Morphological changes during the melting of bulk‐crystallized polymers

The melting behavior of some bulk‐crystallized, high molecular weight polymers has been studied in relation to morphology. Small‐angle x‐ray scattering results from two polymers, polytetramethylene oxide and polyethylene, are presented and discussed. Theoretical correlation functions have been calculated from models based on randomly oriented regions of alternating crystalline and amorphous layers, and have been compared with experimental correlation functions obtained from desmeared x‐ray scattering data. With this approach it has been possible to study changes in the thickness and widths of distributions of both crystalline and amorphous regions, over the temperature range from the crystallization temperature to the melting point. Evidence has been obtained in favor of a “surface melting” mechanism in which fusion occurs by the progressive transfer of chain segments from the “fold surfaces” of the crystalline lamellae to the amorphous or melt‐like phase. This involves the progressive thinning of each lamella towards its center.