Solvation Behavior of Poly(sulfobetaine)-Based Diblock Copolymer Thin Films in Mixed Water/Methanol Vapors

The swelling of thin diblock copolymer (DBC) films is investigated in situ at 22 °C in pure water vapor as well as in mixed water / methanol vapor. The DBC consists of a zwitterionic poly(sulfobetaine) block, poly[3-((2-(methacryloyloxy)ethyl)-dimethylammonio) propane-1-sulfonate] (PSPE), and a nonionic poly(N-sopropylmethacrylamide) (PNIPMAM) block. The swelling in water vapor (either H2O or D2O) and the thin-film response to methanol vapor exchange (i.e., a part of the H2O vapor is exchanged by CD3OH vapor and a part of the D2O vapor is exchanged by CH3OH vapor) are followed with simultaneous time-offlight neutron reflectometry (ToF-NR) and spectral reflectance (SR) measurements. In situ Fourier transform infrared (FTIR)spectroscopy complements these data. Exposure to H2O vapor leads to a slightly higher degree of swelling and amount of absorbed H2O compared to D2O. Upon methanol exchange, the PSPE-b-PNIPMAM thin film undergoes two contractions, which are assigned to the specific responses of the individual polymer blocks of the DBC. Due to its isotope sensitivity, FTIR confirms these two separate contraction processes of the blocks on a molecular level and reveals the role of each polymer block during swelling in water vapor and upon the methanol exchange. Thus, four distinct film regimes with different thicknesses dependencies on the vapor composition can be established, thereby enabling a quarternary nanoswitch.