Effect of molecular weight on the flow-induced crystallization of isotactic poly(1-butene)

The effect of molecular weight on the crystallization behavior of isotactic poly(1-butene) (iPB) under quiescent, isothermal conditions is investigated by rheological and optical microscopy techniques. Small-amplitude oscillatory shear experiments are used to determine the critical gel behavior of the crystallizing melt. The resulting critical gel time is related to the crystallization kinetics during the early stages of the process. While molecular weight does not substantially affect the gel characteristic time, the rheological parameters of the crystalline critical gel are found to be strongly dependent upon molecular weight, thus suggesting that the viscoelastic behavior in the early stages of crystallization is dictated by the molecular properties of the amorphous phase. Optical microscopy observations provide quantitative information on the growth rate and the morphology of the crystallites. It is found that the molecular weight has no effect on the iPB growth rate. Conversely, molecular weight affects the morphological transition from spherulites to square-shaped (hedrites) crystallites. The coupled effects of temperature and molecular weight on the spherulite-to-hedrite transition are presented in the form of a phase diagram which quantitatively confirms recent observations made by Fu et al.