Polymer -clay dispersions as soft glassy materials: Rheology, dynamics and structure
Colloids and polymers are often used in industrial applications to obtain desired properties for formulations such as cosmetics, paints, household cleaners and adhesives. We have studied the rheological modifier laponite, a synthetic disc shaped clay with a diameter of 25 nm and thickness of about 1 nm. It carries a negative charge on the face, while the rim charge is pH dependent. For pH < 9 there is a positive charge along the rim, while at high pH the colloid has a net negative charge. The latter pH conditions lead to the formation of a colloidal glass.
Low laponite concentrations (cl ≤ 2 wt%) initially have a viscoelastic liquid response. The storage modulus becomes frequency independent and escalates over time while the loss modulus shows an upturn at low frequency, both of which are well predicted by the soft glassy rheology (SGR) model. At low cl clusters form due to weak attractions and interact as the constitute elements via a long range repulsion to form the glass. Dynamic light scattering, neutron scattering and rheology were used to study the melting of the glass with the addition of low molecular weight poly(ethylene oxide) (PEO). Excess small polymer chains in solution cause a depletion force resulting in a low viscosity liquid with fast dynamics. Ultra small angle neutron scattering and rheology verify re-entrant behavior with the addition of high molecular weight PEO. We believe a viscoelastic solid is re-formed due to polymer chains bridging between laponite particles. This is the first evidence of re-entrant type behavior in anisotropic colloids.
At high cl (3 wt%) the system is immediately frustrated and a glass is formed due to repulsive interactions between individual particles resulting in G' aging. The aging of G' can be suppressed and tuned with the addition of high molecular weight PEO. Above a critical ratio, ϕ, of the total polymer surface area to total laponite surface area, the PEO dynamics dominate at high frequencies. Thus, it appears that the only parameter needed to tune the rheology of these complex laponite-PEO systems is ϕ.