Optical Characterization of Co-Nonsolvency Effects in Thin Responsive PNIPAAm-Based Gel Layers Exposed to Ethanol/Water Mixtures

Surface-attached, photo-crosslinked gel films of a N-isopropylacrylamide copolymer were investigated in ethanol/water mixtures using a combination of surface plasmon resonance/optical waveguide spectroscopy with reversed WKB analysis. The solvent quality of the pure good solvents drops in their mixture and this co-nonsolvency effect shifts the transition temperature (T(c)) in the mu m-thin gel films from 32.8 degrees C in pure water to 29.7 degrees C with only 0.25 vol.-% ethanol. Between 20 and 70% ethanol and >10 degrees C (the practical temperature limit) the layers existed only in the collapsed state. A reentrant T(c) of 40.2 degrees C was found at 70% ethanol while at higher ethanol volume fraction no T(c) could be recorded.