Toward a Further Understanding of the Decreased β-Phase Content in β-Nucleated Polypropylene Random Copolymer: From Nucleation and Growth Kinetics

Compared with polypropylene homopolymer (PPH), beta-nucleating agent (beta-NA) usually induces less beta-phase in polypropylene random copolymer (PPR) with comonomer inserted. A widely held view is that unfavorable beta-growth kinetically caused by the insertion of comonomer contributes to a decreased relative content of beta-phase (k(beta)) in PPR. To further understand the decreased k(beta) in PPR, we separately investigated the nucleation and growth behavior of various isotactic polypropylenes (iPPs) nucleated with a beta-NA, N,N'-dicyclohexylterephthalamide (DCHT). With the aid of fractionated crystallization, the possibility of DCHT to form beta-nuclei, i(beta)/i, is calculated to be reduced in PPR with ethylene inserted, demonstrating fewer beta-nuclei induced. As for growth, it is found that the insertion of comonomer decreases the relative growth rate of beta-phase (G(beta)/G(alpha)) and weakens the kinetic advantage of beta growth. The study reveals that both i(beta)/i and G(beta)/G(alpha) decrease with the insertion of comonomer. Fewer beta-nuclei induced by DCHT and the subsequent unfavorable beta-growth simultaneously contribute to the final decreased k(beta). Our fundamental insight provides a more comprehensive understanding of the decreased k(beta) as comonomer is inserted into the iPP chain and could have important implications in the control of the polymorphism of polymers.