The measurements of coexistence curves and critical behavior of a binary mixture with a high molecular weight polymer

The critical behavior of solutions of poly (diallydimethylammonium chloride) (PDDAC, Mw=3.2×105) in the critical binary mixture of isobutyric acid (I)+water (W) was studied by the refractive index measurements. The measurements were performed at three different PDDAC concentrations near and far away from the critical point. We observed that the critical temperatures increase linearly with increasing the concentration of PDDAC. As the distance from the critical point increases, the system exhibits a crossover from the renormalized Ising critical behavior to the mean-field one. For the solutions with the highest PDDAC concentration, experiments suggest a crossover to tricritical behavior.