reduction in pH should cause an expanded ionic gel to shrink and collapse. Will 
this collapse also occur rapidly? 
   
 
Figure 3.10. Response of porous gels to pH. The gel swells (expands) at ambient and higher 
pH and shrinks (contracts) at low pH. Repeated cycling between pH 3 and 10 is done, and the 
swelling ratio R during these cycles is plotted. Both swelling and shrinking occur rapidly. A full 
cycle is completed in ?60 s for the first cycle and ?90 s for subsequent cycles. 
 
41 
  
 
 We studied this by repeated incubation of a gel in water at pH 3 (below the pKa) and pH 
10 (well above the pKa). The results, shown in Figure 3.10, are for a gel with the same 
composition as the one studied earlier in Figures 3.3 and 3.4. First, the dry gel was swollen in pH 
10 water and this takes ?20 s as found previously in normal (pH 7) water. Next, the swollen gel 
(R = 300) was removed and placed in pH 3 water. We observed that the swollen gel shrunk to a 
much smaller size (R = 60) in just 30 s. A full first cycle was thus completed in about 60 s. Next, 
we tried a second cycle, and in this case, the shrunken gel reswelled in pH 10 water and took 
?60 s to expand to its initial size (R = 300), while at pH 3, the gel shrunk again to an R of 60 in 
30 s. The timescales and swelling extents were similar for the third and fourth cycles; each full 
cycle was completed in about 90 s. The results demonstrate that the porous gel is capable of fast 
and reversible transitions between expanded and contracted states by changing the pH. Note also 
that the gel is able to mechanically withstand repeated cycling.  
 
 Similar cycling between swollen and shrunken states can be induced in other ways. For 
example, a swollen gel can be shrunk by placing in water (at ambient pH) containing a high 
concentration of salt (NaCl). The shrinking in this case will occur because salt ions will screen 
the electrostatic repulsions between chain segments in the gel.14,16,17 The gel also can be shrunk 
by placing in a water-miscible solvent like ethanol. In this case, the shrinking can be attributed to 
two reasons.20-22 First, the ionic groups along the polymer chains will be less ionized (i.e., revert 
to their undissociated form) in a solvent of lower polarity than water. Second, the affinity of the 
polymer for the solvent (enthalpic contribution to gel swelling) will be reduced in ethanol 
compared to water. 
 
42 
  
 
3.3.6 Mechanical Work from Rapid Gel Expansion 
 Finally, we proceeded to study if gel expansion upon swelling could be harnessed to 
perform mechanical work. As noted in the Introduction, converting the chemical energy in a gel 
to mechanical energy has been a long-standing challenge for polymer scientists.91-96 For our first 
set of studies, we created a porous gel in the form of a long cylinder (?2 cm) with a diameter of 
5 mm. This dry gel (mass of 40 mg) was placed vertically in a glass syringe, and a load of mass 
m was placed on top of the gel (Figure 3.11, Photo A1). Water was then added slowly from the 
top, which induced the gel to expand rapidly in all directions. As the gel expands, it is thereby 
able to lift the load up by a certain height h against gravity (Photo A2). Because of the long 
cylindrical shape of the gel, its expansion in the vertical direction is readily visible in real time. 
 
 The work done over the entire process in Figure 3.11 is given by W = mgh, where g is the 
acceleration due to gravity. Upon increasing the load, the height h to which the gel can lift the 
load decreases (Figure 3.11B), which is to be expected, but more work is done in lifting a heavier 
load (e.g., 6 g) than a lighter one (e.g., 2 to 3 g). A plot of W vs mass m (Figure 3.11C) shows a 
peak in W at m ? 6 g, and for higher m, W decreases. The peak in W corresponds to 0.42 mJ of 
energy, which is when the gel lifts a load of 5.3 g by 8 mm. The fact that this lifting is done in 
?40 s implies a power of 10.5 ?W, and when divided by the gel mass (40 mg), we obtain a 
power density of 260 mW/kg. For comparison, a recent attempt at using microscale gel beads in 
an osmotic engine was able to achieve a slightly lower power density of 230 mW/kg.96 In that 
study, the gel beads were placed on a sieve (i.e., it was not a macroscopic gel), and the work 
done by the gel beads was used to push an external load of 6 kPa over a period of 10 min (i.e., 
their load was larger, while their timescale was 15 times slower than ours).  
43 
  
 
 
 
Figure 3.11. Extracting mechanical work from the expansion of a porous gel. (A) A 
cylindrical porous gel is placed in a syringe and on top of this cylinder, a load of mass m is 
placed (Photo A1). As soon as water is added, the gel swells and expands, thereby lifting the 
weight by a height h (Photo A2) (B) The height to which the load is lifted by the gel is plotted vs 
the mass m of the load. (C) The work done by the gel in lifting the load m by a height h (W = 
mgh) is plotted against the mass m. 
 
44 
  
 
 
Figure 3.12. Reversible lifting and lowering of a load by the expansion and contraction of a 
porous gel. (A) A cylindrical porous gel is placed in a syringe, and a load is placed on it (Photo 
A1). When water is added, the gel absorbs water and expands, thereby lifting the load by a height 
h (Photo A2). Next, when ethanol is added, the gel contracts (by expelling solvent), and thereby, 
the load is lowered to its initial position (Photo A3). (B) Repeated cycling is done in water and 
ethanol, and the position (h) of the load is plotted across three such cycles. A full cycle is 
completed in ?70 s. 
 
 The work done by the porous gel can also be cycled in a reversible fashion, as shown in 
Figure 3.12. The setup is similar to that in Figure 3.11, with a cylindrical gel placed vertically in 
the barrel of a syringe and a load of 1 g on top of it (Photo A1). When water is added from the 
top, the gel expands and lifts the load up by a height of around 10 mm in 30 s (Photo A2). Next, 
45 
  
 
we add ethanol from the top, and this causes the gel to contract, thereby lowering the load to 
roughly its initial position (Photo A3). This contraction occurs in 40 s. We then repeat the cycle 
by adding fresh water followed by ethanol. The plot of load height vs time over three cycles is 
shown in Figure 3.12B. A full cycle takes about 70 s. There is a slight decrease in the height 
reached in cycles 2 and 3 compared to cycle 1, and this is because not all the solvent is removed 
from the gel from cycle to cycle. Nevertheless, we are still able to extract rapid and reversible 
work out of the gel over timescales comparable to those shown earlier in Figures 3.4 and 3.10. 
 
3.3.6 Other Applications 
 Rapid gel expansion can also prove useful in other contexts apart from mechanical lifting 
of loads. One possibility is in blocking the flow of water, which can become important in homes 
that are in danger of flooding. There already exist products in the market (based on gel beads) 
that claim to be able to create a barrier that can block and divert flood water. We constructed a 
setup in the lab to evaluate flow blocking, one in which we could compare our macroscopic 
porous gel with gel beads (Figure 3.13). The setup involves the barrel of a syringe whose bottom 
is covered with a wire mesh that allows water to flow out. A small piece of a paper towel is 
placed on the wire mesh, and a cylindrical gel (40 mg) is placed on it (Figure 3.13A, Photo 1). 
When water is added from the top, initially it flows out of the wire mesh at the bottom (Photo 2 
at t = 3 s). However, by t = 15 s, the gel has expanded and filled up the syringe, thereby 
completely blocking the downward flow of water (Photo 3). No flow of water occurs for 
subsequent times (Photo 4), indicating that the expanded gel has formed an effective barrier. 
Note that there is a column of liquid water above the gel in Photos 3 and 4. 
 
46 
  
 
For comparison, we conducted experiments with gel beads. In the experiment shown in Figure 
3.13B, an equivalent weight (40 mg) of sodium polyacrylate gel beads (that are commonly used 
in diapers) is placed on the paper towel (Photo 1). When water is added from the top, the beads 
start swelling, but water still flows out at the bottom (Photo 2). As more water is added, the 
beads continue to swell and the water column gets thickened as a result. However, water 
continues to flow out through the bottom of the tube (Photos 3 and 4), indicating that the gel 
beads are unable to form a sufficient barrier to water flow. Basically, the more water that is 
added, the more the beads tend to get ?diluted? in the water column. In a variation of this 
experiment, the beads were placed in an enclosed ?pouch? above the wire mesh (water could still 
enter the pouch). In this case, the beads swelled in the pouch, but water still flowed around the 
pouch and out at the bottom. Collectively, the experiments in Figure 3.13 demonstrate the 
advantages of a macroscopic fast-expanding gel over gel beads. 
 
47 
  
 
 
 
Figure 3.13. Gel-expansion as a way to block the flow of water. A comparison is done 
between a macroscopic porous gel and commercial gel-beads (both of the same weight of 40 
mg). The setup involves a syringe with an open bottom that is covered by a wire mesh (see inset) 
and then a small piece of a paper towel. (A) A cylindrical porous gel is placed on the paper towel 
at t = 0 (1). When water is added from the top, initially it flows out through the wire mesh at the 
bottom (2), but by 15 s, the gel is expanded and fills the syringe, thus blocking the flow (3), and 
no further flow is observed even after 5 min (4). (B) Gel-beads are placed on the paper towel at t 
= 0 (1). When water is added from the top, the beads swell and thicken the water column (2, 3), 
but water continues to flow out through the bottom (2, 3, 4). 
48 
  
 
3.4 Conclusions 
 
 We have devised a simple strategy to create robust porous gels that swell and expand 
very rapidly. Our approach involves the in-situ foaming of a monomer solution (mixture of AAc 
and AAm) using a DBS that has acid and base in its two barrels. Gas (CO2) is generated at the 
mixing tip of the DBS by the acid?base reaction, and gas bubbles are stabilized by the 
amphiphilic polymer hmC in the acidic barrel. The monomers are then UV-polymerized to form 
the gel around the bubbles, and the material is then dried under ambient conditions. SEM shows 
a network of interconnected pores in the dried material. When this dry gel is added to water, it 
absorbs water at a rate of 20 g/g?s until an equilibrium is achieved in 15 s at about 300? its 
weight. In the process, the gel size increases by 20%/s until its final sizes are 4? the original ones 
(i.e., there is a 3? increase in size). Such rapid and appreciable expansion can be easily observed 
by the eye. To our knowledge, this is the highest expansion rate reported for gels. Expanded gels 
can be shrunk by decreasing the pH, adding salt, or adding ethanol. Reversible expansion?
contraction cycles, where the gel expands by absorbing 100? water and then contracts by 
expelling 100? water, can be completed in about 60 s. We have used gel expansion to lift loads 
against gravity. A 40 mg gel is able to perform ?0.42 mJ of work over 40 s, which translates into 
a power density of 260 mW/kg. This ability to harness the chemical potential of the gel to do 
useful mechanical work could be a game changer for many applications, including in the creation 
of artificial muscles. 
  
49 
  
 
Chapter 4  
Porous Gel-Sheets for Absorbing Aqueous Liquids    
 
4.1 Introduction 
 Materials that can absorb aqueous liquids (including blood) have been in use throughout 
the course of human history.1-3,5-7 Common absorbent materials include cloth towels (which are 
woven from natural or synthetic fibers) as well as paper towels, napkins or pads. These materials 
generally have a porous structure, which helps in imbibing water through capillary action.12,13 
Sponges or fibrous mats are other classes of porous materials commonly used in homes or labs 
for absorbing spilled liquids. Absorbents also find use in healthcare (e.g., wound dressings made 
of gauze to mop up blood)98,99 and in agriculture.100 Recently, new classes of porous materials 
have been developed as absorbents, and the technical names for these dry materials include 
aerogels,101,102 xerogels,103 and nanofiber mats.104 An ideal absorbent should have a high liquid-
absorption capacity, fast absorption rate, and good mechanical properties (it needs to be strong, 
yet flexible) in both its dry state as well as when full of liquid. To absorb aqueous fluids, the 
material must also have hydrophilic properties.   
 
 Although cloth and paper are the most common absorbents for water, their water-
absorption capacity is limited (Figure 4.1A). It is well-known that much higher extents of 
absorption can be achieved through the use of hydrogels made from superabsorbent polymers 
(often abbreviated as SAPs).16,19,46 Indeed, commercial absorbent pads such as diapers typically 
have SAPs in the form of microbeads sandwiched between sheets of fabric or paper.3,5 Hydrogels 
are three-dimensional (3-D) networks of polymer chains crosslinked by covalent bonds.6,19 SAP 
50 
  
 
hydrogels can swell to more than 100 times their dry weight.16,19,46 However, when a bulk 
(centimeter-scale) SAP gel is dried, it usually becomes a brittle solid. Therefore, for SAPs to be 
used in diapers, they are made in the form of microbeads. The small sizes of these beads ensures 
adequate mechanical properties and also a fast swelling rate. Still, it is worth noting that a diaper 
is very different from a cloth or paper towel. In a diaper, the need for different layers to sequester 
the SAP beads means that the material is thicker and not flexible or foldable like towels. 
 
