ABSTRACT 

 

Title of Dissertation: RADIATION-INDUCED MODIFICATION OF 
ARAMID FIBERS: OPTIMIZING 
CROSSLINKING REACTIONS AND 
INDIRECT GRAFTING OF 
NANOCELLULOSE FOR BODY ARMOR 
APPLICATIONS   

  

 Lorelis González López, Doctor of Philosophy, 
2022  

  

Dissertation directed by: Professor Mohamad Al-Sheikhly, Department of 
Materials Science and Engineering  

 

 

The goal of this dissertation was to design, synthesize, and analyze novel aramid fibers by 

covalently grafting nanocellulose through electron beam irradiation. These nanocellulose 

functionalized fibers showed enhanced strength and larger surface areas, which improves their 

performance and applicability in fiber-reinforced composites. Unmodified aramid fibers have 

smooth and chemically inert surfaces, which results in poor adhesion to many types of resins. 

Nanocellulose was chosen as the ideal filler to functionalize the fibers due to its reactive surface 

and high strength-to-weight ratio. Aramid fibers were further modified by radiation-induced 

crosslinking reactions as a means to avoid scission of the polymeric backbone and to further 

increase the fiber strength. 



An indirect radiation-induced grafting approach was used for synthesizing these novel 

nanocellulose-grafted aramid fibers while avoiding the irradiation of nanocellulose. The fibers 

were irradiated using the e-beam linear accelerator (LINAC) at the Medical Industrial Radiation 

Facility (MIRF) at the National Institute of Standards and Technology (NIST). After the 

irradiation, the fibers were kept in an inert atmosphere and then mixed with a nanocellulose 

solution for grafting. The grafted fibers were evaluated by gravimetric analysis, Fourier-transform 

infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and nuclear magnetic 

resonance (NMR) spectroscopy. The mechanical properties of the synthesized fibers were studied 

by single fiber tensile tests.  

Aramid fibers were also irradiated at the MIRF in the presence of acetylene gas and triacrylate 

solution as a means to induce crosslinking reactions. These fibers were irradiated at both low doses 

and high dose rates at room temperature. A mechanism for the crosslinking of aramid fibers was 

proposed in this dissertation. Mechanical testing of the fibers after crosslinking showed an increase 

in the strength of the fibers of up to 15%.  

Ultra-high molecular weight polyethylene (UHMWPE) fibers were also studied, but due to an 

issue of entanglement of the fibers during the grafting process, their mechanical properties could 

not be analyzed. Future work will focus on using a better set up to avoid entanglement of these 

fibers.  

 To complete the study of the radiation effects on polymers, this thesis explored the 

radiation-induced degradation of aromatic polyester-based resins. The composition of the resins 

studied included phenyl groups and epoxies, which complicate radiation-induced grafting and 

crosslinking reactions. Unlike aramid and polyethylene fibers, polyester-based resins have a C-O-

C bond that is susceptible to degradation. The resins were irradiated at high doses in the presence 



of oxygen. The scission of the polymeric backbone of the polymers was studied using Electron 

Paramagnetic Resonance (EPR) analysis. EPR showed the formation of alkoxyl radicals and C-

centered radicals as the primary intermediate products of the C-O-C scissions. The degradation 

mechanisms of the resins in the presence of different solvents were proposed. Changes in the Tg 

of the polymers after irradiation, as an indication of degradation, were studied by Dynamic 

Mechanical Analysis (DMA). The results obtained from this work show that irradiation of these 

resins results in continuous free radical-chain reactions that lead to the formation of recyclable 

oligomers. 

 

 

 

  



  

 

 

 

RADIATION-INDUCED MODIFICATION OF ARAMID FIBERS: OPTIMIZING 
CROSSLINKING REACTIONS AND INDIRECT GRAFTING OF NANOCELLULOSE FOR 

BODY ARMOR APPLICATIONS 

by 

Lorelis Enid González López 

 

 

 

 

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 Mohamad Al-Sheikhly, Chair 

Professor Lawrence Sita 

Professor Lourdes Salamanca-Riba 

Professor Luz Martínez Miranda 

Professor Robert Briber 



 

 

 

 

 

 

 

© Copyright by 

Lorelis Enid González López 

2022 

 

 

 

 

 

 

 

 



ii 
 

Dedication 

I would like to dedicate my dissertation to my mom, who I admire the most. 

  



iii 
 

Acknowledgements  

I would like to start by thanking my advisor, Dr. Mohamad Al-Sheikhly, for his guidance, 

feedback, and constant encouragement. All his advice, immense knowledge, and leadership 

allowed me to complete this work and become the researcher I am. He was always there for any 

concerns and treated me as part of his family, which I will be forever grateful for.  

I would like to thank Dr. Amanda Forster, who was part of this work from the beginning. 

She guided and encouraged me, and helped me complete all sorts of experiments at NIST. I would 

like to acknowledge the members of my committee for their time spent reviewing this thesis, Dr. 

Lawrence Sita, Dr. Luz Martínez-Miranda, Dr. Lourdes Salamanca-Riba, and Dr. Robert Briber. I 

want to thank Dr. Fred Bateman for the use of the e-beam and his help with all the problems that 

arose during the irradiations.    

I would also like to thank Dr. Zois Tsinas who always gave me his advice and provided 

immense technical assistance throughout my years in the lab. Thank you to Aiysha Ashfaq for 

your support, all the conversations, and friendship. Thank you to everyone who has been a part of 

the Al-Sheikhly lab, Dr. Azadeh Farzaneh, Dr. Najlaa Hassan, Dr. Salimeh Gharazi, and Steven 

Guerin. Thank you to the staff of the Materials Science and Engineering Department, especially 

N. Adaire Parker, for their expert guidance and assistance. 

I would also like to thank my friends Pedro, and Karen, who kept me sane and in the loop 

of everything going on outside my research. I greatly thank my family, here in MD and back home 

in PR; special thanks to Ale whose hugs always made my weekends better. I would like to thank 

my amazing husband Angel, who never doubted I could do this and who was always willing to 



iv 
 

listen to all my worries and doubts. I could not have done this without him. Thank you to my mom 

and dad, for all the love and support, and for always being there. 

 

 

  



v 
 

Table of Contents 
Dedication ....................................................................................................................................... ii 
Acknowledgements ........................................................................................................................ iii 
List of Tables ................................................................................................................................. ix 
List of Figures ................................................................................................................................ xi 
1. Introduction to the field .......................................................................................................... 1 

1.1 High-Performance Polymer Fibers ................................................................................. 1 

1.2 Fiber-Reinforced Polymer Composites........................................................................... 2 

1.2.1 Aramid fibers ............................................................................................................ 3 

1.2.2 Ultra-high molecular weight polyethylene fibers ..................................................... 8 

1.2.3 Nanometric filler ..................................................................................................... 10 

1.2.4 Binder resin ............................................................................................................. 15 

1.3 Effect of Radiation on Polymers ................................................................................... 16 

1.3.1 Types of ionizing radiation ..................................................................................... 17 

1.3.2 Irradiation in aqueous solution................................................................................ 23 

1.3.3 Radiation-induced crosslinking and degradation in polymers ................................ 24 

1.3.4 Radiation-induced polymerization .......................................................................... 32 

1.3.5 Radiation-induced grafting ..................................................................................... 35 

1.3.6 Radiation grafting under different atmospheres ..................................................... 37 

1.3.7 Additional parameters affecting radiation-induced grafting yield .......................... 39 

2. Materials and Experimental Methods ................................................................................... 41 
2.1 Materials ....................................................................................................................... 42 

2.1.1 Polymer Fibers ........................................................................................................ 42 

2.1.2 Nanocellulose .......................................................................................................... 42 

2.1.3 Polymer resin .......................................................................................................... 43 

2.2 Sample preparation ....................................................................................................... 46 

2.2.1 Preparation of samples for grafting......................................................................... 46 



vi 
 

2.2.2 Preparation of fiber samples for crosslinking studies ............................................. 47 

2.2.3 Preparation of polyester-based resins for degradation ............................................ 48 

2.3 E-beam irradiation ........................................................................................................ 49 

2.3.1 Irradiation set-up ..................................................................................................... 49 

2.3.2 Dosimetry ................................................................................................................ 54 

2.4 Post-radiation process ................................................................................................... 59 

2.4.1 Samples for direct grafting...................................................................................... 60 

2.4.2 Samples for indirect grafting .................................................................................. 60 

2.4.3 Samples for crosslinking studies ............................................................................. 61 

2.4.4 Polyester resin samples ........................................................................................... 61 

2.5 Sample Characterization ............................................................................................... 61 

2.5.1 Electron Paramagnetic Resonance (EPR) spectroscopy ......................................... 61 

2.5.2 UV-Vis Spectroscopy ............................................................................................. 65 

2.5.3 Fourier transform infrared (FTIR) attenuated total reflection (ATR) spectroscopy 66 

2.5.4 Gravimetric Analysis .............................................................................................. 66 

2.5.5 Scanning Electron Microscopy (SEM) ................................................................... 67 

2.5.6 Solid-state nuclear magnetic resonance (ssNMR) spectroscopy ............................ 67 

2.5.7 Single fiber testing .................................................................................................. 68 

2.5.8 Gel Content ............................................................................................................. 70 

2.5.9 Dynamic Mechanical Analysis ............................................................................... 70 

3. Results and Discussion ......................................................................................................... 72 
3.1 Effects of irradiation on aramid fibers .......................................................................... 73 

3.1.1 Determining the structures of the radiolytically produced free radicals of aramid 

fibers under anaerobic conditions using EPR spectroscopy ................................................. 73 



vii 
 

3.1.2 Determining the structure of the radiolytically produced free radicals of aramid fibers 

in the presence of N2O-saturated H2O using EPR Spectroscopy .......................................... 78 

3.1.3 Determining the radiation-induced chemical changes of aramid fibers using FTIR 

spectroscopy .......................................................................................................................... 81 

3.1.4 Single fiber testing of irradiated aramid fibers ....................................................... 83 

3.1.5 SEM Morphological changes after irradiation ........................................................ 87 

3.2 Chemical grafting of styrene-containing linker onto nanocellulose ............................. 88 

3.2.1 Confirmation of synthesis of styrene-nanocellulose ............................................... 88 

3.3 Radiation grafting of styrene-nanocellulose onto aramid fibers ................................... 89 

3.3.1 Gravimetric Analysis (DoG calculation) ................................................................ 89 

3.3.2 Mechanical testing .................................................................................................. 97 

