Crystallization of α versus β Phases in Syndiotactic Poly(styrene-stat-3-methylstyrene) and Poly(styrene-stat-4-methylstyrene)

By incorporating different levels of methyl groups at meta- or para-positions (3- vs 4-methylstyrene) as comonomer units into syndiotactic polystyrene (sPS), we examined the effects of comonomer structure/content on the polymorphic behavior. Upon melt-crystallization, wide-angle X-ray scattering (WAXS) profiles revealed that syndiotactic poly(styrene-stat-3-methylstyrene) (sPS-3MS) formed predominantly β-crystals for 3MS content up to 11 mol %, followed by dominance of α-crystals at a higher 3MS level (21 mol %); in the case of syndiotactic poly(styrene-stat-4-methylstyrene) (sPS-4MS), predominant formation of β-crystals occurs only at 2 mol % 4MS content, followed by dominance of α-crystals at higher comonomer levels (5 and 10 mol %). Upon cold-crystallization, both sPS-3MS and sPS-4MS consistently gave only α-crystals. It appears that the formation of α-phase is consistently preferred in these random copolymers. WAXS profiles showed that the characteristic reflections generally shift to lower q-positions with increasing comonomer content, i.e., crystals in copolymers have greater unit cell parameters, except for the β-structure of sPS-4MS. Hence, comonomer units are believed to be incorporated into α-crystals in both sPS-3MS and sPS-4MS or β-structure in sPS-3MS but fully excluded from the β-structure of sPS-4MS. Results of molecular mechanics computation showed that the packing energy increases upon incorporation of comonomer units, the effect being particularly strong for β-crystals of sPS-4MS. The significantly suppressed formation of β-crystals in sPS-4MS may thus be simply attributed to the strongly increased interchain packing energy due to the protruding 4-methyl groups; this steric repulsion is weaker for β-crystals in the sPS-3MS series, consistent with the delayed preference of α-crystals to higher 3MS contents