Interlude of Mesomorphic State in Polyelectrolyte Complexation

Liquid-liquid phase separation of a solution of oppositely charged macromolecules into precipitation upon a decrease in concentration of added salt is well known. Here, using light scattering studies on complexation in aqueous solutions of polycations and oppositely charged organic salt, we report an unusual phenomenon of the emergence of a mesomorphic state. This state occurs with well-defined self-assembled structures in between the homogeneous solution and precipitation as the salt concentration is progressively decreased. Using systematic variations in the concentration c_p and molecular weight of the polymers, charge ratio between the two species, and salt concentration, the mesomorphic state is quantified. For the example of poly(L-lysine) bromide and sodium acrylate, the linear size R of self-assembled structures obeys the universal law of proportionality between R and c_p^(1/6). The mesomorphic state is flanked by precipitation at lower ionic strengths and homogeneous phase at higher ionic strengths. Furthermore, the mesomorphic state disappears by self-poisoning upon increasing polymer concentration at fixed charge ratio. These novel results are interpreted using dipolar interactions and Debye screening.