Direct observation of interfacial profiles of polymer gels during the phase transition by Raman microimaging

The interface profiles between N-isopropylacrylamide (NIPA) hydrogel and wa ter have been directly observed using Raman microimaging technique. As the NIPA hydrogel undergoes the volume phase transition, it is found that the i nterface becomes thicker and smoother. Specifically, the average interfacia l thickness of the NIPA gel at room temperature is about 37 mum, but increa ses to 65 mum at 37 degreesC, the temperature that is higher than the phase transition temperature of T-c = 34 degreesC. The thicker interface may be related to a dense shell formation during the gel shrinking process. The de crease in surface roughness is probably related to hydrophobic properties o f the gel at T > T-c. The repulsive energy between water and the polymer is minimized when the total surface of the interface is reduced, a process, w hich results in a smoother surface. (C) 2000 Elsevier Science Ltd. All righ ts reserved.