Self-induced field model for crystal twisting in spherulites

The observed twisting of crystals about the growth direction in spherulites is modeled as a response to fields generated during the crystal growth process. These fields can be compositional or mechanical. In either case, the high local field value in the melt near the interface acts to reduce the growth velocity. Twisting of the crystal about the growth direction places all portions of the growth surface in contact with melt of lower field value than would be the case for untwisted growth, thereby increasing the growth velocity. The effect of a compositional field is analyzed here, using an extension of the moving point source to compute the composition in the melt ahead of a twisting crystal. The retarding effect of the elastic twisting of the crystal is included in the analysis. Simulations using parameters for a crystallizable/uncrystallizable polyethylene blend are carried out for a crystal with thin, rectangular cross-section. The results are quantitatively and qualitatively consistent with observation. Comments on simplifications within and implications of the model are given.