Isothermic spherulitic growth and the shape of grain boundaries and growth fronts

We investigate experimentally and theoretically the isothermic growth of two spherulites of different modification into a supercooled isotactic polypropylene film. The faster growing-spherulite grows around the-spherulite, and finally the-spherulite is symmetrically and completely included in. In contrast to literature but in agreement with experimental evidence we find that the grain boundary between the teardropshaped-spherulite and the surrounding-spherulite consists of two parts, where one is always an arc of a logarithmic spiral. This-spherulite ends always in a vertex. Its angle depends on the ratio of the two growth rates only. Behind the vertex an intrinsic--grain boundary exists, degenerating to a channel in bulk material. The growth fron of the-spherulite, which ends on the logarithmic spiral or on the intrinsic grain boundary during growth, consists of an arc of a circle continued by an arc of a logarithmic spirial, too.