INTERACTION AND PERMEABILITY OF WATER WITH LIQUID CRYSTALLINE THERMOSET JIANXUN FENG

Transcription

1 INTERACTION AND PERMEABILITY OF WATER WITH LIQUID CRYSTALLINE THERMOSET By JIANXUN FENG A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2001

2 This dissertation is dedicated to my family: my mother, father, brother, and sister for their love and support. I am especially grateful for the patience and encouragement of my wife Yue-xiao and my son Hao, who have been willing to give up precious family time so that I could finish my work. Without their love and support, this work would not have been possible.

3 ACKNOWLEDGEMENTS I am deeply indebted to a number of people. I would like to thank my advisor, Dr. Elliot P. Douglas, without whose enthusiasm, kindness, knowledge, support, and guidance none of this would be possible. I would also like to thank the members of the supervisory committee for their valuable help and input: Dr. Ronald H. Baney, Dr. Christopher D. Batich, Dr. Laurie A. Gower and Dr. Kenneth R. Berger. They are not only my advisors for academic study at the University of Florida but also advisors for my personal life in the past and future. I cannot thank enough all of my colleagues who gave the support and collaboration necessary to make it through this onerous process: Seunghyun Cho, Elizabeth Oborn, Dr. Arthur Gavrin, Dr. Han-ying Zhao, Wei-ling Jia, Tonya Brevaldi, Dongsik Kim, Susan Leander, Stephanie DiFrancesco, Derek Lincoln, and Dr. Gilberto Lunardi. I would thank them for bringing new ideas to our discussions. I cannot express enough gratitude to Dr. Kenneth Berger for his enthusiastic and kindly guidance and support in the measurement of water permeability. I could not have completed the work without his support. I will always remember the happy time when Gilberto and I ran the permeability tests for our samples in Dr. Berger s Package Science Lab. Funding for this project was provided by the Presidential Early Career Award for Scientists and Engineers, administered by the Army Research office. iii

4 TABLE OF CONTENTS ACKNOWLEDGEMENTS...iii LIST OF TABLES... iv LIST OF FIGURES... v ABSTRACT...x CHAPTERS 1 INTRODUCTION REVIEW OF LITERATURE... 5 page 2.1 Introduction Factors on the Permeation in Polymer Materials Effect of Testing Temperature Effect of the Penetrant Effect of the Polymer Effects of Water on Glass Transition Mechanism of Water Absorption Test Method Transmission Rate Methods (Time Lag Technique) Sorption Method Model for Heterogeneous Sorption and Diffusion Dual Mode Transport Diffusion and Permeation in Heterogeneous Media WATER VAPOR PERMEATION IN LIQUID CRYSTALLINE EPOXY Introduction Experiment Synthesis of 4,4 -diglycidyloxy-α-methylstilbene (DOMS) Apparatus and Procedure for Measuring Permeability Mathematical Treatment Results and Discussion Synthesis of DOMS iv

5 3.4.2 Thermal Behavior DOMS Features of Water Permeation in the Studied Epoxies Conclusions INTERACTION OF WATER WITH EPOXY RESINS Introduction Experiment Fourier Transform Infrared Spectroscopy (FTIR) Differential Scanning Calorimetry (DSC) Dynamic Mechanical Spectroscopy (DMS) Results and Discussion Assignment of IR bands Peak Curve Resolving of the Difference Spectrum Thermal Properties Conclusion HETEROGENEOUS MORPHOLOGY OF LCT AND MODEL Introduction Experiment Materials and Sample Preparation Density Measurement Atomic Force Microscopy Results and Discussions Image Analysis Effect of Stoichiometric Ratio on Morphology Effects of Stoichiometry and Microstructure on Density Theoretical Model of the Permeability Formulation of Diffusion Model Formulation of Permeability Mode Molecular Mechanism of Sorption and Transport Conclusion GENERAL CONCLUSIONS AND FUTURE WORK LIST OF REFERENCES BIOGRAPHICAL SKETCH v

6 LIST OF TABLES Table Page 3.1 Elemental analysis results Thermal transition of compounds used in this work The designation of samples Water vapor pressure at the testing temperature Comparison of the characterization temperature of DOMS Activity energy of diffusion and heat of solution Assignment of FTIR band* γ-transition temperature Activation energy of gamma relaxation Thermal properties of DER-SAA and DOMS-SAA systems Comparison of Tg obtained experimentally and by calculation Theoretical Tg using entropy model Parameters for Tapping Mode atomic force microscopy Average theoretical diffusion coefficients calculated from experimental data fitting (at 30 C) Comparison between theoretical and experimental diffusion coefficients at 30 C Theoretical values of the permeability of the two-phase domains iv

7 LIST OF FIGURES Figure Page 2.1 Cartoon representation of molecular packing Schematic design of liquid crystalline thermoset monomer Time lag method Chemical structure of DGEBA The chemical structure of the DER383, DOMS and SAA Curing procedure for DER-SAA and DOMS-SAA systems Profile of half-time method Diagram of the concentration distribution Synthetic scheme for diol Synthetic scheme for DOMS Synthetic mechanism scheme for 4,4 -Dihydroxy-α-methylstilbene (diol) Synthetic mechanism scheme for DOMS monomer DSC thermographs of the liquid crystalline DOMS monomer: (a) heating, (b) cooling (scan rate 20 o C/min) Comparison of FTIR spectra of (a) DOMS-SAA-1.0 and (b) DOMS monomer Typical water vapor permeation rate curves (functional ratio is 1 for both system, test at 37.8 C) Relationship between diffusion coefficient and... amine/epoxide functional ratio at test temperature 37.8 C v

8 3.14 Relationship between diffusion coefficient and amine/epoxide functional ratio at test temperature 30 C Relationship between diffusion coefficient and amine/epoxide functional ratio at test temperature 20 C Relationship between diffusion coefficient and amine/epoxide functional ratio at test temperature 10 C Relationship between solubility coefficient and amine/epoxide functional ratio at test temperature 37.8 C Relationship between solubility coefficient and amine/epoxide functional ratio at test temperature 30 C Relationship between solubility coefficient and amine/epoxide functional ratio at test temperature 20 C Relationship between solubility coefficient and amine/epoxide functional ratio at test temperature 10 C Relationship between permeability amine/epoxide functional ratio at test temperature 37.8 C Relationship between permeability amine/epoxide functional ratio at test temperature 30 C Relationship between permeability and amine/epoxide functional ratio at test temperature 20 C Relationship between permeability and amine/epoxide functional ratio at test temperature 10 C Schematic picture of polar groups near crosslink junction Effect of temperature on diffusion coefficient of DER-SAA system Effect of temperature on diffusion coefficient of DOMS-SAA system Temperature effect on solubility coefficient of DER-SAA system Temperature effect on solubility coefficient of DOMS-SAA system FTIR spectrum of DER-SAA-1.0 sample vi

9 4.2 FTIR spectrum of DOMS-SAA-1.0 sample FTIR spectra of (A) monomer DER383 and (B) DER-SAA Spectra of cured DOMS-SAA-0.8 samples Reaction between epoxy and amine groups Etherfication for epoxy resins Reactivity order of functional groups Curve fitting for DOMS-SAA-0.8 sample. Dotted line is the sum of the three peaks Schematic picture of polar group interaction in the crosslinked network Schematic presentation of hydrogen bond between water and polar groups in resin Ratio of water bonded with amine groups to that with OH groups Theoretical calculation of polar group concentration and ratio Fraction of hydrogen-bonded water of saturated samples DSC thermography of DOMS-SAA-1.0 sample DSC thermography of DER-SAA-1.0 sample Storage modulus and tan δ curves for DER-SAA-0.8 sample Storage modulus and tan δ curves for DOMS-SAA-0.8 sample γ transition for DER-SAA system at frequency f= γ transition for DOMS-SAA system at frequency f= Arrhenius plot of the gamma relaxation Comparison of effect of amine/epoxide functional ratio on the activation energy of Gamma relaxation for dry DOMS-SAA and DER-SAA system Effect of amine/epoxide functional ratio on the activation energy of Gamma relaxation for DER-SAA system vii

10 4.23 Effect of amine/epoxide functional ratio on the activation energy of Gamma relaxation for DOMS-SAA system Five kinds of chemical structure Theoretical mole fraction of three type structures Plot of equal site transition theory for DER-SAA and DOMS-SAA system Glass transition temperature of dry DER-SAA and DOMS-SAA systems Water effect on glass transition temperature of DER-SAA system Water effect on glass transition temperature of DOMS-SAA system Glass transition activation energy of DER-SAA and DOMS-SAA systems Effect of water on glass transition activation energy of DER-SAA system Effect of water on glass transition activation energy of DOMS-SAA system Phase (upper) and topographic (lower) AFM images of DER-SAA-0.8 sample Phase (upper) and topographic (lower) AFM images of DER-SAA-0.9 sample Phase (upper) and topographic (lower) AFM images of DER-SAA-1.0 sample Phase (upper) and topographic (lower) AFM images of DER-SAA-1.1 sample Phase (upper) and topographic (lower) AFM images of DER-SAA-1.2 sample Phase (upper) and topographic (lower) AFM images of DOMS-SAA-0.8 sample Phase (upper) and topographic (lower) AFM images of DOMS-SAA-0.9 sample Phase (upper) and topographic (lower) AFM images of DOMS-SAA-1.0 sample Phase (upper) and topographic (lower) AFM images of DOMS-SAA-1.1 sample Phase (upper) and topographic (lower) AFM images of DOMS-SAA-1.2 sample Representative structure of two-phase epoxy morphology Percentage of hard-phase for DER-SAA and DOMS-SAA systems viii

11 5.13 Density of DER-SAA and DOMS-SAA systems Diffusion coefficient curve fitting for DER-SAA system at 30 C Diffusion coefficient curve fitting for DOMS-SAA system at 30 C Permeability vs. volume fraction for DER-SAA system Permeability vs. volume fraction for DOMS-SAA system Schematic curve of potential energy vs. position during process of water molecule diffusion from one position to another position. (a) non polar groups, (b) low concentration of polar group, (c) high concentration of polar group ix

12 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy INTERACTION AND PERMEABILITY OF WATER WITH LIQUID CRYSTALLINE THERMOSET By Jianxun Feng December, 2001 Chairman: Elliot P. Douglas Major Department: Materials Science and Engineering The complex transport behavior of water in both liquid crystalline thermoset and non-liquid crystalline thermoset systems were investigated. The liquid crystalline thermoset was 4,4 -diglycidyloxy-α-methylstilbene with sulfanilamine (SAA) as the crosslinking agent, the non-liquid crystalline thermoset the diglycidyl ether of bisphenol A. The liquid crystalline thermosets have higher barrier properties than isotropic non-lc epoxy resins. The efficient chain packing of the smectic mesophase of the liquid crystalline thermosets is attributed as the main factor for this difference. Permeation testing results show that the diffusion coefficient, permeability, and solubility coefficient depend on the amine/epoxide functional ratio. FTIR results confirmed that hydrophilic groups in the crosslinked network are one of the major factors that control the sorption and diffusion of water in epoxy resins. Two possible water- x

13 epoxy hydrogen bond configurations are identified, namely hydrogen bond formation of water to amine groups and hydrogen bond formation of water molecules to hydroxyl groups. Thus, diffusion of water molecules into epoxy resins depends on two major factors, namely, the availability of appropriate microvoids in the cured network and the interaction between the water molecules and the epoxy resin matrix. Depression of glass transition temperature was revealed by dynamic mechanical thermal analysis. The intrinsic moisture sensitivity of the epoxy resins is traceable directly to the molecular structure of the network. The presence of polar groups provides the chemical basis for moisture sensitivity. The entropy model can satisfactorily describe the nature of the depression of glass transition temperature. The hypothesis of a heterogeneous network was confirmed by phase images of atomic force microscopy for all of the epoxy samples. High crosslinked domains are surrounded by low crosslinked regions. Quantitative analysis of phase images shows that the relative amount of hard-phase changes with amine/epoxide functional ratio. Based on the analysis of morphology and interaction of water-epoxy resin, a theoretical model for the diffusion coefficient was set up for the first time by considering two factors: morphology and hydrogen bonding. This model relates the fraction of the hard-phase and the hydrogen bonding capacity to the experimental diffusion coefficients. xi

14 CHAPTER 1 INTRODUCTION Liquid crystalline thermosets (LCTs) are a specific class of liquid crystalline materials that posses both the advantages of liquid crystals and the high performance of thermosets. This kind of material has some special properties for not only processing but also application, which makes it the ideal matrix material for advanced composites. However, real life environments during service can be complex and variable. The longterm properties of the LCTs depend on the interaction of the LCTs with environmental factors such as temperature and moisture. Polymer materials are different from other structural materials in that, at ambient temperatures, low molecular weight substances can easily migrate in them freely. The moisture transport characteristics of cured epoxy resin systems is an area of great practical importance in view of the effect of moisture on the properties of the cured resin and of the fiber-resin interphase in structural composites. The degradation of epoxy resins is associated with both moisture induced plasticization and/or micro-mechanical damage and strongly dominates the material response properties under temperature and humidity tests. The damaging process, which is governed by the synergistic action of sorbed moisture and temperature, leads to additional weight gains in samples exposed to a moist environment. Various studies of the kinetics of the moisture transport process[1-6], the 1

15 2 effect of moisture on the dynamic mechanical properties[7, 8], and the possible effect of heterogeneous morphology[1, 3-6, 9-12] have been reported. The physical aspects of the transport process are receiving increasing attention, and the effects of free volume[13] and free volume distribution[1] are being recognized. Adamson postulated[1] that transport of moisture below Tg is a three-stage process in which the absorbed moisture first occupies the free volume present in the form of voids. In the second stage, water becomes bound to network sites causing swelling. Finally, in the third stage, water enters the densely crosslinked regions. The chemical structure of matrix constituents and the processing conditions influence the resulting thermoset networks and hence the properties of cross-linked polymer. In fact, although the concept of homogeneous infinite network has been long erroneously applied to describe the morphology of all network polymers, the hypothesis of highly crosslinked nodules immersed in an internodular matrix of lower crosslinking density seems more reasonable. The role of heterogeneous morphology has been inferred when (1) changes in free volume have apparently not been able to explain the results [11], (2) free volume was not considered as possible parameter[4-6, 9, 10], and (3) new peaks or shoulders appeared in the dynamic transitions[14]. The influence of the matrix nodular structure on durability in aggressive environment, mode of failure and mechanical properties of epoxy resin has been reported in the literature[14-20]. The exposure of heterogeneous materials to humid environments induces different changes of the polymer structure, depending on the affinity and mode of sorption of water. In order to investigate the mechanism of transport of water in a crosslinked epoxy network and the corresponding change of physical and mechanical properties of the

16 3 network, it is crucial to reveal the water epoxy interaction that controls the complex water sorption behavior and degradation mechanism in various combinations of moisture environment and temperature. The moisture dissolved in a film surface diffuses through the film by a series of activated steps. It is clear that both solubility and diffusivity are involved and that the polymer molecular and morphological features will affect the penetrant transport behavior. Chemical and morphological modifications have been observed for some epoxy-water systems to induce changes in the solubility and diffusivity. Unfortunately, there are few reports about the moisture sorption and diffusion[21] in liquid crystalline thermosets, although much research has been conducted on LCTs regarding the synthesis[22-38], orientation[25, 30, 39-41], mechanical properties[22, 25, 30, 35, 36, 40-43], thermal properties[22, 25, 30, 37, 42, 44] and the moisture sorption in conventional epoxies[45]. There are many questions that need to be answered about the transport property of liquid crystalline thermosets. Some of these questions follow: What is the difference in moisture permeability between liquid crystalline epoxy thermosets and conventional epoxies? What is the mechanism of the water sorption and how do diffusion coefficient, permeability and sorption coefficient change with the processing parameters? What is the sorption mechanism for the water molecules diffusing in the LCTs? What is the interaction between the diffusing water molecules and the epoxy matrix? In this investigation, a commercial diglycidyl ether of bisphenol A (DGEBA) epoxy resin DER383, the liquid crystalline monomer 4,4 -diglycidyloxy-αmethylstilbene (DOMS), and the crosslinking agent sulfanilamine (SAA) were selected to

17 4 produce cured networks with different amine/epoxide functional ratios. The goal of this work is to investigate the transport properties of liquid crystalline thermosets. The specific aims of this work are as follows: (1) to test the hypothesis that the liquid crystalline mesophase can contribute to the higher barrier property by close packing of molecules of the smectic mesophase; (2) to investigate the interaction of water to polar groups in the cured epoxy resin network; (3) to study the plasticization effect of water on the glass transition temperature by dynamic mechanical thermal testing; (4) to test the hypothesis that the morphology of a crosslinked epoxy resin is heterogeneous, instead of homogeneous, and this multi-component morphology affects the transport property of water in both epoxy systems; (5) to set up suitable theoretical models to explain the change of diffusion coefficient and permeability with morphology, functional group, and mesophase. We hope to open new research topics about the diffusion in liquid crystalline thermosets and that the research results would gave further insight about the processing-structure-properties relationship in LCTs.

18 CHAPTER 2 REVIEW OF LITERATURE 2.1 Introduction The liquid crystalline (LC) state exhibits molecular order in a size range similar to that of a crystal but acts more or less as a viscous liquid. A crystal may be defined microscopically as a condensed phase in which molecules possess both orientational and three-dimensional long-range order. A liquid is a condensed phase in which there is no long-range order and individual molecules are able to alter their conformations. The shape of a liquid conforms to the vessel in which it is held. Condensed phases with properties intermediate between these two extremes are called mesophases. In other words, molecules within a liquid crystalline phase possess some orientational order and may lack positional order. The degree of molecular order in liquid crystals is intermediate between the three dimensional order in solid crystals and the disorder of an isotropic liquid. Thus, liquid crystals are materials that exhibit long-range ordering in less than three (zero, one, and two) dimensions. The anisotropic mesogenic units within the molecules can arrange themselves into isotropic, nematic, smectic or cholesteric phases due to their degree of molecular order. The cartoon representations of some molecular packing are illustrated schematically in Figure 2.1. In the nematic phase, the direction of the molecules tends to be parallel to a common axis, organized in one dimension only, but there is no long-range positional order. In the smectic phase, the molecules are arranged in two-dimensional layers, within which they posses only orientational order. 5

19 6 Polymer scientists now recognize a large number of smectic mesophases. Figure 2.1 just shows the smectic A structure. Other mesophases not shown in this figure include the smectic C, cholesteric, and discotic structures. Cholesteric liquid crystals have the director ordered in a spiral fashion, and colors appear if the twist of the period of spiral is equal to the wavelength of light. Liquid crystalline thermosets (LCTs) are highly crosslinked and ordered network structures prepared by a polycondensation or polyaddition process. Liquid crystalline states in thermosets are generally classified in the same way as LC states of small molecules. The characteristic of the monomer is that it consists of a mesogenic central part with conventional crosslinking end groups attached to its ends. The schematic design of the monomer is shown in Figure 2.2. For this present study, a special LCT, liquid crystalline epoxy, was used. The special chemical structure is that two epoxide functional groups are attached to both ends of the rigid mesogenic unit. Liquid crystalline thermosets (LCTs) have both the advantages of liquid crystals and the high performance of thermosets. Furthermore, LCTs offer many potential advantages over conventional isotropic networks and liquid crystalline thermoplastics. They incorporate liquid crystalline order into a polymer network. This kind of material has some special properties for not only processing but also applications: anisotropy at the molecular level makes it easier to align under external fields; high mechanical properties make it one ideal candidate matrix for advanced polymer composites; low viscosity during the initial stage makes processing simpler and easier and ensures the quality of the product. Other properties are tuned thermal expansion coefficients, high chemical and corrosion resistance, good mechanical and thermal properties, low

20 7 shrinkage upon cure and good electrical properties. These novel LC materials are expected to find applications as materials for advanced composites, laminates, tooling, adhesives and bonding, protective coatings, electronic packaging, LC displays, and nonlinear optics. Similarly, LCTs can be widely used in the microelectronics industry as an encapsulant agent. The long-term properties of the LCTs during service depend on real life environments and interaction of the LCTs with environments such as temperature and moisture. The water absorption in LCTs should have detrimental effects upon their longterm properties based on knowledge from research results for non-liquid crystalline epoxy matrix composite systems. Generally, polymers are attacked by moisture and other chemicals in their service environment. The degradation of a polymer is an important property that should be taken into consideration during design, processing, and application. Early studies of the sorption mechanism and diffusion of small molecules in plastic materials focused on the desire to seek suitable barrier materials, mainly against gases and moisture, such as PET packaging materials. The resistance of polymer materials to penetration and attack by gases, vapors, and liquid is a primary factor affecting their use in many applications. A number of studies of the transport properties of various substances in polymers has been performed in order to clarify the actual mechanism of the diffusion process and to identify the factors that influence the transport rate of penetrant and equilibrium sorption. The environmental degradation of mechanical properties of the epoxy polymers has been given special attention. Water molecules are reported to act as plasticizers or crazing

21 8 agents for epoxies, strongly influencing the properties of the material subjected to temperature, humidity and stress fatigue tests. (a) Isotropic (b) Isotropic, rigid-rod (c) Nematic (d) Smectic-A Figure 2.1 Cartoon representation of molecular packing Functional group Rigid segment Flexible segment Figure 2.2 Schematic design of liquid crystalline thermoset monomer

22 9 The mechanism and measurement of diffusion, permeation, and solubility is very important to characterize the moisture transport in LCTs. Generally, the diffusion process is described as the condensation and solution of gas at one surface followed by diffusion through host solid and evaporation to gaseous state at the other surface. It is observed that the properties, such as condensation and solubility, play a major role in determining the diffusion processes. The mathematical theory of diffusion in isotropic substances is based on the hypothesis that the rate of transfer of the diffusing substance through a unit area of a section is proportional to the concentration gradient measured normal to the section, that is, Fick s first law of diffusion: J = D C x (2.1) where J is net flux, the rate of transfer per unit area of section; C is the concentration of diffusing substance; x is the space co-ordinate measured normal to the section, and D is called the diffusion coefficient. The minus sign in this equation indicates that mass flows down the concentration gradient, from regions of high concentration to regions of low concentration. If J and C are both expressed in terms of same unit of quantity, for example, gram or gram molecules, then D is independent of the unit and has dimensions length 2 time -1, for example, cm 2 sec -1. Fick s first law assumes that the concentration gradient is independent of time. However, during a real diffusion process, the concentration of the diffusing species within the host solid is changing with time at any given position. By considering the mass-balance of an element of volume it is easy to show that the fundamental differential equation of diffusion takes the form

23 10 C C C ( D ) + ( D ) ( D ) C t = + x x y y z z (2.2) This is known as Fick s second law of diffusion. Frequently, diffusion occurs effectively in one direction only, that is, there is a gradient of concentration only along the x-axis and D is a constant. In such case, the equation reduces to C t = 2 C D 2 x (2.3) On studying the nature of diffusion process of gases, it is usually assumed that the concentration of gases is proportional to the partial pressure around the tested sample. If this assumption is combined with Fick s first law to describe the diffusion process of free gases or liquid, the relationship that follows forms the well-known permeability equation 2 J = DS P1 P L (2.4) where J is the flux, amount of gas passing through unit area per second, D is the diffusion coefficient, S the solubility coefficient and P 1 and P 2 the partial pressure of the permeating gas at the two sides of the membrane of thickness L. The product of D and S is termed the permeability constant P = D S (2.5) The permeability equation has been applied in the study of the permeability of polymers. Under correct boundary conditions, both the solubility and the diffusivity can be determined from the permeability measurements if the time taken to acquire the steady state of permeation is also measured. The so-called time-lag method for determination of the diffusion coefficient and the solubility coefficient for gases in polymers is based on this equation. Equation (2.5) is rigorously correct only when the following conditions apply: (1) diffusion must be truly unidirectional, there exists a concentration gradient

24 11 along the x-axis only; (2) membrane microstructure is homogenous and isotropic; (3) equilibrium is established between the gas phase and the polymer surface--this means that the attainment of sorption equilibrium is much faster than the rate of diffusion, which has been accepted by most investigators and the kinetic theory of gas has shown this assumption is reasonable; (4) diffusion has reached steady state; (5) Henry s law is applicable, the solubility coefficient is not a function of concentration. Fick s law is just a phenomenological description. It does not give any information about the mechanism of diffusion for small molecules through the host materials. Fortunately, detailed studies into the mechanism of the diffusion of small penetrant molecules in polymers show that the mechanism is much simpler with gases, such as nitrogen, carbon dioxide and oxygen. The diffusion process is also much less complicated with gases. In general, Henry s law is obeyed and the diffusion coefficient is independent of concentration. For these reasons gases are preferred for studies into the mechanism of sorption and diffusion and on the effects of polymer structure and morphology and other features. However, the diffusion mechanism is much more complicated with water vapor. Vapors are complicated in that their solubility is not directly proportional to the pressure, that is, Henry s law is not obeyed. Furthermore, the diffusion coefficient is often highly dependent on the concentration of penetrant in the polymer. This means that the diffusion coefficient cannot readily be obtained from the test. Thus, the factors that affect the transport process have to be considered in order to understand the mechanism of permeation of water in crosslinked epoxy resins.

25 Factors on the Permeation in Polymer Materials The various factors affecting the diffusivity and sorption can be discussed in terms of the nature of the gas and nature of the polymer. Diffusion of gases in polymers is free from the complications due to concentration dependence of diffusion coefficient. The simple mechanism of gas in polymer is mainly because of the very low concentrations encountered at all normal pressures, and it also reflects the comparative lack of interaction between simple gases and the polymer. Many of the simple functional relationships found for the permeation and diffusion of gases are due to this lack of interaction. In addition, the effects of various modifications of the polymer should be discussed, such as crosslinking agent, stoichiometry of the mixture, molecular packing, polar functional groups in cured epoxy resin, and so on Effect of Testing Temperature The effect of temperature on the diffusion coefficient is an activated process obeying the relationship similar to the Arrhenius relation: D D e = 0 E d RT (2.6) where D: diffusion coefficient D 0 : the pre-exponential factor E d : activation energy of diffusion The glass transition temperature is a critical point of polymers. The activation energy is found to be dependent on whether the testing temperature is above T g or below T g.

26 13 Meares[46] gave the systematic study of the effect of temperature on the diffusion of gases in the amorphous polymer poly(vinyl acetate). From the Arrhenius plot of ln D vs. 1/T, it was shown that there is a distinct change of slope at the glass transition with a lower energy of activation for polymer in the glassy state[46-48]. There is a less welldefined intermediate region near the glass transition point with a smaller slope than above or below it. To explain the differences above and below the glass transition, it was argued that there was a considerably smaller jump length in the glassy state. The entropies of activation, like the energies, were also less. Above the glass temperature there is greatly increased segmental mobility allowing for a greater zone of activation, leading to the increase both in energy and entropy of activation. Sole et al.[49-51] related the activation energy of diffusion with the combination of hydrogen bonding energy and activation of the gamma relaxation process. In studying water diffusion in cured epoxy resin, they considered the concerted efforts of the specific hydrogen bonds and the molecular relaxation motions governing the transport process. Generally, hydrogen bonds have bond energies in the range of kj/mol and the activation energy for the onset of the gamma relaxation, which they assumed the major relaxation factor to govern the kinetics of transport, is approximately kj/mol. If the kj/mol necessary to overcome the hydrogen bonds is added to this onset activation energy, the resulting value is very consistent with the observed kj/mol activation energies of the diffusion process. By comparing the diffusion activation energy with the sum of gamma relaxation activation energy and hydrogen bonding energy, they concluded that it is the gamma relaxation and water-epoxy interaction that control the mechanism of diffusion of water through epoxy resins.

