Thermodynamic-Dynamic Interrelations in Glass-Forming Polymer Fluids
XL Xu and JF Douglas and WS Xu, MACROMOLECULES, 55, 8699-8722 (2022).
There is a long history of trying to understand the dynamics of glass- forming and other condensed materials exhibiting highly anharmonic interparticle interactions based on their thermodynamic properties. This has led to numerous correlations between thermodynamic (e.g., density, compressibility, enthalpy, entropy, and vapor pressure) and dynamic (e.g., viscosity, diffusion coefficients, and relaxation times) properties, and a steady stream of theoretical models has been introduced to rationalize these correlations in the absence of any generally accepted theory of the dynamics of non-crystalline condensed materials. We view the independent success of these various semi- empirical models of glass-forming liquids as possibly pointing to a greater unity arising from the strong interrelation between thermodynamic properties, which is a matter of interest beyond an understanding of the dynamics of glass-forming liquids. Accordingly, we utilize the lattice cluster theory (LCT) of polymer fluids to show that the configurational entropy, enthalpy, and internal energy are all closely interrelated, as suggested by recent measurements by Caruthers and Medvedev, so that the generalized entropy theory (GET) of glass formation, a combination of the LCT and Adam-Gibbs model, can be recast in terms of any of these thermodynamic properties as a matter of convenience. Thermodynamic scaling, a form of density-temperature scaling exhibited by dynamic and some thermodynamic properties, is used to assess which thermodynamic properties are most naturally linked to dynamics, and we explore the origin of this scaling by both direct calculations based on the GET and molecular dynamics simulations of a coarse-grained polymer model. Through a combination of our comprehensive modeling of thermodynamic properties using the LCT and the highly predictive GET model for how the fluid thermodynamics relate to its dynamics, along with simulation results confirming these theoretical frameworks, we obtain insights into thermodynamic aspects of collective motion and the slow beta-relaxation processes of glass-forming liquids.
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