Mechanical Aging of Thermoset Polymer

Teng Cui

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Department of Mechanical and Industrial Engineering University of Toronto

Teng Cui

Master of Applied Science

Department of Mechanical and Industrial Engineering University of Toronto

2016 Abstract

Polymer aging is a time-dependent process of various changes to the material under the influence of one or more environmental factors. Thermoset polymers experience mechanical properties degradation when subjected to different aging conditions. It is the objective of this research to conduct accelerated tests, supported by limited MD simulations, to determine the degradation behaviors of an epoxy polymer under water immersion, UV radiation, and elevated temperature.

To investigate the effect upon mechanical properties, tensile tests were conducted for more than

270 days. In addition, molecular dynamics (MD) simulations were conducted to investigate the water absorption effect on the elastic properties of the epoxy. The results revealed that water absorption has both plasticization and anti-plasticization effects; UV radiation could lead to post- curing reactions and epoxy embrittlement; thermal aging can also further cure the epoxy initially, but longer thermal aging leads to softening effect.

Acknowledgments

Firstly, I would like to express my sincere appreciation and gratitude to my supervisor Professor Shaker A. Meguid for his continued guidance and cultivation during my master. I would not have been able to achieve these accomplishments without his dedication, encouragement, and instruction. I am deeply impressed by his enthusiasm, dedication, teaching ability, and research expertise. All of these qualities will be my model and will always lead me in my future life. In addition, I also extend many thanks to the Anonymous Sponsor, who provides the financial support for the completion of this research.

Next, I want to thank my lab colleague Mr. Pieter Verberne for his extensive help with the test design and experiments. He also offered me many assistance and suggestions on many problems that I encountered during the completion of my thesis. From him, I have also learned many important qualities that a good researcher should have.

I would like to extend thanks to all the members in Mechanics and Aerospace Design laboratory (MADL). It has been a great pleasure to work in such an excellent group.

Finally, I would like to extend my sincere gratitude to my family, my father Guoqing Cui, my mother Lijun Zhang, my sister Miao Cui, and my brother Xiang Cui, for their wholehearted love and unlimited support. I also want to express my gratitude for my girlfriend Xinyi Zhang for her understanding and encouragements.

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Table of Contents

Abstract ...... ii

Acknowledgments ...... ii

Abbreviations ...... vi

Notations ...... vii

Table of Contents ...... iv

List of Tables ...... viii

List of Figures ...... ix

Introduction and Justification ...... 1

1.1 Aging of Polymers ...... 1

1.2 Problem Statement ...... 2

1.3 Research Objectives ...... 3

1.4 Method of Approach ...... 3

1.5 Thesis Layout ...... 4

Literature Review ...... 5

2.1 Water Aging ...... 5

2.2 UV Aging ...... 8

2.3 Thermal Aging ...... 11

Experimental and Molecular Dynamics Investigations...... 13

3.1 Material Details and Sample Preparation ...... 13

3.2 Aging Methodology ...... 15

3.2.1 Water Immersion ...... 15

3.2.2 UV Radiation ...... 16

3.2.3 Thermal Aging ...... 17

3.3 Characterization of Mechanical Properties ...... 18

3.3.1 Tensile Testing ...... 18

3.3.2 Fracture Surface Examination...... 18

3.4 Molecular Dynamics Modelling of Water Absorption ...... 18

3.4.1 Pure Epoxy Model ...... 19

3.4.2 Epoxy-Water Model...... 21

3.4.3 Water Diffusion Coefficient and Elastic Properties ...... 22

Results and Discussions ...... 25

4.1 Water Aging Results ...... 25

4.1.1 Tensile Properties due to Water Aging ...... 25

4.1.2 Fracture Surface Morphology due to Water Aging ...... 30

4.1.3 Fracture Surface Topology due to Water Aging ...... 32

4.1.4 MD Results ...... 34

4.2 UV Aging Results ...... 36

4.2.1 Tensile Properties due to UV Aging ...... 36

4.2.2 Fracture Surface Morphology due to UV Aging ...... 40

4.3 Thermal Aging Results ...... 43

4.3.1 Tensile Properties due to Thermal Aging ...... 43

4.3.2 Fracture Surface Morphology due to Thermal Aging...... 46

Conclusions and Future Work ...... 49

5.1 Statement of the Problem ...... 49

5.2 Conclusions ...... 49

5.3 Thesis Contributions ...... 50

5.4 Future Work ...... 51

References ...... 52

Abbreviations

AFM Atomic Force Microscopy

CVFF Consistent Valence Forcefield

FTIR Fourier Transform Infrared

RH Relative Humidity

MD Molecular Dynamics

MSD Mean Square Displacement

NMR Nuclear Magnetic Resonance

NPT Isothermal-isobaric

NVT Canonical

PALS Positron Annihilation Lifetime Spectroscopy

SEM Scanning Electron Microscopy

TMPC Tetramethyl Bisphenol-A Polycarbonate

UV Ultraviolet

XPS X-ray Photoelectron Spectroscopy

Notations

퐶푖푗푘푙 General form of elastic coefficient matrix

퐶훼훽 Elastic coefficient matrix for isotropic materials

퐷 Water diffusion coefficient

퐸 Young’s modulus

퐺 Shear modulus

퐾 Bulk modulus

푅 Location of the center of mass of an atom

푡 Time

푇푔 Glass transition temperature

휈 Poisson’s ratio

휆, 휇 Lamé constants

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List of Tables

Table 2.1 UV Wavelength at which various polymers have maximum sensitivity [52] ...... 8

Table 3.1 Chemical composition and concentration in the epoxy resin [81] ...... 13

Table 3.2 Chemical composition and concentration in the curing agent [82] ...... 14

Table 3.3 Number of atoms and water molecules in the epoxy-water systems ...... 22

Table 4.1 Tensile properties and weight gain percentage before and after various aging times in distilled water ...... 29

Table 4.2 Summary of physical and mechanical properties of epoxy for different water absorption contents ...... 34

Table 4.3 Tensile properties before and after UV aging under the irradiance of 6 W/m2 ...... 40

Table 4.4 Tensile properties before and after UV aging under the irradiance of 18 W/m2 ...... 40

Table 4.5 Tensile properties before and after 70℃ thermal aging ...... 46

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List of Figures

Fig. 1.1 Skeletal structures of linear, branched and network polymers ...... 1

Fig. 1.2 Method of Approach ...... 4

Fig. 2.1 Hydrogen bonding states between water and polymer chains, (a) Type I bound water forming one hydrogen bond, (b) Type II bound water forming two hydrogen bonds [31] ...... 6

Fig. 3.1 The molecular structure of (a) DGEBA homopolymer, and (b) Polyoxypropylenediamine [83] ...... 14

Fig. 3.2 (a) Engineering drawing of the designed specimen, all units are in millimeter, and (b) Commission mold for casting specimen ...... 15

Fig. 3.3 Ultraviolet chambers used: (a) UV lamp with irradiance of 6 W/m2, and (b) UV lamp with irradiance of 18 W/m2 ...... 16

Fig. 3.4 Thermal chamber used for thermal aging at 70°C ...... 17

Fig. 3.5 Reaction between epoxide groups and amine groups [89] ...... 19

Fig. 3.6 MD modelling steps of pure epoxy: (a) constituents of molecular structure of epoxy resin and hardener, (b) an epoxy unit, (c) a corresponding structure of low packing density, and (d) final structure of size 53 Å × 53 Å × 53 Å ...... 20

Fig. 3.7 Epoxy modelling procedure to achieve the desired density ...... 21

Fig. 3.8 MD model of the epoxy with different water contents ...... 22

Fig. 4.1 Weight gain percentage with square root of water immersion time ...... 25

Fig. 4.2 Evolution of normalized elastic modulus with water weight content ...... 26

Fig. 4.3 Evolution of normalized tensile strength with water weight content ...... 27

Fig. 4.4 Evolution of normalized fracture strain and tensile toughness with water weight content ...... 28

Fig. 4.5 Typical engineering stress-strain curves before and after water aging ...... 29

Fig. 4.6 SEM micrographs of fracture surfaces for unaged specimen at magnification of (a) 60x, and (b) 1776x ...... 30

Fig. 4.7 SEM micrographs of fracture surface for specimen after 30 days of aging in distilled water at magnification of (a) 59x, and (b) 1776x ...... 31

Fig. 4.8 SEM micrographs of fracture surface for specimen after 180 days of aging in distilled water at magnification of (a) 60x, (b) 1776x in the middle, and (c) 1776x at bottom right ...... 31

Fig. 4.9 AFM height images of fracture surface for (a) unaged specimen, (b) after 30 days of water aging, (c) after 180 days of water aging. Height variation along the diagonal for (d) unaged specimen, (e) after 30 days of water aging, and (f) after 180 days of water aging ...... 33

Fig. 4.10 MSD evolutions during 2 ns MD simulation for five different water contents ...... 35

Fig. 4.11 Experimental and MD results of normalized Young’s modulus change with water content ...... 36

Fig. 4.12 Evolution of normalized elastic modulus with UV aging time ...... 37

Fig. 4.13 Evolution of normalized tensile strength with UV aging time ...... 38

Fig. 4.14 Evolution of normalized fracture strain and tensile toughness with UV aging time ... 39

Fig. 4.15 Typical engineering stress-strain curves before and after different UV aging conditions ...... 39

Fig. 4.16 SEM micrographs of fracture surface after 151 days of UV aging under UV irradiance of 18 W/m2 at magnification of (a) 60x, and (b) 1776x ...... 42

Fig. 4.17 SEM micrograph of fracture surface after 180 days of UV aging under UV irradiance of 6 W/m2 at magnification of 60x ...... 43

Fig. 4.18 Evolution of normalized elastic modulus and tensile strength with thermal aging time ...... 44

Fig. 4.19 Evolution of normalized fracture strain and tensile toughness with thermal aging time ...... 45

Fig. 4.20 Typical engineering stress-strain curves before and after 70°C thermal aging ...... 45

Fig. 4.21 Weight loss percentage during 70℃ thermal aging ...... 46

Fig. 4.22 SEM micrographs of fracture surface after 154 days of 70℃ thermal aging at magnification of (a) 61x, and (b) 1776x ...... 47

Fig. 4.23 SEM micrographs of fracture surface after 300 days of 70℃ thermal aging at magnification of 59x ...... 48

Introduction and Justification

1.1 Aging of Polymers

The application of polymers plays a significant role in almost every industry, including electronics, aerospace, maritime, automotive [1-5]. Polymer can be defined as a substance composed of long molecular chains in which different species of atoms are linked together by covalent bonds [6]. Polymer chains are constituted of many low molecular weight repeating units, which are called monomers. Monomers are linked to each other through chemical reactions to form a polymer [6]. This process of polymer formation from monomers is known as polymerization.

There are three different molecular structures of polymers: linear, branched, and crosslinked structures. A linear chain is similar to a string with two ends. Branched polymeric structures have side chains emanating from the main chains through junction points. Meanwhile, the crosslinked structures have three-dimensional network where the chains are entangled and inter- connected by a number of junctions. Polymers with crosslinked structures are often characterized by the crosslink density, which refers to the number of junction joints per unit volume [6]. A schematic illustration of the three polymeric structures is shown in Fig. 1.1.

