Shear flow and the phase behaviour of polymer blends

The effect of simple shear flow on the phase diagram of partially miscible polymer blends has been analysed through the use of a generalised Gibbs energy of mixing, modified by a stored energy term. The latter is defined as the energy stored by the polymeric molecules during flow, and is seen to have a significant influence on the magnitude and direction of the change in the phase boundary caused by shear. Computer simulations indicate that flow induces mixing when the excess stored energy is negative and phase separation when the excess stored energy is positive. The dependence of the stored energy on the viscosity of the polymer blend was investigated for seven model blends exhibiting different viscosity-composition behaviours. A negative deviation of the viscosity-composition curve from the linear mixing rule was found to be a sufficient condition for a negative excess stored energy, resulting in shear-induced mixing. This prediction is in qualitative agreement with the experimental results on the effect of shear on the phase diagram of poly(styrene-co-acrylonitrile)/poly(methymethacrylate) blends. In contrast, a positive deviation of the viscosity-composition curve from additivity was found to be a necessary but not sufficient condition for a positive excess stored energy which would cause shear-induced phase separation. This analysis seems to indicate that the viscosity-composition behaviour of a polymer blend could be used to qualitatively anticipate the change in the phase diagram of a polymer blend upon flow.