Interaction via Diffusion of Polymer Crystals in Growing Spherulites

Diffusion fields surrounding growing polymer crystals of acicular or lamellar habit are considered. Segregation is modelled by regarding growth faces as "sources" of excess segregatable material in the melt, and variation of diffusity with chain length is taken into account. The overall concentration profile of segregated species closely resembles that corresponding to a single species having the z-average molecular weight of segregated polymer, and can be characterized by an appropriately averaged diffusion range. Results of approximate calculations are related to observed variation of texture in polyethylene spherulites. It is shown that scaling in terms of diffusion range must be confined to situations in which dominant lamellae are spaced no further than about 50 lamellar thicknesses apart, i.e. to regime II. Scaling as observed then depends crucially upon the manner in which lamellar stacks are formed by branching.