Toughening of thermoset/thermoplastic composites via reaction-induced phase separation: Epoxy/phenoxy blends

Phase morphology and phase separation behavior of amine-cured bisphenol-A diglycidyl ether epoxy and phenoxy mixtures have been investigated by means of time-resolved small angle light scattering, optical microscopy, and scanning electron microscopy. The starting reactant mixtures composed of epoxy, phenoxy, and curing agents such as diaminodiphenyl sulfone (DDS) and methylene dianiline (MDA) were found to be completely miscible. Upon curing with DDS at 180 degrees C, phase separation took place in various epoxy/phenoxy blends (compositions ranging from 10-40% phenoxy), whereas the MDA curing showed no indication of phase separation. The mechanical and physical properties of single-phase and two-phase networks were examined, in that the DDS-cured epoxy/phenoxy blends having a two-phase morphology showed improved ductility and toughness without significantly losing other mechanical and thermal properties such as modulus, tensile strength, glass transition and heat deflection temperatures. The energy absorbed to failure during the drop weight impact event was also found to improve relative to those of the single-phase MDA-cured blend as well as of the neat epoxy. Such property enhancement of the DDS-cured blends has been discussed in relation to the two-phase morphology obtained via scanning electron microscopy micrographs of fractured surfaces. (C) 2000 John Wiley & Sons, Inc.