 In recent years, researchers have looked to integrate SAP gels onto fibers (e.g. by coating 
fibers with gel-beads) to achieve a combination of cloth-like flexibility as well as high absorption 
capacity.105-108 However, these studies are yet to translate into commercially viable absorbents 
because the synthesis is difficult or laborious. In parallel, researchers have also tried to introduce 
pores into SAP hydrogels.58-60,109,110 The most common strategy for this is foam-templating, 
where a monomer solution is foamed and the polymerization is then done around the bubbles in 
the foam.60,109 When such a porous gel is ambient-dried or freeze-dried (lyophilized), a dry 
porous solid is obtained. However, these are brittle solids (Figure 4.1B) and moreover, are 
usually prepared in small sizes (with their largest dimension usually being a few millimeters). 
The same limitations hold true for other absorbent materials like aerogels, cryogels, and 
electrospun mats of nanofibers. Summarizing the current state-of-the-art, absorbent materials 
developed thus far do not simultaneously achieve the convenience and durability of cloth or 
paper (Figure 4.1A) as well as the absorbency of SAPs (Figure 4.1B).   
51 
  
 
 
Figure 4.1. Comparison between current water-absorbents and the gel sheets developed in 
this study. Current absorbents fall into two categories: (A) Pads or towels made from cloth or 
paper, which are soft and flexible, but have low absorption capacity. (B) Superabsorbent polymer 
(SAP) gels, which absorb much water, but are hard and brittle. (C) Our gel sheets combine the 
desirable properties of both the above while avoiding their drawbacks: they are soft, foldable and 
flexible, while also exhibiting high water absorption. Note that the sheet expands as it absorbs 
water. Scale bars: 1 cm.   
 
  
 In this paper, we show for the first time that absorbents can be made in a cloth or fabric-
like form (i.e., as flexible sheets, a few mm thick) while still retaining a high capacity to absorb 
water (100? swelling ratio) as well as the ability to absorb rapidly (within seconds). What is 
more, we create these absorbent sheets by a simple and scalable process that allows sheets to be 
prepared at macroscopic sizes (e.g., 10 ? 10 cm) in the lab. Our process involves foam-
templating, where a gel of acrylate monomers is formed around CO2 bubbles, followed by 
ambient drying of this gel. The dried sheets are flexible and soft ?  they can be folded and rolled 
up like fabrics (Figure 4.1C). At the same time, the sheets are robust ? they can sustain a tensile 
stress up to 2 kPa and compression by 85% without being damaged. While the sheets have a 
fabric-like feel, they still behave like hydrogels. Unlike any sponges or absorbents made from 
fabric or paper, our gel sheets expand as they absorb liquids, as shown by Figure 4.1C. The 
sheets can absorb a variety of aqueous fluids, including blood, and their absorption capacity is 
52 
  
 
high. Due to their unique properties, these gel sheets could be useful in cleaning up spilled 
liquids in a variety of locations, including homes, labs, and hospitals. They could also be useful 
tools for absorbing biological fluids during surgeries or other medical procedures.    
 
4.2 Experimental Section 
Materials. Chitosan (medium molecular weight, 250-400 kDa, 99% deacetylated, Product Code 
43020) was obtained from Primex Corp. (Iceland). Palmitic (C16), decanoic (C10), lauric (C12) 
anhydrides and tetraethyleneglycol diacrylate (TEGDA) were purchased from TCI America. 
Ethylene glycol (EG), propylene glycol (PG), polyethylene glycol with an MW of 200 (PEG-
200), and polyethylene glycol with an MW of 400 (PEG-400) were obtained from Thermo Fisher 
Scientific. All other chemicals were from Sigma-Aldrich, including the monomers acrylamide 
(AAm) and acrylic acid (AAc), the crosslinkers polyethylene glycol diacrylate (PEGDA) with an 
MW of 575, N,N?-methylene(bis)acrylamide (BIS), the photoinitiator lithium acylphosiphinate 
(LAP) (precise name: lithium phenyl-2,4,6-trimethylbenzoyl phosphinate), acetic acid 
(CH3COOH), sodium bicarbonate (NaHCO3) and sodium hydroxide (NaOH). All the 
commercial absorbents were either purchased from Amazon Corp. or the local supermarket 
including Sungbo pads, Shamwow? towels, McKesson? gauze dressing, Carrand sponge, 
Bounty? paper towels, and Always? sanitary pads.  
 
Synthesis of hmC. The hmC was synthesized by the same method as in our previous 
studies.68,69,79,109 Briefly, a 1% chitosan solution was first prepared by dissolving in 0.2 M acetic 
acid. Then an equal volume of ethanol was added and the mixture was heated to 65?C. The 
anhydrides (C16, C12 and C8) dissolved in ethanol were added in accordance with the following 
53 
  
 
stoichiometry (with respect to the amines on the chitosan): C16 at 2 mol%, C12 at 5 mol% and 
C8 at 6 mol%. The anhydrides and the chitosan were allowed to react for 18 h, whereupon the 
chitosan was converted to hmC. The reaction is known to follow the stoichiometry and thus a 
total 17% of the amines on chitosan are functionalized with hydrophobes. The hmC was 
precipitated by increasing the solution pH above the pKa of chitosan (~6.5) using NaOH. The 
precipitated hmC was washed three times with ethanol and dried.  
 
Double-Barreled Syringe (DBS) Preparation. DBS syringes with each barrel volume of 3 ml 
and mixing tip of dimensions 3 mm were purchased from J Dedoes, Inc. The DBS were usually 
loaded with 3 mL of solution in each barrel. One barrel was loaded with a solution containing 
monomers, crosslinker, hmC and acetic acid while the other barrel was loaded with a solution of 
0.1% LAP and NaHCO3 (dissolved to its saturation concentration at 25?C, ~ 1.4 M). The DBS 
was covered with aluminum foil until use to prevent degradation of the photo initiator. 
 
Synthesis of Gel Sheets. The foam was injected out of the DBS into a Ziploc bag of size 4? ? 6?, 
as shown in Figure 4.2. The Ziploc bag with the foam was compressed gently between glass 
slabs to spread the foam into a thin layer. It was then placed under UV light for 2 min to 
polymerize the monomers. Upon polymerization, the bubbles get trapped in a polymer network. 
Then unreacted components are washed away by placing this gel in excess water. The water in 
the gel was then replaced with ethanol and plasticizer by performing a solvent exchange for 3 h. 
This gel was dried overnight at room temperature to give a gel sheet. Gels were prepared with 
the following composition unless stated otherwise: a total of 25 wt% monomers, with AAc and 
AAm in a 3:1 weight ratio, and with the crosslinker PEGDA at 2.5 mol% with respect to the total 
54 
  
 
monomers. The hmC concentration was 0.625 wt% and gels were solvent exchanged in a 15/85 
mixture of glycerol/ethanol.  
 
Optical Microscopy. A thin slice was cut from the gel sheet using a sharp blade and the exposed 
structure was captured on a Zeiss Axiovert 135 TV inverted microscope at 100? magnification.  
 
Scanning Electron Microscopy (SEM). A thin slice was cut from the gel sheet using a sharp 
blade and the exposed structure was sputter-coated with gold for 1 min. Images at various 
magnification (100 to 500?) were obtained using a Tescan GAIA FEG SEM. 
 
Tensile Testing. Tensile tests were performed using an Instron Model 68SC-05 instrument. 
Tests were conducted according to the protocol recommended by the American Society for 
Testing and Materials (ASTM). Gel sheets of 4-mm thickness were cut into a dog-bone shape 
with narrow width of 14 mm, an overall width of 19 mm and an inner length 35 mm. Each end of 
the sample was covered with 24-grit sandpaper and gripped between the jaws of the Instron to 
avoid any slippage. The sample was then stretched at a rate of 2 mm/min, and the force was 
recorded during this process. The data were converted to stress vs. strain plots. At least three 
samples were tested for each gel sheet. 
 
Compression Testing. Compression experiments were performed using an AR2000 stress-
controlled rheometer (TA Instruments) in squeeze-test mode.111,112 Experiments were done at 
25?C using a parallel plate geometry (40 mm diameter). Gel sheets of thickness 15 mm were cut 
to a size of 25 ? 25 mm and placed between the parallel plates at the centre. The gel piece was 
55 
  
 
compressed at a constant rate of 50 ?m/s and the normal-stress transducer was used to record 
normal force during this process. The normal force was then converted into stress by dividing 
with gel initial cross section area and strain was calculated using gap between the parallel plates 
data. The experiments were stopped when transducer limit was approached (normal force  ? 50 
N). 
 
Rheological Studies. Rheological experiments were conducted on an AR2000 stress-controlled 
rheometer (TA Instruments) at 25?C. A cone-and-plate geometry (2? cone, 20 mm diameter) was 
used to perform steady-shear rheology on the xanthan gum (XG) solutions. From the plots of 
viscosity vs. shear-rate, the zero shear viscosity of the solutions was obtained from the plateau 
region at low shear-rates. 
 
Absorption Studies with Gel Sheets. For liquid absorption studies specific size gel piece was 
allowed to saturate in water and then held vertical to drain out excess liquid. The amount of 
water absorbed by gel sheet was recorded once it stopped dripping. 
 
 
 
 
 
56 
  
 
4.3 Results and Discussion 
4.3.1 Synthesis of Gel Sheet 
The first step in synthesizing the gel sheets is to make a stable foam containing the 
monomers, which is then polymerized by ultraviolet (UV) light (Figure 4.2). To make the foam, 
we use a double barrelled syringe (DBS). In one barrel of the DBS, we load a solution in acetic 
acid (CH3COOH) of the monomers acrylic acid (AAc) and acrylamide (AAm), the crosslinker 
polyethylene glycol diacrylate (PEGDA), and a polymeric stabilizer. In the other barrel, we use a 
solution of the photoinitiator lithium acylphosphinate (LAP) in sodium bicarbonate (base). When 
both barrels of the DBS are plunged, the acid and base meet at the mixing tip, whereupon the 
following reaction occurs:68,109  
 
R-COOH + NaHCO3 ? R-COONa + CO2 (g) + H2O           (4.1) 
 
This results in the release of carbon dioxide (CO2) gas in the form of bubbles. The 
polymeric stabilizer, which is hydrophobically modified chitosan (hmC), adsorbs on the bubbles 
and thus stabilizes them.69,79,80 We extrude the foam out of the DBS into a Ziploc bag whose end 
is then closed (Figure 4.2A). The Ziploc bag with the foam is then compressed between two 
glass slabs to spread the foams uniformly (Figure 4.2A). Next, the foam is exposed to UV light at 
room temperature for 2 min (Figure 4.2B). The monomers thus get polymerized into a polymer 
network around the gas bubbles (Figure 4.2B). At this stage, we have a gel with pores, and we 
cut the Ziploc bag to take it out. This porous gel is allowed to swell in water, then removed and 
placed in a mixture of glycerol and ethanol. After the solvent exchange is complete, it is dried 
under ambient conditions. The final dried material is a soft fabric-like sheet (Figure 4.2C) and 
we will refer to it as a ?gel sheet? in this chapter.   
57 
  
 
 
 
 
Figure 4.2. Schematic of the procedure used to synthesize gel sheets. (A) A polymerizable 
foam is injected into a Ziploc bag using a DBS. In the foam, bubbles of CO2 are stabilized by the 
polymeric stabilizer hmC. Glass slabs are used to compress the foam into a thin layer. (B) The 
foam is polymerized by UV light for 2 min. The bubbles remain intact and a polymer network is 
formed around the bubbles. (C) The water in the gel sheet is solvent-exchanged with a 15/85 
glycerol-ethanol solution, followed by ambient drying. The dry gel-sheet is soft and flexible.  
 
 
Several aspects of our approach are worth highlighting. As in our previous studies, we 
use a DBS and an acid-base reaction to generate the foams in situ.68,109 The DBS allows the 
foams to be easily injected into a Ziploc bag and spread into a thin layer before polymerization. 
The volume of injected foam and the size of the Ziploc bag determine the dimensions of the gel 
sheet and can be easily controlled. Also, as shown previously, our foams are very stable (foam 
half-life > 25 min) due to the use of hmC as a stabilizer.68,109 Chains of this amphiphilic polymer 
adsorb on the bubbles and ensure that the bubbles remain mostly intact even as the foam is 
compressed between the glass slabs and thereafter during polymerization (which is completed in 
just 2 min). Thus, the porosity from the bubbles is retained in the gel sheet. Lastly, we perform 
solvent exchange with a glycerol-ethanol solution before drying under ambient conditions. 
Glycerol is well known to be a plasticizer for hydrophilic polymers, and it ensures that the dried 
gel-sheet is soft and flexible.113-115 On the other hand, ethanol due to its low surface tension 
prevents the pores from collapsing during drying.   
58 
  
 
 
 
4.3.2 Microstructure of the Gel Sheet. 
 
 
 
Figure 4.3. Microstructure of gel sheets. Representative optical (A) and SEM (B) images of 
the dry gel sheet show a highly porous structure with interconnected pores.    
 
