Hydrogels exhibiting hydrophobicity -high water contact angle and its change on the surface of nanocomposite hydrogel

Surface wettability is one of the most important properties, since it reflects the real structure and chemical compositions at the outer-most surface. We recently developed a novel class of nanocomposite hydrogels (NC gels) with a unique organic / inorganic network structure. Here, we report extraordinarily high hydrophobicity, i.e. abnormally high water contact angle (CA), 100° - 150°, observed at the surface of NC gels consisting of hydrophilic constituents; poly(N-isopropylacrylamide), hydrophilic clay and a large amount of water. High values of θ_W are observed for hydrogels with broad ranges of water and clay contents, and are comparable to or higher than those of well known hydrophobic solid polymers, such as polytetrafluoroethylene (PTFE: CA=110°) and polypropylene (PP: CA=85°). The origin of these high values of CA and their stability are discussed on the basis of gel composition and the surface structure at the gel-air interface.