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

Amphoteric diblock copolymers (S(82)A(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 S(82)A(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 S(82)A(n) was studied at different sodium chloride (NaCl) concentrations. S(82)A(83) precipitated in pure water due to attractive electrostatic interactions with interpolymer chains. Conversely, S(82)A(37) and S(82)A(183) dissolved in pure water. In pure water S(82)A(37) dissolved as a unimer state due to electrostatic repulsion of excess anionic charges in the polymer chain. The long anionic PAMPS block segment in S(82)A(37) covered the short cationic PAPTAC block segment within a single polymer chain. In pure water S(82)A(183) dispersed as polyion complex micelles due to electrostatic repulsion of the cationic PAPTAC shells. The oppositely charged PAMPS and PAPTAC blocks in S(82)A(183) formed a core, while the excess PAPTAC block formed shells. S(82)A(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.