Fig. 1.1 Skeletal structures of linear, branched and network polymers

Polymers are commonly classified into three main categories: thermoplastics, elastomers, and thermosets. Thermoplastics have linear or branched structure that can be melted by heating for further remolding. This is due to the weak intermolecular forces that can be easily broken under 1

elevated temperature. Polyethylene and polyvinyl chloride (PVC) are common examples of thermoplastic polymers. Elastomers are rubbery polymers that have high extension capability. Such extension capability originates from their low crosslinked structure. The elastomer chains are easily deformed under loading due to low crosslink density, and the small amount of crosslinking joints also help to spring back upon unloading. Thermosets are highly crosslinked polymers in which the motion of molecular chains is greatly impeded. Due to such strong crosslinked nature, thermosets cannot be melted, and offer high stiffness and strength. The high strength and stiffness make them suitable materials for various engineering applications. Typical examples of thermoset polymers are various epoxies. Epoxies frequently serve as coatings, fiber reinforced composites and nanocomposites for use in various industries [2, 3].

However, one problem that polymers inevitably encounter is aging. Polymer aging is a time- dependent process of various changes to the material under the influence of one or more environmental factors, such as humidity, heat, radiations, and chemicals. Generally, aging changes the physical, mechanical, and chemical properties of a polymer leading to degradation of its performance. The performance of the polymers and even the entire structures deteriorate when exposed to those aging conditions for an extended period. These problems include but are not limited to swelling [7], blistering [8], and interfacial delamination [9, 10].

1.2 Problem Statement

Specifically, this client-sponsored project is interested in the aging effect of water, UV, and temperature on the mechanical properties of a sponsor-selected epoxy polymer. Existing literature indicates that aging under those conditions can alter both the physical and mechanical properties [11-17]. Typically, aging can influence the quasi-static mechanical properties, viscoelasticity, fracture toughness, thermal expansion coefficient, glass transition temperature

(푇푔), volume relaxation, and moisture/solvent absorption capability. The degradation of these properties largely limits the lifetime of epoxy-based products [10, 18].

There have been extensive studies to investigate effect of different aging factors on polymer degradation. However, due to the complexity of polymer types and aging conditions, the polymer degradation behaviors vary significantly. There is no universal theory to explain all the aging mechanisms. In the context of UV and thermal aging, extremely high UV irradiances and 2

temperatures are commonly applied to accelerate the aging process [19]. However, the aging mechanisms can be very different under different UV irradiances or temperatures. For example, thermal aging above 200℃ can give rise to pure thermolysis that breaks down covalent bonds of polymer backbone chains [20]. Aging between 푇푔 and 200℃ usually leads to thermal oxidation in the presence of oxygen [20]. In view of the current need by the sponsor, a systematic study of the degradation behavior of an epoxy polymer due to water, UV rays, and thermal aging was conducted.

1.3 Research Objectives

It is the objective of this research to determine the changes in the mechanical properties of the considered epoxy as a result of its exposure to aging conditions. The thermoset polymer studied in this research is a two-component commercially available epoxy system having the product name of West System 105 and 206. Specifically, it is the aim of this work to:

(i) Determine the degradation in the mechanical properties of a sponsor-selected epoxy as a result of the aforementioned aging conditions using accelerated tests. The properties of interest include Young’s modulus, tensile strength, fracture strain, tensile toughness, weight change, and fracture surface characterization,

(ii) Evaluate the difference between various aging conditions within a given time frame, and

(iii) Cast light and provide insights on the possible aging mechanisms to explain the epoxy degradation behaviors.

1.4 Method of Approach

In order to achieve the above objectives, detailed experimental and limited MD investigations are conducted. A detailed illustration of the developed method of approach is provided in Fig. 1.2. Simple reduced size dog-bone tensile specimens were selected for water immersion, UV radiation, and thermal aging. The aging is conducted in simple chambers commissioned for each of the specific aging conditions. Once the predefined aging time is reached, the mechanical properties are determined by a series of tests and measurements. A limited MD simulation was 3

also carried out to investigate the water absorption effect on the elastic properties of the epoxy system considered. A comparative analysis between the experimental findings and the MD predictions was also made.

Fig. 1.2 Method of Approach

1.5 Thesis Layout

This thesis is divided into five chapters. Chapter 1 introduces the problem under investigation, objectives, and the method of approach adopted. Chapter 2 summarizes the relevant existing literature with regards to water, UV radiation, and thermal aging. Chapter 3 presents details regarding both the experimental and MD investigations. Specifically, specimen preparation, aging chamber design, aging methodology, mechanical tensile testing, and fracture surface characterization are included in the experimental investigation. In MD investigation, the considered epoxy system is simplified and created. Different amount of water molecules are randomly added into the epoxy system. Then, the elastic properties of the epoxy-water systems are examined and compared with experimental results. Chapter 4 summarizes the results for both the experimental and MD studies. Chapter 5 concludes the research and outlines future work. 4

Literature Review

Summary: This chapter presents a detailed literature review regarding three common types of environmental aging: water aging, UV aging, and thermal aging. For each of the aging conditions, the aging mechanisms, property degradation, and previously employed investigation techniques are included and discussed.

2.1 Water Aging

When polymers are immersed in water, water molecules diffuse into the polymeric network. With more and more water absorbed into the polymer, the molecular chains interact with water molecules leading to structural and property changes.

The physical and mechanical degradation of epoxies due to water absorption have been investigated extensively in the literature. Such studies have utilized dielectric measurements [21- 23], Fourier transform infrared (FTIR) spectroscopy [13, 24-30], X-ray photoelectron spectroscopy (XPS) [25], positron annihilation lifetime spectroscopy (PALS) [28], and nuclear magnetic resonance (NMR) [31] to provide insights to the mechanisms contributing to the properties degradation. It has been found that both free and bound water molecules exist in the epoxy network during water absorption [21-23, 29, 30]. Free water molecules refer to those that do not form hydrogen bond with the polymer network [29], and they usually form clusters and reside in the microvoids of the epoxy [30]. The bound water molecules form hydrogen bonds with the epoxy hydrophilic groups [31]. The hydrogen bonding state between water and epoxy has been further investigated by Zhou and Lucas [31]. They proposed two types of bound water: type I and type II bound water, based on the formation of either one or two hydrogen bonds with polymer chains. Li et al. [28] also reported that water molecules may not form or can form one or two hydrogen bonds with the epoxy network. An illustration of the possible hydrogen bonding states between water molecules and polymeric network is shown in Fig. 2.1. The black lines represent the backbone chains of the polymer, and the water molecules are shown in red color. Hydroxyl and Nitrogen-containing groups are possible sites for hydrogen bonding. Fig. 2.1 (a)

shows the case of only one hydrogen bond for water molecules, and Fig. 2.1 (b) illustrates the two hydrogen bonds situation.

Fig. 2.1 Hydrogen bonding states between water and polymer chains, (a) Type I bound water forming one hydrogen bond, (b) Type II bound water forming two hydrogen bonds [31]

In addition to studies concerning the hydrogen bonding mechanism, chemical reactions were also reported in literature. This is especially prevalent for hygrothermal aging, which is the combined aging effect of both moisture and elevated temperature. De'Nève and Shanahan [13] studied the chemical modifications of epoxy exposed to 100% relative humidity (HR) at 40°C, 55°C, and 70°C, and determined that hygrothermal aging leads to chain scissions. Xiao et al. [25] investigated the irreversible interactions between water and epoxy using both FTIR and XPS. Their results showed that hydrolysis of epoxy backbone chains had occurred after only 6 days of immersion in 90°C deionized water.

Physical properties, such as weight and volume changes, were investigated by many studies [7, 32, 33], indicating either Fickian or sigmoidal water diffusion behaviors. Thermal and mechanical properties also experienced severe degradations during aging in water, such as a reduction of the glass transition temperature, tensile strength, elastic modulus, and fracture strain [13, 14, 33-36]. The loss of these properties was explained by the plasticization effect of the absorbed water. The plasticization effect is due to water-induced swelling. With water absorption, the epoxy volume expands, which increases the segmental mobility of the polymeric chains. A more mobile network is responsible for the decrease of properties.

However, there are a few investigations [33, 37, 38] that have reported an increase of the thermal and mechanical properties at certain water absorption contents. Nogueira et al. [33] observed an

increase of tensile Young’s modulus at 1.9% water content. They attributed that increase to the reactivation of polymer curing in 100℃ water. A decreasing Young’s modulus followed by a partial recovery was also presented in the paper of Papanicolaou et al. [37]. Such anti- plasticization effect may be the result of type II bound water, which forms two hydrogen bonds and increases the crosslink degree, thereby increasing the thermal and mechanical properties of the polymer [31]. However, the anti-plasticization effect of water absorption is still not well- understood.

MD simulations have also been applied to investigate the water/epoxy interactions. There have been many studies calculating the water diffusion coefficient at different temperatures and different water contents by examining the mean square displacement (MSD) of water molecules [32, 39-42]. Many studies [39-41] have observed that the water diffusion coefficient increases with increasing water content, and higher temperature also leads to a higher diffusivity [41]. MD has also been used to predict the glass transition temperature and coefficient of thermal expansion of both pure epoxies and epoxy-water systems [43, 44]. Through the analysis of radial distribution functions between oxygen atoms in water molecules and polar groups in epoxy network, Wu and Xu [39] reported the presence of hydrogen bonding between water and the epoxy. They examined the MSD and rotational time correlation functions of certain polar covalent bond vectors in epoxy. They concluded that high water contents plasticize the epoxy, but low water contents can impede the chain mobility.

Both dynamic [45] and static [46, 47] MD simulations have been conducted to determine the mechanical properties for pure epoxies.. Clancy et al. [48] calculated the elastic modulus of epoxy with different water contents, and concluded that the modulus generally decreases with increasing water content. However, their study only considered 5% and 9.5% water contents along with the neat epoxy. Pandiyan et al. [49] used both static and dynamic approaches to determine the Young’s modulus for two different water contents: 3% and 9%. Their results compared the Young’s modulus for different water contents: 퐸3% > 퐸푝푢푟푒 푒푝표푥푦 > 퐸9%. They attributed the higher modulus of epoxy with 3% water than pure epoxy to anti-plasticization effect. However, the mechanism of the anti-plasticization is not detailed. It should also be noted that random selection of two water contents did not provide a detailed profile of water absorption

effect. In addition, since the epoxies usually saturate within 5% water content under common conditions, more MD investigations should be detailed within this range.

2.2 UV Aging

Exposure to sunlight can lead to modification of epoxy at the molecular level, which results in changes in the macroscopic properties. Among the solar spectrum, the UV portion plays a major role in degrading the mechanical properties of the polymer [50]. The UV spectrum in solar radiation is classified into three categories: UVA, UVB, and UVC. UVA has a wavelength ranging from 400 nm to 315 nm, UVB is the portion from 315 nm to 280 nm, and UVC is from 280 nm to 100 nm. Due to the absorption of earth atmosphere and ozone layer, all the component of UVC and most of the UVB are filtered from reaching earth [51]. Even though, the UVA rays reaching earth are still critical on photo-degradation of epoxy polymers since UVA has shorter wavelength compared with the visible light and infrared. According to the Planck-Einstein relation, the photon energy is inversely proportional to the wavelength, and it is that high quantum energy that can result in electron transition between the different energy levels in the molecule [50]. Since the transition between the different energy levels is dependent on the molecular structure, different polymers have different sensitivity to UV wavelengths. Table 3.1 lists the UV wavelength at which various polymers have maximum sensitivity [52].

