Thermorheological evidence and structure of heterogeneity in syndiotactic polypropylene melts with strong memory effects

Abstract

The memory effect of semi-crystalline polymers is usually related to the heterogeneity in self-nucleated (SN) melts. In this study, the strong memory effects of syndiotactic polypropylene (sPP) and the heterogeneity in SN melts were systematically investigated in a wide temperature range even above its equilibrium melting temperature (Tmo). The rheological measurements revealed that the SN melts exhibited thermorheologically complex behaviors, indicating that the presence of the heterogeneity in the SN melts. From the rheological plots of phase angle (δ) with the complex modulus (G*), the parameter Δ ln G* δ=45° values were determined and used to compare the change of the degree of thermorheological complexity in the SN melts during the heating process. It was found that the memory effect's efficiency to promote the crystallization process improved as the increase of Δ ln G* δ=45° values, which suggested that the memory effect depended on the heterogeneity in the SN melts. After purifying the sample, the strong memory effect still existed, which indicated that it was not caused by the stabilization effect of impurities. The results of wide/small-angle X-ray scattering and infrared spectroscopy revealed that there were no partial residual crystallites or regular chain conformation in our investigated SN melts. Based on the complex chain structure of our investigated sPP, we speculated that the heterogeneity in sPP melts was caused by the separation between the long crystallizable sequence and the atactic sequence.