27 14 An elegant discussion of effects of the relative size of the average cavity volume in the polymer and the size of the penetrant molecules has been presented by Frisch[52]. When the size of penetrant is much less than the average hole size in the polymer, it is suggested that diffusion occurs by localized activated jumps from one pre-existing cavity to another. The activation energy involves the hindered rotation of a few contiguous monomer segments with perhaps some bond stretching to allow the passage of penetrant. As the size of the penetrant increases or the hole size decreases, this mechanism may cease to be dominant since there will be fewer cavities available to accommodate the penetrant molecules. Much larger numbers of monomer segments will therefore need to move for the penetrant to pass from one cavity to another. This process will become more dependent on the macroscopic free volume of the polymer. This argument can be used to explain the change of activation energy with temperature, especially the changes below and above the glass transition temperature. Since the rate of change of free volume with temperature changes at the glass temperature, a change in activation energy will be observed. Solubility of a mobile component in a solid can be described phenomenologically as the distribution of the diffusing component in the polymer matrix. Even in the absence of specific interactions, initial mixing will occur due to gain in entropy and as a result of the van der Waals forces operative between the components. The dependence of the solubility coefficient, S, on temperature of a given polymer-penetrant system can be described in the form of a van t Hoff relationship[47, 48, 53, 54] S S e H s RT = 0 (2.7)

28 15 where S: solubility coefficient S 0 : pre-exponential factor H s : enthalpy of solution of the penetrant Thermodynamic analysis of the temperature variation of the sorption isotherm is helpful in investigations of the physical state of the sorbed water. The sorption process may be considered[55] to consist of two separate thermodynamic processes: (1) condensation of the gas, an exothermic process, and (2) mixing of the condensed gas with polymer, an endothermic process or exothermic process. In other words, the enthalpy of sorption is equal to the sum of the enthalpy of condensation and the partial molar enthalpy of mixing. The heat of sorption is defined[56] by H s = H m H g (2.8) where H m is the heat of mixing for liquid water and H g is the heat of condensation of the vapor phase. The characteristic features of the data for epoxy resins are large negative values[50, 51] of H s. On the basis of the thermodynamic data and other measurements, it appears reasonable to distinguish between at least two forms of the sorbed water. The first form represents that water sorbed at low equilibrium weight gain with little perturbation of the matrix and appears to be attached firmly to specific sites in the amorphous regions and the interfaces for heterogeneous matrix. The second form is the water sorbed at higher equilibrium weight gain usually with change of the matrix, such as swelling of the matrix. The structure of liquid water itself is relatively complex and it is perhaps not so surprising that the physical state of this fraction is not clearly established. For plastics and elastomers[57], H s is usually much less exothermic, and for

29 16 ethycellulose, cellulose, triacetate, and polyethylmethacrylate, H s is close to the heat of condensation for water vapor[47, 48, 54]. With more polar polymers, the initial waterpolymer interactions are stronger and subsequent water-water interaction has less noticeable effect. Interaction of water with epoxy materials shows negative H s values. For example, the research result of diglycidyl ether of bisphenol A crosslinked with six different hardeners shows that the heat of solution of water is negative regardless the hardener[58]. For gases well above their critical point, such as nitrogen, oxygen, hydrogen, etc, at room temperature, the hypothetical value of the condensation heat would be expected to be very small and, therefore, H s is essentially determined by the H m [53]. The heat of solution for permanent gases is small and positive, and the solubility coefficient, S, increases slightly with increasing temperature. For the more condensable gases and vapors, such as water, H s is negative due to the contribution of the heat of condensation and the interaction between water and polar groups in epoxy resin. Therefore, the solubility decreases with increasing temperature. From the definition of permeability (Equation 2.5), it follows that P = DS = D e 0 Ed RT S 0 e H s RT = P e 0 EP RT (2.9) where, E p = E D + H s P 0 = S 0 D 0 : pre-exponential factor Therefore, the change of permeability with temperature is decided by two factors. One is the diffusion factor. Another is the solubility factor. Because permeability is not an independent variable, unlike diffusion coefficient and solubility coefficient, we will not give any further detailed discussion here. Just from the theoretical analysis based on

30 17 Equation (2.9), for some polymers, the permeability coefficient increases with temperature, while for others it decreases with increasing temperature Effect of the Penetrant In the permeation of non-condensable gases in polymeric membranes, equilibrium levels of gas sorption in the solid are low because interactions between solute molecules and the polymer are weak. The diffusion coefficient is independent of penetrant concentration. Application of Fick s law and Henry s law results in a transport equation for permeability P=DS. Because of limited polymer chain mobility, penetrant molecular size and shape are important parameters in determining the diffusion coefficient. Indeed, it is this capacity of polymeric membrane that discriminate between penetrants with subtle steric difference. A second important factor is the chemical similarity between penetrant and the polymer. The qualitative solubility rule of like dissolves like is obeyed. Thus, the gas with a solubility parameter close to that of the polymer material will tend to be more soluble, possibly resulting in higher fluxes. It is clear from the generally accepted picture of the mechanism of the activated diffusion process that larger holes need to be formed in the polymer for the diffusion of larger molecules. These will require a larger energy for their formation and hence the activation energy will be larger for the diffusion of bigger molecules and the diffusivity will be smaller. This is indeed found to be true.[52, 56] It was found[47, 59] that the energy of activation could be expressed as an intermolecular term and an intramolecular term. Consider the extremes of penetrant molecule diameter. If it is equal to or less than the dimension of the free volume, then no energy would be required to open space for the diffusion of the penetrant molecule and

31 18 the activation energy for diffusion would be zero. If the diameter of penetrant molecule is large compared to the free volume dimension and the segment length is much larger than the diameter of the penetrant molecule, then the intramolecular energy depends directly on the square of the penetrant molecule diameter and the intermolecular energy depends on the penetrant molecule diameter. The effect of molecular shape becomes much more evident when larger molecules are considered. It has been shown[59, 60] that, in the case of the linear hydrocarbons, the diffusion coefficient decreases with increasing number of carbon atoms in the chain and levels off to an essentially constant value after five carbon atoms, whereas the branched and cyclic compounds have much lower values of diffusion coefficient. As would be predicted from simple Raoult s law considerations, more easily condensable vapors are more soluble in a given polymer. This is borne out by the approximately linear relationship between the logarithm of the solubility and boiling point of gas or vapor[47, 54]. In strong contrast to gas permeation, polymer materials may be highly swollen by a penetrating liquid[61]. The sorbed volume can be 10 to 20% or even higher, compared to gas-polymer systems where sorbed volumes are very small in most cases[55]. Thus, liquids open up the structure, with the result that the absolute flux rates through the sample can be 2 to 3 orders of magnitude higher for a liquid than for a non-condensable gas. Five types of common solvents, including benzene, toluene, ethyl benzene, methyl acetate, and methyl ethyl ketone, in polystyrene and poly(methyl methacrylate) have been used to evaluate the compositional and temperature dependence of the diffusion coefficient.[61] A mathematical model was proposed for predicting solvent self-diffusion

32 19 coefficient based on free volume theory. It considered the plasticization effects induced by small molecular solvents to correctly estimate the hole-free volume variation. The water molecule is relativly small and in the liquid and solid states is strongly associated through hydrogen bonding formation. This combination of features distinguishes it from the majority of organic penetrants. Whereas the diffusion coefficient generally increases with concentration for an organic vapor, marked decreases have been observed with water in several polymers. Values for the enthalpy of formation of the hydrogen bond in the range 3.4 to 6.6 kcal/mol have been obtained.[62] As a result, strong localized interactions may develop between the water molecule and suitable polar groups of the polymer; on the other hand, in relatively non-polar materials, clustering or association of sorbed water is preferred. As the chemical potential of the water vapor at unit relative pressure is the same as that of the liquid, permeation may be expected to be independent of the physical state of the penetrant. It would appear that differences between liquid and vapor permeabilities are largely due to experimental difficulties in maintaining the vapor phase at unit relative pressure. Real differences may arise if soluble material is extracted from the membrane or if thermal equilibrium is not established throughout the system. On the other hand, water permeability varied with vapor pressure for several polymers.[3-7, 53, 63] Water molecules combine the tendency to cluster, craze and plasticize the epoxy matrix with the characteristic of easy diffusion in the polymer. The water population[3-6, 9, 10] may be divided into three groups: (1) forming a polymer-water solution; (2) absorbed on hydrophilic sites; (3) adsorbed on the surface of free volume elements or microvoids.

33 20 Water clustering[1] has been directly observed on various size scales and has been characterized by several independent techniques such as infrared and dielectric measurements. Intermediate clustering may involve development of opacity in a polymer sample sufficient to scatter visible light from the resulting penetrant complexes. Barrer et al.[48, 64] applied Zimm s relationship between the cluster integral G 11 and volume fraction of the penetrant ν 1 G v γ 1 ( ) 1 11 = φ 1 2 a1 P, T where φ 2 is the volume fraction of the polymer, a 1 is the thermodynamic activity of the water, γ 1 is the activity coefficient of water. For an ideal solution γ 1 is unity. When γ 1 is invariant with concentration as, for example, in the case of an ideal solution, G 11 has the negative value of one molecular volume. This means that a particular water molecule in such a system excludes its own volume to other molecules but otherwise does not affect their distribution. For an athermal polymer solution, γ 1 decreases with an increasing in the water volume fraction so that G 11 / ν 1 is greater than 1 and may actually become positive. This means that the concentration of water molecules is higher than the average concentration around the water molecules. In other words, water molecules cluster together. The first water molecule to enter a polymer structure loosens it and makes it easier for subsequent molecules to enter there. Therefore, G 11 / ν 1 is a clustering function that measures the tendency of sorbed molecules to cluster. (1) G 11 / ν 1 =-1 represents ideal solution. The first molecule excludes one molecular volume to other molecules; (2) G 11 / ν 1 =0 represents sufficient clustering to only overcome the excluding effect of an isolated water

34 21 molecule; (3) For G 11 / ν 1 >0, like molecules tend to cluster, the water molecules prefer to stay together within the polymer; (4) G 11 / ν 1 <-1 means that the water molecules tend to segregate away from each other. A detailed analysis[65] of the sorption equilibrium and diffusion of water vapor under different activities through an unsaturated polyester resin (UPR) was conducted. By differential permeation and microgravimetry techniques, the Zimm-Lundberg approach was used to determine the mean cluster size in the UPR film. It showed a decrease in the Flory interaction parameter when the water activity increased. A decrease in the Flory interaction parameter indicates a larger increase in the water amount absorbed by the polymer with the water activities than that given by the Flory equation. Since the latter is based on a random distribution of the sorbed molecules throughout the polymer volume, the additional sorbed molecules must cluster to already sorbed molecules. The mean cluster size derived from the Zimm-Lundberg cluster integral increases with water activity, especially at high water activities. Starkweather and Merten[66] and Stannett et al.[64] calculated and compared the clustering functions of water for a number of hydrophilic polymers. They all observed that at low relative humidity for water-protein system, the cluster function was strongly negative. However, as the relative humidity approached saturation, these values increased rapidly, becoming positive near saturation. This means that the initial water molecules are sorbed on specific sites, and only when these are filled at higher water concentration does clustering occur. Williams et al.[67, 68] have attempted to ascertain the clustering behavior of penetrant molecules in polymer systems obeying the Flory-Huggins relation. By using the

35 22 liquid lattice approach to treat the random mixing of a disoriented polymer and a solvent, the so-called Flory-Huggins theory is often used to correlate the diffusant activity and the composition of the solution ln a = ln(1 V ) + V p 2 p + χv p where α is the solvent activity, Vp is the polymer volume fraction and χ is the solvent polymer interaction parameter. If the solvent concentration is very small, as in the case of gas sorption, the polymer volume fraction is close to one. The Flory-Huggins relation becomes a s = V e 1+χ constant k d by Thus we can relate the Flory-Huggins interaction parameter χ to the Henry s law k d = e ( 1+ χ ) A relationship was demonstrated between the Zimm-Lundberg cluster integal and the variation of Flory-Huggins interaction parameter χ. This serves as another method of estimating the clustering tendency for a given polymer system. The quality of solvent, expressed as the strength of the polymer solvent interaction, is characterized by the Flory-Huggins parameter. This parameter χ represents the interaction energy per solvent molecule divided by kt. The quantity χkt represents the difference in energy of a solvent molecule immersed in the pure polymer, compared with one surrounded by molecules of its own kind, that is, pure solvent. The correlation between χ and the solvation power of solvent for a given polymer has been rated as follows: (1) if χ<0, there is strong interaction between polymer and solvent, and the water

36 23 will not cluster; (2) if χ is between 0 and 0.5, it is a good solvent; (3) if χ is between 0.5 to 1, it is a borderline solvent; (4) if χ>1.0, the water tends to be in the cluster state. The familiar Flory-Huggins relation for solvent chemical potential in solution µ 1 is given by[55] 0 ( µ µ ) 2 ( ) ln 1 ln 1 1 χφ RT = a = φ + r φ2 + 2 where a 1 : thermodynamic activity of solvent ϕ 1 : volume fraction of solvent in the polymer ϕ 2: volume fraction of polymer r: number of segments in the polymer chain For large values of r, the above equation becomes ln a 1 = lnϕ 1 + ϕ 2 + χϕ 2 2 An application of this clustering function was demonstrated by considering experimental data for several solvent-polymer and gas-polymer systems. For the sorption of water by hydrophilic polymers, the curve shape of sorption versus activity of water was concave downward at low relative humidity. For moisture absorption of epoxy samples, the relationship between maximum moisture content and relative humidity at high relative humidity is not linear[69]. Clustering functions therefore provide an easily understood, unified treatment of the thermodynamics of sorption[55, 65, 70, 71]. The sorption of water and methanol into poly(ethylene terephthalate) was analyzed and compared by the FTIR-ATR method[72]. The two systems were shown to have very different sorption kinetics. The diffusion of liquid water into PET was shown to follow Fickian kinetics with a decrease in the diffusion coefficient with increasing

37 24 crystallinity. No significant evidence of swelling was observed. There is evidence that the relationship between diffusion coefficient and crystallinity is non-linear, and it is implied that the size of the spherulite within the polymer matrix plays an important role in the rate of diffusion. The diffusion and sorption of methanol into PET follows non-fickian or anomalous kinetics. Sorption of methanol is accompanied by swelling, the rate of which is directly related to the diffusion rate Effect of the Polymer In order to understand the mechanisms of transport in various polymers of interest, it is useful to consider features of two principal micro-structural conditions of polymeric materials: the glassy and rubbery states. In the glassy state, a polymer is hard and may be brittle. Rotation about the chain axis is limited and motion within the structure is largely vibratory within a frozen quasi-lattice. Polymers of this type are very dense structures, with very little internal void space (about 2 to 10%). Hence, it is not surprising that penetrant diffusivities through such a structure are low. In contrast, polymers in the rubbery state typically are tough and flexible, with properties associated with free chain motion. In this case, larger segments are thought to participate in the diffusion process due to internal micro-motion of chain rotation and translation, as well as vibration. Basically, then, a larger amount of free volume in which diffusion may take place is more readily accessible. In the realm of semicrystalline polymers, it has been found that the crystalline microphase plays an important role in the transport properties of a film.[55, 59, 70, 71, 73] The presence of crystallinity can be explained by the two-phase model. The morphology of the semicrystalline structure consists of impenetrable crystallites

38 25 dispersed in a rubbery amorphous matrix[59]. Knowing what this microstructure is and how it can be modified by suitable thermal, mechanical or solvent treatment may make possible the development of materials with special permeation properties which are advantageous for a given application. In both glassy and rubbery states, the transport properties can be modified by the presence of a crystalline phase[59, 73] or by stress-induced orientation[73-79]. Both crystalline phase and orientation tend to place additional constrains on the mobility of the amorphous phase through which diffusion takes place. The presence of crystallites in a polymer reduces the effective cross-sectional area for diffusion, increases the effective path length, and may also impose restraints on the amorphous phase. These media are essentially heterogeneous. One of the simple models[56, 73] is as follows P = P a v a κ where P a is the permeability coefficient of the amorphous phase and υ a is the volume fraction of the amorphous phase. The structural factor κ is a function of υ a. Furthermore, crystallinity decreases permeability not only by reducing the volume of amorphous material available for flow but also by creating more restricted pathways for the diffusing molecule. Thus, the diffusion coefficient was found to obey the relationship[59] D = * D τβ where D, the diffusion coefficient in the partially crystalline polymer, is reduced from what it would be in the completely amorphous polymer, D*, by a geometric impedance

39 26 factor τ and a chain immobilization factor β. The latter parameter was initially designed to account for the crosslinking action of the crystallites on segmental mobility. Many attempts have been made to correlate the diffusivity of gases with the nature of the polymer.[79] [80]Unfortunately, it is impossible to change one feature of the polymer without affecting others. It is clear[79] from the hole theory of diffusion that the rate of diffusion will depend on the number and size distribution of pre-existing holes and the ease of hole formation. The former will depend on the ease and degree of the packing of the chains and is related to the free volume and density. The ease of hole formation will depend on the segmental chain mobility, that is, the chain stiffness, and on the cohesive energy of the polymer. These features are also reflected in the coefficient of thermal expansion and in the glass temperature. In addition, the degree of crystallinity and of crosslinking, as well as additives such as fillers and plasticizers, will affect the diffusion process. In general, the more polar groups present in the polymer matrix, the higher its sorptive affinity towards water[81]. However, the accessibility of these groups, the degree of crystallinity of the matrix, and the relative strengths of water-water and waterpolymer bonds are important factors in deciding the total amount of sorption and tend to rule out any simple correlation between the number of polar groups and the solubility. In one study,[82] the values of the diffusion coefficient of water in the polar and non-polar polymers was tested. The polymers were chosen in such a way that they represented different types of polymers, including nonpolar polymers, such as low density polyethylene, polypropylene, polar polymers, such as poly(ethylene glycol tetrephthalate) polyamide 6 and polyamide 12, and copolymers that contained both polar and non-polar

40 27 monomer units. The values of the diffusion coefficient of water in the dry polar polymers were smaller than those in dry polyolefins, but the opposite behavior was found for the permeability because water sorption was much more favorable in polar polymers than in hydrophobic polyolefins. The effect of different stoichiometric ratios on water absorption of diglycidyl ether of bisphenol A cured with stoichiometric variations of triethylenetetramine (TETA) was reported.[83] Analysis of water absorption characteristics of these materials showed the existence of water in two different environments: molecules bound to specific sites in the matrix and molecules clustered in microvoids. Increasing the amine ratio in an epoxy resin has the effect of increasing the rate of water absorption and increasing the equilibrium water uptake. Analysis of the dielectric results at low and high frequency over a range of stoichiometric ratios shows the extent to which the water molecules are bound to the resin or exist as free water. As the amount of amine is increased, the amount of free water increases, which could indicate more free volume in the system. The moisture diffusion process of EPON 828-PACM 20 epoxy system was studied[84, 85] as a function of epoxy-amine stoichiometry and the resulting microstructure. Amine-rich samples generally exhibited Fickians diffusion. Diffusion rates for epoxy-rich and stoichiometric samples changed during absorption such that Fickian diffusion was not obeyed. Differences in diffusion behavior were related to the relative importance of diffusion through the low-density and high-density microstructural phases for different stoichiometries. The changes in saturation level with stoichiometry were explained by competing effects of free volume versus the content of the low-density phase. Increasing the humidity level caused a corresponding increase in saturation level,

41 28 while increasing the temperature caused more pronounced non-fickian behavior. The effects of absorbed moisture on the thermo-mechanical properties of the epoxies were also investigated. Reduction in the glass transition temperature is larger for amine-rich samples than for epoxy-rich and stoichiometric samples. Also from Arrhenius relationships between temperature and diffusivity, activation energies for diffusion were observed to be higher for amine-rich samples than epoxy-rich and stoichiometric samples. This effect was related to the effects of temperature on the relative rates of diffusion through the low density and high-density phases. Saturation levels increased monotonically with an increasing amine content from 20 to 70 parts per hundred (pph) amine but decreased from 14 to 20 pph amine. Changes in the D 0 were dominated by changes in the saturation level. The activation energy increases in almost a quantized fashion with increasing amine content. Reductions in the glass temperature, due to moisture absorption and moisture-induced swelling strains, were measured after hours of exposure of the samples to the three different conditioning environments. These reductions in Tg ranged from 5 to 20 C and were generally larger for amine-rich samples than for epoxy-rich and stoichiometric samples. After desorption, Tg values returned to within ±2 C of their original dry values, except for stoichiometric samples for which values were consistently 3 to 5 C higher than initial dry values due to postcuring reactions occurring during the first DMA tests. Apicella et al. reported[3-6, 9, 10] the relationship between moisture sorption behavior and plasticization of two epoxy systems and hygrothermal and processing variables. The two systems were a low crosslinked epoxy diglycidyl ether of bisphenol A (DGEBA) cured with linear amines and triethylenetetramine, and a highly crosslinked

42 29 system of tetraglycidyldiaminodiphenylmethane (TGDDM) cured with diaminodiphenol sulfone (DDS). Increases in the equilibrium moisture content were observed as a result of thermal cycling in liquid environments. The hygrothermal interactions produced changes both in the epoxy network structures and in the observed moisture sorption behavior. The intrinsic moisture sensitivity arose from the chemical characteristics (hydrophilic groups) as well as from microscopic and macroscopic defects (heterogeneous network structures and fillers). The modes of absorption of the water were found to alter the glass transition temperatures of these two classes of thermosets: free volume changes are principally responsible for the plasticization of the low cross-linked DGEBA resin, while waterpolymer bond formation and polymer-polymer intramolecular hydrogen bond rupture are responsible for the Tg depression in the highly crosslinked TGDDM-DSS systems Effects of Water on Glass Transition The addition of plasticizer to a polymer decreases the cohesive forces between the chains, resulting in an increase in segmental mobility. It is clear that this should result in an increased rate of diffusion and lower activation energy. The function of a plasticizer is to reduce the intermolecular forces which hold the macromolecular structure together. Primary plasticizers normally contain polar groups which reduce or neutralize the field of force of the polymer polar group; secondary plasticizers, which often contain no polar group, are likely to plasticize by pushing apart the chain molecules, thus reduce the secondary intermolecular force. Since the polar groups are not affected, there will be a tendency for chain segments to approach each other, thus squeezing out the secondary plasticizer.

43 30 The study of the effect of the water vapor sorption in polymers on the glass transition temperature forms an important branch of research. Exposure of glassy polymers to high water activity has been shown to plasticize the materials and lower their glass transition temperature. The ability of water molecules to form hydrogen bonds with other water molecules and/or polar groups in the polymer gives rise to plasticization. It further affects the behavior of the sorption and diffusion of water in the polymer. The diffusion coefficient may increase with increasing concentration for hydrophilic polymers or vary inversely for hydrophobic polymers. Epoxy resins absorb moisture from humid environments. This moisture absorption can be attributed largely to the moisture affinity of specific functional groups of a highly polar nature in the cured epoxy resin. Water has been detected in neat epoxy resins and epoxy composites using NMR spectroscopy.[86-92] Since the NMR line width decreases with increasing molecular mobility, the broadening of the water signal in epoxy resins indicates that water is hydrogen-bonded to the resin. Its mobility in epoxy resins is between the value in the solid and free water states. Jelinski et al.[88, 93] investigated the nature of epoxy-water molecule interactions using NMR spectroscopy. From the experiment results, they gave some evidence of the water in the polymer: (1) the water in the epoxy resin is impeded in its movement; (2) no free water exists in the epoxy resin matrix; (3) there is no evidence for tightly bound water; and (4) it is unlikely that the water disrupts the hydrogen-bonded network in the epoxy resin. The water molecules migrate from site to site, but the jumping motion does not involve a specific hydrogenexchange mechanism. Browning[94] proposed that absorbed water molecules can be combined with functional groups of a highly polar nature in cured epoxy resins, such as

44 31 the hydroxyl groups or the amine groups. Using FTIR spectroscopy, Antoon et al. [95] showed that the frequency of the in-plane bending mode of sorbed water in epoxy resins lies between its frequencies in liquid and free gaseous water, another indication of hydrogen bonding. Using dielectric experiments,[96-99] the interaction of water molecules with the polar groups was investigated. The water does not seem to bond to polar groups in the matrix in some results. However, some results have given evidence of interaction between water molecules and polar groups in the resin. It is known that the glass transition temperature of polymers is lowered if a compatible liquid is dissolved into the polymer. The susceptibility of epoxy resins to substantial changes in properties on exposure to humid environments is manifested in particular by a large depression in measured glass transition temperature. Several models for explaining the depression of the glass transition temperature have been proposed. For the free volume model, the glass transition temperature of a dilute system is determined by the dilute volume fraction and the changes of the thermal expansion coefficient Tg = V α T V α p p p gp p Vdα + + d V α T p d V α p d gd + V α d d (2.14) where the subscripts p and d refer to the polymer and the diluent respectively, α is the thermal expansion coefficient, and V is the volume fraction of the polymer or the diluent. The thermal expansion coefficient of water is about o C -1, and the Tg of water is about 4 C. Ellis et al.[100]extended the classical thermodynamic treatment of compositiondependent Tg and gave the equation for the epoxy-water systems. This model has been suggested by applying classical thermodynamic treatment to describe the compositional

45 32 dependence of the glass transition temperature in miscible blends and further extended to the epoxy water system. The plasticization induced by water sorption can be described by theoretical predictions given by T g = T gp C p p X p C pp X p + C pd X d + T gd C pd X d C pp X p + C p d X d (2.15) where X is the weight or mole fraction and Cp is the incremental change in the specific heat at the glass transition. For diglycidyl ether of bisphenol A (DGEBA) and tetrglycidyl-4,4 - diaminodiphenylmethane (TGDDM) systems, Ellis et al.[100] showed that the depression of Tg in a number of epoxy-water samples was in good agreement with theoretical predictions based on equations developed for the composition dependent Tg in polymer diluent systems. The depression of Tg in the stoichiometric compositions shows very close agreement between theory and the experimentally-determined depression of Tg. The investigation of the amine-rich and epoxy-rich non-stoichiometric composition resins shows smaller depression than stoichiometric samples. These studies on epoxy resins with a relatively low Tg have shown that the plasticizing power of water is in complete accord with theoretical predictions and as such does not place water in special category separate from other plasticizers. However, the formation of polymer-water intermolecular bonds and the rupture of polymer-polymer intra-molecular hydrogen bonds, which stiffen the polymer network, strongly depress the glass transition temperature. The glass transition temperature depression observed for the tetrglycidyl-4,4 -diaminodiphenylmethane (TGDDM) and amine crosslinking agent DDS epoxy systems containing 50 PHR of DDS is 20 C greater than that of the resin cured with 20 PHR of hardener even if the same amount of

46 33 water was absorbed at equilibrium. However, the system of intermediate composition, 30 PHR, showed the lowest plasticization, although it sorbed almost the same amount of water[3-6, 9]. The anomalous effect may be considered from the influence that the molecular solution and hydrogen bond formation may have in system of different chemical and physical characteristics. As indicated before, plasticization may depend on the dilution process as well as on rupture of the polymer-polymer hydrogen bonds. However, while the former is governed by the dilute volume fraction, the second depends on the effective concentration of the hydrophilic sites. The occurrence of two mechanisms of plasticization that can be inversely influenced by increase of the DDS concentration, namely decrease of available free volume and increase by hydrophilic sites, determined the lower depression of the glass transition observed for the system of intermediate composition. The strong influence of the prepolymer composition on the water solubility and plasticization may also be related for this system to the increased number of unreacted amines which are able to bond to the water molecules. The calorimetric studies of water-polymer systems showed that complex thermal behavior is often observed for the water in strongly interacting systems. Lee et al.[45] noted the absence of solid-liquid phase transitions of water in a crosslinked network. This effect is generally attributed to the immobilization of water molecules within the polymer. The water that becomes bound to the polymer does not aggregate sufficiently to permit observation of normal phase transition. Various authors[3, 7, 12, 13, ] have sought to quantitatively correlate the observed depression of glass transition temperature to the amount of water sorbed through free volume calculations and special

47 34 interactions. However, the validity of these methods becomes questionable for different polymer and diluent systems Mechanism of Water Absorption The behavior of diffusing moisture is usually affected by the composition, crosslink density, and processing procedure (curing temperature, time, cooling rate). Apicella et al.[3-6, 9, 10] proposed that there are three modes of sorption: (1) bulk dissolution of water in the polymer network; (2) moisture absorption onto the surface of voids which define the excess free volume of the glassy structure; and (3) hydrogen bonding between polymer hydrophilic groups and water. If the first two modes occur consecutively, a dual sorption behavior can be detected.[48, 71] At low activities, sorption of gasses and vapor is usually described by a superposition of Henry s law and Langmuir isotherm. The Henry s term is generally attributed to molecular solution of the penetrants in the glassy matrix, while the Langmuir sorption mode occurs as result of the insertion of the sorbed molecules into a finite number of preexisting gaps in the polymeric matrix.[48, 110, 111] At high activities, however, vapor sorption often involves strong positive deviations from Henry s law, which cannot be simply explained in terms of the Flory-Huggins solution theory, but conveniently interpreted by considering the tendency of molecules to cluster.[4] The intrinsic moisture sensitivity of epoxy resins is traceable directly to the molecular structure. The presence of polar and hydrogen bonding groups, such as hydroxyls and tertiary nitrogen, provides the chemical basis for moisture sensitivity, while the available free volume and nodular network structure represent the physical basis. The evidence given by the broad line NMR analysis indicates that the plasticization

48 35 effect of water on a cross-linked epoxy may also be related to the strong interactions between the dissolved molecules and segments or groups of the polymer, although the exact sorption sites to which the water may be bonded are still uncertain. Adamson[1] postulated that the transport of moisture below Tg is a three-stage process in which the absorbed water first occupies the free volume present in the form of voids. In the second stage, water becomes bound to network sites causing swelling. Finally, it enters the densely crosslinked regions. However, Barrie et al.[111] included the possibility of clustering of water molecules at high activities. Mechanisms based on free volume neglect the existence of specific interactions between water and hydrophilic sites of the network. It has been proposed that the water equilibrium concentration is mainly governed by available free volume[13, 111] or that water molecules essentially occupy microvoids and other morphology defects.[2, 5, 6, 11] Reding, Hara, Hoorn[97, 109, 112] found that water absorption depends on the amount of free volume in the epoxy networks. They showed that the contribution of polar groups in terms of their hydrogen bonding capabilities is reflected by the effect of metachloro, bromo, and methyl substitutes on the water absorption of modified TGDDM epoxy resins cured with DDS hardener, while substitutes in the ortho position adversely affect the hydrogen bonding capability of amine groups and limit the extent of reaction by steric interference. Also, by using O-glycidyl resin systems cured with various amounts of DDS hardener, they proved that free volume plays an important part in determining the level of water absorption. The interpretation based on interaction is the result of NMR and FTIR spectroscopic observations[mckague, 1978 #176; Lachenal, 1999 #181; Morgan, 1977

49 36 #183; Ngono, 1999 #184; Ngono, 2000 #207; Jelinski, 1983 #210; Sammon, 2000 #236; Fieldson, 1992 #338] and suggests that water molecules are linked by strong hydrogen bonds to some hydrophilic groups, mainly hydroxyl or amine groups. The contributions of these groups could be interdependent. Bellenger et al.[113, 114] suggested that water absorption is essentially linked to the concentration of polar groups in the cured resin matrix. However, it decreases with increasing extent of intra-molecular hydrogen bonding. Woo[115] found that water does not appear to be bound to polar groups in the resin, or on hydrogen bonding sites, although dielectric tests indicate that it does not behave as free water, since its polarizability is much reduced. They also observed that the decrease of effective dielectric constant of water is only about 55-77%, which indicates that there is some clustering of the water molecules in the epoxy resin, rather than complete separation. In addition to polar interactions, chain arrangement in the network must be considered in a discussion on the absorption process. Chain arrangement refers to the spatial configuration of the molecular segments in the network. As discussed in Section 2.2.3, different configuration can result in more open structure, which should influence how much moisture is absorbed. For example, the molecules of a polymer crystal pack together very efficiently, and leave very little room for penetrant molecules. Consequently, crystalline polymers absorb very little water. In polymer glasses, such as epoxy, the packing is far less efficient, and there is ample room for small penetrant molecules, such as water, to occupy. This is another aim of the present study: to seek the effect of liquid crystalline mesophases on the sorption and diffusion properties of epoxy systems. The essence of this idea of openness is captured by the free-volume arguments.

