Molecular architecture effect on the microphase separations in supramolecular comb-coil complexes of polystyrene-block-poly(2-vinylpyridine) with dodecylbenzenesulfonic acid: (AB)_nA_n block-arm star copolymer

We studied the supramolecular comb-coil block copolymers formed by stoichiometric complexations of an amphiphilic surfactant, dodecylbenzenesulfonic acid (DBSA), with the poly(2-vinylpyridine) (P2VP) blocks in a linear polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) and a nonlinear block-arm star copolymer of the type (PS-b-P2VP)₅PS₅. The effect of the block copolymer molecular architecture on the hierarchical structures in the ordered state and the relevant order-disorder transitions (ODT) have been revealed using small-angle X-ray scattering (SAXS). Both the linear and the block-arm complexes exhibited structure-within-structure morphology in which the larger-scale PS microdomains were embedded in the matrix consisting of the smaller-scale lamellar mesophase organized by the P2VP(DBSA) comb blocks. The order-disorder transition temperature (T_ODT) of the copolymer domain in the linear complex was significantly higher than that of neat PS-b-P2VP due to the stronger interblock repulsion caused by the increase in the polarity of P2VP blocks upon complexation with DBSA. In sharp contrast, the corresponding T_ODT of (PS-b-P2VP)₅(PS) ₅(DBSA) complex was approximately the same as that of the neat copolymer. In this case the disruption of the PS microdomains induced a concurrent disordering of the smaller-scale lamellar mesophase because the entropic loss due to excessive stretching of short PS block chains in the microdomains overwhelmed the polar-nonpolar repulsion responsible for the formation of the lamellar mesophase. We have also observed a significant reduction in the interdomain spacing of the copolymer domains in the (PS-b-P2VP)₅(PS)₅(DBSA) complex compared to that in the neat copolymer. The smaller interdomain spacing was attributed to the lower aggregation number of PS star arms in the microdomains due to chain-crowding effect.