Analysis of reversible melting in polytetrafluoroethylene |
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Authors: | H.-J. Radusch |
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Affiliation: | (1) Martin-Luther-University Halle-Wittenberg, Institute of Material Science, Geusaer Str., D-06217 Merseburg, Germany |
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Abstract: | Summary The reversibility of crystallization and melting of polytetrafluoroethylene (PTFE) has been investigated as function of crystallization conditions and temperature by temperature-modulated differential scanning calorimetry (TMDSC). The total and average specific reversibility of the melt-crystallized PTFE is considerably larger than in case of as-polymerized powder. This experimental observation must be attributed to different coupling between crystallized sequences of the molecules within the globally semi-crystalline superstructure. The crystallinity of as-polymerized PTFE is close to 100%, and the crystals melt in a narrow temperature interval close to the equilibrium melting temperature. Melt-crystallized PTFE, in turn, shows a crystallinity of about only 40% and melts at lower temperatures. The morphology of the melt-crystallized PTFE allows molecule segments to melt and crystallize reversibly as a function of temperature. The extended-chain conformation, evident in as-polymerized powder, inhibits reversible melting due to required molecular nucleation after complete melting of a molecule. The experimental findings are discussed within the framework of a similar investigation on polyethylene of different crystal morphology and support both the concepts of lateral-surface activity and molecular nucleation. |
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Keywords: | DSC TMDSC reversible and irreversible crystallization and melting specific reversibility of crystals polytetrafluoroethylene |
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