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

In a previous report the internal energy, U, enthalpy, H, Helmholtz free energy, A, and Gibbs free energy, G, of polyethylene (PE) were determined when two of the thermodynamic factors X or Y were constant, where X and Y were two of the four factors, pressure, P, volume, V, temperature, T, and entropy, S. In this work these various energies were evaluated when one of the four thermodynamic factors is constant based on the thermodynamic conditions. In the case of the isothermal process with T = const, the internal energy was expressed by UT, enthalpy by HT, Helmholtz free energy by AT and Gibbs free energy by GT. The heat, TST, and the work, PTVT, in the isothermal process were evaluated based on the thermodynamic relation that HT−GT=TST and HT−UT=PTVT. In the isothermal process the changes of PTVT were large and dominant compared to those of TST when VT, ST and PT increased independently under constant T, while the changes of TST were small and almost constant at constant T. The isobaric process with P = const was characterized by the changes of TPSP being large and dominant compared to those of PVP when VP, SP and TP increased independently under constant P, while the isoentropic process with S = const was characterized by the changes of PSVS being large and dominant compared to those of TSS when TS, PS and VS increased independently under constant S where the subscripts, T, P, S, mean the isothermal, isobaric and isoentropic ensembles, respectively.