VARIATION OF DYNAMIC VISCOELASTICITY DURING YIELDING AND POST-YIELD RELAXATION IN A GLASSY POLYMER

A key factor causing the non-linear viscoelastic behaviors in largely deformed glassy polymers has been considered to be shear-induced structural change. To obtain further information on the molecular kinetics of the structural change, the dynamic viscoelastic properties of glassy polymethyl methacrylate were measured during its uniaxial elongation and subsequent relaxation processes. The dynamic data obtained in the yielding process showed that the structure of the quasi-equilibrium glass was successively changed into another stable one exhibiting much faster stress relaxation. In the post-yield relaxation process, on the other hand, the dynamic properties gradually reverted with time, presumably indicating structural recovery in the glass. Furthermore, some hardening mechanism was directly observed in a fully stress-relaxed state of the largely deformed glass.