Small-Angle Neutron Scattering Study of Shear-Induced Phase Separation in Aqueous Poly(N-isopropylacrylamide) Solutions

The influence of shear flow on the structure of concentrated aqueous poly(N-isopropylacrylamide) solutions near the lower critical solution temperature was investigated by means of small-angle neutron scattering. Two samples, both in the semi-dilute regime above the overlap concentration, were studied. The scattering curve of the less concentrated sample was not influenced by shear flow, although high shear rates were reached. The more concentrated 4 wt.-% sample, however, displayed shear-induced demixing under strong shear flow conditions. Experiments at different shear stresses indicated the existence of a threshold shear stress and the phase separation process became faster with increasing stress. The two-dimensional scattering patterns remained isotropic even during the phase separation process and the correlation length as obtained from an Ornstein- Zernike plot increased. The influence of shear flow on the phase separation process is thus similar to a temperature increase. The results are in excellent agreement with data from recent rheo-optical experiments where shear-induced phase separation was also observed for the concentrated solution at high shear rates. Apparently, strong shear flow exerts an effect analogous to a temperature increase.