Investigation of the Temperature Dependence of Activation Volume in Glass-Forming Polymer Melts under Variable Pressure Conditions

Motivated by empirical observations suggesting that changes in the activation volume Delta V-# upon varying temperature (T), determined from the pressure (P) dependence of the segmental structural relaxation time tau(omega), might be related to changes in the extent of collective motion in glass-forming liquids, we investigate the temperature dependence of Delta V-# and the extent of collective motion in a coarse-grained polymer melt using molecular dynamics simulation. After confirming the validity of the string model of the dynamics of glass-forming liquids in predicting the T and P dependence of tau(omega,) we find that there is unfortunately no direct relation between Delta V-# and the average string length L quantifying the extent of collective motion. This negative finding is also in qualitative accord with predictions from the generalized entropy theory of glass formation, in which the extent of collective motion is quantified indirectly through the configurational entropy of polymer melts. By drawing an analogy between the definitions of Delta V-# and the fragility of glass formation, we instead find that both Delta V-# and fragility are complex quantities involving a convolution of energetic effects associated with the variation of activation free energy, along with changes in the extent of collective motion with T and P. We further demonstrate that the extent of collective motion can be estimated quantitatively by normalizing the activation free energy by its value at the onset temperature of glass formation. It is suggested that this normalized measure of the change of collective motion with T should be superior to fragility in quantifying the role of collective motion in condensed materials giving rise to non-Arrhenius relaxation.