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.
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