STRUCTURAL TRANSITION IN POLY(METHYL METHACRYLATE) DUE TO LARGE DEFORMATION AT TEMPERATURES BELOW THE EQUILIBRIUM 2ND-ORDER TRANSITION-TEMPERATURE

To analyse the structural change in glassy Polymers under large deformation at low temperatures, Poly(methyl methacrylate) specimens were uniaxially compressed at temperatures below the equilibrium second-order transition temperature T2 with varying strain rates. The state of steady plastic flow which appeared in the lower yield range of the stress-strain curve was analysed using the Eyring equation in a novel way. This analysis provided the following results: at low temperatures, the volume of a flow unit decreased with temperature, probably approaching the least critical value; and a functional relation between the activation enthalpy DELTA-H and the activation entropy DELTA-S for the glass deviated gradually from that for the melt derived from the WLF equation with decreasing temperature. This deviation could be attributed to the structural transition of the glass into liquid-like structures of the melt at elevated temperatures above T(g) + 100 K where the WLF equation is no longer available.