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 beta-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 C1g and C2g. C1g, related to the free volume fraction available at T(g), mostly depends on cross-link density, whereas the product C1gC2g, related to the free volume expansion coefficient, is a function of both the chain flexibility and the cross-link density. Motions responsible for the beta-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-delta, and loss modulus, E". This effect is responsible for the decrease of the elastic modulus, E', at room temperature with increasing cross-link density.
