Poly(sulfobetaine) versus Poly(N-isopropylmethacrylamide): Co-Nonsolvency-Type Behavior of Thin Films in a Water/Methanol Atmosphere

The swelling and co-nonsolvency behaviors in pure H2O and in mixed H2O/CH3OH atmosphere of two different polar, water-soluble polymers are studied in-situ in thin film geometry. Films of a zwitterionic poly(sulfobetaine), namely poly[3-((2-(methacryloyloxy)ethyl)dimethylammonio) propane-1-sulfonate] (PSPE), and a polar nonionic polymer, namely poly(N-isopropylmethacrylamide) (PNIPMAM), are investigated in real time by spectral reflectance (SR) measurements and Fourier transform infra-red (FTIR) spectroscopy. Whereas PSPE is insoluble in methanol, PNIPMAM is soluble but exhibits co-nonsolvency behavior in water/methanol mixtures. First, the swelling of PSPE and PNIPMAM thin films in H2O vapor is followed. Subsequently, CH3OH is added to the vapor atmosphere and its contracting effect on the water-swollen films is monitored, revealing a co-nonsolvency-type behavior for PNIPMAM. SR measurements indicate that PSPE and PNIPMAM behave significantly different during the H2O swelling and the subsequent exposure to CH3OH, not only with respect to the amounts of absorbed water and CH3OH, but also to the co-solvent induced contraction mechanisms. While PSPE thin films exhibit an abrupt one-step contraction, the contraction of PNIPMAM thin films occurs in two steps. FTIR studies corroborate these findings on a molecular scale and reveal the role of the specific functional groups, both during the swelling and the co-solvent induced switching of the solvation state.