Table 2.1 UV Wavelength at which various polymers have maximum sensitivity [52]

Polymer Wavelength (nm) Energy (kcal/mol) Styrene-acrylonitrile copolymer 290, 325 99, 88 Polycarbonate 195, 345 97, 83 Polyethylene 300 96 Polystyrene 318 90 Poly(vinyl chloride) 320 89 Polyester 325 88 Vinyl chloride-vinyl acetate copolymer 327, 364 87, 79 Polypropylene 370 77

The detrimental effect of UV is more prominent with the presence of oxygen, such as in earth’s atmospheric environment, which causes photo-oxidation in polymers. The general photo- oxidative degradation mechanism of polymer in Rabek’s book includes the following steps [53]:

hv Initiation PH → P ∙ (or P ∙ +P ∙) + (H ∙)

Chain propagation P ∙ +O2 → POO ∙

POO ∙ +PH → POOH + P ∙

Chain branching POOH → PO ∙ + ∙ OH

PH +∙ OH → P ∙ +H2O

PO ∙ → Chain scission processes

Termination P ∙ +P ∙→ PP

P ∙ +PO ∙→ POP Crosslinking reactions P ∙ +POO ∙→ POOP to inactive products POO ∙ +POO ∙→ POO − OOP

(or POOP + O2) where PH is the polymer, ‘P ∙’ is the polymer radical, ‘H ∙’ is the hydrogen radical, 푂2 is oxygen molecule, ‘PO ∙’ is the polymer oxy radical, ‘POO ∙’ is the polymer peroxy radical, POOH is the polymer hydroperoxide, and ‘HO ∙’ is hydroxyl radical. In the initiation stage, UV radiation leads to polymer absorption of high-energy photons, which gives rise to photolysis. The initial photolysis produces polymer radicals (P ∙). During the propagation stage, with the participation of oxygen, polymer radicals keep consuming polymer molecules (PH) and producing polymer hydroperoxide (POOH). In the chain branching stage, polymer hydroperoxide (POOH) is UV unstable, and it decomposes into polymer oxy radical (PO ∙) and hydroxyl radical (HO ∙). The polymer oxy radical (PO ∙) is the main source for chain scission. It is generally accepted that the formation of carbonyl groups (C = O) during the decomposition of polymer oxy radicals is one of the main chain scission reactions [54-56]. An example of the chain scission reaction is as follows [53]:

The termination stage is the process of radical recombination into inactive molecules. These reactions increase the molecular length and chains crosslinking.

According to Rabek [53], this free radical mechanism applies to almost all types of polymers. In addition to the direct photolysis of polymer molecule to initiate the degradation, internal and/or external impurities can also produce radicals under UV exposure. These radicals can further react with the polymer molecules to initiate a series of the aforementioned reactions. Some of the internal impurities include unsaturated bonds, catalyst residues, and chromophores. The external impurities include, but are not limited to, additives, solvents, and pollutants.

Based on the above mechanism, the effect of UV on polymer’s mechanical properties is a competence between chain scission and crosslinking reactions. Even though the detailed chemical reactions during UV radiation vary from different epoxy systems, Chain scission caused by photolysis and photo-oxidation leads to shorter molecular chains and lower molecular weight, which gives rise to properties degradation [54, 55, 57-60]. Chain scission jeopardizes the physical and mechanical properties of the epoxy, such as discoloration, micro-cracks, and reduction in strength [16, 61]. The crosslinking reactions have the opposite effect that increases the epoxy properties. In addition to photo-oxidation, UV also has curing effect on epoxies, which could increase the polymerization and crosslink degree [59, 62, 63]. The more crosslinked structure results in enhanced mechanical properties, such as Young’s modulus and tensile strength [15], but also embrittles the polymer. The curing effect by UV was typically characterized by the decrease of epoxide intensity in FTIR spectra [16, 59].

When polymers are exposed to UV radiation, different depth of the material receives different UV intensity due to the attenuation during UV penetration and oxygen diffusion. Therefore, the material properties change is not homogeneous in the bulk sample [64, 65]. The surface layers typically experience the most severe degradation. In order to study the heterogeneity of epoxy

after UV radiation, many researchers [15, 64-68] have attempted to obtain the stiffness and hardness depth profile using nanoindentation. Mailhot et al. [65] examined the chain scission and crosslinking effects of UV radiation by studying epoxy and tetramethyl bisphenol-A polycarbonate (TMPC). They conducted nanoindentation from the surface to the core by gradually cutting thin top layers using microtome. In their results, the stiffness of the epoxy within the top 60 μm is lower than that of the middle region after 30 h UV aging, which was explained by the chain scission mechanism. However, TMPC had a higher surface stiffness than the core region after UV radiation, which was attributed the crosslinking reactions. A higher surface modulus than the core was also obtained by Woo et al. [15]. Nanoindentation results [15, 65, 67] on modulus depth profile show that the thickness of the oxidation layer generally ranges from 20 휇푚 to 250 휇푚 due to different aging conditions.

2.3 Thermal Aging

Thermal aging is another important category of polymer degradation caused by elevated temperature. During the manufacturing, fabrication, and storage processes, polymers experience a series of thermal treatments, which causes various properties degradation [69]. A general mechanism of thermal aging has been reported by Le Huy et al. [20, 70]. When the ambient temperature is lower than 푇푔, polymers are in a hard solid state in which the polymeric structural changes are extremely slow. Between 푇푔 and around 200℃, thermal oxidation plays a dominant role during the aging process, if under oxygen-rich environment. The thermal oxidation reactions are very similar to the UV-induced photo-oxidations, which have been described earlier [53]. The main difference between the two processes lies in the initiation stage. In thermal aging, high temperature initiates the chemical bond dissociation. Meanwhile, in photo-degradation, UV absorption gives rise to bond dissociation [53]. For both of the aging conditions, the dissociation of external impurities, additives are also responsible for the initiation. At temperatures higher than 200℃, pure thermolytic processes can happen [20], which breaks the covalent bonds of backbone chains and causes huge mass loss. It can even char the polymer.

One of the most prevalent techniques in the study of thermal oxidation chemistry applies FTIR to provide insights on the detailed reactions. Through shining beams consisting of different

combinations of frequencies, FTIR is able to obtain the absorbance profile at different wavenumbers for the material of interest. In polymers, the vibration of different bond is assigned to a certain wavenumber. By analyzing the absorbance change at different wavenumbers before and after aging, people can infer the chemical bond changes within the material. Based on the chemical bond changes, many possible reaction schemes have been proposed [54, 71-73]. Many investigations have suggested that thermal oxidation of epoxy can cause post-curing reactions [72, 74] and chain-scission [75, 76].

Physical and mechanical properties of epoxies can be modified due to these chemical changes. An increase of Young’s modulus, tensile strength, and glass transition temperature has been reported due to thermal treatment [76-79]. One reason for the property enhancement is caused by the post-curing reactions. It is common that epoxy systems have not reached fully cured state before aging, which means there are still residual epoxide groups and primary or secondary amines groups [80]. Under elevated temperature, the mobility of the polymeric chains increases, which gives rise to more thorough reactions of reactive groups. Therefore, a more crosslinked polymer network is reached. More crosslinked polymeric structure presents higher stiffness, higher strength, and higher glass transition temperature [74].

In addition to the properties enhancement effect due to post-curing, elevated temperature can also lead to the reduction of mechanical properties. Barral et al. [17] aged their epoxy system in forced air convection oven set at 156℃, and their results show that both tensile strength and Izod impact strength decrease with increasing thermal aging time. A decrease of tensile strength was also obtained by Unnikrishnan and Thachil [79]. Such property degradation can be explained by chain-scission mechanism, which is partly proved by measurable mass loss during thermal aging. Another fact observed by many studies is the change from ductile to brittle failure after thermal aging [17, 20, 79]. Both post-curing reactions and thermo-oxidative crosslinking lead to epoxy embrittlement. It should also be noted that the degradation behaviors vary significantly for different types of polymers and different aging temperatures.

Experimental and Molecular Dynamics Investigations

Summary: In this chapter, a detailed account of the experimental work conducted is outlined and discussed as well as the techniques used for measuring the mechanical properties degradation. The thermoset polymer studied in this research is a two-component commercially available epoxy system having the product name of West System 105 and 206. In addition, limited MD work was conducted to explain the effect of water absorption on elastic properties.

3.1 Material Details and Sample Preparation

The thermoset polymer studied in this research is a two-component commercially available epoxy system having the product name of West System 105 and 206. Table 3.1 and Table 3.2 summarize the chemical compositions of the resin and the curing agent, respectively, as per supplier material data sheet [81, 82]. The main concentration of the resin is Propane, 2,2-bis[p- (2,3-epoxypropoxy)phenyl]-, polymers, also known as DGEBA homopolymer. The hardener is an amine-based curing agent, where Polyoxypropylenediamine has the highest concentration within the compound. The molecular structures of DGEBA and Polyoxypropylenediamine are illustrated in Fig. 3.1 (a) and (b) [40].

Table 3.1 Chemical composition and concentration in the epoxy resin [81]

Ingredient Name Concentration (%) Propane, 2,2-bis[p-(2,3- 60-100 epoxypropoxy)phenyl]-, polymers Benzyl alcohol 10-30 Phenol-formaldehyde polymer glycidyl 1-10 ether

Table 3.2 Chemical composition and concentration in the curing agent [82]

Ingredient Name Concentration (%) Polyoxypropylenediamine 30-50 Polymer of epichlorohydrin, bisphenol- 10-30 A, and diethylenetriamine Tetraethylenepentamine 10-30 Diethylenetriamine 5-20 Reaction products of triethylenetetramine and propylene 5-20 oxideTriethylenetetramine Triethylenetetramine 1-10

Fig. 3.1 The molecular structure of (a) DGEBA homopolymer, and (b) Polyoxypropylenediamine [83]

Due to the extensive number of tests necessary to investigate the aging effect on the epoxy considered, a large number of specimens are required. The designed reduced dog-bone shaped tensile test specimen (based on ASTM: D638-10 standard) with minor modifications to the gauge length, to allow for strain measurement using an attachable extensometer, was used. The engineering drawing of the developed specimen is shown in Fig. 3.2 (a). A weight ratio of 5.36:1 resin to curing agent was used to obtain the desired properties. The resin-hardener mixture was degassed in a vacuum of 76 cm Hg for 10 minutes to facilitate the removal of air bubbles. Afterwards, the mixture was injected into the PTFE molds (Fig. 3.2 (b)) coated with mold release

agent. After 4 days room temperature curing, the specimens were removed from the molds. Then, the cured samples were visually inspected to ensure consistent quality between specimens. Afterwards, the specimens were placed in the respective aging condition. Details concerning the aging conditions are provided in the following subsections.

Fig. 3.2 (a) Engineering drawing of the designed specimen, all units are in millimeter, and (b) Commission mold for casting specimen

3.2 Aging Methodology

3.2.1 Water Immersion

The immersion of the specimens was conducted in a glass container filled with distilled water. The water temperature was stabilized at 24±1°C under room conditions. To allow water to envelop all the specimens’ surfaces, the specimens were elevated 1 mm from the container bottom by placing specimen’s grip regions on glass spacers. The glass container was sealed to reduce contamination and evaporation of water. In addition, care was taken during immersion and extraction of the specimens to avoid water contamination. The specimens were aged in distilled water for up to 270 days, during which various properties was examined periodically. After the specimens were removed from the water, they were firstly surface dried for weight recording and then for mechanical testing.