50 37 Researchers have tried to relate both the equilibrium uptake and the diffusion coefficient to the free volume content of the material. From a nano-scale point of view, the crosslink junctions are clearly vital for establishing the nanopore networks. The steric restrictions at crosslink junctions cause packing to suffer and increase the openness. The covalent bond restricts the arrangement of the epoxy and/or amine monomer in the threedimensional space. This covalent bond is directional in nature and the connection between epoxy and amine must conform to prescribed bond angles and length. This causes local packing density to decrease, leading to unoccupied space, or extra free volume, formation nearby crosslink junctions. Positron annihilation lifetime spectroscopy (PALS) can reveal much useful information of the atoms arrangement at the crosslink junction point. This size scale is on the order of 2 to 20 Å in diameter[50]. Free volume caused by steric hindrances at the crosslink junctions provides both unoccupied volume and polar sites that water molecules prefer to occupy. However, from an experimental point of view, free volume is a very elusive quantity, and such theories are based on assumptions. 2.3 Test Method Many techniques have been used to measure the transmission rate of low molecular-weight penetrants through a polymer membrane. In general, chemical methods are neither as convenient nor as versatile as physical methods. Chemical methods usually are discontinuous procedures which often suffer from lack of quantitative reaction between reactants. Most of the methods that determine the amount of penetrant which have passed through the membrane per unit time involve measuring the increase in concentration, or pressure, or volume, of the permeated substance as a

51 38 function of time. For gas permeability measurements, the most widely used methods are the PVT techniques where the volume at constant pressure or the pressure at constant volume of the transmitted penetrants is measured as a function of time and temperature. For the PVT method, polymer samples are initially completely free of penetrant by using the high vacuum system. At time zero, a known penetrant pressure is introduced to one side of the membrane, and the pressure increase in a constant-volume receiving section is followed as a function of time. The apparatus can be modified for use with condensable vapor. Instead of measuring the increase in pressure in a constant-volume receiving section, the increase in volume of transmitted gas against a constant pressure can be determined. This general method is used for several of simpler permeation apparatus which are widely employed for quality-control Transmission Rate Methods (Time Lag Technique) The time lag technique has been applied successfully to a wide variety of polymer-gas systems. This method combines evaluation of steady state transport with the analysis of earlier transients. It allows a monitoring of diffusion from time zero until the diffusing species attains a constant permeation rate. Under most measurement conditions, the subject membrane is initially free of gas. Gas is then introduced on the upstream side and the accumulated amount of transported species is measured on the downstream side. The necessary experimental conditions are an initially gas free film, provisions for the equilibration at the inlet gas-polymer interface, and essentially zero concentration of gas held at the polymer outflow face. The first condition is assured by evacuation of the thin film prior to introduction of the test gas, the second has been experimentally checked for many polymer-gas systems through sorption isotherms, and

52 39 the third condition is automatically satisfied because of the extremely slow rates of permeation encountered experimentally. Its importance lies in the fact that the two quantities permeability P and diffusion coefficient D can be evaluated from a single experiment. Sorption coefficient S can be deduced from Equation 2.5. Under above boundary conditions, using a constant value for D, the solution of Fick s law can be solved as the relationship Q t = DC l n= 2 n ( ) 2 2 l 2lC1 1 Dn π t [ t ] 2 2 ( 2 ) 1 exp 6D π n l n= 1 (2.16) where: Q t is the amount of test gas passing through a film in time t; l is the film s thickness; C 1 is the concentration at the upstream face. When the steady state is achieved, t becomes large enough to make the exponential term negligibly small. The Equation 2.16 can be simplified to 2 l ( t ) DC 1 Qt = l 6D (2.17) A plot of Q t vs. t yields a straight line whose intercept θ on the t axis is represented by the equation θ = 2 l 6D (2.18) where: θ = time lag; l is the thickness of the membrane; D is the diffusion coefficient From equation 2.18, D can be determined. The slope of the steady state portion of the plot of equation 2.17 can be directly related to the permeability, P. Once the values for P and D are known, the solubility coefficient, S, can be calculated from P = D*S. Figure 2.3 is a typical example of time lag experiment where the early time segment is featured to illustrate the transient transport. The accumulated diffusive flux is plotted as a function of time. The initial curve shows the changing diffusion rate in the

53 40 unsteady states and the beginning of the trend toward linear relation becomes much more apparent at longer times. Linearity demonstrates constant flux conditions when steady state is reached. For more complicated sorption and diffusion mechanism than simple Fick s law diffusion of a single species, the time lag function may not be so readily obtained, nor will it always be possible to precisely calculate the time to reach steady state. However, an experimental time lag can always be obtained from its definition. While the numerical ratio between time lag and the time required to reach steady state varies from case to case, for each specific system there should exist a certain fixed ratio for purposes of approximation. Q t θ Time Figure 2.3 Time lag method

54 Sorption Method A fair estimate of the solubility of a penetrant in a solid can be obtained quite simply by the weighing bottle method or its variations. The sample is either completely immersed in the test liquid or is suspended in the vapor above the liquid surface. In the latter case, it is often possible to vary the vapor pressure of the penetrant by the addition of nonvolatile solutes to the liquid. More satisfactory procedures for determining the rate of sorption and de-sorption in solids by gravimetric methods are due to the development of the sorption balance by McBain and Baker. In its usual form, this balance consists of a sensitive helical spring suspended in an all-glass thermo-stated jacket. The spring extension with load is accurately calibrated using platinum weights at the temperature of the experiment. With a well-made spring, Hooke s law is obeyed over wide ranges of extension within the accuracy that the change in extension can be determined with a sensitive cathetomer. Polymer samples are mounted on a quartz spring microbalance and the change in mass is recorded as a function of time. The recorded weight is corrected for buoyancy to obtain the mass of the sample. This technique can be extremely accurate provided a sensitive microbalance is used. Quartz spring microbalances are difficult to apply to liquid sorption kinetics. Since the density of the liquid medium is close to the density of polymer, correction for buoyancy results in considerably less accurate values of sorbed mass. Instead, the socalled pat-and-weigh technique is used for liquid sorption. This involves immersing the polymer in the liquid penetrant and periodically removing the sample, blotting the surfaces to remove excess liquid, and then weighing the sample on a conventional laboratory balance. If the sample is too thin or the diffusion coefficient is too high, a

55 42 significant amount of penetrant may desorb during the time it is not immersed. This can be remedied by using thicker samples, but this can significantly increase the time required for a single experiment. The repeated handling of the sample can result in an erroneous rate of mass uptake. Volumetric determination of the amount of vapor sorbed or desorbed by the solid generally is not as versatile as the gravimetric method. It is not well suited for dynamic measurements and requires lengthy calculations and correction factors. Appreciable errors may arise from the adsorption of vapors on the wall of equipment and from the extreme sensitivity of volume to temperature fluctuations. However, it does have an advantage when sorption is quite small. A very large amount of the solid in the form of thin sheets can be used to obtain measurable sorption. In principle, high-pressure sorption experiments are nearly identical to those performed at lower pressure. An evacuated sample of polymer is pressured with gas and the solubility is calculated from the observation of initial and equilibrium pressure. The concentration of the sorbed gas in the polymer is determined from a mass balance on the gas phase. Sorption kinetics experiments are commonly used to determine diffusion coefficients for penetrants in polymers. For non-condensable gases such as methane, nitrogen and carbon dioxide, pressure decay techniques are often used to monitor sorption kinetics. For more condensable gases or vapors, gravimetric techniques that directly follow mass changes with time are used. Use of FTIR-ATR spectroscopy to characterize penetrant diffusion in polymers has been investigated[yi, 2000 #222; Sammon, 2000 #236; Illinger, 1980 #293; Fieldson,

56 #338; Snively, 1999 #408]. This technique utilizes quantitative Fourier transform infrared-attenuated total reflection spectroscopy to determine the amount of penetrant that has sorbed as a function of time. In addition to the sorption kinetics, a spectroscopic technique also provides information regarding the molecular state of the penetrant at various times in the transport process. The method can be applied to systems of thin polymer films with penetrants that are spectroscopically active, including multicomponent systems, provided each penetrant absorbs in a unique region of the infrared spectrum. Fieldson et al.[fieldson, 1992 #338] studied the diffusion of liquid water in polyacrylonitrile (PAN). The diffusion coefficients that were measured were in good agreement with those reported in the literature for high-activity water vapor in PAN. The test also demonstrated both the reproducibility and accuracy of FTIR-ATR experiments for measuring the diffusion coefficient. A model[56] selected for obtaining diffusion coefficients from the sorption studies is that of diffusion into a plane sheet. This model assumes that (1) the initial concentration of the gas in the polymer is uniform, (2) the surface of the polymer film is kept at constant concentration, (3) the amount of solute taken up by the polymer sheet is a negligible fraction of the whole. All these conditions are met at the beginning of any sorption experiment. For short times, the solution of the Fick s law can be approximated as M t = M 4 l Dt ( ) 0. 5 π (2.19) A plot of M t /M versus (t/l) 0.5 is initially linear and from the slope of 2(D/πl 2 ) 0.5, D can be evaluated.

57 Model for Heterogeneous Sorption and Diffusion For the simple liquid system, the motion of the molecule A may take place in many different ways. Suppose one of A s neighboring space is unoccupied for some reason, for example thermal fluctuation. If molecule A acquires sufficient thermal energy and moves in the proper direction during the time interval that the neighboring position is vacant, molecule A can move from its own position to a new equilibrium position in the adjacent position. Such a displacement gives rise to diffusive motion if another molecule jumps into the vacancy left by molecule A before A can return to its original position. On the average, a molecule has the jumping frequency f with the jumping distance of d per jump. The diffusion constant D is then related to the jumping frequency f and the jumping distance d by D = 1 2 f d 6 (2.20) It can be shown that the energy required for the formation of a hole of the size of the molecule in liquid is the same as the energy of vaporization per molecule. The energy barrier which the diffusing molecule must overcome to move from its original equilibrium position is usually small in comparison to the hole energy. Also, a molecule ordinarily vibrates about its equilibrium position at a frequency of to vibrations per second; a given molecule has many vibrations during the time of a diffusion jump. It is to be expected that if a hole is present, the molecule will have a sufficient number of vibrations in the proper direction during the time interval of existence of the hole to enter the hole. Therefore, the probability that a diffusional jump takes place is proportional to the probability that a hole of sufficient size is adjacent to the molecule. The hole or the

58 45 free volume exists in the system. The dimensions of these holes are not necessarily identical. The hole size distribution can be expressed by the Boltzmann function. In reality, there is fluctuation in the local density. Also, even if an individual hole may not be large enough to accommodate a diffusing molecule, the co-operative motion of several neighboring molecules may allow two or more holes to merge into one hole large enough for a diffusing jumping to occur. The concept of the co-operation of free volume is particularly important in understanding polymer materials. In fact, the idea that molecular transport in polymers occurs mainly through the co-operation of several degree of freedom was advanced many years ago by Barrer.[54] For polymers, the probability of the existence of a hole or free volume depends on the structure, morphology, and properties of the polymer. Much work has been done to set up the models of the diffusion in heterogeneous polymer systems Dual Mode Transport It was stated by Meares et al.[46] that a second mechanism of sorption may be implicated in the solution and diffusion behavior of small molecules in amorphous polymers. His investigations indicated that the glassy state contains a distribution of microvoids frozen into the structure as the polymer is cooled through its glass transition temperature. Free segmental rotations of the polymer chains are restricted in the glass state, resulting in fixed microvoids or holes throughout the polymer. It was also observed that the sorption isotherm curve showed characteristic curvature concave to the pressure axis. This suggested two concurrent mechanisms of sorption: ordinary dissolution and hole-filling.

59 46 The dual mode sorption model is now widely used to describe sorption behavior of polymers. This model describes the sorption mechanism in terms of one population of ordinarily dissolved gas or vapor which resides within the polymer matrix and is described by Henry s law. The second population is considered to occupy un-relaxed free volume within the polymer and is described by the Langmuir isotherm[70, 71]. All of the above pertains to microstructural conditions which are thought to prevail just below glass transition temperature. The equilibrium part of the dual sorption model[70, 71] is simply expressed by the following equation for the isotherm CH bp C = C D + C H = k D p + 1+ bp ' (2.20) where C: solubility, cc(stp)/cc polymer; k D : Henry s law dissolution constant; cc(stp)/cc*atm; b: hole affinity constant, atm-1; C H : hole saturation constant, cc(stp)/cc polymer; P: pressure, atm The first term, C D, represents sorption of normally diffusible species, while the second term, C H, represents sorption in microvoids or holes. The hole affinity constant, b, is related to the ratio of rate constant sorption and de-sorption of penetrant in the holes. When b*p is much less than 1, the sorption isotherm reduces to a linear form ' C = ( k D + C H b) p (2.21) At sufficiently high pressure, the microvoids become saturated and will no longer absorb additional penetrant. When b*p is much larger than 1, sorption in microvoids reaches the saturation limit, C H, and equation 2.20 again reduces to a linear form C = k p + ' D C H (2.22)

60 47 Thus, the dual sorption model predicts that an isothermal plot of C vs. p will consist of low-pressure linear region and high-pressure linear region connected by a nonlinear region. Based on this dual sorption, a model was set up to describe the transient sorption of penetrant in a polymer, called the kinetic part of the original dual sorption theory. The basic assumptions applied to the transport model are: (1) Two modes of sorption, Henry s law sorption and Langmuir sorption, occur simultaneously; (2) Local equilibrium between the two modes is maintained throughout the membrane; (3) The gas sorbed in the Langmuir mode is completely immobilized; (4) Diffusion occurs only in the Henry s mode; (5) The true diffusion coefficient is a constant, independent of concentration or position in the membrane. Eilenberg and Vieth[116] suggested that if one were to assume that diffusing gas exists at roughly its liquid density in polymer microvoids, then the Langmuir capacity constant would provide a means of measuring un-relaxed volume. The Langmuir capacity term for the excess volume of the glassy state is expressed by ' C H = ( V V ) g V g l ρ * (2.23) where ρ* is the liquid-like molar density of diffusing gas at the point of complete saturation of the Langmuir capacity of the polymer, Vg is the specific volume of the glass and V l is extrapolated from the equilibrium liquid line at the same temperature. Therefore, the unrelaxed or excess free volume fraction is equal to [V g -V l ]/V g.

61 Diffusion and Permeation in Heterogeneous Media Heterogeneous materials of technical usefulness are perhaps as common as homogeneous ones. The chemical structure, functionality, and composition of the matrix chemical constituents, as well as the processing conditions, influence the resulting networks and hence the properties of the cross-linked polymer. In fact, although the concept of homogeneous infinite network has long been erroneously applied to describe the morphology of all thermosetting polymers, the hypothesis of highly cross-linked nodules immersed in an internodular matrix of lower cross-linking density seems more reasonable. The influence of the matrix nodular structure on durability in aggressive environments, model of failure, and mechanical properties of TGDDM-DDS based composites has been reported in the literature by Morgan et al.[17, 117, 118]. Formation of a heterogeneous network is favored by diffusion restraints, due to the vitrification of the system and presence of differently activated cross-linking mechanisms. The exposure of heterogeneous materials to humid environments induces different morphological changes of the polymeric structure, depending on their affinity to water molecules and their mode of sorption. Diffusion and transport of matter in heterogeneous systems is important in such processes of moisture transport in polymers. Heterogeneity may be considered under the groups: (1) disperse phase in a continuum: the disperse phase could be regularly arranged with all particles having the same shape, size and orientation, or particles in regular position but having varying shape, size or orientation; (2) Two or more interpenetrating continuous phases, for example, regular lattice-like capillary networks or irregular

62 49 continuous networks of phase A in a continuum of phase B; (3) laminates, such as simple pair, sandwich type, or multiple laminates. In order to describe the transport properties, such as the diffusion coefficient, and permeability, the following parameters would be required: (1) the geometry of the dispersed phase (shape, size, and size distribution, concentration and concentration distribution, orientation, topology); (2) the composition, state of matter, and other properties of the disperse phase; (3) the composition, state of matter, and other properties of the continuous phase. Even with all the information above, it would still be difficult to predict new heterogeneous phase media more desirable for a particular purpose, because the properties of mixtures are not usually additive. Moreover, the nature of the polymerdisperse phase interface and interaction may require investigation where this interface is of high area. The solubility model of two-phase morphology was suggested[119] for the gas sorption in block copolymers. The solubility of He, H 2, O 2, CO 2, CH 4, C 2 H 6, C 3 H 8, and n-c 4 H 10 was investigated in a systematic series of phase separated polyether-polyamide segmented block copolymers containing either poly(ethylene oxide) (PEO) or poly(tetramethylene oxide) (PTMEO) as the rubbery polyether phase and nylon 6 (PA6) or nylon 12 (PA12) as the hard polyamide phase. The contribution of each phase to the overall sorption in a phase separated block copolymer was suggested as follows S = Φ A S A + Φ B S B (2.24) where S is the copolymer solubility, Φ A and Φ B are the volume fractions of components A and B in the copolymer, and S A and S B are the solubility of pure homopolymers A and B. Extrapolation of a line through the two PE-b-PA copolymer data points to the limit of

63 50 100% PTMEO and 100% PA successfully provides an estimated value that is consistent with experimental data values. The proper theoretical description of the permeability or analogous properties of composite polymeric materials is of interest, particularly in view of the growing technological importance of these materials. It is usually assumed that media is composed of distinct domains of non-interacting components A and B, usually of microscopic size, but characterized by permeability coefficients of P A and P B not appreciably different from those of corresponding bulk phases A and B. Heterogeneous polymer blends, block or graft copolymers exhibiting well-developed domain structures, foams, and filled or even partially crystalline polymers are examples of the materials of interest here. The problem is to determine the functional dependence of overall permeability coefficient of the medium P on P A and P B (assumed hereafter to be constant for a given test gas/vapor, that is, independent of the concentration of the vapor/gas), the composition (expressed in terms of the volume fraction V A, V B of components A and B), and the structural characteristics of the composite medium. Given P a and P b, the difficulty of evaluating P as a function of V A increases very rapidly as the structure of medium becomes more complex. Accordingly, a number of approaches leading to what are at best partial solutions of the problem have been developed in various fields. The simple composite medium structures to be dealt with are those giving rise to strictly unidimensional permeation flow, as in the case of a laminate of A and B oriented so that the laminations are parallel to the direction of flow. Simple averaging formulas then hold, namely,

64 51 P = V P + V A A B P B (2.25) where P: the arithmetic mean permeability; V A : the volume fraction of phase A; V B : the volume fraction of phase B When a laminate of A and B is oriented so that the laminations are normal to the direction of flow, the harmonic mean permeability is P = VAPA + VB PB (2.26) Other types of structure require more sophisticated treatment. Most approaches developed for this purpose concern media consisting of a micro-particulate dispersion of one component in a continuous matrix of the other. In what follows, we shall assume that A is the disperse phase and B the continuous one. The structure of such a composite medium can be specified in terms of particle shape, size, orientation, and mode of packing. The relevant formulas have been developed principally for long cylindrical rods and are presented here in terms of the dimensionless variables P/P B, V A, and α=p A /P B. For dispersions of spheres in continuous matrix, in which distances between particles are sufficiently large to ensure that the flow-line pattern around any one sphere is practically uni-distributed by the presence of others, the following relation, first derived by Maxwell,[56, 99] holds P / P B 1 α + α 3vA = + ( 2) /( 1) v A (2.27) This model was used in the analysis of polypropylene/polyamide 6 blends.[120] In order to extend the application of equation 2.27 to practical morphology, the treatment of more concentrated dispersions has been attempted in either of two ways[56]. On the

65 52 one hand, relatively simple closed expressions have been obtained by the introduction of various kinds of approximations. The formulas below are some of the research results. They are applicable to random dispersions of spherical particles. ( α P / P B )( P / P B ) 1/ 3 = (1 v A )( α 1) (2.28) PB 2P ( 1 )( α + ) = 3v ( α 1) P P B A (2.29) P P B 10/3 14/3 1 α + 2 k va k3 ( α 1) va v + ) 1 7 /3 α A 6 ( α + 4 ) /( α 1) k v α + = 1+ 3v ( L A 3 2 A 5 1 (2.30) According to the most complete derivation available for packing in a simple cubic, body-centered cubic, and face-centered cubic lattices, the three constants are different. Membranes which contain impermeable flakes or lamellae show permeabilities much lower than conventional membranes, and hence can serve as a barrier for oxygen, water, and other solutes. For polymers filled with plate-like particles, a model[120] was also proposed that considers the permeability of the filler to be zero P = (1+ (1 Φ P A B 0.5Φ B L W ) ) (2.31) where P A is the permeability of component A, Φ B is the volume fraction of the nonpermeable component in the blend, and L/W is the ratio between length and thickness of the hypothetical uniformly dispersed plates of non-permeable phase in the matrix. This model was used in the analysis of polypropylene/polyamide 6 blends. Cussler et al.[121] developed and verified theories[122] predicting the properties of these barrier membranes. The membrane consists of thin polymer films filled with

66 53 flakes aligned with the plane of the film. Micrographs of such a film would look like a bed of wet leaves imbedded in a continuous polymer phase. Four models differ in the geometry assumed for the flakes. The most realistic model has flakes which are randomly shaped and randomly distributed throughout the plane of the film. The idealized model consists of flakes that are not randomly located in the film, but rather occur periodically in a discrete number of planes within the film. Three geometries of the flakes were assumed. In the first geometry, the flakes were assumed to be rectangles of uniform size, but great width, regularly spaced like the bricks in a wall. The second geometry had each layer of flakes as a single flake perforated with regularly spaced pores. The diffusion takes place through the pores rather than slits. The third geometry assumed that flakes are randomly sized rectangles randomly located in the discrete planes. The theory developed based on these models predicts the variation of flux across a flake-filled polymer with loading and the aspect ratio of the flakes. The diffusion flux can vary with flake loading volume in two ways. First, if diffusion path through the flakes is paramount, the relationship holds: J J N 2 ϕ = + 1 ϕ 0 1 C (2.32) where J 0 is the flux of pure polymer without flakes, J N is the flux with n flakes in the polymer film, C is constant and ϕ is the volume fraction of flakes. The correct prediction occurs in spite of the major approximations in idealizing the geometry of these films. If diffusion through gaps between flakes is slow, then the linear relationship holds:

67 54 J 0 = 1+ C1ϕ J N (2.33) These models and theories represent an extension of the earlier studies of transport phenomena in composite media. The special morphology of liquid-crystalline polymer (LCP) presents a unique example to study heterogeneous diffusion processes because the efficient chain packing of mesophase is of remarkable gas barrier properties. Weinkauf et al.[76] studied the transport properties of thermotropic liquid-crystalline copolyester. The physical structure of LCPs is commonly described by a hierarchical model involving both microscopic and macroscopic scales. The copolyester has several morphological features, such as skincore structure, polydomain texture, and crystallinity within the mesophase. These features represent several levels of ordering which may affect transport. Based on the special morphology of LCPs and their transport properties[75-77], they used a two-phase model to investigate the extreme case of non-conducting nematic mesophase. The core of this two-phase model is that the nematic mesophase domains have a high aspect ratio and are dispersed in a continuous phase. They assumed that the permeability coefficient of the small volume fraction of conducting boundary regions could be large enough to account for the observed transport behavior. This two-phase model gives the expression of the permeability of the liquid crystalline polymer if the nematic phase is assumed impermeable: P = P B 4 5 ( ) 1 1 Φ N (2.34) where Φ N is the volume fraction of nematic phase, P B is the permeability of the conducting boundary phase, and P is the permeability of the bulk transport property.

68 55 This model fits the experiment data successfully. Some conclusion can be derived from the analysis using this model: (1) the mechanism dominated by transport in a small volume fraction (about 10%) of boundary regions could possibly account for the total transport properties of these materials, (2) it is possible that homogeneous transport processes inside the mesomorphic domains can occur at the same time as those occurring throughout the boundary region. (3) a more complete model is needed to better understand the complex morphology of this kind of LCP and its nature of permeability.