3.3.3 SEM Morphological changes after grafting .......................................................... 100 

3.3.4 Identifying the chemical changes of the aramid fiber using FTIR spectroscopy . 102 

3.3.5 Solid state NMR (ss-NMR) .................................................................................. 104 

3.4 Radiation grafting of nanocellulose onto UHMWPE fibers ....................................... 108 

3.5 Crosslinking of aramid fiber ....................................................................................... 109 

3.5.1 Crosslinking degree after irradiation in acetylene and triacrylate (TMPTA) 

environments ....................................................................................................................... 109 

3.5.2 Single fiber testing of crosslinked aramid fibers .................................................. 111 

3.6 Radiation degradation of polyester-based resins ........................................................ 114 

3.6.1 Identification of the radiolytically produced free radicals of polyester resins in 

different environments using EPR spectroscopy ................................................................ 115 

3.6.2 Changes in viscoelastic properties from irradiation in solvent environments ...... 127 

4. Conclusions: ........................................................................................................................ 132 
4.1 Design, synthesis, and analysis of nanocellulose grafted polymer fibers ................... 132 



viii 
 

4.2 Radiation-induced crosslinking of aramid fibers ........................................................ 134 

4.3 The fundamental differences between the radiation chemistry of aromatic polyester and 

aramid – Radiation-induced degradation of polyester-based resins for upcycling ................. 135 

5. Contribution to Science....................................................................................................... 136 
5.1 Contribution to grafting .............................................................................................. 137 

5.2 Contribution to crosslinking ....................................................................................... 138 

5.3 Contribution to polyester-based resin upcycling ........................................................ 139 

6. Future Work ........................................................................................................................ 140 
References ................................................................................................................................... 144 
 

  



ix 
 

List of Tables 

Table 1. Radiation chemical yield of species formed after the irradiation of aqueous solutions.100

....................................................................................................................................................... 23 

Table 2. G-values for selected polymers. Table taken from Andrew. 97 ...................................... 25 

Table 3. Effects of quaternary C-atoms for irradiation of polymers.105 ........................................ 26 

Table 4. Polyester-based resins and their composition. ................................................................ 45 

Table 5.  Weibull scale and shape parameters for fibers irradiated dry in inert atmosphere at 

different doses. .............................................................................................................................. 85 

Table 6. Weibull scale and shape parameters for fibers irradiated in water saturated with N2O at 

different doses. .............................................................................................................................. 86 

Table 7. Direct radiation grafting results of nanocellulose onto aramid fibers, varying type of 

nanocellulose, irradiation temperature, and total dose. All samples were purged before irradiation 

with N2O. ...................................................................................................................................... 91 

Table 8. Indirect radiation grafting results of nanocellulose onto aramid fibers varying type of 

nanocellulose and total dose. All irradiations were done in sub-ambient temperatures. .............. 93 

Table 9. Indirect radiation grafting results of styrene-nanocellulose onto aramid fibers varying the 

total dose. Main change was preparing fresh styrene-nanocellulose solution to avoid 

homopolymerization before grafting. All irradiations were done at sub-ambient temperature, while 

the grafting temperature refers to the temperature at which the samples were mixed after 

irradiation. ..................................................................................................................................... 95 

Table 10. Direct radiation grafting results of nanocellulose onto UHMWPE fibers, varying type of 

nanocellulose and total dose. All samples were purged before irradiation with N2O. ............... 108 



x 
 

Table 11. Indirect radiation grafting results of nanocellulose onto UHMW PE fibers varying type 

of nanocellulose and total dose. All irradiations were done in sub-ambient temperature, and at a 

grafting temperature of 40 °C. .................................................................................................... 109 

Table 12. Schematic representation of radical species formed after the radiation-induced scission 

of polyester chains, and calculation of the number of peaks expected from the hyperfine splitting 

of each radical. ............................................................................................................................ 117 

Table 13. Schematic representation of radical species formed after the radiation-induced scission 

of polyester chains, and calculation of the number of peaks expected from the hyperfine splitting 

of each radical. ............................................................................................................................ 124 

Table 14. Summary of key mechanical property changes as a function of treatment environment. 

Reference samples are recorded as absolute values while the dosed samples are computed as the 

relative change. ........................................................................................................................... 128 

 

  



xi 
 

List of Figures 
Figure 1. Measured fiber modulus versus the theoretical crystal modulus for several high-

performance polymer fibers. Figure taken from Park.5 .................................................................. 2 

Figure 2. Poly(p-phenylene terephthalamide), chemical structure of Kevlar. ................................ 4 

Figure 3. Schematic of the condensation reaction between 1,4-phenylene-diamine and 

terephthaloyl chloride to produce para-aramid. .............................................................................. 4 

Figure 4. Representation of the difference in behavior between flexible and rigid polymers during 

the spin drawing process. Figure taken from Hancox.32 ................................................................. 5 

Figure 5. Schematic of the chain polymerization of ethylene, resulting in polyethylene. (n 

represents the degree of polymerization). ....................................................................................... 8 

Figure 6. Chemical structure of cellulose. (n represents the degree of polymerization) .............. 11 

Figure 7. Schematic of (a) nanocrystalline, and (b) nanofibrillated cellulose, which can be 

extracted from cellulose fibrils using acid hydrolyzed amorphous regions, and mechanical 

processing, respectively. Cellulose nanofibers are also called nanofibrillated cellulose or cellulose 

nanofibrils. Figure taken and modified from Phanthong et al.83................................................... 13 

Figure 8. Schematic of supermass colloider in which the grinding process of cellulose to produce 

cellulose nanofibers takes place. Image taken and modified from Nechyporchuk.87 ................... 14 

Figure 9. Chemical structure of 4-vinylbenzyl chloride. .............................................................. 15 

Figure 10. Chemical structure of LRF 092 (top) and E51 (bottom) epoxy resins. ....................... 16 

Figure 11. Schematic of the photoelectric interaction. ................................................................. 18 

Figure 12. Schematic of pair production interactions. .................................................................. 20 

Figure 13. Mechanisms for radiation-induced (a) crosslinking, and (b) degradation. .................. 24 

Figure 14. Schematic of the initiation and propagation steps in the polymerization mechanism. 33 



xii 
 

Figure 15. Schematic of the termination and chain transfer steps in the polymerization mechanism.

....................................................................................................................................................... 33 

Figure 16. Schematic of the processing steps for the radiation-induced grafting, (a) direct, and (b) 

indirect, of nanocellulose onto aramid fibers and the subsequent testing procedure. ................... 41 

Figure 17. Ester bond - common bond in all the resins studied. ................................................... 43 

Figure 18. Schematic of the proposed mechanism for the grafting of 4-(chloromethyl)styrene onto 

nanocellulose................................................................................................................................. 47 

Figure 19. Example of powder and bulk resin samples. ............................................................... 48 

Figure 20. Schematic of the MIRF. In the figure the following are highlighted: (1) two -stage 

traveling wave rf LINAC, (2) collimator head for medical treatment beam, (3) 380 V motor 

generator to convert to 50 Hz, (4) 8 MW Klystron and waveguide for 3000 MHz rf, (5) water 

cooling system, and (6) operator’s console and data acquisition system. .................................... 49 

Figure 21. Set-up of sample irradiation in front of the e-beam. ................................................... 50 

Figure 22. The inside of the “hot box” insulated irradiation chamber used for high temperature 

irradiations. ................................................................................................................................... 51 

Figure 23. The inside of the cooler used as an insulated irradiation chamber for conducting the 

irradiation experiments at sub-ambient temperature. .................................................................... 52 

Figure 24. Turntable set-up used to irradiate polymer resin samples at the same time. Picture was 

taken during the dosimetry measurements and the caps and alanine strips were removed prior to 

starting the experiments. ............................................................................................................... 54 

Figure 25. Alanine strips placement for dosimetry measurements. .............................................. 56 

Figure 26. Bruker Alanine Dosimeter Reader. Alanine strips were inserted into the middle cavity 

after irradiation.............................................................................................................................. 57 



xiii 
 

Figure 27. Faraday cup in reference to the sample placement. ..................................................... 58 

Figure 28. Radiochromic films used for dosimetry. ..................................................................... 59 

Figure 29. Visible Spectrophotometer used for radiochromic film dosimetry. ............................ 59 

Figure 30. Absorption of energy by the spins, or resonance, occurs when the magnetic field is 

scanned and the energy difference between the states is equal to the energy of the spectrometer.

....................................................................................................................................................... 62 

Figure 31. Bruker EMX EPR spectrometer .................................................................................. 64 

Figure 32. FAVIMAT single-fiber tester used for the mechanical testing of aramid fibers. Inset on 

the right shows the set-up, with a mini spring clamp placed on the end of the fiber which kept them 

taught............................................................................................................................................. 69 

Figure 33. 1st derivative EPR spectrum of non-irradiated aramid fiber purged with inert gas. ... 74 

Figure 34. 1st derivative EPR spectra of non-irradiated and irradiated aramid fibers treated at 50, 

100, and 150 kGy in inert conditions (left), and the concentration of radicals generated at the 

different irradiation doses (right). ................................................................................................. 75 

Figure 35. (a) Absorption, (b) first derivative, and (c) second derivative EPR spectrum of aramid 

fibers irradiated dry in N2 environment at 150 kGy. ..................................................................... 77 

Figure 36. Proposed mechanisms for the formation of radicals after irradiation of aramid fibers in 

inert atmosphere. ........................................................................................................................... 78 

Figure 37.  1st derivative EPR spectra of irradiated aramid fibers treated at 50, 100, and 150 kGy 

in an aqueous environment and saturated with N2O. .................................................................... 79 

Figure 38. Proposed mechanisms for the formation of radicals after irradiation of aramid fibers in 

water and saturated with N2O. ...................................................................................................... 80 

Figure 39. FTIR spectrum of non-irradiated aramid fibers and assigned IR peaks. ..................... 82 



xiv 
 

Figure 40.FTIR spectrum of aramid fibers irradiated at 200 kGy, dry in N2 atmosphere (black) and 

in the presence of water saturated with N2O (blue) and assigned IR peaks. ................................ 83 

Figure 41. Weibull probability plots of failure strength for fibers irradiated to 0, 50, and 300 kGy 

dry in N2 atmosphere, tested with a 20 mm gauge and at 0.75 mm/min....................................... 84 

Figure 42. Weibull probability plots of failure strength for fibers irradiated to 0, 50, and 80kGy in 

the presence of water, tested with a 200 mm gauge and at 2 mm/min. ........................................ 85 

Figure 43. SEM images of (left) unirradiated, and (right) 100 kGy irradiated aramid fibers ...... 87 

Figure 44. FTIR spectrum of nanocellulose (black) and styrene-grafted nanocellulose (blue) and 

assigned IR peaks. ......................................................................................................................... 89 

Figure 45. UV-Vis spectrum of rinsing solution after first (black) and last (red) wash with DI. 

