A surface plasmon resonance study of volume phase transitions in N-isopropylacrylamide gel films

Cross-linked N-isopropylacrylamide (NIPAAm) gel is covalently attached to a substrate, and the resulting interface is probed using surface plasmon resonance (SPR) as a function of hydrostatic pressure and temperature. SPR provides a direct measurement of the local refractive index, which changes with the swelling ratio of the gel film. Similar to bulk NIPAAm. gel, the transition temperature increases and the volume phase transition becomes broader as pressure increases. The width of the transition ranges from less than 0.5 degreesC at 1 bar to as much as 10 degreesC at 1000 bar, and the transition temperature increases by as much as 7 degreesC over the same range of pressures, However, the presence of a fixed substrate effectively confines the volume phase transition near the interface to one dimension, perpendicular to the substrate. This has significant effects on the transition temperature, particularly at high cross-linking density and high concentration of an ionizable comonomer. Furthermore, the swelling effect of the ionic groups is reduced, and the water content of the swollen gel does not change with increased ionic content. While the volume phase transition of the corresponding bulk gels can have a total volume change as large as 100-fold, the gel films have a total volume change around 15-fold.