Yielding behaviour in model epoxy thermosets - II. Temperature dependence

The temperature and strain-rate dependence of yielding in compression has been investigated for a series of model thermosets in which crosslink density is varied without compositional change. The yield stress was only indirectly influenced by the crosslink density through its effect on the molecular mobility and the glass transition temperature. The Eyring activation volume was relatively insensitive to changes in crosslink density or composition but increased as the temperature was raised. The Robertson theory of yielding was found to give an approximate prediction for the temperature and strain rate dependence of the yield stress and the adjustable molecular parameter in this theory was close to the theoretical value. Neither the Argon nor the Bowden theories yielded significantly improved fits to the data. The use of these structurally related polymer systems appears to provide a very sensitive test of yielding theories. (C) 1998 Elsevier Science Ltd. All rights reserved.