Solvents Triggered Coil-to-Globule-to-Coil Transition of Dual Nanocomposite Hydrogels with Inorganic Hybrid Crosslinking

In this paper, the reentrant solvation of dual nanocomposite hydrogel poly-N-isopropylacryl-amide/Laponite/SiO2 (PNIPAM/Laponite/SiO2) upon shrinkage/reswelling process has been investigated. Depending on the unique hierarchical microstructure of inorganic hybrid crosslinking of Laponite and SiO2 as well as the preferential interaction of polar solvents with PNIPAM chains, the hydrogel exhibited rapid coil-to-globule-to-coil transition in water-polar solvent mixtures. The solvation behavior could be controlled through varying types of organic solvents. Shrinkage in water-polar solvent mixtures occurred as a conse-quence of strong interaction between polar solvents and PNIPAM chains, whereas reswelling resulted from the direct interaction of the solvent molecules with the intermolecular water in the hydrogel. The attractive competing effects on forming hydrogel-water and hydrogel-polar solvent hydrogen bonds were considered to be indispensable to the solvation. The rapid response rate was attributed to the synergistic effect of the unique heterogeneous microstructure with inorganic hybrid crosslinking and preferential interaction of polar solvents with polymer chains. The mechanism proposed in this paper provides a new reference on design of smart soft matter systems. Moreover, several solvation effects described in this paper can be incorporated in theory of cononsolvent-induced conformational transitions in the nanocomposite hydrogels with inorganic hybrid crosslinking.