C. Krause, R. Horst and B. A. Wolf

"Shear Effects on the Phase Diagrams of Solutions of highly Incompatible
Polymers in a Common Solvent 2: Experiment and Theory"

*Macromolecules* **1997**, *30*, 890-895.

**Abstract:** 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.

preprint number: 173