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Poly(propylene) Heterophasic Copolymers: Molecular Structure Analysis through Fractionation Techniques



Heterophasic poly(propylene) copolymers obtained by sequential polymerization are multi-component products formed by a crystalline matrix of poly(propylene) and an ethylene-propylene rubber phase. Some crystalline polyethylene and poly(ethylene-co-propylene) are formed in the rubber synthesis step by copolymerization of ethylene and propylene. These multi-component systems present a complicated microstructure and heterophasic morphology. In this study, two heterophasic copolymers with the same total amount of rubber but different composition and molecular weight, were thoroughly analyzed. The morphology was evaluated by SEM and AFM techniques. The complexity of the system required some attention in the fractionation methodology to perform an accurate interpretation of results and different methodologies of fractionation, especially for obtaining PE fractions as free of PP as possible, were performed in order to better characterize the crystalline polyethylene fractions in the copolymers. Preparative TREF was used to obtain fractions for further analysis by 13C-NMR, DSC and GPC. Also samples were submitted to Cross Fractionation Chromatography (CFC). Results obtained by the different methodologies were compared.