Development and application of pH-responsive polymers

This review summarizes the development of pH-responsive polymers and their main applications. Random pH-responsive copolymers have been prepared via conventional free radical polymerization from a pH-responsive pendant fatty acid-containing monomer (AaU) and a permanent water-soluble pendant sulfonate-containing monomer (AMPS). In water, the resulting copolymers, P(A/AaU), form single polymer chain (unimer) micelles under acidic conditions due to intrapolymer hydrophobic interactions between the protonated AaU units. The surfaces of the unimer micelles are covered with hydrophilic AMPS units, which provide colloidal stabilization. Under basic conditions, the P(A/AaU) polymer chains expand as a result of the electrostatic repulsive interactions between the pendant ionized fatty acids and the sulfonate anions. Through the use of a pH-responsive AaU homopolymer, pH-responsive sunscreen was developed. Although pH-responsive sunscreen shows waterproof properties under neutral conditions, it disperses under weakly basic conditions such as soap water. Therefore, pH-responsive sunscreen resists sweat but can be washed off using soap water. pH-responsive diblock copolymers composed of a PAMPS block and a pH-responsive pendant fatty acid-containing block were prepared via controlled radical polymerization. In water, the block copolymers form interpolymer micelles under acidic conditions due to hydrophobic interactions between the protonated pendant fatty acid groups, whereas the polymer micelles dissociate under basic conditions. Finally, this review also discusses pH-responsive gelling agents based on ABA triblock copolymers.