Dielectric polarization of a partially neutralized poly(methacrylic acid) in dilute aqueous solutions: Conformational transition

Dielectric permittivity epsilon' (at a frequency of 1 MHz) of aqueous solutions of a partially neutralized poly( methacrylic acid) Na-PMAA was studied with varying the degree of ionization alpha(i) from 0 to 0.78 and concentrations w(2) (g/g) from 1.25 x 10(-4) to 6 x 10(-3) at 25 degrees C. It was shown that the concentration dependence of epsilon' for Na-PMAA aqueous solutions at all alpha(i) values contains two regions corresponding to different changes in epsilon', and the above regions are separated by the crossover point. In the first region, epsilon' increases with the increasing concentration Delta epsilon'/Delta w(2) > 0; in the second region, epsilon' decreases with the increasing concentration Delta epsilon'/Delta w(2) < 0. The sign inversion of Delta epsilon'/Delta w(2) is explained by different structures of water and mechanisms of hydration in regions I and II. In the region corresponding to the ascending branch of the epsilon'-w(2) curve, the dipole moments of macromolecules per repeating unit, mu = (< M-2 >/N)(1/2), were calculated according to the Buckingham theory. The mu value is high; as a macromolecule is saturated with ionized units, this value nonmonotonically changes from similar to 10 D at alpha(i) = 0 to similar to 18 D at alpha(i) similar to 0.24. This dependence peaks at similar to 30 D at alpha(i) similar to 0.1. As is assumed, (< M-2 >/N)(1/2) depends on the vector sum of partial dipole moments of hydrated nonionized monomer units (mu similar to 10 D) and fluctuation-induced dipole moments of ionized monomer units. The profile of mu as a function of the degree of ionization of Na-PMAA indicates the occurrence of the conformational transition in an individual macromolecule. This transition takes place at the degree of ionization alpha(i) similar to 0.1. An analysis of molecular interactions in the (ionized coil)-solvent system explans the conformational transition in Na-PMAA.