Molecular simulation technique together with spectroscopic methods were utilized to study the phase separation behavior of thermoplastic elastomers. Several kinds of systems were investigated, which included diacetylene-containing polyurethane elastomers, polyurethane elastomers with monodisperse hard segments, and a bio-degradable polyester elastomer. For the semi-rigid segmented polyurethane elastomers, we found that the chain rigidity of hard segment is the dominating factor responsible for their phase separation behavior. Chemical immiscibility, crystallization, and presence of hydrogen bonding are not necessary to drive phase separation even though they can promote it. The phase diagrams for semi-rigid polyurethane elastomers associated with different lengths in hard/soft segments were explicitly calculated using molecular simulation technique based on rod-coil model. It was found that longer hard segment or shorter soft segments have higher degree of phase separation than their counterparts. This result was verified by vibrational spectroscopy. It was also shown that DSC is not appropriate to evaluate the phase composition of polyurethane elastomers. The phase separation kinetics and the ultimate degree of phase separation for ultra-thin films of polyurethane elastomers are different from their bulk forms. Hard segments will also show preferential orientation onto the substrate surface. These are the direct consequence of hard segment chain rigidity effect. All the experimental results were successfully reproduced by Monte Carlo simulation based on rod-coil model. Finally, the phase transformation process and mechanical deformation process of a bio-degradable thermoplastic elastomer, PHO, was investigated by FT-Raman spectroscopy and normal coordinate analysis. The long side-chains of PHO will form more extended conformations when PHO undergoes crystallization. It was also found that the strain-induced crystallization and crystalline break-up are not significant for deformed sample. We proposed that the high permanent tensile set associated with PHO comes from the amorphous part rather than from the crystalline part of the system.

The addition of a low concentration of dissolved high molecular weight polymer can greatly modify the rheological properties of a simple. Many of the modification can be attributed to flow-induced departures of the average chain conformation from its isotropic value at quiescence. The statistical deformation and orientation of polymer chains in flow has been predicted by various molecular models, but these predictions have not been adequately tested. This research provides an important molecular-level understanding of chain conformation in dilute solutions undergoing elongational flow; the work applies light scattering and birefringence techniques to probe chain conformation in situ. We have investigated the influences of chain stiffness and solvent quality on the chain conformations produced in and around the stagnation point of opposed jet flow. By light scattering, the average radius of gyration of the examined polymers has been probed both parallel and perpendicular to the stretching axis for flows of various strength. Flexible polymers are not deformed affinely under any circumstances, with statistical coil deformation falling much below this limit. Although solvent quality has little impact, slightly more coil deformation is observed in a theta solvent than in a good one. For a relatively stiff polymer of approximately 15 persistence lengths, the behavior of chain deformation/orientation in opposed jet flow is different; Because less strain is required to orient a stiff polymer than to deform an analogous flexible polymer, conformation changes are less localized, extending outside the region between the jets. Nevertheless, the overall conformational changes remain less than that predicted by a rod model.

A complete characterization of the mechanical, thermal and physical properties of acrylic-based latex blends films and a thermodynamic analysis of their deformation is presented in this study. These blends are composed of a glassy poly(methyl methacrylate-co-ethyl acrylate) $\rm(T\sb{g}=45\sp\circ$C), and a rubbery poly(methyl methacrylate-co-butyl acrylate) $\rm(T\sb{g}={-}5\sp\circ$C). Blend films are prepared, in different proportions of the two copolymers, by drying at temperatures high enough to ensure complete coalescence of the latex particles. Thermo-mechanical characterization provides evidence for the phase separation of the blend components by the existence of two distinct glass transitions. Effective blend moduli and Poisson's ratios exhibit sigmodial shaped profiles with composition, indicating the transformation of a continuous rubbery phase, with dispersions of the glassy phase, to a continuous glassy phase, with dispersions of a rubbery phase. Although not precisely measured, a range of 30-40% hard phase in the blend is identified as the interval of this transformation, bridged by a co-continuous morphology. A large amount of water is absorbed by these blends, which turns them white and opaque from their transparent dry state. The impact on mechanical properties is relatively minor as absorbed water is located in separate domains. Redrying at ${-}70\sp\circ$C preserves this whiteness, while redrying at elevated temperatures returns the blends to their original transparency. A qualitative model associates the absorbed water molecules with phase separated domains of residual surfactant within the dry films. Deformation calorimetry of these blends measures the work, heat and change in internal energy of isothermal deformation. An optimal combination of stiffness and extensibility maximizes the blend toughness by a synergistic distribution of energy between the two phases in their respective energy absorbing and energy dissipating mechanisms. The work of deformation increases at higher strain rates but the change in internal energy over fixed extensions remains constant. The additional work, consequently, is dissipated as heat by rate-dependent viscous effects. In summary, these blends provide an excellent model system to study the energy balance of deformation of two phase systems. The results highlight the need of a shift in focus when designing blends for optimum toughness and stiffness, by providing for a simultaneous maximization of energy dissipation and absorption.

