Competitive swelling forces and interpolymer complexation in pH- and temperature-sensitive interpenetrating network hydrogels

Abstract

Interpenetrating polymer network (IPN) hydrogels composed of a pH-sensitive poly(acrylic acid) (PAA) network and a temperature-sensitive poly(N-isopropylacrylamide) (PNIPAAm) network were prepared via sequential, UV-initiated polymerization. The molecular interactions and responsive behavior of these hydrogels were investigated using swelling studies under various conditions of pH and temperature. Results indicate the existence of hydrogen-bonded complexation between the two polymer networks, which competes with polymer–water interactions, resulting in lower water content. The effect is most significant in hydrogels with an equimolar ratio of the polymer components. We have also found that ionization of the PAA network creates a swelling force that opposes heat-induced deswelling of the PNIPAAm network. We have shown that the use of ternary contour plots is an efficient way of analyzing the swelling behavior of these complex materials.