The tensile mechanical properties and fracture toughness of a Bisphenol‐A type difunctional epoxy resin, cured with different amounts of metaphenylene diamine, using two cure cycles, were determined over a range of temperature. The tensile modulus in the glassy state was seen to be predominantly related to intermolecular packing, while in the rubbery state crosslink density was the important factor. Yielding appeared to be due to an increase in free volume as a result of dilatation during the tensile test and was related to a critical shear stress. The large strain properties like tensile strength, elongation‐to‐break, and toughness showed a more complex dependence on chemical structure, molecular architecture, intermolecular packing, and crosslink density. The roles played by the relaxation processes in determining mechanical properties are highlighted.

SynopsisThe moisture transport characteristics of a difunctional epoxy resin cured with different amounts of metaphenylene diamine, using two cure cycles, are reported. Besides studying the kinetics of moisture sorption at 20, 50, 75, and lWC, the investigations also included measurement of thermal expansion coefficients and dynamic mechanical transition of the dry and wet samples. The moisture sorption of the sample is shown to be related to its specific volume and hence to its fractional free volume. In the glassy state, the free volume is apparently in the form of frozen voids, and moisture sorption/desorption at this temperature is of the Langmuir type with little or no bond formation. At higher temperatures the free volume is generated predominantly through segmental motion of the a transition. The Henry's Law mode becomes operative, and the moisture can now form bonds. "he possible effect of nonuniform crosslink density on moisture sorption is also considered.

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The processing of polymer matrix composites requires the formation of an autohesive bond between the surfaces of adjacent polymer entities as well as with the fiber surface. This autohesive bond strength development in amorphous polymers has been extensively studied and modeled. This paper describes a study undertaken to describe the development of the autohesive bond between polymer surfaces in semi-crystalline matrix composites. The results show that the development of the autohesive bond competes with the formation of crystals and actually results in the decrease of fracture strength at long processing times.

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