Laponite/poly(2-methyl-2-oxazoline) hydrogels: Interplay between local structure and rheological behaviour.

Hypothesis: Dispersions of Laponite in water may form gels, the rheological properties of which being possibly tuned by the addition of polymer chains. Laponite-based hydrogels with poly(ethylene oxide) (PEO) were the most widely investigated systems and the PEO chains were then found to reduce the elastic modulus.

Experiments: Here, hydrogels based on Laponite and poly(2-methyl-2-oxazoline) (POXA) were considered. The adsorption behavior and the local structures within these nanocomposite gels were investigated by small-angle neutron scattering and NMR. The same materials were macroscopically characterized using rheology.

Findings: An original evolution of the storage modulus G' with the POXA concentration is evidenced compared to Laponite/PEO hydrogels. At low POXA concentrations, a continuous reduction of G' is observed upon increasing the polymer content, as with PEO, due to the screening of electrostatic interactions between the clay platelets. However, above a critical value of the POXA concentration, G' increases with the polymer content. This difference with PEO-based hydrogels is correlated to the stronger affinity of POXA chains for the clay surfaces, which results in the reduction of the inhomogeneities for the Laponite disks within the gels. Steric repulsions would then counterbalance the effect of electrostatic repulsions and lead to the strengthening of the POXA-based hydrogels.