This dissertation describes the use of supercritical carbon dioxide (SC CO2) as an aid in fabricating polymer/polymer composites. Monomers and initiators were infused into solid polymer substrates using SC CO 2. The monomers were subsequently polymerized within the substrates to form composites. CO2 swells the polymer substrate and increases the diffusively of reactants within the substrate. The solvent strength of SC CO2 is tunable allowing control over the degree of swelling and over the partitioning behavior of the reactants. CO2 can be easily removed from the final products. First, polystyrene/polyethylene bulk composites were investigated. Styrene and a radical initiator were infused into and reacted throughout the bulk of polyethylene substrates. The composite composition was controlled by controlling infusion time, reaction time and partitioning conditions. Characterization of the composites showed that the crystalline domains of the polyethylene were unaffected. Styrene infused into and polymerized within only the amorphous domains of polyethylene. Polyethylene and polystyrene are immiscible; the semicrystalline nature of polyethylene frustrated gross phase separation of the polystyrene. The resulting “kinetically trapped” phase morphology gave the composites interesting mechanical properties. The phase morphology was characterized, and the polystyrene was found to reside within the interlamellar regions and the centers of the polyethylene spherulites. The polystyrene formed a continuous “scaffold” that reinforced the polyethylene. The reinforcement provided efficient and dramatic improvement in the composite modulus and strength. However, the composites fracture toughness decreased with increasing polystyrene content. The fracture behavior was correlated to the microstructural damage mechanisms in the composites. Second, surface composites were investigated. Using a two-stage process, ethyl 2-cyanoacrylate (ECA) monomer was anionically polymerized in the surface regions of poly(tetrafluoroethylene-co-hexafluoropropylene) substrates. An investigation of the anionic polymerization of ECA in CO 2 established the viability of that system. The composite fabrication process involved first infusing a basic initiator into the substrate using SC CO2. In the second step, monomer was introduced (using SC CO 2) to the substrate. As the monomer absorbed into the initiator-containing substrate, it would polymerize. The composite surfaces were characterized using surface-sensitive techniques. The mechanical performance of the composites were determined by measuring the adhesive fracture toughness.

Gelatin has been used as a binder or dispersing agent for light-sensitive and non-light-sensitive photographic layers. The ability to keep the silver halide crystals finely dispersed and to protect the silver halide crystals and other additives from abrasion and other mechanical and chemical influences make gelatin desirable in photographic applications. However, gelatin is very sensitive to changes in humidity. Although this sensitivity to moisture is favorable when the film must be processed, it is also a drawback to the use of gelatin in an emulsion layer. The absorption of moisture can induce swelling stresses, causing dimensional instability commonly observed as bending or curling in the photographic films. This dissertation focuses on the effects of moisture on the state of stress and dimensional stability of gelatin coatings. The hygroscopic effects on the thermal, mechanical, and transport properties were also investigated. Two types of polymer latices, poly(ethyl acrylate) and poly(ethyl methacrylate), were studied as additives to gelatin. The effects of latex concentration, latex particle size, drying condition at vitrification, and gelatin concentration at set point were examined as a function of relative humidity. The goal is to develop an understanding of these properties and assist in controlling or selecting conditions which will minimize the dimensional instability of photographic films over a wide range of use conditions. A vibrational holographic interferometry method and a thermomechanical analyzer were adopted to measure the stresses and dimensional changes as a function of relative humidity. The incorporation of a polymer latex can reduce the moisture sensitivity, and hence increases the dimensional stability of the emulsion layer exposed to the moisture. Composite theories for an isotropic composite filled with spherical particles were applied to determine the humidity expansion coefficient and elastic moduli of the gelatin-latex films. The experimental data were in excellent agreement with the theories. The decrease in swelling stress with an addition of the polymer latex was explained by the incremental linear elasticity theory. Based on this theory, the best material (i.e., minimum swelling stress, lowest Eβ value) was found to be the gelatin film with 40 parts PEA and 15% gelatin concentration at set point, and was dried at the LMERH condition (130F/5.5% RH).

