A re-entrant glass transition in colloidal disks with adsorbing polymer

A re-entrant colloidal glass transition has been observed in dispersions of a model disk shaped colloid, laponite^®, with the addition of an adsorbing low molecular weight polymer, poly(ethylene oxide) (PEO). In basic solutions laponite^® forms a repulsive colloidal glass. With the addition of low molecular weight PEO, we observe a retardation in formation of the glass, and the system behaves as a viscous fluid. Upon aging, a glassy phase re-forms, with the time scale depending on the concentration of polymer. For samples with polymer concentration high enough to saturate the laponite ^® surface, we believe this arrested state to be an attractive glass where excess PEO chains in the suspension lead to a weak interparticle attraction. For the polymer molecular weight we considered, all the samples eventually age to form a disordered state. We combine our results with previous work on laponite^®-PEO dispersions to propose qualitative state diagrams with attractive glass, repulsive glass, liquid and gel regimes as a function of polymer molecular weight, polymer concentration, and aging time. To our knowledge, the progression of repulsive glass-liquid-attractive glass states has not been previously observed in colloids with adsorbing polymers.