Synthesis and Characteristics of Nanocomposite Gels Prepared by In Situ Photopolymerization in an Aqueous System

Nanocomposite (NC) gels consisting of poly(N-isopropylacrylamide) (PNIPA) and inorganic clay were prepared by photoinitiated free-radical polymerization in the presence of exfoliated inorganic clay in an aqueous solution. To obtain NC gels with superior properties, very low concentrations (e.g., 0.1 to 0.4 wt % relative to a monomer) of a hydrophobic photoinitiator were used. The resulting photopolymerized NC gels (photo-NC gels) were uniform and transparent, irrespective of their clay and polymer contents. In contrast with chemically cross-linked hydrogels (photo-OR gels), which arc mechanically fragile, photo-NC gels exhibit excellent mechanical properties with large elongations at break of similar to 1000% and high tensile strength, that is, up to 50 times that of photo-OR gels. In addition, the modulus and the tensile strength could be controlled over a wide range by altering the clay content. From the analytical results and mechanical and swelling properties, it was concluded that photo-NC gels are composed of an organic (PNIPA)/inorganic (clay) network structure similar to that of NC gels prepared by redox-initiated free-radical polymerization and that the network structure is fabricated through the formation of clay-brush particles. Photo-NC gels exhibit a number of outstanding characteristics. They can exist in various forms including thin films and large monoliths with a wide range of thicknesses (10(-3)-10(2) mm) and can form thin coatings on a substrate. In addition, they can be patterned using photolithography, their sliding frictional behavior can be controlled by the thermosensitivity, cell harvesting can be achieved without enzymatic treatment, and microchannels can be fabricated with a controllable flow system.