Hydrophobic surface characteristics of nanocomposite hydrogels

The surface of nanocomposite hydrogels (NC gels), consisting of poly(N-isopropylacylamide) (PNIPA), exfoliated clay (hectorite) and a large amount of water, showed an extraordinarily high contact angle for water (θ_w) at ambient temperature, despite the fact that all of its constituents are hydrophilic under this condition. The θ_w value varied within the range 100° ∼150°, depending on composition, i.e. the water content and the clay concentration. High θ_w values were stable in short-term measurements (≤ 10 s), but during long-term measurements ( ∼20 min), they underwent unique changes which strongly depended on the clay concentration (i.e. the network density). The unusually high hydrophobicity was only observed in PNIPA-based NC gels and was attributed to the alignment of the N-isopropyl groups at the gel-air interface. Also, the hydrophobic surface of the NC gel changed to hydrophilic when the gel came into contact with water and rapidly reverted to hydrophobic on subsequent drying. Furthermore, it was found that the high θ_w values of the PNIPA-NC gel were hardly affected by modifications of the network through copolymerization with or interpenetration of a hydrophilic polymer. The adsorption of a hydrophilic (cationic) dye also hardly affected the high θ_w value.