69 CHAPTER 3 WATER VAPOR PERMEATION IN LIQUID CRYSTALLINE EPOXY 3.1 Introduction Liquid crystalline thermosets (LCTs) are a specific class of liquid crystalline materials that posses the advantages of both liquid crystals and high performance thermosets. This kind of material has some special properties for not only the processing but also the application. There are two special aspects of morphology needed to be mentioned for liquid crystalline thermosets. First, the molecules are arranged on one or two dimensional or positional regularity. Second, unlike liquid crystalline polymers, the molecules are crosslinked by covalent bond forming three-dimensional networks. Therefore, they possess excellent mechanical properties. However, real life environments during the service can be complex and variable. The long-term properties of the LCTs depend on the interaction of the LCTs with the environment conditions such as the temperature or moisture. The water absorption in LCTs has detrimental effects upon their long-term properties. These effects, based on the research results for the epoxy matrix composite systems, have been associated with moisture-induced micromechanical damages in the epoxy resin and/or at epoxy-fiber interfaces as well as internal stresses, relaxation of mechanical properties and plasticization. Unfortunately, the interaction of water with liquid crystalline thermosets has remained unclear [21]. Much research has been reported for LCTs about the synthesis[22-38], orientation[25, 30, 39-41], mechanical properties[22, 25, 30, 35, 36, 40-56

70 57 43], thermal properties[22, 25, 30, 37, 42, 44], and moisture sorption for conventional epoxies[45]. Many questions need to be answered, such as: differences in moisture permeability between the liquid crystalline epoxy thermosets and the conventional epoxy; the mechanism of the water absorptions, and how the diffusion coefficient, permeability and sorption coefficient change with the processing parameters? The aim of the present work is to answer to the above questions by a study of the sorption characteristics of conventional epoxide-amine samples and liquid crystalline epoxide-amine samples. We hope to gain further insight about processing-structure-properties relationships in the LCTs. The measurement of diffusion, permeation, and solubility is very important to characterize moisture transportation in LCTs. Diffusion and sorption coefficients are usually determined by recording the weight change of sample during sorption, while diffusion coefficients and permeability are obtained by using the constant pressure apparatus or the Manometric apparatus to follow flow through a sample film. These methods have several disadvantages: such as, requiring mechanical support for the test film, vacuum-tight seals. And the most serious limitation is that the measuring precision is usually not sufficient for a detailed analysis of the data in terms of transient permeation. Present work uses a dynamic method, which is largely free of the above drawbacks, to measure permeability and diffusion coefficients. 3.2 Experiment Synthesis of 4,4 -diglycidyloxy-α-methylstilbene (DOMS) The synthesis of 4,4 -diglycidyloxy-α-methylstilbene (DOMS) followed the procedure of Earls s patent[123] although some steps were modified according to the

71 58 work in our group[124]. Phenol (99 %), chloroacetone (96%), dichloromethane (99.9%), concentrated sulfuric acid, epichlorohydrin, isopropanol (99%), and sodium hydroxide were used as obtained from Fisher without further purification. Sythesis of 4,4 -dihydroxy-α-methylstilbene (diol). Phenol (5.31mol, 500g) and chloroacetone (2.65 mol, 211 ml) are added to a 1000 ml flask with solvent of dichloromethane (300 ml) and cooled to 10 C. With stirring, concentrated sulfuric acid (147 ml) is added drop-wise to the stirred solution over a two-hour period in order to maintain the reaction temperature at 10 C. After two and half hours of post reaction at 10 C, the viscous orange solution product is mixed with 1000 ml of iced deionized water. The oil product is separated then washed with a second 1000 ml portion of chilled deionized water. The product was washed repeatedly with water for about 10 times. Then, the recovered product is added to a 2-liter beaker along with 700 ml of ethanol and 500 ml water and stirred to dissolve the product into solution. Deionized water (300 ml) is added to the stirred solution and heating commenced. As the temperature of the mixture increases, the mixture begins to clear. Each time clearing is observed, sufficient deionized water is added to induce cloudiness, followed by continuation of mixing and heating. Once the temperature reaches about 90 C, massive precipitation of white crystalline plates occurs. Heating and stirring ceases at this time. The solution is then slowly cooled down to room temperature. Then the mixture is chilled to 5 C and held there for 12 hours to let the crystal precipitate. The crystalline product is recovered by filtration, re-crystallized again in a mixed solvent of 300 ml ethanol and 500 ml deionized water. The re-crystallization is repeated until the sulfuric acid in solution had been

72 59 washed out. The ph paper is used to test the neutral condition. The solid product is then filtered and dried in a vacuum oven at 35 psi and 70 C. Epoxidation of 4,4 -dihydroxy-α-methylstilbene. The mechanism of this epoxide reaction is nearly the same as the epoxide reaction for bisphenol A referred to as the taffy process. The process is the reaction of excess epichlorohydrin, diol, and a stoichiometric amount of NaOH. 4,4 -dihydroxy-α-methylstilbene (100 g, 2.2 mol) prepared above, epichlorohydrin (344 ml, 10 times in mole of 4,4 -Dihydroxy-αmethylstilbene), deionized water (24.6 ml, 8.0 percent by weight of the epichlorohydrin), and isopropanol (270 ml, 53 percent by weight of the epichlorohydrin) are added to a three neck flask and heated to about 55 C with stirring under a nitrogen atmosphere. Once the 55 C reaction temperature is achieved, one third of sodium hydroxide solution (38.7 grams, dissolved in 155 ml deionized water) is added drop-wise to the reactor over a 50 minute period in order to maintain a reaction temperature between 55 and 59 C. Ten minutes after completion of the aqueous sodium hydroxide addition, the stirring is stopped and the aqueous layer which separates from the reaction mixture is drawn off. Heating and stirring is resumed. This step is repeated twice, to add the remaining two thirds of the NaOH solution. After a three hours reaction, the solution is poured into a beaker with 600 ml deionized water and washed twice. The resulting solid is washed with 350 ml acetonitrile and 150 ml water solution (70%/30% volume fraction). The white solid product is filtered and dried in a vacuum oven. The final product is recovered as crystalline off-white solid with an epoxide equivalent weight of 181. Elemental analysis and theoretical data are shown in Table 3.1.

73 60 Material Table 3.1 Elemental analysis results Theory Value Experiment Value C (%) H (%) O (%) C (%) H (%) O (%) Diol DOMS Measurement of EEW. The epoxy equivalent weight (EEW) is defined as the monomer weight per epoxy group. In the case of a di-epoxide such as DOMS, the EEW is half of its molecule weight. There are many methods to determine the EEW of epoxy. The requirement that determine the applicability of a special analysis method are the solubility of the sample in the respective reagent, a sufficient rate of reaction, and the absence of side reactions. Because of their limited solvent properties, water, alcohols, and diethyl ether are not suitable media for the determination of common epoxy resins. We employed dimethylformamide (DMF) as the hydrochlorination solvent. The most frequently applied methods are those based on addition of hydrogen chloride to the epoxy group. It was found that the hydrogen chlorination method to be specific for the epoxy group. In the present study, to make sure the reaction takes place completely, excess hydrochlorination reagent is added to obtain the best results. The amount of hydrogen chloride was at least two to three times the amount of the epoxide. The EEW is measured via end-group titration following the procedure described by Jahn[125]. A sample of DOMS weighting between 3 to 5 meq of the expected EEW was dissolved and reacted, with stirring, in 25 ml of a hydrochlorination reagent of approximately 0.2N. The hydrochlorination solution was made by dissolving 26 ml of concentrated aqueous hydrochloric acid added to 1 L of dimethylformamide. This

74 61 reaction mixture stands at room temperature for 20 min to allow sufficient reaction time. At the end of reaction period, a few drops of indicator solution (0.2wt% bromophenol blue in dimethylformamide) were added. Then excess acid was back-titrated with standard sodium hydroxide solution. Solution of NaOH, 0.1 N in methanol, was added drop-wise to the solution until the color changes from yellow to blue. The end-point of titration was at a ph of 4.5, as measured by a Hanna Instruments Checker pocket sized ph meter, which had an accuracy of ±0.1. The volume of NaOH solution required was recorded. To get precise contribution of the hydrogen chloride solution, three blank tests were also run in an identical manner, which was 25 ml of the dimethylformamide/hydrochloric acid solution without any added sample. The weight per epoxy equivalent is calculated as follows EEW = C N ( B A) Where A is the volume (in milliliters of sodium hydroxide solution used for titrating the sample; B is the volume (in milliliters) of sodium hydroxide solution used for titrating the blank; The difference between the amount of hydrogen chloride added and the amount unconsumed, as determined by titration with a standard base, is the amount of hydrogen chloride reacted with epoxy groups. C is the weight (in milligrams) of sample used, and N is the normality of the titrating solution. Polarized optical microscopy (POM). POM experiments were conduced using a Nikon Fluorophot microscopy and a Linkham Scientific Instruments HFS91 hot stage controlled by a TMS 91 temperature controller. Small amounts of DOMS were placed between two 12 mm round, glass microscope cover slips and observed between crossed polarizers. The samples were heated at rate of 10 C/min to 400 C. The cover glass that

75 62 was placed on the top of the sample was pressed to make a thin film of the sample. The clearing temperature and liquid crystalline phase were observed by optical texture. The nematic-isotropic transition temperature was taken at the disappearance of birefringence from the sample. To study structural development, film samples of mixtures of the LC epoxy and the curing agent SAA were prepared by casting 2% acetone solutions of the mixture on glass slides, and drying in a vacuum oven at room temperature for about 4 hours. To observe the structural development of the LC epoxy during curing, the samples were put onto a preheated hot stage at 150 C, and held isothermally for up to 4 hours. Differential scanning calorimetry (DSC). DSC analysis was performed using a Seiko Instruments SSC5200 DSC220C with an SDM5600H Data station. Calibration was done with alumina standard. The sample size was approximately 10 to 20 mg. DSC nitrogen flow rate 100 ml/min. Samples were sealed in aluminum pans and heated/cooled in the DSC at a rate of 10 C/min. The melting point for the uncured monomer was determined from the extrapolated onset of a DSC experiment in a nitrogen gas flow with a heating rate of 10 o C/min. Sample preparation. To conduct this work, it is necessary to select a system that includes a LC mesophase and non-lc. This approach facilitates the investigation and comparison of water permeation in liquid crystalline and conventional epoxy. The liquid crystalline epoxy monomer, 4,4 -diglycidyloxy-α-methylstilbene (DOMS), and a commercial, non-lc epoxy monomer of similar molecular weight, diglycidyl ether of bisphenol A (DGEBA) (DER383), generously supplied by Dow Chemical Company, were selected for comparison. DER383 is a liquid reaction product of epichlorohydrin and bisphenol A. The EEW of DER383 was reported by DOW to be between

76 63 An average EEW value of 180 was used to calculate the amount of sulfanilamide needed during formulation. Based on general chemical structure (Figure 3.1) and EEW value, the index n can be calculated. For the EEW of DER383, the index is about Its chemical structure is very similar to that of DOMS, except the central part of the structure. O CH 2 CH CH 2 CH 3 OH CH 3 O O C O CH 2 CH CH 2 O C O CH 2 CH CH 2 CH 3 n CH 3 Figure 3.1 Chemical structure of DGEBA O CH 3 O CH 2 CHCH 2 O C O CH 2 CH CH 2 CH 3 DER383 O CH 3 O CH 2 CHCH 2 O C CH O CH 2 CH CH 2 DOMS NH 2 O S NH 2 O SAA Figure 3.2. The chemical structure of the DER383, DOMS and SAA

77 64 Sulfanilamide (SAA), purchased from Aldrich Chemical Co., was selected as the cross-linking agent. The SAA curing agent is a di-functional amine, containing two primary amines, is capable of forming four bonds per molecule to yield a three dimensional network. SAA was used without further purification. DOMS was synthesized in our lab as mentioned above. The chemical structures and characteristics are shown in Figure 3.2 and Table 3.2. Table 3.2 Thermal transition of compounds used in this work MW Chemical Formula T (g/mol) m ( C) T i ( C) T k ( C) LC epoxy monomer (DOMS) Non-LC epoxy monomer (DER383) Cross-linking agent (SAA) Note: T m is melting point, T in is the isotropic-to-nematic transition temperature, and T k is the crystallization temperature. Preparation of cast films for DOMS-SAA and DER-SAA. DOMS or DER383 was first melted at 150 C in a convection oven. The curing agent, sulfanilamide, was then added to the resin. This mixture was periodically stirred over the next 15 minutes to dissolve all the sulfanilamide into the resin. Once all the sulfanilamide had dissolved, the mixture was degassed and then poured onto a preheated aluminum plate (dimension=300 mm 300 mm 15mm) with a Teflon spacer (thickness=0.1 mm). Sufficient degassing is critical to obtain bubble-free films. After about 10 minutes, a second aluminum plate was put on the top of the first one. The curing cycle followed this procedure: after 4 hours at 150 C, the temperature of the oven was increased to 175 C at 2 C/minute and held for one hour, the temperature was then increased to 200 C at 2 C/min and held for 4 hours, as shown in Figure 3.3. Each aluminum plate was sanded with silicon carbide

78 65 sandpaper, polished, cleaned with water and acetone, sprayed with nonstick release agent (Crown 6075) purchased from Fisher Scientific, baked at 200 C for 2 hours, then cleaned to sweep out the extra Teflon powder. 200 C Temperature 150 C 175 C 1 Hour 4 Hours 4 Hours Time Figure 3.3 Curing procedure for DER-SAA and DOMS-SAA systems After curing, an opaque, cream colored casting film was obtained for the DOMS- SAA system. The DER-SAA system was transparent. Specimens were removed from the mold. Tested film samples were cut into the shape of circle. The radius was about 30 mm, and the thickness 100 µm. Because of the homemade nature of the molds, each asprocessed specimen had slight variation in thickness. To ensure measurement precision, only samples with a thickness standard deviation less than ±5 µm were used for the test. The samples were masked with aluminum foil around the edges on both sides and temporarily placed in a vacuum desiccator until further measurement. The designation of the samples is listed in Table 3.3

79 66 Table 3.3 The designation of samples Assignation Monomer Amine/epoxide functional ratio DER-SAA-0.8 DER DER-SAA-0.9 DER DER-SAA-1.0 DER DER-SAA-1.1 DER DER-SAA-1.2 DER DOMS-SAA-0.8 DOMS 0.8 DOMS-SAA-0.9 DOMS 0.9 DOMS-SAA-1.0 DOMS 1.0 DOMS-SAA-1.1 DOMS 1.1 DOMS-SAA-1.2 DOMS Apparatus and Procedure for Measuring Permeability A MOCON Permatran-W3/31 Water Vapor Permeation Measurement System was selected to measure and analyze the water vapor transmission rate of the tested material. In the operational procedure, the test film was first put into the test cell, flushed with dry nitrogen gas on both sides of the film to degas moisture absorbed during sample installation to set the detector to zero. HPLC-grade water was then introduced into sponges, located in the outer cover of the test cell, by syrup injection through the hole on the outside of the cell. As the water vapor diffused through the test film, carried by nitrogen carrier gas to the detector, the water vapor transmission rate (WVTR) was continuously recorded. The schematic procedure of the test is shown in Figure 3.4 To achieve higher sensitivity, the nitrogen gas flow was set at 10 sccm (standard cubic centimeters per minute). Exam time=30 minutes; Re-zero cycle=5; Test method=convergence; Initial condition time=0 minutes; Relative humidity of test cell =

80 67 100%; Individual zero =NO; Cycle count=infinite; Temperature determination mode=auto; Nitrogen flow rate determination mode=auto. To ensure system accuracy in determining water vapor transmission rates and to compensate for environmental variations that affect the system, such as room temperature, a calibration procedure was performed to compensate for these variables in test environments. A certified reference film (WVTR= g/package/day, cm 2, 10sccm, 37.8 C, 100%RH) was selected for the calibration. This film was obtained from Modern Controls Inc. The reference film was individually tested with test equipment traceable to NIST, National Institute of Standards and Testing. Phase One: Outgas Film Phase Two: Standard Test WVTR Permeation Level Half Level Permeation Half-Time Time Figure 3.4 Profile of half-time method 3.3 Mathematical Treatment The diffusion process is generally described by the following expression: 2 C C D t = (3.1) 2 x

81 68 To simplify the mathematical treatment, it is assumed that the diffusion coefficient D is not a function of concentration, that the surface concentration is proportional to the pressure of the water vapor, and that swelling of the membrane is negligible. sample C=C 1 C=0 X=l X=0 Figure 3.5 Diagram of the concentration distribution Based on the experiment conditions, the following initial and boundary conditions are assumed: c = 0 0<x<l, t = 0 c = c 1 at x = l, t 0 c = 0 at x =0, t 0 During the non-steady state condition, the concentration distribution is given by c = c x 2 c 1 + l π n sin cos nπ Dn π t / nπx l e l 2 (3.2) Based on this equation, we can get the flux at the surface for x=0,

82 69 c x = + 2 nπx ) l c1 cos( nπ )cos( l Dn π t / c l + e l 2 c c 2c = 0 = + x x l l cos( ) 1 nπ e 2 Dn π 2 t / l 2 J ( x = 0) = D + c c D 2c1D x x= 0 = l + l 1 1 cos( n ) π e 2 2 Dn π t / l 2 (3.3) When it goes to equilibrium, Dc l 1 J ( x = 0, t = ) = When the flux equals half of the equilibrium flux value, we have (just considering the first order) D = ln 4 π 2 l t 2 1 / 2 (3.4) Our approach of finding the diffusion coefficient D, when permeability (P) is known, is the half time method which requires finding the time (t 1/2 ) at half of the final permeation rate. For the present work, the MOCON Permatran W3/31system recorded the water vapor transmission rate (WVTR), which is the time rate of water vapor flow, normal to the surface, through unit area of the test film. The water vapor permeability (P) was calculated using Equation (3.5): P = WVTR Thickness VP R 1 R ) ( 2 (3.5)

83 70 Where: VP: water vapor pressure at the test temperature. Some of the values are listed in table 3.4. R 1 : relative humidity at the source expressed as fraction, for the present test, it is 100% relative humidity. So, R 1 =1 R 2 : relative humidity of the vapor in the inner cell. For the present test, R 2 =0 Once the permeability P and diffusion coefficient D were calculated using Equations 3.4 and 3.5, the solubility coefficient S can be calculated by using: S = P D (3.6) Of course, the half-time method requires the film to be out-gassed and then continued to equilibrium, but this method yields high precision for P, D, and S. Temperature ( C) Vapor pressure (mmhg) Table 3.4 Water Vapor Pressure at the Testing Temperature Note: Data from the CRC handbook of Chemistry and Physics 3.4 Results and Discussion Synthesis of DOMS Epoxy resins are characterized by three-member rings known as epoxy, epoxide, oxirane, or ethoxyline groups. Commercial epoxy resins contain aliphatic, cycloaliphatic, or aromatic backbones. The capability of the epoxy ring to react with a variety of substrates imparts versatility to the resins. Treatment with curing agents yields an insoluble and intractable thermoset polymer. The synthetic schemes used to prepare the DOMS monomer are shown in Figure 3.6 and 3.7.

84 71 O Cl CH 2 C CH 3 Chloroacetone + OH Phenol H 2 SO 4 CH 2 Cl 2-10 o C CH 3 HO CH C OH 4,4'-dihydroxy-alpha-methylstilbene (diol) Figure 3.6 Synthetic scheme for diol CH 3 HO CH C OH O + Cl CH 2 CH CH 2 4,4'-dihydroxy-alpha-methylstilbene (diol) Epichlorohydrin CH 3 NaOH, 110 o C, Isopropanol CH 3 CH OH O CH 3 O CH 2 CH CH 2 O CH C O CH 2 CH CH 2 4,4'-diglycidyloxy-alpha-methylstilbene (DOMS) Figure 3.7 Synthetic scheme for DOMS O OH Cl CH 2 C CH 3 Dichloromethane -10 o C HO H CH 2 C CH 3 O HO CH C CH 3 O H 2 SO 4 2 hours HO CH C OH CH 3 Figure 3.8 Synthetic mechanism scheme for 4,4 -Dihydroxy-α-methylstilbene (diol)

85 72 OH NaOH (catalyst amount) ONa H 2 O epichlorohydrin O CH 2 CH CH 2 Cl NaOH (stoichiometric) O CH 2 CH O CH 2 NaCl H 2 O Figure 3.9 Synthetic mechanism scheme for DOMS monomer It is a two-step synthesis. The first step is to synthesize the diol. Reaction of chloroacetone with phenol in the presence of a sulfuric acid catalyst yields the diol. The next step is epoxidation of the diol compound, to convert the diol to monomer DOMS. As with most other epoxy resins, the liquid crystalline monomer DOMS is also prepared by the reaction of compounds containing an active hydrogen group with epichlorohydrin, followed by dehydrohalogenation. DOMS is a low weight crystalline molecule. Epichlorohydrin is most widely used to synthesize epoxy resins. The reaction sequence involves the attack by a phenolate anion at the least hindered carbon of the epoxide ring, followed by intramolecular displacement of chloride to regenerate an epoxide. Reaction with excess epichlorohydrin leads to an epoxideterminated oligomer. The end groups are commonly referred to as glycidyl ether groups. The molecular weight of the oligomer can be varied by suitable adjustment of the epichlorohydrin excess.

86 Thermal Behavior DOMS The 4,4 -dihydroxy-alpha-methylstilbene (diol) was not liquid crystalline, however, the DOMS proved to exhibit a monotropic nematic mesophase on cooling from 105 to 65 C. The characteristic of the DOMS monomer can be seen in the DSC heating and cooling traces. This thermal behavior was entirely reproducible over many heating and cooling cycles. Typical DSC heating and cooling thermographs are shown in Figure The comparison with data from different resources is listed in Table 3.5. Table 3.5 Comparison of the characterization temperature of DOMS Resource T m ( C) T in ( C) T k ( C) Lin[22] Barclay[126] This study Note: T m is melting point, T in id the isotropic-to nematic transition temperature, and T k is the crystallization temperature The melting temperature, isotropization temperature and nematic-crystalline transition are higher than in data given by Lin[22]. The temperatures are nearly the same as those in the literature of Barclayet al.[126]. The differences in temperature are due to (1) different synthesis methods of the DOMS monomer and (2) the purity of the DOMS tested. These factors will influence the molecular weight of the DOMS monomer. The values of the average of molecule weight (or the EEW) are the same for all these resources. The lower transition temperature given by Lin may be due to the fact that the LC epoxy used in the study was less pure than the one used by Barclay et al., according to the author.[22] As shown in the DSC curve, the liquid crystalline epoxy, DOMS, is monotropic. Upon heating, it melts directly from a crystalline state into an isotropic state

87 74 at 135 C. Upon cooling from the isotropic state, it shows a nematic phase from 105 to 65 C. In the liquid crystalline state, the epoxy shows a typical nematic schlieren texture on cooling from the isotropic phase. For the present study, the aromatic tetrafunctional diamine sulfanilamide, SAA, was chosen for the curing reaction, since it was of lower reactivity than an aliphatic amine, thereby allowing the oligomers to melt completely before a substantial degree of crosslinking had occurred. For determination of the stoichiometry of the curing reaction, the LC DOMS was treated as difunctional and the SAA amine as tetra-functional. To investigate the effect of stoichiometric ratio on the structure and properties of cured LC epoxy, different functional ratio of amine/epoxide were selected. The commercial non- LC epoxy DER383 monomer, diglycidyl ether of bisphenol A (DGEBA) of similar molecular weight, was also selected for this comparison DOMS-monomer Heat Flow (relative) (b) (a) Temperature ( o C) Figure 3.10 DSC thermographs of the liquid crystalline DOMS monomer: (a) heating, (b) cooling (scan rate 20 o C/min)

88 75 To certify the completion of the reaction between the amine and the epoxy groups, FT infrared spectroscopy was used. In all cases, the characteristic C-O epoxy absorption at 914 cm -1 was nearly eliminated on curing, except with the epoxy-rich samples. As an example, spectra of both DOMS monomer and DOMS-SAA-1.0 cured network are shown in Figure The disappearance of the epoxy peak is clearly evident. Further evidence of the effectiveness of the curing reaction was the fact that all the resulting networks prepared from DOMS-SAA and DER-SAA were insoluble in some organic solvent, such as toluene or chloroform. No visible swelling was observed (a) Absorbance (b) 914 cm Wavenumber (cm -1 ) Figure 3.11 Comparison of FTIR spectra of (a) DOMS-SAA-1.0 and (b) DOMS monomer Using cross-polarized optical microscopy, it was observed that thin films of the resultant epoxy network were extremely birefringent. The effect of curing on the LC

89 76 texture of the networks could be observed by hot stage microscopy. The DOMS-SAA system exhibited typical smectic texture. However, texture observation alone was not effective in identifying the type of mesophase retained by the high crosslink density network. The smectic texture in the cured liquid crystalline epoxy resin is very stable. DSC studies of these crosslinked oligomers certified its stability. The cured networks with high crosslink density showed no isotropic transition below decomposition, which means the molecular organization of smectic LC phase had been frozen into the thermoset. This is consistent with other research results. Barclay [126] reported that the networks with high crosslink density remained highly birefringent until decomposition without exhibiting a clearing temperature and showed no change in disclination structure. Some WAXD results showed the same conclusion for the DOMS-SAA system.[127] In the present DOMS-SAA system, the nematic monomer DOMS can yield a network with smectic ordering. In these LC epoxy systems, the tendency to form a smectic-like network on crosslinking seems predominant for the lower molecular weight oligomers. Similar results have been noted by Barclay[126, 128] in triazine-based thermoset materials. A possible explanation for this observation is that a narrower polydispersity of the lower weight oligomers resulted in a more uniform distance between crosslink sites. Thus, as the crosslinking reaction proceeded, the repetition of this uniform distance between crosslinking resulted in a smectic-layered organization Features of Water Permeation in the Studied Epoxies General features. Typical water vapor permeation rate curves for the present work were shown in Figure The water vapor transmission curves were apparently

90 77 Fickian, and the water concentration reached equilibrium after one or two days. There is no evidence that the absorbed water in both DER-SAA and DOMS-SAA systems induce the non-fickian behavior. This seems reasonable because non-fickian behavior has been explained by the viscous nature of polymers. The relaxation time is a characteristic parameter of the response of a polymer to a change in an internal or external variable at a given temperature. Thus, changes in the internal moisture concentration during sorption could induce polymer relaxation, which causes the non-fickian behavior.[100, 101, 129] Both the DER-SAA and DOMS-SAA system are highly cross-linked. Thus, high crosslinking constrains the relaxation induced by changes in the internal moisture concentration of the network during sorption. In other words, the length of the segment is too short for water to have an obvious effect on chain relaxation. Accordingly, this liquid crystalline thermoset DOMS-SAA system is a rigid rod network. It is not flexible. Even if the glass transition temperature is depressed, due to the plasticizing effect of absorbed H 2 O, there should be no obvious effect on the transport properties. In other words, the absorbed water will not change the transport characteristics from Fickian behavior to non-fickian behavior. Neither DER-SAA nor the DOMS-SAA has any apparent deviation from Fickian behavior. The theoretical equation (3.3) fits well with the experimental data (using the first order)(figure 3.12). Effect of liquid crystalline structure on the transport properties of water. Comparison of the diffusion coefficient, solubility coefficient, and permeability of the conventional thermoset DER-SAA system and liquid crystalline thermoset DOMS-SAA system are shown in Figure 3.13 to 3.24.

