Dynamic Response of Anchored Poly(N-isopropylacrylamide-co-methacrylic acid-co-benzophenone methacrylate) Terpolymer Hydrogel Layers to Physicochemical Stimuli

The use of responsive thin hydrogel films as the active matrix in sensor applications requires detailed knowledge of their structural and dynamical properties and their responsiveness to external conditions. Here, the structure and dynamics in photo-crosslinked, surface-attached films (≈1 μm dry thickness) of a poly(N-isopropylacrylamide) (PNIPAAm)-based terpolymer (containing 5% methacrylic acid and 1% benzophenone methacrylate comonomers) at different temperatures and salt (NaCl) contents are reported. The swelling is monitored by surface plasmon resonance/optical waveguide spectroscopy (SPR/OWS), and the hydrogel dynamics are studied by micro-photon correlation spectroscopy. In addition to the expected volume phase transition above a critical temperature of ca. 32-35 °C and at NaCl concentrations above 1 m, SPR/OWS reveals subtle thickness variations at lower salt concentrations. The fast cooperative diffusion of the polymer network and a slow mode for the relaxation of the thermal concentration fluctuations in the grafted hydrogel layer exhibit a non-monotonic dependence on the salinity and a strong slowdown with temperature near the collapse transition. These dynamics respond sensitively and distinctly to structural changes induced by increasing temperature or added monovalent salt in comparison to the only subtle changes of the polymer fraction.