Huan-Jun Li

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.

The unique optical and physical properties of soft, transparent, stimulus-sensitive nanocomposite gels

A new type of polymer hydrogel with a unique organic (polymer)/inorganic (clay) network structure has been synthesized by the in-situ free-radical polymerization of N-isopropyl acrylamide (NIPA) in the presence of exfoliated clay platelets in an aqueous medium. The resulting nanocomposite hydrogels (NC gels) consisting of PNIPA and clay (hectorite) exhibit extraordinary optical, mechanical and swelling properties. NC gels also show a clear phase transition due to the coil-to-globule transition of the PNIPA chains. It was observed that the phase-transition temperature (lower critical solution temperature: LCST), defined as the onset temperature of a steep transmittance drop, shifts to a lower or higher temperature than that of pure water (≅ 34 °C) when conditions are altered. When an inorganic salt, such as NaCl, CaCl2 and AlCl3, was added to the surrounding water, the LCST of the NC gels generally shifted to a lower temperature, in a manner almost inversely proportional to the salt concentration. On the other hand, when the NC gels adsorbed cationic surfactant, e.g. hexadecyl trimethyl ammonium chloride, the LCST shifted toward a higher temperature, although the shift and its profile strongly depended on the adsorption conditions, such as the surfactant concentration and the adsorption time. Consequently, non-thermo-sensitive NC gel was obtained by using a surfactant aqueous solution with a concentration higher than the critical micelle concentration.