Optical and SEM micrographs of a typical gel sheet are presented in Figure 4.3. For this, 
the monomers used were 18.75% AAc and 6.25% AAm, while the concentration of PEGDA 
crosslinker was 2.5 mol% with respect to the total monomer. The aqueous gel (with pores) was 
solvent-exchanged in a 15/85 glycerol/ethanol solution and then dried at ambient conditions. The 
dried gel sheet looks white and is shown in a folded form in Figure 4.3. The micrographs reveal 
59 
  
 
the porous nature of the sheet, with the pores being interconnected and forming a network of 
open microchannels. The porosity ?gel of the dry gel-sheet can be estimated by eq. 4.2 from 
density measurements:109  
?gel
 ?gel =1?   (4.2) 
?bulk
where ?gel is the density of the gel-sheet and ?bulk is the density of the bulk solid (without pores). 
We find ?gel to be 84%, indicating that it is highly porous. Analysis using ImageJ revealed the 
average pore size to be 240 ?m. 
 
 4.3.3 Gel Sheets: Tactile and Mechanical Properties. 
We typically created gel sheets of dimensions 10 ? 8 cm and a thickness of ~ 4 mm 
(Figure 4.4A) with the composition indicated above. The sheet?s robust nature is shown by 
Photos A1?A5: it can be folded and unfolded several times, or it can be rolled up and twisted ? 
in all cases, there is no tearing or visible damage even after multiple cycles of such deformations. 
In terms of touch and feel (texture), the gel sheet is very much like a sheet of cloth or fabric. 
Figure 4.4B shows that the gel sheet can be easily cut using a pair of scissors. Here again, the cut 
edges are smooth and clean, much like a fabric (Photos B1?B5). We also prepared a gel sheet 
with a higher thickness of 15 mm, and a piece of 2.5 ? 2.5 cm size from this sheet is shown in 
Figure 4.5 alongside a cotton ball. Both these materials have a similar look and feel ?  e.g., both 
can be squeezed repeatedly between one?s fingers and still remain unchanged. 
 
60 
  
 
 
Figure 4.4. Fabric-like nature of gel sheets. (A) A gel sheet (10 ? 8 ? 0.4 cm) can be folded 
and unfolded several times, without showing cracks or tears. (B) A gel sheet is cut cleanly and 
smoothly like a fabric using a pair of scissors. Scale bars: 1 cm.    
 
 
Next, we proceeded to characterize the mechanical properties of the gel sheet under 
tension and compression. For tensile tests, the sheet was cut into a dog-bone shape with an 
overall length of 35 mm and a width in the narrow region of 14 mm. The piece was gripped on 
each end by the jaws of the instrument and stretched at a constant rate of 2 mm/min. The 
corresponding stress vs. strain plot (Figure 4.6A) shows a tensile strength of 2 kPa (i.e., the 
maximum stress at break), a tensile modulus of 4.8 kPa, and a tensile strain of 45% before 
failure. For testing under compression, we worked with a thicker piece having the same 
dimensions as in Figure 4.5 (15 mm thickness). A plot of compressive stress vs. strain 
corresponding to a 50 ?m/s rate of compression is shown in Figure 4.6B. The piece can be 
compressed up to 85% of its size, at which point the stress reached the instrument limit and the 
experiment was stopped. Upon removing the compression, the piece recovers instantly to its 
61 
  
 
uncompressed state, as can be noted from the photos in Figure 4.6B. Even after several such 
compressive cycles, no damage or plastic deformation is seen, which is consistent with the visual 
observations from Figure 4.5. Collectively, the gel sheet is shown to have robust mechanical 
properties. In its thick form, it can be likened to a sponge or cotton, whereas in its thin form it is 
similar to cloth or fabric. We are not aware of any previous hydrogel that has been reported to 
have such tactile or mechanical properties.   
 
 
 
Figure 4.5. Texture of a thick gel sheet. A 15-mm thick gel sheet in cube form (2.5 ? 2.5 cm) is 
compared side-by-side with a cotton ball of similar dimensions. Both materials can be squeezed 
between fingers several times (10 cycles) without any lasting changes in size or structure. Scale 
bars: 1 cm. 
 
62 
  
 
 
Figure 4.6. Mechanical properties of gel sheets. (A) Tensile stress vs. strain. The tensile 
(Young?s) modulus is 4.8 kPa and the gel sheet can be stretched by 45% until failure. (B) 
Compressive stress vs. strain. The gel sheet is a soft, spongy material that can sustain more than 
85% compression without damage. The photos show that the compressed gel returns instantly to 
its initial size upon removing the load. 
 
 
Next, we proceeded to characterize the mechanical properties of the gel sheet under 
tension and compression. For tensile tests, the sheet was cut into a dog-bone shape with an 
overall length of 35 mm and a width in the narrow region of 14 mm. The piece was gripped on 
each end by the jaws of the instrument and stretched at a constant rate of 2 mm/min. The 
corresponding stress vs. strain plot (Figure 4.6A) shows a tensile strength of 2 kPa (i.e., the 
maximum stress at break), a tensile modulus of 4.8 kPa, and a tensile strain of 45% before 
failure. For testing under compression, we worked with a thicker piece having the same 
dimensions as in Figure 4.5 (15 mm thickness). A plot of compressive stress vs. strain 
corresponding to a 50 ?m/s rate of compression is shown in Figure 4.6B. The piece can be 
compressed up to 85% of its size, at which point the stress reached the instrument limit and the 
experiment was stopped. Upon removing the compression, the piece recovers instantly to its 
uncompressed state, as can be noted from the photos in Figure 4.6B. Even after several such 
compressive cycles, no damage or plastic deformation is seen, which is  
63 
  
 
 
 
Figure 4.7. Identifying the optimal plasticizer concentration and type. (A) Photos showing 
that a gel sheet prepared without glycerol as the plasticizer in the solvent-exchange step is brittle. 
This shows the need for glycerol as a plasticizer. (B) Tensile (Young?s) modulus of gel sheets 
prepared with various glycerol concentrations in the solvent-exchange step. The optimal content 
of glycerol is 15% (circled), i.e., glycerol: ethanol = 15:85. If more glycerol is used, the tensile 
modulus becomes too low. (C) Gel sheets plasticized by propylene glycol (PG), ethylene glycol 
(EG), glycerol, PEG-200 and PEG-400 are compared. In all cases, the sheet is solvent-exchanged 
with a 15:85 plasticizer: ethanol solution. The sheets are then heated in an oven at 70?C, and the 
weight drop over time is the plasticizer lost by evaporation. PG and EG are completely removed 
within 3 h whereas only ~ 30% of the other plasticizers are removed. This data again show 
glycerol to be an optimal plasticizer. 
64 
  
 
consistent with the visual observations from Figure 4.5. Collectively, the gel sheet is shown to 
have robust mechanical properties. In its thick form, it can be likened to a sponge or cotton, 
whereas in its thin form it is similar to cloth or fabric. We are not aware of any previous 
hydrogel that has been reported to have such tactile or mechanical properties.   
 
The use of glycerol as a plasticizer is key to the above properties. Plasticizers are small, 
non-volatile molecules that distribute between polymer chains and decrease inter-chain 
interactions, thereby improving the flexibility of the material.113-115 If no plasticizer is used (i.e., 
the gel is dried in pure ethanol), the dry gel is brittle and breaks into pieces when slightly 
deformed (Figure 4.7A). We varied the concentration of glycerol as a plasticizer: i.e., in the 
solvent exchange step, the gels were soaked in glycerol/ethanol solutions containing 5, 15, 30, 
50, and 100% v/v of glycerol before drying under ambient conditions. From these studies, 15% 
glycerol was determined to be optimal. If the glycerol content was higher, the tensile modulus of 
the gel-sheet became too low (Figure 4.7B). 
 
 We also examined a range of plasticizers in addition to glycerol: specifically, ethylene 
glycol (EG), propylene glycol (PG), and polyethylene glycol (PEG) of molecular weights (MW) 
200 and 400 Da. After the ambient drying step with each of these plasticizers (15% v/v in 
ethanol), we analyzed the residual plasticizer in the samples by measuring the weight loss upon 
heating to 70?C (Figure 4.7C). In the cases of EG (boiling point, BP = 197?C) and PG (BP = 
188?C), the plasticizers evaporate quickly, leading to a > 95% weight loss ? and in turn, the 
samples become brittle. With glycerol (BP = 290?C), PEG-200 and PEG-400 (BP > 300?C), the 
weight loss plateaus at around 30%. The PEG-plasticized samples still did become brittle after 
65 
  
 
the 70?C heating step. In contrast, the glycerol-plasticized gel-sheets remained soft and flexible 
even after 3 days at 70?C. To summarize, glycerol is found to be an excellent plasticizer for our 
gel-sheets due to it being a small molecule with a high BP (low volatility).113-115 Even after a 
year at room temperature, these gel-sheets remained soft and flexible ? indicating that the sheets 
are stable and have a long shelf life.   
 
4.3.4 Tuning the Properties of Gel Sheets. 
We now discuss the effects of compositional variables on the gel-sheet properties. We 
were interested in optimizing the fabric-like texture and robustness of the sheets as well as their 
ability to absorb liquids (which is discussed in the next section). The main conclusions from 
these studies are:  
1. A 3:1 ratio of ionic monomer (acrylic acid, AAc) to non-ionic monomer (acrylamide, 
AAm) is optimal for our gel sheet to ensure high water absorption while retaining 
good strength in the swollen state. Ionic gels are known to swell more than nonionic 
gels, as also noted in our previous study.109 However, a pure ionic gel-sheet absorbs 
so much water that it is difficult to lift up by hand, and hence this is to be avoided.  
2. Foams created with the above monomers are stabilized by 0.625% of the polymeric 
stabilizer hmC. This concentration is sufficient to ensure that the foams remain stable 
during the UV polymerization. In turn, it ensures that the gel sheet has a highly 
porous structure with interconnected pores.  
3. PEGDA (MW of 575 Da) at 2.5 mol% of the total monomers is the optimal 
crosslinker concentration for obtaining soft fabric-like sheets. If the PEGDA 
concentration is lower, the dried sheets are sticky and SEM reveals collapsed pores 
66 
  
 
(Figures 4.8A, 4.9). If the PEGDA content is higher, the dried sheets are stiff and 
absorb less water.  
4. Shorter-chain crosslinkers have been tried, but are not optimal. Examples include 
tetra-ethylene glycol diacrylate (TEGDA) and N,N?-methylene(bis)acrylamide (BIS). 
However, when the corresponding gels are dried, they shrink by ~ 60% whereas with 
PEGDA as the crosslinker, the gel shrinks by less than 10% (Figure 4.8B,C). Thus, a 
relatively longer-chain crosslinker like the 575 Da PEGDA is necessary to get fabric-
like sheets. 
 
 
 
67 
  
 
 
 
Figure 4.8. Identifying the optimal crosslinker concentration and type. (A) Gel sheets with 
different concentrations of PEGDA crosslinker are prepared and their porosities are calculated 
using eq 4.2. If the PEGDA content is too low, the porosity is low, indicating that most of the 
pores collapse during drying. This is consistent with the SEMs in Figure 4.3. For this reason, the 
optimal PEGDA is 2.5 mol% (circled), which is used in the rest of the studies. (B) Gel sheets 
crosslinked with three different crosslinkers: BIS, TEGDA, and PEGDA (all at the same 
concentration of 2.5 mol% of total monomers). BIS and TEGDA are small molecules whereas 
the PEGDA has an MW of 575. The bar graph shows the ratio of  gel sheet volume after ambient 
drying (Vdry) to the volume before drying (Vwet). When crosslinked by PEGDA, the dry gel-sheet 
retains 90% of its volume, indicating that the pores are mostly intact (not collapsed).  (C) Photos 
of the gel sheets before and after drying, corresponding to the data in (B). The PEGDA gel-sheet 
shrinks the least, consistent with the data shown in (B).  This indicates that most of the pores in 
the material are intact. Scale bars: 1 cm. These observations indicate that a long-chain crosslinker 
like PEGDA is optimal for the gel sheets.   
68 
  
 
 
Figure 4.9. Effect of PEGDA content on gel-sheet porosity. SEMs at two magnifications are 
shown for three different PEGDA concentrations: (A) 1.5 mol%, (B) 2.5 mol%, and (C) 5 mol%. 
Pores are collapsed in (A), whereas both (B) and (C) show open, interconnected pores. This is 
consistent with the data in Figure 4.8A. PEGDA of 2.5 mol% (highlighted) is identified as the 
optimum, and this is used in the rest of the studies.  
 
 
4.3.5 Gel Sheets: Absorbent Properties. 
We now focus on the ability of our gel-sheets to absorb liquids. First, a 25-mL spill of 
deionized (DI) water is created on the countertop and a gel-sheet (10 ? 8 ? 0.4 cm) is used to 
absorb it. Figure 4.10A, Photos A1-A4 show that the gel-sheet absorbs all the spilled water 
within 20 s. The water-filled sheet remains intact and can be held up by hand (Photo A5); note 
that there is no water dripping down from the sheet. For comparison, the same experiment is 
done with a commercially available absorbent pad (Sungbo Corp.) made of cloth. The pad is 
folded in two to reach the same dimensions as our sheet, and then contacted with an identical 25-
69 
  
 
mL water spill (Figure 4.10B).  Photos B1-B4 show that the pad absorbs only some of the water: 
even after a minute, only ~ 60% of the initial spill is absorbed. Moreover, when the pad is lifted 
up, water starts dripping out of it.  
 
