Binding interaction and gelation in aqueous mixtures of poly(N-isopropylacrylamide) and hectorite clay.

The binding interaction between poly(N-isopropylacrylamide) (PNIPAm) chains and hectorite clay platelets was directly detected by quartz crystal microbalance with dissipation (QCM-D) to explore the cross-linking mechanism in the strong nanocomposite hydrogel (NC gel), which is in situ polymerized with NIPAm in the clay suspension. PNIPAm chains were allowed to be adsorbed on the gold surface of the QCM electrode. A large frequency shift Δf in the QCM as introducing the clay indicated that a large amount of clay platelets were adsorbed on the deposited PNIPAm layer. The relationship between the dissipation shift ΔD and Δf revealed that the adsorption included two steps of fast initial buildup and following densification of the clay platelets. In dilute aqueous mixtures, the PNIPAm chain and clay formed aggregates as observed from the hydrodynamic diameter. A gelation state diagram was established for concentrated aqueous PNIPAm-clay mixtures. The Raman spectrum pointed out the conformation change of the PNIPAm chains in aqueous solutions when the clay was added, which would be caused by the adsorption of PNIPAm chains to the clay platelets.