A One-Component Model for the Phase Behavior of Dispersions Containing Associative Polymers

The statistical mechanical theory developed herein predicts the effects of associative polymer, random walk chains with adhesive end groups, on the interaction between parallel plates at full and restricted equilibrium conditions. The Derjaguin approximation and Barker-Henderson perturbation theory then determine interparticle potentials and macroscopic thermodynamic variables, respectively, with the compositions of coexisting phases following from classical thermodynamic criteria for equilibrium in one-component systems. Weakly or nonassociative polymers induce depletion attractions that result in reversible phase transitions. Increasing the strength of the adhesion to the plates diminishes the range and strength of the attraction, so higher polymer concentrations are needed to induce flocculation. Sufficient adsorption to the plates may cause a repulsive barrier that stabilizes dispersions against depletion flocculation at full equilibrium. Strong adsorption, however, results in a bridging attraction that grows with polymer concentration until the surface is saturated and then diminishes with additional polymer. This leads to flocculation at low polymer concentrations and restabilization at a higher value that depends on the solvent quality. The interactions at restricted equilibrium mimic the long-range features of the full equilibrium potentials but contain an additional repulsive core.