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 βnucleating 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.

Role of dicumyl peroxide on the morphology and mechanical performance of polypropylene random copolymer in microinjection molding

“…The values of the polydispersity ( M w / M n ) of CRPPR decreased as compared with that with undegraded CRPPR0 resin, thereby indicating a narrow molecular weight distribution. It has been reported that in the process of the degradation reaction, the radicals generated by the DCP attack the tertiary carbon that is adjacent to the ethylene units on the PP backbone, thereby resulting in the degradation of the PPR chains …”

“…A "controlled-rheology polypropylene" technology provides an approach to improve the processing ability of polypropylene resin in μIM, which is performed using a peroxide-initiated scission (β-scission) reaction. Present studies 10,11 indicate that the degradation mechanisms of polypropylene homopolymer (PPH) and polypropylene random copolymer (PPR) are different due to the difference in their respective molecular structures. For the PPH chemical degradation reaction, hydrogen abstraction from PPH backbones proceeds at the site of the tertiary carbon atom, which results in a very extensive degradation, lower melt strength, and lower mechanical properties as compared with the original PPH.…”

“…The values of the polydispersity (M w /M n ) of CRPPR decreased as compared with that with undegraded CRPPR0 resin, thereby indicating a narrow molecular weight distribution. It has been reported that in the process of the degradation reaction, the radicals generated by the DCP attack the tertiary carbon that is adjacent to the ethylene units on the PP backbone, thereby resulting in the degradation of the PPR chains 10. Moreover, as compared with shorter molecular chains, the longer molecular chains were more easily captured by radicals as they arereleased from the decomposing DCP and are present in the β-scission reaction.…”

Influence of melt structure on the crystallization behavior and polymorphic composition of polypropylene random copolymer

“…In such copolymer, the co-monomers randomly distribute into the long propylene sequences, resulting in the decrease of crystallization ability of homo-propylene sequences and thus the decline of total crystallinity and finally leading to an excellent balance of stiffness and toughness. It should be noted that the polymorphic behavior of PPR is somewhat different from that of the iPP homo-polymer: it is even more difficult for PPR to increase the b-crystallization ability by the common methods mentioned above [27][28][29][30][31]. Therefore, it is very important to explore useful and effective way to enhance the crystallization ability and b-phase content of PPR.…”

Dynamic mechanical properties, crystallization behaviors, and low‐temperature performance of polypropylene random copolymer composites

“…Polypropylene random copolymer (PPR) is prepared with the addition of 3–7 wt % ethylene embedded randomly in the propylene matrix. PPR is a widely used matrix component in pipes, daily necessities, packaging materials, automobile parts, furniture, thin films, and other industrial fields for its excellent comprehensive mechanical properties, good processing performance, and relatively low costs . However, industrial applications of PPR are greatly hindered because of its insufficient impact toughness, especially at low temperatures.…”