 
Figure 4.10. Water mopping by gel sheet compared to controls. At t = 0, a gel sheet (A) or a 
commercial cloth pad (Sungbo Corp.) (B) of identical size (10 ?  8 ?  0.4 cm) are placed over a 
spill of 25 mL water. Snapshots at various stages are shown. The gel sheet absorbs all the water 
and the swollen sheet does not drip when held vertically. The commercial pad only absorbs 60% 
of the water, and moreover, the water drips out when held vertically. Scale bars: 2 cm.    
 
We also compared with paper towels. A paper towel (Bounty? brand, made by Procter & 
Gamble Corp.) is again folded to dimensions of 10 ? 8 ? 0.4 cm and placed over a 25 mL water-
spill (Figure 4.11). The paper towel absorbs only 48% of the initial spill (Photos 1 to 3), which is 
even less than the commercial pad. Moreover, like the pad, the paper towel fails to hold onto the 
70 
  
 
absorbed water: i.e., when it is lifted up, water drips out of it (Photo 4). Thus, Figures 4.10 and 
4.11 show that our gel-sheet is superior in two ways: it absorbs more water, and furthermore this 
water is held tightly, such that there is no drip-off from the swollen sheet.   
 
 
Figure 4.11. Water mopping by Bounty? paper towel. At t = 0, a folded Bounty? paper 
towel of 10 ? 8 ? 0.4 cm size is placed over a spill of 25 mL water. Snapshots at various stages 
are shown. The towel only absorbs 48% of the water, and moreover, the water drips out when 
held vertically. Scale bars: 1 cm.  
 
 
 
Figure 4.12. Water absorption limit for gel sheets. The absorption limit (or ?dripping limit?) is 
the amount of water that can be held by a sheet at saturation ? before it starts to drip. (A) This 
quantity is plotted vs. sheet size for gel sheets as well as a commercial cloth pad (Sungbo Corp.). 
(B) This quantity is compared for various sheets, all having a size of 10 ? 8 ? 0.4 cm. The gel 
sheet exhibits 3? the absorption limit of the others.       
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Figure 4.12 further quantifies the differences between our gel sheet and other absorbents. 
First, we varied the gel sheet dimensions and recorded the volume of water absorbed. Sheets 
with 4-mm thickness and varying sizes (3 ?  2, 4 ? 3, 6 ?  4, 8 ?  6, and 10 ?  8 cm) were placed 
in DI water and allowed to attain equilibrium absorption. The saturated sheets were taken out and 
held vertically to remove excess water. When the dripping stopped, the sheets were weighed and 
the amount of absorbed water (termed the ?water absorption limit? or ?dripping limit?) is plotted 
in Figure 4.12A. The same experiment is repeated with different sizes of the cloth pad (Sungbo) 
and those data are also plotted in Figure 4.12A. As expected, the absorption limit increases with 
sheet size in both cases. However, the values are much higher for the gel sheet. From the slopes 
of the lines in Figure 4.12A, we calculate a water-absorption ?capacity? of 2.2 mL/cm3 for the 
gel sheet vs. 0.67 mL/cm3 for the cloth pad. When translated to a weight basis, the gel sheet 
absorbs about 30 g of water per gram of dry material. This is the conventional ?swelling ratio? 
used in comparing SAP hydrogels. Note that the weight basis can be misleading when dealing 
with thin sheets, which is why we have preferred to use a size (volume) basis for the above data. 
  
 Figure 4.12 B compares the water absorption limit (dripping limit) of the gel sheet 
and three commercial products, viz. the Sungbo pad, a Shamwow? towel, and Bounty? paper 
towels, all at a 10 ? 8 ? 0.4 cm size. Shamwow? towels are a popular commercial product and 
are stated to be made of chamois cloth (a type of cotton). Bounty? paper towels (tagline: ?the 
quicker-picker-upper?) are commonly used in homes and labs. Our data show that the absorption 
limit of the gel sheet is 70 mL while it is 15 to 20 mL for the others. Thus, the gel sheet absorbs 
more than 3 times as much water as the commercial absorbents tested.      
 
72 
  
 
 
Figure 4.13. Expansion of gel sheets upon absorbing water. 3-cm discs of a gel sheet 
and a paper towel (Bounty?) are compared after adding given amounts of water. (A1-A3) Photos 
at different times. (B) Plot of diameter vs. time. The gel sheet expands by 80% whereas the paper 
towel remains at the same size. Scale bars:1 cm.      
 
One unique aspect of the gel sheets is that they still respond like gels ? specifically, as 
they absorb water, they swell and expand. The swelling occurs because the anionic chains repel 
each other and the counterions also increase the internal osmotic pressure. In contrast, typical 
absorbents imbibe water by capillary action12 and do not swell. To demonstrate these differences, 
we cut the gel sheet into a disc of 3-cm diameter and did the same with a Bounty? paper towel. 
Then we added water dropwise at the center of both discs. The gel sheet starts expanding (Figure 
4.13A) and the diameter of the disc vs water amount is plotted (Figure 4.13B). In contrast, the 
paper towel remains the same size and it gets saturated with just 3 mL of water, after which the 
water just pools around the disc. The gel sheet expands by 80% in its diameter until it gets 
saturated at ~ 20 mL of water. Similar expansion is observed with the gel sheet, regardless of the 
geometry. We had previously shown a rectangular gel-sheet in Figure 4.1C and this expands 
73 
  
 
from initial dimensions of 10 ? 8 ? 0.4 cm to final dimensions of 16.2 ? 13.6 ? 0.6 cm. 
Conversely, none of the commercial pads or towels expanded upon contact with water. 
 
 
Figure 4.14. Blood mopping by gel sheet compared to controls. At t = 0, a gel sheet (A) or a 
gauze wound dressing (McKesson?) (B) of identical size (10 ? 8 ? 0.4 cm) are placed over a 
pool of 40 mL blood. Snapshots at various stages are shown. The gel sheet absorbs 99% of the 
blood and the swollen sheet does not drip when held vertically. The commercial gauze only 
absorbs 55% of the blood, and moreover, the blood drips out when held vertically. Scale bars: 2 
cm.  
 
Next, we tested absorption with other liquids. One liquid of importance is blood,79 which 
has a viscosity about 4 times that of water. We poured 40 mL of citrated bovine blood on a 
benchtop and used a gel sheet (10 ? 8 ? 0.4 cm) to mop up the spill. Figure 4.14A demonstrates 
that the gel sheet absorbs 99% of the blood within a minute (Photos A1-A4). The blood-soaked 
sheet is then lifted up and all the blood is retained within it, i.e., there is no dripping. Also, note 
74 
  
 
that as the sheet absorbs blood, it expands, much like in the previous case with water. For 
comparison, we conducted the same test with a commercially available wound dressing of the 
same dimensions (McKesson? 4-ply polyester/rayon nonwoven gauze). Figure 4.14B shows that 
the gauze dressing absorbs only 55% of the blood pool (Photos B1-B4). Moreover, when the 
blood-soaked gauze is lifted up, blood starts dripping out of it ? i.e., the gauze is not able to hold 
the blood tightly.    
 
Currently many products are available for cleaning up blood from minor or traumatic 
wounds (dressings), during surgeries, and during menstrual bleeding (sanitary pads). We 
compared blood absorption by our gel sheet against three such products: the McKesson? gauze 
dressing from Figure 4.14, a sanitary pad (Always? brand, made by Procter & Gamble Corp.), 
and a polyurethane (PU) sponge (made by Carrand Co.). We cut each of the materials to a size of 
2 ? 2 ? 0.4 cm and soaked them in citrated bovine blood. Once saturated, the materials were 
removed and held vertically till the dripping stopped; this gives the blood absorption limit. 
Figure 4.15 plots this quantity for the different materials. We note that the gel sheet absorbs 4.6 
mL of blood whereas the three commercial products all absorb around 1.5 to 1.8 mL of blood. 
Thus, once again, the gel sheet absorbs about 3? the blood compared to the other products.    
 
In everyday life, we encounter spills of even more viscous liquids than blood. Can we use 
the gel sheet to clean up such spills? To test this, we prepared viscoelastic solutions of a 
polymer, xanthan gum (XG) in water, with the XG concentration ranging from 0.01 to 2%. The 
zero-shear viscosity (?0) of the solutions, i.e., the viscosity in the low-shear Newtonian limit, 
was measured by rheometry and ranged between 1 mPa.s (which is the viscosity of water) and 
75 
  
 
4400 Pa.s (which is 4 million times that of water). Note that solutions beyond a ?0 of 100 mPa.s 
are not only viscous but also shear-thinning. Also, even the Newtonian solutions show 
viscoelastic effects such as rod-climbing.  
 
 
Figure 4.15. Blood absorption limit for gel sheet and commercial products.  This quantity is 
the amount of blood that can be held by a sheet at saturation and it is compared for a gel sheet, a 
gauze dressing (McKesson?), a polyurethane (PU) sponge, and an Always? sanitary pad. All 
have a size of 2 ? 2 ? 0.4 cm. The gel sheet absorbs about 3? the blood compared to the others. 
  
Gel sheets of size 2 ? 2 ? 0.4 cm were placed in each XG solution and the absorption 
limit was determined in each case. For comparison, the experiments were repeated with the same 
size of the Sungbo pad. The results (Figure 4.16) show that, regardless of the fluid rheology, the 
gel sheet is able to absorb aqueous fluids. The absorbed amount actually increases with an 
increase in ?0 up to about 4 Pa.s and then decreases with further increase in ?0. The initial 
increase is because viscous liquids tend to drip less from the sheet, i.e., more of the liquid 
remains in the sheet. However, if the liquid is too viscous, it does not penetrate into the sheet in 
the first place (it forms a gooey puddle on the countertop). Still, the gel sheet was able to absorb 
4.1 mL of a XG solution having a viscosity 4 million times that of water. For comparison, the 
76 
  
 
Sungbo pad only absorbed less than 1 mL of such a highly viscous fluid. Similar absorption 
experiments were also conducted with other viscoelastic aqueous fluids, including those based 
on surfactants (wormlike micelles), and the results again showed the superior absorbency of the 
gel sheets. 
 
 
Figure 4.16. Absorption limit for viscoelastic solutions. Solutions of xanthan gum (XG) with 
varying zero-shear viscosities (Pa.s) were tested. The absorption limit is the amount of liquid that 
can be absorbed by a sheet at saturation (without dripping). It is compared for a gel sheet and a 
cloth pad (Sungbo), both of size of 2 ? 2 ? 0.4 cm. 
 
 
 
 
4.4 Conclusions 
Innovation. We have shown, for the first time, how to prepare SAP hydrogels in an unusual, yet 
useful form: as large, flexible sheets. The chemistry used to make these gels is the conventional 
one, involving polymerization of acrylate monomers. However, the unique sheet-like geometry 
arises because we do the UV-polymerization around the bubbles of a foam, which is first 
introduced into a Ziploc bag and flattened to a thin layer prior to UV exposure. The resulting gel 
is then plasticized with glycerol and then ambient-dried to give the final ?gel sheet?. The 
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macroporous sheets have fabric-like properties: i.e., they can be folded, rolled up, and cut with 
scissors. At the same time, like hydrogels, they have the ability to rapidly absorb aqueous fluids. 
As they absorb fluid, the sheets expand, which is a remarkable property not observed with any 
other absorbents based on paper, cloth, or sponge.  
 
Impact and Applications. We have compared our gel sheets with commercial absorbents such 
as paper or cloth towels; gauze dressings for wounds; and sanitary pads. These comparisons have 
been done with water, blood, and viscous or viscoelastic liquids. In all cases, the gel sheets 
outperform their commercial counterparts. The water absorption limit with the gel sheet is 
around 3 times that of other materials. Absorbed liquids are retained within the gel sheet when it 
is lifted up whereas with controls, excess liquid drips down. These findings suggest that the gel 
sheet is truly a ?better picker-upper? and could excel at cleaning up spills in homes, labs, and 
hospitals. Gel sheets could also be useful for absorbing biological fluids during surgeries or other 
medical procedures (note that most bodily fluids are viscous or viscoelastic). The sheets could 
also find use in personal hygiene products (diapers and sanitary pads). Lastly, there is still an 
important need for hemostatic materials that can staunch bleeding from severe wounds. Due to 
their high absorbency and flexible nature, these gel sheets could function as hemostats, and this 
is a direction we are actively pursuing in our lab.  
 
