Phase Separation and Gelation in Solutions of A–B Associative Polymers

An equilibrium theory for reversible network formation in two-component solutions of associative polymers is presented to account for the phase behavior due to hydrogen bonding, metal–ligand, electrostatic, or other pairwise associative interactions. We consider polymers of types A and B with many associating groups per chain and consider only A–B association between the groups. A simple analytical expression for the free energy is derived and is shown to be consistent with the classical Flory–Stockmayer gelation theory. It is shown that association and formation of a reversible network is always accompanied by a tendency for phase separation, even at good solvent conditions, a significant difference from self-associative polymers. Homogeneous networks are most easily stabilized near stoichiometric conditions between A and B associative groups, resulting in a sol–gel–sol transition as the overall composition is altered. Chemical incompatability between the A and B polymers drives a competition between attractively and repulsively-driven phase separation, leading to microphase formation and eutectic behavior.