Molecular Architecture Effect on Microphase Separation in Supramolecular Comb−Coil Complexes of Polystyrene-block-poly(2-vinylpyridine) with Dodecylbenzenesulfonic Acid: AnBn Heteroarm Star Copolymer

Abstract

The microphase separation in a supramolecular comb−coil block copolymer formed by the stoichiometric complexation of an amphiphilic surfactant, dodecylbenzenesulfonic acid (DBSA), with the poly(2-vinylpyridine) (P2VP) blocks in a nonlinear heteroarm (PS)5(P2VP)5 star copolymer has been investigated. The hierarchical structure in the ordered state and the relevant order−disorder transitions (ODT) of the complex have been revealed using small-angle X-ray scattering (SAXS). The results were compared with those obtained previously for linear PS-b-P2VP(DBSA) and block-arm (PS-b-P2VP)5(PS)5(DBSA) complexes [Macromolecules 2005, 38, 10117] to illustrate the effect of block copolymer architecture on the self-assembly behavior of the supramolecular comb−coil complexes. The heteroarm (PS)5(P2VP)5(DBSA) complex exhibited structure-within-structure morphology in which the larger-scale PS cylindrical microdomains were embedded in the matrix consisting of the smaller-scale lamellar mesophase organized by the P2VP(DBSA) comb blocks. As observed previously for the linear PS-b-P2VP(DBSA) complex, the ODT temperature (TODT) of the copolymer domain in the heteroarm complex was appreciably higher than that of neat (PS)5(P2VP)5 due to the strong interblock repulsion caused by the increase in the polarity of P2VP blocks upon complexation with DBSA. Most interestingly, a significant increase in the TODT of the smaller-scale lamellar mesophase was observed in the heteroarm complex compared to that of its linear counterpart, and this was attributed to the lower entropy of transition due to the junction constraint. The interdomain distance of the copolymer domains in the heteroarm complex was smaller than that in the linear complex due to lower aggregation number of PS star arms in the cylindrical domains. However, the corresponding interdomain distance was significantly larger than that in the block-arm complex due to different type of intermolecular segregation occurring in the two kinds of star copolymers.