Mechanism and Kinetics of Ultra-High Molecular Weight Polytetrafluoroethylene Sintering

The effect of sintering time on the melt evolution of Ultra-High Molecular Weight Polytetrafluoroethylene was studied in situ by high temperature Wide Angle X-ray Scattering, and by cyclic thermal loading profiles within a Differential Scanning Calorimeter (DSC) and Thermo-Mechanical Analyzer (TMA). Results obtained from these techniques support the concept of molecular ordering in the melt state as a function of sintering time well above the melting temperature. TMA, which is not a conventional technique for monitoring thermal transitions, is shown to be sensitive enough for such purposes. Both DSC and TMA exhibit nonequilibrium melt behavior even 30 degrees C above its equilibrium melting temperature for long time periods. A correlation between the DSC and TMA results is established. The cyclic thermal profile leads to a dramatic growth in enthalpy of crystallization/melting. The mechanism for this growth is associated with two independent processes; isothermal annealing at the sintering temperature and lamellar thickening in the solid state. (C) 2014 Wiley Periodicals, Inc.