Isotactic polypropylene/ethylene‐ co ‐propylene blends: Influence of the copolymer microstructure on rheology, morphology, and properties of injection‐molded samples

Meltrheological behavior, phase morphology, and impact properties ofisotactic‐polypropylene (iPP)‐based blends containingethylene–propylene copolymer (EPR) synthesized by means of atitanium‐based catalyst with very high stereospecific activity(EPR Ti ) were compared to those of iPP/EPR blends containingEPR copolymers synthesized by using a traditional vanadium‐basedcatalyst (EPR V ). The samples of EPR copolymers weresynthesized ad hoc. They were characterized by comparablepropylene content, average molecular masses, and molecular massdistribution in order to assess the effects of distribution ofcomposition and sequence lengths of the structural units on thestructure–properties correlations established in the melt and in thesolid state while studying different iPP/EPR pairs. 1–5 Differential scanning calorimetry, (DSC), wide‐angle X‐ray spectroscopy(WAXS), small‐angle X‐ray (SAXS), and scanning electron microscopy(SEM) investigations showed that the EPR Ti chain ischaracterized by the presence of long ethylenic sequences withconstitutional and configurational regularity required forcrystallization of the polyethylene (PE) phase occurring, whereas amicrostructure typical of a random ethylene–propylene copolymer wasexhibited by the EPR V copolymer. The different intra‐ andintermolecular homogeneity shown by such EPR phases was found to affecttheir melt rheological behavior at the temperatures of 200 and 250°C;all the EPR Ti dynamic–viscoelastic properties resultingwere lower than that shown by the EPR V copolymer. As far asthe melt rheological behavior of the iPP/EPR V andiPP/EPR Ti blends was concerned, both the iPP/EPR pairs areto be classified as “negative deviation blends” with G ′and G " values higher than that shown by the plaincomponents. The extent of the observed deviation in the viscosityvalues and of the increase in the amounts of stored and dissipatedenergy shown by such iPP/EPR pairs was found to be dependent oncopolymer microstructure, being larger for the melts containing theEPR Ti copolymer. The application of the Cross–Buecheequation also confirmed that, in absence of shear, the melt phaseviscosity ratio is the main factor in determining the viscosity ofiPP/EPR blends and their viscoelastic parameters. The generalcorrelation established between EPR dispersion degree (range ofparticle size and number‐average particle size), as determined ininjection‐molded samples, and melt phase viscosity ratio (μ) wasratified; the type of dependence of EPR size upon μ value was inqualitative agreement with the prediction of the Taylor–Tomotikatheory. Contrary to expectation, 1–5 for test temperatureclose to iPP T g , EPR V particlesranging in size between 0.75 and 1.25 μm resulted and were moreeffective than EPR Ti particles, ranging in size between0.25 and 0.75 μm, in promoting multiple craze formation. Also takinginto account the SAXS results, revealed that the molecularsuperstructure (i.e., crystalline lamellar thickness and amorphousinterlayer) of the iPP matrix is unaffected by both the presence ofEPR Ti and EPR V phase. The above finding wasrelated to the ethylenic crystallinity degree shown by theEPR Ti copolymer. In particular, such a degree ofcrystallinity was supposed to deteriorate toughening by decreasing thetie molecules density in the EPR Ti domains, notwithstandingthe beneficial effect of the ethylenic lamellar buildup. For testtemperature close to room temperature, the ductile behavior exhibitedby the iPP/EPR Ti blends was accounted for by a predominantshear yielding fracture mechanism probably promoted by a highconcentration of interlamellar tie molecules among iPP crystallites inagreement with DSC results. Nonisothermal crystallization experimentsshowed, in fact, that the crystallization peak of the iPP phase fromiPP/EPR Ti melt is shifted to higher temperaturesnoticeably, thus indicating a material characterized by a comparativelyhigher number of spherulites per unit value grown at lower apparentundercooling values. Accordingly, WAXS analysis revealed comparativelyhigher iPP crystal growth in the directions perpendicular to thecrystallographic planes (110) and (040) of theiPP. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 701–719, 1999