  The quality of solvents of polymers is often described in terms of the Flory
chi parameter typically assumed to depend only on the temperature, T. In
certain polymer-solvent systems fitting the experimental data enforces the
replacement of chi(T) by a concentration dependent chieff. In turn, this
modifies the swelling and collapse behavior. These effects are studied, in the
framework of a mean-field theory, for isolated coils and for planar brushes.
The phi dependence of chieff gives rise to three main consequences: (i) Shift
in the cross-over between Gaussian and self-avoidance regimes; (ii) A
possibility of first-order collapse transition for isolated flexible coils;
(iii) The possibility of a first-order phase transition leading to a vertical
phase separation within the brush. The discussion relates these effects
directly to thermodynamic measurements and does not involve a specific
microscopic model. The implementation for the case of
Poly(N-isopropylacrylamide) (PNIPAM) is discussed.
