Isotactic poly(propylene) crystallization: Role of small fractions of high or low molecular weight polymer

Two well-characterized metallocene isotactic poly(propylene) (iPP) samples, one of high and one of low molecular weight, were blended together for studying the effects of a second component on quiescent and shear-induced crystallization. A small amount of added high molecular weight (HMW) (up to 10 wt.-%) polymer increases the quiescent crystallization rate. This was observed as a decrease in characteristic halftime of transmitted light intensity. The crystallization halftime increases again when adding more than 10 wt.-% of HMW polymer. The crystallization halftime of pre-sheared samples decreases with increasing HMW fraction and is lowest for the HMW sample by itself. For the specific shearing conditions (gamma = 600, T-x = 145 degreesC), wide-angle X-ray diffraction (WAXD) images show the presence of the gamma (gamma) crystalline phase of iPP for samples containing 25 wt.-% of HMW and higher. DSC thermograms demonstrated higher crystalline fractions with increasing HMW fraction in pre-sheared samples.