pH-Induced Association and Dissociation of Intermolecular Complexes Formed by Hydrogen Bonding between Diblock Copolymers.

Poly(sodium styrenesulfonate)⁻block⁻poly(acrylic acid) (PNaSS⁻b⁻PAA) and poly(sodium styrenesulfonate)⁻block⁻poly(N-isopropylacrylamide) (PNaSS⁻b⁻PNIPAM) were prepared via reversible addition⁻fragmentation chain transfer (RAFT) radical polymerization using a PNaSS-based macro-chain transfer agent. The molecular weight distributions (Mw/Mn) of PNaSS⁻b⁻PAA and PNaSS⁻b⁻PNIPAM were 1.18 and 1.39, respectively, suggesting that these polymers have controlled structures. When aqueous solutions of PNaSS⁻b⁻PAA and PNaSS⁻b⁻PNIPAM were mixed under acidic conditions, water-soluble PNaSS⁻b⁻PAA/PNaSS⁻b⁻PNIPAM complexes were formed as a result of hydrogen bonding interactions between the pendant carboxylic acids in the PAA block and the pendant amide groups in the PNIPAM block. The complex was characterized by ¹H NMR, dynamic light scattering, static light scattering, and transmission electron microscope measurements. The light scattering intensity of the complex depended on the mixing ratio of PNaSS⁻b⁻PAA and PNaSS⁻b⁻PNIPAM. When the molar ratio of the N-isopropylacrylamide (NIPAM) and acrylic acid (AA) units was near unity, the light scattering intensity reached a maximum, indicating stoichiometric complex formation. The complex dissociated at a pH higher than 4.0 because the hydrogen bonding interactions disappeared due to deprotonation of the pendant carboxylic acids in the PAA block.