Influence of migrating ionic groups on the solubility of polyelectrolytes: phase behavior of ionic poly(N-isopropylacrylamide) copolymers in water

The influence of the migration of charges bound to polymer chains on the solubility of the charged polymer is studied theoretically and experimentally. It is found that, due to the migration of charges, the solubility of such a polymer is lower than the solubility of a polyelectrolyte having the same fraction of nonmigrating charged groups. It is shown that the solubility of a copolymer with an ionizable component depends on the ratio of the degree of ionization i to the fraction x of monomer units B able to be ionized. If this ratio equals unity (i = x), the solubility of the copolymer increases with an increase in the fraction of monomer units B. When the fraction x of monomer units B is much higher than the actual degree of ionization i, the ions bound to the polymer chains can be easily transferred from one chain to another in an optimum way so that to minimize the loss in translational entropy of the counterions. As a result, demixing of the polymer solution for i ≠ x occurs at lower values of the polymer concentration φ than in the case of a polymer with i = x; the corresponding value of φ is still higher than the demixing concentration for electroneutral polymer. The theoretical results are supported experimentally by a study of the solubility of copolymers of N-isopropylacrylamide, NIPAM, with N,N-[(dimethylamino)propyl]methacrylamide, MADAP, with different degrees of ionization. It is found that the migration of ions bound to the polymer chains has a significant influence on the solubility of such copolymers. Because of an increase in the hydrophilicity of the copolymers with increasing fraction of MADAP units, the dependence of the solubility of these copolymers on the MADAP fraction has a complex nonmonotonic character.