ガラス状エポキシネットワークの塑性流動

抄録

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.