Shear-induced crystallization of polypropylenes: effect of molecular weight

The crystallization kinetics of three polypropylenes with different molecular weights was studied during shear under isothermal condition with a fibre pull-out device. Nucleation and growth under shear were observed and compared to static conditions. The crystalline growth rate was measured both in static condition and under shear. In static condition, the morphologies are alpha-phase spherulites and are formed from nuclei which are randomly distributed. Under shear alpha-phase morphologies are still observed but the nucleation density and the growth rate depend on the shear-rate. The nucleation density is strongly enhanced by shear and acts as the main factor on the overall kinetics. The growth rate increases with the shear-rate, but the basic growth mechanisms seem to be unmodified. beta phase appears after shear during the relaxation of the orientation. (C) 1999 Kluwer Academic Publishers.