IN-SITU INTERFEROMETRY STUDIES OF THE DRYING AND SWELLING KINETICS OFAN ULTRATHIN POLY(N-ISOPROPYLACRYLAMIDE) GEL FILM BELOW AND ABOVE ITSVOLUME PHASE-TRANSITION TEMPERATURE

The temperature influence on the drying and swelling kinetics of ultra thin poly(N-isopropylacrylamide) gel film were studied by in-situ inte rferometry. The drying rate is greatly dependent on temperature. When T less than or similar to 31 degrees C, the original gel film was swol len and contained more than 99% of water before drying. Our results sh owed that the initial drying rate was a constant, However, it became f aster and faster toward the end of the drying. On the other hand, when T greater than or similar to 32 degrees C, the gel film was in the co llapsed state, but still contained similar to 70% of water, before dry ing. Three distinct drying processes were observed, namely, a very fas t initial shrinking, a slow transition period, and a normal drying pro cess similar to that of the gel film when T less than or similar to 31 degrees 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 longit udinal modulus were determined From a least-squares fitting of the swe lling experiment at data according to the swelling equation for a larg e disk. A critical slowing down of the gel swelling for the thin film was observed in the range of 33-35 degrees C.