Samples were washed multiple times after the grafting procedure, before DoG measurements were 

taken. ............................................................................................................................................. 90 

Figure 46. Indirect radiation grafting results of styrene-nanocellulose onto aramid fibers varying 

the total dose and grafting temperature. All irradiations were done at sub-ambient temperature. 96 

Figure 47. Strength as a function of irradiation dose for the styrene-nanocellulose onto aramid fiber 

at the different grafting temperatures. ........................................................................................... 98 

Figure 48. Styrene-nanocellulose onto aramid fiber: Fiber strength as a function of degree of 

grafting for the samples grafted at various temperatures. The strength of the nongrafted samples is 

shown for comparison. .................................................................................................................. 99 

Figure 49. SEM images of aramid fiber before (left) and after (right) grafting. ........................ 101 

Figure 50. SEM image of nanocellulose-grafted aramid fibers. ................................................. 102 

Figure 51. Proposed mechanism for the grafting of styrene-nanocellulose onto the aramid fibers.

..................................................................................................................................................... 103 



xv 
 

Figure 52. FTIR-ATR spectra of nontreated (black) and nanocellulose-grafted (red) aramid fibers.

..................................................................................................................................................... 104 

Figure 53. 13C NMR spectrum of untreated aramid fiber. The peaks assigned to the chemical 

structure of the aramid are highlighted. ...................................................................................... 105 

Figure 54. Schematic showing the regular nomenclature for identifying the C-atoms in cellulose.

..................................................................................................................................................... 106 

Figure 55. 13C NMR spectrum of styrene-nanocellulose. The peaks assigned to the chemical 

structure of both the styrene and the cellulose are highlighted. .................................................. 107 

Figure 56. 13C NMR spectra collected from the nanocellulose-grafted aramid fiber (green), 

styrene-nanocellulose (red), and untreated aramid fibers (blue). ............................................... 108 

Figure 57. Residual Mass of irradiated fibers as function of dose after dissolving in H2SO4. ... 110 

Figure 58. Proposed mechanism for irradiation of aramid in acetylene environment. ............... 111 

Figure 59. Strength as a function of irradiation dose for the samples irradiated in an acetylene 

environment. ............................................................................................................................... 112 

Figure 60. Strength as a function of irradiation dose for the samples irradiated in a triacrylate 

(TMPTA) environment. .............................................................................................................. 113 

Figure 61.  Strength of the fibers after irradiation in TMPTA and acetylene environment as a 

function of residual mass. ........................................................................................................... 114 

Figure 62. EPR spectra of neat 1R, 2R, 3R, and 4R resins electron beam-irradiated at a dose-rate 

200 kGyh-1 in the absence of solvent and in the presence of oxygen. The total electron beam dose 

is 1000 kGy. ................................................................................................................................ 116 

Figure 63. Typical ionizing radiation-induced scission in polyester chains in the presence of O2.

..................................................................................................................................................... 116 



xvi 
 

Figure 64. EPR spectra of irradiated 1R, 2R, 3R, and 4R resins in water by electron beam at a 

dose-rate 200 kGy/h, to a total dose of 1000 kGy. ..................................................................... 118 

Figure 65. Degradation of the polyester through an SN2 substitution reaction. ......................... 120 

Figure 66. Schematic of the formation of the tetroxide after high-dose irradiation in the presence 

of oxygen. ................................................................................................................................... 121 

Figure 67. Decomposition of tetroxide formed from the polyester-based resin without (a) and with 

(b) two water molecules by “Bennet mechanism”.201,202 ............................................................ 122 

Figure 68. EPR spectra of 1R, 2R, 3R, and 4R resins irradiated in a DMSO-and-water mixture 

(50:50 ratio) by electron beam. All samples were irradiated to a total dose of 1000 kGy at a dose 

rate of 200 kGyh-1. ...................................................................................................................... 123 

Figure 69. DMSO interaction with •OH radicals203(top), and  reaction of methyl radical from 

irradiation in DMSO/water solution with 1R resin (bottom). ..................................................... 124 

Figure 70. EPR spectra of irradiated 1R, 2R, 3R, and 4R resins in IPA by electron beam at a dose-

rate 200 kGy/h, to a total dose of 1000 kGy. .............................................................................. 125 

Figure 71. EPR spectra of 1R, 2R, 3R, and 4R resins irradiated in (a) no solvent, (b) water, (c) 

IPA, and (d) DMSO and water, 5 months after being irradiated by electron beam.................... 127 

Figure 72. Representative data from dynamic mechanical testing. (a) Resin system 4R (terephthalic 

acid based) showing changes in G’ as a function of temperature and dosing environment. (b)Tanδ 

as a function of temperature for each resin formulation. ............................................................ 130 

Figure 73. Chemical structure of aramid and UHMWPE showing the C-N and C-C bonds that lead 

to the formation of C-centered radicals after irradiation. ........................................................... 137 

Figure 74. Schematic of the indirect irradiation grafting approach. Only the polymer fibers are 

irradiated and irradiation of the nanocellulose is avoided. ......................................................... 138 



xvii 
 

Figure 75. Proposed mechanism for the irradiation of aramid fibers in an acetylene environment. 

The acetylene molecules react with the C-centered radicals generated on the aramid and brings the 

chains closer, facilitating crosslinking. ....................................................................................... 139 

Figure 76. Chemical structure of nylon-6,6 as a potential fiber for future grafting experiments.

..................................................................................................................................................... 141 

Figure 77. Schematic of the proposed template to keep the fibers taut during the grafting 

experiments. ................................................................................................................................ 142 

  



1 
 

1. Introduction to the field 

1.1 High-Performance Polymer Fibers 

High-performance fibers are engineered to have extraordinary mechanical and chemical 

properties. They are derived from materials with unique molecular structures that give them many 

attractive characteristics. These fibers are designed with a specific performance goal, usually with 

high strength, stiffness, and heat and chemical resistance.1,2 Other outstanding properties of high 

performance fibers include high elastic modulus, strain to failure, and toughness.3  

The beginning of the high-performance polymer fiber industry is often accredited to 

DuPont, where the first aromatic polyamide, or aramid fiber, was invented.1 The 

commercialization of Kevlar, a para-aramid fiber, in the late 1960’s led to the interest in producing 

several types of fibers, including ultrahigh molecular weight polyethylene (UHMWPE) fiber. In 

part due to their application in fiber-reinforced polymer composites, the growth of these fibers has 

been consistently increasing since their introduction. Significant advancement in their production 

has been driven by their use for both military and commercial applications. In terms of their 

military applications, it includes soft and flexible fiber panels for body armor, and as reinforcement 

in rigid polymer composites for lightweight vehicle armor.3 The aerospace industry has also played 

an important part in the development of these fibers. There, the fibers are used in reinforced 

composite structural materials, including pressure vessels and satellites.4  

Optimization of the structural and ballistic properties of high-performance fibers is of great 

interest.1 Although the theoretical strength and modulus of these fibers is high, synthesized fibers 

usually do not exhibit values as high as predicted.  For example, polyamide fibers have shown a 

maximum moduli only 1/20 of their theoretical value.2 Figure 1 shows the tensile modulus of 



2 
 

various high performance fibers which are still below their theoretical values.5 Further research on 

increasing these values is an attractive area of research that has gotten a lot of attention in recent 

times.  

 

Figure 1. Measured fiber modulus versus the theoretical crystal modulus for several high-
performance polymer fibers. Figure taken from Park.5 

1.2 Fiber-Reinforced Polymer Composites  

Fiber-reinforced polymer composites have been established as an important type of 

engineering material for high performance applications due to their excellent mechanical 

properties. Some of the attractive properties of these composites are a high strength to weight ratio, 

high toughness to weight ratio, and ease of fabrication.6,7 These composites are typically made of 

high-strength fibers, such as aramid fibers, and a binder material, including but not limited to resins 

or elastomers. They have a vast number of applications including ballistic armor8–10, wind turbine 

blades11–13, and sports good14–16. Depending on their use, various aspects of the composite such as 



3 
 

the fiber orientation, ratio of fiber to matrix, and fiber-matrix adhesion can be tailored to meet 

these demands.17–19  

The excellent mechanical properties of the composites are in part due to the use of high-

performance fibers. However, effective attachment between the fibers and the resin is also a huge 

contributor to their attractive properties.20,21 A strong interface is key to achieving the composite’s 

theoretically expected properties which are based on the properties of its constituents. The main 

interactions that determine fiber-resin adhesion at the interface are mechanical interlocking and 

chemical bonding. Mechanical interlocking is a type of physical attachment between the fiber and 

the resin, while chemical bonding is due to the electrostatic attraction between their chemical 

groups.17,22–26 To improve the properties of the composite, it is necessary to understand and 

increase these interactions. 

The degree of mechanical interlocking can be increased by modifying the surface 

morphology of the fibers. Fiber roughness, or the existence of irregularities on the surface of the 

fiber, results in more contact points between the fiber and the resin, increasing the interactions at 

this interface. These irregularities act as a mechanical anchor, resulting in better physical contact 

or attachment between the fiber and the resin at the interface.23,27 In terms of the chemical bonding, 

adding a nanomaterial such as nanocellulose, which has surface functional groups such as hydroxyl 

that are very reactive, allows for the possible formation of hydrogen bonding between the 

nanocellulose and the resin, creating a stronger fiber-nanocellulose-resin interface.27–30  

1.2.1 Aramid fibers 

Aramid, or aromatic polyamides fibers, are a class of high-strength synthetic fibers, first 

introduced by DuPont in 1965.31 Their commercial product Kevlar remains one of the best-known 

aramids in the market. Aramids may be divided into two types: para- and meta-aramids, depending 



4 
 

on where the linkage is attached to the aromatic ring. Kevlar is a para-aramid, and its chemical 

structure consists of large phenyl rings, linked together by amide groups in a close and compact 

molecular structure. This gives them rigidity and low solubility (Figure 2). These polymer chains 

are held together by hydrogen bonds between adjacent chains, and π-π stacking of its aromatic 

rings, giving them high strength, high modulus, toughness, and thermal stability.32 The term 

aramid will be used to refer to Kevlar fibers in this work. 

