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 so lubility 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 sam e fraction of nonmigrating charged groups. It is shown that the solubility of a copolymer with an ionizable component depends on the ratio of the degr ee 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 increa ses 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 ch ain to another in an optimum way so that to minimize the loss in translatio nal entropy of the counterions. As a result, demixing of the polymer soluti on for i not equal x occurs at lower values of the polymer concentration ph i than in the case of a polymer with i = x; the corresponding value of phi is still higher than the demixing concentration for electroneutral polymer. The theoretical results are supported experimentally by a study of the sol ubility of copolymers of N-isopropylacrylamide, NIPAM, with N,N-[(dimethyla mino)propyl]methacrylamide, MADAP, with different degrees of ionization. It is found that the migration of ions bound to the polymer chains has a sign ificant influence on the solubility of such copolymers. Because of an incre ase in the hydrophilicity of the copolymers with increasing fraction of MAD AP units, the dependence of the solubility of these copolymers on the MADAP fraction has a complex nonmonotonic character.