REACTION-MECHANISMS, MICROSTRUCTURE, AND FRACTURE PROPERTIES OF THERMOPLASTIC POLYSULFONE-MODIFIED EPOXY-RESIN

The microstructure and fracture properties of diglycidyl ether of bisp henol A (DGEBA) epoxy resins modified with phenolic hydroxyl-terminate d polysulfone (PSF) and cured with diaminodiphenyl sulfone (DDS) harde ner have been investigated as a function of the molecular weight and c oncentration of PSF. The microstructure changed from a typical particu late structure to a phase-inverted structure as the molecular weight a nd/or the concentration of the modifier increased. The fracture toughn ess, measured by compact tension tests, increased with the microstruct ural changes toward the phase-inverted structure. The microstructural changes observed have been interpreted in terms of variation in the re action mechanisms as determined by near-infrared spectroscopy. The lev el of minor reactions such as etherification and homopolymerization re actions increased with increasing molecular weight and/or concentratio n of the modifier, in line with the tendencies observed in microstruct ure and fracture toughness. In the system containing 20 wt % of M(n) 1 0,000 PSF, about 30% of the epoxy groups were consumed by etherificati on and homopolymerization reactions, whereas none of these reactions o ccurred in the unmodified system. The increase in minor reactions in t he modified systems may be to be due to the restricted molecular mobil ity, resulting from the increase of system viscosity caused by the mod ification. This increase in viscosity also reduced the rate of phase s eparation. The degree of heterogeneity in the epoxy network must incre ase with the increasing side reactions. The formation of the heterogen eous epoxy network and the slowdown of phase separation will prevent a uniform precipitation of the modifier and finally result in a heterog eneous partially phase-inverted structure or the completely phase-inve rted structure, depending on the amount of modifier incorporated over the critical concentration. (C) 1993 John Wiley & Sons, Inc.