  
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Chapter 5   
Porous Oleo-Sheets for Absorbing Organic Solvents  
5.1 Introduction 
Spills of crude oil or leakages of organic solvents onto water bodies cause severe water 
contamination and adversely affect the surrounding marine ecosystem.116-119 Over the past 30 
years, major oil spills have included the Exxon Valdez spill in 1989117 and the Deepwater 
Horizon event in 2010.118 The continued disposal of solvents such as toluene, cyclohexane, 
dichloromethane, etc. by the petroleum industry is another key source of water pollution.116 
Common methods to clear immiscible liquids from water are by using dispersants,120-122 
skimmers,123 in situ burning,124 and absorbents.9,125 Recently, porous materials have emerged as 
promising absorbents for water remediation because of their effectiveness in removing oils and 
solvents.11,126-128 Porous materials are three-dimensional solids containing numerous pores that 
allow a large volume of solvent to be absorbed rapidly.12,13,126 By tuning surface wettability 
towards oils and organic solvents (i.e., making them superhydrophobic and oleophilic), these 
materials can show high selectivity in separating immiscible liquids from water.11,127,128     
 
Currently, to develop porous absorbents, commercially available sponges such as those 
based on polyurethane or melamine are being explored rigorously.127,129-135 These sponges are 
cheap, easily available, and highly porous. However, the surfaces of these sponges is inherently 
hydrophilic due to amino and carboxyl groups. Therefore, to effectively absorb oils from water, 
the surfaces of these sponges have to be hydrophobically modified. This is done by coating a 
layer of nanomaterials130,133-135 or grafting hydrophobic functional groups.127,134 Further, with 
appropriate choice of nanoparticles, other interesting properties such magnetic response and fire 
79 
  
 
resistance can be endowed to these sponges.129-133,135 Despite these merits, these materials have 
not been considered for water remediation on a large scale for many reasons. First, the 
hydrophobic modification of these sponges is a tedious and complicated process. Second, the 
nanoparticles are adhered only physically on the surface and thus might start leaching out and 
possibly cause secondary contamination. Also, such leaching may cause the material to lose 
hydrophobicity and thereby its selectivity for oil. Lastly, the amount of solvent absorbed by the 
sponges is limited by the pore volume.126 The polymer constituting the substrate (sponge) has 
minimal effect on solvent absorption.  
 
When it comes to water-absorption, polyelectrolyte gels are known to be superabsorbent 
polymers (SAPs) ? they can swell, expand and absorb up to several hundred times their dry 
weight in water.15,16,109 Recently, lipophilic polyelectrolyte gels have also been reported that can 
swell and absorb nonpolar organic solvents.53,54 Thus, could we design porous absorbents for oils 
based on SAP-like polymers that are inherently hydrophobic? Recently, we have made porous 
gel-sheets based on SAP polymers that can absorb high amounts of water. We made these sheets 
porous by foam templating. Could the same methods be used for oleophilic polymer gels? 
Porous gels of oleophilic polymers have been made in only a few studies: by porogen leaching in 
which sugar/salts granules are used as templates for pores,136,137 high internal phase emulsions 
(water in oil) polymerization,138-140 and foam-templating using supercritical CO .141-1432  Foam-
templating is the most simple strategy and involves making a foam, i.e., a dispersion of gas 
bubbles in a monomer, prior to polymerization. However, while aqueous foam stability is well 
understood, not much research has been done on foams in which the liquid is non-aqueous 
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(either polar or non-polar).71-73 In fact, producing stable foams in the latter case is very 
challenging. 
 
 
Figure 5.1. Foam stability in aqueous phase vs non aqueous phase. (A) Foams rapidly get 
produced when acidic solution is added to water containing sodium bicarbonate and surfactant 
Tween (TW) 80. In presence of TW80, aqueous foams show very high foamability. (B) When 
water is replaced with oil, TW80 fails to stabilize non-aqueous foams and thus show low 
foamability. 
 
 
Typically, foams are stabilized by surfactant molecules that adsorb at the liquid-air 
interface and stabilize bubbles against coalescence. In water, surfactants strongly adsorb at the 
air-water interface due to the high surface tension ? of water (72 mN/m) and thus significantly 
reduce ? (to ~ 30 mN/m). This is conducive to foaming: as shown in Figure 5.1A, when acetic 
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acid is added to an aqueous solution containing NaHCO3 and the surfactant Tween 80 (T80), the 
NaHCO3 reacts with acid to produce bubbles of CO2 that get stabilized by T80. Thus, a stable 
foam is produced: note that the foam volume is much higher than the initial liquid volume and 
remains so for at least 5 min. In contrast, oils (such as aliphatic monomers) have a low ? (20-30 
mN/m), and well-known surfactants like T80 and sodium dodecyl sulfate (SDS) hardly reduce ?. 
Thus, it is difficult to foam oil using conventional surfactants, as shown by Figure 5.1B.   
  
Here, we present a non-aqueous foam templating approach to make porous oleo-sponges 
and oleo-sheets, which absorb significant amounts of organic solvents. Our approach involves 
the foaming of a solution containing hydrophobic monomers (dodecyl acrylate and a 
polyurethane crosslinker). The foam is generated in situ via the reaction of an acid with NaHCO3 
to produce CO2 gas bubbles, which are stabilized by a silicone surfactant, VorasurfTM DC 5164. 
The monomers in the foam are then polymerized to form a polymer network around the bubbles. 
Subsequently, the material is dried under ambient conditions to give a porous solid. We call this 
an oleo-sponge or oleo-sheet, depending on its dimensions. When this material is contacted with 
organic solvents such as toluene, dichloromethane, and chloroform, it absorbs the solvent up to 
more than 50 times of its initial dry weight within 2 s. Oleo-sheets could be used to absorb a 
large spill of organic solvents. Moreover, we designed a hybrid omni-absorbtent sheet that can 
selectively absorb water on one side and oil from the other side.   
 
  
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5.2 Experimental Section 
Materials. Monomer dodecyl acrylate and accelerator N,N-dimethyl-para-toluidine (DMPT) 
were purchased from TCI America. Crosslinker Ebecryl 230 was a gift from Allnex. It is an 
aliphatic urethane segment with two acrylates at its ends. Foam stabilizer VorasurfTM DC 5164 
additive was also a gift from Dow chemicals. It is a silicone surfactant. Iron (III) oxide 
nanoparticles were purchased from Alfa Aesar. All other chemicals were obtained from Sigma-
Aldrich including Benzoyl peroxide (BP), acetic acid, sodium bicarbonate (NaHCO3), 
chloroform, tetrahydrofuran (THF), dichloromethane (DCM), toluene, methanol, ethanol, 
cyclohexane, decane kerosene, oil redo and methylene blue. Polyurethane (PU) sponge (product 
details: Carrand 40102) and magnet were purchased from Amazon Corp. and K&J magnetics 
Corp. respectively. Materials for preparing the hydrophilic porous sheet was described in our 
chapter 4.   
 
Synthesis of oleophilic absorbents. First, monomer dodecyl acrylate crosslinker Ebecryl 230 
were mixed in 2:3 wt% ratio. Then 3.5wt% of initiator (benzoyl peroxide) was added to the 
above solution and allowed to dissolve (Figure 5.2). To prepare oleophilic absorbent, typically in 
2g of monomer solution, first we dispersed 0.2 g of sodium bicarbonate and mixed 0.5 g of the 
surfactant (VorasurfTM DC 5164). Then added 0.5 ml of 4.7M acetic acid and 20 ?L of 
accelerator (N,N dimethyl-para-toluidine, DMPT). The above solution was mixed well using 
spatula until foaming started. At this stage, foamed solution was quickly transferred either into a 
cylindrical or sheet mold.  The polymerization was completed in ~ 3 minutes and liquid foamed 
turned into a porous solid. Further, this porous solid was washed once with water and ethanol to 
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remove any unreacted components followed by ambient drying to give a dry porous solid, named 
oleosponge or oleosheet in this chapter. 
 
Optical Microscopy. For optical microscopy, monomer foams were prepared in the absence of 
an accelerator (DMPT) to avoid any foam polymerization during imaging. A small quantity of 
monomer foam was placed on glass slide using spatula and then images were taken on Zeiss 
Axiovert 135 TV inverted microscope at 100? magnification. This process was repeated a few 
times to capture several images. Then using ImageJ software, approximately 5 images were 
analyzed to get an average bubble size, standard deviation, and overall bubble size distribution 
graphs. Additionally, we also obtained images of oleosponge structure (porous solid).  
 
Scanning Electron Microscopy (SEM). Three dried oleosponge were cut using a sharp blade to 
expose the inner porous structure. The exposed area was sputter coated with gold for a minute 
and several images were captured in a Tescan XEIA FEG SEM at 50 to 500?. Then using the 
ImageJ program, we analyzed five images to obtain pore size distribution. 
 
Tensile Test. Tensile tests were conducted on Instron Model 68SC-05 instrument in accordance 
with American Society for Testing and Materials (ASTM) protocol. Briefly, oleosheets were cut 
into a dog-bone shape with following specifications: narrow and overall width of 14 mm and 19 
mm respectively and an inner length of 35 mm. Both ends of each sample were first covered with 
24-grit sandpaper and gripped between Instron jaws to prevent any slipping of the sample. Then 
the sample was stretched with a 2 mm/min rate until it teared into two pieces. Recorded force 
and strain data was converted into stress and strain% date. Total four samples were analyzed.  
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Compression Test. Compression tests were performed in a Instron Model 68SC-05 instrument 
in cyclical compression mode. Experiments were done at 25?C using a parallel plate geometry 
(lower plate of 150 mm diameter and upper plate with 50 mm diameter).  Cylindrical oleosponge 
of 2 cm and length 2.5 cm was compressed at a rate of 500 ?m/s. The oleosponge was 
compressed until 70% strain and then released to get a full cycle of material performance. This 
process was repeated 5 times.    
 
Contact Angle Measurements. The wetting properties and hydrophobicity of the sample was 
determined using contact angle measurements. For this, a 2?L DI water droplet was placed on 
the oleosponge flat surface at room temperature. Then droplet images were captured using Dino-
Lite USB Digital Microscope AM4113T at 50? and using ImageJ analysis, contact angle was 
measured. We measured contact angle on replicas of 5 samples.  
 
Solvent absorption kinetics. For determining expansion (i.e., volume in swollen state to initial 
dry form), the same size oleosponge piece was placed in a solvent and a slow-motion movie was 
recorded. Linear dimension expansion was obtained by analyzing stills from slow-motion movie 
and extrapolated to get volume expansion. We also measured absorption capacity and efficiency. 
For these studies, oleosponge was immersed in toluene solution and allowed to absorb solvent. 
Once the sample was completely swollen, it was removed from the toluene and excess solvents 
was wiped prior to its weight measurement. A ratio of swollen to dry oleosponge weight was 
calculated using these weight measurements. For absorption efficiency test, the completely 
swollen oleosponge was squeezed to remove solvent and then was placed back into solvent to 
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absorb again. This cycle was repeated 100 times and absorption capacity for each cycle was 
recorded. All swelling kinetics studies were repeated three times for each sample and an average 
absorption ratio is reported.  
 
  
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5.3 Results and Discussion 
5.3.1 Synthesis of Oleophilic Absorbent. 
 
 
 
Figure 5.2. Schematics of the method used to create oleophilic absorbent. (A) A foam is 
generated in the oil phase by mixing acidic aqueous solution with base. The oils phase contains 
monomer, crosslinker, thermal initiator, silicone surfactant and dispersed sodium bicarbonate 
(NaHCO3). An accelerator is also added to speed up the polymerization process. (B) The acid 
and base react to produce CO2 gas bubbles. Simultaneously, oil in water emulsions are formed 
due to mixing of two immiscible phases. Both foam and emulsion are stabilized by surfactant. 
Foams are quickly transferred into either a sheet or cylindrical mold and polymerized in 3 min.  
Photos of porous solid in the form of a sheet and cylinder reveals that the material is flexible and 
has a sponge-like texture. 
 
The procedure for synthesizing oleophilic sponges or sheet is a simple two step method 
as shown schematically in Figure 5.2. First, monomer dodecyl acrylate (DDA) and crosslinker 
Ebecryl 230 and initiator benzoyl peroxide (BP) are mixed to form a homogenous monomer 
solution. In second step, we first disperse sodium bicarbonate (NaHCO3) in monomer solution 
and then add foam stabilizer VorasurfTM DC 5164, aqueous acetic acid solution and accelerator 
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N,N dimethyl-para-toluidine (DMPT) as demonstrated in Figure 5.2A. Using a spatula, we mix 
this solution until it starts foaming. During mixing, the acidic and NaHCO3 solutions come into 
contact, whereupon the following reaction occurs: 
 
R-COOH + NaHCO3 ? R-COONa + CO2 (g) + H2O           (5.1) 
 
As a result, carbon dioxide (CO2) gas is released in form of bubbles. Simultaneously oil 
in water type emulsions are formed upon mixing due the presence of two immiscible phases 
(water and monomer solution).  Both bubbles and emulsions get stabilized by silicone surfactant, 
VorasurfTM DC 5154 present in the solution. The foam-emulsion solution is then added into 
either a sheet or cylindrical mold and spread uniformly to polymerize as illustrated in Figure 
5.2B. The DMPT expedites the thermal polymerization process at room temperature and a 
crosslinked polymer network is formed around bubbles. In ~3 minutes, the foam-emulsion 
solution is completely polymerized into a solid porous material. We washed this material in 
ethanol and water to remove any unreacted substances and then let it dry at ambient conditions. 
The final dried material is a yellowish-white porous solid. We synthesized this material both in 
the form of a thin sheet and a cylindrical sponge as shown in photos in Figure 5.2. We will refer 
to this material as ?oleosponge or oleosheet in this study.  
 
