Thermodynamic–Dynamic Interrelations in Glass-Forming Polymer Fluids

Abstract

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 β-relaxation processes of glass-forming liquids.

Molecular Dynamics Investigation of the Pressure Dependence of Glass Formation in a Charged Polymer Melt

“…We find that t * increases less rapidly than τ α upon cooling, implying that the time scale associated with the mobile particles decouples from the structural relaxation time. In line with previous works, ,, a power law can be used to describe the relation between t * and τ α , namely, t * / τ f ∝ false( τ α / τ normalf false) 1 − ζ , where the exponent ζ quantifies the degree of “decoupling” between these two time scales and τ f is the “fast” β-relaxation time on the order of 1 ps Figure d indicates that 1 – ζ deviates considerably from unity and depends on P and charge interactions rather weakly.…”

Influence of Functionalization on the Crystallinity and Basic Thermodynamic Properties of Polyethylene

“…99−101 This trend is exactly the opposite from antiplasticizer additives, where the material becomes stiffer in its solid state in comparison to the pure material. 65,102 This inverted behavior of material stiffness above and below the solidification temperature is symptomatic of additives that alter the fragility of glass formation to be lower in the case of the antiplasticizer and higher in the case of additives whose properties are opposite from that of an antiplastizer, 96 and we expect the same phenomenon to arise in our model PE, despite the capacity of all these materials to crystallize.…”

“…The densification due to the more attractive interactions in PES and PA is expected to decrease the mobility compared with PE. The T dependence of mobility or relaxation for glass-forming liquids above the glass transition temperature T g can be described by the localization models, and small molecular additives can result in plasticization or antiplasticization in a way to vary the fragility index. ,, However, the change of mobility with additives is expected to be more complicated in case of semicrystalline polymers …”

“…The T dependence of mobility or relaxation for glass-forming liquids above the glass transition temperature T g can be described by the localization models, and small molecular additives can result in plasticization or antiplasticization in a way to vary the fragility index. 65,96,97 However, the change of mobility with additives is expected to be more complicated in case of semicrystalline polymers. 98 The Debye−Waller parameter ⟨u 2 ⟩ is found to decrease with increasing f at T higher than T c for PE, as shown (Figure 10).…”

Thickness Dependence of Segmental Dynamics in Free-Standing Thin Films Predicted by a Dynamically Correlated Network Model

“…Glassy materials have been used for several thousand years. When supercooled liquids, including polymeric liquids, are cooled, their viscosity increases rapidly, and these eventually undergo vitrification. , Such a noticeable increase in viscosity upon cooling is essential to the glass transition in a supercooled state; however, the mechanism of this slowdown phenomenon has not yet been elucidated, although many theoretical models have been proposed to explain this phenomenon. − In the early stages, the concept of a simple free volume was proposed based on structural pictures in the liquid state. − Later, a theory based on the idea of cooperative rearrangements of molecules (or segments) was proposed by Adam and Gibbs, where configurational entropy plays a vital role in viscous slowdown . This entropic view was further combined with the mode coupling theory − and developed into the random first-order transition (RFOT) theory. − However, pure dynamical theories such as kinetically constrained models − and dynamic facilitation models, which do not use thermodynamic ideas, have also been developed.…”