Synthesis and characterization of pH- and thermo-responsive hydrogels based on poly(2-cyclopropyl-2-oxazoline) macromonomer, sodium acrylate, and acrylamide

New graft copolymer hydrogels based on sodium acrylate, acrylamide, and styryl-terminated poly(2-cyclopropyl-2-oxazoline) macromonomer (MM) were synthetized by free radical polymerization using N,N′-methylenebisacrylamide as cross-linker. The polymerization was carried out in water at 5 °C and was initiated by sodium peroxodisulfate/N,N,N′,N′-tetramethylethylenediamine. The MM and the hydrogels were characterized by NMR and FTIR spectroscopy. In the hydrogels, the sodium acrylate provided the sensitivity to changes in pH value while the MM provided sensitivity to temperature. In dependence of their composition, the bi-sensitive hydrogels showed conformational transitions with variation of temperature or pH value. This property was shown macroscopically as a hydrogel volume contraction or expansion as it was determined by swelling experiments in water at different pH values and temperatures. Due to phase separation within the hydrogels facilitated by the graft copolymer network structure, both sensitivities could be addressed individually by both triggers and defined swelling states could be addressed over a wide range by adjusting both temperature and pH.