ON THE DEVELOPMENT OF OBLONG PARTICLES AS PRECURSORS FOR POLYMER CRYSTALLIZATION FROM SHEAR-FLOW - ORIGIN OF THE SO-CALLED FINE-GRAINED LAYERS

Recent studies concerning shear-induced crystallization after isotherm al short-term shearing enabled a theoretical explanation for the forma tion of highly oriented surface layers in isotactic polypropylene. Und er these special conditions also an occurrence of so-called fine grain ed layers was observed but has not been understood, so far. For slight ly supercooled melts of polyethylene the formation of oblong crystalli tes, which were oriented perpendicularly to the flow direction, was re cently reported for very slow continuous shear flow. This phenomenon w as explained by the rotational component of this flow. The present wor k describes the growth of similar structures in isotactic polypropylen e melts after fast short-term shearing at mild degrees of supercooling . The pertinent crystallites, which are finally formed on transverse p recursors, are observed in cross-sections perpendicular to the directi on of previous flow. Their anisotropic growth was monitored with the a id of light scattering. The present work also emphasizes that these st ructures are formed as a consequence of shear rates that occur in indu strial processes. In fact, in injection molded parts they are formed i n the neighborhood of the highly oriented surface layers. It is sugges ted that the transverse thread-like particles act as activators for th e formation of the precursors of the highly oriented surface layers, a s soon as they switch into the flow direction.