Swelling and Exchange Behavior of Poly(sulfobetaine)-Based Block Copolymer Thin Films

The humidity-induced swelling and exchange behavior of a block copolymer thin film, which consists of a zwitterionic poly(sulfobetaine) [poly(N,N-dimethyl-N-(3-(methacrylamido)propyl)ammoniopropanesulfonate) (PSPP)] block and a nonionic poly(N-isopropylacrylamide) (PNIPAM) block, are investigated by time-of-flight neutron reflectometry (TOF-NR). We monitor in situ the swelling in the H₂O atmosphere, followed by an exchange with D₂O. In the reverse experiment, swelling in the D₂O atmosphere and the subsequent exchange with H₂O are studied. Both, static and kinetic TOF-NR measurements indicate significant differences in the interactions between the PSPP₈₀-b-PNIPAM₁₃₀ thin film and H₂O or D₂O, which we attribute to the different H-and D-bonds between water and the polymer. Changes in the chain conformation and hydrogen bonding are probed with Fourier transform infrared spectroscopy during the kinetics of the swelling and exchange processes, which reveals the key roles of the ionic SO₃^- group in the PSPP block and of the polar amide groups of both blocks during water uptake and exchange.