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

Abstract

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 (VPTTfs), and where contraction starts (VPTTc). 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 VPTTc 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.