3.2.2 UV Radiation

To replicate the UV spectrum of sunlight within a laboratory setting, an artificial 40-watt UVA- 340nm lamp was used as the UV source. This lamp is typically used for this application [59] and has its peak spectrum at wavelength of 340 nm. The specimens were placed in two chambers (Fig. 3.3) equipped with the aforementioned lamp. Two chambers were constructed so that UV irradiance could be varied to examine the effect of the intensity on mechanical aging. The chambers are constructed of half-inch plywood. The inside of the chambers is finished with a thin layer of aluminum to increase reflection. For the first chamber, the specimens were placed at a distance of 275 mm away from the lamp, which resulted in UV irradiance of about 6 W/m2, as detected by a UVA-340 light meter (Lutron Electronic Enterprise Co., Ltd). Such irradiance level is comparable with the daily average solar UV irradiance in the Sponsor-designated region. In order to accelerate the aging, the specimens were located 70 mm away from the UV source in the second chamber, which produced an elevated UV irradiance of about 18 W/m2. The temperature in both UV chambers was monitored and stabilized at 30 ± 3℃. All the specimens were located in close proximity to keep the UV irradiance as similar as possible for all the specimens. The side surface of the sample directly facing the UV lamp was also labelled. The specimens were aged up to 270 days before taken out for properties characterization.

Fig. 3.3 Ultraviolet chambers used: (a) UV lamp with irradiance of 6 W/m2 , and (b) UV lamp with irradiance of 18 W/m2

3.2.3 Thermal Aging

Thermal aging was conducted in the thermal chamber depicted in Fig. 3.4. The heat source for the chamber is a 100-watt infrared heating lamp, which was selected for its high efficiency of heat generation. Through preliminary testing of the heating chamber, it was shown that one lamp could attain and maintain a temperature of 90°C. As indicated by the sponsor, the operating and storage temperatures of the epoxy can reach 70℃. To maintain the chamber temperature at 70 ± 1℃ , a STC-1000 microcomputer temperature controller was used. The chamber walls are constructed with half-inch plywood due to its ease of manufacturing and modification, and its heat insulation properties. To increase the heat insulation property of the chamber, the inside of the chamber was covered with half-inch Styrofoam sheeting. Finally, aluminum foil is adhered to the Styrofoam to reflect the radiant heat being produced, thereby minimizing the heat conduction through the walls. The specimens were placed in the thermal chamber for a period of 300 days, during which they were taken out periodically for properties characterization. Similar to UV aging, the specimens were also placed together in close proximity to ensure temperature consistency to the greatest extent.

Fig. 3.4 Thermal chamber used for thermal aging at 70°C

3.3 Characterization of Mechanical Properties

3.3.1 Tensile Testing

Tensile tests were conducted using an Instron 8522 mechanical tester with an attachable extensometer to record the strain. Tensile properties, including Young’s modulus, tensile strength, fracture strain, and tensile toughness, were evaluated before and after aging. The testing was performed using displacement control at a rate of 1 mm/min. All of the tensile tests were conducted at room temperature, and all the results are an average of five tests.

3.3.2 Fracture Surface Examination

Post-mortem examinations of the morphological and topological features of the fracture surfaces were conducted to provide insights of the influence of aging on the epoxy. Quanta FEG 250 environmental SEM was used to examine the overall fracture surface morphology. SEM micrographs can show the fracture initiation site, the crack formation and the propagation, as well as various aging features. The SEM was operated under low vacuum mode at 10 kV. In order to gain a better understanding of the fracture surface, such as topological features and surface roughness, and to provide a correlation with SEM images, a Bruker Multimode 8 AFM [84] was used to scan the fracture surface. Scanning was done in PeakForce tapping mode, with a silicon nitride probe having a normal spring constant of 0.4 N/m, and normal resonant frequency of 70 kHz [85]. The topology was measured over an area 60×60 μm near the middle of the fracture surface with a resolution of 512 samples per line. The scan rate was set at 0.1 Hz, and all the images were processed in NanoScope Analysis software.

3.4 Molecular Dynamics Modelling of Water Absorption

In this section, details regarding the MD simulations conducted in this study are discussed. The molecular models of the epoxy system were constructed in Materials Studio software. Meanwhile, all the MD simulations were conducted using LAMMPS with consistent valence forcefield (CVFF) which is commonly used to investigate the mechanical properties of polymers [86-88]. Non-bonded interactions were calculated based on van der Waals (vdW) forces with a

cut-off distance of 12.5 Å. All the minimization procedures were conducted using conjugate gradient algorithm. Periodic boundary conditions were imposed on all the directions for all the MD simulations.

3.4.1 Pure Epoxy Model

Due to the proprietary nature and composition complexity of the experimentally investigated epoxy, the material composition in the MD simulations was simplified. To construct the crosslinked polymeric structure, only the highest concentration in the epoxy resin and amine hardener, i.e., DGEBA and Polyoxypropylenediamine, were used. The curing reaction between epoxide and amine groups was investigated and reported by other researchers [47, 89]. In this simulation, all the active hydrogen atoms were reacted. Each hydrogen atom in the amine group of the hardener reacted with one epoxide group of the resin. There are four active hydrogens in the Polyoxypropylenediamine molecule; therefore, four DGEBA molecules can react and bond with it. Fig. 3.5 illustrates the reaction between the epoxide groups and active hydrogens in the amine groups. In the figure, only the epoxide groups of the resin and amine groups of the curing agent are shown for clarification, the rest of the polymeric chain is denoted by R.

Fig. 3.5 Reaction between epoxide groups and amine groups [89]

Based on the reaction depicted in Fig. 3.5, epoxy resin and hardener molecules were firstly created (Fig. 3.6 (a)), and then we manually constructed the resultant of the reaction between four resin molecules and one hardener molecule. The constructed structure is set as a representative epoxy unit, illustrated in Fig. 3.6 (b). Then, the entire polymeric network was

generated by loosely packing 50 representative epoxy units in the simulation box. The simulation box was then gradually compressed in incremental steps in LAMMPS to a final size of 53 Å × 53 Å × 53 Å. The compression brings the epoxy density close to the desired density of about 1 g/cm3 . Such polymer creation method in MD has previously been applied by many researchers, including Yu et al. [90] and Alian et al. [87]. After each compression, minimization was carried out followed by 100 ps canonical (NVT) relaxation at 298 K. Fig. 3.6 (c) and 3.7 (d) present the initial packing configuration and the final structure. After reaching the desired density, the whole structure was allowed to equilibrate for 4 ns under isothermal-isobaric (NPT) ensemble at 298K and at 1 atm. The final NPT relaxation allows the structure to expand or contract to avoid unrealistic internal stresses. A time step of 1 fs was applied throughout the MD simulation. A summary of the modelling steps is presented in the flowchart in Fig. 3.7.

Fig. 3.7 Epoxy modelling procedure to achieve the desired density

3.4.2 Epoxy-Water Model

The procedure for modelling the epoxy-water system is similar to that of the pure epoxy described above. However, before reducing the simulation box size, water molecules were randomly added into the initial loosely packed epoxy structure as per the earlier work of Clancy et al. [48], and Pandiyan et al. [49]. This structure was gradually compressed to the same size as that of the pure epoxy; a cube with all dimensions being 53 Å. Similar to the pure epoxy modelling, the compressed structure was then subjected 4 ns of NPT relaxation at 298K and 1 atm to allow for volume adjustment. The pure epoxy network together with epoxy containing 2% and 4% water contents is presented in Fig. 3.8 as representatives. The hydrogen and oxygen atoms of the water molecules (represented by pink spheres) are exaggerated to highlight the location within the polymeric structure. Table 3.3 lists the total number of atoms and water molecules in each of the epoxy-water systems.

Fig. 3.8 MD model of the epoxy with different water contents

Table 3.3 Number of atoms and water molecules in the epoxy-water systems

H2O weight percentage (%) Total number of atoms Number of water molecules 0.0 11050 0 1.0 11176 42 2.0 11305 85 3.0 11434 128 4.0 11569 173 5.0 11704 218

3.4.3 Water Diffusion Coefficient and Elastic Properties

During the water absorption process, the polymeric network and the absorbed water interact. With additional water molecules permeating inside the polymeric structure, the volumetric swelling results in the rearrangement of the molecular chains. The adjustment of polymer chains also influences the water diffusion behavior. Examining the water diffusion coefficient can partly reflect the changes in polymer’s internal structure. In the current MD simulation, the water diffusion coefficient is calculated using the following Einstein relation [39]:

1 d 2 DRRlim(t)(0) 6 t dt where R is the location of the center of mass of an atom, t is the time, and the part in the angle bracket denotes MSD for a group of atoms.

There are several different methods to determine the elastic properties of polymers in MD, of which the dynamic method and static method are most commonly used. The dynamic approach allows us to obtain the stress-strain curve of the system in a continuous fashion, which is analogous to experimental testing [49]. However, the strain rate typically used for the dynamic method ranges between 107/s and 1010/s [45, 49, 91], which is much higher than the experimental values. In addition, the resulting stress-strain curve fluctuates significantly because of the instantaneous response of the dynamic method. Another common approach is the static method, which (i) deforms the simulation box to a certain strain, (ii) minimizes the system energy, (iii) calculates the average stress tensor of the system, and (iv) further deforms the system and repeats the procedure. The stress-strain response in the static method is usually more stable. The average stress tensor in MD is calculated based on the definition of the virial stress.

In this research, the static method was used to determine the elastic coefficient matrix. The simulation box was loaded up to 0.5% strain at an increment of 0.1% strain in each of the normal and shear directions. While loading in one direction, all the remaining directions were fixed. After each straining, the system was minimized and the stress tensor of the system was calculated. The elastic coefficient matrix can be calculated by:

∂σij Cijkl = ∂εkl Due to the symmetry of stress tensor and strain tensor, as well as the continuity of strain energy, the elastic coefficient matrix 퐶푖푗푘푙 can be written in a second order form 퐶훼훽. If the material is homogeneous and isotropic, 퐶훼훽 can be determined by two Lamé constants 휆 and 휇, and it is in the form of:

 2000    2000   2000  00000  00000   00000 

Then the elastic properties can be calculated from the two constants: 휇(3휆+2휇) 휆 2 퐸 = , 휈 = , 퐺 = 휇 , 퐾 = 휆 + 휇 휆+휇 2(휆+휇) 3 where 퐸,휈,퐺, 퐾 are the elastic modulus, Poisson’s ratio, shear modulus, and bulk modulus, respectively. In this thesis, 휆 and 휇 are calculated using the least square fitting method [92] given by the following equations: 4a − 2b + 3c μ = 33 2a + c − 15μ λ = 6

a = C11 + C22 + C33

b = C12 + C13 + C23 + C21 + C31 + C32

c = C44 + C55 + C66

Results and Discussions

Summary: In this chapter, the results of water, UV, and thermal aging are summarized and discussed. The degradation of the tensile properties, and fracture surface characterization after different aging conditions are presented and discussed. In addition, the MD simulation results of water absorption effect on the epoxy’s elastic properties are provided and compared with experimental results and earlier contributions.

4.1 Water Aging Results

4.1.1 Tensile Properties due to Water Aging

Through tensile testing, the degradation of the epoxy’s mechanical properties due to water absorption has been determined. Fig. 4.1 depicts the weight gain percentage of the specimen with respect to the square root of time. The weight gain percentage increases linearly with the square root of time during the first 1600 hours of water absorption. This agrees with a typical Fickian diffusion behavior [93]. All the error bars in the results represent standard error.

Fig. 4.1 Weight gain percentage with square root of water immersion time

After obtaining the weight gain profile with immersion time, all the tensile properties are presented in terms of water content and normalized with respect to the initial unaged properties. It was found that Young’s modulus has a decreasing trend with water absorption when water content is less than 1.6% as shown in Fig. 4.2. At this point, the elastic modulus of the epoxy has decreased by 25% from the initial value. Between 1.6% and 2.8% water absorption content, the elastic modulus recovers some of the experienced losses. Afterwards, the modulus decreases once again.

