Effects of cononsolvency on preferential adsorption phenomenon in poly(N-isopropylacrylamide) ternary solutions

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

The water swelling and subsequent solvent exchange including co-nonsolvency behavior of thin films of a doubly thermo-responsive diblock copolymer (DBC) are studied via spectral reflectance, time-of-flight neutron reflectometry, and Fourier transform infrared spectroscopy. The DBC consists of a thermo-responsive zwitterionic (poly(4-((3-methacrylamidopropyl) dimethylammonio) butane-1-sulfonate)) (PSBP) block, featuring an upper critical solution temperature transition in aqueous media but being insoluble in acetone, and a nonionic poly(N-isopropylmethacrylamide) (PNIPMAM) block, featuring a lower critical solution temperature transition in water, while being soluble in acetone. Homogeneous DBC films of 50-100 nm thickness are first swollen in saturated water vapor (H₂O or D₂O), before they are subjected to a contraction process by exposure to mixed saturated water/acetone vapor (H₂O or D₂O/acetone-d6 = 9:1 v/v). The affinity of the DBC film toward H₂O is stronger than for D₂O, as inferred from the higher film thickness in the swollen state and the higher absorbed water content, thus revealing a pronounced isotope sensitivity. During the co-solvent-induced switching by mixed water/acetone vapor, a two-step film contraction is observed, which is attributed to the delayed expulsion of water molecules and uptake of acetone molecules. The swelling kinetics are compared for both mixed vapors (H₂O/acetone-d6 and D₂O/acetone-d6) and with those of the related homopolymer films. Moreover, the concomitant variations of the local environment around the hydrophilic groups located in the PSBP and PNIPMAM blocks are followed. The first contraction step turns out to be dominated by the behavior of the PSBP block, whereas the second one is dominated by the PNIPMAM block. The unusual swelling and contraction behavior of the latter block is attributed to its co-nonsolvency behavior. Furthermore, we observe cooperative hydration effects in the DBC films, that is, both polymer blocks influence each other's solvation behavior.

Thermoreversible Gelation of Associating Polymers in Hydrogen-Bonding Mixed Solvents

The thermoreversible gelation of associating polymers in hydogen-bonding mixed solvents is studied theoretically on the basis of the model ternary solution in which polymers carrying functional group A are dissolved in a primary solvent SB (mainly water) and cosolvent SC. Functional group A forms intermolecular cross-links (A-A) leading to gelation. It can also form hydrogen bonds with either solvent molecule SB or SC (A-SB, A-SC). If bound to a solvent molecule, A becomes inert. Solvent molecules form complexes with variable association numbers (SB-SC). Ternary phase diagrams for sol-gel transition lines and their cross sections on the temperature-solvent composition plane are constructed. It is shown that a minimum in the polymer concentration along the sol-gel transition line appears at a special solvent composition which depends upon the structure of the complexes. At such an optimal gel point, the sol-gel transition temperature becomes the highest, thus leading to stable, strong gels. The degree of adsorption, preferential adsorption coefficient, average molecular weight of the complexes, and cross-link probability are calculated as functions of the solvent composition. At the optimal gel point, the preferential adsorption coefficient changes its sign and the average molecular weight of the complexes reaches a maximum. Results are compared with the experimental data on poly(vinyl alcohol) in a mixed solvent of water/dimethyl sulfoxide, dimethyl sulfoxide/urea, or water/methanol.

Preferential adsorption of the additive is not a prerequisite for cononsolvency in water-rich mixtures

NMR studies reveal the distinct molecular interactions accounting for cononsolvency.

Buffers more than buffering agent: introducing a new class of stabilizers for the protein BSA

In this study, we have analyzed the influence of four biological buffers on the thermal stability of bovine serum albumin (BSA) using dynamic light scattering (DLS).

1H MAS NMR studies of the phase separation of poly(N-isopropylacrylamide) gel in binary solvents

Preferential interactions of solvents with poly(N-isopropylacrylamide) (PNIPAM) gel networks in binary water/alcohol (water/methanol and water/ethanol) mixtures have been investigated using variable-temperature high-resolution 1H MAS NMR. NMR results for PNIPAM gel in the binary solvents reveal the existence of two distinct types of water/alcohol mixtures above the LCST: confined binary solvents bound inside the gel, and free binary solvents expelled from the gel. It is interesting to find that the alcohol concentration in confined solution is significantly higher than that in free solution. Moreover, of the two alcohols, ethanol is more significantly concentrated in the confined solution. These results demonstrate that the polymer preferentially interacts with alcohol molecules over water and that the alcohol with higher hydrophobicity exhibits higher preferential absorption on PNIPAM. Our results also show that 1H NMR measurements made on two distinct types of solution provide a convenient, direct means of characterizing the preferential adsorption of solvent on polymer.