Reentrant Behavior of Poly(N-isopropylacrylamide) Br

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 (xm) reaches 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 reswel领 of the collapsed chains at xm > 50 mol %. The sharp reentrant swel领-to-collapse-to-swel领 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.