Observation of a New High-Temperature Transition in Polytetrafluoroethylene

We wish to report that in sintered polytetrafluoroethylene (PTFE), there is an order → disorder transition at 370 ± 10 °C which otherwise has been known to show the highest temperature (melting) transition at 327 °C since its discovery over 50 years ago. The intensity of the 370 °C transition varies from sample to sample, and at present we are not fully aware of the factors responsible for its selective formation except that a high molecular weight and moderate shear stresses in the solid ↔ melt region are required. Limited experimental data available suggest that the loss of helical order occurs at ~327 °C and at ~370 °C the loss of equatorial order takes place; the latter existing in the presence of molecular constraint created from shear stresses on a high molecular weight resin. Differential scanning calorimetry, rheology, hot-stage optical microscopy, variable-temperature X-ray diffraction, and infrared data are presented to demonstrate these findings.