Understanding Activation Volume in Glass-Forming Polymer Melts via Generalized Entropy Theory
Wen‐Sheng Xu, Jack F. Douglas, Wenjie Xia, Xiaolei Xu
Abstract
The generalized entropy theory (GET) of glass formation provides an analytic framework for the structural relaxation time and thermodynamic properties of glass-forming polymers in terms of molecular parameters and variable thermodynamic conditions (e.g., variable pressure). Here, we utilize the GET to elucidate the physical nature of the activation volume ΔV# estimated from the leading pressure dependence of the structural relaxation time. We start by analyzing the temperature dependence of ΔV#, for which the predictions from the GET are shown to broadly accord with experimental and simulation results. This analysis is followed by establishing general trends in the variation of ΔV# with molecular parameters describing chain stiffness, cohesive interaction strength, chain length, and monomer structure. Our calculations further show that ΔV# is related to the differential change of the activation free energy as a function of temperature and thus bears a close relationship to the fragility of glass formation. The GET is also used to demonstrate that the variations of fragility and the glass transition temperature Tg with molecular parameters in polymer materials having similar cohesive interaction strength, i.e., a similar chemical nature, can be understood to arise from the relative efficiency of molecular packing near Tg, which we quantify through the experimentally measurable thermal expansion coefficient and isothermal compressibility. A structural understanding of the variation of fragility translates into a better understanding of ΔV# by virtue of the approximate relationship between these properties independently established by the GET.