Poly(N,N-dimethylaminoethyl methacrylate) Brushes: pH-Dependent Switching Kinetics of a Surface-Grafted Thermoresponsive Polyelectrolyte

The temperature-dependent switching behavior of poly(N,N-dimethylaminoethyl methacrylate) brushes in alkaline, neutral, and acidic solutions is examined. A novel microscopic laser temperature-jump technique is employed in order to study characteristic thermodynamic and kinetic parameters. Static laser micromanipulation experiments allow one to determine the temperature-dependent variation of the swelling ratio. The data reveal a strong shift of the volume phase transition of the polymer brushes to higher temperatures when going from pH = 10 to pH = 4. Dynamic laser micromanipulation experiments offer a temporal resolution on a submillisecond time scale and provide a means to determine the intrinsic rate constants. Both the swelling and the deswelling rates strongly decrease in acidic solutions. Complementary experiments using in situ atomic force microscopy show an increased polymer layer thickness at these conditions. The data are discussed on the basis of pH-dependent structural changes of the polymer brushes including protonation of the amine groups and conformational rearrangements. Generally, repulsive electrostatic interactions and steric effects are assumed to hamper and slow down temperature-induced switching in acidic solutions. This imposes significant restrictions for smart polymer surfaces, sensors, and devices requiring fast response times.