This dissertation describes the characterization of post-yield deformation processes in thin polyethylene films under biaxial stress states. In particular, research focused on the development of test methodologies and analyses associated with post-yield deformation is described. Considerable emphasis is placed on the initiation and evolution of a unique heterogeneous, post-yield deformation process observed in thin polyethylene films under biaxial stress states. This process closely resembles the necking phenomenon noted in uniaxial tests, and hence initial efforts have primarily involved characterizing the new phenomenon and comparing it to uniaxial necking. The effects of initial molecular orientation and stress state on the formation of either heterogeneous or homogeneous post-yield deformation are investigated and discussed. The evolution of the heterogeneous deformation zones, which are referred to as dilatational bands (DB), is modeled within a thermodynamic framework. The results from the phenomenological characterization are used to determine how the thermodynamic model is to be applied. A kinematic analysis of DB's on the local and global level is also presented. The global kinematics are obtained by measuring changes in the overall dimensions of the DB's as they evolve, while local kinematics are obtained by measuring the principal draw ratios. Results from these studies dictate how the thermodynamic model is to be applied. The thermodynamic model accounts for the energy associated with DB evolution. Based on conclusions derived from phenomenological and kinematic observations, the model is reduced to a single parameter model manifesting expansion of the DB. Correspondingly, the M-integral is the energy release rate describing DB evolution. A material property known as the specific enthalpy of transformation is obtained for several polyethylene films under various biaxial stress states. This property is a measure of the material's resistance to undergoing a post-yield drawing from an initial state to the final, drawn state. A practical outcome for the test methodology might be a scheme to optimize operating conditions for solid-state drawing in a film processing line. The final part of this dissertation presents the development and application of an in-situ birefringence technique able to monitor the molecular orientation direction. The technique is employed to determine the molecular orientation direction field in the process zone of a crack tip for a polyethylene film subjected to a Mode I loading condition. The orientation direction is determined from an analysis of digital images obtained in-situ, in which the tested sample is placed between a pair of constantly rotating crossed polarizing films.

A novel manufacturing process called high-temperature high-pressure sintering was studied and explored. Solid fiber reinforced composites are produced by consolidating and compacting layers of polymeric fabrics near their melting temperature under high pressure. There is no need to use an additional matrix as a bonding material. Partial melting and recrystallization of the fibers effectively fuse the material together. The product is called a “matrix free” fiber reinforced composite and essentially a one-polymer composite in which the fiber and the matrix have the same chemical composition. Since the matrix is eliminated in the process, it is possible to achieve a high fiber volume fraction and light weight composite. Interfacial adhesion between fibers and matrix is very good due to the molecular continuity throughout the system and the material is thermally shapeable. Plain woven Spectra ® cloth made of Spectra® fiber was used to comprehensively study the process. The intrinsic properties of the material demonstrate that matrix free Spectra® fiber reinforced composites have the potential to make ballistic shields such as body armor and helmets. The properties and structure of the original fiber and the cloth were carefully examined. Optimization of the processing conditions started with the probing of sintering temperatures by Differential Scanning Calorimetry. Coupled with the information from structural, morphological and mechanical investigations on the samples sintered at different processing conditions, the optimal processing windows were determined to ensure that the outstanding original properties of the fibers translate into high ballistic performance of the composites. Matrix free Spectra® composites exhibit excellent ballistic resistance in the V50 tests conducted by the US Army. In the research, process-structure-property relationship is established and correlations between various properties and structures are understood. Thorough knowledge is obtained for this creative process regarding the procedures, outcomes, advantages and capabilities. Two other ultra high molecular weight polyethylene fiber containing materials, Dyneema Fraglight® nonwoven felt and Spectra Shield® Plus PCR prepreg, were also carefully studied using the process of high-temperature high-pressure sintering. Their structures, morphologies and thermo-mechanical properties were compared with consolidated Spectra® cloth. The results clearly demonstrate that Spectra® cloth is the best candidate for making ballistic protective shields.

