Morphology of banded spherulites in poly(epsilon-caprolactone)/poly(styrene-ran-acrylonitrile) blends

The occurrence of banded spherulites in blends of poly(epsilon-caprolactone) with poly(styrene-ran-acrylonitrile) is studied by means of optical-, scanning electron-, environmental scanning electron-, and atomic force-microscopy as well as small-angle X-ray scattering. The measurements reveal that lamellae are confined in fibrils that bend periodically from the center of spherulites in a radial direction. The fibrils show also a regular spacing in the lateral direction. Single bent lamellae can be observed by atomic force microscopy after permanganic etching. The enrichment of amorphous material near to the growth front of the spherulites can be directly observed by polarized light microscopy after temperature jump experiments. The excluded amorphous material on the surface of the growing spherulites has the shape of droplets. This might be the main reason for the initiation of the bending process because the result is a non-uniform stress distribution acting on the growing lamellae at the interface between spherulites and the surrounding melt. The amorphous material of the blends can be found in three areas as observed by small-angle X-ray scattering experiments and microscopical methods: i) between the lamellae, ii) excluded from the interlamellar region but within the spherulite, and iii) for high contents of non-crystallizable material, it can be excluded from the spherulite. The banding periodicity of spherulites as a function of the crystallization temperature can be described in terms of two models, based either i) on the inherent twisting of lamellae perpendicular to the axis of a screw dislocation or ii) on the dependence of the diffusion of amorphous material away from the growth front of lamellae and the temperature dependence of the rate of crystallization Both models fail at very low supercoolings near to temperatures where the banding disappears completely