Study on conformation change of thermally sensitive linear grafted poly(N-isopropylacrylamide) chains by quartz crystal microbalance

The temperature-induced dehydration and hydration of linear thermally sensitive poly(N-isopropylacrylamide) (PNIPAM) chains grafted on a gold surface in water were measured in real time by using a quartz crystal microbalance. Our results reveal a gradual dehydration process during the heating from 20 to 35 degreesC. The dehydration starts at similar to23 degreesC, much lower than the well-known coil-to-globule transition temperature (similar to32 degreesC) of individual PNIPAM chains free in water, suggesting that dehydration occurs when PNIPAM chains are at the crumpled state. On the other hand, in the cooling process, there exists a hysteresis in the hydration, and such hysteresis only disappears at similar to20 degreesC, at which water becomes a sufficiently good solvent. This is because some additional hydrogen bonding formed in the collapsed state at higher temperatures could not be completely removed as long as the temperature is not far away from the Theta-temperature (similar to30 degreesC).