91 78 It is obvious that the diffusion coefficient and permeability of the DOM-SAA system are much lower than that of conventional epoxy DER-SAA system. As stated in the experimental section, the chemical structures of the DOMS and DER 383 are very similar. However, the permeation properties, such as D and P, of cross-linked networks are very different. The liquid crystalline thermoset DOMS-SAA system has a higher barrier than that of DER-SAA system for the all amine/epoxide functional ratios tested. The higher barrier properties are beneficial for the LCTs to be used as high demanded materials, for instance, the matrix of advanced composites. 14 Permeability, P (10-9, g*mm/(m 2 *Pa*Sec)) DER-SAA test data DOMS-SAA test data DER-SAA fit curve DOMS-SAA fit curve Time (minute) Figure Typical water vapor permeation rate curves (functional ratio is 1 for both system, test at 37.8 C) Similar chemical structures can exhibit different properties. This phenomenon can be attributed to special characteristics of the phase transition during the curing process for DOMS-SAA system, that is, the liquid crystalline structure is locked in the heavily cross-linked network. The DOMS-SAA system is initially isotropic at a curing

92 79 temperature of 150 C. Smectic A phase forms during the curing process. Formation of a more ordered smectic phase during isothermal cure has been seen previously in several studies on a variety of LCTs. The reaction-induced isothermal phase transition can be explained by principles of step-growth polymerization and the packaging of rod-like molecules[130]. As a cure reaction proceeds, DOMS monomers are linked end-to-end. Thus the aspect ratio of growing rod-like molecules increases. At some critical aspect ratio, the formation of an ordered molecular arrangement becomes easier. This happens because the entropy for packing of the rods drives to the more ordered liquid crystalline phase direction. The transition of liquid crystalline texture from isotropic to smectic has been completed. Liquid crystalline structures remain almost unchanged to the final stage of the reaction. In other words, the skeleton of the ordered network has been built before the crosslinking. The crosslink agent SAA also plays an important role in the formation of the smectic mesophase. SAA is an aromatic diamine with amine non-equivalent groups. SAA is much less reactive than other aromatic amine compounds. Electrons on the nitrogen atom in the amine group are strongly withdrawn by the sulfone function. Mulliken electron density populations were calculated using the PM3 method for several model epoxy resin hardeners. This means that the basicity of the amine is quite low. In fact, because the nitrogen atom on the SAA amide group is negatively charged, hydrogen atoms attaching at the nitrogen become acidic. The hydrogen atom on the sulfonamide group presumably acts as a weak acid. Thus, the two reactive groups on SAA have opposite and unequal reactivities much like a weak base and a weak acid.

93 80 Diffusion Coefficent, D (10-10 cm 2 /s) o C DER-SAA DOMS-SAA Amine/Epoxide Functional Ratio Figure Relationship between diffusion coefficient and amine/epoxide functional ratio at test temperature 37.8 C Diffusion Coefficient, D (10-10 cm 2 /s) o C DER-SAA DOMS-SAA Amine/Epoxide Funtional Ratio Figure 3.14 Relationship between diffusion coefficient and amine/epoxide functional ratio at test temperature 30 C

94 81 Diffusion Coefficient, D(10-10 cm 2 /s) o C DER-SAA DOMS-SAA Amine/Epoxide Functional Ratio Figure Relationship between diffusion coefficient and amine/epoxide functional ratio at test temperature 20 C Diffusion Coefficient, D (10-10 cm 2 /s) o C DER-SAA DOMS-SAA Amine/Epoxide Functional Ratio Figure Relationship between diffusion coefficient and amine/epoxide functional ratio at test temperature 10 C

95 o C DER-SAA DOMS-SAA Solubility Coefficient, S (g/m 3 Pa) Amine/Epoxide Functional Ratio Figure Relationship between solubility coefficient and amine/epoxide functional ratio at test temperature 37.8 C o C DER-SAA DOMS-SAA Solubility Coefficient, S (g/m 3 Pa) Amine/Epoxide Functional Ratio Figure Relationship between solubility coefficient and amine/epoxide functional ratio at test temperature 30 C

96 o C DER-SAA DOMS-SAA Solubility Coefficient, S (g/m 3 Pa) Amine/Epoxide Functional Ratio Figure 3.19 Relationship between solubility coefficient and amine/epoxide functional ratio at test temperature 20 C Solubility Coefficient, S (g/m 3 Pa) o C DER-SAA DOMS-SAA Amine/Epoxide Functional Ratio Figure Relationship between solubility coefficient and amine/epoxide functional ratio at test temperature 10 C

97 84 Permeability, P (10-9, g*mm/(m 2 *Pa*sec)) o C DER-SAA DOMS-SAA Amine/Epoxide Functional Ratio Figure Relationship between permeability amine/epoxide functional ratio at test temperature 37.8 C Permeability, P (10-9, g*mm/(m 2 *Pa*sec)) o C DER-SAA DOMS-SAA Amine/Epoxide Functional Ratio Figure Relationship between permeability amine/epoxide functional ratio at test temperature 30 C

98 85 Permeability, P (10-9, g*mm/(m 2 *Pa*sec)) o C DER-SAA DOMS-SAA Amine/Epoxide Functional Ratio Figure Relationship between permeability and amine/epoxide functional ratio at test temperature 20 C Permeability, P (10-9, g*mm/(m 2 *Pa*sec)) o C DER-SAA DOMS-SAA Amine/Epoxide Functional Ratio Figure Relationship between permeability and amine/epoxide functional ratio at test temperature 10 C

99 86 Lin et al.[22, 131] reported that the amine function of SAA is more reactive than the amide function of SAA. The reaction mechanism can be described by four different reaction steps: (1) reaction between the SAA primary amine and an epoxide, (2) reaction between the SAA secondary amine and an epoxide, (3) reaction between the primary amine of SAA amide and an epoxide, and (4) reaction between the secondary amine of SAA amide and an epoxide. When a rigid-rod epoxy compound is reacted with the curing agent SAA, the formation of an ordered phase during cure is enhanced by using this diamine in which the two amine groups have unequal reactivity. This in-equality in reactivity encourages chain extension to occur prior to branching and eventual cross-linking. The more reactive group of the dual-reactivity curing agent behaves as a chain extension agent while the group with lower reactivity subsequently completes the crosslinking reaction. It gives rise initially to the formation of linear polymer sections, allowing enough time for liquid crystalline organization to occur at the elevated cure temperatures, leading to the formation of smectic structure, then through cross-linking, locks the liquid crystalline order. WAXS patterns[22, 41, 127] for the DOMS-SAA shows a layer periodicity with a d spacing of 21 Å. This reflection is consistent with the length of the DOMS molecule without the crosslinker, suggesting that SAA is attached as a side-group and it is not a part of the smectic layer structure. The lateral packing distance of stilbene-like mesophases is about 4.5 Å. The 5.7 Å length of the long axis of SAA approximately corresponds to a maximum of a diffraction ring in the azimuthal direction. The efficient chain packing of the smectic liquid crystalline order is presumably responsible for DOMS-SAA remarkable gas barrier properties.

100 87 The difference between transport properties of water in DER-SAA and DOMS- SAA systems can also be attributed to the flexibility of the chain segment. An understanding of this is attained by considering the subtle change in accessibility of polar groups in the cured epoxy resin, such as hydroxyl groups and amine groups, to the diffusing water molecules. When an epoxide group reacts with an amine group, the connection creates micro-opening near the crosslink junction because of the restricted bond angles and lengths (Figure 3.24). This opening leaves the polar groups near the crosslink junction potentially exposed to the water molecules. This is the case when a single junction is considered. In reality, multiple junctions exist in the cured epoxy resin. Some of the open volume created by junctions may be occupied by flexible molecular segments from other junctions[49-51]. This will fill in the open volume and sterically exclude the polar site from association with water. N CH 2 CH OH CH 2 CH 3 O C O CH 3 O S O N H 2 C OH CH CH 2 CH 3 O C O CH 3 Figure 3.25 Schematic picture of polar groups near crosslink junction The free volume near the crosslink junction clearly demonstrates how the rigidity affects the ability of water to associate with the polar groups in the cured resin. As is well

101 88 known, moisture is an effective plasticizer in that it will reduce the glass transition temperature. Systematic investigation of the degree of plasticization shows this trend. However, only in the more flexible networks will water significantly alter the effects of physical aging. So, during the absorption process, the water will cause more plasticization by creating more free volume in the DER than in DOMS. With regard to moisture uptake, it is likely that water will be able to open up the structure and reveal more polar sites for association, especially in the more flexible networks. Thus the sorption of water in the DER-SAA system is higher than that in the rigid liquid crystalline DOMS-SAA system. FTIR results (see Chapter 4) from the study of the polar group interaction with water molecules in the cured resin also supports the effect of free volume near crosslink junctions. The Effect of Temperature on the Diffusion Coefficient. The relationship between the diffusion coefficients and temperature for five stoichiometric ratios is shown in Figures 3.25 and 3.26 for DER-SAA and DOMS-SAA systems respectively. The diffusion coefficients increased with increasing test temperature for all samples in both systems. The experimental result is not unexpected. This trend is consistent with the basic Arrhenius relationship. The Arrhenius equation predicts that the diffusion coefficient will increases with increasing temperature. D D e = 0 E d RT (3.7) where D: diffusion coefficient D 0: the pre-exponential factor E d : activation energy of diffusion

102 89 If the difference between the DER-SAA and DOMS-SAA systems is checked, it is obvious that the increase of the diffusion coefficient, D, changes more rapidly with temperature in the DER-SAA system versus the DOMS-SAA system. The activation energies listed in Table 3.5 show the activation energy for DER-SAA system is also higher than in the DOMS-SAA system. Just by definition, diffusion activity energy is an energy barrier for the diffusing molecules to overcome for the diffusion to proceed. At first glance, it seems that the activation energy for the DOMS-SAA system should be higher than that of the DER-SAA system based on the higher molecular packing of the liquid crystalline DOMS-SAA system. However, this phenomenon is indeed the reflection of the molecular structure of these two systems. As stated above, the liquid crystalline thermoset consists of rigid monomers linked together by the SAA crosslinking agent. Mobility of the rigid segments is not easily affected by external factors, such as testing temperature. Thus the effect of temperature on this LC thermoset is much less than on the flexible DER-SAA system. Because the diffusion of water molecules in the cured resin depends on the movement of segments in the matrix, it is feasible that water molecule diffusion is less dependent on temperature for DOMS-SAA system. Thus the change of D with temperature is much less rapid for the DOMS-SAA system. For the flexible DER-SAA system, the segment movement is sensitive to the temperature. The change in temperature causes a great deal of change in the properties of this flexible matrix material. Therefore, the diffusion coefficient changes rapidly with temperature for DER-SAA system.

103 90 Diffusion coefficient (cm 2 /s X10-10 ) DER-SAA-0.8 DER-SAA-0.9 DER-SAA-1.0 DER-SAA-1.1 DER-SAA-1.2 DER-SAA system Temperature ( o C) Figure 3.26 Effect of temperature on diffusion coefficient of DER-SAA system Diffusion coefficient cm 2 /s (X10-10 ) DOMS-SAA-0.8 DOMS-SAA-0.9 DOMS-SAA-1.0 DOMS-SAA-1.1 DOMS-SAA-1.2 DOMS-SAA system Temperature ( o C) Figure 3.27 Effect of temperature on diffusion coefficient of DOMS-SAA system

104 91 The effect of temperature on solubility coefficient. The temperature effect on the solubility coefficient, S, is different from the effect on the diffusion coefficient. As shown in Figure 3.27 and Figure 3.28, S decreases with increasing temperature. The dependence of the solubility coefficient, S, on temperature of a given polymer-penetrant system can be described in the form of a van t Hoff relationship S = S 0 e H s RT (2.7) where S: solubility coefficient S 0 : pre-exponential factor H s : change in enthalpy of solution of the penetrant The values of H s are given in Table 3.6. For the present study systems, H s takes a negative value. The decreasing of S with increasing of temperature means that the enthalpy of the solution of penetrant water has a negative instead of a positive value. As stated in section 2.2.1, the sorption process consists of two separate thermodynamic processes: (1) condensation of the water vapor gas, an exothermic process, and (2) mixing of the water with epoxy resin. Overall, the enthalpy of sorption is equal to the sum of the enthalpy of condensation and the partial molar enthalpy of mixing. H s = H m H g where H m is the heat of mixing for liquid water and H g the heat of condensation of the vapor phase. If the water in the epoxy matrix is free water, then H m should be positive. If the water molecules form hydrogen bonds with polar groups in the network, H m should be negative. Comparing the general data of hydrogen bonding enthalpy (10 to 30 kj/mol) and H g (40 kj/mol) with experimental data in Table 3.6, it is

105 92 reasonable to state that there are two kinds of water present. The first represents the free water that has no interaction with the epoxy matrix. The second is the water sorbed at special polar groups in the cured epoxy resin, such as the hydroxyl groups. If all sorbed water were in free form, then H s would be positive, which is not consistent with experimental data. If all sorbed water were in a hydrogen bonded form, then H s would reach the value of -50 to -70 kj/mol (H m + Hg ). This is not consistent with experimental data either. The special thermodynamic characteristic reveals that the interaction between the absorbed water and the cross-linked network is an exothermic processing. This is due to the formation of strong hydrogen bonds between the water molecules and the polar groups of the networks. This moisture absorption can be attributed largely to the moisture affinity of specific functional groups of highly polar nature in the cured epoxy resin. This has been verified by FTIR spectroscopy results in Chapter 4. Table 3.6. Activity energy of diffusion and heat of solution Activity energy Amine/epoxide Heat of solution System of Diffusion functional ratio (J/mol) (J/mol) DER-SAA DOMS-SAA

106 93 The change of S with temperature can be explained in terms of the interactions between water molecules and the polar groups in the matrix. Such interactions will be exothermic in nature, as described by the equation: Gaseous H 2 O molecule + Polar groups Bound H 2 O molecule + Heat As absorption proceeds, the available polar sites will form hydrogen bonds with water and an equilibrium will be established between bound water and free water. Increasing the temperature increases the external heat and pushes the above reaction to the left, decreasing the amount of bound water and thus the equilibrium uptake. This is consistent with the experimental data. Solubility coefficient (g/m 3 Pa) DER-SAA system DER-SAA-0.8 DER-SAA-0.9 DER-SAA-1.0 DER-SAA-1.1 DER-SAA Temperature ( o C) Figure 3.28 Temperature effect on solubility coefficient of DER-SAA system

107 94 Solubility coefficient (g/m 3 Pa) DOMS-SAA system DOMS-SAA-0.8 DOMS-SAA-0.9 DOMS-SAA-1.0 DOMS-SAA-1.1 DOMS-SAA Temperature ( o C) Figure 3.29 Temperature effect on solubility coefficient of DOMS-SAA system The effect of stoichiometry on the transport properties of water. The effect of the stoichiometric ratio on the diffusion coefficient, solubility coefficient and permeability at various temperatures is shown in Figures 3.13 to With regard to the trends of D, S, and P with the stoichiometric ratio, both of the systems, DER-SAA and DOMS-SAA, display the same trend, despite the DOMS-SAA system s liquid crystalline network. This suggests that the transport process is governed by diffusion and sorption mechanisms of similar nature for these two systems. There are two common factors in both of the epoxy systems. First, the morphology of both DER-SAA and DOMS-SAA systems is heterogeneous. For most epoxy-amine systems, the reactions between the epoxy and the amine cross-linking agent generically involve the opening of the epoxide ring by reaction with an amine hydrogen,

108 95 dominated by primary amine reactions with some secondary amine reactions. A twophase morphology (see chapter 5), consisting of a highly crosslinked domain phase surrounded by low crosslink density dispersed phase, is obtained. In other words, it has morphological heterogeneity. As our results show (detailed discussion in Chapter 5), there is an observable change in the relative amounts of the soft phase with the stoichiometric ratio. For epoxy-rich samples, the percentage of the hard phase increases with the stoichiometric ratio, the higher the ratio, the greater the percentage of the hard phase. However, when the stoichiometric ratio is larger than one, the percentage of hard phase decreases with the stoichiometric ratio. Second, there is hydrogen-bond interaction between water molecules with polar groups in the cured epoxy resin. The concentration of polar sites (hydroxyl and amine groups) increases with increasing amine/epoxide functional ratio in the cured mixture. Some of the penetrating water molecules form hydrogen bonds with the polar groups, some of the water molecules are in the free state (non-hydrogen bonded). All of the trends seen for these two factors are the same for the DER-SAA and DOMS-SAA systems. The only difference between the DER-SAA system and the DOMS-SAA system is caused by the molecular packing density, as explained previously. For the diffusion coefficient, D, a monotonic decrease with increasing amine/epoxide functional ratio at different test temperatures is seen for all samples. The solubility coefficients at all temperatures reach a minimum at stoichiometric ratio. The permeability also reveals a minimum when this functional ratio is unity. This illustrates that stoichiometric epoxy-saa samples have the best permeability properties. The effect of stoichiometry on moisture diffusion could be explained by the above two factors. The

109 96 detailed discussions of these effects will be given in Chapter 4 and Chapter 5 based on the FTIR, dynamic mechanical analysis, and atomic force microscopy. For now, the major argument is outlined below. The trend of D with stoichiometric ratio can be attributed to the two-phase factor and to hydrogen bond interaction between water and the polar groups in the cured resin. For epoxy-rich samples, the fraction of hard-phase increases with increasing functional ratio. The hard phase acts as a blockade against the diffusing water molecules, effectively slowing their advance. Therefore, the diffusion coefficient decreases with increasing crosslink density. For amine-rich samples, however, the fraction of hardphase decreases with increasing of functional ratio. Surprisingly, the diffusion coefficient decreases with the amine/epoxide functional ratio. This is an unexpected result. If the two-phase morphology played a major role in determining D, it would be expected that D would increase with SAA concentration. But, this is not the actual case. The experimental result can be attributed to the increasing number of polar groups. Under the hypothesis, water molecules are fixed to certain polar sites through hydrogen bonding. Indeed, the strength of hydrogen bonds would influence the diffusion process. As the amine/epoxide functional ratio increases, more of the hydrophilic groups exist, and greater amount the absorbed water is bonded at these sites. Thus the dissociation of the polymer-water complex requires a great energy input. So these polar sites acts as sink wells. It will fix water molecules and make them immobile. The effective diffusion process is slowed down. The diffusion coefficient will, therefore, decrease with an increasing amine/epoxide functional ratio. The sink effect of these polar sites may control the diffusion mechanism of water molecules. This assumption was supported by

110 97 FTIR, DMA and AFM results. Detailed explanations will be given in Chapter 4 and Chapter 5. The hydroxyl and tertiary amine groups play an important role in water bonding to the network. The general form can be expressed as two factors. One factor, S 1, is effect of morphology. The other, S 2, is the effect of polar groups in the cured network. S = S 1 (morphology)*s 2 (hydrogen bonding) = S1(morphology)*k*(X*H epoxide + Y*H amine ) Where S is the solubility coefficient. k is a constant, dependent on experimental conditions, such as temperature and the molecular weight of structural unit of the crosslinked network. The polar group effect, S 2, can be expressed a simple additive relationship linking water absorption to network composition can be applied to the present work. X and Y are stoichiometric parameters. H epoxide and H amine are the functional contributions the hydrogen bonding of the water to the network. This expression illustrates also that solubility consists of two parts. One part is the solubility of water forming normal solution with the epoxy resin matrix. This is accounted in the term of S 1. The other part is due to the water penetrating into the matrix forming hydrogen bonds with hydrophilic groups. From this relationship and the twophase model, the change in the solubility coefficient with the amine/epoxide functional ratio of the network can be explained. (1) For epoxy-rich samples, there are two important characteristics. One is that the crosslink density is lower than stoichiometric samples. Thus the movement of chain segments is easier than in higher crosslinked networks. The free volume fraction in these samples is greater which provides the necessary space for water molecules to inhabit.

111 98 From the two-phase morphology model, it is reasonable to assume that the absorbed water prefers to occupy the low crosslink density internodular phase because the free volume is higher in that region. The fraction of this phase decreases with the amine/epoxide functional ratio. So the amount of water should decrease with increasing stoichiometric ratio. The second characteristic of epoxy-rich samples is that the polar group concentration is lower. As we know, these polar groups prefer to form hydrogen bonds with penetrant water molecules. A low concentration of polar groups means that they will not contribute much to the solubility. Thus polar groups play a minor part in determining the solubility of epoxy-rich samples. Therefore, the two-phase morphology will determine the change in the solubility coefficient with amine/epoxide functional ratio. In Chapter 5, the quantitative analysis reveals that the fraction of hard-phase increases with amine/epoxide functional ratio. The hard-phase is a highly crosslinked phase. Its free volume fraction is lower. So the solubility decreases with increasing SAA content in the cured mixtures, that is, S decreases with increasing functional ratio for the epoxy-rich cross-linked networks. (2) For amine-rich samples, increasing amine/epoxide functional ratio increases the percentage of soft phase and free volume. Thus, there is more space for water molecules to occupy in the cured resin. Therefore the solubility increases with stoichiometric ratio. Simultaneously, the concentration of polar groups increases. So not only can penetrating water molecules can occupy the greater free volume, water molecules can also bind at the extra polar sites. This allows more water molecules to absorb into the resin. Thus S increases with functional ratio for the amine rich networks. Sole et al.[49] also showed that the isothermal equilibrium weight gains are a function of

112 99 the free volume fraction and polarity for rigid, semi-rigid and flexible epoxy networks. Polarity plays an important part in determining the equilibrium weight gain, as is easily seen for the three types of epoxy resins: monoamine resins, difunctional resins, and trifunctional resins. The monoamine epoxy resins are the least polar and absorb the least amount of water, about 1 wt%. The difunctional resins take on a wider range of moisture, in the range of 1 to 3wt%. The trifunctional resins are the most polar and clearly absorb the most water, greater than 3 wt%. 3.5 Conclusions The moisture permeation process for both the conventional epoxy-amine system and liquid crystalline epoxy system follows Fick s law. There is no obvious evidence that the absorbed water changes its transport behavior from Fickian to non-fickian for either the DER-SAA or the DOMS-SAA systems. The high cross-linking density and/or the rigid rod structure are responsible for the Fickian behavior. The liquid crystalline epoxy network exhibits higher barrier properties to the moisture permeation than the conventional epoxy network, even though the monomer chemical structures of the DOMS and DER are similar. The efficient chain packing of the smectic mesophase of the liquid crystalline thermosets (LCT) is the main factor for this remarkable difference. This was ascribed to the microstructure of liquid crystalline epoxy resins, which consists of closely packed anisotropic domains. This close packing will act as a barrier for the diffusion of the penetrant through the matrix. The segments arrange such that little free volume exists for the penetrant molecules to occupy. All of these special characteristics of the anisotropic liquid crystalline thermosets make the macro-scale properties, such as

113 100 the diffusion coefficient and sorption coefficients different from the isotropic DER-SAA epoxy system. The curing conditions, such as the stoichiometry, have a strong effect on the moisture permeation properties. The diffusion coefficient decreases monotonically with increasing amine/epoxide functional ratio. The permeability (P) and solubility coefficients (S) reach a minimum when the functional ratio equals one. For epoxy rich networks, P and S decrease with that ratio, while for amine rich networks, P and S increase with that ratio. As to the influence of the stoichiometry of the amine/epoxide functional group on D, P and S, the two-phase morphology model and the hydrogen bond interaction between absorbed water and the network are useful tools for the explanation of the test data.

114 CHAPTER 4 INTERACTION OF WATER WITH EPOXY RESINS 4.1 Introduction Understanding the effect of moisture on epoxy resins is critical for the design of reliable composite structures. The size of water molecule is relatively small. It is well known that in the liquid and solid state the water molecules are strongly associated through hydrogen bond formation. Hydrogen bonding is a phenomenon in which hydrogen atoms are bonded to very electronegative elements and show strong attraction between molecules. As a result of hydrogen bonding, the boiling point of water is considerably higher than other liquids with similar sized molecules. In these substances, the hydrogen atoms are normally located between two electronegative atoms, but closer to one than the other. The combination of these special features distinguishes it from the majority of organic penetrants and penetrant gases, such as nitrogen or carbon dioxide. Whereas the diffusion coefficient generally increases with concentration of organic vapor, marked decreases have been observed with water in several polymers.[barrer, 1958 #294; Barrie, 1984 #298; Barrker, 1961 #329; Crank, 1968 #56; Dixon-Garret, 2000 #226; Dutheillet, 1999 #206; Fieldson, 1992 #338; Fieldson, 1995 #215; Frisch, 1969 #295; Greenfield, 1993 #380; Gurtin, 1979 #374; Hayward, 1997 #364; Hietala, 2000 #213; Hong, 1997 #59; Marais, 1999 #246; Marais, 2000 #242; Marcovich, 1999 #366; Mateo, 2000 #255; McConville, 2000 #237; Merkel, 2000 #218; Nagasubramanian, Reimschuessel, H. K. #187; Neogi, 1996 #257; Netz, 1994 #362; Pasternak, 1970 #54; 101

115 102 Paul, 1999 #71; Paul, 1999 #82; Paul, 1999 #118; Peterson, #197; Rajagopalan, 2000 #200; Sammon, 2000 #236; Tamagawa, 2000 #239; Tonge, 2000 #230; Vanlandingham, 1999 #60; Vanlandingham, 1999 #65; Vieth, 1991 #306; Vrentas, 1975 #302; Wang, 2000 #201; Yi, 2000 #222] The moisture affinity of epoxy resins and composites is generally attributed to the polar functional groups in the resin. Values for the enthalpy of formation of the hydrogen bond in the range 3.4 to 6.6 kcal/mole have been obtained. As a result, strong localized interactions may develop between water molecules and suitable polar groups of the epoxy; on the other hand, in relatively non-polar materials, clustering or association of sorbed water is encouraged. The structure of liquid water as revealed by x-ray and neutron diffraction studies is consistent with a picture in which each water molecule is surround by four others, to which it is hydrogen-bonded, with approximately tetrahedral coordination. There is, of course, dispersion in the distribution of molecular separations, and also in the distribution of relative molecular orientations. Indeed, this ordering extends only about two molecular diameters from a given molecules, and orientational ordering does not extend beyond the first shell of neighbors. Despite intensive study, our understanding of how the molecular interactions determine the structure and properties of water remains incomplete. At room temperature, at the typical spacing of water molecules in the liquid, the molecular interaction is dominated by hydrogen bonding with energy of about 10 k B T per bond, and this dominant water-water interaction becomes saturated when four other molecules surround the central molecules. The analysis of interaction between epoxy resins and water has been a particular challenge for the polymer chemist because of the complexity of the three-dimensional

116 103 network, the types of monomers and initiators, the mechanism of polymerization reactions, the insoluble nature of the product and the susceptibility of the network to hydrolysis and other types of chemical attack. Consequently, there has been little knowledge of the structural basis of the physical, chemical and ultimate mechanical properties of the epoxy resins. However, it is essential that knowledge of the structures and interaction with moist environments be obtained in order to optimize the performance of epoxy resins. It is observed[3-6, 9, 10] that property differences between desiccated and soaked samples exist, which supports the concept that physical and/or chemical modifications are introduced into the polymer network by moisture-temperature aging. Although the exact mechanism of this change has not been completely identified yet, previous investigations offered explanations related to the microvoiding and crazing of the polymer[5, 9] or by assuming a dual state of water[6] in the cured epoxies. For the dual state model, the material would retain the penetrant even at high temperatures due to some strong mutual molecular interactions generated by the polar nature of the epoxy-water system, which have been identified in several spectroscopic studies.[86-92, ] Some studies, however, suggested that the effect of water can be related to the normal behavior theoretically predicted for the compositional dependence of glass transition by considering water in epoxy resin as miscible blends. The approach based exclusively on the free volume concept often fails to fit the experimental glass transition temperature data. The failure of fit was attributed to the value of Tg chosen for water or the water solubility in epoxies. However, even when correct water solubility and water thermal parameters were used, large discrepancies were observed by examining systems of

117 104 different composition and extent of cure. This anomalous plasticization could be explained only by considering strong mutual interactions between the dissolved molecules and segments of the polymer. The hydrogen bonding of water-penetrated epoxy resins and other polymers has been tested on a diglycidyl ether of bisphenol A epoxy cured with different types or/and different amounts of amine curing agents[67, ]. The increased water solubility and glass transition temperature depression observed for the sample cured with increased content of amino hardener were attributed to the high hydrophilic character of amine. The comparison between the experimentally determined and theoretical values of wet glass transition temperatures indicates neither the free volume model nor the entropy model can fit the experimental data in the full range of variables. The free volume better describes the behavior of low amino content resins while the entropy model becomes effective at higher amino contents. Aside from these phenomenological characterizations of moisture sorption, very little molecular level information exists concerning the interaction of water molecules and the epoxy matrix. Although water molecules have very simple chemical structures, there is difficulty in characterization of water in epoxy resins. When using basic techniques, such as diffraction of x-rays and scattering of neutrons, to study polymeric materials at the molecular level, there are difficulties localizing such small and versatile molecules, which can rapidly change their interactions with neighboring molecules. Fortunately, Fourier transform infrared spectroscopy (FT-IR) and high resolution nuclear magnetic resonance (NMR) of solids, which are certainly the most sensitive and precise techniques to study hydrogen bonding, have proved to be very powerful techniques to

