Rheology of the Pluronic P103/water system in a semidilute regime: evidence of nonequilibrium critical behavior

The linear and nonlinear rheological behaviors of semidilute aqueous solutions of the amphiphile triblock polymer Pluronics P103 in water are reported here. For C(surf) < or = 20 wt%, micelles are spherical at temperatures lower than ca. 27 degrees C and grow with increasing temperature to form long polymer-like micelles. These polymer-like micelles exhibit strong viscoelasticity and a shear-banding region that shrinks as the cloud point is approached. Master time-temperature-concentration curves were obtained for the dynamic moduli using traditional shifting factors. In the nonlinear regime, P103 polymer-like micellar solutions follow the master dynamic phase diagram proposed by Berret and colleagues, in which the flow curves overlap in the low-shear-rate homogeneous flow region. Within the nonhomogeneous flow region (confirmed by flow birefringence and small-angle light-scattering measurements), oscillations and overshoots are detected at the inception of shear flow, and two main relaxation mechanisms are apparent after cessation of steady shear flow. Evidence for nonequilibrium critical behavior is presented, in which the order parameter is the difference of critical shear rates that limit the span of the plateau stress. Most of the steady-state and transient features of the nonlinear rheology of the P103 polymer-like micelles are reproduced with the Bautista-Manero-Puig (BMP) model, including the predictions of nonequilibrium critical behavior under flow.