Relative Enthalpy of Polytetrafluoroethylene From 0 to 440 °C

Using a drop method and an ice calorimeter, precise measurements of enthalpy relative to 0 °C were made on a sample of granular polytetrafluoroethylene which was initially 95 percent crystalline. The measurements were at temperatures every 50 degrees from 50 to 300 °C (both before and after melting and quenching); and also at 340, 400, and 440 °C in the liquid range, where it appeared that structural equilibrium of the polymer was reached only slowly. Marked upturns in the heat capacity-temperature curves of the crystalline and quenched polymer above about 200 °C were treated as corresponding to gradual but reversible fusion of the type commonly caused by impurity components ("premelting"). Consideration was given also to the possibility of melting of thin crystals. The (total) heat of fusion could not be determined calorimetrically by the method used, but the additional assumption of three alternative approximations in all cases led to 327 °C as the crystalline melting point of the polymer, which agrees exactly with the accepted value based on direct observation. The heat capacity and relative enthalpy and entropy of the polymer derived from the data were joined smoothly with precise low-temperature values measured earlier at the Bureau on the same and two other samples of polytetrafluoroethylene, and these properties are represented by equations and tabulated as functions of temperature.