Particle Concentration Promotes Flow-Induced Crystallization of High Molecular Weight Isotactic Polypropylene

Flow-induced crystallization occurs when semicrystalline polymer melts are subjected to large deformations prior to supercooling. The entropy reduction and chain orientation involved in this process can form flow-induced precursors, leading to faster crystallization kinetics and changes in crystalline morphology. With increasing levels of shear stress, isotactic polypropylene can form highly oriented structures, which contribute to greatly improved material properties. Herein, the effect of particle concentration (specifically, catalyst residue) on the flow-induced crystallization of two samples of isotactic polypropylene was investigated through shear rheology and ex-situ simultaneous small/wide angle X-ray scattering (SAXS/WAXS). Upon the application of flow, the sample with higher particle concentration crystallized at faster rates relative to the sample with fewer heterogenous impurities. The nucleation ability of these particles was particularly pronounced at lower levels of deformation, while flow effects became prominent as larger deformations were applied. For sufficiently strong flows, a lower critical shear stress was observed for the formation of shish-kebab structures in the sample with higher concentrations of particles. At equivalent levels of specific work within both flow regimes, the morphology of these anisotropic structures were found to be characteristically distinct from one another.