Poly(N-isopropylacrylamide) Cross-Linked Gels as Intrinsic Amphiphilic Materials: Swelling Properties Used to Build Novel Interphases

The hydrophilic nature of hydrogels allows their swelling in aqueous solutions. In that way, any substance loaded inside the gel is exposed to the aqueous media and could be released if it is soluble in water. However, only substances that are soluble in water can be loaded inside a gel, which can be swelled only in water. In this work, we studied the swelling of poly(N-isopropylacrylamide) (PNIPAM) gels in nonaqueous solvents and their solutions with water. PNIPAM gels swell strongly in highly polar solvents, but they do not swell in slightly polar solvents (e.g., toluene). However, it is possible to swell the gel in mixtures containing toluene. The observed properties of PNIPAM gels allow describing them both as solvogels or amphigels. When the loaded substance is soluble in one solvent (e.g., water) and not in another (e.g., chloroform), the substance is not released but exposed to the new media. As a proof of concept, a colorimetric pH sensor active in CHCl3 and a Cu1+ sensor in water were built. Moreover, using a ternary solution containing toluene linear polystyrene can be loaded inside the gel, making a semi-interpenetrated network. Because PNIPAM swells in water and solvents immiscible in water, a liquid/liquid interphase can be set inside a gel. A near-infrared absorbing dye (soluble in CHCl3) is loaded in only half of a thermoresponsive PNIPAM gel. Upon near-infrared irradiation, only the region where the dye is loaded heats up driving the phase transition of PNIPAM.