Hydrogels with hydrophobic surfaces: Abnormally high contact angles for water on PNIPA nanocomposite hydrogels

The surfaces of polymeric nanocomposite (NC) hydrogels consisting of hydrophilic constituents such as poly(N-isopropylacrylamide) (PNIPA), hydrophilic clay, and water, were found to exhibit high contact angles. The PMIPA-NC gels were prepared by free-radical polymerization of NIPA monomer in the presence of exfoilated clay uniformly dispersed in aqueous medium. The sessil-drop contact angle measured at the surfaces of N-NC6 gels shows a distinctly hydrophobic surface for the N-NC6 gel despite the hydrophilic nature of all the constituents. A gradual decrease of contact angle observed for all samples is due to the evaporation of water from droplets under the same conditions of a constant solid-liquid contact area. The results also show that the rough surface of NC hydrogels are effective in producing ultrahigh values of contact angle and the high contact angles on the surfaces of N-NC gels are derived from the combined effects of amphiphilicity of PNIPA and network structure.