  The Flory chi parameter is typically assumed to depend only on the
temperature, T. Experimental results often require the replacement of this
chi(T) by chieff, that depends also on the monomer volume fraction, phi,
chieff(phi,T). Such chieff(phi,T) can arise from two state-models, proposed for
polyetheleneoxide (PEO) and other neutral water-soluble polymers. The predicted
phi dependence of chibar=chieff-(1-phi)\partial \chieff/\partial phi,
obtainable from colligative properties, differs qualitatively between the
various models: (i) The model of Karlstrom (J. Phys. Chem. 1985, 89, 4962)
yields \partial chibar/\partial phi > 0 while the model of Matsuyama and Tanaka
(Phys. Rev. Lett. 1990, 65, 341) and of Bekiranov et al (Phys. Rev. E 1997, 55,
577) allows for \partial chibar/\partial phi <0 (ii) chibar(phi) as calculated
from the Karlstrom model, utilizing the parameters used to fit the phase
diagram of PEO, agrees semiquantitatively with the experimental values. On the
other hand, chibar(phi) similarly calculated from the model of Bekiranov et al.
differs qualitatively from the measured results. Altogether, chibar(phi)
provides useful measure for the performance of a model.
