Epoxy-cyanate ester shape memory thermoset: some aspects of phase transition, viscoelasticity and shape memory characteristics

A shape memory thermoset comprising of a co-reacted system of epoxy resin (diglycidylether of bisphenol A), cyanate ester (bisphenol A dicyanate ester) and phenol telechelic poly(tetramethylene oxide) (PTOH) was investigated for its morphology, viscoelasticity and shape memory characteristics at the transition temperature regime. The system exhibited a switching temperature (Tswitch) centered at about 105 degrees C. Atomic force microscopy analyses at different temperatures provided evidences for the existence of a discrete phase at Tswitch regime. Polarized light microscope images gave evidence for the birefringence and tubular crystal formation due to PTOH segments in the shape memory thermoset. It is concluded that the Tswitch has its origin from melting transition of PTOH and Tg of the thermoset matrix, the latter being lowered through plasticization by PTMO segments. Reversibility of Tswitch, and stress relaxation behavior of the blend were investigated by dynamic mechanical analysis (DMA). The reversibility of transition temperature was ascertained by cyclic DMA. Temperature dependency of shape memory properties implied fast recovery of original shape above the Tswitch. The cured system manifests shape memory properties even below Tswitch though it is a slow process. The extent of shape recovery increased with temperature and became faster in league with the trend in temperature dependency of stress relaxation of the polymer. Copyright (c) 2013 John Wiley & Sons, Ltd.