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 (theta(w)) on newly-created, cross-sectional surfaces produced by cutting prior to measurement. Values of theta(w) for N-NC gels were observed in the range of 100 degrees-131 degrees and changed depending on the composition, the environment and the measuring time. It was shown that hydrophobic surfaces (high theta(w)) are formed most effectively in N-NC gels with specific clay and water contents. Also, during long-term measurements, high values of theta(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 theta(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.