Structures of thin ionomer films in solvent mixtures

We have used neutron reflectivity to measure the concentration profiles of polystyrenesulfonated acid (PSSA_x) films with three different degrees of sulfonation (x = 3.4%, 12.8%, and 27.0%) in water, CCl₄, and a mixture of the two solvents. The data show that, except for the x = 3.4% films where CCl₄ is a good solvent, the largest degree of swelling occurred in the mixed solvent. Contrast matching the water to the polymer layer enabled us to profile the CCl₄ concentration. The results showed that CCl₄ and water were mixed within the polymer film in a ratio of 1:2 and 1:4 for 12.8 mol % and 27.0 mol % PSSA, respectively. Self-consistent-field calculations indicated that the number of adsorbed sulfonated blocks scales linearly with the degree of sulfonation. Using the interaction parameters between the PS and SA blocks obtained by fitting to the data in pure solvents, excellent agreement is obtained for the profiles of the polymer and the solvent mixtures for all values of x.