Influence of flow on the crystallization kinetics of polymers

Abstract

In the cross-sections of injection-molded parts made from crystallizable polymers there are, in general, at least two types of morphologies. In the interior, spherulitic crystallization predominates, as also occurs with solidification of quiescent melts. Near the surfaces we find highly oriented, often optically structureless layers caused by shear-induced crystallization. — With the aid of an isothermal extrusion experiment at just a few degrees below the thermodynamic melting temperature, where the typical crystallization time for quiescent crystallization has the magnitude of some hours, it is possible to trace the shear-induced crystallization process. Polypropylene is extruded during a short shearing time through a rectangular duct, and the birefringence across the end of the duct is measured during and after flow. If the shearing time is not too large, nucleation and growth process of the shear-induced layers are separated. Nucleation of thin precursor sheets takes place during shear, whereas after stop of flow one gets a process of Avrami index 1 due to one-dimensional growth upon the sheets. By variation of the experimental parameters it is possible to trace the nucleation process during shear.