Empirical Determination of the Internal Energy, Enthalpy, Helmholtz Free Energy and Gibbs Free Energy of Polyethylene When Two of the Thermodynamic Factors, Pressure, Volume, Temperature and Entropy, are Constant

The internal energy, U, enthalpy, H, Helmholtz free energy, A, and Gibbs free energy, G, of polyethylene (PE) have been determined when two of the thermodynamic factors X and Y are constant where X and Y are pressure, P, volume, V, temperature, T, and entropy, S. We call an ensemble with the thermodynamic factor X being constant an iso-X ensemble, such as that with X = T is an iso-thermal ensemble, iso-T ensemble, while an ensemble with the thermodynamic factors X or Y being constant is an iso-X or iso-Y ensemble, such as that with X = T or Y = P being constant is an iso-thermal or iso-baric ensemble, iso-T or iso-P ensemble. In the case of the iso-T or iso-P ensemble, an increment of the internal energy expressed by dUT,P, was evaluated based on thermodynamic conditions, T = const and P = const, and was given by dUT,P = dUT + dUP where dUT and dUP are the increments of the internal energy for the iso-T and iso-P ensembles, respectively, and the increments occur with the increment of dUT,P simultaneously.