Equation of state for polymer solids

A simple, three-constant pressure-volume-temperature PVT equation of state, previously applied to polymer liquids, is applied to polymer solids, both crystalline and amorphous. In terms of reduced variables P̄=P/B0, V̄=V/V0, and T̄=T/T0, the equation P̄ V̄5=T̄3/2−ln V̄ fits experimental volume data taken from the literature for 19 polymers to within the accuracy of the measurements, about 0.1%. Using the different sets of reducing parameters for the liquid and glassy states of an amorphous polymer, a second order phase transition is predicted. At zero pressure, this transition temperature T2 lies about 20 K below the glass transition temperature Tg but has a greater pressure dependence. It was also found that the combination of reducing parameters RT0/B0V0 is approximately constant for amorphous and crystalline polymers.