In-Situ Interferometry Studies of the Drying and Swelling Kinetics of an Ultrathin Poly(N-isopropylacrylamide) Gel Film below and above Its Volume Phase Transition Temperature

The temperature influence on the drying and swelling kinetics of ultrathin poly( N -isopropylacrylamide) gel film were studied by in- situ interferometry. The drying rate is greatly dependent on temperature. When T ≲ 31 °C, the original gel film was swollen and contained more than 99% of water before drying. Our results showed that the initial drying rate was a constant. However, it became faster and faster toward the end of the drying. On the other hand, when T ≳ 32 °C, the gel film was in the collapsed state, but still contained ∼70% of water, before drying. Three distinct drying processes were observed, namely, a very fast initial shrinking, a slow transition period, and a normal drying process similar to that of the gel film when T ≲ 31 °C. As for the swelling process, it can be better described by the first-order kinetics developed by Li and Tanaka. The cooperative diffusion coefficient and the ratio of the shear modulus to the longitudinal modulus were determined from a least-squares fitting of the swelling experimental data according to the swelling equation for a large disk. A critical slowing down of the gel swelling for the thin film was observed in the range of 33−35 °C.