Signatures of a concentration dependent Flory chi parameter: swelling and collapse of coils and brushes

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V. A. Baulin and A. Halperin

Macromolecules, 35, 6432 – 6438 (2002)

The quality of solvents of polymers is often described in terms of the Flory χ parameter typically assumed to depend only on the temperature, T. In certain polymer-solvent systems fitting the experimental data enforces the replacement of (χT) by a concentration-dependent χeff. 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 ϕ dependence of χeff 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-isopropylamide) (PNIPAM) brushes is discussed.