KCl-Modulated Hydration and Subsequent Thermoresponsive Behavior of Poly(sulfobetaine)-Based Diblock Copolymer Thin Films

The KCl-modulated swelling of double thermoresponsive diblock copolymer (DBC) thin films in D₂O atmosphere and their subsequent responsive behavior are investigated via spectral reflectance (SR), in situ time-of-flight neutron reflectometry (ToF-NR), and Fourier-transform infrared (FT-IR) spectroscopy. The copolymer consists of a short zwitterionic (poly(4-(N-(3-methacrylamidopropyl)-N,N-dimethylammonio) butane-1-sulfonate)) (PSBP) block and a long non-ionic poly(N-isopropylmethacrylamide) (PNIPMAM) block. DBC thin films are prepared via spin-coating from solutions containing 5 mM or devoid of KCl. The addition of KCl to the DBC thin films leads to a higher swelling ratio and D₂O content during vapor treatment with D₂O. Upon the subsequent heating of the hydrated DBC films, the films swell further and reach a maximum thickness before contracting. Two separate volume phase transition temperatures (VPTTs) are observed, namely where a further swelling plateau is reached (VPTT_fs), and where contraction starts (VPTT_c). Based on complementary SR studies of the effect of KCl on the swelling behavior of the respective PSBP and PNIPMAM homopolymer thin films, we conclude that the “further-swelling” period is mainly a consequence of the UCST-type phase transition of PSBP, whereas the “film contraction” period is due to the LCST-type phase transition of PNIPMAM in thin-film geometry. We observe that KCl reduces the VPTT_c of the PNIPMAM blocks. Moreover, the salt migrates or aggregates inside the thin film upon heating, thereby forming a KCl enrichment layer in the intermediate section of the film. Furthermore, the observations by FT-IR prove that macroscopic and mesoscopic D₂O absorption and desorption are correlated with the effect of KCl on hydration and de-hydration of the hydrophilic groups.