Changes in the structure of PTFE produced by different cooling rates from the sintering temperature and following permanent mechanical deformation have been studied with the electron microscope. Slow cooling rates resulted in longer and wider striated bands (which constitute the PTFE) than fast cooling rates, however, there was no apparent change in the relative amount of crystalline material as x-ray and infrared absorption studies indicate. Permanent strain resulted in severe distortion of the band structure; the striations either slipped past one another or became kinked. The manner in which the deformation occurred indicated it was controlled by a viscous noncrystalline phase. It is proposed that the band structure itself consists of two phases, crystalline striations or platelets separated by viscous noncrystalline matrix.
Mechanical properties and microstructure of PTFE at various temperatures were correlated through the effects of temperature and band size on the flow stress and microstructure deformation. Electron microscopic examination of deformed microstructure showed that at low temperatures the crystalline deformation modes (kinking and bowing) were favored, whereas noncrystalline modes (sliding and rotation) were predominant at high temperatures. At low temperatures PTFE showed a crystalline size effect and stress drop phenomenon in its flow-stress relations not manifested at high temperatures.
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