Short time properties, dynamic fragility and pressure effects in deeply supercooled polymermelts

Abstract

Our activated barrier hopping theory of segmental relaxation in deeply supercooledpolymer melts is applied to compute short time properties including the glassy shearmodulus, localization length and vibrational frequency. Numerical calculations for specificpolymers suggest the theory simultaneously predicts a reasonable elastic modulus, localizedstate vibrational frequency, dynamic fragility and dynamic crossover and glasstransition temperatures. The theory also provides explicit connections between shorttime-/length-scale properties and the slow alpha relaxation process. The extension of thetheory to elevated pressures is initiated. Pressure is found to broaden the deeplysupercooled regime and reduce the dynamic fragility. However, the predictedRossler–Sokolov universal supra-Arrhenius law for the temperature dependence of thealpha relaxation time remains accurate at all pressures. A common theme is theessential role played by the ratio of the dynamic crossover temperature (idealmode coupling critical temperature) and kinetic glass transition temperatureeven in the deeply supercooled regime where activated processes are dominant.