STATISTICAL THERMODYNAMICS OF POLYMER CRYSTAL AND MELT

A crystalline-state theory recently developed by Midha and Nanda is commented on and applied to the isobars of polyethylene, poly(vinylidene fluoride), and poly(chlorotrifluoroethylene) at atmospheric pressure, and to an isotherm of polyethylene. Satisfactory agreement with experimental results. This includes the volume change at the melting point T//m and the volume difference DELTA V between crystal and melt below T//m, when crystal and the earlier liquid-state theory are combined. An inverse proportionality between T//m and alpha //l, the expansivity of the melt at T//m, derived much earlier for low-molecular-weight solids, is recovered with an identical numerical coefficient. The thermodynamic functions of polyethylene are investigated in both phases. An additional temperature-dependent term in the entropy and free energy is introduced and tentatively attributed to a volume- and temperature-dependent short-range ordering. Good agreement with experiment, including the entropy and temperature of fusion, ensues.