Ratio between Random Coil Size and Crystalline Spacing Determines Mesophase Formation in Quenched Isotactic Polypropylene

The relationship between the molecular weight of isotactic polypropylene (iPP) and mesophase formation during quenching was successfully established. Surprisingly, a unique number-average molecular weight (Mn) dependent mesophase formation was identified that iPP with a lower or higher Mn preferred to form a pure mesophase but the ones with moderate Mn failed in inducing a high fraction mesophase when the same quenching processes were applied. It was interpreted by the effect of coupling between the sizes of random coils and the crystalline spacing on the crystallization rate, which further affected the formation of mesophase. The Mn represents the largest population of units possessing the same molecular weight in the system. Therefore, the crystallization rate was mainly controlled by the molecular chains with similar length of the largest fraction. A comparison between the radius of gyration of random coils in the melt and the final spacing of crystals induced during quenching suggests that the molecular chains in iPP with a lower Mn needed to be disentangled before crystallization and the ones in iPP with a higher Mn must be folded forth and back in crystallites during growth stage. Both cases led to a slow crystallization rate, benefiting the mesophase formation during quenching.