Role of Cohesive Energy in the Thermodynamics and Dynamics of Glass-Forming Polymer Melts
Jack F. Douglas, Karl Freed, Wen-Sheng Wu
Monomer molecular and chemical structures represent the most important characteristics of polymers that directly determine the basic molecular parameters (such as cohesive energy) influencing the physical properties of polymer materials. However, a generally accepted theory of how these molecular parameters influence polymer glass-formation remains elusive. Here, we particularly focus on the role of cohesive energy in polymer glass-formation based on molecular dynamics simulations for a coarse-grained model of unentangled glass-forming polymer melts, and we analyze several thermodynamic and dynamic properties relevant to glass-formation under both constant volume and constant pressure conditions. Our results indicate that the characteristic temperatures of glass-formation increase with the strength of cohesive interactions, but we find a rather weak dependence of cohesive energy strength on the fragility of glass-formation. We further explore how cohesive energy strength alters the dynamic heterogeneity of glass-forming polymer melts as indicated by non-Gaussian parameter α2 and string-like cooperative motion. In particular, our analysis reveals an interesting link between the average string length L and the peak height of α2 in glass-forming polymer melts. Finally, we demonstrate that our simulation data for all cohesive interaction strengths can successfully be described by the string model of glass-formation, which predicts that the structural relaxation in glass-forming liquids is governed by the high temperature activation free energy and L.