Study on β-Nucleated Controlled-Rheological Polypropylene Random Copolymer: Crystallization Behavior and a Possible Degradation Mechanism

Abstract

Controlled-rheological polypropylene random copolymer (CRPPR) and β-nucleated CRPPR were prepared through peroxide-initiated reactive extrusion and their crystallization behaviors were comparatively investigated. Rheological experiments indicated that all degraded samples acquired better flow properties than undegraded samples and the addition of β-nucleating agent has little effect on the flowability. Unlike conventional controlled-rheological polypropylene homopolymer of which the shortened molecular chains are unfavorable for β-nucleation, the structure characterizations in this work demonstrated an unexpected increase in the β-phase content of degraded β-nucleated CRPPRs with elevated peroxide concentration. Successive self-nucleation and annealing thermal analysis revealed the generation of thicker lamellar in highly degraded samples, which implied that the stereo regularity improved when the molecular chain reacted with peroxide. Based on the experimental results, a possible degradation mechanism was proposed that free radicals preferentially attack the tertiary carbon atoms adjacent to ethylene co-units during the degradation reaction, which resulted in a reduction of stereo errors, and, consequently, improvement of the β-crystallization ability.