H
N C

O

C

O

H
N

n  

Figure 2. Poly(p-phenylene terephthalamide), chemical structure of Kevlar. 

Aramid fibers are synthesized via the condensation reaction of the monomers 1,4-

phenylene-diamine and terephthaloyl chloride. N-methyl pyrrolidone (NMP) and calcium chloride 

are used as solvents for the polymerization reaction, and calcium chloride acts as the ionic 

component to occupy the hydrogen bonds of the amide group. Hexamethylphosphorous triamide, 

a known carcinogenic, was originally used as solvent, but replaced by NMP for safety concerns. 

A schematic of this reaction is shown in Figure 3.  

NH2

H2N

+

O

Cl

O

Cl
H
N C

O

C *

O

H
N*

Terephtaloyl chloride1,4-phenylene-diamine

-HCl

Poly(p-phenylene terephtalamine)

n

 

Figure 3. Schematic of the condensation reaction between 1,4-phenylene-diamine and 
terephthaloyl chloride to produce para-aramid. 

Most synthetic fibers are made of flexible chain polymers that require their molecules to 

be in an extended chain configuration and largely crystalline packing, in order to achieve the best 



5 
 

mechanical properties. Because of this, the production of these fibers commonly involves 

mechanically drawing the fiber after melt spinning. However, the drawing process of flexible chain 

polymers requires disentanglement of the chains followed by their orientation in the solid phase. 

This affects the final mechanical properties of the produced fibers, making them less than 

theoretically predicted. In the case of aramid fibers, the polymer poly-p-benzamide forms a liquid 

crystalline solution which allows for the formation of a “rod-like” molecular structure, instead of 

the flexible-chains, when it’s dissolved. As the concentration of the polymer is increased, the 

chains start to associate in a parallel alignment and produce highly oriented polymer chains. Figure 

4 shows the difference in behavior between flexible and rigid polymers during the spinning 

process. 

 

Figure 4. Representation of the difference in behavior between flexible and rigid polymers 
during the spin drawing process. Figure taken from Hancox.32 

Since their introduction, one of the most important uses of the aramid fibers has been as a 

reinforcement in composite materials.33 Application of these composites include bulletproof 



6 
 

vests3,34, the aerospace and aircraft industry35,36, and even as straighteners in fiber optic cables37. 

Unfortunately, the use of aramid fibers in composites is limited by their smooth and chemically 

inert surface, which results in poor adhesion to many types of resins. As a result, functionalization 

of the fibers must be done prior to their addition into the composites in order to enhance their 

overall performance and maximize their potential.  

Attempts to modify the surface of aramid fibers by generating functional groups on their 

surface, and increasing its surface’s roughness, include plasma treatment38,39, acid chemical 

treatment40,41, and fluorination20,28. The main challenge with these treatments is that they cannot 

be used in an industrial scale and usually result in an unwanted decrease in the intrinsic fiber 

strength due to backbone breaking of the polymer chains.42 Another limitation of this type of 

approach is that it is based on creating functional groups in the fiber’s surface that are specific to 

only one type of resin, making it unsuitable for application with other resins.43 Improvements to 

the adhesion between the fibers and resins can be measured in terms of its interfacial shear strength 

(IFSS). For example, an increase of up to 42.07% in the IFSS of aramid/epoxy composites was 

observed by Zhao after treatment with phosphoric acid solutions.44 However, even using low 

concentrations of the solutions resulted in a decrease of the mechanical properties of the fibers.  

High energy irradiation of aramid fibers has also been studied as a way to enhance the 

interfacial properties in the composite, by improving aramid fibers’ adhesion to resins.45–48 

Irradiating polymers with ionizing radiation such as γ-rays or accelerated electrons leads to the 

formation of very reactive intermediates, free radicals, and excited states.47 Research has shown 

that irradiated fibers have a more uneven surface, which contributes to the degree of mechanical 

interlocking, and helps in the fibers’ bonding with the resin. By controlling the irradiation 

parameters, such total dose and dose rate, the mechanical properties of the fibers are not 



7 
 

significantly reduced, making this one of the most efficient, clean, and energy-saving methods for 

fiber modification. Functionalization by irradiation is usually done at high doses (up to 2,200 kGy), 

but although improvement of up to 17.7% in the IFSS of aramid/epoxy composites has been 

accomplished47, these high doses lead to damage in the tensile strength of the fibers. In terms of 

the chemical components, the formation of radicals on the surface of the fiber allows for the 

grafting of a nanometric filler onto the fiber. This nanometric filler is chosen to increase the 

chemical bonding at the fiber-resin interface. Lower doses, from 20 to 40 kGy, can also result in 

the generation of radicals49, although no proposed mechanism, or radical formation information is 

found in the literature.  

It is also of interest to increase the tensile strength of these fibers, and to make them more 

resistant to irradiation. As mentioned above, one of the main uses of these fibers is in the aerospace 

industry, where they are exposed to high doses of space radiation.50 The grafting of various 

monomers onto aramid fibers have been reported to not only increase the mechanical properties of 

the subsequently made fiber-reinforced composites, but also the tensile strength of the fibers.43,48,51 

Zhang et al. successfully grafted hyperbranched polysiloxane onto aramid fibers through  a 

chemical grafting procedure.52 The study showed that grafted fibers were more resistant to UV 

irradiation, with their modulus and break extension staying at 95-97% after 168 h of irradiation. 

Also, the grafted fibers had higher tensile properties than the non-grafted fibers. The increased 

UV-resistance of the grafted fibers is attributed to the UV-resistant quality of the hyperbranched 

polysiloxane. An increase in the tensile properties was reported to be due to the hyperbranched 

polysiloxane mending the defects present on the surface of the fibers.  



8 
 

1.2.2 Ultra-high molecular weight polyethylene fibers 

Ultra-high molecular weight polyethylene (UHMWPE) fibers are another important type 

of high-performance fibers. They are commercially produced by DSM under the name Dyneema, 

and by Honeywell under the name Spectra. Polyethylene is a polymer produced from the monomer 

ethylene. Depending on the polymerization conditions, the properties of the produced polyethylene 

can be tuned. A schematic of the chain polymerization of polyethylene from the ethylene monomer 

is shown in Figure 5. 

C C
H

H

H

H

C C

H

H H

H n  

Figure 5. Schematic of the chain polymerization of ethylene, resulting in polyethylene. (n 
represents the degree of polymerization). 

Coordination catalysts can be added to the reaction to produce linear polyethylene with a 

high molecular weight. The use of these catalysts leads to a living polymerization, where the 

polymerization continues until the ethylene is completely depleted. That is how UHMWPE, which 

has a molecular weight of at least 1 million g/mol, is produced.53 Another benefit of incorporating 

the catalysts in the production of PE is that it produces highly crystalline polymers by preventing 

branching during the polymerization.54 The crystallinity of linear PE produced by this method 

ranges from 70-90%.55 This is a major factor to which the excellent physical and mechanical 

properties of UHMWPE fibers is attributed. UHMWPE fibers are produced through a gel spinning 

process in which UHMPE resin is dissolved and extruded through a spinneret into a water bath. 

After removing the solvent, the polymer goes through the drawing process discussed above. 



9 
 

UHMWPE fibers have a high strength to weight ratio, high chemical resistance, and good 

mechanical properties, including hardness and Young’s modulus due to their high crystallinity. 

These properties make them ideal for use in fiber-reinforced polymer composites. Similar to 

aramid fibers, UHMWPE reinforced composites are used in a wide range of industries including 

ballistic protection, aerospace, and medical devices.56–58 Despite their extensive use, these fibers 

exhibit certain drawbacks that limits their applications and overall performance. Some of these 

drawbacks include low surface energy, poor creep resistance, and lower thermal stability. As is the 

case for aramid fibers, UHMWPE fibers also show poor adhesion in composites due to their non-

polar, inert nature.  

UHMWPE fibers are commonly modified before their use in composites to improve the 

degree of adhesion between the fiber and the resin at the interface. Functionalizing the fiber by 

introducing polar groups is one way to achieve this. Li et al. observed that chromic acid modified 

UHMWPE fiber had not only an increased interfacial adhesion strength when preparing a 

composite, but also showed increased elongation at break and tear strength.59 Characterization of 

the fibers showed a higher surface roughness and an increase in oxygen-containing groups on the 

surface of the fibers, which impact its behavior in the composite. The main issue with this 

technique is that the strength and tensile properties of the fibers are decreased with increasing 

treatment time. The introduction of nanomaterials as reinforcements has also become a common 

method for the functionalization of these fibers. The challenge is to have an even distribution of 

the nano-reinforcer in order to achieve good adhesion at the fiber-resin interface, and not affect 

the properties of the composite. When choosing a nano-reinforcement, materials with good 

mechanical properties, such as carbon nanotubes and nanoclays, are most attractive.60 

Mohammadalipour et al. reported that introduction of nanoclay onto the resin improved the tensile 



10 
 

strength, modulus, and overall adhesion of the composite.61 Most of work reported on the 

introduction of nano-reinforcements onto FRPCs center on their addition to the resin, with less 

emphasis on the fibers.60,62–64 

Radiation-induced grafting is an alternative to incorporate these nano-reinforcements onto 

the fiber surface. The high crystallinity of UHMWPE fibers makes it more difficult to graft 

nanoparticles when compared to bulk material, which has been previously explored.65,66 

Yakusheva et al. grafted acrylic monomers onto UHMWPE fibers after irradiating the fibers in a 

pulsed ion-beam installation at doses of up to 10-16 ions/cm2.67 A direct grafting approach was 

followed by Kondo et al. as they used a radiation-induced graft-polymerization technique to graft 

N-vinyl formamide monomer onto UHMWPE fibers.68 Studies like this focus on the mechanical 

properties of the composite produced after fiber modification rather than on the effects of the 

modification on the fibers, which must be maintained in order to achieve the best performance. 

