Gruneisen parameters of bead-spring chains: MD simulation and theory

CS Stevenson and JG Curro and JD McCoy, JOURNAL OF CHEMICAL PHYSICS, 153, 244903 (2020).

DOI: 10.1063/5.0035451

Molecular Dynamics (MD) simulations were carried out in a microcanonical ensemble to compute the Gruneisen parameter (denoted as gamma) of a liquid of bead-spring chains having 10 beads/chain. gamma was studied over a wide range of temperatures below and above the glass transition temperature. We found that the Gruneisen parameter varied in the range of 2.1-3.1 and was significantly higher than typically observed experimentally in real polymers. In the glass, a theory was developed for gamma using a cell model in which the beads are harmonically bound to their respective cell centers. The resulting Gruneisen parameter is predicted to increase slightly with temperature. Above the glass transition temperature, we employed the generalized Flory dimer equation-of-state and the polymer reference interaction model theory to calculate gamma. In these calculations, we found that gamma decreased with temperature in the liquid. The theoretical predictions for gamma were found to be in good qualitative agreement with our MD simulations, without any adjustable parameters, both above and below T-g. In experiments on real polymers, gamma undergoes a sharp discontinuity at the glass transition. By contrast, in our MD simulations, gamma varies smoothly over a broad transition region.

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