Solution Properties of Architecturally Complex Multiarm Star Diblock Copolymers in a Nonselective and Selective Solvent for the Inner Block

In comparison to the behavior of linear block copolymers, much less is known about the structure and properties of highly branched polymeric materials. Motivated by this, the solution properties of a series of 26- and 40-arm polystyrene poly(2-vinylpyridine) (PS-PVP) star diblock copolymers of different weight-average molecular weight (M-w) and styrene to 2-vinylpyridine (S/V) ratios are studied. These stars are investigated in tetrahydrofuran (THF), a thermodynamically good solvent for both blocks, and in toluene, a solvent that is selective for the inner PS blocks. It is found that in both THF and toluene, the 26- and 40-arm stars remain dispersed as unimolecular star block copolymers across the concentration range studied, 0.001< c < 10.0 mg/mL. The hydrodynamic radius, R-h, increases with PS M-w and number of arms, with the stars of highest M-w and number of arms having the largest R-h. The characteristic rho ratio, R-g/R-h, is similar to 1.0, suggesting that these stars do not behave as homogeneous hard spheres, but rather exhibit Gaussian soft sphere characteristics. TEM images indicate that these stars adopt an unusual asymmetric structure in toluene due to intramicellar microphase segregation of the arms: The PVP blocks collapse and aggregate within the unimolecular structure while the PS blocks stretch and shield the aggregated PVP domain. Despite the strong tendency of PVP to drive aggregation in toluene, repulsive steric interactions between solvated PS blocks and this rearrangement prevent PVP domains of selectively solvated stars from assembling into multimolecular aggregates. This morphological behavior is consistent with inferences drawn from analyses of the concentration dependence of the z-average diffusion coefficient, which suggests that frictional interactions are stronger than star-star interactions. In total, these results shed new light on how topologically complex, amphiphilic block copolymers organize in solution.