The melting behavior of n-alkanes up to carbon number 390 and molecular weight fractions of linear polyethylene up to M" = 5600 have been analyzed. Surprisingly, it is found that the melting temperature-molecular weight relation is the same for both types of chain molecules. The results are explained in terms of premelting of the n-alkanes, with the disordering of sequences of end units, so that the structure prior to melting is the same as that of the polymer. Support of this conclusion is given by theoretical considerations and experimental results. The difficulties in extrapolating the present data base to the equilibrium melting temperature of the infinite molecular weight chain are discussed.

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The morphology of solvent-cast or melt-pressed isothermally crystallized poly(ether ether ketone) (PEEK) films was investigated by polarized optical microscropy as a function of crystallization temperature. For crystallization temperatures below 300 °C, only spherulitic morphologies were present whereas, at higher temperatures, a morphological transition from spherulitic to crystal-aggregate-like structures could be observed during isothermal crystallization. As the crystallization temperature is increased, the content in the second type of structures was observed to increase. The temperature dependence of the relative population in these two morphologies can be qualitatively correlated with that of the double endothermic behavior observed when PEEK is heated above its crystallization temperature. The observation of this new morphology for PEEK cannot be explained in terms of a polymorphic behavior, a degradation process, or a fractionation effect during crystallization. At this point we can only speculate that it may be related to the structure of the amorphous liquid phase from which the crystallinity develops.

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