Direct observation of polymer gel surfaces by atomic force microscopy

We report here, for the first time, the direct observation of the submicron structure of gel surfaces in water by using an atomic force microscope (AFM). We present also its change in response to external stimuli; we investigated, among the variables that affect the topography of the gel surface, the effect of the network density of poly(acrylamide) gels and the effect of the temperature change of poly N-isopropylacrylamide gels. Gels were prepared with disklike shape of thickness ranging from 10 to 50 mu m, and one of the gel surfaces was chemically adhered onto a glass plate. Spongelike domains of submicrometer scale were found here on the gel surfaces, which was strongly affected by the cross-linking density (nature of the gel network) as well as the osmotic pressure (environmental condition), and also thickness (condition of constraint). The qualitative properties of the surface microscopic structure of gels are discussed in relation to a hypothetical model of two-dimensional gels based on the Flory-Huggins theory. These results disclose that the surface microstructures of polymer gels in solvent as well as the nanometer scale structural changes are associated with the gel phase transition. Moreover, they indicate that the potential for a new technology to control the domain size of the gel surface as well as its function by external stimuli could emerge, which would find a variety of applications in many fields, such as engineering, medicine, and biology. (C) 1996 American Institute of Physics