Making Hydrophilic Polymers Thermoresponsive: The Upper Critical Solution Temperature of Copolymers of Acrylamide and Acrylic Acid

Homopolymers of acrylamide and acrylic acid are known to be hydrophilic and soluble in water. Their copolymers are soluble in water at neutral pH but can be turned into polymers possessing an upper critical solution temperature (UCST) behavior when the carboxylic acid groups are protonated (pH < 3). At temperatures below the UCST, the formation of interchain hydrogen bonds between the acid and amide groups makes the copolymers insoluble. These hydrogen bonds are disrupted when the temperature is heated above the critical temperature (Tc), exhibiting a UCST behavior. In this study, linear water-soluble random copolymers of acrylamide and acrylic acid of different compositions and molar masses have been synthesized. The thermoresponsive properties of the copolymers were studied to elucidate the effects of their composition, molar mass, and concentration by measuring their cloud point temperatures (Tcp) in acidic buffer solutions to keep the carboxylic acid groups in the protonated state. The effects of pH and added salts or urea were also studied.