Growth rate and morphology of polyoxymethylene supermolecular structures

Abstract

Abstract The connection between the growth rate and morphology of supermolecular structures was investigated for polyoxymethylene which was isothermally crystallized from the melt at various temperatures. Optical and electron microscopy showed that below 158°C the crystallization occurred in the spherulite form, while above this temperature hedrites and, sporadically, ovoids were obtained. Although the radial growth rate itself does not exhibit a break around the critical temperature (158°C), a more detailed analysis based on the kinetic theory of crystallization has revealed a sudden drop in the interfacial free energy of the fold surface around this temperature. For the growth model represented by two‐dimensional surface nucleation, which seems the most probable representation, values of 183 and 95.5 erg/cm 2 were found for the temperature regions of growth of spherulites and hedrites, respectively. Morphologically, the two types differ by a larger extension of monocrystalline lamellae in hedrites, the mode of their arrangement, and the final external shape. The increasing extension of the lamellae, due to a decrease in the radial growth rate, which can be understood in terms of the segregation theory by Keith and Padden, is probably the reason that at a certain critical size (near 158°C), the hedritic growth becomes stabler than the spherulitic growth. A further investigation will be needed to elucidate the part played by a simultaneous drop in the free energy of the fold surface.