Thermo-Responsive Behavior of Amphoteric Diblock Copolymers Bearing Sulfonate and Quaternary Amino Pendant Groups

Amphoteric diblock copolymers (S₈₂A _n) composed of poly(2-acrylamido-2-methylpropanesulfonic acid sodium salt) (PAMPS) with poly(3-(acrylamido)propyl trimethylammonium chloride) (PAPTAC) blocks were synthesized via reversible addition-fragmentation chain transfer (RAFT) radical polymerization. Three S₈₂A _n were prepared with a fixed degree of polymerization (DP) for the PAMPS block (= 82) and different DP values for the PAPTAC blocks ( n = 37, 83, and 183). The solubility of S₈₂A _n was studied at different sodium chloride (NaCl) concentrations. S₈₂A₈₃ precipitated in pure water due to attractive electrostatic interactions with interpolymer chains. Conversely, S₈₂A₃₇ and S₈₂A₁₈₃ dissolved in pure water. In pure water S₈₂A₃₇ dissolved as a unimer state due to electrostatic repulsion of excess anionic charges in the polymer chain. The long anionic PAMPS block segment in S₈₂A₃₇ covered the short cationic PAPTAC block segment within a single polymer chain. In pure water S₈₂A₁₈₃ dispersed as polyion complex micelles due to electrostatic repulsion of the cationic PAPTAC shells. The oppositely charged PAMPS and PAPTAC blocks in S₈₂A₁₈₃ formed a core, while the excess PAPTAC block formed shells. S₈₂A _n showed lower critical solution temperature (LCST)-type thermo-responsive behavior at certain NaCl concentrations, and the LCST increased with the NaCl concentration. The mechanism of LCST behavior involves hydrogen bonding interactions between the pendant amide groups and water molecules.