118 105 determine the interaction configurations that these water molecules establish with neighboring molecules, provided there is no saturation of bands and no band overlap in the range of interest due to these molecules. These two techniques are not inhibited appreciably by the insoluble nature of the cured resin. Consequently, substantial structural information at the molecular level can be obtained. Studies have been made to identify the location on the cured resin structure at which the absorbed moisture interacts. These studies used NMR[86-92, 108, 133, 135, 136] and IR spectroscopy[bellenger, 1987 #387; Fieldson, 1992 #338; Moilanen, 2000 #243; Sammon, 2000 #236; Yi, 2000 #222] and showed that interaction occurs at a primary or secondary amine site or at a hydroxyl site. Danieley et al.[104] showed that the absorbed moisture interacts primarily with the hydroxyl sites. NMR data have shown that the interaction between the absorbed moisture and resin structure is weak and on the order of hydrogen bonding. Danieley et al.[104] suggested a two-dimensional bonding model representation of the polymer chain to support their conclusion that the major interaction is at the hydroxyl site. They argued that, if all the remaining amine and epoxy groups were to interact to form additional hydroxyl groups, the average number of atoms between crosslink junctions which affects flexibility, thus Tg, would be unchanged. The moisture absorption, on the other hand, would continue to increase in direct proportion to the number of additional sites created in further reaction between epoxy and amine groups. In order to investigate the permeability properties of water in the cured network, this chapter discusses the interaction of water with DER-SAA epoxy system and liquid crystalline epoxy DOMS-SAA system. This chapter will focus on the characterization of

119 106 the interaction of epoxy resin with water by using three powerful techniques, FTIR, DSC and DMA. (1) FTIR will reveal interaction information at the molecular level; (2) DSC will could give valuable information about the thermodynamic properties, and (3) dynamic mechanical analysis (DMA) could show, at segment chain level, the effect of segment movement on the interaction. It is interesting to compare the different type of epoxy systems because of the special mesophase state of the DOMS-SAA system. A direct comparison between the two systems has not been made before. 4.2 Experiment Fourier Transform Infrared Spectroscopy (FTIR) FTIR analysis was done on a Magna-IR E.S.P. System 760 spectrometer with Nicolet s OMNIC software. DTGS KBr was selected as the detector. The cured epoxy film, prepared by the same method mentioned in Chapter 3, having dimensions of about 1.5 cm 1.5 cm 8 µ, was mounted in the sample holder. Absorbance spectra at 4 cm -1 resolution were obtained with a Fourier transform infrared spectrometer using 256 sample and reference scans. With FT-IR analysis, it should be remembered, especially for quantitative analysis, that the highest signal-to noise ratio rather than the highest signal is the most important factor to be considered during sample preparation. Of course, linear optical signal requires that the infrared samples have no residual orientation, no voids or holes, and a uniform distribution of material. Careful control of the sample preparation procedure must be achieved in order that reproducible samples are obtained for the infrared examination. In the present research, the FTIR spectra data were further processed for spectral subtraction, curve fitting, deconvolution etc., and it is absolutely

120 107 necessary to keep the absorbance in the linear range of the Beer-Lambert law or spectral artifacts will be generated during the data processing steps. To minimize Beer s-lambert law deviations for subsequent spectral manipulations, the epoxy thickness is such that the maximum absorbance in any spectrum is less than Differential Scanning Calorimetry (DSC) DSC analysis was performed using a Seiko Instruments SSC5200 DSC220C with an SDM5600H Data station. Calibration was done with an alumina standard. The nitrogen flow rate was100 ml/min. Samples were sealed in aluminum pans and heated/cooled in the DSC at a rate of 10C/min. Sample weight range was 10 to 20 mg. Determination of glass transition temperature and changes in heat capacity were determined using the software on this DSC station Dynamic Mechanical Spectroscopy (DMS) DMS was done on Seiko SDM/5600 DMS 110 thermomechanical analyzer interfaced with a Seiko SDM5600H Rheostation for data collection and analysis. The mode was set to the double cantilever flexure type. DMS imposes a sinusoidal strain on a sample over a range of temperatures and frequencies. All samples were run at 0.75 C/minute at five frequencies (0.1, 0.5, 1, 5, and 10 Hz). The information was collected in the form of storage modulus (E ), loss modulus (E ), and the loss dispersion (tan δ) as a function of temperature and frequency. The sample size used was approximately rectangular with dimensions of 45 mm in length by 10 mm in width by 1 to 1.5 mm in thickness. Samples were clamped in the flexure with a torque wrench using 9.4 kgf/cm,

121 108 as suggested by the manual, to provide sufficient clamp force and to prevent slippage of the sample during testing. The sample chamber was purged with nitrogen gas at rate of 200 ml/minute. In order to capture all three major relaxation of the sample, the temperature range was 100 to 250 C. The temperature rate was relatively slow (0.75 C/minute) to ensure that all five frequency tests occurred at the same temperature for each data point. Transition temperatures are given as the peak in the tan δ curve. The activation energy for two of the relaxations, γ and α, were calculated using an Arrhenius relationship and the tan δ curve. ln( f ) = ln( f 0 ) + E a RT where f is the test frequency, f 0 is a pre-exponential factor, T is the relaxation temperature in absolute scale (K), R is the gas constant, and Ea is the activation energy. The activation energy was calculated from the slope of the plot of ln(f) versus 1/T. 4.3 Results and Discussion Assignment of IR bands The typical spectra for the DER-SAA and DOMS-SAA systems are shown in Figure 4.1 and Figure 4.2. In order to interpret the spectra, it is necessary to have some insight into the structural origin of the numerous infrared bands in order to use them effectively. Unfortunately, the complexity of the molecules involved made complete band assignments impossible and only major assignments are given here.

122 109 To facilitate assignment of the numerous IR bands of epoxy resins, we performed two preliminary experiments. The first consisted of recording the IR spectra of epoxy monomers of DER 383 and DOMS (before addition of amine SAA to epoxy). The secondary series consisted of dried cured samples. A comparison between these dry samples and the monomer spectra conveyed useful information on the main spectra bands. Spectra of dried samples: Figure 4.3 shows the infrared spectrum recorded at room temperature, in the range cm -1, of the monomer DER383 and the DER- SAA-1.0 cured resin. In common with other polymers containing aromatic rings, the infrared spectrum is characterized by relatively sharp bands attributed to the predominantly conformationally insensitive modes of aromatic rings and relatively broad bands attributed to comformationally sensitive backbone modes together with hydrogen-bonded groups. In the spectrum of DER-SAA-1 cured resin, we can distinguish three groups of bands. At wavemumbers higher than 3100 cm -1, we find a band that is quiet broad. We can logically deduce some important information from this special band. From the Figure 4.3, it is obvious that this band consists of several bands. If it were due to only one kind of vibration, the width of the band would be very sharp. From the shape of this band, we are sure that several band types contribute to the effect. There are several possibilities of assignment of this group. One possibility is the assignment to the O-H stretch in hydrogen-bonded O-H. groups. Another possibility is the assignment to the O-H stretch in non-hydrogen bonded O-H group.

123 Absorbance Wavenumber (cm -1 ) Figure 4.1 FTIR spectrum of DER-SAA-1.0 sample Absorbance Wavenumber (cm -1 ) Figure 4.2 FTIR spectrum of DOMS-SAA-1.0 sample

124 111 The characteristics of these two kinds of bands are different. For the hydrogen bonded O-H stretch, the hydrogen bond is relatively weaker than the covalent bond. The hydrogen bond changes with the temperature. Ngono, Marechal, and Mermilliod[142] investigated the possibility of the assignment by runing the spectra at different temperatures to see the change of this O-H group. They selected the aromatic DGEBA- DDM and DGEBA-TETA systems to investigate. After comparison of the spectra obtained at room temperature and at a temperature above the boiling point of water, they reached some conclusions: (1) The difference spectrum subtracted the spectra of 120 C from the spectra of room temperature is only composed of differential-type bands, displaying a minimum and maximum at close wavenumbers. This proves that no evaporation of water molecules occurs upon raising the temperature, as evaporation would appear in this spectrum in the form of full bands of water; (2) The presence of the positive ν(o-h ) intense band around 3365 cm -1 and negative ν(o-h) band around 3590 cm -1 shows that at 120 C hydrogen bonds are broken. As the amount of water is very small in the dried samples, these hydrogen bonds are established by COH groups of alcohols. The changes of hydrogen-bonded bands into free bands upon rupture of hydrogen bonding was proven by the weak features displaying a maximum at 1127 cm -1 and minimum at 1085 cm -1. However, the case is not so simple. Primary and secondary amines also have the characteristic N-H stretching absorptions in the cm -1 range of the IR spectrum. It is located in the same range as alcohols, and these amine absorption bands are generally sharper and less intense than hydroxyl bands. Primary amines show[143] a pair

125 112 of bands at about 3350 and 3450 cm -1, and secondary amines show a single band at 3350 cm -1. Tertiary amines show no absorption in this region because they have no N-H bonds. From the spectra of Figure 4.3, it is obvious that the DER383 spectrum has no peaks beyond the wavenumber of 3100 cm -1. However, there is obvious broadband in the spectrum of SAA cured DER systems. This results from the curing reaction between the DER and SAA. During curing the epoxy group interacts with the SAA amine groups. The epoxy ring is opened to produce a hydroxyl group and secondary/tertiary amine groups. The middle and well-identified group of bands is that one which falls in the region between cm -1. These bands are due to C-H stretch (ν C-H ) in the alkyl groups. Comparison of these bands that appear in both spectra of resins with these in the paper by Ngono et al.[44, 142] allows all the sub-bands of this group to be assigned Absorbance B 0.2 A Wavenumber (cm -1 ) Figure 4.3. FTIR spectra of (A) monomer DER383 and (B) DER-SAA-1.0

126 113 The last group consists of the set of more compact bands that appear below 1650 cm -1. We assign the most important components of this group in Table 4.1, following usual assignments[62, 144] and after a comparison of two spectra of Figure 4.3. We assigned the intense band around 1250 cm -1 to ν(c-o-c) mode that appears in aromatic ethers. We have labeled it ν(c(φ)-o-c(alkyl). The other ν(c(φ)o-c(alkyl) band appears around There is no doubt about the assignment of the band at 915 cm -1 to the C-O vibration in epoxide groups. Hydration. After collecting the spectra of the dry samples, the samples were soaked in deionized water and allowed to equilibrate. The soaking time was estimated according to the diffusion coefficient and the samples were given extra time to assure equilibrium had been reached. At room temperature, the diffusion coefficient is 1.0 to cm 2 /s, indicating that only a few minutes are required for complete sorption of vapor into the film. At room temperature, more than 120 minutes was allowed for the epoxy film to equilibrate in the liquid water environment. Water molecules were uniformly distributed inside the film. Then the samples were put into the FTIR spectra chamber to collect the spectra of the wet samples. No base-line correction was performed on the spectra. In Figure 4.4, we display the spectrum of water sorbed DOMS-SAA-0.8 resin films and that of the corresponding dry DOMS-SAA-0.8 sample film. Also included is the hydration spectrum of DOMS-SAA-0.8 epoxy resin film, which is equal to the difference between the spectrum of water-sorbed sample and the spectrum of dry sample. In other word, the bands in the hydration spectrum are a reflection of the effect of water

127 114 in the resin. Finally, it must be emphasized that the spectra are displayed on an absolute absorbance scale. In other words, they are not arbitrarily scale expanded. Table 4.1 Assignment of FTIR band* Wave-number Characteristic Assignment (cm -1 ) ~3500 strong ν(oh) 3067 ν as (C H2) epoxy 3038 ν(c-h) phenyl ring 2968 strong ν (CH) alkyl group 2929 strong ν (CH) alkyl group 2874 strong ν (CH) alkyl group ~2068 weak ϕ, di-substituted 1887 weak ϕ, di-substituted ~1759 weak ϕ, di-substituted 1608 strong ν (C=C) phenyl ring 1583 strong ν (C=C) phenyl ring 1511 strong ν (C=C) phenyl ring 1457 ν (C=C) ϕ + δ as (CH3) 1431 weak δ (OCH2) 1414 weak δ (CH) epoxy 1385 δ s (CH3) gem-dimethyl 1363 weak δ s (CH3) gem-dimethyl 1347 δ (CH) epoxy,δ (C-N) secondary amine 1298 ν (C-O) + ν (C-C) 1250 strong ν (C ϕ -O-C alky l) 1185 δ (ϕ-h) in plane, stretch of SO 2 group 1086 δ (ϕ-h) in plane 1036 ν (ϕ-o-c) 1012 δ(ϕ-h) in plane bending 916 epoxy ring 831 δ(ϕ-h) out-of plane *Data is assignment is based figure 4.3 and from [Bellenger, 1987 #387; Fieldson, 1992 #338; Moilanen, 2000 #243; Sammon, 2000 #236; Yi, 2000 #222]

128 115 The hydration spectrum in Figure 4.4 consists of full bands due to H 2 O and of differential bands due to the modification of absorption bands induced by the addition of water molecules. The full bands due to water molecules are consequently easily identified. These are the broad band at 3274 cm -1 and the sharp band at 3648 cm -1 in the spectra Absorbance A:Dry B: Wet B-A Wavenumber (cm -1 ) Figure 4.4 Spectra of cured DOMS-SAA-0.8 samples. The broad and intense hydrogen-bonded O-H stretch band which culminated at 3274 cm -1 may be due to either to hydrogen bonding formed either between water molecules and hydroxyl groups or between water molecules and nitrogen atoms on the amine groups or to both of them. The band at 3648 cm -1 is the O-H stretch band due to O- H groups of H 2 O molecules that do not establish hydrogen bonds. We will discuss the relative number of such free OH groups of H 2 O.

129 116 The bending band δ of H 2 O, which is known to be much less sensitive to hydrogen bonds than the O-H stretch band, is composed of a shoulder at 1605 cm -1 which we assign to H 2 O molecules that establish no hydrogen bonds. In water vapor, this band is also found[95, 145] at 1595 cm -1. The band at 1635 cm -1 was assigned to bending vibrations in H 2 O molecules that establish two hydrogen bonds. In liquid water, this band falls in close vicinity to this value. Ngono et al. [142] found that the integrated intensity of this bending band of O-H in DGEBA-DDM is the same as that found in the spectrum of a film of liquid water 0.18 um thick. Assuming that the integrated intensity of this bending mode scarcely depends on whether the H 2 O molecules establish hydrogen bond or not, they deduced that the number of H 2 O molecules in the film corresponding to spectrum of DGEBA-DDM is of same order as the number of molecules in a film of water 0.18 um thick, that is molecules/cm 2. They deduced further that the number of water molecules is 0.18 molecules per alcoholic group (both hydrogen bonded and non-hydrogen bonded). Based on the estimation that the ratio of hydrogen-bonded alcohol groups to free alcohol groups falls between 2 to 3, they concluded that only some of the free alcohol groups in equilibrium with the ambient atmosphere establish a hydrogen bond with a H 2 O molecule embedded in the resin Peak Curve Resolving of the Difference Spectrum From the above spectrum (Figure 4.4), it is obvious that the water absorbed in the epoxy resins has two states. One is free water state. The other is water forming hydrogen bonds with polymer functional groups, especially the polar groups on the structure of the cured epoxy, regardless of its liquid crystalline nature. Therefore, we need to answer some relevant questions. (1) How much of the absorbed water is free water that has no

130 117 interaction with the polymer network; (2) for the hydrogen-bonded water, how much is bonded to the different polar groups, ie, what percentage of the water is bonded to the hydroxyl groups and what percentage is bonded with the amine polar groups; (3) Do other polar groups exist in the cured three dimensional network that also form hydrogen bonds with water; (4) What trends are seen with changes in the stoichiometric ratio of epoxy to curing agent; (5) Are there differences between the DOMS-SAA and the DER- SAA systems? In order to answer these questions, a quantitative study of hydrogen bonded versus non-hydrogen bonded water was performed on all the DER-SAA and the DOMS- SAA systems for different stoichiometric ratios of amine groups to the epoxy groups. Polar groups forming hydrogen bonds with water. The curing processing is very critical in terms of the analysis of the polar groups in the crosslinking network. Many researchers[118, 146] [147] have confirmed that the epoxy-amine system forms exclusively from epoxide-amine addition reactions. There are no epoxide homopolymerization reactions. The reaction of epoxides with primary or second amines involves the following reactions: RNH 2 O CH 2 CH RNH CH 2 CH OH R 1 R 2 NH O CH 2 CH R 1 R 2 N CH 2 CH OH Figure 4.5 Reaction between epoxy and amine groups

131 118 It has been found that the epoxy ring opens in one of two ways[135]: anionically or cationically. In the reaction between an epoxy group and an amino group in the presence of a proton donor, one of the C-O covalent bonds in the epoxy ring is weakened by active hydrogen of a proton donor so that the carbon becomes slightly positive. The nitrogen in the amino group attacks the partially positive carbon in the epoxy ring to produce a new chemical bond and a hydroxyl group. The problem of the general feasibility of a non-catalytic occurrence of this reaction, that is, via a direct amine-epoxy group interaction, is more complex. For the present study, we will not give further discussion about this mechanism. We will focus on whether the nascent hydroxyl groups react further with others or keep in the cured resin. The answer is very important for our spectra curve deconvolution, especially for epoxy-rich mixtures. A possible side reaction is etherfication, especially when the amine is present in less than stoichiometric concentrations. Reaction of epoxide and hydroxyl may occur to produce an ether group: O R CH CH 2 H R O CH CH 2 R OH R R CH CH 2 O CH CH 2 R Figure 4.6 Etherfication for epoxy resins Hydroxyl groups for this reaction are supplied by previously formed glyceryl units. However, Williams[68] has shown that this reaction cannot occur to a significant extent under the normal cure conditions. He formed an adduct by reacting one mole diamine with four moles phenyl glycidyl ether. This adduct was mixed with epoxy resin and was heated for an adequate amount of time. There was no significant increase in

132 119 viscosity. Infrared spectroscopy of the mixture showed no significant reduction in the intensity of the absorption band due to the epoxide group near 910 cm -1. It is concluded, therefore, that the etherification reaction is not catalyzed by any of the products of the epoxide-amine reaction and cannot be postulated to contribute to the cure of systems containing less than stoichiometric amounts of amine. Rozenberg[147] also gave the same conclusion about the side reaction of hydroxyl groups. It was concluded that for aromatic amine and epoxide systems no reaction takes place below 200 C. At high temperatures, one can observe a slow reaction. But the reaction rate is practically low. This side reaction may be catalyzed when primary or secondary aliphatic or aromatic amines containing a heterocylic nitrogen atom, such as diaminopyridine, is used as a curing agent. However, even in this case the reactivity of primary and secondary amino groups is significantly higher than that of the hydroxyl group. This means that the contribution of this side reaction at stoichiometric ratios of functional groups or excess epoxy groups can be ignored. From the point view of group reactivity, we can also reach the same conclusion. In the DER-SAA or DOMS-SAA system, there are at least five types of reactive groups because of the unequal reactivity of the two-amine groups: aromatic primary amine, aromatic secondary amine, primary sulfonamide, secondary sulfonamide, and the nascent hydroxyl formed during curing. The tertiary amines formed by the reaction of two epoxy groups with a primary amine are ineffective catalysts because of the steric factor. Obviously, the reactivity of these groups are different from each other. In order to establish a meaningful curing picture and our curve deconvolution for this complex

133 120 system, it is first desirable to characterize the reactivity differences of these reactive groups with the epoxy ring in the given temperature range. The nucleophilicity of the functional groups can be characterized by the dissociation constant (Ka) of their conjugated acid. The higher the pka, the higher the nucleophilicity of the reactive groups. As the data shown in Liu s paper[135], the reactivity order of the groups is as follows: R O NH 2 NH S NH 2 CH 3 CH 2 OH O pka Active Inactive Figure 4.7 Reactivity order of functional groups From this order of reactivity, one can obtain some clues about the cure reaction. First, the primary amine group is more reactive than secondary amine groups. It is also known that primary amine groups react about twice as fast as secondary amines even though there is no evidence indicating that they do not react in parallel. Second, the reactivity of the amine group is higher than that of the sulfonamide group. Third, the hydroxyl group is more inactive than the sulfonamide group or the amine group. Furthermore, the reaction between the OH and the epoxy is relatively slow, about two times slower than that of epoxy and amine reactions. So it is reasonable to assume that the OH produced during the curing reaction will kept in the final network. However, we can not exclude the hydroxyl group acting as a catalyst. The cure reaction can be catalyzed by compounds containing active hydrogens, such as Lewis

134 121 acids, phenols, alcohols, as well as hydroxyl groups formed by amine epoxide addition. Therefore, the curing reaction usually shows autocatalysis in the early stages. The widely accepted mechanism[135] is the formation of an amine-proton donor adduct. Different models have been developed to describe the curing reactions of epoxies and primary amines. For the liquid crystalline thermoset DOMS-SAA system, the curing reaction is nearly the same as that of the non-liquid crystalline epoxy DER-SAA system. Lin et al. and Liu et al.[22, 135, 148] reported that the amine function of SAA is more reactive than amide function of SAA. The reaction mechanism, therefore, can be described by four different reactions, which are: (1) reaction between the SAA primary amine and an epoxide; (2) reaction between the SAA secondary amine and an epoxide; (3) reaction between the primary amine of SAA amide and an epoxide; and (4) reaction between the secondary amine of SAA amide and an epoxide. The curing process is very critical in terms of the formation of the liquid crystalline phase in the crosslinked network. Besides chemical reactions associated with the traditional epoxy chemistry, the phase transitions are also involved in the curing process. The reaction of DOMS and SAA is catalyzed not only by its nascent product but also by its nascent mesophase structure.[135] However, the final concentration of polar groups would not be much different from that in the non- LC epoxide resin. Based on the above analysis, it is nearly certain that the polar groups in the cured epoxide resin systems of DER-SAA and DOMS-SAA are the hydroxyl group, the secondary amine group, and the tertiary amine group. The ether bond can not have much hydrogen bonding capacity because it is a weak hydrogen bond acceptor[34]. If the

135 122 water interacts with the polar groups, it will form hydrogen bonds with the hydroxyl and amine groups. Therefore, it is logical to split the broad band of the hydrogen-bond peak of water into two sub-peaks. One is attributed to the water hydrogen-bonded to the hydroxyl group. The other is identified as the band of hydrogen-bonded water attached to the amine polar groups. For the curve subtraction and deconvolution, we are fully aware of the inherent problems involved in curve resolution. Even if the procedure is performed intelligently, it is always difficult to convince the skeptics. Without a priori knowledge of the band shape, position, width, and number of component curves comprising the complex band envelope, as well as the problem of drawing a base line, the results obtained from curve resolving are always open to debate. Nevertheless, using a set of reasonable and consistent assumptions, we have resolved the hydrogen-bonded and free water region into three components, all of which can be justified from the analysis given above and the trial of curve fitting. Rather than permitting the computer to obtain a best fit by changing a multitude of variables, we have restricted the options by making following assumptions. (1) The region of the absorbed water consists of three bands attributable to OH vibration mode which are attached to nitrogen atoms, hydroxyl groups, and free water. In fact, to double check our conclusion drawn from the analysis of the chemical reaction, we tried the least-square curve fitting of this region by assuming it consisted two, four or five bands. These trials yield a bad fit and the positions of these peaks have no physical meaning. Based on the analysis of the chemical reaction of the curing and trial of these fittings, we concluded that, in our view, a more logical approach to the problem of curve

136 123 fitting is to fit the region into three peaks. The crosslinked structure is also consistent with this assumption. The trend observed in the spectra as a function of stoichiometric ratio is also consistent with systematic changes in the intensity and position of the threepeak contribution. (2) The band shape was assumed to be Lorentz. The curve fitting trial curve also certified that the best fitting could be obtained by using alorentz function instead of a Gauss function. (3) A linear base line was assumed from 3700 to 3100 cm -1, where there is no significant underlying absorbance. (4) Curve fitting was limited to the spectral data available between 3700 and This circumvents the problem of overlapping contributions in the wings of the region. Using a curve fitting procedure of the least-square method, we resolved the water region into three components by using the Lorentz multi-peak fitting. An example of the curve fitting is shown in Figure 4.8. The left peak is attributed to the free water. The middle peak of the curves corresponds to the water hydrogen-bonded to the hydroxyl group. The right peak band is assigned to the hydrogen-bonded OH stretching modes that are bonded to nitrogen atoms of the amine group in the network. A flat base line was chosen from 3700 to 3000 cm-1 where there were no underlying bands. Given the inherent broadness of the band, this result is not unexpected. The goodness of fit can be seen in Figure 4.8, which shows the original spectrum obtained from the sample of DOMS-SAA-0.8 and the sum of the three fitting component spectra. Fitting error is well within experimental error limit. Similar results were obtained for other samples.

137 Absorbance Wavenumber(cm -1 ) Figure 4.8 Curve fitting for DOMS-SAA-0.8 sample. Dotted line is the sum of the three peaks Nature of the hydrogen bond. The absorbance maximum for a free hydroxyl group is near 3650 cm -1. The peak at this position was not observed in any of the DOMS- SAA or DER-SAA system spectra, whether dry or wet samples were examined. This peak is only observed in the hydration spectra. Two types of hydrogen bond can be postulated in this system: one type is situated at about 3400 cm -1, which is designated to the hydrogen bond between absorbed water and hydroxyl group. The second hydrogen bond is located at about 3250 cm -1, which is assigned to the hydrogen bond between a water molecule and a nitrogen on the amine group. This assignment is consistent with the assumption of the work of Bellenger et al.[138]. The structure of the polar section of a crosslinked polymer is shown in Figure 4.9. Several types of hydrogen bonds could be postulated within this system as follows: a hydrogen bond may be formed between the hydroxyl group and the closest nitrogen atom of the tertiary amine at the crosslink junctions (type 1); or a hydrogen bond between the hydrogen on a hydroxyl group and the closest ether oxygen (type 2); or the formation of a

138 125 hydrogen bond between two hydroxyls (type 3). There are two factors affecting the strength and probability of formation of intermolecular hydrogen bonds. These are the acidity of the donor and basicity of the acceptor, and the distance between the donor and acceptor. Comparison of the basicity of the acceptor groups for the three types of hydrogen bond postulated shows that type 1 bonds are most likely, and type 2 bonds are least likely. O S CH 2 N 1 O CH O H CH 2 2 CH 3 O C O CH 3 3 N H 2 C OH CH CH 2 CH 3 O C O CH 3 Figure 4.9 Schematic picture of polar group interaction in the crosslinked network When H 2 O molecules penetrate into the matrix, there are two kinds of polar groups in the cured epoxy. One is the hydroxyl group. The other is the nitrogen group. The H 2 O molecules can form hydrogen bonds with either of these groups. As mention previously, the hydrogen bond strength between water molecules and ether groups is very weak, therefore the possibility of forming this kind of hydrogen bond is less. The amount of water attached to the polar groups depends on the concentration of each of these two polar groups, hydroxyl and nitrogen.

