Shear-induced crystallization of polypropylenes: effect of molecular weight

The crystallization kinetics of three polypropylenes with different molecul ar weights was studied during shear under isothermal condition with a fibre pull-out device. Nucleation and growth under shear were observed and compa red 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 dist ributed. Under shear alpha-phase morphologies are still observed but the nu cleation density and the growth rate depend on the shear-rate. The nucleati on 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 b asic growth mechanisms seem to be unmodified. beta phase appears after shea r during the relaxation of the orientation. (C) 1999 Kluwer Academic Publis hers.