Abstract
The single relaxation time of glassy epoxy networks having different crosslink densities was evaluated during uniaxial compression processes by using a simple nonlinear single relaxation model to study the effect of crosslinked molecular structures on the transient nonlinear viscoelastic behavior of glassy epoxy networks. The model consisted of two elastic springs expressing linear viscoelastic behavior and a dashpot with variable viscosity as a single parameter representing strain-induced structural change. We calculated the strain-dependent relaxation time τSS during the compression by fitting the model to the experimental stress-strain curves obtained at various strain rates and at temperatures T 18℃ lower than the glass transition temperature Tg of the material being compressed. With increasing strain εn, τSS obtained for each epoxy network steeply decreased in the pre-yield region of strain and then reached to low steady values appearing in strain ranges larger than the yield strain, reproducing previously reported typical behavior observed for glassy polymers under large deformation. When compressed at an identical strain rate, τSS-εn relations for all epoxy networks almost coincided each other despite the difference in the crosslink density. This result indicates that, if the deformation is performed at the same condition i.e., strain rate and T-Tg, strain-induced variation of relaxation time in the epoxy glasses is almost not influenced by crosslinked molecular structures.