Fig. 4.2 Evolution of normalized elastic modulus with water weight content

This behavior has been previously reported by Nogueira et al. [33] and Papanicolaou et al. [37]. The general decreasing trend of Young’s modulus is caused by the plasticization effect of the absorbed water. Water absorption leads to epoxy volume swelling, increasing the mobility of the epoxy chain. Epoxy with more mobile structural network shows lower stiffness. However, at certain water contents, in our case between 1.6% and 2.8%, there is a partial recovery of the modulus. Such anti-plasticization effect can be explained by type II bound water proposed by Zhou and Lucas [31]. Type II bound water molecules form two hydrogen bonds with the epoxy chains. Compared with the mobile water (no hydrogen bond) or Type I bound water (forming one hydrogen bond), Type II bound water is more stable and requires higher activation energy to

be removed from the epoxy networks. During the water absorption process, more Type II water binds with the polymeric chains, thus increasing the crosslinking and rigidity of the epoxy. Therefore, when the anti-plasticization effect of Type II bound water is predominant over the plasticization effect, the stiffness can be partly recovered. After all the available sites for hydrogen bonding are occupied, further water absorption will continue to mobilize the epoxy network and decrease the modulus.

The tensile strength in Fig. 4.3 has a generally decreasing trend, experiencing a 14% loss after 270 days of water immersion. The decreasing strength can be caused by two mechanisms. Firstly, water-induced plasticization increases the chain mobility and lowers the strength [33]. Secondly, absorption and accumulation of water molecules in the polymeric network lead to formation of microvoids. This effect is evidenced by the SEM examination of the fracture surfaces and will be discussed in the following section. Since these two factors contribute to the loss of tensile strength, the anti-plasticization effect does not seem to play a significant role in the recovery of the tensile strength of the polymer.

Fig. 4.3 Evolution of normalized tensile strength with water weight content

Fig. 4.4 shows that the fracture strain and tensile toughness share a very similar trend. The fracture strain and tensile toughness underwent a 49% loss and 54% loss of their initial values

after 270 days of aging in distilled water, respectively. This indicates that embrittlement of the epoxy has occurred during water immersion. The decrease of fracture strain and tensile toughness can be caused by the formation of micropores due to water ingress [94]. Water molecules permeate and aggregate to form microvoids, and reside in them. The existence of mobile water in polymer microvoids has been confirmed by spectroscopic analyses [30]. The permeation of mobile water in microvoids increases the formation of crazes and cracks, leading to reduced elongation. This effect is confirmed and will be discussed in the following section. The loss of tensile strength and ductility can also be observed from the typical engineering stress-strain curves before and after aging in water, as shown in Fig. 4.5. A summary of all the mean values of the properties that were measured are provided in Table 4.1.

The scatter of the results may originate from the micro-defects within specimens and variation of different aging conditions. The micro-defects include but are not limited to geometrical defects (uneven surface, edge defects) and impurities. In spite of the fact that concentrated efforts were devoted to casting defect-free specimens, many micro defects developed and existed in the samples. In terms of aging conditions, although the specimens were placed in close proximity, they still may not receive the same aging treatments. Both factors contributed to the scatter of the results. Unfortunately, the large scatter impedes the accuracy of the results and leads to large deviations in the same tests.

Fig. 4.4 Evolution of normalized fracture strain and tensile toughness with water weight content

Fig. 4.5 Typical engineering stress-strain curves before and after water aging

Table 4.1 Tensile properties and weight gain percentage before and after various aging times in distilled water

Young’s Tensile Fracture Tensile Weight Immersion Time Modulus Strength Strain Toughness Gain (Day) (GPa) (MPa) (%) (MJ/m3) (%)

0 3.2 69.2 11.1 5.5 0

8 2.9 66.0 7.6 3.7 0.97

30 2.4 65.8 9.1 4.4 1.67

60 2.6 66.4 6.7 3.3 2.31

120 2.8 63.9 6.1 2.8 2.80

180 2.9 59.5 5.6 2.5 2.95

270 2.8 59.4 5.7 2.5 3.13

4.1.2 Fracture Surface Morphology due to Water Aging

Examination of the fracture surface after various aging durations can provide insights of the possible degradation mechanisms occurring in the epoxy. SEM fracture surface micrographs for a representative unaged specimen are shown in Fig. 4.6. The figure depicts that the fracture surface of an unaged specimen is smooth except for one long crack spanning the surface. The pristine surface reveals that the samples are free of many large defects.

Representative fracture surfaces for specimens after 30 and 180 days of aging are presented in Fig. 4.7 and Fig. 4.8, respectively. It is apparent from the fracture surfaces that there are crack lines and aggregates of micropores for both aging times. However, the fracture features do vary for the two aging times. After 30 days of water immersion, the fracture surface in Fig. 4.7 (b) shows some straight and light crack formations. After 180 days of water immersion, the crack formation becomes significantly more pronounced as shown in Fig. 4.8 (c). Another noticeable feature is the local aggregation of micropores; e.g., see Fig. 4.8 (b). For the unaged specimen, this feature is less obvious. In addition, after 30 days of immersion, more micropores are located in the regions closer to the outer surfaces of the specimen as shown in Fig. 4.7. In contrast, for 180 days case presented in Fig. 4.8, such features are discovered across the entire section. The appearance of more crack lines, aggregates of micropores, and crack branches are evidence of microvoids formation, which contributes to tensile strength loss.

Fig. 4.6 SEM micrographs of fracture surfaces for unaged specimen at magnification of (a) 60x,

and (b) 1776x

Fig. 4.7 SEM micrographs of fracture surface for specimen after 30 days of aging in distilled water at magnification of (a) 59x, and (b) 1776x

Fig. 4.8 SEM micrographs of fracture surface for specimen after 180 days of aging in distilled water at magnification of (a) 60x, (b) 1776x in the middle, and (c) 1776x at bottom right

4.1.3 Fracture Surface Topology due to Water Aging

To provide further details regarding the fracture surface, AFM was used to examine the topological features of the surface as a complement to the morphology. The height images of the fracture surfaces for both unaged and aged specimens are shown in Fig. 4.9 (a)-(c). The height profiles in Fig. 4.9 (d)-(f) are plotted along the diagonals of the height images. All of the images were taken near the middle of the fracture surface. It can be observed that the unaged specimen has a maximum peak to trough variation of about 250 nm along the diagonal. The specimens after 30 and 180 days of water absorption have height differences of 1.2 µm and 1.5 µm along the same scan distance. Therefore, it can be concluded that the surface roughness of fracture surface increases after water absorption. In addition, the unaged specimen shows relatively flat topography with gradual changes in the fracture surface. However, step or stacked lamellar topological features are presented after aging in water in Fig. 4.9 (e) and Fig. 4.9 (f), respectively. Contrasting the AFM images with the SEM images, we can conclude that the white crack lines in the SEM images (Fig. 4.7 (b) and Fig. 4.8 (c)) are the edges of the steps or stacked lamellar features in the AFM images.

4.1.4 MD Results

Table 4.2 summarizes the MD results for the physical and the elastic properties of the epoxy containing different water contents. From Table 4.2, it can be observed that the volume of the system increases with increasing water content, and the density has a decreasing trend. It is known that the mechanical properties of an epoxy are very sensitive to its density [39]. Our results show a decreasing trend of the mechanical properties with decreasing density within the current scope of the investigation. The decreasing density is caused by the water-induced swelling. Such plasticization effect further separates epoxy molecular chains and increases the network mobility, leading to property degradation. The decreasing trend of moduli agrees with the earlier published work in the literature [13, 14, 33]. However, the density does not decrease monotonically. The epoxy density with 4% water content is higher than that for the case of 3% water content, and the mechanical properties for 4% water content are higher than that of 3% case, respectively. This agrees with Type II bound water theory that two hydrogen bonding state results in secondary crosslinking and a denser structure. This could possibly explain the partial recovery of the mechanical properties.

Table 4.2 Summary of physical and mechanical properties of epoxy for different water absorption contents

Elastic Shear Bulk Poisson’s Diffusion Water Volume Density content Modulus Modulus Modulus Ratio Coefficient (%) (Å3) (g/cm3) (GPa) (GPa) (GPa) (× 10−5 cm2/s)

0.0 114895.64 1.079 4.94 1.88 4.49 0.32 --

1.0 119926.65 1.045 4.50 1.71 4.14 0.32 0.68

2.0 123735.58 1.023 4.44 1.72 3.53 0.29 7.53

3.0 132779.67 0.963 3.67 1.41 3.06 0.30 22.95

4.0 133265.23 0.970 4.22 1.66 3.08 0.27 9.44

5.0 135317.4 0.965 3.19 1.26 2.27 0.26 16.01

The water diffusion coefficient was determined by calculating the MSD of water molecules. Fig. 4.10 presents the MSD evolution for five different water contents. The water diffusion coefficient was calculated based on the steady diffusion region [40] from 500 ps to 1500 ps, and the results are shown in Table 4.2. In correlating the density with the diffusion coefficient, it can be noticed that the diffusion coefficient increases with a decreasing density, indicating that higher density impedes the mobility of both the epoxy molecular chains and the water molecules.

Fig. 4.10 MSD evolutions during 2 ns MD simulation for five different water contents

A comparison between MD results and experimental results for normalized Young’s modulus is shown in Fig. 4.11. The experimental results are currently limited to about 3% of water content, and longer aging data are still required. Experimental results show a decreasing trend of Young’s modulus within 1.6% water absorption, then an increasing trend from 1.6% to 2.8% water content followed by further decrease. In MD simulation, the decreasing trend of Young’s modulus continues to 3.0%, and the increase of Young’s modulus happens from 3.0% to 4.0%. From 4.0% to 5.0% water absorption, Young’s modulus continues to drop. Comparing both experimental and MD results, it further confirms that water absorption has both plasticization and anti-plasticization effects. Though there are some discrepancies, the experimental results and MD results have similar trends.

The discrepancies between MD and experimental results may be associated with the simplifications employed in the MD model. First, the epoxy network was simplified. There are many other chemical compounds in the experimental material, while only the components with the largest concentration were considered. Second, the crosslinked structure was manually constructed, which may deviate from the real curing situation. Therefore, the mechanical properties of the MD epoxy system are different from experimental properties. However, since we are more interested in the water-induced properties degradation, the property change percentage due to water absorption is more critical. Third, the force field used may not be able to simulate the real atomistic interactions. In addition, the water molecules were randomly added into the epoxy system, which deviates the real water distribution. Even though, the MD simulation can still reflect the plasticization and anti-plasticization effects of water on the epoxy, which agrees with experimental results.

Fig. 4.11 Experimental and MD results of normalized Young’s modulus change with water content

4.2 UV Aging Results

4.2.1 Tensile Properties due to UV Aging

The evolution of normalized elastic modulus with UV aging time is shown in Fig. 4.12 for both 6 W/m2 and 18 W/m2 irradiances. The elastic modulus generally increases with UV radiation for

both irradiances and reaches about 15% higher than the initial unaged value after 270 days. From the current results, there is no significant difference between the two UV irradiances on the elastic modulus change. The increase in elastic modulus is an indication of the epoxy being stiffer due to the crosslinking reactions in the specimens. Since the specimens were cured at room conditions, many epoxide groups and amine groups would remain unreacted. UV has curing effect on the epoxy. Absorption of UV spectrum provides energy for further reactions of the reactive groups, which gives rise to higher crosslink density [59, 62]. Epoxy with higher crosslink density possesses higher stiffness and strength [15].

