Morphological and fracture studies of alloys of thermosets and thermoplastics

To improve the fracture toughness of brittle epoxies, the incorporation of various thermoplastics was studied. These materials exhibited multiphase structures, which were found to have significant influences on effective toughening.;Initially, simple epoxy/polyethersulfone (PES) systems were investigated. The objective was to establish the relationships (1) between PES chemical structures and the resultant morphologies and (2) between these morphologies and fracture toughness. Modification of the chemical structures included variations of the end groups and the molecular weight. The effects of the concentration of PES and the processing conditions were also evaluated. All variables showed significant effects on the structural development, resulting in a wide range of multiphase microstructures. Despite the various morphologies, only the inverted phase structure coupled with the weak interface exhibited substantial fracture toughness improvement. Debonding and microcracking at the interface prior to the main crack were observed. Additionally, plastic deformation of the continuous phase, which was mainly comprised of ductile PES, was confirmed. Fracture analysis revealed that microcracking was effective only to relieve triaxial stress state ahead of the crack tip, and the major contribution to toughening seemed to be attributed to the yielding of the ductile continuous phase.;Next, phase separation and the structural development of the epoxy/PES systems were examined theoretically and experimentally. Although phase separation was proven to be a natural consequence of the Flory-Huggins theory, this theory does not account for the effect of the end groups and the processing conditions.;Finally, amorphous-crystalline block copolymers were evaluated as novel toughening agents. These materials, which incorporated crystalline phases in the epoxy matrix, utilized an alternative toughening mechanism, namely phase transformation toughening. Our experimental results confirmed that polybutyleneterephthalate (PBT) segments of the copolymers could form either closely packed crystal lamellae or finely dispersed single lamella. The crystallization behavior of the PBT segments was found to be greatly constrained by the epoxy network as well as the PES segments. Several characteristic morphologies were obtained upon failure, however, improvement in fracture toughness was marginal.