139 126 O S N O H H O CH 2 CH 2 CH 3 CH O C O OH H CH 3 O H H O H OH CH 3 N CH O C O H 2 C CH 2 CH 3 Figure 4.10 Schematic presentation of hydrogen bond between water and polar groups in resin Effect of stoichiometric ratio. Figure 4.11 shows the effect of the change in the ratio of the amount of H 2 O hydrogen-bonded to the nitrogen of amine groups to the amount of H 2 O hydrogen-bonded to the hydroxyl group. It is obvious that this ratio is dependent on the chemical structure of the epoxy resin matrix. When the stoichiometric ratio is less than 1, the ratio is nearly independent of the stoichiometric ratio. This phenomenon seems a little weird. However, if we analyze the reaction of the epoxy group and the amine, this trend gives us much more valuable information about the structure of the epoxy resin matrix. When the stoichiometric ratio is less than 1, the reaction mixture has excess epoxy. There is not enough amine groups to open all the epoxy rings. From the chemical structure of SAA, it is easy to see that to each nitrogen, there are two hydrogens attached. When the amine group reacts with the epoxy group in the epoxy rich mixture, two of the hydrogens on each nitrogen will open two epoxy groups to produce two hydroxyl groups. So the number of hydroxyl groups is equal to the number of hydrogens in the amine groups. Therefore, the number of hydroxyl groups is twice the

140 127 number of the nitrogen groups. The ratio of the number of amine groups to the number of hydroxyl groups is 0.5 for the epoxy rich curing mixtures. It is not dependent on the value of the stoichiometric ratio, whether it is 0.8, 0.9 or 1.0. The ratio of the number of nitrogens to the number of hydroxyl groups is always 0.5 regardless the stoichiometric ratio. In other words, the concentration ratio of the nitrogen to that of hydroxyl is 0.5. For the stoichiometric ratio larger than 1, the trend is different from that for the epoxy-rich cured matrix. For DER-SAA-1.1, DER-SAA-1.2, DOMS-SAA-1.1 and DOMS-SAA-1.2, the amine group is in excess instead of the epoxy groups. Let us use the example of DER-SAA-1.1 to calculate the concentration ratio of nitrogen atoms to hydroxyl groups. The functionality ratio of the hydrogen to the epoxy group for DER-SAA-1.1 is 1.1. The formula of this mixture are 100 epoxy groups mixing with 110 hydrogen groups in SAA. So the number of nitrogen atoms in this mixture is 55, half the number of hydrogen atoms. After curing, all of the 100 groups of epoxy have reacted with SAA to produce 100 hydroxyl groups. So after curing, the ratio of the number of nitrogens to that of the hydroxyl groups is 55/100, that is Thus the concentration ratio of nitrogen to hydroxyl in the DER-SAA-1.1 cured sample is Using the same method, the concentration ratio of DER-SAA-1.2 sample can be calculated. The calculated theoretical value for DER-SAA-1.2 is So from the theoretical point, the absorbance ratio of OH that is hydrogen-bonded to nitrogen and hydroxyl groups increases with increasing of stoichiometric ratio. This is the same trend as seen in the experimental data.

141 128 The concentrations of the [OH] and [N] groups in the DER-SAA system for different amine/epoxide functional ratios can be calculated theoretically by the expressions: [ 4 r OH ] epoxy rich = 4 EEW + r M a (4.1) [ OH ] 4 a min e rich = 4 EEW + r M a (4.2) [ 2 r N ] = 4 EEW + r M a (4.3) where M a is the molar mass of SAA, r is the amine/epoxide functional ratio. EEW is the epoxy equivalent weight. The changes in [OH] and [N] concentrations with stoichiometric ratio based on theoretical calculation using Equation 4.1 to 4.3 are plotted in Figure Comparing Figure 4.11 and 4.12, there exists a similarity of the shapes between the theoretical polar group concentration ratio of [N]/[[OH] and the experimental ratio of water-nitrogen bond to water-oh hydrogen bond. This similarity further confirms our curve fitting assumptions. From the theoretical point of view, the [N]/[OH] ratio should be the same regardless of whether the DER-SAA or DOMS-SAA system are examined. However, the real experimental data show there is difference for these two systems. This phenomenon can be explained from the structure of the cured network. The molecular packing factor should also be considered for analysis of experimental data. First, concentration of water absorbed in DER-SAA resins is relatively low. For the DOMS-SAA system, this saturation concentration is even lower because of the higher

142 129 degree molecular packing in the liquid crystalline mesophase. The molecular chains arrange into a smectic mesophase. The chains are packed more closely than the chains of DER-SAA system. It is difficult for the water molecules to diffuse into a liquid crystalline mesophase. Thus, not all of the nitrogen and hydroxyl polar sites are occupied by the water molecules. The water molecules prefer the hydroxyl site to the nitrogen polar site for both DER-SAA and DOMS-SAA systems. Therefore the experimental ratio of [N]/[OH] is lower for the more-closely packed DOMS-SAA system versus the DER-SAA system. Second, as will be discussed in Chapter 5, the real morphology is not homogeneous in the cured epoxy resin. It consists of highly crosslinked regions dispersed in a lesser crosslinked phase (like a boundary). The free volume fraction in the low crosslink density region is higher. It is easy for water molecules to penetrate into this low density region. For the highly crosslinked region, the concentration of water is relatively lower. For the liquid crystalline DOMS-SAA system, the special packing of molecules in the smectic mesophase is even higher. Therefore a high percentage of the water penetrating into the matrix will stay in the boundary region. Thus there is relatively higher percentage of water existing in a cluster in the micro-void region. The cluster of water is the segregation of water molecules interacting with each other through OH-OH, not N-OH hydrogen bonds. With a lower percentage of water existing in the densely packed region, a higher percentage of the water molecules will remain in the water cluster. Keep mind that FTIR can not distinguish between water-water bands and waterhydroxyl band in the cured network. The area of the peak under the HO-HO band in the

143 130 spectra includes the contribution of both water-water band and water-hydroxyl groups. Thus, the percentage of water bound to nitrogen atoms calculated from FTIR experimental results becomes lower for DOMS-SAA samples. However, for the DER-SAA system, the chains are more flexible. The portion of water forming a solution with the matrix is larger. The relative percentage of water that stays in the void is lower. Thus the absorbance ratio of hydrogen-bonded OH to nitrogen and hydroxyl group is high. Therefore the experimental ratio of [N]/[OH] is lower for the DOMS-SAA than for the DER-SAA system. 1 DOMS-SAA DER-SAA Experiment data Relative [N]/[OH] Amine/epoxide functional ratio Figure 4.11 Ratio of water bonded with amine groups to that with OH groups

144 (N) (mol/kg) [OH] (mol/kg) (N)/(OH) Theoretical value Ratio or concentration amine/epoxide functional ratio Figure 4.12 Theoretical calculation of polar group concentration and ratio For FTIR, absorbance is determined by Beer s law. A = a b c where a is the specific absorptivity, b is the internal cell or sample thickness, and c is the concentration of the absorbing substance. For the absorbed H 2 O that is attached to the hydroxyl groups or nitrogen atoms, A = a OH OH b OH c OH A = a N N b N c N The thickness b is same as the sample thickness, b OH = b N The ratio of A N to A OH can be expressed as:

145 132 A A N OH = a a OH b b N N OH c c N OH = a a N OH c c N OH (4.4) So the ratio of absorbance is proportional to the ratio of the concentrations of these two polar groups in the cured matrix. Based on the analysis above, the ratio of the concentrations of the N and OH groups remains 0.5 for all epoxy-rich cured resin matrix. So the absorbance ratio stays the same in this range of epoxy resins for both DER-SAA and DOMS-SAA systems. If we go further to assume that the absorbance coefficient for the hydrogenbonded OH bonds are the same without considering it is either attached to hydroxyl or nitrogen, then the absorbance ratio would be 0.5. From the experimental data, we have found the absorbance ratios to be 0.45 and 0.35 for DER-SAA and DOMS-SAA respectively. This coincidence indirectly certifies that it is correct to deconvolute the hydrogen-bond peak into two peaks. From the results of the deconvolution of the spectra, we can obtain the change of the percentage of hydrogen-bonded water with changes in the stoichiometric ratio. As seen in Figure 4.8, the range from 3100 to 3700 cm -1 can be split into three peaks, two of them are OH bands associated with hydrogen bonding, the third is the free OH band of the H 2 O. By using Beer s law, we tried to determine how much of the absorbed water is hydrogen-bonded and how much is in the free state. Before we go further, we want to emphasize that this method of calculation is not perfect. In order to make it easy to calculate, we assume that the absorbance coefficient of free OH is same as the absorbance coefficient of hydrogen-bonded OH. In fact, the coefficients of these two kinds of OH groups may not be absolutely equal. However, it is a good practice to use

146 133 quantitative numbers to express the trend. We want to find the qualitative trend of the free water in the matrix with the change of stoichiometric ratio. We used the area corresponding to the three peaks to calculate the percentage of the hydrogen-bonded water. The formula is as follows: A OH N + OH OH P bond = AOH N + AOH OH + Afree A (4.5) where P bond : percentage of water that is in the hydrogen-bonded state A OH-N : the area on the spectra corresponding to the peak of hydrogen-bonded OH to nitrogen atoms A OH-OH : the area on the spectra corresponding to the peak of hydrogen-bonded OH to hydroxyl groups A free : the area on the spectra corresponding to the peak of the free OH band Figure 4.13 shows the relationship between the percentage of hydrogen-bonded water calculated using Equation 4.5. Relative fraction of hydrogen-bonding DOMS-SAA DER-SAA amine/epoxide functional ratio Figure 4.13 Fraction of hydrogen-bonded water of saturated samples

147 134 For both the DER-SAA and DOMS-SAA systems, the percentage of hydrogenbonded water increases with increased stoichiometric ratio. This is not unexpected. As the amount of SAA increases in the mixture in the cured resin, the amount of polar groups increases because the concentration of hydroxyl group and/or the amine groups increases (see Figure 4.12). These polar groups tend to interact with water molecules to form hydrogen bonds. As we stated earlier, the assumption that the absorbance coefficients are equal may cause some error in the absolute value of the percentage. However, this assumption will not affect the trend of this plot. The same argument used to explain Figure 4.11 can be applied here to explain the difference in the trend between the DER-SAA and DOMS-SAA systems. As stated before, the morphology of the cured epoxy is a two-phase heterogeneous network. Penetrating water can stay in the low crosslink density region in a cluster because the void fraction is high and void size is large. Water can also diffuse into the highly crosslinked domain region, where water forms hydrogen bonds with polar groups or exists in the free state as free water. For closepacked liquid crystalline DOMS-SAA samples, it is difficult for water molecules to get inside the domain region. Most of the absorbed water is in the low crosslink density region as clusters of water. Thus the fraction of water that stays inside the domain is relatively low. In other words, the fraction of hydrogen-bonded water is relatively higher. However, DER-SAA forms flexible networks. Water molecules can diffuse into the domain and remain in the free water state. Thus the fraction of free water is relatively high. Correspondingly, the hydrogen-bonded fraction is relatively lower than that of the DOMS-SAA system.

148 Thermal Properties Differential Scanning Calorimetry (DSC): For the DSC measurement, the samples were not given special treatment. Samples were cut from bulk samples that had been stored in the lab room environment (about 28 C and 30 to 50% RH). Because the mass of the sample required by the DSC is very small, just about 10 mg, the moisture in the sample may have diffused out during the test procedure. So one major concern for the DSC analysis is whether there is a possible loss of water from the sample. With respect to water vapor diffusing out of the sample during thermal analysis, some factors have to be considered: (1) the relative rate of the water diffusion to the rate of the DSC scan rate, (2) the thickness or the size of the tested sample. It had previously been assumed that for relatively thick samples (1-2 mm) contained in sealed sample containers, the time scale of DSC scan was sufficiently short to allow only an insignificant quantity of water to diffuse out of the sample. The moisture remains in the sample during the scan. If the moisture diffused out during the DSC scan, the water molecules would be captured in the sealed sample pan. If this were the case, there would be a significant ice melting endotherm during the scan of the second run. There is no such evidence from the thermographs for our present study. The DSC thermographs of these samples are given from Figure 4.14 to Figure However, some papers[100] have given opposite results. When quench cooled from higher temperature, such as above Tg, a significant ice melting endotherm was observed, an evidence of the presence of free water produced by moisture diffusing out of the sample during thermal analysis. This difference is probably caused by the temperature effect. For their experiment, the samples were first raised to a high temperature. At high

149 136 temperatures, the diffusion process increases significantly enhancing the diffusion of water out of the sample. One interesting phenomenon is easy to see from the DSC thermograph. At about 10 to 20 C, which depending on the samples, there is indeed an endothermal peak on the curves when the DSC was run from low temperature to high temperature. The first scan and the second scan of DSC are nearly the same. Neither the first scan nor the second scan shows the free water, as noted by the absence of an ice melting endotherm at 0 C. This is consistent with other research results[149]. The temperature of the endothermal peak is below 0 C temperature. It is reasonable to assume that the moisture is in the range of microscopic scale. The smaller the scale of the water, the lower the freezing point of the water. This effect is generally attributed to the immobilization of water molecules within the resin. The water that becomes bound to polar sites does not aggregate sufficiently to permit observation of normal phase transitions of water. The moisture did not diffuse out of the sample during the thermal analysis. Instead, it stayed inside the sample, remaining in the micro-void to form a microscopic mass segregate. This is consistent with the analysis of the FTIR analysis. From the FTIR result, as mentioned above, there are three kinds of water molecules in the epoxy sample: free water, water bonded to amine groups, and water bonded to OH groups. Part of the water bonded to OH groups exists as water-water clusters, in form of microscopic-scale water clusters. It is the latter part that contributes to the endothermal peak in the DSC thermograph.

150 DOMS-SAA Heat Flow (relative) Temperature ( o C) Figure 4.14 DSC thermography of DOMS-SAA-1.0 sample 0 DER-SAA Heat Flow (relative) Temperature ( o C) Figure 4.15 DSC thermography of DER-SAA-1.0 sample

151 138 Dynamic mechanical spectroscopy (DMS). Many experiments have shown that the properties of an epoxy-amine system are sensitive to absorbed moisture as well as to changes in the crosslinking density. The present study examines the influence of these effects on the dynamic mechanical response of this epoxy system at 5 different frequencies in the temperature range of 100 to 200 C. The behavior of these transitions as a function of moisture or stoichiometry provides new insight into the structure of these materials. Five distinct stoichiometric formulations of the epoxy system considered were studied. These five formulations were made by the method mentioned in Chapter 3. Film samples from these formulations were subjected to desiccation and soaking in deionized water for at least a month to ensure that moisture absorption had reached equilibrium. Dynamic mechanical experiments can provide information that reveals sorption and plasticization mechanisms. Epoxy resins have been described to exhibit relaxation transitions at high temperatures associated with the glass transition and secondary transitions over quite broad low-temperature ranges. While the glass transition requires large scale movements, the secondary transitions are often a combination of molecular rotation of some main chain side groups, motion of some segments of the main chain or motion of small molecules dissolved in the polymer. Thus, changes in a polymer network structure are manifested by transitions exhibited in dynamic mechanical spectra. The broad low temperature transition indicates a wide spectrum of motions and activation energies. The water conditioning of epoxy resins results in an increase in the magnitude of the transition and a slight drop in the transition temperature of about 20-25K, with

152 139 increasing moisture content.[9] Researchers have found that liquid crystalline and nonliquid crystalline bisphenol-based epoxies generally undergo three distinct transitions. The three major transitions or relaxations are typically denoted as α, β, γ in order of decreasing temperature. However, there is much disagreement on the source of these relaxations. The dynamic mechanical properties of the DER-SAA and DOMS-SAA systems were monitored in the 100 to 200 C range. Both the dry samples and the samples saturated with deionized water at room temperature were used in the dynamic mechanical property test. Typical plots of the logarithm of shear storage (G ) and loss (G ) modulus and tan δ versus temperature for different frequencies are shown in Figure 4.16 to Figure Figure 4.16 and 4.17 show typical tan δ and storage modulus curves for the DER- SAA series and for DOMS-SAA series. The tan δ and storage modulus curves clearly show the presence of the three major transitions typical of epoxy materials. The glass transition temperature is in the range of 160 C to 180 C and 180 to 200 C for the DER-SAA and DOMS-SAA systems respectively. The β transition appears in the range of 40 to 100 C. The γ transition temperature is lower, about 80 to 20 C. Figure 4.16 and 4.17 show typical tan δ and storage modulus curves for the DER- SAA series and for DOMS-SAA series. The tan δ and storage modulus curves clearly show the presence of the three major transitions typical of epoxy materials.

153 140 1e+12 1e+11 DER-SAA-0.8-Dry 0.1 Hz 0.5 Hz 1 Hz 5 Hz 10 Hz 1e+10 Log E' 1e+9 1e+8 1e Temperature ( o C) Hz 0.5 Hz 1 Hz 5 Hz 10 Hz DER-SAA-0.8-Dry Tan theta Temperature ( o C) Figure 4.16 Storage modulus and tan δ curves for DER-SAA-0.8 sample

154 Hz 0.5 Hz 1 Hz 5 Hz 10 Hz DOMS-SAA-0.8-Dry Tan D Temperature ( o C) 1e+11 1e+10 DOMS-SAA-0.8-Dry 0.1 Hz 0.5 Hz 1 Hz 5 Hz 10 Hz Log E' 1e+9 1e+8 1e Temperature ( o C) Figure 4.17 Storage modulus and tan δ curves for DOMS-SAA-0.8 sample

155 142 Gamma (γ) relaxation transition. The gamma transition has been associated with a number of motions in the network. Pogany[150] associated the relaxation with the wriggle motion of the aromatic ring, contrary to others suggestion that rotation of aromatic ring is responsible for the relaxation. In the DOMS-SAA system, Lin et al.[22] suggested that the rotation of the mesogenic unit about the para ether linkage leads to little or no disturbance of the neighboring units and does not change the length of the chain. Lin et al.[22] also showed the peak does not shift with differences in curing conditions, which suggests that the motion is due to mesogenic rotation and not to molecular motion of the segments. Other authors have associated the relaxation with a crankshaft type motion within the mesogenic unit[68]. The effect of variation of stoichiometry can best be explained by assuming that the relaxation contains a contribution from the diphenylpropane nucleus. The magnitude of this contribution is independent of the degree of cure of the resin, while if insufficient amine is added, the contribution from active glyceryl groups is decreased. Expanded views of the gamma (γ) relaxation portion of the DMS scans of the for DER and DOMS-SAA systems are shown in Figures 4.18 to 4.19, to show the effect of varying composition on the mechanical damping, typical of the response of dynamic mechanical spectra to changes in amine/epoxide functional ratio. These figures show that the effect of stoichiometric ratio on the gamma relaxation is not negligible. All samples have been fully cured according the procedure in Chapter 3. As the amount of curing agent increases, the temperature corresponding to the peaks of the tan δ curves changes. However, the onset of the peak is not very sensitive to the amine/epoxy functional ratio. Not only the flexible DER-SAA system shows this kind behavior, an analogous behavior

156 143 has also been observed for the rigid liquid crystalline DOMS-SAA system. In fact, virtually all epoxides exhibit identical γ relaxation behaviors in the onset region, regardless of the architecture of the epoxy or curing agent; this similarity has been noted by several authors[50] [109] [97]. Table 4.2 γ-transition temperature Sample Dry Tγ ( C) Wet DER-SAA to to 54.3 DER-SAA to to 47.3 DER-SAA to to 46.8 DER-SAA to to 47.7 DER-SAA to to 44.9 DOMS-SAA to to 30.1 DOMS-SAA to to 34.2 DOMS-SAA to to 29.2 DOMS-SAA to to 25.5 DOMS-SAA to to 30.0 It is well known that variation of curing agent effects the temperature of the alpha maximum associated with the Tg process. However, from these two figures, it is obvious that the amine content in the resin does not significantly influence the onset position of γ

157 144 transition process. Therefore, this gamma relaxation process should be associated with the relaxation of some segment that its movement is not very sensitive to the crosslink density. Reding et al.[109] studied a series of polycarbonates. They compared the poly(bisphenol-a carbonate) with (1) addition of an ortho methyl group on the benzene ring, (2) substitution of a phenyl group for a methyl of normal polycarbonate, and (3) a tetra-ortho-chloro derivative. All these materials showed the gamma and glass relaxations. The carbonate group was attributed to the lower temperature transition, that is, the gamma transition. The phenyl group was suggested to cause the main chain glass transition. Hara et al.[97] used the same materials, poly(bisphenol-a carbonate) and poly(tetrachloro bisphenol-a carbonate) to study the dielectric properties of the relaxation of these polymers. They suggested the glass transition is attributed to the reorientation of dipoles due to the segmental micro-brownian motion of the molecular main chain. The γ transition was suggested to be due to the side group and it was noted that the location of gamma absorption was not affected by its environment. It has been demonstrated that the γ peak is not inherent in either the epoxy monomer or the amine component. The most obvious cause of the gamma peak, then, is the chemical reaction between the epoxy and the amine molecules, which creates flexible segments after the epoxy ring opens. It has been suggested that the CH 2 -CH(OH)-CH 2 - O- group is responsible for the γ relaxation in the amine cured epoxy resin. It is probable that the relaxation is caused by a crankshaft type rotation of the group. The four central carbon atoms can rotate without requiring cooperative motion from others. The bond

158 145 angles of carbon-carbon bonds (109.5 ) and that of C-O-C bonds (105 ) are very similar and the group could form a crankshaft. The rotation is hindered by the pendant OH group, which is also capable of forming hydrogen-bonds. The hindrance of the OH group is probably responsible for the gamma relaxation taking place at about 66 C as in some other polymers. The beginning of the gamma peak is observed at roughly the same temperature for the DER-SAA and DOMS-SAA systems under study. Although a precise understanding of the molecular mechanisms responsible for the γ relaxation mechanism remains unclear, it is generally accepted that the primary, or most fundamental, motions responsible for the relaxation are activated by the onset of relaxation. These results can be explained by considering the movement of the CH 2 -CH(OH)-CH 2 -Ohydroxypropylether sequence. The mobility of this group only depends on the microregion environment, the small volume near the crosslink junction. This group is able to undergo motions in identical temperature and frequency conditions, whatever the crosslink density and the chemical nature of the network may be. This micro-scale free volume is mainly determined by the covalent bond angle around the crosslink junction. It is unlikely that macroscopic scale properties, such as density fluctuations, account for the gamma relaxation behavior of networks. Thus the onset temperature of this gamma relaxation is independent of the crosslink density. All of the onset temperatures for the DER-SAA or DOMS-SAA samples (except DER-SAA-0.8) are same. However, the peak position depends on the cooperation or extent of the motion. In three-dimensional crosslinked networks, the whole covalent linked network constitutes a highly coupled system. Because of the heterogeneous crosslink density, some CH 2 -CH(OH)-CH 2 -O-

159 146 groups in the networks stay in one crosslink environment, while others stay in different crosslink environments. Some segments could move at low temperatures, but others need higher temperatures to entirely activate the relaxation mechanism. Thus, the peak position changes with stoichiometric ratio in the resin mixture. Now, regarding the difference in the tan δ peak position between DER-SAA and DOMS-SAA systems, we can account for this by assuming that the presence of a liquid crystalline mesophase partly impedes the motions of the CH 2 -CH(OH)-CH 2 -O- groups. Owing to the close packing of molecular segments, the tighter the packing, the more prevalent is the hindrance of the surroundings on the motions of these groups. As a consequence, the corresponding activation energy barrier is increased, and finally, the appearance of the motions of these groups is delayed to higher temperatures. It is interesting to note the peculiarity of the onset of the γ relaxation of DER- SAA-0.8 sample. Its onset temperature is below that of the other samples. Although it would be necessary to obtain more experimental data to confirm this phenomenon, it is reasonable, under the present case of limited data, to assume the extra dangling epoxy group is responsible for this lower onset temperature. The movement of a dangling epoxy end group is easier than a CH 2 -CH(OH)-CH 2 -O- group, thus, the onset temperature is lower. This phenomenon becomes more evident for samples of higher excess epoxy cured networks. However, the close packing of molecules in the smectic mesophases will impede the movement of this epoxy end group, and defer its absorption peak to higher temperature, so that its peak overlaps with the peak of the CH 2 -CH(OH)- CH 2 -O- group. However, there is need for further experiment to determine the mechanism of γ transition in epoxy-rich resins.

160 tan δ DER-SAA-0.8 DER-SAA-0.9 DER-SAA-1.0 DER-SAA-1.1 DER-SAA Temperature ( o C) Figure 4.18 γ transition for DER-SAA system at frequency f= DOMS-SAA-0.8 DOMS-SAA-0.9 DOMS-SAA-1.0 DOMS-SAA-1.1 DOMS-SAA-1.2 tan δ Temperature ( o C) Figure 4.19 γ transition for DOMS-SAA system at frequency f=0.1

161 148 Activation energy of γ transition. Analysis of temperature and frequency dependence of the loss peak is commonly based on the determination of activation energy from the Arrhenius relationship: f = f 0 e E a / RT where f is the frequency in Hertz, Ea is the activation energy, R is the ideal gas constant ( J/K/mol) and T is the temperature of the transition. A typical Arrhenius plot is shown in Figure The plot is fairly linear, although the limited frequency range makes it impossible to determine whether or not the data actually follows an Arrheniustype of dependence. The activation energy data are listed in Table 4.3 and displayed in Figure 4.21 and The gross average of activation energies of the polymers possessing the glycidyl ether segment is 59.8 kj/mole. Since the accuracy of determination is roughly 6.3 kj/mole, the spread of activation energy values may be considered fairly small. The similar value of activation energy and transition temperature found for DER-SAA or DOMS-SAA systems leads to the conclusion that the transition has the same mechanism for the epoxy resins with different amine/epoxide functional ratio. Further, the transition must be due to a change in the molecular mobility of their common structural feature, the glycidyl ether segment. The values of these activation energies are in good agreement with the results of Garard et al.[107]. Also, Horn et al.[112] reported that the gross average of activation energies of the polymers possessing the glycidyl ether segment is 14.3 kcal/mole.

162 149 Table 4.3 Activation energy of gamma relaxation Samples Ea of γ transition (dry sample) (kj/mol) Ea of γ transition (wet sample) (kj/mol) DER-SAA DER-SAA DER-SAA DER-SAA DER-SAA DOMS-SAA DOMS-SAA DOMS-SAA DOMS-SAA DOMS-SAA Arrhenius plot of DER-SAA-0.8-dry ln (f) Temperature (1/T) Figure 4.20 Arrhenius plot of the gamma relaxation

163 γ Τransition Activity Energy (dry samples) Activation Energy Ea (kj/mol) DER-SAA DOMS-SAA amine/epoxy functional ratio Figure 4.21 Comparison of effect of amine/epoxide functional ratio on the activation energy of Gamma relaxation for dry DOMS-SAA and DER-SAA system 80 γ-transition Ea of DER-SAA system 70 Ea (kj/mol) Dry samples Wet samples amine/epoxide functional ratio Figure 4.22 Effect of amine/epoxide functional ratio on the activation energy of Gamma relaxation for DER-SAA system

164 γ-transition Ea of DOMS-SAA 80 Ea (kj/mol) 70 Dry samples Wet samples amine/epoxy functional ratio Figure 4.23 Effect of amine/epoxide functional ratio on the activation energy of Gamma relaxation for DOMS-SAA system From the results, the tendency towards highest activation energy at stoichiometry can be clearly seen. This is in agreement with the trend observed by other authors[151]. For epoxy-rich resins, this trend can be interpreted as the result of excess epoxy, which remains a fraction of epoxy ring groups that are present as dangling ends. It is easier for these dangling groups to move than segments crosslinked on both ends. The concentration of these groups is proportional to the probability that di-epoxide molecules are reacted at only one end. The five types of these arrangements connected to two nitrogen atoms are listed in Figure For epoxy-rich samples, the chemical structures are dominated by type A structure and these unreacted dangling groups. For amine-rich samples, there are three kinds of glyceryl unit and five types of possible diamine structures. One kind of glyceryl units is linked to a nitrogen atom which has one other glyceryl group directly linked to it and at least one other glyceryl group

165 152 indirectly linked to it. The second kind is directly linked to a nitrogen atom that retains an unreacted hydrogen. The third kind is bonded to a nitrogen atom which is also directly bonded to another glyceryl group but has two unreacted hydrogens at the opposite end of the molecule. OH CH CH OH OH H 2 C CH 2 CH N R N H 2 C CH 2 CH OH (a) OH CH CH OH OH H 2 C CH 2 CH N R N H 2 C H (b) OH CH CH OH H 2 C N R N H 2 C (c) H H CH OH H H N R N H 2 C CH 2 CH OH (d) CH OH H N R N H 2 C (e) H H Figure 4.24 Five kinds of chemical structure

166 153 In the present study, the crosslinking agent SAA has two different amine functional groups, one is an amine group and the other is a sulfonamide group. Because the reactivity of these groups is different and the reactivity of the amine group is higher than that of sulfonamide group, we would expect that the chemical structure is dominated by type (a) arrangement when the amine/epoxide functional ratio is equal to 1. With the increasing of amine/epoxide functional ratio from 1 up to 1.2, the percentage of chemical structures of type (b) and type (c) will increase. As we know, the CH 2 -CH(OH)-CH 2 -Ogroup is responsible for the gamma relaxation. The relaxation depends on the steric constraint of nearby similar groups. In the type (b) and (c) structures it is easier for the gamma relaxation to occur and the corresponding activation energy is relatively lower. Thus, by increasing the amine/epoxide functional ratio, the average activation energy is lowered due to the increase in the percentage of type (b) and type (c). This trend is consistent with the experimental data shown in Figure This hypothesis can be tested since the fraction of glyceryl units of each type in these non-stoichiometric resins can be determined by calculating the probability of each type[151]. Based on our DER-SAA or DOMS-SAA system, the fraction of type (a), type (b), and type (c) is given by the following expressions, modified according to our present resin systems. f a = (1/r) 4 f b = 4(1/r) 3 (1-1/r) f c = 6 (1/r) 2 (1-1/r) 2

167 154 The plot of these fractions as a function of amine-rich stoichiometric ratio is shown in Figure It is obvious that the effect of functional ratio on the mole fraction of each type of chemical structure is consistent with present experimental results. mole fractions of three type structures (f) type a type b type c Amine/epoxide functional ratio Figure 4.25 Theoretical mole fraction of three type structures relaxations: Our present γ relaxation data also fits the equal site transition theory of ω = 2 υ RTγ e Ea where ω is the measuring frequency, Ea is activation energy, and ν is the frequency of vibration of the segments. If we plot Ea vs. T γ for different kinds of samples (Figure 4.26), it should be a linear relation. The change in temperature of this gamma relaxation, T γ, measured by the position on the tan δ curve, shows a good agreement with changes in the activation energy for this relaxation process. This is illustrated by the data.

