Empirical Determination of the Pressure-Temperature Phase Diagram of Polyethylene Based on the Pressure-Volume-Temperature-Entropy Gibbs Free Energy Equation of State

Abstract

The pressure-temperature (P-T) phase diagram for polyethylene (PE) was calculated based on the empirical Gibbs free energy (G) equation of state, GFE-Eos, for the crystalline PE (crys-PE) and the amorphous PE (amor-PE) where the GFE-Eos consists of variables such as the pressure (PG ), volume (VG ), temperature (TG ), and entropy (SG ) which satisfy the condition that the G is constant. The GFE-Eos for the crys-PE and the amor-PE can be expressed by three dimensional (T G-P G-G) planes, from which the melting temperature (Tm ) line was evaluated. The thermodynamic quantities of V m and S m for the crys-PE and amor-PE at Tm and their discontinuous changes, and along the melting line were evaluated and discussed based on the Clapeyron equation for the phase transition.