Copolymer nanocomposite hydrogels: Unique tensile mechanical properties and network structures

Copolymer-based nanocomposite hydrogels (NC gels) were synthesized through in situ copolymerization of two monomers, N-isopropylacrylamide (N) and N,N-dimethylacrylamide (D), in the presence of inorganic clay in aqueous solutions. The thermoresponsive transition of N-copolymer NC gels shifted toward higher (lower) temperature by copolymerization with hydrophilic (hydrophobic) monomers. A specific copolymer-NC gel (N/D-NC gel) showed an abnormal enhancement in tensile mechanical properties as compared with the corresponding homopolymer-NC gels. The swelling properties and glass transition temperature of the N/D-NC gels also showed a different trend compared to those of the other copolymer-NC gels. These unusual changes in the gel properties were observed at all clay concentrations, and could be attributed to the formation of a unique organic/inorganic network structure consisting of physical crosslinks due to the exfoliated clay and chemical crosslinks due to self-crosslinking of the N/D copolymer. The formation of a self-crosslinked network was also confirmed by the clay-decomposition technique.