Xing et al. worked on the grafting of methyl acrylate (MA) monomer onto UHMWPE by a gamma-

ray pre-irradiation induced graft polymerization technique.69 They studied the effects of radiation 

dose on the mechanical properties of the fibers and observed a decrease in the tensile strength as a 

function of radiation dose. Although higher doses lead to a higher amount of monomer grafted, it 

also resulted in a lower tensile strength of the fiber. The low dose rate achieved by gamma-ray 

irradiation could be a significant issue causing this decrease in strength. The benefits of using 

electron beam irradiation over gamma-ray will be discussed in a following section.  

1.2.3 Nanometric filler 

The addition of nanomaterials, such as nanocellulose, to either part of a polymer composite 

can strengthen the interface by providing better contact between the fiber and the resin. 

Furthermore, this nanometric filler may enhance a property of the composite and add 



11 
 

multifunctionality. Attaching a nanometric filler as a reinforcement to the fibers improves the 

adhesion between the fibers and the resin by (1) creating irregularities on the fiber’s surface, 

allowing for mechanical interlocking, and (2) introducing functional groups for chemical bonding. 

It also leads to a significant increase in the surface area for interactions, and increases the 

roughness of the otherwise smooth fibers.17,22,43–46,70 This could widely improve the fibers’ 

adhesion to many kinds of resins. 

Nanomaterials commonly used to modify fiber-reinforced composites include 

nanocellulose30, multi-walled carbon nanotubes (CNTs)71,72, silica43, and zinc oxide73. Research 

on the addition of CNTs to aramid fiber composites show an increase in the IFSS attributed to a 

high density of nanotubes distributed in the fiber surface, seen as surface irregularities, that provide 

more contact points between the resin and the fibers.22,71  A challenge with using CNTs is their 

unreactive nature, which means they need to be treated in harsh conditions to functionalize, and 

the low purity of most available nanotubes.74,75 This is also true for silica and zinc oxide 

nanoparticles, where labor-intensive methods are needed to manufacture the modified 

composites.18,70,76 Nanocellulose, on the other hand, is a cleaner, more reactive, and highly 

available material that can be attached to the fibers by a simple immersion process.23,77–79  

O
O

O

OH

OH

OH

HO

OH
OH

n  

Figure 6. Chemical structure of cellulose. (n represents the degree of polymerization) 



12 
 

Cellulose is a homopolymer comprised of β-1,4-linked anhydro-D-glucose units (Figure 

6), which can be derived from a wide variety of sources including wood, grass, algae, and fungi. 

Nanocellulose can be obtained from cellulose to produce two main types: (1) cellulose 

nanocrystals, usually produced by an acid treatment of the cellulose, and (2) cellulose nanofibers, 

commonly produced through mechanical disintegration. A schematic of these two processes is 

shown in Figure 7. This work will focus on cellulose nanofibers, and their grafting onto polymer 

fibers. Cellulose nanofibers, which will be referred to simply as nanocellulose in this work,  are 

commercially available and have generated high interest for industrial applications, especially after 

the discovery of pretreatments methods that facilitate the mechanical disintegration process.80  

Because of their excellent mechanical properties, low cost, low toxicity, and renewable resource 

origin, nanocellulose is often regarded as an ideal candidate for reinforcing composites.81–83 In 

addition to their high aspect ratio and strength, an attractive property of nanocellulose is their 

highly reactive, hydroxyl-full surface.84–86 These functional groups would not only help the 

cellulose attach to the fibers, but also be a source of reactive sites for the resin to bond to.  



13 
 

 

Figure 7. Schematic of (a) nanocrystalline, and (b) nanofibrillated cellulose, which can be 
extracted from cellulose fibrils using acid hydrolyzed amorphous regions, and mechanical 
processing, respectively. Cellulose nanofibers are also called nanofibrillated cellulose or 

cellulose nanofibrils. Figure taken and modified from Phanthong et al.83 

Nanocellulose is prepared by mechanical disintegration of dry cellulose pulp or cellulose 

in an aqueous medium. To produce nanocellulose with a high degree of polymerization, 

crystallinity, and aspect ratio, aqueous solution is preferred. In this environment the interfibrillar 

hydrogen bonding of the cellulose is loosened, enabling the delamination of the fibrils instead of 

their chopping. The fibers used for this work were produced via a supermass colloider grinder. In 

this process, a cellulose slurry is passed between static and rotating grinding stones. The cellulose 

is separated into its elemental form of nanofibers by the shearing forces that are generated between 

the stones. During the process the cell wall is delaminated, and the nanofibers are individualized. 

The size of the nanofibers can be varied by adjusting the space between the stones.87,88  

a 

b 



14 
 

 

Figure 8. Schematic of supermass colloider in which the grinding process of cellulose to produce 
cellulose nanofibers takes place. Image taken and modified from Nechyporchuk.87 

Nanocellulose has been used to improve the performance of composites both by attaching 

it to the fibers and by dispersing it in the resin.82,89 As reported in a paper by Uribe, et al.30, after 

incorporation of nanocellulose in carbon fiber-reinforced laminates, the composite’s ultimate 

strength increased by 28, 20 and 86% under tensile, flexural, and shear strength testing, 

respectively. In the experiment, the nanocellulose was incorporated on the carbon fibers by a 

simple dipping process in a nanocellulose solution, which was followed by a vacuum-assisted 

liquid resin infusion to fabricate the laminates. The use of nanocellulose in composites was also 

demonstrated by Asadi, et al.82 who immersed chopped glass fibers in an aqueous nanocellulose 

solution, and dried them at room temperature. They reported an improvement of 69% in the IFSS 

of coated fibers, and an improvement of 43% in their strength modulus.82 Although nanocellulose 



15 
 

has been successfully incorporated in various fiber-reinforced composites, there is a missing gap 

in the literature about their incorporation onto the surfaces of high performance fibers, including 

aramid and UHMWPE, and the mechanisms by which they are attached.   

1.2.3.1 Linker 

One way to improve the attachment of the nanocellulose to the polymer fiber is by grafting 

a polymer linker to the nanocellulose. The linker is used to create covalent graft points between 

the nanocellulose and the fiber. Yatvin, et al.90 used a sulfonyl azide monomer to covalently bond 

various copolymers to aramid fabric via thermal grafting. For this process, the material that is 

going to be grafted onto the aramid, the nanocellulose, is first bonded to a more reactive material. 

For example, a polymer such as 4-vinylbenzyl chloride (VBC) (Figure 9) contains carbon-carbon 

bonds that are more likely to react with radicals generated on the polymer fiber surface. Thus, a 

simple chemical grafting approach can be followed to attach the VBC to the nanocellulose, while 

separately, radicals are created on the fiber surface by irradiation. The two are then mixed, with 

the double bond in the VBC being suitable to react and covalently bond to the fiber via the 

produced radicals.  

CH2

Cl  

Figure 9. Chemical structure of 4-vinylbenzyl chloride. 

1.2.4 Binder resin 

The final part of the fiber-reinforced composite is the binding material that holds it 

together, in this case a thermoset resin. Epoxy resins are used in most aramid fiber-reinforced 



16 
 

composites, including in composite pressure vessels used in space shuttles91, in high speed boats92, 

and vehicle armor3. Epoxy resins are defined as having one or more epoxide groups in the 

molecule. Depending on their application, they can be reacted, or cross-linked with themselves or 

other reactants (curing agent) to make resins with specific properties. In addition to acting as a 

binder, the resin also acts as a stress transfer medium for the reinforcing fibers. 

Typical epoxy resins used in aramid fiber reinforced composites include LRF 092, a highly 

crosslinked epoxy anhydride/tertiary amine resin, and E51 a bisphenol-A type epoxy resin (Figure 

10).42,46,47,91 

H2C
O

C
H

H2
C O C O

H2
C CH CH2

O

H2C
O

C
H

H2
C O C O

H2
C C

H

OH

H2
C C O

H2
C CH CH2

O
O

 

Figure 10. Chemical structure of LRF 092 (top) and E51 (bottom) epoxy resins. 

1.3 Effect of Radiation on Polymers  

The effects of electromagnetic radiation on any material depends on the power and frequency 

of the applied radiation. Radiation types with a low frequency, such as microwave and infrared 

radiation, are considered non-ionizing radiations. Non-ionizing radiations only have enough 

energy to excite the electrons of the material, without ionizing them.93,94  Ionizing radiation, such 

as gamma ray (γ-ray) and electron beam (e-beam), has enough energy to modify the chemical, 

physical, and biological nature of the material. When the material is irradiated, if the energy is 

higher than that of a particular orbital electron, an electron is ejected, resulting in ionization. 

Ionizing radiation is currently used for the sterilization of medical equipment, disinfestation and 



17 
 

pest control of food and agricultural products, and materials modification.95 This dissertation will 

focus on the use of ionizing irradiation to modify and improve the mechanical properties of 

polymers. The use of ionizing irradiation with the goal of polymer recycling will also be 

investigated.  

To describe the interaction of ionizing radiation with matter it is important to define the 

following parameters: dose, dose rate, and radiation chemical yield. The dose is the amount of 

energy absorbed per unit mass of matter and it’s expressed with the SI unit gray (Gy); 1 Gy is the 

amount of radiation required to deposit 1 Joule (J) of energy per 1 kg of matter Gy = J
kg

. The dose 

rate is the absorbed dose per unit time (Gy/s), and the radiation-chemical yield represents the 

number of moles produced or destroyed per unit of absorbed energy (mol/J).96,97 

1.3.1 Types of ionizing radiation 

The main types of ionizing radiation are high energy electrons (e-beam), gamma (γ) rays, 

and x-rays.96,97 

1.3.1.1 Interactions of gamma (γ) and x-rays with matter 

In most gamma irradiation facilities, a radioactive isotope, commonly cobalt-60 (60Co), is 

used as a power source. γ-rays are high energy photons emitted when an electron in the valence 

shell drops to a lower energy state, a process known as radical decay. One of the issues with gamma 

irradiation is that once that radical decay is started, it will continue until the atom reaches a stable 

state. At this point, the source will need to be replaced. 60Co for example, has a half-life of 5.3 

years. Additionally, in comparison with the electron beam, gamma irradiation can achieve only 

low dose rates on the order of 10-3 kGy/s. In terms of the penetration depth, gamma rays have the 



18 
 

advantage of achieving high penetration, which is useful when irradiating bulk materials. The 

absorbed dose decreases exponentially with penetration, according to Beer-Lambert’s law: 

It = I0e−𝑎𝑎𝑎𝑎 (1.1) 

where 𝐼𝐼𝑎𝑎 is the radiation intensity after passing through thickness 𝑡𝑡, 𝐼𝐼0 is the initial radiation 

intensity, and 𝑎𝑎 is the coefficient of linear absorptivity.   

