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Intercalated nanocomposites of modified montmorillonite clays in a glassy epoxy were prepared by crosslinking with commercially available aliphatic diamine curing agents. These materials are shown to have improved Young's modulus but corresponding reductions in ultimate strength and strain to failure. The results were consistent with most particulate‐filled systems. The macroscopic compressive behavior was unchanged, although the failure mechanisms in compression varied from the unmodified samples. The fracture toughness of these materials was investigated and improvements in toughness values of 100% over unmodified resin were demonstrated. The fracture‐surface topology was examined using scanning electron and tapping‐mode atomic force microscopies and shown to be related to the clay morphology of the system. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1137–1146, 2001

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Microcellular polystyrene foams have been prepared using supercritical carbon dioxide as the foaming agent. The cellular structures resulting from this process have been shown to have a significant effect on the corresponding mechanical properties of the foams. Compression tests were performed on highly expanded foams having oriented, anisotropic cells. For these materials an anisotropic foam model can be used to predict the effect of cell size and shape on the compressive yield stress. Beyond yield, the foams deformed heterogeneously under a constant stress. Microstructural investigations of the heterogeneous deformation indicate that the dominant mechanisms are progressive microcellular collapse followed by foam densification. The phenomenon is compared to the development of a stable neck commonly observed in polymers subjected to uniaxial tension, and a model that describes the densification process is formulated from simple energy balance considerations.

PMMA-layered silicate intercalated nanocomposites are synthesized using supercritical carbon dioxide (scCO2) to produce ordered materials with significant levels of reinforcement. The scCO2 is used to homogeneously distribute monomer as well as act as a low-viscosity solvent for MMA polymerization. This route allows for synthesis of nanocomposites containing significant levels of organically modified layered silicates (OMLS). Below 40 wt % OMLS, the intercalated nanocomposites exhibit a d spacing commensurate with dimensions of the fully extended surfactant chains. Above 40 wt % OMLS, the composite volume is saturated with inorganic material, and the d spacing decreases to homogeneously distribute the polymer volume. A model for estimating this transition concentration is presented. At concentrations approaching the homogeneously intercalated morphology, the basic mechanical and physical properties of the composite are investigated.

A new approach to prepare and characterize prestressed competitive double network elastomeric systems was investigated. A styrene−butadiene−styrene (SBS) triblock copolymer system containing physical cross-links was used to achieve a double network by additional chemical cross-linking using ultraviolet (UV) light. Properties measured from conventional monotonic tensile tests, stress relaxation, thermomechanical, hysteresis, and swelling analysis were investigated and related to their network structure. These double network elastomers show a transition between competitive and collaborative behavior in their mechanical properties at different strain regimes. These elastomers also show lower permanent set in both low and high strain regimes along with lower hysteresis. These networks exhibit lower modulus along with lower coefficient of thermal expansion, still showing lower swelling ratios, which results from a competition between the networks.

SYNOPSISThe tensile fatigue behavior of two engineering thermoplastics (polyacetal and nylon,,,) were studied by measuring changes in the dynamic viscoelastic response together with changes in potential energy density, strain energy density, and irreversible work. The results show that both stress softening and hardening can occur in controlled load cyclic conditions. At high stress levels and/or frequencies, both the polyacetal and nylon,,, show evidence of thermal softening as characterized by changes in their dynamic viscoelastic properties and decrease in storage modulus with corresponding increases in loss modulus and loss tangent. This effect is supported by observed decreases in the overall crystallinity as measured in DSC experiments. At lower stress levels (the mechanically dominated region), all results indicate that, although fatigue crack propagation (FCP) is one of the mechanisms governing the fatigue life, its contribution is minor and crack initiation time constitutes the majority of the fatigue life. Also, during the initiation stage, both materials become less viscoelastic and more elastic. This phenomenon is evidenced by overall reductions in the loss modulus, loss tangent, and irreversible work densities while the storage modulus is maintained. I NTRO DU C T l O N Today, engineering thermoplastics are commonplace in many structurally demanding applications. They are used for pipes, for automotive components, and a wide variety of other structural and mechanical components (e.g., gears pumps). Usually these materials are chosen for either their high specific strength and toughness, their inherent resistance to corrosion in aggressive environments, or for fabrication advantages. Yet in comparison to other structural materials (e.g., metals) little is known about the mechanisms which govern their fatigue life.The mechanisms governing the fatigue life in metals have been the topic of discussion in the engineering community for over a century.'-' Hence, a wealth of knowledge exists which enables engineers to confidently design metal parts for applications where the loads are expected to fluctuate or cycle * To whom correspondence should be addressed.Journal of Applied Polymer Science, Vol. 58,869-879 (1995) 0 1995 ,John Wiley & Sons, Inc.CCC 0021-8995/95/050S69-11 over their service lives.8 A variety of studies have been conducted in metals which clearly define the effects of grain morphology and dislocations on fatigue For example, the decrease in internal damping together with the increase in modulus during cyclic loading of steels have been attributed to the gradual restriction of dislocation motions.'In contrast to metal alloys, relatively little is published describing the mechanisms which control the fatigue life in polymers. Many of the earlier studies are phenomenological in nature and typically focus on the development of stress-lifetime (S-N) data for specific polymer systems and loading frequencies. However, some studies have investigated the effects of different polymer morphologies...

This is the first of a two-part series investigating the degradation mechanisms of PBO fiber and approaches to alleviating degradation and improving fiber properties. Poly-p-phenylenebenzobisoxazole (PBO) fiber is a high strength and modulus fiber with remarkable thermal stability. Recent in-service failures of this fiber have revealed that the fiber degrades rapidly in relatively mild environmental conditions of moisture and heat. In this work the mechanisms of degradation due to moisture, the presence of acid, and the effect of radiation from the UV-vis spectrum are investigated. It is found that exposure to moisture results in the loosening of the fiber morphology leading to an increase in the number and size of defects. The presence of aqueous acid causes both loosening of the fiber structure and hydrolysis of the oxazole ring structure. The effect of UV-vis radiation is primarily hydrolysis of the material near the fiber surface with attendant formation of amide linkages.