Molecular analysis of the viscoelastic properties of epoxy networks as deduced from the study of model systems

Abstract

Various model systems were built from a diepoxide and different amines (or mixtures of amines) in order to investigate the dependence of molecular motions on the cross-link density and network chain flexibility. Dynamic mechanical experiments were performed over the frequency range 0.01–85 Hz at temperatures covering the β-relaxation process and the glass transition region. The glass transition temperature, T g, markedly depends on both the cross-link density and chain flexibility. The frequency-temperature superposition principle (WLF equation) was used to determine the viscoelastic coefficients C 1 g and C 2 g. C 1 g, related to the free volume fraction available at T g, mostly depends on cross-link density, whereas the product C 1 g C 1 g, related to the free volume expansion coefficient, is a function of both the chain flexibility and the cross-link density. Motions responsible for the β-process begin to develop at the same temperature, whatever the cross-link density and chain flexibility may be. However, an increase in cross-link density is accompanied by an increase in amplitude and a broadening towards high temperatures of both damping, tan δ, and loss modulus, E′. This effect is responsible for the decrease of the elastic modulus, E′, at room temperature with increasing cross-link density.