Effect of molecular weight and pressure on crystallization of Cis-Polyisoprene

Cis-polyisoprene from hevea rubber has been fractionated by non-solvent precipitation. Crystallization in fractions has been studied at atmospheric pressure and at 0.5 kilobar using transmission electron microscopy of osmium tetroxide stained thin films. Analyses of kinetics data at atmospheric pressure using Secondary Nucleation Theory showed that fractions of molecular weight greater than or equal to 897,000 show only regime III kinetics, a fraction of molecular weight 543,000 showed both regime II and regime III kinetics and the lower molecular weight fractions showed all three regimes together. Studies at 0.5 kilobar demonstrated the possibility that the regime III-II transition is kinetic in nature. Morphological studies provided additional evidence for the regime I-II transition. An increase in branching was observed below that transition temperature. In the regime I region, spherulites, axialites and single crystals grow concurrently in the film showing molecular weight fractionation during crystallization.