5.3.2 Microstructure of the Oleosponge. 
In our synthesis procedure, we are mixing two immiscible phases i.e., water and 
monomer solution along with acid-base reaction to generate bubbles. Therefore, we expect to 
have foams and emulsion formation simultaneously in the foamed solution. Such systems have 
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been studied previously and are known as foamed-emulsion systems in the literature.144-146 As 
shown in Figure 5.3A1, the monomer foamed solution appears white and turbid indicating 
formation of bubbles and emulsion. The schematic representation of the foamed emulsion 
solution is shown in Figure A2 in which oil-in-water (O/W) emulsions are formed. When this 
foamed - emulsion solution is looked under microscope, we indeed see both foam and emulsion 
present together. The optical micrograph is shown in Figure A3. As can be seen, O/W emulsions 
are very small in comparison to bubbles and are present at very high concentration. It should be 
noted that, foamed-emulsion solution was allowed to dissipate and coalesce for a few minutes 
before observing under microscope. This was done because O/W emulsions in fresh solution are 
very small and present at very high concentration and thus we could not see them clearly under 
microscope initially. When foamed-emulsion solution is polymerized, the monomer emulsion 
droplets polymerize to give a continuous polymer network around bubbles. The photo of solid 
polymerized foam referred as ?oleosponge? is shown in Figure 5.3B1. The corresponding 
schematic and optical micrograph are presented in Figure B2 & B3 respectively. The optical 
micrograph reveals numerous pores present in the oleosponge. 
 
The microstructure of the non-aqueous foam and corresponding polymerized dried 
oleosponge is presented in Figure 5.4. Typically, foam was prepared by mixing 2 g of monomer 
solution containing 38.7 wt% dodecyl acrylate, 57.8wt% Ebecryl 230, and 3.5wt% of initiator 
with 0.2 g of NaHCO3, 0.5 ml of 4.7 M acetic acid solution and 0.5g VorasurfTM DC 5164 
(silicone surfactant) was used a non-aqueous foam stabilizer. All components were mixed well 
together using a spatula and upon foaming, optical micrographs were taken. The bubbles appear 
nearly closely packed (Figure 5.4A). We analyzed bubble size from five optical images using 
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ImageJ program and the bubble size distribution is plotted. As we can note, most of the bubbles 
fall in the range of 200 ?m - 400 ?m. The average foam diameter calculated from this 
distribution is shown in the same plot. The mean bubble diameter is 267 ?m and standard 
deviation is 122 ?m.  
 
 
 
Figure 5.3. Microstructure and schematic of foamed-emulsion system and oleosponge (A) 
The monomer solution and water are immiscible and so the monomer foams are expected to have 
both bubbles and oil in water (O/w) emulsions (1). A schematic of this foam solution is shown 
(2). Representative optical micrograph of the foam confirms the presence of emulsion around 
bubbles (3). (B) When these monomer foams are polymerized, the monomer droplets polymerize 
to give a continuous network around bubbles (1&2). Optical micrograph reveals numerous pores 
surrounded by continuous polymer network (3). 
  
These foams will convert into pores once the monomer solution around bubbles is 
polymerized followed by ambient drying. The SEM micrograph in Figure 5.4 B shows numerous 
pores on the oleosponge surface and pores seem interconnected. The pore size is estimated using 
ImageJ software and distribution is plotted. It appears that most of the pores fall in the range of 
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200-400 ?m. The average pore diameter is 277 ?m with standard deviation of 147 ?m. By 
comparing pore size distribution in SEM with bubbles size in optical micrograph, it seems that 
both plots have a similar size distribution. Also, the pore size is very close to the average bubble 
size indicating that the majority of bubbles got locked-in the polymer network before getting 
dissipated away or coalesced. This is achieved by accelerating the polymerization process using 
20 ?L of DMPT in monomer foams (completed in ~ 3 minutes) otherwise polymerization would 
have taken hours. The porosity ?foam foam of the oleosponge can be estimated by equation 5.2 
using density measurements:109 
?
 ? foam =1?
foam
  (5.2) 
?bulk
where ?foam is the density of the dry oleosponge and ?bulk is the density of the bulk dry solid (non-
porous). The measured values are ?foam = 0.097 g/cm3 and ?bulk = 0.813 g/cm3. The porosity ?foam 
is 88%, indicating that it is a highly porous material.   
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Figure 5.4. Micrographs and size distribution of monomer foam and oleosponge (A) An 
optical micrograph of monomer foam, prior to polymerization, shows nearly closely packed 
bubbles. Five optical micrographs of such monomer foam were analyzed to get bubble size 
distribution which is shown as histogram. (B) The SEM micrograph of oleosponge shows porous 
structure with interconnected pores. The corresponding pore size distribution is obtained by 
analyzing pores in three samples.  
 
5.3.3 Mechanical Properties of Oleophilic Absorbent 
We typically synthesized oleophilic absorbents in two shapes: a flat sheet and cylinder. 
Cylindrical oleosponges have diameter between 1-4 cm and length 5 cm.  The sheets are of 
dimensions 10 ? 7 cm and a thickness of ~ 4 mm as shown in Figure 5.5A. The sheet is 
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mechanically robust and can undergo multiple cycles of folding-unfolding or rolling-bending 
without any visible tearing as demonstrated by Photos A1-A6. The oleosheet appears the same as 
initial even after several such deformation cycles. 
 
 
 
Figure 5.5. Mechanical characterization. An oleosheet of dimensions 10  ? 7 cm and thickness 
4 mm folded/unfolded and rolled several times (1-5). The photos show no visible cracks and 
tearing on the final sheet. (B) Mechanical strength of the oleophilic absorbent is characterized 
using tensile and compression tests. The sheet can be stretched by 35% strain before tearing and 
has a very high tensile modulus (1). It can sustain more than 70% compression strain for several 
cycles and yet shows no structural damages (2). 1 cm scale bar.   
 
We characterized the mechanical properties of the oleophilic absorbents by performing 
tensile and compression tests. For tensile test, the sheet was cut into a dog-bone shape with a 
width of 14 mm in narrow region and longitudinal inner length of 35 mm. Both ends of the 
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pieces were first wrapped in 24 grit sandpaper and then gripped between the jaws of the 
instruments. The sandpaper was used to prevent any slippage on gripped jaws. The sheet was 
stretched with a constant rate of 2 mm/ min until failure i.e., sheet got torn into two pieces. The 
corresponding tensile stress vs strain is plotted in Figure 5.5B. The sheet has a tensile strength of 
12 kPa at break point and can withstand 35% tensile strain. The material shows high tensile 
modulus of 40kPa. The compression tests were performed on cylindrical oleosponge of diameter 
2 cm and length 2.5 cm. For this, material was placed between parallel plates of the Instron and 
compressed at a rate of 500 ?m/s until 70% strain for 5 cycles.  The corresponding stress vs 
strain for cycle 1 and cycle 5 are plotted (Figure 5.5B). The cycle 5 (blue curve) overlaps with 
cycle 1 (red curve) indicating no plastic deformation of the oleosponge. The material shows no 
fracture and no collapsed pores. It can be compressed for several cycles and upon releasing 
compression, the sponge recovers instantly back to its initial non compressed form. The stress is 
~4.2 kPa at 70% strain.  
 
5.3.4 Hydrophobicity of Oleosponge 
To effectively separate oils or volatile solvents from water surface, the absorbent material 
should have hydrophobicity towards water and lipophilicity for oils and solvents. To illustrate 
the hydrophobicity of oleosponge, droplets (2?L) of various liquids such as water, acidic 
solution (pH 2), basic solution (pH 12), toluene and ethanol are placed on flat oleosponge 
surfaces as displayed in Figure 5.6A1. The volatile organic solvents toluene (red droplet) and 
ethanol (yellow droplet) instantly wetted the surface and get absorbed. In contrast, all aqueous 
droplets remained on the surface and showed a small contact area. Thus, oleosponge exhibits 
super hydrophobicity for water and lipophilicity for solvents/oils. The measured contact angle 
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with the water droplet is 130? and its photo is shown in Figure 5.6A2. For control, a similar 
hydrophobicity test is conducted on commercial polyurethane (PU) foam which are regularly 
used for cleaning purposes. As can be observed from the photo in Figure 5.6B1, aqueous droplets 
slowly wetted the surface and eventually got absorbed. Also, as expected, toluene and ethanol 
droplets completely wetted the surface and got absorbed immediately. The PU foam surface 
displays hydrophilicity with contact angle of 75?.   
 
 
 
Figure 5.6. Contact angle of oleosponge and PU foam. Droplets of various liquids (acidic, 
basic, water, ethanol, and toluene are placed on a flat (A) oleosponge and (B) PU foam surface. 
Aqueous droplet remains spherical on oleosponge surface whereas droplet slightly settle down 
on PU foam surface. In both foam, solvent droplets get absorbed. The oleosponge and PU foam 
show a contact angle of 130? and 75? with water respectively. The corresponding digital photos 
are shown underneath.  
  
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5.3.2 Oil-Water Separation and Oil Absorption Capacity 
Next, we proceeded to evaluate selective solvent/oil absorption capacity of the oleophilic 
absorbents. For this, a piece of 2 ? 2 cm and a thickness of ~ 6 mm was dipped in a toluene 
(dyed with oil redo)-water mixture bath. The bath was prepared by adding 40 ml of DI water and 
15 ml of toluene containing oil-redo for visualization. As demonstrated in Figure 5.8A, the 
oleosponge started absorbing toluene immediately and within 2 seconds, it absorbed 15 ml of 
toluene. The underneath water was turned clear indicating complete toluene absorption. The 
oleosponge is hydrophobic/lipophilic and highly porous with interconnected open pore networks. 
The lipophilicity of the oleosponge endows it with excellent wettability for oil and porous 
structure provides channels for fast solvent absorption. The oleosponge absorbed toluene within 
a matter of seconds and swelled spontaneously while absorbing toluene. Each dimension of the 
piece almost became double. This implies that the oleosponge can absorb considerably more 
solvent than its initial total pore volume.  
 
The PU foam has a 3D porous structure and appears very similar to oleosponge in terms 
of material texture. However, this material is inherently hydrophilic (Figure 5.6) Thus, we expect 
PU foam to absorb water as well as solvents. When a piece of 2 ? 2 cm and a thickness of ~ 6 
mm was placed in toluene-water mixture, PU foam also absorbed toluene instantly but absorbed 
only a few ml of it and thus the mixture still looks red due to remaining toluene floating on water 
surface (Figure 5.7B). Its size increases by less than 10% (Figure 5.7B). Even though PU foams 
are explored for oil absorption capacity, the polymer skeleton in PU foam is not flexible enough 
to expand upon absorbing solvents like our oleosponge. Low absorption capacity will remain one 
of the limitations for such PU foam-based absorbents. In another test, we dipped oleosponge and 
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PU foam in water (dyed with methylene blue). As expected, our oleopsonge absorbed no water 
(no blue spots on surface) whereas PU foam absorbed water and appeared bluish due to 
hydrophilic in nature. For comparison, photos of both oleosponge and PU foam are shown after 
absorbing water and toluene in Figure 5.8.  
 
 
 
Figure 5.7. Solvent absorption. (A) An oleosponge of dimensions 2 ? 2 ? 0.6 cm is added to 
toluene-water mixture (1). The oleosponge selectively absorbs toluene (dyed with oil redo) 
completely and underneath water turns clear. The oleosponge expands as it absorbs solvent 
(2&3). (B) When PU foam of the same size is added to toluene-water mixture, it fails to absorb 
toluene completely indicated by the toluene layer (dyed with oil redo) on the water surface.   
 
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Figure 5.8. Oleosponge and polyurethane (PU) foam after absorbing water and toluene. 
Initially both PU foam and oleosponge are of the same size with dimensions 2 ?  2 ? 0.6 cm. 
When both are added to water which is dyed with methylene blue, PU foam absorbs water and 
becomes bluish in color. Contrarily, oleosponge did not absorb water and thus appeared white. 
Upon absorbing toluene (dyed with oil redo), PU foam expanded slightly whereas oleosponge 
size increased a lot. This difference was clearly visible in the photo. The scale bar is 0.5 cm.  
 
The swelling kinetics and solvent absorption capacity is quantified in Figure 5.9. As 
mentioned earlier our porous oleosponge swelled almost instantaneously regardless of initial 
size. So, to evaluate swelling (volume expansion), a piece of size 2 ? 2 ? 0.6 cm was added into 
toluene solution and slow-motion movie was captured. We measured the dimensions of the 
oleosponge as it expanded by analyzing snapshots from the movie. A plot of volume expansion 
(defined as final volume/initial volume of oleosponge) vs. time is shown in Figure 5.9 A.  Our 
oleosponge expands at rate of 15 cm3/cm3.s (blue graphs) whereas, it is 0.7 cm3/cm3.s for PU 
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foam (red graphs). Remarkably both foams are completely swollen within a second. Figure 5.9B 
shows volume expansion in various solvents and oils. The oleosponge (red color) expanded 
almost 6 times in both chloroform and toluene. The expansion in oils i.e., decane and kerosene is 
~ 2?. The PU foam (marked in yellow) shows relatively low swelling ratio in all solvents and 
oils (lower than 1.5?).  
 
