The Formation of Curved Polymer Crystals: Polyoxymethylene

An optical and electron microscopical study is presented of the habits exhibited by chain-folded polyoxymethylene crystals grown from hot 0.02 percent solutions of the polymer in orthodichlorobenzene when these solutions are cooled to temperatures (T ) between 140 and 80 °C inclusive. In contrast with the lamellar crystals formed in the T = 140 and 120 °C preparations, which crystals were six-sectored and nearly planar, the crystals formed below T = 120 °C exhibited multisectored dendritic habits and were all the more pronouncedly curved the lower the temperature at which they grew. The most pronouncedly curved crystals which were observed were hollow bowl shaped dendrites having a radius of curvature of ~ 2 μm which were formed in the 80 °C preparations. The possible origins of why the lamellar crystals of polyoxymethylene were all the more pronouncedly curved the lower the crystallization temperature are considered in the light of conjectures which have been previously advanced concerning the formation of curved crystals of poly(4-methylpentene-1). Among the features which are discussed is the role played by the bulkiness of the chain folds.