Interferometric imaging of hydrogels undergoing volume changes

A polyacrylamide hydrogel undergoing a volume change was studied by using i nterferometric imaging. The method produces the radial distribution of the matrix density for the cylindrical hydrogel at different times during the v olume transition caused by solvent change. When the water-swollen gel was i mmersed in a water-methanol mixture, contraction of the gel matrix occurred first in the edge and propagated to the center. The edge did not reach the equilibrium density immediately after the solvent change. The gel contract ed rather evenly, although the edge maintained a higher density during the change. After reaching the uniform density, the center of the gel contracte d further whereas the edge remained unchanged. A similar pattern was observ ed in the. reswelling in pure water. These findings indicate that the gel s tudied had a shear modulus not small compared with its bulk modulus.