Interplay between Chain Relaxation Time and Melt Crystallization Time in Microinjection Molding of Polyoxymethylene

Abstract

Microinjection-molding subjects the polymer melt to high cooling and shear rates, which significantly affects the crystallization behavior during solidification. In this work, fast scanning chip calorimetry, conventional differential scanning calorimetry, melt rheology experiments, and polarized light optical microscopy permitted drawing conclusions about the crystallization of polyoxymethylene under such processing conditions. Computer simulations and Cox–Merz experiments predict orientation of molecular segments and shear-induced formation of crystal nuclei in all regions of the component, that is, in both the skin and core. However, as the result of the interplay between the cooling rate/crystallization temperature and the chain relaxation time, the survival of nuclei is restricted to skin-near layers. In contrast to the fast cooling skin, in the slowly cooling core region, the initially oriented structure recovers to the random coil state due to the much shorter relaxation time compared to the crystallization time. The study suggests that structure formation of crystallizable polymers during melt processing, including microinjection molding, largely depends on the (temperature-dependent) ratio between the chain relaxation and chain crystallization time.