Long-term aging effects on the rheology of neat laponite and laponite-PEO dispersions

We observe aging behavior of neat laponite systems over the course of 1,000 or more days. Under basic conditions, low laponite concentrations (1 wt%) slowly evolve from a viscoelastic liquid to a glass made of clusters acting as constituent elements interacting via long-range repulsion. Higher concentrations of laponite (3 wt%) quickly form a glass of individual particles. Intermediate concentrations of laponite form a glass that is a combination of clusters and individual particles. The aging rheological response and upturn of the loss modulus at low frequencies are well predicted by models of soft glassy systems (Fielding et al., J Rheol, 44(2):323-369, 2000; Sollich, Phys Rev E, 58(1):738-759, 1998). If low amounts of high-molecular-weight (M (n >=)163 kg/mol) poly(ethylene oxide) (PEO) are added, the aging behavior follows the dynamical response of the clay. Above a critical ratio, phi, of the free polymer chains in solution to the total laponite surface area, the PEO dynamics dominate at high frequencies. It appears that the dynamics of these complex laponite-PEO systems are governed by the parameter phi.