Steady plastic flow in specimens of tightly cross-linked glassy epoxy network was analyzed as a rate process to discuss the mechanism of nonlinear viscoelastic behavior of glassy network polymers under large deformation. Specimens were uniaxially stretched at temperatures below the glass transition temperature with various strain rates. True stress σ and birefringence in the specimen were recorded during the deformation as functions of nominal strain ε n . The glassy stress σ G was extracted from the σ-ε n relations by means of modified stress-optical rule (MSOR). The obtained σ G -ε n relations had a steady flow region, which was observed as an almost constant stress state in the post-yield strain range. The flow stress in this region was analyzed by using the Eyring equation in a special way proposed by Nanzai. The analysis gave an experimental relation between the activation enthalpy ΔH and the activation entropy ΔS, which agreed fairly well with that derived from WLF equation for the linear viscoelastic relaxation of the material in the molten state. This agreement provides an experimental evidence verifying structural change of the glass into liquid-like, highly non-equilibrium structure in the glassy network polymer under large deformation. Independently of the presence of crosslinked molecular structures, the essential mechanism of nonlinear viscoelasticity of glassy polymers is found to be strain-induced structural change.

Steady plastic flow of glassy epoxy networks having various crosslink density was analyzed with Eyring equation to discuss the effect of crosslinked molecular structures on nonlinear viscoelastic behavior of glassy polymers in terms of strain-induced structural change. Steady flow stresses of the glasses were calculated with modified stress optical rule (MSOR) from stress and birefringence data observed during uniaxial stretching. Activation enthalpy DH, activation entropy DS and activation volume va of the steady flow for each material were obtained as functions of stretching conditions by means of a special fitting method of Eyring equation proposed by Nanzai. As having been reported for thermoplastic glassy polymers, DH, DS and va for each material were in unique functional relations each other. The DH-DS relation for each material agreed fairly well with that derived from WLF equation for the linear viscoelastic relaxation of the material in the molten state. This result confirms that strain-induced change of glassy structures into liquid-like ones is the essential mechanism of the nonlinear viscoelastic behavior of glassy polymers independently of crosslink density. DH-DS relations for epoxy networks showed only a weak dependence on the crosslink density, whereas va markedly increased with increasing crosslink density. The steady flow stresses at an identical straining condition was almost the same for epoxy networks with different crosslink density except for materials with extremely high crosslink density, which showed a lower flow stress. An increase of crosslink density probably makes va enlarge due to constraints introduced by crosslinked structure, resulting in the reduction of flow stresses especially at extremely high crosslink density.