Solvent uptake capacity in terms of mass is another important parameter that has been 
used in previous studies to evaluate material?s absorbent properties. We measured our 
materials?s absorption capacity (defined as ratio of swollen absorbent weight to initial dry 
weight) for a wide range of volatile solvents and oils. A plot of this absorption capacity is 
presented in Figure 5.9C. The oleosponge shows very high absorption capacity in chloroform 
(90g/g), DCM (~65 g/g), THF and toluene (55 g/g). In oils (decane and kerosene) and methanol 
the absorption capacity is around 20 g/g. The absorbed solvents and oils can be easily squeezed 
out and material can be reused for many cycles. Figure 5.9D shows the absorption efficiency of 
our material for chloroform, toluene and decane for 100 cycles.  In chloroform we observed 
slight reduction in absorption capacity. But in toluene and decane, the absorption capacity 
remains the same for all cycles. The oleosponge retains its initial hydrophobicity for so many 
cycles and the dried material still shows a similar water contact angle (~130?).  This implies that 
our material could be reused several times without significant reduction in absorption efficiency. 
Also, the material is mechanically robust enough to withstand so many solvent squeezing cycles. 
Reusability of the material is a very crucial factor for potential applications in removing oils and 
volatile solvents at lower cost. 
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Figure 5.9. Swelling kinetics and absorption capacity in solvents and oil. (A) An absorbent 
(dimensions 2 ? 2 ? 0.6 cm in all cases) was added to toluene and linear dimension expansion 
was measured. Then this expansion was extrapolated to get volume in a swollen state at any time 
and volume expansion (ratio of final to initial volume) vs. time is plotted for our oleosponge and 
commercial PU foam. The inset shows photos of initial and fully swollen materials. (B) 
Equilibrium volume expansion in various solvents is plotted for both materials. (C) The 
absorption capacity of oleosponge (weight of fully saturated to initial dry material) is also 
measured for a wide range of solvents and oil. (D) For material reusability efficiency test, 
solvent was squeezed out of fully saturated oleosponge and reused. This process was repeated for 
100 cycles in chloroform, toluene and decane and corresponding absorption capacity is plotted.  
 
100 
  
 
 
 
Figure 5.10. Oleopsonge and polyurethane (PU) foam expansion (A) To measure material 
expansion as solvent is absorbed, toluene was added dropwise directly to the material and 
corresponding linear size was recorded. A plot of linear size increase % which is defined as 
change in linear dimensions/initial lengthx100 is plotted as a function of added toluene amount. 
The oleopsonge attained equilibrium expansion at ~ 10 ml even though it got saturated at a 
higher amount. Contrarily, PU foam got saturated in less than 5 ml. (B) Photos of both 
absorbents are shown at different states during this process. Scale bar: 1 cm. 
 
The expansion of oleopsonge is also measured in terms of solvent absorbed amount. For 
this, toluene was added dropwise to the oleopsonge of size 2 ? 2 ? 0.6 cm and corresponding size 
increment was plotted in Figure 5.10A. The oleosponge starts expanding very rapidly (length 
change of ~15%/ml) as toluene is added until 70% expansion is reached after which expansion 
plateau (blue curve). The maximum expansion of ~ 75% is reached in absorbing 10 ml of 
toluene. Interestingly, oleopsonge gets saturated much later after absorbing almost twice the 
amount of toluene (21ml). On the other hand, PU foam length increased by ~ 10% and it became 
saturated in just 4 ml of toluene (red curve). This can be visually observed from photos in Figure 
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5.10B. Initially both absorbents are of the same size and upon adding 4 ml of toluene on both, 
white patches can be seen on oleosponge indicating incomplete saturation whereas excess 
toluene can be seen around PU foam in Photo 2. After adding 21 ml of toluene, some amount 
became visible around the oleopsonge indicating saturation.   
 
 
 
Figure 5.11. Large oil spill clean-up using oleosheet. (A) An oleosheet of dimensions 10 cm ? 
7 cm and thickness 4 mm was spread over a 40 ml toluene spill. The sheet absorbs the toluene 
quickly and expands to a size of dimensions ~ 14 ? 10 ? 8 cm. The sheet is robust enough to be 
picked by hand. It holds the solvent within the polymer network when held upright and no 
dripping occurs. Scale bar: 1cm. 
 
 
To absorb solvents from large spills, large size absorbent materials need to be developed. 
On this front, however, most of the material developed over the last a few decades fails even 
though these materials show good oil absorption capacity. This is because those methods are 
either very laborious or very energy intensive to scale up. Here our oleophilic absorbent 
synthesis technique overcomes all these challenges. As illustrated in Figure 5.2, we can easily 
create these in the form of large sheets if required by simply polymerizing monomer foam 
between two flat glass slabs. To demonstrate potential use of our oleosheet in large spill cleanup, 
we spilled 40 ml of toluene on a glass slab and created a large pool as demonstrated in Figure 
5.11. An oleosheet of dimensions 10 ? 7 ? 0.4 cm is spread and moved around to absorb spilled 
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toluene. The sheet absorbed all the solvent quickly and its dimensions expanded. The swollen 
sheet is mechanically strong enough to be lifted by hand. Moreover, it holds the absorbed 
solvents within the polymer network when held vertically and no solvent dripping occurred. 
 
 
 
Figure 5.12. Magnetic responsive oleosponge. (A) The oleosponge was synthesized by 
incorporating iron (III) oxide particles in the monomer solution while keeping all the 
composition and mixing steps the same. This oleosponge was added to a bath containing toluene 
(dyed with oil redo) and water. It sticks to magnetic very strongly and swells in toluene as 
expected. The swollen oleopsonge is picked up by magnet. Scale bar: 1 cm. 
 
 
In addition to large oleosheet design, we could utilize the simplicity of our method to 
endow other interesting properties in our material. For example, nanoparticles of interest can be 
dispersed in the monomer solution prior to foam polymerization which then permanently get 
trapped in the crosslinked polymer network. To show this proof of concept, we dispersed 5 wt% 
iron (III) oxide nanoparticles in the initial monomer solution (Figure 5.2A) while keeping all the 
ingredient composition and mixing steps the same. In the synthesized oleosponge, nanoparticles 
indeed get incorporated, and it exhibits magnetic response as shown in Figure 5.12. The 
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oleosponge sticks to the magnet very strongly when brought in contact (Photo 1). The magnetic 
oleosponge absorbs toluene and swells similar to regular oleopsonge. Remarkably, swollen 
oleopsonge can be picked up using a magnet as shown in Photo 3.  This type of magnetic 
behavior in absorbents could be helpful for collecting solvent absorbed material from spill sites 
remotely. 
 
Lastly, we designed a hybrid sheet that can selectively absorb water immiscible solvents 
on one side and aqueous fluid on the other. This sheet is made by combining hydrophilic gel 
sheet from chapter 4 and an oleosponge sheet. As described in chapter 4, gel sheet is also 
synthesized using foam templating techniques and is made of hydrophilic polyacrylic acid and 
polyacrylamide. Gel sheet is particularly devised for absorbing aqueous fluids such as water 
spills on counter tops and excess blood during surgeries. Thus, here we fabricated a universal 
sheet by integrating both kinds of layers for practical application in everyday life. 
 
We spread this hybrid sheet first on a 30 ml toluene spill and once it completely imbibes 
the whole solvent, the same sheet is spread on a water spill from the other side (Figure 5.13). In 
the initial state (photo 1), we can clearly notice both layers (white layer-hydrophilic and light 
red- lipophilic). When this sheet is spread on toluene spill, the sheet absorbs it and expands on 
this side while still adhering to the hydrophilic side which remains white color indicating no 
toluene uptake by the hydrophilic side (Photos 2-5). The same sheet is spread on a 30 ml water 
spill (dyed blue) with hydrophilic side facing water. As reported in chapter 4, the hydrophilic 
side also imbibed water within seconds and expanded. The hybrid sheet can be lifted by hand 
and when held vertically, both water and toluene do not drip back implying that both layers can 
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hold the liquids within. Interestingly, even after swelling both layers remain adhered and do not 
come off when lifted up. This kind of hybrid sheet could find many potential applications in 
automobile industry, cosmetic industry, and households. 
 
 
 
Figure 5.13. ?Janus omni-absorbent sheet? for absorbing both oil and water selectively. (A) 
A hybrid sheet that has two layers with opposite wettability for water is used to clean up both 
solvent and water spill. The sheet is prepared by gluing the oleosheet to a gel sheet from chapter 
4. In the initial photo 1 both layers are clearly visible. This sheet is first spread on a 30 ml 
toluene spill. It absorbs the solvent and expands while the hydrophilic side remains adhered and 
does not absorb any toluene. (B) Then the sheet is spread on water (dyed with methylene blue) 
(1-4). The hydrophilic side also swells upon absorbing water. No solvent and water drips back 
from this sheet when held vertically. The sheet photo in final state is intact one piece with both 
layers swollen (5). Scale bar: 2 cm. 
 
 
 
 
 
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5.4 Conclusions 
 In this study, we have proposed a simple strategy to fabricate porous absorbents by 
templating non-aqueous foams for water remediation applications. In this approach, an acid and 
a base are reacted in the monomer solution to generate CO2 gas bubbles which get stabilized by 
silicone surfactants present in the monomer solution. These monomer foams are quickly 
polymerized to give a highly porous solid named ?oleosponge or sheet?. We have created this 
material either in a cylindrical or thin sheet mold. The optical micrograph of monomer foam 
shows nearly closely packed bubbles which upon polymerizing, get converted into 
interconnected pores with an average size of 277 ?m.  
 
Our oleosheets are flexible and can be folded-unfolded several times without any visible 
damages. The sheet has tensile modulus of 40kPa and can sustain more than 90% compression 
strain without plasticizing. It is hydrophobic and can selectively absorb volatile solvents 
instantaneously from water. The oleosponge volume expands ~ 6 times in chloroform and 
toluene. It shows an absorption capacity of more than 50 g/g for a range of solvents and could be 
reused for more than 100 cycles. In comparison to our oleosponge, commercial PU foam hardly 
expands and gets saturated in 0.2? lower solvent amount. Further, using our technique we can 
easily endow magnetic properties in these oleosponge. Our oleosheets could be used to clean 
large oil spill. We also designed a hybrid sheet consisting of two layers with opposite wettability 
for water. These kinds of sheets could potentially be used in cosmetics, automobile industry and 
household.  
 
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Chapter 6  
Recommendations and Future Work 
 
6.1 Project Summary 
 
 In this dissertation, we have developed advanced porous materials with an unprecedented 
combination of swelling rate and extent both in aqueous and non-aqueous liquids. The materials 
are synthesized using a foam templating method where gas bubbles constituents the pores in the 
final material. The obtained porous structures have interconnected open pores that facilitate fast 
liquid absorption. Moreover, we have designed porous materials in the form of large sheets 
(length ~10 cm) in addition to small sponges. These materials could potentially be used in 
homes, labs and hospitals as absorbent sheets. Hydrogel based porous sheets particularly could 
find application in stopping bleeding. The oleosheets could be used for removing oils and 
solvents from water bodies.  
 
 
 In chapter 3, we have devised a simple technique to fabricate porous hydrogels with 
remarkable swelling rate and extent. In this technique, we used a double barrel syringe to 
generate monomer foams in situ. One barrel of the syringe contains monomers, crosslinker, 
stabilizer and acetic acid while the other have sodium bicarbonate along with the UV initiator.  
Upon mixing, the acid-base reacts to generate CO2 gas bubbles. We used a polymeric foam 
stabilizer called, hydrophobically modified polymer that adsorbs at the liquid-gas interface and 
keeps monomer foams stable during polymerization. Further, polymerization was quickly 
completed in 2 mins by using a UV initiator to trap all the bubbles in the polymer network before 
they get dissipated away. The obtained porous gel was solvent exchanged in ethanol and dried to 
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give a porous solid. The final material has open and interconnected pores. When this gel is added 
to water, it rapidly swells to an equilibrium in ~ 20 s.  and absorbs water 300 times of its initial 
weight. The swelling ratio can be tuned by easily varying either ionic monomer concentration or 
crosslinking density. We exploited the rapid and high extent swelling of these gels to do useful 
mechanical work. These gels could be really helpful in designing mechano-chemical engines 
such as artificial muscles, soft actuators and osmotic engines. 
 
