Reentrant Behavior of Poly(N-isopropylacrylamide) Brushes in Water-Methanol Mixtures Investigated with a Quartz Crystal Microbalance

The solvent composition induced reentrant behavior of poly(N-isopropylacrylamide) (PNIPAM) chains grafted on a SiO₂ surface in water-methanol mixtures was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D) at 20°C. The frequency and energy dissipation responses showed that the grafted PNIPAM chains sharply collapse when the methanol content (x_m) reaches about 17 mol %. In the range 17-50 mol %, the grafted chains remain in a collapsed state. Further increase of the methanol content leads to an abrupt reswelling of the collapsed chains at x_m > about 50 mol %. The sharp reentrant swelling-to-collapse-to-swelling transition was attributed to the water-methanol complexation instead of the preferential adsorption effect. Our results also suggest that the water-methanol complexation is not induced by hydrophobic interaction but by hydrogen bonding.