The interactions of γ and x-rays can be categorized into the following: 

1. Photoelectric effects: 

In this type of interaction, the γ photons lose all of their energy upon interacting with the 

electrons of atoms or molecules. The electron is then ejected, and its kinetic energy (EEe) is equal 

to the energy of the incident photons (Eγ) minus the binding energy of the electrons with the 

nucleus of the atom (BEe). 

EEe  =  Eγ   −  BEe  (1.2) 

 

Figure 11 illustrate the photoelectric interaction. 

 

Figure 11. Schematic of the photoelectric interaction. 

 



19 
 

2. Compton scattering  

In the Compton scattering interactions, the incident γ photons do not lose all of their energy 

upon interactions with the electrons of matter. The energy of the scattered γ photons and the energy 

of the ejected electron can be determined with the following equations: 

E′ =
E

1 + E
m0c2 (1 − cosθ)

(1.3) 

where E is the initial photon energy (MeV), E′ is the scattered photon energy (MeV), m0c2 is the 

electron rest mass energy (0.511 MeV), and θ is the angle of the scattered photon.  

If the photon scattering angle is known, the energy of the recoil electron (KEe−) can be calculated 

as: 

KEe− =
E

m0c2
E(1 − cosθ)

1 + E
m0c2 (1 − cosθ)

(1.4) 

3. Pair Production 

In regions of high atomic numbers (Z) and higher energies, pair production interactions begin 

to dominate. In this mechanism, an atom emits a positron/electron pair upon absorption of an 

incoming photon in the nucleus. After the individual particles attenuate and lose a significant 

amount of energy, the two will then recombine to produce two photons each of energy 0.511 MeV. 

These photons are called annihilation radiation and are characteristic of pair production. In 

compliance with the conservation of energy, the pair production interaction has a low-energy 

threshold of 1.022 MeV – the rest mass energy of the positron and electron. A schematic of the 

process is shown in Figure 12. 



20 
 

 

Figure 12. Schematic of pair production interactions. 

Conservation of mass for the pair production is as follows:  

Eγ = 2m0c2 + Ee+ + Ee− (1.5) 

The pair production interaction coefficient (κ) is proportional to the square of Z of the attenuating 

material and increases rapidly for E ≥ 1.022 MeV.  

1.3.1.2 Interactions of high-energy electrons with matter  

E-beam technology was originally developed in the 1950s and had the ability to produce 

electrons with an energy up to 2.3 MeV.97,98 During e-beam irradiation fast electrons, with the 

energy to excite and ionize other molecules, are generated in a vacuum usually by a heated cathode 

or a radio frequency (RF) ion source. 97,99 Particle accelerators are used for this process. The fast 

electrons generated are then accelerated and shaped by an electrostatic field formed between the 

anode and cathode. An electron optical system is commonly used to focus the accelerated electrons 

to the window plane. The electrons with high enough energy can then exit the accelerator through 

a 10 – 15 μm thick titanium window foil, towards the target. In addition to avoiding the handling 

of radioactive isotopes, the advantages of the e-beam over gamma irradiation include the 

possibility of higher dose rates, in the order of 10 kGy/s, and the ability to control the emission of 

electrons through the use of magnets.  



21 
 

As a high-energy particle traverses through a medium, a fraction of its initial energy is lost 

through each interaction with an electron or nucleus in the medium. As these particles collide, 

electrons are knocked free of their orbit to produce an ion pair, a positively-charged ion and a 

negatively-charged free electron. 

M
e−p+a
�⎯⎯⎯�M+ + ne− (1.6) 

Charged particles interact via charged Coulombic interactions represented by Coulomb’s Law. 

This law takes into account both charge (higher charge yields a stronger force) and distance (force 

decreases as 1/r2). 

F = k
q1q2

r2
(1.7) 

where F is the electric force acting on q1and q2, q1and q2 are the charge of particles 1 and 2, 

respectively, and r is the distance between the charges. 

Rate of Energy Loss 

As a charged particle travels through a medium, Coulomb forces between the moving 

particle and the electrons of the stationary atoms result in millions of interactions before the 

particle is completely stopped. Because the particle is scattered through many angles and each 

interaction has a distinct probability of occurring, it is impossible to calculate the exact total energy 

loss of the particle. For this reason, an average energy loss per unit distance or stopping power 

(dE/dx) will be defined for several types of charged particles. Stopping power may also be used to 

calculate the energy loss rate (-dE/dx) through various media. Depending upon the mass of the 

particle, the track may be jagged (for small, charged particles such as electrons and positrons) or 

relatively straight (for heavy ions such as alpha particles). 



22 
 

While an electron travels through the medium, a significant portion of the energy released will 

be in the form of bremsstrahlung radiation which is deposited some distance away from the particle 

track. 

1. Stopping power for electrons: 

dE
dx

�
MeV

m
� = 4πr02

mc2

β2
NZ �ln�

βγ�γ − 1
I

mc2� +
1

2γ2
�
(γ − 1)2

8
+ 1 − (γ2 + 2γ − 1)ln2��  (1.8) 

 

2. Stopping power for p, d, t, a: 

dE
dx

�
MeV

m
� = 4πr02z2

mc2

β2
NZ �ln�

2mc2

I
β2γ2� − β2� (1.9) 

 

3. Stopping power for positrons: 

dE
dx

�
MeV

m
� = 4πr02

mc2

β2
NZ

�ln�
βγ�γ − 1

I
mc2� −

β2

24
�23 +

14
γ + 1

+
10

(γ + 1)2 +
4

(γ + 1)3� +
ln2
2
� (1.10)

 

 

where r0 = e2

mc2
= 2.818 x 10−15m (electron radius), 4πr02 = 10−28m2 = 10−24cm2, mc2 =

0.511 MeV (electron rest mass energy), γ = T+mc2

mc2
= 1

�1−β2
,  WT = (γ − 1)mc2 (kinetic 

energy), β = v
c

, c is the speed of light in m/s, N is the number of atoms per m3 in a material, Z is 

the atomic number of the material, z is the charge of incident particle, and I is the mean excitation 

potential of the material. 

For Z > 12, I can be approximated as: 

I(eV) = (9.76 + 58.8Z−1.19)Z (1.11) 



23 
 

1.3.2 Irradiation in aqueous solution  

When dilute aqueous solutions are irradiated, the radiation interacts with the water and 

produces various reactive species. The observed chemical changes are the indirect result of the 

reactions of the radicals generated from the water, with the material. The following species are the 

product of exposing water to ionizing radiation:100 

H2O
Irradiation
�⎯⎯⎯⎯⎯⎯� OH 

• , eaq− , H3O+, H•, H2, H2O2 (1.12) 

During the irradiation of aqueous solutions, the ionized molecules react rapidly to form 

hydroxyl radicals ( OH 
• ), while the electrons become hydrated (eaq− ).100,101 These radical species 

react together or with the hydrogen ion (H3O+) to form smaller amounts of hydrogen radical (H•), 

hydrogen molecules (H2), and hydrogen peroxide (H2O2). The radiation-chemical yield, or yield 

(G), of these species have been established for electrons with energies in the 0.1 – 20 MeV range 

and are shown in Table 1.  

Table 1. Radiation chemical yield of species formed after the irradiation of aqueous solutions.100 

Species Yield (μmol/J) 
𝐻𝐻2 0.04 
𝐻𝐻2𝑂𝑂2 0.08 
𝑒𝑒𝑎𝑎𝑎𝑎−  0.29 
𝐻𝐻• 0.064 
𝑂𝑂𝐻𝐻 

•  0.29 
𝐻𝐻2𝑂𝑂2  0.08 
H3O+ 0.29 

  

These radical species are of great importance in the use of radiation for grafting. They 

promote the generation of radicals on the substrate and facilitate the grafting of the selected 

monomer. The hydroxyl radical ( OH 
• ) is the most important since it will readily abstract hydrogen 



24 
 

or attach to a C-C double bond and initiate the polymerization process. It has been shown that the 

generation of hydroxyl radicals can be enhanced by the use of nitrous oxide.102–104  

1.3.3 Radiation-induced crosslinking and degradation in polymers 

Ionizing radiation can result in either crosslinking or degradation of polymers, caused by 

the reactive species generated. Chemical crosslinking is the process in which two or more polymer 

chains form bonds between themselves and see an increase in their molecular weight.  When 

polymer chains are broken, that process is known as chain scission. The polymer undergoes 

degradation and a reduction in its molecular weight is observed. The mechanisms for both 

processes are shown in Figure 13. 

 

Figure 13. Mechanisms for radiation-induced (a) crosslinking, and (b) degradation. 

The G-value, or radiation chemical yield, is used to quantify the polymer’s response to 

irradiation in terms of crosslinking (G(X)) or chain scission (G(S)). In this case the G-value 

represents the chemical yield of crosslinking or degradation from radiation by the number of 

molecules per 100 eV of energy dissipated in the material. Polymers are commonly classified 

depending on their response to ionizing radiation, whether they exhibit predominantly crosslinking 

or chain scission. Table 2 shows the G-values of a variety of polymers. 



25 
 

Table 2. G-values for selected polymers. Table taken from Andrew. 97 

 

The structure of the polymer influences its response to ionizing radiation. It has been 

observed that polymers with an α-hydrogen, like PE, favor crosslinking. While vinyl polymers 

with two side chains attached to the backbone carbon, such as polymethylmethacrylate, tend to 

degrade. Aromatic polymers, such as aramid, are usually resistant to ionizing radiation.97 The 

effects of quaternary C-atoms are shown in Table 3. 

