Toughening mechanisms in melt manipulated thermoplastics (PS and PC) and in novel modified epoxies.

摘要

This study examined the deformation mechanisms associated with the fatigue and fracture of polystyrene (PS) and polycarbonate (PC) processed using a novel polymer melt manipulation technique, vibration-assisted injection molding (VAIM), as well as the toughening mechanisms of several epoxy resins modified with several novel block co-polymers.; VAIM processed PS was found to possess a tensile strength 28% greater than conventionally processed PS, which is in agreement with previous research. The increase in tensile strength is due to the higher craze initiation stress required to overcome the thicker "frozen in" layer of the VAIM processed specimens. Also, the residual orientation present in VAIM specimens appears to retard craze propagation and this is evident by shear banding. Unfortunately, VAIM processed PS did not result in a fatigue lifetime improvement. In fatigue, the PS specimens processed either way usually resulted in the initiation and propagation of a single "killer craze" that caused failure.; In contrast, the VAIM processed PC did not exhibit a significant improvement in tensile strength compared against the conventionally molded specimens. In cyclic loading, for the two stress levels examined, both the VAIM and conventional PC specimens were found to perform equally---when examining the average lifetimes and their corresponding standard deviations.; In addition, this work studied the deformation mechanisms associated with novel block co-polymer modified epoxies. It was found that the plane strain fracture toughness, KIC, was on the order of 3.0MPa√m for the NanoStrength(TM) E20 modified AEP and PIP cured systems, which is comparable to micro-segregated core-shell particles. Toughness improvements significantly depended on cross-link density of the epoxy matrix, curing agent, and on the amount of polybutadiene present in the toughening agents examined.; The system with the highest, KIC, was selected to make a conductive adhesive using 30nm diameter Ag spheres and 50nm length Ni chains. The adhesive strength was studied as a function of bondline thickness. It was found that the addition of 20wt% nanosilver spheres to the DGEBA/AEP/E20(10) system resulted in a reduction in resistivity in excess of 4X and in an adhesive strength increase of 2.5X for small, 5mum, bondline thicknesses.