Abstract
Effects of crosslinked molecular structures in glassy epoxy networks on the nonlinear viscoelastic behavior have been studied in terms of strain-induced structural change. We prepared four epoxy glasses with different crosslink densities from the same epoxy precursor by controlling fractions of chain-extender and crosslinker in curing agents. The epoxy glasses were subjected to uniaxial compression at a temperature 18K below the glass transition temperature of each sample. With increasing crosslink density, initial elastic moduli and yield stresses decreased, whereas flow stresses appearing after the strain softening almost unchanged, except for the most tightly crosslinked sample showing lower flow stresses. Results of density measurement and DSC showed that an increase in the crosslink density made the glassy structures more unstable. These results show that unstable glassy structures due to constraints on segmental mobility introduced by crosslinked molecular structures lessen the initial elastic modulus and the yield stress. On the other hand, the variation of crosslink density had almost no effect on the flow stresses of the glasses. This observation presumably indicates that molecular crosslinks have little effect on the strain-induced structural change. When the crosslink density comes to quite high, however, the crosslinks are likely to start affecting the flow stress, i.e. strain-induced unstable glassy structures.
