Surface roughness of polymer gels

Polymer gel surfaces in water have been studied by using an atomic force microscopy (AFM) and it was established that the surface conformation is characterized by the spongelike domains with submicrometer scale. The surface structure and roughness of polymer gels depend not only on the nature of the system but also on the preparation condition: the domain of the hydrophilic network is much smaller than that of the hydrophobic one, and a gel prepared in a poor solvent forms a rough surface with much larger domains than that prepared in a good solvent. Surface roughness of a gel can change in response to the external condition: when the bulk polymer network enters in the collapsed phase, the conformation of the spongelike domains on the surface will change, but it can be restored when the gel swells again through the volume phase transition. Surface roughness due to the spongelike domains was discussed in terms of the autocorrelation function and of the power spectral density calculated from the topographies. The morphology change is consistent with the hypothetical model of two-dimensional gels based on the Flory-Huggins theory.