Shear-induced phase separation in polyelectrolyte/mixed micelle coacervates

A quantitative study of the shear-induced phase separation of a polycation/anionic-nonionic micelle coacervate is presented. Simultaneous rheology and small-angle light scattering (SALS) measurements allow the elucidation of micrometer-scale phase separation under flow in three coacervate solutions. Below 18 degrees C, all three of the coacervate solutions are optically clear Newtonian fluids across the entire shear rate range investigated. Once a critical temperature range and/or shear rate is achieved, phase separation is observed in the small-angle light scattering images and the fluid exhibits shear thinning. Two definitive SALS patterns demonstrate the appearance of circular droplets at low shear rates near the critical temperature and ellipsoidal droplets at higher temperatures and shear rates. The shear-induced droplets range in size from approximately 1 to 4 mum. The ellipsoidal droplets have aspect ratios as high as 4. A conceptual picture in which shear flow extends the polyelectrolyte chains of the clear coacervate liquid phase is proposed. The extended chains create interpolyelectrolyte-micelle interactions and promote expulsion of small ions from the complex, resulting in the formation of micrometer-scale phase-separated droplets.