Lyotropic Microphase Separation in Aqueous Solutions of Double Sulfobetaine Diblock Copolymers Induced by Charge Separation Length Mismatch

Microcompartments produced via the liquid–liquid phase separation of polymer solutions are promising as transient microenvironments that can be generated on demand with minimal energy consumption. Lyotropic microphase separation in aqueous solutions of a double hydrophilic diblock copolymer composed of two distinct polysulfobetaine chains, poly(2-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonioethanesulfonate (PSB2) and poly(4-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammoniobutanesulfonate (PSB4), with very similar chemical structures but different charge separation lengths (CSL) within the sulfobetaine moieties (PSB2_n-b-PSB4_m) was investigated. PSB2_n-b-PSB4_ms with narrow molecular weight distributions were synthesized through radical addition–fragmentation chain transfer polymerization, followed by fractional precipitation. The phase behavior was studied as a function of the polymer concentration (φ) and block copolymer composition (f_PSB4). The aqueous solutions of PSB2_n-b-PSB4_m undergo phase separation due to specific interactions between identical sulfobetaines, triggered by differences in their CSL. The structural similarity of the zwitterions induces increased miscibility, lattice distortion, and morphological change. These results can be rationalized in terms of the low Flory–Huggins interaction parameter (χ) for PSB2/PSB4, the poor hydrophilicity of PSB2, and the weak interactions between PSB2 and PSB4. These unique mesoscale aqueous molecular assemblies, produced by microphase separation of double-hydrophilic block copolymers with high chemical symmetry, are promising as novel aqueous compartments for molecular reservoirs.