Signatures of a Concentration Dependent Flory chi Parameter: Swelling and Collapse of Coils and Brushes

TL;DR

This paper explores how a concentration-dependent Flory chi parameter influences polymer coil and brush behavior, revealing potential phase transitions and collapse phenomena through mean-field theory.

Contribution

It introduces a mean-field framework incorporating a concentration-dependent chi parameter to analyze polymer swelling and collapse, highlighting new phase transition possibilities.

Findings

Concentration dependence of chi affects coil and brush swelling.

First-order collapse transition for isolated coils is possible.

Vertical phase separation can occur within polymer brushes.

Abstract

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.