Morphology of an acetal resin

Lamellae are basic structural elements of the acetal resin, polyoxymethylene. Lamellae, similar to those observed after precipitation of polymer single crystals from solution, are found both in the interior and on the surface of samples crystallized from the melt. Lamellae crystallized from the melt are organized into spherulites or hedrites. The latter are hexagonal structures, several microns thick, grown by slow crystallization of thin films. The thickness of the hedrite depends directly on the crystallization temperature and only indirectly on the thickness of the original film. New lamellae originate at screw dislocations on previously established lamellae at the centers of both spherulites and hedrites, as well as on the periphery of growing spherulites. A mechanism based on these observations is postulated for the crystallization of polyoxymethylene from the melt.