TRANSITION MECHANISM FROM ELASTIC-DEFORMATION TO PLASTIC-FLOW IN POLY(METHYL METHACRYLATE)

Sudden changes of compressive strain‐rate on glassy poly(methyl methacrylate) lead to the conclusion that the post‐yield state under a constant stress is a state of steady flow in the polymer. Non‐linearity between the stress divided by temperature and the logarithm of strain rate for this steady plastic flow can successfully be analyzed using the Eyring equation with structural factors variable depending on stress and temperature. This analysis gives a unique functional relationship between the activation entropy and the activation enthalpy, which agrees fairly well with that for the melt derived from the WLF equation. This agreement provides a direct evidence verifying structural change of the glass into liquid‐like structure and enables us to recognize the conformational arrangement as an essential structural parameter controlling molecular mobility. Another experimental relation between the Eyring factors—the activation volume and the activation enthalpy—permits us to estimate the magnitude of an elementary volume for the molecular movement in the melt as a function of temperature. Moreover, the experimental finding of the steady flow in the glassy polymer motivated us to propose a transition mechanism from elastic deformation to plastic flow in which the internal viscosity or the activation volume is introduced as a single parameter representing the transient state of the structure. Copyright © 1990 Society of Plastics Engineers