Study On Crystallization Mechanism Of Isotactic Polypropylene

In this thesis,the crystallization behaviors and structures of sheared isotactic polypropylene?iPP?with and without the ?-nucleating agents?TMB-5?were investigated.The variations of ?-crystallinity under different conditions were studied.The formation mechanism of crystals in iPP was proposed.Shear flow accelerates the crystallization kinetics,modifies the polymorphic morphology,and induces the formation of ?-iPP.The ?-crystallinity increases with shear stray,molecular weight and decreases with shearing temperature.Shearing before high-temperature pre-crystallization is not favor in of ?-crystallization,while shearing after high-temperature pre-crystallization is.In situ synchrotron radiation wide-angle X-ray scattering?SR-WAXS?was used to trace the crystallization process of sheared iPP.A prior formation of?crystal to?crystal was observed with appropriate shear temperature.Synchrotron radiation small-angle X-ray diffraction?SR-SAXS?was used to characterize the lamellar orientation.Adding ?-nucleating agent is the most efficient method to produce ?-iPP.The ?-content increases with increasing NA concentration(C_NA)before C_NA?0.1 wt%.The ?-inducing efficiency of TMB-5 reduces above 145?C,and is invalid at 150?C,accompaniedwiththe?-accerleratingbehavior,performingasa temperature-dependent dual-inducing ability.Different morphologies were obtained after treating TMB-5 with different solvents,accompanied with different nucleation efficiencies.The?-inducing efficiency has no positive relationship with the nucleation efficiency.Shearing and ?-NA do not interplay in a definitely synergetic or antagonisitic manner to induce the formation of ?crystals,but depend on the ?-NA loading.With a low NA concentration,shearing can compensate the ?-content to some extent,while with a high NA loading,shearing inhibits the formation of ?-iPP,and the ?-fraction decreases with the increasing shear rate.The?-inducing efficiency of the original TMB-5 particles is insusceptible to shear flow,while the columnar-like TMB-5 is seriously affected by shear.Based on the experimental results,we interpret the shear and ?-NA induced iPP crystallization from the perspective of molecular helical conformation of the mesophase.Shear flow and ?-NA interrupt the local ratio of the molecular helical conformation in different manners,leading to the formation of ? and ?crystals.