Isotactic polypropylene/ethylene-co-propylene blends: effects of composition on rheology, morphology and properties of injection moulded samples

Abstract

Melt rheology, phase morphology and properties of (30/70) isotactic polypropylene/ethylene-co-propylene (iPP/EPR) blends, containing EPR copolymer synthesised by means of a titanium based catalyst with very high stereospecific activity (EPRTi), were compared to that of (30/70) iPP/EPR blends containing EPR copolymer synthesised by using a traditional vanadium based catalyst (EPRV). It was found that both the melts are to be classified “negative deviation blends” and that, for such a composition ratio, the EPR represents the continuous phase. Scanning electron microscopy (SEM) investigations confirmed that the iPP component segregates in spherical-shaped domains with average size lower than 1.0μm. Such a fine degree of dispersion of the minor component in solid state is found independent of melt phase viscosity ratio and has been correlated with the interfacial tension between iPP and EPR phases. The presence of 70% of EPR copolymer dramatically interfered with the iPP crystallisation process, as strong reduction in iPP crystallinity index (Xc) values is shown by both the iPP/EPR blends. Comparatively lower Xc value is exhibited by iPP phase crystallised in presence of EPRTi copolymer. Differential scanning calorimetry (DSC) experiments revealed, in fact, that the crystallisation process of iPP phase and that of ethylenic sequences along EPRTi chains hinder each other. By small angle X-ray scattering (SAXS) it was shown that the iPP/EPRTi materials are characterised by amorphous inter-lamellar layer (La) considerably higher than that shown by the plain iPP and iPP phase crystallised in presence of EPRV copolymer. This strong La increase was mainly ascribed to entrapping of EPRTi crystalline domains into the iPP amorphous inter-lamellar layer. Wide angle X-ray scattering (WAXS) studies demonstrated that the EPR content determines the ratio between the apparent crystal size of iPP phase grown in a direction perpendicular to the (110) crystallographic plane and the apparent crystal size of iPP phase grown in a direction perpendicular to the (040) crystallographic plane. The higher the EPR content, the higher the thickness of the iPP lamellae in the direction of the radial growth of iPP spherulites, in agreement with increased nucleation density shown by DSC results. The different modulus performance shown by the materials at room temperature was accounted for by distinguishing between effects of phase crystallinity and super-molecular structure.