 

 

 

 



26 
 

Table 3. Effects of quaternary C-atoms for irradiation of polymers.105 

H2
C C

R1

R2

n

 

Polymer 
 R1 R2 

Radiation-induced 
change 

Polyethylene 
Polypropylene 

Poly(vinyl chloride) 
Polyacrylonitrile 

Polyacrylates 
Polystyrene 

Poly(vinylidene chloride) 
Polyisobutylene 

Polymethylstyrene 
Poly(methyl methacrylate) 

Natural rubber 
Cellulose and derivatives 

H H Cross-linking 
CH3 H Cross-linking 
Cl H Cross-linking 
CN H Cross-linking 

CO2R H Cross-linking 
C6H5 H Cross-linking 

Cl Cl Degradation 
CH3 CH3 Degradation 
C6H3 CH3 Degradation 

CO2CH3 CH3 Degradation 
  Cross-linking 
  Degradation 

 

1.3.3.1 Radiation-induced crosslinking  

As discussed above, depending on the irradiation conditions and the properties of the 

polymer being irradiated, crosslinking can occur when the radicals generated on the polymer 

preferably react with each other. This creates new permanent intermolecular bonds between 

molecules that were previously separate. In addition to the chemical structure and chain mobility 

of polymers, the association between its molecules have a huge effect on its mechanical properties. 

Some advantages of radiation-induced crosslinking over chemically-induced crosslinking include 

little to no temperature dependence, no unwanted chemical residue, and a quick turnaround 

period.106   

Electron beam irradiation is commonly used for the radiation-induced crosslinking of many 

polymers, with the goal of improving their chemical and physical properties including the 



27 
 

polymer’s Young’s modulus, hardness, and heat and chemical resistance.107 High radiation dose 

rate tends to favor crosslinking over chain scission reactions. However, it is important to note that 

even for polymers in which crosslinking is dominant, some degradation is still observed.108,109 This 

is due to the radiation still breaking some bonds along the backbone of the polymer, thus reducing 

their properties. For semi-crystalline polymers, crosslinking occurs in the amorphous phase of the 

polymer. The crosslinking ‘strengthens’ the polymer and reduces mobility of the chains in the 

amorphous phase.  

The degree of degradation can be minimized, by the addition of certain chemicals that 

enhance the crosslinking of the polymer.110–112. For example, the addition of a mediating alkyne 

gas during irradiation prevents degradation of polymeric material and enables the introduction of 

additional molecular and functional characteristics. Furthermore, keeping the polymer system in 

an inert atmosphere will protect the radicals from reacting with oxygen, which often leads to 

oxidative degradation. 

Crosslinking enhancement 

The effect of e-beam irradiation on PE fibers in an acetylene environment was reported by 

Klein et al. in 1993. They found that the degree of crosslinking achieved was dependent on the 

crystallinity of the fibers and concluded that crosslinking provided an effective mechanism for 

stress to be transmitted through the polymer.113 However, a study on the gamma irradiation of 

UHMWPE fibers in various environments found that even in acetylene, the tensile strength and 

elongation of the fibers decreased substantially.114 It is to be expected that the higher crystallinity 

of the UHMWPE fibers would make crosslinking harder to achieve. Also, the low dose rate of 

gamma irradiation may play a big role in these results. Deng et al. also studied the mechanical and 

thermal effects of irradiation environment (air, nitrogen, and acetylene) on UHMWPE fibers. They 



28 
 

found an initial increase in the tensile strength for samples irradiated in the acetylene environment, 

but saw a decrease after 160 days.114 This could be a result of the radicals, which promote cross 

linking and chain scission, staying on the fibers after the initial irradiation. These radicals could 

cause oxidation and thus damage of the fibers. One solution to this is to anneal the fibers after 

irradiation and thus removing any unreacted radicals.115 

The e-beam irradiation of aramid fibers in acetylene gas has been scarcely reported. It has 

been found that coating a layer of acetylene on aramid fibers and cords, increases the adhesion to 

various polymers and epoxy resins.116 The same group reported that plasma polymerization of 

acetylene on aramid cords increased their adhesion to rubber compounds.117 Benzoic acid has been 

used as a crosslinker for aramid fibers, with the goal of increasing their adhesion to epoxy resins 

and increasing their compressive strength.118 

1.3.3.2 Radiation-induced degradation 

Although radiation-induced degradation might be an undesired reaction in most cases, it 

can lead to the optimization of recycling processes. Polymer recycling is an important issue, since 

polymer waste is a daunting challenge being faced all over the world.119–123 Ionizing radiation can 

be used to modify the chemical structure and the overall properties of all types of polymers. There 

are three main ways in which ionizing radiation can aid in dealing with the polymer waste problem: 

(1) enhancing the mechanical properties and performance of the recycled polymers, (2) degrading 

or increasing the degradability of polymers, and (3) producing advanced polymer materials with 

better environmental compatibility.121,124–127 For this work, the focus will be on using e-beam 

irradiation for the degradation of polymer resins, which are an important part of fiber-reinforced 

polymer composites. As discussed above, the main effects of polymer irradiation are crosslinking 



29 
 

and scission of the polymeric backbone. In this case, scission of the polymer is the desired outcome 

of the process.  

One common approach for recycling polymer waste is chemical recycling.  Chemical 

recycling is based on the decomposition or conversion of the large macromolecular structure of 

polymers into smaller molecules. The concept is based on the depolymerization of the material, 

where the size of the molecule is reduced, but the monomer unit remains unchanged. Thus, the 

resulting polymer can be used for the production of new materials. The main issue with chemical 

recycling is that it requires high temperatures and the use of catalysts to drive the reaction. This 

leads to large energy consumption, which would also be an environmental issue. The pre-treatment 

of polymers by irradiation before chemical recycling results in a decrease in the energy 

consumption and promotes the chemical recycling reactions.  

Zhao (1996) et al. found that chain scission caused by irradiation of polypropylene results 

in the polymer being more susceptible to thermal degradation, as observed by a decrease in its 

thermal degradation temperature. As a result of the irradiation, a decrease in the molecular weight 

of the polypropylene was observed, which they attribute to chain scission of the backbone of the 

polymer. Additionally, since the polymer was irradiated in air it also led to the formation of polar 

groups on the surface of the polymer, increasing its oxidative degradability. After irradiation, the 

temperature needed for pyrolysis of the polypropylene was greatly reduced.128 In another case, a 

decrease in the onset temperature for mass loss of butadiene-containing polymers was achieved 

after irradiating copolymers in the presence of air.129 Their comparison between the irradiation in 

the presence and absence of oxygen, shows that scission and polymer degradation may be 

promoted by conducting the irradiation in the presence of air.  



30 
 

In addition to lowering the energy required for further chemical recycling of these 

polymers, irradiation of polymer waste can also be used for creating new and better materials. 

High density polyethylene (HDPE) waste was recycled into filler by first irradiating the HDPE via 

the e-beam and then grinding it into a powder. This powder was used as a filler for low density 

polyethylene, creating a HD/LD polyethylene material with improved mechanical properties when 

compared to LDPE.130  Vignon et al. made a polymer blend (representing thermoplastic waste) 

with polycarbonate, to synthesize a polycarbonate-based extruded thermoplastic.131 The samples 

were exposed to gamma irradiation, in the range of 0 – 150 kGy and the mechanical properties of 

the product were studied. It was found that irradiating the material led to more homogeneous 

blends with higher thermal stability.  

Unsaturated polyester resin 

One of the most widely used types of thermosetting polymers are unsaturated polyester 

resins (UPRs).132,133 UPRs have a relatively low cost, and good mechanical, electrical, and 

chemical properties.133–135  It is expected that the global UPRs market will grow from $ 9.6 billion 

in 2021 to $ 12.9 billion by 2026.136 This makes UPRs an interesting candidate in the search for 

polymer recycling methods.137 Polyester resins are produced by the condensation of a polyol with 

a combination of saturated and unsaturated anhydrides, and their properties can be tuned by the 

proportion and choice of the starting materials.138–142 As thermoset resins, they are thermally and 

chemically stable, which complicates their recycling process.143,144 Current efforts to recycle 

thermoset resins include mechanical methods, which involve a grinding process of the material, 

thermal methods where the resin is decomposed by heating it up to high temperatures, and 

chemical methods, which mostly concentrate on depolymerization of the resin.145–147 The 

advantages of ionizing irradiation over these methods include: (1) high rates of radical formation, 



31 
 

which influence the degradation process, (2) no need of initiators or additives, and (3) can be done 

at room temperature.148  

Promoting polymer degradation 

As previously discussed, the effect of irradiation on the polymer depends heavily on the 

environment in which the polymer is irradiated and the chemical structure of the polymer. 

Irradiation of the polymer in air leads to the formation of peroxyl radical species and results in a 

major enhancement effect on the radiation degradation of most polymers.149 Dispersing the 

polymer in a solvent prior to irradiation will result in the absorption of the high-energy radiation 

by both the polymer and the solvent, and in the generation of radicals from both. All the radicals 

generated will then induce reactions in the polymer and influence its degradation.  

The physicochemical characteristics of the polymer in combination with the irradiation 

environment, e.g., presence of oxygen, proximate solvents, irradiation temperature, total dose, and 

dose-rate, can either induce scissions, crosslinking, or both in the irradiated pure polymers.96  The 

distinctive backbone bonds in polyesters, comprised of the C-O-C bond sequence is highly  

sensitive to ionizing radiation and generally favors scission.109 The mechanism of radiation-

induced scissions is complicated by the presence of additional moieties common in representative 

polyesters including double bonds, phenyl, and carbonyl groups. While the presence of the C-O-

C bonds enhance scissions, the π structure of the phenyl groups provides protection via the reaction 

of the electron with the π structure, ultimately reducing effective bond breakage. The presence of 

double bonds plays an important role in the structures of the radiation-induced free radicals, their 

kinetics, and the mechanisms by which they decay. As for polyester resins used in industry to 

prevent corrosion, which contain unsaturated vinyl monomers such as vinyl toluene, styrene, and 

other additives, the radiation effects become even more complicated since these residual vinyl 



32 
 

monomers undergo radiation polymerization, which contribute to an increase to the Tg of the 

polyester-resins. These challenges in the use of ionizing radiation to degrade the unsaturated resins 

can be overcome by tuning the irradiation conditions such as high dose rate to impede the radiation 

polymerization of these residual vinyl monomers. In addition, the presence of O2 during the 

irradiation is critical to inhibit the radiation polymerization of the vinyl monomers present in the 

resins, and also to prevent the crosslinking reactions of the radiolytically produced polyester 

radicals.109 

1.3.4 Radiation-induced polymerization   

Ionizing radiation can also induce polymerization reactions, which are initiated when a 

monomer or oglomer is irradiated. There are two main types of polymerization reactions: free 

radical and cationic polymerization.150 Radiation-induced free-radical polymerization reactions 

usually follow the chain addition mechanism. As excited species, such as free radicals, are 

produced on the irradiated material, they can initiate the chain polymerization mechanism. This 

technique enables the initiation of polymerization reactions at conditions in which conventional 

initiation methods do not work to start the reaction.  

