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

Intervals of shear flow that stretch polymer chains form flow-induced precursors which accelerate crystallization and transform the crystalline morphology from isotropic spherulites to anisotropic structures. The flow-induced crystallization of two commercial samples of isotactic polypropylene with nearly identical molecular weight distributions, differing in concentrations of catalyst residue particles, was investigated by using dynamic rheology and ex situ Synchrotron X-ray scattering. Upon the application of flow, the sample with higher particle concentration crystallized at faster rates relative to the sample with lower levels of heterogeneous 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 (γ̇ ≤ 145 s–1), a lower critical shear stress (∼0.096 MPa) was observed for the formation of shish–kebab structures in the sample with higher concentrations of particles. In this work, we have also identified the formation of shish–kebab structures in the presence of weak flow (γ̇ ≤ 0.3 s–1) when sheared for long durations of time. For equivalent levels of specific work within both flow regimes, the morphologies of these anisotropic structures were found to be characteristically distinct from one another. The long period and degree of crystallinity were also found to increase with shear stress above the stress level needed for the formation of shish–kebab structures.