Effect of Polymer Charge Density on the Phase Behavior of Sodium Poly(acrylate-co-acrylamide)−DTAB Systems

The phase behavior of a series of sodium poly(acrylate-co-acrylamide)−dodecyltrimethylammonium bromide (DTAB)−water systems has been investigated. The polyacrylate content in the copolymer (and hence the ionic group content) was varied from 16 to 80%. Both binodals and spinodals in the ternary aqueous systems were determined at 25 °C, spinodals for the 80, 40, 30, and 16% acrylate content polymer−DTAB systems, and binodals for the 80, 40, and 30% acrylate content polymers. Phase separation is found to be associative in all systems; ie., the binodal curves are the result of phase equilibrium tie-lines in which a concentrated polymer−surfactant gel is in equilibrium with a dilute supernatant. The two-phase equilibrium area in the phase diagram decreases with decreasing ionic content of the polymer. Experimental results for the spinodals are compared to calculate spinodals and plait points using a Flory−Huggins type interaction model with surfactant micelles as the second polymer component. Using reasonable values for the Flory−Huggins interaction parameters χ i j and micellar aggregation number m, calculated plait points confirm the associative nature of phase separation as indicated by the orientation of the experimental tie-lines.