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 degrees 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 X(ij) 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.