On the observation of a new morphology in poly(arylene ether ether ketone). A further examination of the double endothermic behavior of poly(arylene ether ether ketone)

The morphology of solvent-cast or melt-pressed isothermally crystallized poly(ether ether ketone) (PEEK) films was investigated by polarized optical microscopy as a function of crystallization temperature. For crystallization temperatures < 300 deg 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 it can only be speculated that it may be related to the structure of the amorphous liquid phase from which the crystallinity develops.