(621i) Effect of Particle Concentration on the Flow-Induced Crystallization of High Molecular Weight Isotactic Polypropylene

Flow-induced crystallization, ubiquitous in polymer melt processing techniques, 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 a highly oriented structures, which contribute to greatly improved material properties. Herein, the effect of particle concentration (catalyst residue) on the flow-induced crystallization of two samples of isotactic polypropylene was investigated through shear rheology and and ex-situ simultaneous small/wide angle X-ray scattering (SAXS/WAXS). After shearing, flow-induced crystallization was monitored in a rotational rheometer using oscillatory time sweeps and temperature ramps. The sample with higher concentrations of catalyst residue showed faster crystallization kinetics and crystallized at higher temperatures for moderate levels of specific work. The effects of increased particle concentration diminished with increasing levels of work, where upon reaching a saturation threshold flow effects were predominant. Under conditions of short term shear, the X-ray data confirm that the highly oriented shish-kebab morphology could be induced when a critical shear stress of ~0.1 MPa is attained. Interestingly, we found that these shish-kebab structures could also be formed at low shear stresses when sheared for very long times.