Shear effects on the phase diagrams of solutions of highly incompatible polymers in a common solvent .2. Experiment and theory

Cloud point temperatures (T-cp) were measured at different constant shear rates for three representatives of the ternary system cyclohexanone/polystyrene/poly(n-butyl methacrylate) (CHO/PS/PBMA) by means of a newly constructed rheo-optical apparatus that can be operated in the temperature range from 0 to 100 degrees C up to maximum shear rates of 1440 s(-1) and maximum stresses of 384 Pa. In all cases one observes an extension of the homogeneous region as the shear rate gamma is raised. With the system CHO/PS 196w/PBMA 2050 (the figures denote the molar masses of the polymers in kilograms/mole) the effects become maximum for high concentrations of PBMA, where the demixing temperatures increase by more than 25 degrees C per 100 s(-1). For a pronounced predominance of one polymer in the mixture, T-cp is a linear function of gamma in the entire range of shear rates. At blend compositions in between, the slope of T-cp versus gamma is largest at the lowest shear rates and diminishes as gamma is increased until the dependence becomes linear again at sufficiently large values. Possible effects of polymolecularity were studied by exchanging the broadly distributed PS 196w against the narrowly distributed PS 207; no differences beyond experimental error could be detected. A substitution of the high molecular weight PBMA 2050 by the lower molecular PBMA 335 leads to a pronounced reduction of the effects; in this case the extent of shear-induced mixing passes a minimum for a blend composition of approximately 1:1. Phase diagrams of the flowing systems of interest were also calculated theoretically on the basis of a generalized Gibbs energy of mixing (value for stagnant solutions plus energy stored under stationary conditions in the sheared state) by direct minimization of this quantity. The information concerning equilibrium and rheological behavior required for that purpose was obtained as described in part 1 of this series. The sign and magnitude of all theoretically predicted effects plus their variation with molar masses and composition are in very good agreement with the experimentally observed features of shear-induced changes of the phase state. Possible reasons for the lack of complete quantitative agreement are discussed.