With short term shearing at low degrees of supercooling a method has been developed by which nucleation during shearing can effectively be separated from the subsequent crystal growth, which, in these experiments, mainly occurs after the cessation of the flow. In this way the kinetics of nucleation by shear could be unravelled for isotactic polypropylene. The required measurement consists of the monitoring of the optical retardation which increases with the monitoring time and levels off by saturation. From these measurements conclusions could be drawn with respect to the modes of formation of the nuclei and of their type. Model predictions could be confirmed with the aid of electron micrographs.

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Recently, the mechanism of shear induced crystallization in melts of isotactic industrial polypropylene has been unravelled for sufficiently high rates of shear [1]. This was achieved with the aid of, what we called, short term shearing. This shearing was applied in a rectangular duct of large aspect ratio. The development of crystallization subsequent to this short term shearing was monitored with the aid of the growing optical retardation. Other traits of this process could be deduced from the cross-sections of the samples extracted from the duct after complete solidification. Mainly one of these traits is the subject of the present paper. As with injection molding, highly oriented areas are found in the cross-sections. The laws governing the limitations of these areas are discussed. In other words, the dependence of the thickness of the layers on the previous shear rate and shearing time is described with the aid of a model containing the relaxation times of threadlike (liquid) precursors. These relaxation times are assumed to depend on the size of the precursors: The larger these precursors have grown the more stable they are assumed. Nucleation and growth are considered as statistical processes.

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