On the development of oblong particles as precursors for polymer crystallization from shear flow: Origin of the so-called fine grained layers

Abstract

Recent studies concerning shear-induced crystallization after isothermal short-term shearing enabled a theoretical explanation for the formation of highly oriented surface layers in isotactic polypropylene. Under these special conditions also an occurrence of so-called fine grained layers was observed but has not been understood, so far. For slightly supercooled melts of polyethylene the formation of oblong crystallites, which were oriented perpendicularly to the flow direction, was recently reported for very slow continuous shear flow. This phenomenon was explained by the rotational component of this flow. The present work describes the growth of similar structures in isotactic polypropylene melts after fast short-term shearing at mild degrees of supercooling. The pertinent crystallites, which are finally formed on transverse precursors, are observed in cross-sections perpendicular to the direction of previous flow. Their anisotropic growth was monitored with the aid of light scattering. The present work also emphasizes that these structures are formed as a consequence of shear rates that occur in industrial processes. In fact, in injection molded parts they are formed in the neighborhood of the highly oriented surface layers. It is suggested that the transverse thread-like particles act as activators for the formation of the precursors of the highly oriented surface layers, as soon as they switch into the flow direction.