Microstructural effects and the toughening of thermoplastic modified epoxy resins

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Abstract
We have studied an epoxy resin formulation consisting of the diglycidyl ether of bisphenol‐A (DGEBA), modified with phenolic hydroxyl‐terminated polysulfone (PSF) and cured with an aromatic amine curing agent, diaminodiphenyl sulfone (DDS). A range of microstructures and fracture properties have been obtained by controlling the formulation cure conditions (cure temperature and cure cycle in an isothermal mode). The chemical conversion of the cured resins has been monitored by near‐infrared spectroscopy (NIR). Although only a single material formulation was used, three distinct types of microstructure were identified by scanning electron microscope (SEM) observations on samples prepared at different cure temperatures. Surprisingly, the thermal and fracture properties of the cured samples did not vary noticeably, in spite of the significant microstructure variations. The consistency of these fracture toughness results with cure temperature changes was an unexpected result in the light of our earlier observations of a strong dependence of fracture toughness on cure temperature in neat resin systems. The difference in behavior between neat and modified resins reveals that the fracture toughness of the latter is dependent on a combination of the microstructure and the matrix resin properties. This hypothesis was also supported by an observation of high fracture thoughness in a sample cured in a two‐step process, which we believe is due to the optimum microstructure and matrix resin properties, being achieved separately during precure and postcure, respectively. The increase in fracture toughness values caused by the modification (ΔGIC) was calculated from the fracture toughness values of neat and modified resins, prepared under the same cure conditions, using a proposed theoretical equation. © 1993 John Wiley & Sons, Inc.