 In Chapter 4, we have created porous hydrogels in the form of sheets that are flexible and 
robust in their dried form. Here, we used a similar double barrel syringe strategy to create 
monomer foams that are stabilized by hmC. However, to get the sheet form, monomer foams are 
generated in a Ziploc bag and compressed between two glass slabs followed by UV 
polymerization. After this, we solvent exchanged the porous sheet in ethanol-glycerol solution. 
The glycerol acts as plasticizers and retains initial flexibility of the sheet even after drying 
otherwise the gel will turn into a brittle solid. The dried sheets are soft, foldable, compressible, 
and robust like fabrics. These gel sheets could absorb very rapidly a high volume of various 
aqueous liquids including water, blood, and even viscous fluids. In all these cases, our gel 
outperformed many commercially available absorbents sheets such as cloth pads, paper towels 
and blood adsorbents including wound dressings and sanitary pads. The ability of our material to 
absorb high capacity of aqueous fluids very rapidly could be useful in cleanup of spilled liquids 
from countertops at home, labs, hospitals and surgeries. Because our gel sheet shows very high 
absorption capacity, less of it would be used in clean up, thereby will have lower impact on the 
environment in terms of storage and disposal. 
 
108 
  
 
 In chapter 5, we have synthesized porous materials for absorbing oils and organic 
solvents. Again, we employed foam templating technique but here we generated foams in non-
aqueous phase which consists of monomers, crosslinker, initiator and special silicone-based 
surfactants. Silicone surfactants ensure foam stability in nonaqueous phase during 
polymerization. We can create these porous materials in the form of cylindrical sponges and 
sheets, termed ?oleosponge or oleosheet?. These shows inherent hydrophobicity and lipophilicity 
which allows selective removal of oil/solvents from water. It can absorb a wide range of solvents 
very quickly from water and has good reusability. Our oleosheet can be easily scaled up to clean 
oil or solvent leakages from water bodies due to the ease of synthesis techniques and ingredients. 
Moreover, material can be made magnetic responsive which will facilitate removal of absorbed 
solvents remotely. We also developed a ?Janus omni-absorbent sheet? containing two layers with 
opposite wettability for water: one layer selectively absorbs water while the other can absorb 
only oil or solvents. Such hybrid sheets could potentially be useful in household, cosmetic 
industry, and automobiles. These sheets offer benefits of both oil absorbent and water absorbent 
material in one sheet.  
 
 
 
 
 
 
 
 
 
 
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6.2 Recommendations for Future Work 
6.2.1 Porous Gels for Wound Care 
 
 In chapter 3, and 4 we use a double barrel syringe (DBS) to first create monomer foams 
which are stabilized by hmC and then quickly polymerize under UV light to get porous 
hydrogels. We also demonstrate that our porous material can rapidly absorb blood. All these 
interesting aspects of our strategy and materials could be exploited for stopping bleeding. 
Uncontrolled bleeding is the number one cause of deaths both in military and civilians involved 
in serious accidents.68,69,147 Our lab has been particularly interested in stopping such bleeding.  
 
A lot of efforts have been put by other researchers as well in developing hemostatic 
material for severe injuries.148-150 The point these studies emphasize is that a material should be 
able to uptake blood quickly to cover the wound cavity thereby form a barrier to the blood flow. 
Porous materials are ideal as they can absorb liquid faster than non-porous ones. Therefore, the 
fast and high extent of swelling capabilities of our gel could be beneficial for stopping bleeding 
in severe injuries. Additionally, hmC (a blood gelling agent) is present in porous gel system. This 
could be advantageous as hmC present in our gel can synergically work with the victim body?s 
auto blood clotting cascade to stop bleeding. Thus, it would be interesting to explore our porous 
material for hemostatic applications. We performed some preliminary in vitro and vivo studies in 
this regard. Figure 6.1 shows blood absorbing capabilities of our gel sheet in comparison to 
commercial porous hemostatic material, ?Gelfoam?. We can see that our gel sheet has a blood 
absorption capacity of 3g/cm3 and gets completely saturated within a minute, in contrast 
?Gelfoam? took more than 10 minutes to absorb just 0.2 g/cm3 blood. Our gel size significantly 
increases (could cover wound cavity) whereas ?Gelfoam? size remains the same. To test our 
110 
  
 
material?s ability to stop bleeding, we performed in vivo studies. For this, we created severe 
liver, spleen and artery hemorrhage on swine model and applied our gel sheets. In all these cases, 
we observed that our material was indeed able to stop bleeding significantly in contrast to 
Gelfoam. Snapshots from the liver injuries treatment using our gel sheet vs. Gelfoam are shown 
in Figure 6.2. Our gel sheet absorbed blood and provided a physical barrier to the blood flow 
whereas GelFoam could not hold the blood back and got easily pushed away by the flowing 
blood from injury. In our case hemostasis was achieved in ~3 minutes but no hemostasis was 
attained in the Gelfoam case. Currently we are performing more studies to confirm the 
superiority of our material in treating severe hemorrhage. 
 
 
Figure 6.1 Blood absorption by our porous gel compared to a commercial blood-absorbent 
(?Gelfoam??). The blood absorption capacity vs. time is plotted for the two materials when 
brought in contact with bovine whole blood. Photos before and after swelling are also shown. 
111 
  
 
 
Figure 6.2. Porous materials for stopping bleeding from a severe liver hemorrhage. (A) A 
severe injury was created on a swine liver and a gel sheet piece was compressed against injury 
for two minutes. The gel piece absorbed blood and no further bleeding occurred upon removing 
compression. (B) Contrarily on a similar severe injury, Gelfoam failed to absorb blood and got 
pushed away by flowing blood from the wound. No hemostasis was achieved in this case.  
 
 
Previously we have created hemostatic foams that are elastic and mechanically 
robust.68,69 We could combine these hemostatic foam?s expansion ability and our current fast 
gelation strategy to solidify the foams at wound site. In other words, we could induce in situ 
gelation of foams at the wound cavity. This will provide more mechanical barrier to the blood 
flow and simultaneously polymerized porous networks will absorb released blood. In-situ 
gelation can be induced in many ways. First, synthetic monomer foams can be quickly 
polymerized under visible light by using higher concentration of LAP initiator or other initiator 
which can generate free radicals under visible light. Second, hmC itself can also be modified 
with acrylate groups which can then undergo free radical polymerization without need for any 
112 
  
 
synthetic monomers. Lastly, electrostatic interactions between polymer chains could result in 
foam gelation. One such example is gelling polyvinyl alcohol (PVA) foams using tannic acid. 
Previously, PVA hydrogels have been crosslinked with tannic acid in the literature.151 It would 
be interesting to try similar chemistries to gel a foam at wound site. 
 
 As demonstrated in our previous studies, hmC can gel blood instantly and acts as a 
hemostatic agent. It would be interesting to fabricate a double layer porous sheet where a thin 
layer of hmC is formed on our gel sheet. The hmC will interact with blood and our gel sheet will 
absorb released blood which will concentrate blood clotting factors. Thereby both hmC and our 
gel will work simultaneously but independently to speed up the hemostasis process. 
Commercially available ?TachoSil? (manufactured by Baxter?) a fibrin sealant patch is an 
example of a double layer hemostatic material. The top layer is made from collagen whereas the 
button layer is based on thrombin and fibrinogen proteins which helps in blood coagulation as 
shown in Figure 6.3. These patches are generally made by freeze drying and also blood clotting 
proteins are very expensive. Thus, our material could provide a cheap alternative to TachoSil.   
 
 
Figure 6.3. TachoSil structure. Image of Tachosil,(Baxter?) a commercial hemostatic patch 
with two layers: collagen and blood clotting proteins. 
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6.2.2. Hybrid Porous Gels That Change Shapes 
 Our foam templating technique can be extended to synthesize double layer porous 
hydrogels with both layers showing distinct properties. This can be achieved first by forming a 
uniform layer with type one monomer foam and then spreading another layer of different 
monomer foam on top of it. Foams show solid like behavior and therefore will not mix with 
bottom layer foam.68 Only monomers at the interface will diffuse into each other. Upon 
polymerization, it will result in a porous gel with two layers that are connected strongly at the 
interface as shown in Figure 6.2. These kinds of hydrogels could be designed to undergo shape 
change rapidly. For example, we can make one layer with stimuli responsive polymers such as 
NIPA, and the other based on responsive polymers. When such gel is heated above 32 ?C, NIPA 
side will shrink and because both layers are connected at the interface, the gel will curl. 
Similarly, we could have the same polymer but different crosslinking density. In this case, more 
densely crosslinked will swell less and curling will occur.  It would be very interesting to try 
different variations of monomers, crosslinking density and foam arrangement prior to 
polymerization to obtain other shape changes. 
 
 
Figure 6.4. Hybrid porous gel with two layers. (A) Schematic showing a hybrid sheet with two 
porous gel layers. (B) A photo of hybrid gel: top layer based on acrylic acid and bottom layer is 
NIPA. 
114 
  
 
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126 
  
 
List of Publications 
  
Publications:  
1. Choudhary, H.; Raghavan, S. R. Superfast-Expanding Porous Hydrogels: Pushing New 
Frontiers in Converting Chemical Potential into Useful Mechanical Work. ACS Appl. 
Mater. Interfaces 2022, 14, 11, 13733?13742. 
2. Choudhary, H.; Rudy, M. B.; Dowling, M. B.; Raghavan, S. R. Foams with Enhanced 
Rheology for Stopping Bleeding. ACS Appl. Mater. Interfaces 2021, 13, 13958? 13967. 
3. Zhang, J; Cui, C; Wang, P.-F; Li, Q; Chen ,L; Han, F; Jin, T; Liu, S; Choudhary, H; 
Raghavan, S.R; et al."Water-In-Salt" Polymer Electrolyte for Li-Ion Batteries. Energy 
Environ. Sci 2020, 13 (9), 2878?2887. 
4. Mozil Devan Padmanathan,A.; Sneha Ravi, A. Choudhary, H. ; Varanakkottu, S.;Dalvi 
S., Predictive Framework for the Spreading of Liquid Drops and Formation of Liquid 
Marbles on Hydrophobic Particle Bed. Langmuir 2019 35 (20), 6657-6668 
5. Choudhary, H.; Zhou, C.; Raghavan, S.R., A Better Picker-Upper: Superabsorbent ?Gel 
Sheets? with Fabric-Like Flexibility Submitted to Matter 2022. 
 
Manuscripts in Preparation:  
1. Choudhary, H.; Zhou, C.; Raghavan, S.R., ?Universal Method to Make Any Brittle Gel 
Robust Using a Simple Double-Network Strategy?. 
2. Choudhary, H.; Raghavan, S.R., ?Porous Organogels by Templating Non-Aqueous 
Foams: Ability to Absorb a Variety of Organic Solvents.?  
3. Choudhary, H.; Dowling, M. B.; Raghavan, S.R., ?Novel Wound Dressings/Sponges 
Based on Superporous Hydrogels for Severe Bleeding Control.?  
4. Choudhary, H.; Dowling, M. B.; Raghavan, S.R.,  Injectable Expanding Powder for 
Treatment of Non-compressible Hemorrhage. 
5. Choudhary, H.; Srivastava, M.; Raghavan, S.R., Polymer Films with On-Demand 
Degradability: A New Material for Compostable Trash Bags. 
  
127 
  
 
List of Presentations 
 
 
1. Choudhary, H., Rudy, M., Dowling, M.B., & Raghavan, S.R. Foams with Enhanced 
Rheology for Stopping Bleeding. Research Symposium on Environmental and Applied 
Fluid Dynamics Friday, June 03, 2022, The George Washington University.  
2. Choudhary, H., and Raghavan, S.R., Robust and flexible gel-sheet that can rapidly absorb 
blood. ACS Fall 2021, August, Atlanta, Georgia. 
3. Choudhary, H., and Raghavan, S.R., Polymeric Foams for Stopping Bleeding from 
Severe Injuries. ACS Fall 2021, August, Atlanta, Georgia. 
4. Choudhary, H., and Raghavan, S.R., Flexible Towel-like Polymer That Can Rapidly Mop 
up Blood. AIChE Annual Meeting 2021,November, Boston, Massachusetts. 
5. Choudhary, H., and Raghavan, S.R., Polymeric Foams Capable of Arresting Bleeding 
from Non-Compressible Injuries. AIChE Annual Meeting 2021,November, Boston, 
Massachusetts. 
6. Choudhary, H., Rudy, M., Dowling, M.B., & Raghavan, S.R., Foams with Enhanced 
Rheology for Stopping Bleeding. 95th ACS Colloid and Surface Science Symposium 
June 2021,Virtual.  
7. Choudhary, H., Raghavan, S.R., Flexible and Robust Polymer Gel-Sheet with Ideal 
Properties for Hemostasis. 95th ACS Colloid and Surface Science Symposium June 
2021,Virtual.  
8. Choudhary, H., Rudy, M., Dowling, M.B., & Raghavan, S.R., Rheology of Foams 
Capable of Arresting Bleeding from Non-Compressible Injuries. International Congress 
on Rheology 2020 (2020, December),Virtual.  
9. Choudhary, H., Raghavan, S.R., Robust and Flexible Gel-Sheet That Can Rapidly Absorb 
Water and Blood. MRS Fall Meeting 2020 (December),Virtual. 
10. Choudhary, H., Rudy, M., Dowling, M.B., & Raghavan, S.R., Elastic and Mechanically 
Robust Polymeric Foams to Stop Bleeding. Society of Rheology 2019 (October), 
Raleigh, North Carolina.  
 
 
128