This thesis focuses primarily on the structure-property relationships of poly (styrene-ethylene-butylene-styrene) triblock copolymer TPEs. First evidence for strain-induced crystallization occurring in certain SEBS block copolymers has been established using unique techniques like deformation calorimetry, combined in-situ small angle X-ray and wide angle X-ray diffraction (SAXD/WAXD). Also the ramifications of such strain-induced crystallization on the mechanical properties like cyclic hysteresis, stress relaxation/creep retention of these SEBS systems have been studied. In addition, the structural changes in the morphology of these systems on deformation have been investigated using combined SAXD/WAXD setup. Small angle X-ray diffraction probed the changes at the nano-scale of polystyrene (PS) cylinders, while wide angle X-ray diffraction probed the changes at molecular length scales of the amorphous/crystalline domains of the elastomeric mid-block in these systems. New structural features at both these length scales have been observed and incorporated into the overall deformation mechanisms of the material. Continuous processing techniques like extrusion have been used to obtain ultra long-range order and orientation in these SEBS systems. Thus well ordered crystal like hexagonal packing of cylinders, where in each element in this hexagonal lattice can be individually addressed without any grain boundaries can be realized using these robust techniques. The effect of long-range order/orientation on the mechanical properties has been studied. In addition, these well ordered systems serve as model systems for evaluating deformation mechanisms of these SEBS systems, where the relative contributions of each of the phases can be estimated. EPDM/i-PP thermoplastic vulcanizates (TPVs) have micron size scale phase separated morphologies of EPDM rubber dispersed in a semicrystalline i-PP matrix as a result of the dynamic vulcanization process. Confocal microscopy studies, along with scanning electron microscopy (SEM) studies show that the morphology of these EPDM/i-PP systems resembles a microcellular "filled" foam in which i-PP occupies the strut regions and EPDM the inner core. Based on this, an analytical model has been developed that takes into account composition information, molecular weight, cure state and morphology into account.

This thesis is focused in three areas: An investigation of a thermodynamic criterion for failure by environmental stress cracking using observations of the wetting behavior of stress-cracking liquids on glassy polymer substrates; Determination of the dominant chemical and physical degradation mechanisms associated with exposure of poly-p-phenylenebisbenzoxazole fiber to moisture moisture and UV-Vis spectrum light; And finally, the effect of constraint on fracture at a bi-material interface is investigated using a model epoxy-metallic adherend specimen. The wetting behavior of an ESC liquid on polycarbonate substrates has been evaluated as a function of substrate stress using a variation of Contact Adhesion Testing, a novel method of measuring small contact angles by refraction and conventional goniometry. The inelastic and elastic strain condition and time to the onset of crazing were also observed. A normalization of the time to onset of crazing using stress state, solubility difference and diffusion coefficients was shown to collapse the kinetic observations. A comprehensive study of the degradation mechanisms of PBO AS fiber exposed in a controlled manner to challenging chemical environments, moisture and UV-Visible spectrum light was undertaken. Fibers were characterized using a broad range of mechanical and physical tests including tensile testing, Elemental Analysis, scanning electron microscopy, small angle X-ray diffraction, wide angle X-ray diffraction and attenuated total reflectance infrared spectroscopy. Degradation by moisture is found to be primarily due to a loosening of the fiber's fibrillar structure. Degradation by UV-Visible spectrum light is found to be chemical in nature involving hydrolytic disruption of the oxazole ring and possible subsequent conversion to an amide bond. Approaches to alleviation of PBO AS fiber degradation were studied including super-critical carbon dioxide extraction of residual acid, the use of UV-Vis blocking coatings, compaction of the fiber microstructure and PBO AS/Siloxane composites prepared in super-critical carbon dioxide. Finally, the effect of constraint on fracture at the interface between a polymer and adherend having orders of magnitude larger stiffness was studied using a model epoxy/metallic adherend system. Fracture energy was measured using an Elastic Wedge Opened Double Cantilevered Beam test and the process zone imaged using photoelastic methods.

Radiation crosslinked samples of poly(vinylidene fluoride) (PVF$\sb2$) were obtained from the Raychem Company, heated to above their crystalline melting point, stretched and isothermally recrystallized at 100$\sp\circ$C. The crystalline and amorphous orientation functions were studied as a function of crosslink density and extension ratio by X-ray diffraction and birefringence. The crystalline phase orients with its c-axis parallel to the deformation direction and the amorphous phase orients perpendicular to the deformation direction. The degree of orientation increases with both the crosslink density and the extension ratio. Values of both the crystalline and amorphous intrinsic birefringences of PVF$\sb2$ are offered. The intrinsic birefringence of the amorphous phase, calculated from bond polarizabilities and birefringence measurements using rubber elasticity theory, equals 0.098 $\pm$ 0.017. The intrinsic birefringence of the alpha crystalline phase is calculated from bond polarizabilities using the differentiated Lorentz-Lorenz equation and is equal to 0.145 $\pm$ 0.002. Each of the samples are characterized in terms of their polymorphic form by wide angle X-ray diffraction, their molecular weight using Flory-Rehner theory, and their degree of crystallinity from density measurements. In addition a value of 0.15 is offered for the polymer-solvent interaction parameter in N,N-dimethylacetamide. Finally, attempts at drawing a correlation between the amorphous orientation functions of molten PVF$\sb2$ samples and the orientation functions of these same samples after crystallization indicate that the crystalline orientation functions indeed depend upon the amorphous orientation functions of the deformed molten network rather than upon the crosslink density or extension ratio of the samples taken separately.