The heat of fusion of polytetrafluoroethylene

Abstract

Abstract The heat of fusion of virgin and melt‐processed polytetrafluoroethylene (PTFE) was determined using the Clapeyron equation. Experimental data were obtained from PVT experiments and high‐temperature x‐ray diffraction measurements. For virgin, as‐polymerized PTFE, the melting temperature is given by where, for T m in degrees Celsius, A = 346.3±1.2, B = 0.095±0.003, and P is the pressure in kilograms per square centimeter. At the end of the atmospheric‐pressure melting interval, the amorphous and crystalline specific volumes V 1 and V c are 0.6517 and 0.492 cm 3 /g, respectively. Thus the heat of fusion is 24.4 cal/g, or nearly twice the value reported previously. The increases in enthalpy and volume at the melting point both indicate a degree of crystallinity of about 75–80% although infrared, x‐ray, and NMR data give much higher levels. Data from calorimetry, NMR, and dynamic mechanical measurements indicate that in virgin PTFE some of the crystals continue to experience torsional oscillations at temperatures below the room‐temperature transitions. This indicates that there are at least two kinds of crystalline regions. For previously melted PTFE, T m is determined by A = 328.5±0.7 and B = 0.095±0.002, the volumes are V am = 0.6349 and V cr = 0.4855 cm 3 /g, and the heat of fusion is 22.2 cal/g. The entropy of fusion for PTFE is much closer to that of polyethylene than was previously believed.