168 155 Qualitatively, the experimental data agrees with this theory. The relaxation temperature has a good linear relation. The activation energy increases with increasing of characteristic relaxation temperature. 90 Activation Energy Ea (kj/mol) Tγ ( o C) Figure 4.26 Plot of equal site transition theory for DER-SAA and DOMS-SAA system Glass transition. The definition of the glass transition is defined as the peak in the tan δ curve. Glass transition temperature can be defined in terms of the temperature position of the α peak maximum exhibited in the E and tan δ plots. All these definitions of Tg suffer from the fact that they are based on dynamic mechanical quantities which are quite sensitive to the frequency of the experiment as well as the sample heating rate[8]. Consequently, the Tg values defined from dynamic mechanical experiments are invariably higher than the Tg values usually obtained by the more conventional methods such as dilatomeric experiments. It is the contention of this part of the study to offer a

169 156 clear indication of the role of matrix-sorbed water, whether it exists as free water or it is hydrogen-bonded, in the depression of the glass transition. Thermal analysis data from both DSC and DMTA for all the epoxy systems are summarized in Table 4.4. In Figure 4.27, the Tg s of DER-SAA and DOMS-SAA epoxides, determined from DMS measurements, are plotted versus SAA concentration. Our samples exhibited a strong dependence of Tg on the SAA composition. The glass transition temperatures increase with increasing amounts of crosslinking agent for the epoxy-rich systems. The general increase in Tg with increasing SAA content suggests an increase in crosslink density. However, the glass transition temperature decreases with increased amount of SAA for the amine-rich epoxy systems. When the stoichiometric ratio of epoxy functional groups to hydrogen functional groups reaches to unity, the glass transition temperature attains its highest level. The maximum in Tg for the amine-rich samples supports the contention that unreacted SAA provides a plasticization effect on Tg. It is also generally believed that a change in the glass transition temperature is caused by the crosslink density. The physical properties of the DER-SAA and DOMS- SAA series as a function of SAA concentration can be understood in terms of the network structures discussed previously[112]. The temperature at which the glass-rubber transition takes place will depend on the internal chain stiffness of a single molecule, for which only a few monomer segments in succession need to be considered, and on interactions between a part of one molecule and other molecules or more distant parts of the same molecules. The trend in Figure 4.27 is consistent with studies on other epoxy systems. Some results[83, 117] showed that for the DGEBA/TETA system, the highest value of Tg is observed for an equi-molar ratio. When comparing the DOMS-SAA and

170 157 DER-SAA system, it is obvious that the glass transition temperature of the DOMS-SAA is significantly higher than that of the DER-SAA across the entire series. Although further investigation is needed to determine the cause of this difference, it is reasonable to suggest that the special morphology of the liquid crystalline resin is a major factor. The close packed molecules in LCTs allow much less free volume for the chain segments in the three dimensional network, hindering relaxation. Thus the transition temperature of the DOMS-SAA system is higher than that in the DER-SAA system DER-SAA DOMS-SAA Tg ( o C) amine/epoxide functional ratio Figure 4.27 Glass transition temperature of dry DER-SAA and DOMS-SAA systems Effect of water on thermal properties. When solvents are mixed into a polymer, they usually alter the polymer s rheological and/or mechanical properties, seen as a depression of the glass transition temperature, and/or changes in stiffness, toughness and strength. Since moisture is easily absorbed by polymers, especially those that have polar groups on their molecular chains, such as polyamides and epoxy resins. Many

171 158 experimental results have shown that for epoxy-amine systems the Tg is reduced by sorbed moisture. Our results also show the depression of the glass transition temperature by the absorbed water. Figure 4.28 and 4.29 show the glass transition temperatures of the DER-SAA and DOMS-SAA systems. Figure 4.30, 4.31, and 4.30 show the activation energy of the glass transition. From Table4.4 and Figure , it is easy to see that the water has a plasticization effect on the epoxy systems. This is not a surprise because of the polar nature of the epoxy-water system and the results obtained from FTIR spectroscopy. These studies indicate some form of molecular association. The most widely accepted explanation has been that this depression in Tg is the result of specific interactions. It has been suggested that in systems where such interactions are present, greater than normal changes in Tg could be expected. 180 DER-SAA Tg ( o C) dry samples wet samples amine/epoxide functional ratio Figure 4.28 Water effect on glass transition temperature of DER-SAA system

172 DOMS-SAA Tg ( o C) dry samples wet samples amine/epoxide functional ratio Figure 4.29 Water effect on glass transition temperature of DOMS-SAA system 1100 Glass transition E a (dry samples) Activation energy Ea (kj/mol) DER-SAA DOMS-SAA amine/epoxide functional ratio Figure 4.30 Glass transition activation energy of DER-SAA and DOMS-SAA systems

173 Glass transition Ea of DER-SAA system Activation energy Ea (kj/mol) Dry samples Wet samples amine/epoxide functional ratio Figure 4.31 Effect of water on glass transition activation energy of DER-SAA system 1100 Glass transition Ea of DOMS-SAA system Activation energy Ea (kj/mol) Dry samples Wet samples amine/epoxide functional ratio Figure 4.32 Effect of water on glass transition activation energy of DOMS-SAA system

174 161 Table 4.4 Thermal properties of DER-SAA and DOMS-SAA systems Sample Tg (dry) Tg (wet) Cp (J/deg/g) DER-SAA DER-SAA DER-SAA DER-SAA DER-SAA DOMS-SAA DOMS-SAA DOMS-SAA DOMS-SAA DOMS-SAA The intrinsic moisture sensitivity of the epoxy resins is traceable directly to the molecular structure. The presence of polar and hydrogen bonding groups, such as the hydroxyl, amine, sulfone and tertiary nitrogen, provides the chemical basis for moisture sensitivity, while the available free volume and two-phase network structure represent its physical aspect. Theoretical models based on the analysis of free volume and entropy of water saturated polymers are commonly used to describe and test the nature of the polymer-dilute interactions. For the free volume model, the glass transition temperature of a dilute system is determined by the dilute volume fraction and the changes of the thermal expansion coefficient

175 162 Tg = V α T V α p p p gp p + Vdα + d V α T p d V α p d gd + V α d d (4.6) where the subscript p and d refer to the polymer and the diluent respectively, α is the thermal expansion coefficient, V is the volume fraction of the polymer or the diluent. The thermal expansion coefficient of water is about o C -1 and the Tg of water is about 134K. The theory based on the changes of free volume satisfactorily applies to polymerdiluent systems whose glass transition temperatures and thermal expansion coefficients are well defined, but fails for systems and temperature ranges where uncertain evaluations or arbitrary assignments of these parameters must be made. Another model[100] has been suggested for the epoxy-water system that applies classical thermodynamic treatment to describe the compositional dependence of the glass transition temperature in miscible blends and further is extended also to the epoxy/water system. The plasticization induced by water sorption can be described by theoretical predictions given by: T g = T gp C p p X p C pp X p + C pd X d + T gd C pd X d C pp X p + C p d X d (4.7) where X is the weight or mole fraction and C pp is the incremental change in the specific heat at the glass transition. Actually, the value of the C pp used should refer only to the units capable of activation; however, the experimentally measured DSC value may be used in the case of epoxy systems. In fact, when highly crosslinked networks are produced, it can be assumed that all units are involved in cooperative molecular motion at Tg. The lower the value of C pp, the greater is the depression of the glass transition temperature, especially at low concentrations of plasticizer. Ellis and Karasz have

176 163 theoretically calculated Tg depressions of 10 to 15 K/wt% water for the less crosslinked DGEBA systems and 25K/wt% water for the high Tg TGDDM-based systems using a value of C pd for the water of 1.94 kj and values ranging from 0.34 to 0.53 for the epoxies[100]. These expectations were generally confirmed by their experiments on samples equilibrated at high temperatures. For the present study, the depression of Tg in the DER-SAA series calculated using the above equation is detailed in Table 4.5 below. The glass transition temperature of water (Tg) is selected as 134 K, Cp is 1.94J/g/K, as suggested by Ellis [100]. Table 4.5 Comparison of Tg obtained experimentally and by calculation Samples Cpp (J/mol/K) Tg (dry) ( C) Tg (wet)-exp ( C) Tg-cal ( C) DER-SAA DER-SAA DER-SAA DER-SAA DER-SAA DOMS-SAA DOMS-SAA DOMS-SAA DOMS-SAA DOMS-SAA It is clear that there is a large difference between the experimentally obtained Tg and the calculated Tg. The differences vary from about 30 to 100 C, which indicates that this calculation model is not suitable for the present epoxy systems. Several factors may contribute to this large difference. First, this model does not consider the specific interaction between the water and the epoxy, such as the hydrogen-bonding. Second, the

177 164 disparities are probably due in part to the value chosen for the Tg of water, as has been noted by others.[100] Based on our characterization by FTIR, there is interaction between the water and the epoxy matrix. Thus a simple free volume model is not suitable for the analysis of the glass transition temperature depression. An alternative description of wet glass transition temperature, which designates configurational entropy rather than free volume as the temperature dependent function, has been proposed for these epoxides and other polymers in which the water may be adsorbed by localization at a strongly polar molecular group. Evidence[86] has been given by broad line nuclear magnetic resonance analysis that the plasticization effect of the water on a crosslinked epoxy may be related to the strong interactions between the dissolved water and segments of the polymer, although the exact sorption sites to which water may be bonded are still uncertain. The entropy model[103] assumes the configurational entropy as the temperature dependent function. T g = T 1 g 0 [ ] R y( r) M s C p (4.8) and 1 1 () + (1 r) ln 1 y ( r) r ln r r r M s f M w where R is the universal gas constant (8.314 J/(mol*K), M w is the formula weight of water (M w = 18 g/mol), M s = N a /N s (g/mol) is the effective formula weight of hydrogen bonded sites with N a the Avogadro s number ( mol -1 ). N s is the number of hydrogen bond sites. The meaning of r is the ratio of the number of water molecules to

178 165 hydrogen bonded sites. Cp (J/g/ C) is the difference of specific heat due to glass transition in the dry resin, and f is the grams of moisture per gram of dry resin. The formation of intermolecular bonds between the water molecules and the polymer s hydrophilic groups during sorption may strongly depress the glass transition temperature of a system with high hydrogen bonding capacity. We will attempt to use the entropy model to emphasize the hydrogen-bonding characteristic of the interaction of water with the epoxy. The results of the DER-SAA and DOMS-SAA series are listed in Table 4.6. As mentioned before, the change in SAA content will affect the crosslink density. Table 4.6 Theoretical Tg using entropy model Samples r Tg (dry) ( C) Tg (wet)-exp ( C) Tg (wet)-cal( C) DER-SAA DER-SAA DER-SAA DER-SAA DER-SAA DOMS-SAA DOMS-SAA DOMS-SAA DOMS-SAA DOMS-SAA The entropy model seems more adequately to describe the plasticization behavior of the DER-SAA system. However, for the liquid crystalline systems, the theoretical predicted value of the wet glass transition temperature is much lower than the

179 166 experimental data. This means that this model is not suitable for describing the plasticization of liquid crystalline thermoset system. One reason, we deduced from the specific morphology of this smectic matrix, is that this model did not consider this liquid crystalline mesophase property. The special arrangement of the molecules in the mesophase state would change the relative role of the absorption on the site of hydrophilic groups. The interaction of water with the epoxy matrix would also change due to the special mesophase. Now let us focus on the discussion of the DER-SAA series. As we have discussed, the high crosslink density decreases the free volume of the matrix. Based on the free volume model, the most highly crosslinked sample, the DER-SAA-1.0, should be less sensitive to water plasticization than the epoxy-rich mixtures, if the single mechanism of molecular solution is effective. For the amine-rich epoxy samples, however, this free volume can not explain the trend of the glass temperature depression. The DER-SAA-1.2 has a large glass temperature depression after the sample was saturated with water. Its glass transition depression is 5 C higher than for the DER-SAA- 1.0 even though the crosslink density of DER-SAA-1.2 is nearly same as that of DER- SAA-1.0 samples. The anomalous effect may be explained by considering the influence of the molecular solution and the hydrogen bond formation on the plasticization of the different stoichiometric ratio systems. Based on the entropy model, the water stays in the epoxy matrix not only as solution molecules but also bonded to the hydrophilic group sites. While the solution state decreases the glass transition temperature by increasing the free volume, the hydrogen-bonding mechanism is governed by the concentration of the hydrophilic sites. The occurrence of two mechanisms of plasticization, which can be

180 167 influenced by the increase of the crosslink agent (SAA) concentration in the cure mixture, causes the additional 5 C depression for DER-SAA-1.2 than DER-SAA-1.0 samples. The changes in the glass transition for both epoxy systems are opposed to those predicted by considering the free volume changes only. The observation of the unexpected trend confirms the presence of a second mechanism of plasticization, the hydrogen-bonding mechanism. 4.4 Conclusion FTIR results reveal that polar groups in the crosslinked network are one of the major factors that control the sorption and diffusion of water in epoxy resins. Two possible hydrogen bond configurations are identified, namely N HO and O HO interactions. Evidence for each of these bonds exists in the spectra of FTIR. The diffusion of water molecules into epoxy resins depends on two factors, namely, the availability of appropriate microvoids in the polymer network and the attractive forces between the water molecules and the epoxy resin matrix. The second factor concerns the chemical nature of the water versus that of the polymer. This factor determines the polymerpenetrant affinity. Water molecules are hydrogen-bonded and form clusters within the polymer. In epoxy resins, the presence of hydroxyl and amine groups affects the water absorption of the cured network. The FTIR results verify this argument. It was proposed that sorption and transport of water in the epoxy in its glassy state is controlled predominantly by several features (the crosslink density, the liquid crystalline mesophase, and the two-phase crosslinked network) and by the concentration of possible sorption sites. It was suggested that steric conditions imposed by the

181 168 crosslinked network, heterogeneous systems with regions of higher density dispersed in a matrix of lower density, and smectic mesophase are responsible for the lack of accessibility to all sorption sites. With increasing concentration of amine in the mixture, crosslink density and/or polar group concentration increase. The crosslink junction provides extra micro-space for the sorption of water molecules. The polar character of some amine groups and nascent hydroxyl groups provide extra positions for the formation of hydrogen-bonds between water molecules and these polar groups. These two synergetic effects of amine concentration result in greater water absorption in the epoxy-amine resin samples. Thus the equilibrium moisture content increases with the amine concentration. Dynamic mechanical spectra results show that the glass transition temperature also depends on the amine/epoxide functional ratio. The glass transition temperature is a maximum at stoichiometry. This relationship between Tg and functional ratio has the same trend as the relationship between sorption and functional ratio. The combination of the free volume fraction, two-phase morphology, and polar group concentration accounts for these common relations.

182 CHAPTER 5 HETEROGENEOUS MORPHOLOGY OF LCT AND MODEL 5.1 Introduction As mentioned in the previous chapters, the permeability of the polymer is dependent on the its composition, structure and morphology. Sorption and transport of water vapor in the glassy state of the epoxy is controlled not only by the crosslink density and number of possible sorption sites but also by morphological features. Several investigations of epoxy microstructure have been made in which techniques such as optical[18] and electron microscopy[14, 16, 17, 19] were used to observe polymer surfaces that had been etched[16, 18], strained, or fractured at low temperatures[14, 16, 19]. Theoretical modeling of the curing of an epoxy-resin using the Monte Carlo method has also been published[20]. Thermosetting epoxy polymers were shown to consist of two phases. These phases differed in hardness, density, and refractive index. The difference in refractive index between the dispersed phase and the dispersing medium is not sufficient to define the domains in the epoxy systems optically except by solvent action (swelling, etching). The domain size determination by photographic means was ambiguous because of possible solvent swelling. The mechanical means of definition depends upon the relative hardness and rigidity of the dispersed domains with respect to the interstitial dispersing 169

183 170 phase. Cuthrell et al.[152] used the micropenetrometer to measure the bisphenol A epoxy and showed the average size was about 27.6µ. Electron microscopy studies of epoxies as well as other thermoset resins have suggested that these are heterogeneous systems with regions of higher density dispersed in matrix of lower density. These density fluctuations are thought to arise from differences in the crosslink density in the polymer. TEM of microtomed sections of these epoxy samples show them to be complexed-structured materials. This makes straight forward interpretation of results difficult. Scanning electron microscopy, however, show a two-phase structure, consisting of a high-density nodular phase surrounded by a lowdensity dispersed phase. The size of nodular structure increases with increasing curing temperature. The nodular structure observed in these studies varies in size from 6 nm to 7 um. In general, small, highly crosslinked microgel particles with diameters on the order of 10 nm are the main micro-structural feature with large structures consisting of aggregates of these gel particles. Within the larger nodular structures, smaller nodular structures, referred to as microgels, are observed with size ranging from nm. Well-defined regions of density differences are not readily apparent in these studies. More detailed work is necessary before quantitative correlation of water sorption process to the morphological features of epoxy system can be made. Characterization of the textures of LCTs is the key to the identification of the specific phases present, understanding the structure, and its relationship to the solid-state properties. Dynamic hot stage microscopy experiments with viedo tape recording provide images of the texture changes associated with phase changes, as a function of temperature and time. Because of the fine-grained nature of the LC textures in the final network, the

184 171 distinction between nematic and smectic phase could not be determined solely by POM. The smectic networks appear opaque in bulk form with a high defect density, Schlieren texture (LC domain size ~1 um) under the polarizing optical microscope. The nematic networks appear translucent in bulk with LC domain size approaching the resolution of the POM, producing a weakly birefringent uniform texture. The isotropic, rigid-rod DGGHMS/MDA networks and isotropic DGEBA/MDA networks are transparent and featureless under the POM.[18] Recently, the development of the atomic force microscopy (AFM) methods has provided a new capability for determining polymer morphology with nm or better spatial resolution. The contact, tapping, and lateral force modes of operation have been successfully used to view crystallites and nano-fibrillar structures in a variety of polymeric films. Furthermore, these AFM modes have been successfully used on occasions in which significant topographic and lateral force differences exist between phases, in order to determine the morphology of polymeric blends and block polymers. Using phase contrast techniques, the different polymer and inorganic phases can be distinguished regardless of the topological changes. These techniques have not yet developed into routine techniques for the analysis of thermoset polymers. The application of AFM to characterize the morphology of crosslinked epoxy has not been fully undertaken. Vanlndingham et al.[85, 129, ] reported the result of application of AFM on the EPON 828 epoxy system. Changes in microstructure and mechanical properties are investigated as a function of stoichiometry. That crosslinked epoxy system exhibits a two-phase structure consisting of hard microgel phase and dispersed phase of soft, unreacted and/or partially reacted material.

185 172 However, not much study of the morphology of liquid crystalline thermosets on the micro-scale has been done. LCTs posse a unique hierarchical microstructure in the absence of an external field, they tend to form disordered multi-polydomain morphology. In this chapter, changes in morphology are investigated as a function of epoxyamine stoichiometry. The epoxy-amine systems for both the liquid crystalline thermosets (DOMS-SAA) and the non-liquid crystalline thermosets (DER-SAA) systems will be discussed. Based on the analytic results of morphology, the diffusion coefficient model and permeability model for water transport for epoxy resins were set up to express the change in these quantitatively with morphology and polar group concentration Experiment Materials and Sample Preparation For morphology study, all of the liquid crystalline DOMS-SAA system and non- LC DER-SAA system were included. It will be easy to investigate and compare the morphology in liquid crystalline epoxy and conventional epoxy. The chemical structures and characteristics are shown in Figure 3.2. The cure procedure is the same as stated in Chapter Density Measurement Samples used for density measurement were made according to the procedure stated in chapter 3. However, to minimize the sorption effect on the density measurement, these samples were thicker. The general size of these samples was about mm. Sample densities were measured using the Archimedes principle. Using an electric analytic digital balance, the dry weight of the samples was determined to within a tenth of

186 173 a milligram. Each sample was then suspended from the balance and immersed in distilled water of known temperature. The weights of the sample in air and in water were measured, and the density of the water was known, this allowed the density of the sample to be calculated. Weighing the dry sample before and after immersing in water confirmed that moisture uptake during the experiment to be negligible Atomic Force Microscopy Atomic force microscopy (AFM) was performed using a Digital Instruments Nanoscope III. The mode selected was the tapping mode. This mode operates by scanning a tip attached to the end of an oscillating cantilever across the sample surface. A single silicon probe with dimensions of um and a tip radius of 5-10 um was selected. The cantilever was oscillated at or near its resonance frequency. When scanning, the tip lightly taps on the sample surface, contacting the surface at the bottom of its swing. The feedback loop remained at constant oscillation amplitude by maintaining a constant root mean square (RMS) of the oscillation signal acquired by the split photodiode detector. For this tapping mode, the vertical position of the scanner at each (x, y) data point, in order to maintain a constant set-point amplitude, was stored by the computer to form the topographic image of the sample surface. The phase and height images were recorded for each sample examined. The height images represent the topography of the sample surface. The phase images represent the difference between the free oscillating phase of the AFM tip and the phase of the tip as it interacts with the sample. Thus, the phase image is representative of the difference in the viscosity property. In this case, the crosslink densities of different areas on the sample surface are

187 174 represented by the phase contrast. After all, the height and phase images correspond to changes in the amplitude and phase, respectively, of the oscillating AFM cantilever tip. Table 5.1 Parameters for Tapping Mode atomic force microscopy Panel Parameter Range of adjustment Scan size 1.0 µm Scan rate 1 to 2 Hz X offset 0 nm Scan Control Y offset 0 nm Scan angle 0 degree Number of samples 512 Z limit 440V Integral gain 0.2 to 0.6 Proportional gain 0.3 to 1.2 Feedback Control Look ahead gain 0.0 Set point 0.5 to 2.0 V Derive frequency 250 to 350 KHz Drive amplitude 0.5 to 1.0 V

188 175 Phase images were obtained using as-processed samples without any sample preparation. By adjusting the set-point, the distance between tip and sample was controlled. Correspondingly, the interaction force between the tip and sample can be adjusted by setting different set-point parameter. High force is beneficial for contrast of the phase image, which is a function of elastic and viscoelastic properties. Lower force is suitable for getting the correct topographical image. For the present study, the phase image is emphasized to reveal the microstructure or morphology of different cured epoxy resins. Table 5.1 is a compilation of feed-back parameters that were used to perform the scans. Assigning phase and height components to atomic force microscopy image should be done carefully. The height and phase images depend on the settings of the experimental parameters: the free oscillation amplitude of the tip, the set-point amplitude, the tip shape, the cantilever force constant, the compliance of the sample to the tip, and the frequency of the oscillation. The set-point amplitude is the amplitude that is maintained during scanning by adjustment of the vertical position of the tip. The free oscillating amplitude is the amplitude of the AFM tip oscillation when is not in contact with the surface. Assigning regions of different phase shifts to the image picture of special polymeric phases requires knowledge of the sample-tip interaction occurring during imaging. The interaction between the tip and the sample is complex. It depends on the distance of the tip from the surface of the sample, the properties of the sample, and the environmental media around the tip and the surface of the sample.

189 176 The tip encounters different forces as it approaches a sample surface. The first boundary encountered by probes is fluid film damping. A damping air film is developed when an oscillating probe comes to within 10 microns of the sample surface. At this distance, air is squeezed between the probe and the surface during each down-stroke of the tip. Conversely, as the tip rebounds upward, a partial vacuum results. This pumping effect dampens probe motion and may lead to false engagement of the surface. The surface tension effect results from the presence of condensed water vapor at the surface. This is an attractive force, and can pull a tip down toward the sample surface with a force strong enough to indent some materials. Depending upon how much water is present, surface tension effects begin at nm above the surface. Usually, tapping mode is employed to alleviate surface tension attraction. At the angstrom level above the surface, Van der Waals forces cause weak attraction between the atoms in the tip and the sample. This attraction is detectable and is used to monitor non-contacting tip-sample interactions. The tip and sample are said to contact when their respective atoms encounter the others Coulombic forces. At this level, electron shells from atoms on both tip and sample repulse one another, preventing further intrusion by one material into the other. Pressure exerted beyond this level leads to mechanical distortion of one or both materials. 5.3 Results and Discussions Image Analysis The phase and height image of the DER-SAA and DOMS-SAA systems are shown in Figure 5.1 to Because of the mechanical interaction between the tip and

190 177 the sample, many factors influence the phase image. Accordingly, the AFM phase images were only qualitative in nature. Therefore, scanning conditions were set to be similar in order to have meaningful comparisons drawn between the images. As stated previously, the topography is less likely to have an effect on the phase image. However, to make high quality phase images, the effects of surface topography on the phase image were carefully minimized. The samples were made between two glass plates. This mold surface ensures that the surface of the sample is very flat. For these 1 um square scans, the imaged regions of the samples that were very smooth, with topographic changes of less than 50 nm over the scanned region. Further, by comparing the height image and the phase image, we were able to conclude that there was no relation between the height image and the phase image. Thus, the phase contrast should not be influenced significantly by differences in sample topography. The phase contrast between white and black on these images is 90, which is an extremely large range. Therefore, the property difference between the two phases is quite significant. The different regions of phase shift represent the high and low crosslink density phases of the resins. Examination of phase and topography images from Figure 5.2 to 5.11shows that, at large scales, the phase images are independent of the topographic features. Racich[16] et al. also investigated the influence of high- and low-energy mold substrates on sample morphology. Examination of both free and fracture surfaces of diglycidyl ether of bisphenol A (DGEBA) and diethylene triamine (DETA) epoxy systems indicate that bulk-cured epoxy resins have a nodular morphology whether the substrate is silicone rubber or Teflon poly(tetrafluoroethylene).

191 178 Figure 5.1 Phase (upper) and topographic (lower) AFM images of DER-SAA-0.8 sample

192 179 Figure 5.2 Phase (upper) and topographic (lower) AFM images of DER-SAA-0.9 sample

193 180 Figure 5.3 Phase (upper) and topographic (lower) AFM images of DER-SAA-1.0 sample

194 181 Figure 5.4 Phase (upper) and topographic (lower) AFM images of DER-SAA-1.1 sample