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

Abstract

Summary 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.

Microstructure Characterization of Amorphous Phase of Polypropylene Impact Copolymers

“…This crystalline fraction can be associated to an ethylene homopolymer or ethylene rich copolymers (ethylene-propylene). [1,2] Based on the 13 C-NMR characterization, a significant amount of blocky ethylene ([EEE]) was observed in the amorphous phase of ICP30 and ICP00. The concentration of these triads as well as the [EPE] concentration is higher for ICP00.…”

“…Extensive studies have shown that some crystalline ethylene rich polymers are formed during the rubber polymerization step of the ICPs, which vary in a broad range of compositions from ethylene homopolymer to blocky crystalline propylene/ethylene copolymers. [1][2][3][4] The amount of these crystalline structures is related to the ratio of ethylene and propylene monomers in the rubber polymerization step. These crystalline structures are formed inside the rubber domains and can be visualized by SEM when the rubber (random propyleneethylene copolymer) is extracted by n-hexane.…”

“…These polymers are used in a wide variety of applications, including flexible and rigid packaging, nonwoven fabrics for diapers and medical garments, bumpers, panels and trims for automotive industry, among others. [1][2][3] Propylene impact copolymers (ICPs), also known as heterophasic copolymer (HECO) or high impact polypropylene (hiPP), are reactor blends of propylene and ethylene-propylene copolymers, generally prepared using a two-step sequential polymerization process, in which the constituents are produced in the first and second reactor, respectively. The final propylene impact copolymer is composed of a crystalline PP (major component), an amorphous ethylene-propylene random copolymer (ethylene propylene rubber -EPR) and a series of crystalline ethylene-propylene copolymers with a wide composition range, varying from ethylene homopolymer (PE) to a highly modified ethylene or propylene copolymer.…”

“…The final propylene impact copolymer is composed of a crystalline PP (major component), an amorphous ethylene-propylene random copolymer (ethylene propylene rubber -EPR) and a series of crystalline ethylene-propylene copolymers with a wide composition range, varying from ethylene homopolymer (PE) to a highly modified ethylene or propylene copolymer. [1][2][3][4] The amorphous and crystalline propylene-ethylene copolymers with different monomer sequence distributions and molar masses generate a blend with a very complex molecular structure. [4] DOI: 10.1002/masy.202000133…”

“…[2,3,[6][7][8][9] The rubber size is strongly affected by the rubber composition and molecular weight. [1] Given the importance of rubber size and distribution, composition, and morphology in improving mechanical properties of ICPs, most of the work developed in this paper is focused on characterization of the rubber fraction.…”