The purpose of this investigation was to determine thermodynamic properties for linear polyethylene, polypropylene and an ethylenepropylene copolymer, from previously determined p-v-t and calorimetric data.

The entropy and enthalpy were calculated from calorimetric and p-v-t data using a datum of entropy and enthalpy equal to zero at 14. 7 pounds per square inch pressure absolute and 32 °F. Entropy and enthalpy were plotted against pressure for a series of isotherms. The values of entropy and enthalpy were then corrected for pressure to arrive at the resultant diagrams.

The results are shown in Figures 2, 3, 4, 5, 7, 8, 9, 10, and 11. ln Figures 6, 7, 9, and 10 it can be seen that the thermodynamic properties of polyethylene and polypropylene are very similar up to 240 °F.