On the Correlation of Rheology and Morphology of Bimodal Polypropylene Reactor Blends Synthesized by Homogeneous Binary Metallocene/Metallocene Catalysts

Abstract

ABSTRACTIn this study, morphological and rheological characteristics of bimodal polypropylene reactor blends synthesized by binary catalysts based on I: rac-Me2Si(2-Me-4-PhInd)2ZrCl2 and II: (2-PhInd)2ZrCl2 at different molar ratios were investigated. Gel permeation chromatography, scanning electron microscopy, and rheometry analysis were performed to evaluate the effect of molar ratio of catalyst II, responsible for the formation of elastomeric stereoblock polypropylene with low molecular weight, to catalyst I, responsible for production of high molecular weight isotactic (i) polypropylene, on the molecular weight, molecular weight distribution, morphological characteristics and rheological behavior of the synthesized products. The gel permeation chromatography results indicated that once a hybrid of the two catalysts is used, a broad and bimodal molecular weight distribution would be obtained, and the molar ratio of the catalysts governs the values of molecular weight and molecular weight distribution. 13C NMR results suggest that the different polypropylene tacticity resulting from the two catalysts (stereoblock vs. isotactic) is hardly influenced using a binary system. The effect of molecular weight enhancement and molecular weight distribution broadening was confirmed through the linear rheological data (G′ at lower frequencies, crossover modulus and crossover frequency) due to the impeded molecular motions of chains with high molecular weights which play an important role in the elasticity of chains. The zero shear rate viscosity and relaxation time, determined by fitting the Carreau–Yasuda model, are in great conformity with the experimental data at low-frequency region. Likewise, the miscibility and increased level of heterogeneity of microstructure, which is a result of changing the molar ratios of catalysts II/I, are confirmed through the Cole–Cole and Han plots and were further corroborated through the obtained scanning electron microscopy results.