pH- and ionic-strength-induced contraction of polybasic micelles in buffered aqueous solutions

We report the synthesis and characterization of poly(dimethylaminoethyl methacrylate)-block-poly(styrene) (PDMAEMA-b-PS) diblock copolymers by RAFT polymerization. These polymers form uniform spherical micelles with dispersities less than 0.05 upon addition of aqueous buffer to polymer solutions in DMF. Potentiometric titrations under constant ionic strength conditions yield the first rigorous ionic-strength-effective pK_a correspondence for PDMAEMA homopolymers and micelles. We demonstrate that the effective polymer pK_a increases monotonically toward the monomer pK_a with increasing ionic strength, but decreases slightly upon association of polymer chains into micelles. We further characterize the pH- and ionic-strength-induced contraction of the micelle coronas in buffered aqueous solutions. In monoprotic buffers, the micelle corona behaves as a salted osmotic brush, as has been observed for other block polyelectrolyte micelle systems in unbuffered solutions. In polyprotic buffers, however, we observe an anomalously high degree of corona contraction. We demonstrate through a simple two-domain equilibrium model that this contraction likely arises from concentration of the charged buffer species in the micelle corona, which shifts the buffer dissociation equilibrium farther toward multivalent species than in the bulk.