Shear flow and large strain oscillation of dense polymeric micelle suspension

Poly(ethylene oxide) (PEO) hydrophobically end-capped with an octadecyl group forms spherical micelles in water that resemble multiarm star polymers. The micelles jam (gel) at high concentrations after cooling below a critical temperature showing a discontinuous liquid-solid transition. The rheology of jammed micelle suspensions was studied utilizing both shear flow and oscillation. The flow rate was found to decrease with decreasing stress following a power law down to at least 10(-5) s(-1), and neither a yield stress nor a flow rate independent viscosity was observed. Shear flow induces a first normal stress difference that is proportional to the shear stress and relaxes exponentially after cessation of flow. When the shear stress approaches a critical value, the flow rate increases more sharply and becomes independent of the shear stress at the critical value. The response to oscillatory shear is linear only for deformations less than 1%. The nonlinear response to large strain oscillation can be viewed as a superposition of the linear viscoelastic response and the strongly shear stress dependent flow. At a given shear stress the flow rate increases with increasing temperature.