Unusually high hydrophobicity and its changes observed on the newly-created surfaces of PNIPA/clay nanocomposite hydrogels

The surface wettability of cross-sections of polymeric hydrogels was studied, focusing particularly on poly(N-isopropylacrylamide) (PNIPA) hydrogels below their lower critical solution temperature (LCST). It was found that nanocomposite hydrogels (N-NC gels) with organic (PNIPA)/inorganic (clay) network structures exhibit extraordinarily high contact angles for water (θ_w) on newly-created, cross-sectional surfaces produced by cutting prior to measurement. Values of θ_w for N-NC gels were observed in the range of 100°-131° and changed depending on the composition, the environment and the measuring time. It was shown that hydrophobic surfaces (high θ_w) are formed most effectively in N-NC gels with specific clay and water contents. Also, during long-term measurements, high values of θ_w showed unique changes which strongly depended on the clay concentration (i.e. network density). Further, the hydrophobic surface of N-NC gels changed to hydrophilic in contact with surface water and rapidly reverted to hydrophobic on subsequent drying. Also, contrary to the conventional hydrophobic surfaces of solids, a water droplet on the hydrophobic surface of an N-NC gel did not fall, even on a vertical surface, because of the strong interaction between the droplet and the gel surface. The mechanism for creating high values of θ_w was attributed to the amphiphilicity of PNIPA chain in PNIPA/clay networks below the LCST and, more specifically, to the spontaneous alignment of N-isopropyl groups of PNIPA chains at the gel-air interface.