Nonlinear Viscoelastic Deformation in Polymeric Glasses

Abstract

AbstractThe dielectric, mechanical and volumetric relaxation data, and stress-strain data in tension, shear and uniaxial compression for polymeric glasses are compared. The relaxation phenomenon is viewed as a thermodynamically irreversible response to a perturbation in the configurational state. The relaxation behavior is apparently linear only in the limit of a very small perturbation, but otherwise generally nonlinear. Adam-Gibbs' entropy equation [1] has previously been shown to describe the dielectric and volumetric relaxation data better than Doolittle's free volume equation [2]. The temperature and the structure, i.e., history, dependence of mechanical relaxation time is shown to agree with the A-G theory better Than the Doolittle theory, in that the former predicts an Arrhenius type dependence on the temperature when the structural parameter, in this case the entropy, is fixed, while the latter predicts the temperature independence if the fractional free volume is fixed in the glassy state. The temperature dependence of the distribution of relaxation times is shown to agree qualitatively with the dynamic Ising model of Fredrickson and Andersen [3] (F-A model) involving the cooperativity of molecular motions in dense liquids.