Solvation Dynamics of Thermoresponsive Polymer Films: The Influence of Salt Series in Water and Mixed Water/Methanol Atmosphere

Understanding the salt effects on solvation behaviors of thermoresponsive polymers is crucial for designing and optimizing responsive systems suitable for diverse environments. In this work, the effect of potassium salts (CH₃COOK, KCl, KBr, KI, and KNO₃) on solvation dynamics of poly(4-(N-(3'-methacrylamidopropyl)-N,N-dimethylammonio) butane-1-sulfonate) (PSBP), poly(N-isopropylmethacrylamide) (PNIPMAM), and PSBP-b-PNIPMAM films is investigated under saturated water and mixed water/methanol vapor via advanced in situ neutron/optical characterization techniques. These findings reveal that potassium salts enhance the films' hygroscopicity or methanol-induced swellability. Interestingly, the anions effects do not mirror the empirical Hofmeister series, which describes the salting-in effects for such polymers in dilute aqueous solution, particularly evident in PSBP films with an approximately inverted order. PNIPMAM and PSBP-b-PNIPMAM exhibit pronounced deviations from such an inverted correlation and vary somewhat for water-rich and methanol-rich atmospheres. Molecular dynamics (MD) simulations suggest that the observed orders of solvation result from the accessibility of the hydrated solvation shells close to the PSBP-b-PNIPMAM chains.