pH-induced association and dissociation of intermolecular complexes formed by hydrogen bonding between diblock copolymers

Abstract

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.