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 °C at 1 bar to as much as 10 °C at 1000 bar, and the transition temperature
increases by as much as 7 °C 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.