Micromechanical deformation processes responsible for toughening mechanisms in ultrafine monospherical inorganic particle-filled polyethylene were investigated in situ by a field-emission gun-environmental scanning electron microscope (FEG-ESEM) with low-voltage techniques. In general, the ultimate properties of polymer composites are largely dependent on the degree of dispersion of filler particles into the matrix. Very often, the agglomeration is one of inevitable occurrences in polymer composites, mixed with ultrafine filler particles. In the present work, the effects of agglomerates, consisting of ultrafine monospherical filler particles, were reexamined in polymer composites on the toughening mechanism. The results show that the dominant micromechanical deformation processes are the multiple debonding processes inside agglomerates, in which the ratio of the matrix strand and the size of agglomerate plays a great role of matrix yielding. In the specimen, where the agglomerates are isolated in the matrix, deformation begins at the equatorial region of agglomerates and propagates through them. However, in the case of closely placed agglomerates, deformation occurs homogeneously within the whole area inside the agglomerates. In both cases, in conjunction with the multiple debonding processes, the major part of energy during the deformation dissipates through the shear-flow processes of the matrix material. In particular, the micromechanical deformation processes observed in this work confirm that the agglomerates do not always have negative effects on the mechanical properties-at least, in the shear deformable semicrystalline polymer matrices. The agglomerates may be effectively used for the improvement of toughness. Furthermore, the FEG-ESEM with low-voltage techniques offers an extremely promising and efficient alternative method to study the morphology as well as in situ micromechanical deformation processes in nonconducting polymer systems.

The biocompatibility, processing ease, and mechanical properties of freeze-thawed poly(vinyl alcohol) (PVA)-based hydrogels have encouraged significant research toward developing this material for various biomedical applications. Crystallization that occurs during the freeze-thawing process is cited in the literature as the primary mechanism responsible for the resultant mechanical properties. Further analysis, however, shows the presence of two unique mechanisms that contribute to PVA's mechanical properties. During freeze-thaw cycling water freezes causing phase separation, which facilitates crystallization. The impact of phase separation during freeze-thaw cycling was investigated by comparing freeze-thawed and aged PVA hydrogels. Aged hydrogels were not prepared by freezing and, therefore, did not exhibit significant phase separation. The amount of phase separation was discerned using optical microscopy in the hydrated state. Crystallinity and mechanical properties were also evaluated as a function of the number of cycles (for freeze-thawed gels) and aging time (for aged gels).For freeze-thawed hydrogels, crystallinity deviated significantly from the trend observed in compressive modulus, indicating that crystallinity was not the only factor determining the hydrogel's mechanical properties. Phase separation was found to occur during freeze-thaw cycling independently of crystallization, especially at later freeze-thaw cycles (after the third). The trends observed for both crystallinity and modulus for aged hydrogels, however, were in better agreement with each other.Further evaluation of the mechanical properties of aged and freeze-thawed hydrogels with similar crystallinities indicated that freeze-thawed hydrogels have significantly higher modulus values (p < 0.05). As a result, phase separation, independently of crystallization, was determined to have a significant effect on gelation during freeze-thaw cycling. In particular, PVA-rich regions that are formed during phase separation, without additional cross-linking, are believed to have a significant effect on the resultant mechanical properties.

The combination of awareness of harmful industrial processes and environmental issues and depleting petroleum-based resources has spurred much research in developing materials from renewable sources. Epoxy resins are common pre-polymers used in a variety of industries, such as adhesives, coatings, insulations, and high performance composites. To transform epoxy resins into crosslinked networks with desirable thermal and mechanical properties, the resins must be cured with a curing agent. This review encompasses recent developments using bio-based epoxy resins and bio-based epoxy curing agents. Resins and curing agents synthesized from modified plant oils, sugars, polyphenols, terpenes, rosin, natural rubber, and lignin are highlighted and their thermal and mechanical properties reviewed.

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The relationships among cure temperature, chemical kinetics, microstructure, and mechanical performance have been investigated for vinyl–ester resins. Fourier transform infrared spectroscopy was used to follow the reactions of vinyl–ester and styrene during isothermal curing of Dow Derakane 411‐C‐50 at 30 and 90°C. Reactivity ratios of vinyl–ester and styrene vinyl groups were evaluated using the copolymer composition equation. The results indicate that the ratio of vinyl–ester to styrene double bonds incorporated into the network is greater for 30 than for 90°C cure. Mechanical properties were obtained for systems subjected to isothermal cures at 30 and 90°C and postcured above ultimate Tg. The results show that the initial cure temperature significantly affects the mechanical behavior of vinyl–ester resin systems. In particular, values of strength and fracture toughness for postcured samples initially cured isothermally at 30°C are significantly higher than those obtained for samples cured isothermally at 90°C. Examination of fracture surfaces using atomic force microscopy revealed the existence of a nodular microstructure possessing characteristic nodule dimensions that are affected by the temperature of cure. Such features suggest the existence of phase separation during cure. A binary interaction model in conjunction with chemical kinetic data and estimated solubility parameters was used to evaluate enthalpic interactions between the growing polymer network and monomers of the vinyl–ester system. The results indicate that the interaction energy becomes increasingly endothermic as cure progresses and that this energy is affected by the temperature of cure through differences in copolymerization behavior. Hence, in addition to entropic factors, the changes in enthalpic contribution to the Gibbs free energy suggest that the probability of phase separation increases with extent of cure and that its onset is potentially affected by cure temperature. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 725–744, 1999