Fig. 4.12 Evolution of normalized elastic modulus with UV aging time

The normalized tensile strength change with UV aging time is provided in Fig. 4.13. After 60 days of UV exposure, the tensile strength experienced 25% and 20% increase at the irradiance of 6 푊/푚2 and 18 푊/푚2, respectively. The initial increase of strength is still caused by the post- curing effect. Then the tensile strength drops quickly to only about 60% of its unaged value. The reduction of the strength is caused by photo-oxidation reactions. UV radiation initiates free radicals formation and results in a series of chain-scission and crosslinking reactions. This reaction process produces a thin brittle oxidation layer and leads to crazes and cracks formation [56], which can greatly reduce the strength and ductility. SEM images in the following subsection provide insights on the UV embrittlement effect. The higher UV irradiance of 18

푊/푚2 generally has a larger effect on the degradation of tensile strength than that of the 6 푊/푚2, although the difference is not very significant.

Fig. 4.13 Evolution of normalized tensile strength with UV aging time

Fig. 4.14 shows the normalized fracture strain and tensile toughness for both irradiances. The fracture strain and tensile toughness decrease with increasing UV aging time. After 270 days of aging, the fracture strain experienced about 90% loss for both irradiances. The tensile toughness and the fracture strain follow a similar degradation trend and overlap well with each other. It is also hard to differentiate the influence of the two UV irradiances in terms of the fracture strain and the tensile toughness. Fig. 4.15 presents typical engineering stress-strain curves after different UV aging conditions. It can be observed from the stress-strain curves that 270 days of UV aging greatly reduced the elongation and strength. The mechanical properties of specimens after different UV aging times are summarized in Table 4.3 and Table 4.4.

Fig. 4.14 Evolution of normalized fracture strain and tensile toughness with UV aging time

Fig. 4.15 Typical engineering stress-strain curves before and after different UV aging conditions

Table 4.3 Tensile properties before and after UV aging under the irradiance of 6 W/m2

Young’s Tensile Fracture Tensile UV Aging Time Modulus Strength Strain Toughness (Day) (GPa) (MPa) (%) (MJ/m3) 0 3.2 69.2 11.1 5.5

60 3.4 86.7 6.3 4.1

120 3.7 76.9 3.3 1.6

151 3.7 59.6 1.9 0.6

210 3.7 49.9 1.5 0.4

270 3.7 43.6 1.3 0.3

Table 4.4 Tensile properties before and after UV aging under the irradiance of 18 W/m2

Young’s Tensile Fracture Tensile UV Aging Time modulus Strength Strain Toughness (Day) (GPa) (MPa) (%) (MJ/m3) 0 3.2 69.2 11.1 5.5

60 3.5 82.7 6.4 4.0

120 3.5 66.3 2.3 0.9

151 3.6 60.6 1.9 0.6

213 3.6 46.0 1.4 0.3

270 3.7 37.7 1.1 0.2

4.2.2 Fracture Surface Morphology due to UV Aging

Fig. 4.16 shows the representative SEM micrographs of fracture surface after 151 days of aging under UV irradiance of 18 W/m2. Compared with the unaged fracture surface in Fig. 4.6, there are more crack formations after UV radiation, which increased the surface roughness. From the low magnification image of the fracture surface in Fig. 4.16 (a), it can be noticed that the fracture initiates mainly from the bottom-left corner and propagates to top-right. A detailed image in Fig.

4.16 (b) shows the deep cracks and branches in the fracture surface. Fig. 4.17 shows the fracture surface of specimen after 180 days of aging under the irradiance of 6 W/m2. Similar to Fig. 4.16 (a), the fracture initiation and propagation features are also clearly indicated in the fracture surface.

Another common phenomenon for the UV-aged samples is that there is a distinctive morphology between the area close to the surface and the core region. Both Fig. 4.16 (a) and Fig. 4.17 show that the area close to the surface presents a relatively smoother morphology, as indicated by the red bars in Fig. 4.17. Meanwhile, the core part displays coarser surface. Woo et al. [15] noticed a similar phenomenon of the fracture surface, and they verified that the stiffness of the outer smooth region is higher than the stiffness of the core region using nanoindentation. Compared with the unaged, water-aged, or thermally aged specimens (discussed in next section), the thin smooth strip region close to the side surface is a unique feature for the UV-aged specimens. The average length of the six red bars in Fig. 4.17 is about 150 휇푚, which falls in the range of a typical photo-oxidation layer (20 휇푚-250 휇푚) [15, 65, 67]. However, it is still difficult to state that the smooth region is an oxidation layer. Further chemical analyses are required to make a firm statement.

In addition, the fracture surface after UV aging is much rougher than the fracture surface of the unaged specimen. However, the AFM has a limited vertical measuring range of 5 휇푚. Therefore, it is not appropriate or necessary to conduct AFM scanning for surface roughness characterization of UV-aged specimens.

Fig. 4.16 SEM micrographs of fracture surface after 151 days of UV aging under UV irradiance of 18 W/m2 at magnification of (a) 60x, and (b) 1776x

Fig. 4.17 SEM micrograph of fracture surface after 180 days of UV aging under UV irradiance of 6 W/m2 at magnification of 60x

4.3 Thermal Aging Results

4.3.1 Tensile Properties due to Thermal Aging

The evolution of normalized Young’s modulus and tensile strength are both depicted in Fig. 4.18 due to shared aging trend. After 154 days of thermal aging at 70℃, the elastic modulus and the tensile strength experienced an enhancement of approximately 11% and 15% higher than the unaged values, respectively. During the following 146 days of aging, Young’s modulus and the tensile strength decrease with thermal aging time and are close to the initial properties after 300 days. In terms of the fracture strain and the tensile toughness, as shown in Fig. 4.19, the epoxy undergoes about 55% loss of elongation and toughness during the initial 210 days of thermal

aging, and followed by a full recovery of the ductility. The typical engineering stress-strain curves before and after 70°C thermal aging are provided in Fig. 4.20 as further illustration.

The above thermal behaviors can be attributed to the following mechanisms. The initial increase of stiffness and strength, as well as the reduction of ductility is the outcome of post-curing effect under elevated temperature. High temperature accelerates the motion of polymeric segments and leads to a more complete reaction between epoxy resin and curing agent, which gives rise to a more crosslinked structure. A more crosslinked polymeric network presents higher stiffness and strength, as well as shorter elongation. After the maximum achievable crosslink degree is reached, the effect of thermal oxidation starts to stand out. The reduction of stiffness, strength, and increase of ductility shows that further thermal aging softens the epoxy. Thermal oxidation can cause scission of side chains [17]. The removal of the side chains results in decrease of strength. The weight loss percentage shown in Fig. 4.21 indicates the elimination of small molecules or volatiles during thermal aging. Since there is less entanglement due to the loss of dangling chains, the ductility of the epoxy can increase. However, it should be stressed that thermal aging is a very complicated process, and different polymers and different temperatures can present drastically different aging behaviors. Table 4.5 summarizes the tensile properties before and after thermal aging.

Fig. 4.18 Evolution of normalized elastic modulus and tensile strength with thermal aging time

Fig. 4.19 Evolution of normalized fracture strain and tensile toughness with thermal aging time

Fig. 4.20 Typical engineering stress-strain curves before and after 70°C thermal aging

Fig. 4.21 Weight loss percentage during 70℃ thermal aging

Table 4.5 Tensile properties before and after 70℃ thermal aging Young’s Tensile Fracture Tensile Thermal Aging Time Modulus Strength Strain Toughness (Day) (GPa) (MPa) (%) (MJ/m3) 0 3.20 69.2 11.1 5.5

120 3.30 75.2 7.6 4.6

154 3.56 79.8 7.8 5.0

210 3.39 72.0 4.8 2.4

240 3.35 68.9 6.7 3.6

300 3.24 68.1 11.5 6.7

4.3.2 Fracture Surface Morphology due to Thermal Aging

The SEM micrographs of specimens after 154 days and 300 days of thermal aging at 70℃ are shown in Fig. 4.22 and Fig. 4.23. It can be observed in both figures that the fracture is initiated at a corner, and it propagates outside in a radial distribution. In addition, most of the fracture

surfaces after thermal aging highlight the presence of different morphological regions. Right from the initiation site is a mirror-like region surrounded by an arc and the surface edges. At this smooth region, the cracks are propagating in a stable manner. Beyond that region, the cracks are more prominent. Crack branches and parabolic marks are observed further away from the initiation site. More crack branches or secondary cracks indicate fast and unstable crack propagation during the fracture process [17]. The cracks start to branch out when reaching the maximum propagation speed in the material, since the energy has to be dissipated by more crack formations under these circumstances [95].

Compared with the pristine fracture surface in Fig. 4.6, the specimens after 154 or 300 days of thermal aging present a coarser fracture surface with more cracks. A more detailed picture in Fig. 4.22 (b) shows the typical stacked lamellar feature, as evidenced in the region enclosed in a red box. Similar to UV-aged specimen, the rough surface after thermal aging is also not appropriate for AFM scanning.

Fig. 4.22 SEM micrographs of fracture surface after 154 days of 70℃ thermal aging at magnification of (a) 61x, and (b) 1776x

Fig. 4.23 SEM micrographs of fracture surface after 300 days of 70℃ thermal aging at magnification of 59x

Conclusions and Future Work

Summary: In this chapter, we re-state the problem, summarize the research findings and outline some issues worthy of future research.

5.1 Statement of the Problem

Thermoset polymers are often subjected to various detrimental conditions during their service life. It is of great importance to know how the mechanical properties of them degrade when they are exposed to those conditions. In this thesis, we investigate the effect of water absorption, UV aging, and thermal aging on the mechanical properties of a sponsor-selected epoxy. Extensive tensile tests were conducted to determine Young’s modulus, tensile strength, fracture strain, and tensile toughness prior to and after different periods of aging. Fracture surface morphology and topology were examined using SEM and AFM to cast light on the aging mechanisms. Molecular dynamics simulation was also conducted to study the water absorption effect on the mechanical properties of the epoxy considered.

5.2 Conclusions

A summary of the major conclusions in terms of aging in water, UV aging, and thermal aging is provided below:

(i) Water absorption has plasticization effect, which reduces Young’s modulus, tensile strength. The plasticization effect is caused by water-induced swelling, which increases the mobility of polymeric chains. At certain water absorption content, the modulus can be partly recovered due to anti-plasticization effect. Anti-plasticization effect results from Type II bound water that forms two hydrogen bonds with epoxy chains, leading to an increase in polymer’s crosslinking degree.

(ii) The epoxy becomes more brittle after water absorption. Such embrittlement effect is evidenced by the reduction of the fracture strain with increased water content. SEM

micrographs show the formation of micropores in the fracture surfaces after water aging, which results in loss of ductility. The fracture surface after water aging shows step or stacked lamellar pattern, and the fracture surface roughness increases after aging in water.

(iii) UV aging is conducted in two chambers with different UV irradiances, i.e., 6 W/m2 and 18 W/m2. UV radiation has post-curing effect, which increases the stiffness and tensile strength of the epoxy. UV radiation also leads to embrittlement of the epoxy. SEM micrographs show different morphological features between the surface layers and the core region.

(iv) The thermal aging results indicate that temperature at 70℃ can further cure the epoxy and increase its stiffness and strength. After 150 days, longer time of thermal aging can soften the epoxy, as shown by the reduced strength and stiffness, together with increased elongation. SEM images illustrated different regions of radially propagating cracks after thermal aging, including initiation site, smooth mirror-like region, and rough region with more crack branches.

5.3 Thesis Contributions

The main contributions of this thesis are as follows:

(i) Developed experimental setups, methodologies for water, UV, and thermal aging.

(ii) Conducted extensive tests and measurements to determine the aging behavior of a specific epoxy during the aforementioned three types of aging conditions; Various properties were examined, including Young’s modulus, tensile strength, fracture strain, tensile toughness, and fracture surface characterization.

