Thermosensitive poly(N-isopropylacrylamide-co-acrylic acid) hydrogels with expanded network structures and improved oscillating swelling-deswelling properties

In this paper, a P(NIPAAm-co-AAc) hydrogel with rapid temperature sensitivity and improved oscillatory properties over small temperature cycles around the physiologic temperature of 37 °C was synthesized in an alkaline solution (Tris/HCl solution, pH 8.8 and I = 0.5 M). SEM micrographs revealed that the unique properties achieved can be attributed to the expanded network structure generated in the alkaline solution during the copolymerization reaction. As a result of the dissociation of the carboxyl groups (-COOH) of AAc to carboxylate anions (-COO-), the electrostatic repulsion between carboxylate anions was strong and led to the expanded conformations of polymer chains. Therefore, the network of the hydrogels thus obtained was extremely expanded and exhibited fast temperature sensitivity and improved oscillating swelling-deswelling properties.