VISCOELASTIC RESPONSE OF MODEL EPOXY NETWORKS IN THE GLASS-TRANSITION REGION

Six epoxy networks with various structures built up from a diepoxy prepolymer, DGEBA, and three different diamines or mixtures of a monoamine and a diamine were studied by dynamic mechanical analysis in the glass transition region. The systems were designed in order to investigate the dependence of glass transition T(g) on both crosslink density and network chain flexibility. The time (frequency)-temperature superposition principle (WLF equation) was used to determine the viscoelastic coefficients C1g and C2g which are related to some free volume characteristics on the molecular scale. C1g, related to the free volume fraction available at T(g) depends mainly on crosslink density, even though the product C1gC2g, related to the free volume expansion coefficient, is dependent on both chain flexibility and crosslink density. Thus, viscoelastic properties determined over large temperature and frequency ranges are shown to yield more precise information on epoxy network structure than the simple analysis of glass transition temperature.