Rheo-Optical Studies of the Effect of Shear Flow on the Structure of Elastomer Blends.
The effect of shear flow on the structure of a phase-separated, near-critical blend of 50/50 (w/w) poly(styrene-ran-butadiene) and polybutadiene was studied with two different custom-built rheo-optical instruments that combined polymer melt flow and small-angle light scattering (SALS). The deformation of the phase domains during shear flow was nonaffine, and the SALS patterns evolved from a spinodal ring (SR) pattern to a squashed SR with two high-intensity lobes, to an H-pattern, to a butterfly pattern with a dark streak along the equator, and finally to a steady-state, elliptical pattern. The SALS patterns were explained in terms of a network model, in which the strands of the network first orient in the flow direction, then extend in this direction, and finally break up into droplets aligned in the flow direction. According to this picture, the strands in the vorticity direction do not deform until relatively high strains, after which the periodicity of the network begins to disappear. Supporting this model was the observation that the transitions between the different SALS patterns corresponded to inflections and/or maxima in the shear stress or first normal stress difference. Increasing the shear rate changed the kinetics of the structure evolution and reduced the size of the phase-separated droplets in the steady state. No evidence was obtained for flow-induced miscibility. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1725–1738, 2004