Pressure effects on the vibrational spectra of amorphous polystyrene. II. Grüneisen parameters

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

Molecular Dynamics (MD) simulations were carried out in a microcanonical ensemble to compute the Gruneisen parameter (denoted as γ) of a liquid of bead-spring chains having 10 beads/chain. γ 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 γ 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 γ. In these calculations, we found that γ decreased with temperature in the liquid. The theoretical predictions for γ 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, γ undergoes a sharp discontinuity at the glass transition. By contrast, in our MD simulations, γ varies smoothly over a broad transition region.