Study On Crystallization Mechanism Of Isotactic Polypropylene

In this thesis,the crystallization behaviors and structures of sheared isotactic polypropylene（iPP）with and without the β-nucleating agents（TMB-5）were investigated.The variations of β-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 β-iPP.The β-crystallinity increases with shear stray,molecular weight and decreases with shearing temperature.Shearing before high-temperature pre-crystallization is not favor in of β-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 ofβcrystal toαcrystal was observed with appropriate shear temperature.Synchrotron radiation small-angle X-ray diffraction（SR-SAXS）was used to characterize the lamellar orientation.Adding β-nucleating agent is the most efficient method to produce β-iPP.The β-content increases with increasing NA concentration(C_NA)before C_NA≈0.1 wt%.The β-inducing efficiency of TMB-5 reduces above 145?C,and is invalid at 150?C,accompaniedwiththeα-accerleratingbehavior,performingasa temperature-dependent dual-inducing ability.Different morphologies were obtained after treating TMB-5 with different solvents,accompanied with different nucleation efficiencies.Theβ-inducing efficiency has no positive relationship with the nucleation efficiency.Shearing and β-NA do not interplay in a definitely synergetic or antagonisitic manner to induce the formation of βcrystals,but depend on the β-NA loading.With a low NA concentration,shearing can compensate the β-content to some extent,while with a high NA loading,shearing inhibits the formation of β-iPP,and the β-fraction decreases with the increasing shear rate.Theβ-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 β-NA induced iPP crystallization from the perspective of molecular helical conformation of the mesophase.Shear flow and β-NA interrupt the local ratio of the molecular helical conformation in different manners,leading to the formation of α and βcrystals.