Structure Changes of Polymeric Materials under Large Deformation

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Time evolution of Poisson ratio of polystyrene during elongation deformation with constant rates was measured by a newly developed tensile tester equipped with a strain gage based on LASER speckle method in order to study cohesive structure changes induced by deformation around the glass transition zone. Time evolution of the ratio measured at low strain rates, at which viscosity growth function showed linear response, could be superimposed to a composite curve by the method of reduced variables with the same shift factor for the viscoelastic function. The obtained composite curve changed from 0.33 in the glassy zone to 0.5 in the rubbery zone. However, the experimental curve was not in accord with the prediction by a simple linear response theory. This result may be attributed to strong nonlinear nature of volume relaxation phenomena. At higher strain rates, where viscosity growth function showed nonlinear response, Poisson ratio in the short time region increased.