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

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 (TG-PG-G) planes, from which the melting temperature (Tm) line was evaluated. The thermodynamic quantities of Vm and Sm for the crys-PE and amor-PE at Tm and their discontinuous changes, ΔVm and ΔSm, along the melting line were evaluated and discussed based on the Clapeyron equation for the phase transition.