ON THE LIMITATIONS OF SHEAR-INDUCED CRYSTALLIZATION IN POLYPROPYLENE MELTS

Recently, the mechanism of shear induced crystallization in melts of i sotactic industrial polypropylene has been unravelled for sufficiently high rates of shear [1]. This was achieved with the aid of what we ca lled, short term shearing. This shearing was applied in a rectangular duct of large aspect ratio. The development of crystallization subsequ ent to this short term shearing was monitored with the aid of the grow ing 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 ar e found in the cross-sections. The laws governing the limitations of t hese areas are discussed. In other words, the dependence of the thickn ess of the layers on the previous shear rate and shearing time is desc ribed with the aid of a model containing the relaxation times of threa dlike (liquid) precursors. These relaxation times are assumed to depen d on the size of the precursors: The larger these precursors have grow n the more stable they are assumed. Nucleation and growth are consider ed as statistical processes.