Polyelectrolyte Multilayers with Reversible Thermal Responsivity

Abstract

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 45 °C, 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 10−55 °C. 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.