Free volumes and structural relaxations in diglycidyl ether of bisphenol-A based epoxy–polyether amine networks

Abstract

Two types of polyether diamines were used to prepare model rubbery and glassy epoxy–amine networks with diglycidyl ether of bisphenol-A; α,ω-diamino terminated polyoxypropylene (POP) diamines and α,ω-diamino terminated poly(oxypropylene)-block-poly(oxyethylene)-block-poly(oxypropylene)s (POP-POE-POP). The structural relaxations in the glassy and rubbery epoxy–amine networks at segmental (α relaxation) and local (β relaxation) levels were investigated by modulated differential scanning calorimetry (MDSC) and dynamic mechanical analysis (DMA). The characteristic length of glass transition of the networks ξ(T_g) was evaluated from MDSC using Donth's thermal fluctuation approach. While the POP diamine networks showed ξ(T_g) values of 2.0 and 2.07 nm, for POP diamine molecular weights of 230 and 400, respectively, the corresponding values for POP-POE-POP diamine networks were found to be 1.41 and 1.58 nm for POP-POE-POP diamine molecular weights of 600 and 900. This implied diminishing size of the cooperatively rearranging regions with decreasing crosslink density. DMA measurements were used to evaluate the crosslink density of the networks, characteristic features of the α and β transitions in terms of the width, intensity of transitions, and activation energy of the β relaxation. The studies revealed highly cooperative sub-T_g β relaxations for the glassy networks and a truncated but pronounced β relaxation for the rubbery networks. Positron annihilation lifetime spectroscopy (PALS) was used to characterize the molecular topology of the networks in terms of the free volume nanohole sizes and their distribution. The difference of the average distance between crosslink points and the free volume nanohole size was seen to increase with the chain length of the diamines, indicating the fluctuational nature of the networks influenced by the sub T_g relaxation.