Non-Newtonian Flow in Polymethyl Methacrylate Glass under Simple Shear Condition

Large deformation of polymethyl methacrylate (PMMA) glass was examined under the condition of simple shear. Post-yield steady plastic flow was observed over a wide range of strain rate. For the post-yield plastic flow, the logarithm of shear viscosity plotted against the logarithm of shear strain rate showed a non-Newtonian behavior quite resembling the shear thinning behavior ob-served in polymer solutions and melts. Rate analysis applied to the non-Newtonian flow showed that the fully annealed equilibrium structure of undeformed PMMA glass has been changed into non-equilibrium meltlike structures in the state of steady plastic flow. Comparison of experimental values of the activation enthalpy ΔH and activation entropy ΔS for the plastic flow in simple shear with those in uniaxial tension and compression led us to the conclusion that the meltlike structures under quite different deformation conditions were uniquely expressed by a single characteristic equation relating the activation enthalpy ΔH to the activation entropy ΔS. Evaluation of shear activation volume in simple shear proved that the experimental functional relation of activation volumes in simple shear, uniaxial tension and uniaxial compression was in good agreement with theoretical relations predicted by the Eyring rate theory.