(iii) Conducted MD simulations of pure epoxy and an epoxy with different water contents for elastic properties determination, and obtained similar trend with experimental tests.

(iv) Provided insights on various aging mechanisms that could explain the reasons for the deterioration of the mechanical properties as a result of aging.

5.4 Future Work

The following areas are worthy of future research efforts:

(i) Obtain longer aging data to provide a more complete/realistic degradation profile of the epoxy’s mechanical properties.

(ii) Investigate the coupled aging effect of the epoxy properties and compare them with natural aging conditions. These coupled aging results and natural aging data can be possibly used to develop life prediction models for the epoxy.

(iii) Develop a more realistic MD model to investigate the different types of aging effects.

References

[1] S. Rimdusit and H. Ishida, "Development of new class of electronic packaging materials based on ternary systems of benzoxazine, epoxy, and phenolic resins," Polymer, vol. 41, pp. 7941-7949, Oct 2000.

[2] A. Toldy, B. Szolnoki, and G. Marosi, "Flame retardancy of fibre-reinforced epoxy resin composites for aerospace applications," Polymer Degradation and Stability, vol. 96, pp. 371-376, Mar 2011.

[3] L. Yan and N. Chouw, "Crashworthiness characteristics of flax fibre reinforced epoxy tubes for energy absorption application," Materials & Design, vol. 51, pp. 629-640, Oct 2013.

[4] A. S. Haynes, C. J.A., and K. J., "Thermomechanical Impact of Polyurethane Potting on Gun Launched Electronics," Journal of Engineering, vol. 2013, p. 11, 2012.

[5] S. Farris, I. U. Unalan, L. Introzzi, J. M. Fuentes-Alventosa, and C. A. Cozzolino, "Pullulan- Based Films and Coatings for Food Packaging: Present Applications, Emerging Opportunities, and Future Challenges," Journal of Applied Polymer Science, vol. 131, p. 12, Jul 2014.

[6] R. J. Young and P. A. Lovell, Introduction to polymers. London: CHAPMAN & HALL, 1991.

[7] G. Z. Xiao and M. E. R. Shanahan, "Swelling of DGEBA/DDA epoxy resin during hygrothermal ageing," Polymer, vol. 39, pp. 3253-3260, Jun 1998.

[8] Y. Tu and J. K. Spelt, "Blistering as a form of degradation in adhesive joints," Journal of Adhesion, vol. 72, pp. 359-372, 2000.

[9] J. N. Myers, C. Zhang, K. W. Lee, J. Williamson, and Z. Chen, "Hygrothermal Aging Effects on Buried Molecular Structures at Epoxy Interfaces," Langmuir, vol. 30, pp. 165- 171, Jan 2014.

[10] A. P. Mouritz, "Environmental durability of z-pinned carbon fibre-epoxy laminate exposed to water," Composites Science and Technology, vol. 72, pp. 1568-1574, Aug 2012.

[11] B. Dao, J. Hodgkin, J. Krstina, J. Mardel, and W. Tian, "Accelerated Aging Versus Realistic Aging in Aerospace Composite Materials. V. The Effects of Hot/Wet Aging in a Structural Epoxy Composite," Journal of Applied Polymer Science, vol. 115, pp. 901- 910, Jan 15 2010.

[12] J. M. Hutchinson, "Physical aging of polymers," Progress in Polymer Science, vol. 20, pp. 703-760, 1995.

[13] B. Deneve and M. E. R. Shanahan, "Water-absorption by an epoxy-resin and its effect on the mechanical-properties and infrared-spectra," Polymer, vol. 34, pp. 5099-5105, 1993.

[14] A. Zafar, F. Bertocco, J. Schjodt-Thomsen, and J. C. Rauhe, "Investigation of the long term effects of moisture on carbon fibre and epoxy matrix composites," Composites Science and Technology, vol. 72, pp. 656-666, Mar 2012.

[15] R. S. C. Woo, H. G. Zhu, C. K. Y. Leung, and J. K. Kim, "Environmental degradation of epoxy-organoclay nanocomposites due to UV exposure: Part II residual mechanical properties," Composites Science and Technology, vol. 68, pp. 2149-2155, Jul 2008.

[16] H. G. Kumar, R. P. Singh, and T. Nakamura, "Degradation of carbon fiber-reinforced epoxy composites by ultraviolet radiation and condensation," Journal of Composite Materials, vol. 36, pp. 2713-2733, 2002 2002.

[17] L. Barral, J. Cano, J. Lopez, P. Nogueira, and C. Ramirez, "Effect of thermal degradation on the mechanical properties of a diglycidyl ether of bisphenol A 1,3- bisaminomethylcyclohexane (DGEBA/1,3-BAC) epoxy resin system," Journal of Applied Polymer Science, vol. 63, pp. 1841-1849, Mar 28 1997.

[18] C. Lin, J. C. Suhling, P. Lall, and Ieee, Isothermal Aging Induced Evolution of the Material Behavior of Underfill Encapsulants. New York: Ieee, 2009.

[19] X. Buch and M. E. R. Shanahan, "Influence of the gaseous environment on the thermal degradation of a structural epoxy adhesive," Journal of Applied Polymer Science, vol. 76, pp. 987-992, May 16 2000.

[20] H. M. Le Huy, V. Bellenger, J. Verdu, and M. Paris, "Thermal-oxidation of anhydride cured epoxies—III. Effect on mechanical properties," Polymer Degradation and Stability, vol. 41, pp. 149-156, 1993 1993.

[21] D. Hayward, E. Hollins, P. Johncock, I. McEwan, R. A. Pethrick, and E. A. Pollock, "The cure and diffusion of water in halogen containing epoxy/amine thermosets," Polymer, vol. 38, pp. 1151-1168, Mar 1997.

[22] I. D. Maxwell and R. A. Pethrick, "Dielectric studies of water in epoxy-resins," Journal of Applied Polymer Science, vol. 28, pp. 2363-2379, 1983.

[23] C. Grave, I. McEwan, and R. A. Pethrick, "Influence of stoichiometric ratio on water absorption in epoxy resins," Journal of Applied Polymer Science, vol. 69, pp. 2369-2376, Sep 1998.

[24] G. Z. Xiao and M. E. R. Shanahan, "Irreversible effects of hygrothermal aging on DGEBA/DDA epoxy resin," Journal of Applied Polymer Science, vol. 69, pp. 363-369, Jul 1998.

[25] G. Z. Xiao, M. Delamar, and M. E. R. Shanahan, "Irreversible interactions between water and DGEBA/DDA epoxy resin during hydrothermal aging," Journal of Applied Polymer Science, vol. 65, pp. 449-458, Jul 1997.

[26] S. Cotugno, D. Larobina, G. Mensitieri, P. Musto, and G. Ragosta, "A novel spectroscopic approach to investigate transport processes in polymers: the case of water- epoxy system," Polymer, vol. 42, pp. 6431-6438, Jul 2001.

[27] G. A. Luoma and R. D. Rowland, "Environmental degradation of an epoxy-resin matrix," Journal of Applied Polymer Science, vol. 32, pp. 5777-5790, Nov 1986.

[28] L. Li, Y. Yu, H. Su, G. Zhan, S. Li, and P. Wu, "The Diffusion Mechanism of Water Transport in Amine-Cured Epoxy Networks," Applied Spectroscopy, vol. 64, pp. 458- 465, Apr 2010.

[29] S. Cotugno, G. Mensitieri, P. Musto, and L. Sanguigno, "Molecular interactions in and transport properties of densely cross-linked networks: A time-resolved FT-IR spectroscopy investigation of the epoxy/H2O system," Macromolecules, vol. 38, pp. 801- 811, Feb 8 2005.

[30] P. Musto, G. Ragosta, G. Scarinzi, and L. Mascia, "Probing the molecular interactions in the diffusion of water through epoxy and epoxy-bismaleimide networks," Journal of Polymer Science Part B-Polymer Physics, vol. 40, pp. 922-938, May 15 2002.

[31] J. M. Zhou and J. P. Lucas, "Hygrothermal effects of epoxy resin. Part I: the nature of water in epoxy," Polymer, vol. 40, pp. 5505-5512, Sep 1999.

[32] Y. C. Lin and X. Chen, "Investigation of moisture diffusion in epoxy system: Experiments and molecular dynamics simulations," Chemical Physics Letters, vol. 412, pp. 322-326, Sep 5 2005.

[33] P. Nogueira, C. Ramirez, A. Torres, M. J. Abad, J. Cano, J. Lopez, et al., "Effect of water sorption on the structure and mechanical properties of an epoxy resin system," Journal of Applied Polymer Science, vol. 80, pp. 71-80, Apr 4 2001.

[34] Y. H. Hu, X. C. Li, A. W. Lang, Y. P. Zhang, and S. R. Nutt, "Water immersion aging of polydicyclopentadiene resin and glass fiber composites," Polymer Degradation and Stability, vol. 124, pp. 35-42, Feb 2016.

[35] S. Kajorncheappunngam, R. K. Gupta, and H. V. S. GangaRao, "Effect of aging environment on degradation of glass-reinforced epoxy," Journal of Composites for Construction, vol. 6, pp. 61-69, Feb 2002.

[36] M. G. Lu, M. J. Shim, and S. W. Kim, "Effects of moisture on properties of epoxy molding compounds," Journal of Applied Polymer Science, vol. 81, pp. 2253-2259, Aug 2001.

[37] G. C. Papanicolaou, T. V. Kosmidou, A. S. Vatalis, and C. G. Delides, "Water absorption mechanism and some anomalous effects on the mechanical and viscoelastic behavior of an epoxy system," Journal of Applied Polymer Science, vol. 99, pp. 1328-1339, Feb 15 2006.

[38] J. M. Zhou and J. P. Lucas, "Hygrothermal effects of epoxy resin. Part II: variations of glass transition temperature," Polymer, vol. 40, pp. 5513-5522, Sep 1999.

[39] C. Wu and W. Xu, "Atomistic simulation study of absorbed water influence on structure and properties of crosslinked epoxy resin," Polymer, vol. 48, pp. 5440-5448, Aug 24 2007.

[40] S. G. Lee, S. S. Jang, J. Kim, and G. Kim, "Distribution and Diffusion of Water in Model Epoxy Molding Compound: Molecular Dynamics Simulation Approach," IEEE Transactions on Advanced Packaging, vol. 33, pp. 333-339, May 2010.

[41] D. R. Xin and Q. Han, "Investigation of moisture diffusion in cross-linked epoxy moulding compound by molecular dynamics simulation," Molecular Simulation, vol. 39, pp. 322-329, Apr 2013.

[42] B. Wunderle, E. Dermitzaki, O. Holck, J. Bauer, H. Walter, Q. Shaik, et al., "Molecular dynamics approach to structure-property correlation in epoxy resins for thermomechanical lifetime modeling," Microelectronics Reliability, vol. 50, pp. 900-909, Jul 2010.

[43] D. R. Xin and Q. Han, "Study on thermomechanical properties of cross-linked epoxy resin," Molecular Simulation, vol. 41, pp. 1081-1085, Sep 2015.

[44] V. Varshney, S. S. Patnaik, A. K. Roy, and B. L. Farmer, "A molecular dynamics study of epoxy-based networks: Cross-linking procedure and prediction of molecular and material properties," Macromolecules, vol. 41, pp. 6837-6842, Sep 23 2008.

[45] D. R. Xin and Q. Han, "Molecular dynamics study on the tensile deformation of crosslinking epoxy resin," Journal of molecular modeling, vol. 21, pp. 2567-2567, 2015-Jan 2015.

[46] A. Shokuhfar and B. Arab, "The effect of cross linking density on the mechanical properties and structure of the epoxy polymers: molecular dynamics simulation," Journal of Molecular Modeling, vol. 19, pp. 3719-3731, Sep 2013.

