Graphene derivatives in responsive hydrogels: Effect of concentration and surface chemistry

Reduced graphene oxide (RGO) containing composite hydrogels, based on poly(N-isopropyla-crylamide) (PNIPA) were prepared by two different methods: (i) by incorporating RGO directly into the polymer matrix; (ii) applying a post-synthesis reduction of the graphene-oxide (GO) already incorporated into the polymer. The samples were compared by various microscopic (small angle neutron scattering, differential scanning calorimetry, H-1 NMR spectroscopy, thermogravimetry) and macroscopic (kinetic and equilibrium swelling properties and mechanical testing) techniques. Results from microscopic and macroscopic measurements show that the dispersity of the nanoparticles as well as their interaction with the polymer chains are influenced by their surface chemistry. Incorporation of nanoparticles limits the shrinkage and slows down the kinetics of the thermal response. Both thermogravimetric and solid-state NMR measurements confirmed strong polymer - nanoparticle interaction when hydrophilic GO was used in the synthesis. In these cases, the slow thermal response may be explained by the decrease of the free volume inside the nanocomposite matrix caused by a hypernodal structure. Our results imply that both the chemistry and the concentration of incorporated graphene derivatives are promising in tuning the thermal responsivity of PNIPA.