STUDY OF SHEAR THINNING IN HIGH POLYMER-SOLUTION USING DYNAMIC NMR MICROSCOPY

Dynamic NMR imaging has been used to measure velocity profiles for water solutions of WSR301 poly(ethylene oxide) in laminar flow through a 700-mu-m-i.d. capillary as a function of concentration and pressure gradient. A transition from Poiseuille flow to power law shear thinning is apparent as the concentration is increased above c*. Equilibrium self-diffusion coefficients have been measured as a function of concentration for WSR301 and as a function of molar mass for monodisperse poly(ethylene oxide) standards in D2O, and it is apparent that the data are consistent with the usual reptative scaling behavior. A measurement of self-diffusion in the presence of the flow field indicates that shear thinning is associated with significant enhancement of the polymer Brownian motion along the axis of shear, and these data are interpreted by using a simple adaptation of the entanglement/blob model.