Large Deformation Mechanism of Glassy Polymers

Abstract

Large deformation mechanism of glassy polymers was investigated through examination of stress-strain relations with the aid of a nonlinear single-relaxation model, measurement of dynamic viscoelasticity during deformation, and examination of static fracture properties of well-yielded polymers. A simple mechanical model with variable single-relaxation time as a parameter quantifying strain-induced structural change successfully reproduced stress-strain relations during yielding process. The single relaxation time started to vary at a strain much smaller than the yield strain, and it finally reached low constant values in the post-yield regime of strain. Thus, it was concluded that glassy structures continuously changed in the course of yielding. Variation of dynamic viscoelasticity experimentally observed during yielding process strongly supported this conclusion. Large deformation given at a very low strain rate as well as stress relaxation history after deformation were found to lessen the ductility of glassy polymer. This observation presumably indicates that the mechanism of nonlinear stress relaxation, which provides polymeric glasses with fluidity, also gives brittleness to the glassy system.