The effect of particle-scale dynamics on the macroscopic properties of disk-shaped colloid-polymer systems

Particle-scale rearrangements occurring in mixtures containing anisotropic disk-shaped colloids and polymers often lead to the development of unusual viscoelastic features that can play a crucial role in applications. Here, we report the early aging behavior of a series of mixtures containing laponite, a model disk-shaped colloid, and poly(ethylene oxide) (PEO) chains of three different molecular weights, 35. kg/mol, 20. kg/mol and 4.6. kg/mol. The concentrations of PEO were varied within their respective dilute and semi-dilute un-entangled regions. Rheological experiments were utilized to describe the effects of chain number and length on the macroscopic behavior of suspensions and dynamic light scattering (DLS) was used as a tool to establish a correlation between the macroscopic behavior and dynamics seen at smaller length scales. For dilute concentrations of PEO, the rheology is consistent with a glass-liquid-glass phase transition, a trend that resembles the re-entrant behavior observed in many colloid-polymer systems. However, in more concentrated PEO solutions, samples re-stabilize and remain stable for a longer period of time. Interestingly, at lower length scales the stabilization that is seen in the concentrated region is characterized by an increase in the first and second relaxation timescales, suggesting that tightly bound stable clusters diffuse through the medium. To our knowledge, this type of behavior in an anisotropic colloid-polymer system has not been previously observed.