Nanocomposite Gels by Initiator-Free Photopolymerization: Role of Plasma-Treated Clay in the Synthesis and Network Formation

Nanocomposite hydrogels (NC gels) composed of polymer–clay networks are successfully synthesized via initiator-free (IF) photopolymerization in aqueous media using plasma-treated clay. IF-NC gels with a high tensile strength, strain at break, and thermoresponsiveness, which are almost identical to those of NC gels prepared by redox and photoinitiator methods, are obtained by optimizing the plasma-treatment conditions and exfoliation in water. The clay nanosheets play important roles as initiators, multifunctional cross-linkers, and auxiliary agents for facilitating in situ free-radical polymerization toward IF-NC gels. The mechanism for the formation of the polymer–clay network in the IF-NC gels is clarified through Fourier transform infrared and electron spin resonance studies and involves the formation of hydroperoxides and radicals on the clay surface by plasma treatment and subsequent UV irradiation, respectively, and through designed free-radical polymerization experiments in the presence or absence of the clay nanosheets. Because of their simple and versatile syntheses, form and size diversities, and superb characteristics, IF-NC gels can be used in a variety of applications. The results provide important insight into super-hydrogels and organic–inorganic nanocomposites, as well as polymer synthesis and photochemistry.