Cyclic Water Storage Behavior of Doubly Thermoresponsive Poly(sulfobetaine)-Based Diblock Copolymer Thin Films

The cyclic swelling and collapse behavior of a doubly thermoresponsive diblock copolymer thin film, consisting of a zwitterionic poly(sulfobetaine), poly(N,N-dimethyl-N-(3-methacrylamidopropyl)-ammoniopropane sulfonate) (PSPP), and a nonionic poly(N-isopropylmethacrylamide) (PNIPMAM) block, is investigated in situ at three characteristic temperatures with time-of-flight neutron reflectometry. With increasing temperature, the thin film becomes less hydrophilic, which leads to a decreased but faster water uptake. This response of the block copolymers in the thin-film geometry differs greatly from their known aqueous solution behavior. In the cyclic experiments at constant temperature, the behavior is reproducible in terms of mesoscopic parameters such as swelling ratio and water content, even though Fourier transform infrared spectroscopy reveals altered swelling mechanisms, which are attributed to a complex interplay between different water species. Thus, the overall reduced hydrophilicity affects the overall swelling behavior of the thin film but not the hydration of particular functional groups of the diblock copolymer PSPP-b-PNIPMAM.