Polyelectrolyte multilayers with reversible thermal responsivity

Thermally responsive polyelectrolyte multilayers were made from charged poly(N-isopropylacrylamide) (PNIPAM) copolymers. The temperature-dependent water content of the thin film, studied in situ using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, revealed microscopic and macroscopic transitions at 33 and 45degreesC, respectively. About seven water molecules per NIPAM repeat unit were found to be reversibly lost from, or recovered by, the film upon cycling over a temperature range of 10-55degreesC. Assuming each ion pair represents a cross-link, swelling theory was used to translate these results into polymer-solvent interaction parameters and enthalpies of mixing for the various polymer components. The flux of a charged probe molecule, ferricyanide, through the NIPAM-rich multilayer was assessed with rotating disk electrode voltammetry. Thermally reversible modulation of ion transport was demonstrated.