Shear-induced demixing in polystyrene/poly(vinyl methyl ether) blends. 1. Early stages of shear-induced demixing

Light scattering has been used to investigate the effect of shear-induced d emixing (SID) in polystyrene/poly(vinyl methyl ether) blends of diverse mol ecular weight distributions down to -30 K below the quiescent spinodal temp erature of any blend. The early stage of the demixing process, observed alo ng the vorticity direction, presents features remarkably similar to spinoda l decomposition. These include an initial exponential increase of scattered intensity with time and a maximal growth rate for q = q(m). This demixing occurs after a shearing time tau(d), which decreases as shear rate increase s. The shear-induced demixing process appears to be strongly dependent on t he applied shear rate, the experimental temperature, and the polydispersity of the components of the blend. For a given shear rate, this process is fa ster (higher growth rates and lower tau(d)) for blends with a higher polydi spersity and for lower experimental temperatures. These results allow quant ification of the SID process and give access to data which can test the the ories predicting SID by taking into account the effect of stored energy on the thermodynamics or the influence of shear flow on concentration fluctuat ions.