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 μm-thin gel films from 32.8 °Cin pure water to 29.7 °C with only 0.25 vol.-% etha- nol. Between 20 and 70% ethanol and >10 °C (the practical temperature limit) the layers existed only in the collapsed state. A reentrant Tc of 40.2 °C was found at 70% ethanol while at higher ethanol volume fraction no Tc could be recorded.