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

The interface profiles between N-isopropylacrylamide (NIPA) hydrogel and water have been directly observed using Raman microimaging technique. As the NIPA hydrogel undergoes the volume phase transition, it is found that the interface becomes thicker and smoother. Specifically, the average interfacial thickness of the NIPA gel at room temperature is about 37 μm, but increases to 65 μm at 37°C, the temperature that is higher than the phase transition temperature of Tc=34°C. The thicker interface may be related to a dense shell formation during the gel shrinking process. The decrease in surface roughness is probably related to hydrophobic properties of the gel at T>Tc. The repulsive energy between water and the polymer is minimized when the total surface of the interface is reduced, a process, which results in a smoother surface.