[47] C. Wu and W. Xu, "Atomistic molecular modelling of crosslinked epoxy resin," Polymer, vol. 47, pp. 6004-6009, Jul 26 2006.

[48] T. C. Clancy, S. J. V. Frankland, J. A. Hinkley, and T. S. Gates, "Molecular modeling for calculation of mechanical properties of epoxies with moisture ingress," Polymer, vol. 50, pp. 2736-2742, Jun 5 2009.

[49] S. Pandiyan, J. Krajniak, G. Samaey, D. Roose, and E. Nies, "A molecular dynamics study of water transport inside an epoxy polymer matrix," Computational Materials Science, vol. 106, pp. 29-37, Aug 2015.

[50] D. Feldman, "Polymer weathering: Photo-oxidation," Journal of Polymers and the Environment, vol. 10, pp. 163-173, Oct 2002.

[51] J. Frederick, H. Snell, and E. Haywood, "Solar ultraviolet radiation at the earth's surface," Photochemistry and Photobiology, vol. 50, pp. 443-450, 1989.

[52] T. Kelen, Polymer Degradation. New York: Van Nostrand Reinhold Co., 1983.

[53] J. F. Rabek, Photodegradation of polymers: physical characteristics and applications. Germany: Springer, 1996.

[54] N. Mailhot, S. Morlat-Theias, M. Ouahioune, and J. L. Gardette, "Study of the degradation of an epoxy/amine resin, 1 photo- and thermo-chemical mechanisms," Macromolecular Chemistry and Physics, vol. 206, pp. 575-584, Mar 4 2005.

[55] P. Musto, G. Ragosta, M. Abbate, and G. Scarinzi, "Photo-oxidation of high performance epoxy networks: Correlation between the molecular mechanisms of degradation and the viscoelastic and mechanical response," Macromolecules, vol. 41, pp. 5729-5743, Aug 12 2008.

[56] R. S. C. Woo, Y. Chen, H. Zhu, J. Li, J. K. Kim, and C. K. Y. Leung, "Environmental degradation of epoxy-organoclay nanocomposites due to UV exposure. Part 1: Photo- degradation," Composites Science and Technology, vol. 67, pp. 3448-3456, Dec 2007.

[57] G. Zhang, W. G. Pitt, S. R. Goates, and N. L. Owen, "Studies on oxidative photodegradation of epoxy-resins by IR-ATR spectroscopy," Journal of Applied Polymer Science, vol. 54, pp. 419-427, Oct 1994.

[58] H. Kim and M. W. Urban, "Molecular level chain scission mechanisms of epoxy and urethane polymeric films exposed to UV/H2O. Multidimensional spectroscopic studies," Langmuir, vol. 16, pp. 5382-5390, Jun 2000.

[59] F. W. Liu, M. X. Yin, B. Y. Xiong, F. Zheng, W. F. Mao, Z. Chen, et al., "Evolution of microstructure of epoxy coating during UV degradation progress studied by slow positron annihilation spectroscopy and electrochemical impedance spectroscopy," Electrochimica Acta, vol. 133, pp. 283-293, Jul 2014.

[60] F. Awaja and P. J. Pigram, "Surface molecular characterisation of different epoxy resin composites subjected to UV accelerated degradation using XPS and ToF-SIMS," Polymer Degradation and Stability, vol. 94, pp. 651-658, Apr 2009.

[61] D. Kotnarowska, "Influence of ultraviolet radiation and aggressive media on epoxy coating degradation," Progress in Organic Coatings, vol. 37, pp. 149-159, Dec 1999.

[62] C. Decker, "Kinetic study and new applications of UV radiation curing," Macromolecular Rapid Communications, vol. 23, pp. 1067-1093, Dec 20 2002.

[63] A. Mariani, S. Bidali, S. Fiori, M. Sangermano, G. Malucelli, R. Bongiovann, et al., "UV-ignited frontal polymerization of an epoxy resin," Journal of Polymer Science Part a-Polymer Chemistry, vol. 42, pp. 2066-2072, May 2004.

[64] X. Gu, C. A. Michaels, P. L. Drzal, J. Jastnin, D. Martin, T. Nguyen, et al., "Probing photodegradation beneath the surface: a depth profiling study of UV-degraded polymeric coatings with microchemical imaging and nanoindentation," Journal of Coatings Technology and Research, vol. 4, pp. 389-399, Dec 2007.

[65] B. Mailhot, P. O. Bussiere, A. Rivaton, S. Morlat-Therias, and J. L. Gardette, "Depth profiling by AFM nanoindentations and micro-FTIR spectroscopy for the study of polymer ageing," Macromolecular Rapid Communications, vol. 25, pp. 436-440, Jan 2004.

[66] P. Delobelle, L. Guillot, C. Dubois, and L. Monney, "Photo-oxidation effects on mechanical properties of epoxy matrixes: Young's modulus and hardness analyses by nano-indentation," Polymer Degradation and Stability, vol. 77, pp. 465-475, Sep 2002.

[67] N. D. Mailhot, S. Morlat-Therias, P. O. Bussiere, and J. L. Gardette, "Study of the degradation of an epoxy/amine resin, 2 kinetics and depth-profiles," Macromolecular Chemistry and Physics, vol. 206, pp. 585-591, Mar 2005.

[68] X. H. Gu, C. A. Michaels, P. L. Drzal, J. Jastnin, D. Martin, T. Nguyen, et al., "Probing photodegradation beneath the surface: a depth profiling study of UV-degraded polymeric coatings with microchemical imaging and nanoindentation," Journal of Coatings Technology and Research, vol. 4, pp. 389-399, Dec 2007.

[69] N. S. Allen and M. Edge, "Thermal Stabilisation of Polymers," in Fundamentals of Polymer Degradation and Stabilisation, ed England: Elsevier Applied Science, 1992, p. 22.

[70] H. M. Le Huy, V. Bellenger, J. Verdu, and M. Paris, "Thermal-oxidation of anhydride cured epoxies. I—mechanistic aspects," Polymer Degradation and Stability, vol. 35, pp. 77-86, 1992.

[71] C. Galant, B. Fayolle, M. Kuntz, and J. Verdu, "Thermal and radio-oxidation of epoxy coatings," Progress in Organic Coatings, vol. 69, pp. 322-329, Dec 2010.

[72] A. Cherdoud-Chihani, M. Mouzali, and M. J. M. Abadie, "Study of crosslinking AMS/DGEBA system by FTIR," Journal of Applied Polymer Science, vol. 69, pp. 1167- 1178, Aug 1998.

[73] B. Dao, J. Hodgkin, J. Krstina, J. Mardel, and W. Tian, "Accelerated aging versus realistic aging in aerospace composite materials. II. Chemistry of thermal aging in a

structural composite," Journal of Applied Polymer Science, vol. 102, pp. 3221-3232, Nov 2006.

[74] C. Bockenheimer, D. Fata, and W. Possart, "New aspects of aging in epoxy networks. I. Thermal aging," Journal of Applied Polymer Science, vol. 91, pp. 361-368, Jan 2004.

[75] X. Buch and M. E. R. Shanahan, "Thermal and thermo-oxidative ageing of an epoxy adhesive," Polymer Degradation and Stability, vol. 68, pp. 403-411, 2000.

[76] Y. Pei, K. Wang, M. Zhan, W. Xu, and X. Ding, "Thermal-oxidative aging of DGEBA/EPN/LMPA epoxy system: Chemical structure and thermal-mechanical properties," Polymer Degradation and Stability, vol. 96, pp. 1179-1186, Jul 2011.

[77] T. D. Chang and J. O. Brittain, "Studies of epoxy-resin systems .C. Effect of sub-Tg aging on the physical-properties of a fully cured epoxy-resin," Polymer Engineering and Science, vol. 22, pp. 1221-1227, 1982.

[78] W. Fan and J.-l. Li, "The Effect of Thermal Aging on Properties of Epoxy Resin," in Progress in Materials and Processes. vol. 602-604, Z. Shi, J. H. Dong, and W. Ma, Eds., ed, 2013, pp. 798-801.

[79] K. P. Unnikrishnan and E. T. Thachil, "Aging and thermal studies on epoxy resin modified by epoxidized novolacs," Polymer-Plastics Technology and Engineering, vol. 45, pp. 469-474, 2006 2006.

[80] R. J. Morgan, "Effect of thermal history and strain rate on the mechanical-properties of diethylenetriamine-cured bisphenol-A-diglycidyl ether epoxies," Journal of Applied Polymer Science, vol. 23, pp. 2711-2717, 1979 1979.

[81] "West System 105 Epoxy Resin," Material Safety Data Sheet, Apr 26 2013.

[82] "West System 206 Slow Hardener," Material Safety Data Sheet, Apr 26 2013.

[83] S. Masoumi, B. Arab, and H. Valipour, "A study of thermo-mechanical properties of the cross-linked epoxy: An atomistic simulation," Polymer, vol. 70, pp. 351-360, Jul 2015.

[84] Bruker, "MultiMode 8 Specifications," 2011.

[85] Bruker. (2016). Scanasyst-Air AFM Probe. Available: http://www.brukerafmprobes.com/Product.aspx?ProductID=3726

[86] N. Fasanella and V. Sundararaghavan, "Atomistic modeling of thermomechanical properties of SWNT/Epoxy nanocomposites," Modelling and Simulation in Materials Science and Engineering, vol. 23, p. 16, Sep 2015.

[87] A. R. Alian, S. I. Kundalwal, and S. A. Meguid, "Multiscale modeling of carbon nanotube epoxy composites," Polymer, vol. 70, pp. 149-160, Jul 23 2015.

[88] A. R. Alian, S. I. Kundalwal, and S. A. Meguid, "Interfacial and mechanical properties of epoxy nanocomposites using different multiscale modeling schemes," Composite Structures, vol. 131, pp. 545-555, Nov 2015.

[89] E. Girardreydet, C. C. Riccardi, H. Sautereau, and J. P. Pascault, "Epoxy-aromatic diamine kinetics. 1. Modeling and influence of the diamine structure," Macromolecules, vol. 28, pp. 7599-7607, Nov 1995.

[90] S. Yu, S. Yang, and M. Cho, "Multi-scale modeling of cross-linked epoxy nanocomposites," Polymer, vol. 50, pp. 945-952, Jan 2009.

[91] D. Hossain, M. A. Tschopp, D. K. Ward, J. L. Bouvard, P. Wang, and M. F. Horstemeyer, "Molecular dynamics simulations of deformation mechanisms of amorphous polyethylene," Polymer, vol. 51, pp. 6071-6083, Nov 26 2010.

[92] U. W. Suter and B. E. Eichinger, "Estimating elastic constants by averaging over simulated structures," Polymer, vol. 43, pp. 575-582, Jan 2002.

[93] G. Z. Xiao and M. E. R. Shanahan, "Water absorption and desorption in an epoxy resin with degradation," Journal of Polymer Science Part B-Polymer Physics, vol. 35, pp. 2659-2670, Nov 1997.

[94] K. I. Ivanova, R. A. Pethrick, and S. Affrossman, "Hygrothermal aging of rubber modified and mineral filled dicyandiamide cured digylcidyl ether of bisphenol A epoxy resin. I. Diffusion behavior," Journal of Applied Polymer Science, vol. 82, pp. 3468- 3476, Dec 2001.

[95] W. J. Cantwell and H. H. Kausch, "Fracture Behaviour of Epoxy Resins," in Chemistry and Technology of Epoxy Resins, B. Ellis, Ed., ed London: Blackie Academic & Professional, 1993.