Information given by slow melting on phase content and maximum drawability of high molecular weight polyethylene films
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The technique of slow melting (v = 1 Kh−1) used previously on nascent high molecular weight PE (UHMWPE) is now applied to films prepared in different solvents. Besides the expected endotherm of fusion of the orthorhombic crystals, new endotherms and phase changes are observed at low and high temperature. The total enthalpy of phase change is near the enthalpy of melting of perfect orthorhombic crystals. The new endotherms with slower kinetics can be distinguished on the melting trace from the melting of the orthorhombic crystals. The trace permits the definition of a calorimetric phase composition of a sample. The different films have the same amount of orthorhombic crystals but different phase compositions. Their maximum drawability λmax is solvent-dependent and varies between 260 (n-C12) and 60 (trichlorobenzene). The comparison of calorimetric spectra and values λmax suggests the existence of an optimum in the range of phase-change temperatures to obtain a high λmax. The melting traces of drawn films show that the temperature of the high- T sendotherms is lowered by drawing. The phase content of nascent UHMWPE is dramatically changed by melting-recrystallization. The origin of the non-orthorhombic phase changes is discussed.
KeywordsEnthalpy High Molecular Weight Phase Composition Phase Change Material Processing
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