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

Abstract

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-isopropylmethacrylamide) (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-of-flight 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.

Poly(sulfobetaine)-Based Diblock Copolymer Thin Films in Water/Acetone Atmosphere: Modulation of Water Hydration and Co-nonsolvency-Triggered Film Contraction

“…We noted this phenomenon already in previous work and attributed it to residual H 2 O inside the films, which cannot be removed. 61 After the water vapor swelling in experiments A and B, the thickness and water content of both DBC films are increased. The film thickness in experiment A increases from 66 to 124 nm, and the SLD value decreases from 0.66 to 0.06 × 10 −6 Å −2 , indicating a strong hydration of the thin film.…”

“…Furthermore, the water and acetone-d6 content can be calculated from the SLD profile following our previous approach mentioned below. 61,62 The overall water content is calculated using thickness and SLD as…”

“…22 However, no distinctive co-nonsolvency-type behavior could be observed for thin films of the corresponding DBC at the studied ratio of the mixed water/methanol vapor atmosphere. 61 In the present study, the polysulfobetaine block in the studied DBC is changed to polymethacrylamide PSBP as its UCST behavior is more stable with respect to the cloud point compared to the hitherto employed polysulfobetaine blocks. 4 This change in the DBC enables the use of a rather symmetrical copolymer made of two blocks of about equal size.…”

“…52−59 Its co-nonsolvency behavior in certain water−alcohol mixtures in solution was demonstrated recently 19,60 and was also confirmed for thin films. 13,22,61 However, co-nonsolvency studies on thin thermo-responsive polymer films are still rare and in particular limited to lower alcohols, while studies using other organic co-solvents, such as acetone, have not been reported.…”

“…73 In fact, while acetone is a good solvent for PNIPMAM, a recent study pointed to the cononsolvency behavior of PNIPMAM in water−acetone mixtures, 13 in addition to featuring co-nonsolvency in aqueous methanol and ethanol. 22,61,63,74 In contrast, acetone is a nonsolvent and effective precipitant for PSBP.…”