The polymerization process can be divided into 4 steps: initiation, propagation, 

termination, and chain transfer. The chain initiation reactions start when the material is irradiated, 

and the initiator decomposes into a free radical (X∗). The rate of reaction of this step is controlled 

by the decomposition of this initiator. The chain initiation ends when the free radical adds to the 

first monomer unit and produces the chain initiating species (M∗). This initiating species, or 

initiating monomer, starts the propagation reactions. It goes through an iterative growth and after 

successive addition of monomer molecules, macroradicals are formed. Both steps are shown in 

Figure 14. 



33 
 

X X*

X*+M M*

Mn+M* Mn+1-M*

Initiation

Propagation

 

Figure 14. Schematic of the initiation and propagation steps in the polymerization mechanism. 

The active free radical species are highly unstable and tend to self-annihilate through two 

termination reactions: combination and disproportionation. Combination reactions occur when two 

macroradicals react resulting in the formation of a polymer comprised of the two species. On the 

other hand, in disproportionation reactions one of the species abstracts a H-atom from the other 

which results in a H saturated macromolecule together with its omega unsaturated counterpart.150 

Chain transfer reactions compete with the propagation reactions as the reactive macromolecule 

(𝑀𝑀 −𝑀𝑀∗) may react with a component present in the experiment, that is not the monomer. In this 

case the active site is transferred to that other non-monomer species, terminating the 

polymerization of that specific chain, but starting a new chain polymerization. A schematic of 

these two steps is shown in Figure 15.   

2Mn-M* M2n

Mn-M* + Mm-M* Mn Mm+

Mn-M* + X X*Mn +

Termination

Disproportionation

Combination

Chain Transfer

 

Figure 15. Schematic of the termination and chain transfer steps in the polymerization 
mechanism. 



34 
 

 In cationic polymerization reactions, an electrophile reacts with the monomer which has a 

π bond. The reaction is initiated by adding the electrophile which results in cleavage of that double 

bond, and a cation is left on one end of the original double bond. For example: 

H CH3

CH3H

H Cl

CH3

CH3

H

H
H

Cl

 

Propagation then proceeds with the reaction of the monomer with the active species.  

A +

n n+1
A

monomer
active species  

The polymerization is terminated generally by combination the nucleophilic anion with the 

cationic chain. Another way in which the chain is terminated is by chain transfer. In this case, 

hydrogen can be abstracted by the counterion or by the monomer.  

 

 

 

 



35 
 

A

n
H n

A

Termination by combination

 

H

A

H n
H

+ HA

H n

H n
H

+
H n

+

Chain transfer termination

 

1.3.5 Radiation-induced grafting 

Radiation-induced grafting refers to the process in which a functional monomer may be 

covalently bonded to a polymer substrate with the use of radiation. This is a rapid method that 

allows for the functionalization of a variety of polymers, and that can be upgraded from the 

laboratory to the industrial scale.96 Some of the advantages of radiation-induced grafting over 

chemical grafting include, not requiring any hazardous reactants, and no limitations in the polymer 

shape. The degree of grafting (DoG) is used to quantify the amount of grafting achieved through 

the following equation:  

DoG (%) =
mf − mi

mi
× 100 (1.13) 

where mf is the mass of the sample after grafting, and mi is the mass of the sample before grafting.  

There are two main ways to graft monomers onto a substrate: direct and indirect grafting. These 

will be discussed in the following sections.  



36 
 

1.3.5.1 Indirect Grafting 

In the process of indirect grafting the polymer substrate is irradiated first in a vacuum or in 

an inert atmosphere, to protect the radicals formed during the irradiation. The substrate is kept in 

that inert atmosphere until it is mixed with the monomer that will be grafted onto it. At that moment 

the radicals generated will start the reaction with the monomer or polymer that is being grafted. 

Radicals are then generated on the monomer, through which a covalent bond between the monomer 

and the substrate is formed. In the case of using a monomer, polymerization grafting can also be 

achieved. One of the advantages of this type of grafting is that there is a low degree of 

homopolymerization of the monomer since the radicals are formed through a chain transfer 

mechanism. This in turn leads to an easier process of washing the grafted material, since less 

homopolymer will need to be removed. In addition, this allows for the grafting of virtually any 

monomer, since it would not be subjected to any irradiation that may damage it. The disadvantage 

of indirect grafting involves the need to maintain the irradiated substrate in an inert atmosphere 

and at cold temperatures to conserve the radicals before the monomer is introduced. If kept at room 

temperature, the radicals generated will quickly begin to decay by recombination.  

1.3.5.2 Direct Grafting 

The process of direct grafting is that in which both the polymer substrate and the monomer 

are irradiated together. Grafting is achieved through the simultaneous generation of radicals in the 

substrate and in the monomer, which then react to form covalent bonds between the two. The 

irradiation may be done in air, in inert atmosphere, or in an environment that promotes the 

generation of a specific kind of radical, i.e., nitrous oxide. The main disadvantage of this method 

is that it is limited to certain substrate-monomer pairs. A similar number of radicals is desired for 

both the substrate and the monomer. However, if they have different radiation-chemical yields (G) 



37 
 

more radicals of one will be generated over the other. This is especially concerning if the monomer 

has a higher yield since it will lead to a high degree of homopolymerization and a low DoG. Ways 

to promote grafting and avoid homopolymerization include: (1) polymerization inhibitor in the 

monomer solution, (2) high temperature environment, and (3) lower dose rates.151 

1.3.6 Radiation grafting under different atmospheres 

As discussed above, it is of outmost importance to control the type and yield of radicals 

formed during the irradiation to achieve high degrees of grafting. In this section, the effect of 

different types of irradiation atmospheres on the final product will be discussed. 

1.3.6.1 Presence of oxygen 

When a polymer is irradiated under a natural atmosphere, the presence of oxygen around 

it will have significant effects on the type of radicals formed. The result is the oxidation of the 

polymer from the reaction of the polymer molecule with the peroxyl radicals generated from the 

oxygen molecules. The following chain reaction scheme describes the oxidative degradation of 

polymers (RH) in air:152 



38 
 

RH R + H
Initiation

Propagation
R + O2 ROO

ROO + RH ROOH + R

ROOH RO + OH

OH + RH

H2O+RO

RO + RH ROH + R

RO chain scission reactions

Termination
2R R-R

ROO + R ROOR

ROOH

chain branching

crosslinking reactions 
to non-radical products

radical conversion

H2O + R

 

During e-beam irradiation the presence of oxygen not only increases the rate of degradation 

of the polymer, but also increases the possibility of cross-linking.109 For the purpose of the grafting 

experiments done in this work, the irradiations were conducted in the absence of oxygen.  

1.3.6.2 Absence of oxygen 

Irradiations under vacuum or in an inert atmosphere allow for the formation of radicals in 

the polymer without the interference of oxygen molecules. In the case of indirect grafting 

procedures, the absence of oxygen slows the decay of the radicals generated on the polymer until 

the monomer is introduced. If direct grafting is done in non-aqueous solution, the absence of 

oxygen will also be beneficial to achieve a high DoG because it will avoid the reaction of the 

radicals with species other than the polymer.  



39 
 

1.3.6.3 Nitrous oxide environment 

As discussed previously, during the irradiation of the polymer in aqueous solution the 

hydroxyl ( OH 
• ) radical plays an important role in the formation of radicals on the backbone of the 

polymer. These radicals are generated when the OH 
•  radical abstracts hydrogen atoms from the 

backbone of the polymer or adds to a C-C double bond. Thus, it is beneficial to have a high 

concentration of hydroxyl radicals in the solution. In 1960 Dainton and Peterson104, showed that 

the addition of nitrous oxide into an aqueous solution led to the increase in concentration of 

hydroxyl radicals. When nitrous oxide is introduced to the solution the aqueous electrons react 

with it which leads to the generation of more hydroxyl radicals trough the following reaction: 

eaq− + N2O
H2O�⎯�N2+−OH + OH 

•  (1.14) 

This reaction is very fast (k = 9.1  ×  109 M−1s−1) which allows for the majority of the aqueous 

electrons to be converted to hydroxyl radicals, before reacting with the other species.153  

1.3.7 Additional parameters affecting radiation-induced grafting yield 

The DoG can be controlled by various irradiation parameters including dose, dose rate, and 

linear energy transfer (LET). The irradiations done in this work were completed with the use of an 

e-beam accelerator. In general, a higher irradiation dose leads to a larger number of radicals 

generated and thus a higher DoG. However, the effects of high doses on the polymer substrate 

must be taken into account when selecting the irradiation dose for grafting. In terms of the dose 

rate, the inverse is observed; a high dose rate leads to a decrease in the DoG. This is due to high 

dose rates increasing the density of radicals which promotes their recombination and the formation 

of gels.154  



40 
 

Other conditions that affect the DoG are the chemistry of the polymer substrate, monomer 

concentration, and grafting temperature. The substrate chemistry is important because it 

determines the reactivity of the radicals produced during irradiation. The nature of the radical is 

usually determined by electron paramagnetic (EPR) spectroscopy to determine its reactivity with 

the monomer. The presence of functional groups that can either stabilize or quench the radicals 

generated must also be considered. Mostly, a higher concentration of the monomer leads to a 

higher DoG. However, an increase in the monomer concentration might lead to an increase in the 

viscosity of the solution and thus a decrease in the diffusion of the monomer to the polymer 

substrate which might reduce the DoG. Finally, an increase in the temperature can increase the 

DoG for both direct and indirect grafting procedures. Heating the