Shear flow behavior of a dynamically symmetric polymeric bicontinuous microemulsion

We have investigated the shear flow behavior of a dynamically symmetric polymeric bicontinuous microemulsion using rheology and in situ small angle x-ray scattering. The microemulsion consists of a ternary blend of poly (ethylene-alt-propylene) (PEP), poly (butylene oxide) (PBO), and a PEP-PBO diblock copolymer. Steady shear experiments reveal an unusual shear thickening behavior at the onset of the non-Newtonian regime, which is consistent with the strain hardening and frequency thickening (at large strains) under oscillatory shear. Scattering experiments indicate development of anisotropy in the bicontinuous structure within the thickening regime. Subsequent shear thinning is observed at intermediate shear rates. Shear-induced bulk phase separation is detected at very high rates. This work complements previous studies on a dynamically extremely asymmetric bicontinuous microemulsion, and thereby establishes the universal rheological properties of polymeric microemulsions. Possible underlying molecular mechanisms for these rich rheological phenomena are discussed.