Melt rheology, phase structure and impact properties of injection-moulded samples of isotactic polypropylene/ethylene-propylene copolymer (iPP/EPR) blends: influence of molecular structure of EPR copolymers

Abstract

Isotactic polypropylene (iPP) based blends containing as second component ethylene-propylene copolymers (EPR) having, for constant propylene (C 3) content (wt/wt), different average molecular masses ( M ̄ w and/or M ̄ n ) and, for constant average molecular mass, different molecular mass distributions ( MMD) were investigated. The study was undertaken to establish the influence of the melt phase viscosity ratio μ in determining the average particle size of the EPR phase in the vicinity of the minimum expected according to the Taylor-Tomotika theory for the average particle size versus log μ function, when μ is about equal to unity (in previous studies we have in fact reached μ values far above 1). Moreoever, we also report the effects of molecular mass and molecular mass distribution of the EPR phase on the melt rheological behaviour of iPP/EPR blends, on the mode and state of dispersion of the EPR phase in the melt, as well as, in the solid state after iPP crystallization in injection moulded samples, on the crystalline lamellar thickness and the thickness of the amorphous interlayer of iPP phase, and finally on the impact properties of blend materials. It should be pointed out that the apparent viscosity of all the iPP/EPR blends investigated is expected to obey the logarithm additivity rule that applies at constant temperature and shear rate. The application of the Cross-Bueche equation revealed that the zero-shear viscosity η o of these iPP/EPR blends deviates positively from the logarithm additivity rule. Assuming that the crystallization of the iPP phase freezes the morphology of the EPR phase, a strict correlation is confirmed to exist between the values of EPR particle size and EPR particle size range, as measured by scanning electron microscopy on samples in the solid state, and μ value. The number average particle diameter ( D ̄ n ) and the particle size range of the EPR phase ( D) are found to increase with increasing μ value as expected according to the Taylor-Tomotika theory. Finally, when the iPP phase crystallizes from its blends with EPR under non-isothermal conditions, the phase structure developed in the blends is characterized by lamellar thickness and interlamellar amorphous layer thickness, respectively, lower and higher than that shown by plain iPP.