LIGHT-SCATTERING STUDY OF IONOMERS IN SOLUTION .4. ANGULAR MEASUREMENTS OF SULFONATED POLYSTYRENE IONOMERS IN A POLAR-SOLVENT (DIMETHYLFORMAMIDE)

The salt-free polyelectrolyte behavior of random ionomers, i.e., partially sulfonated polystyrene (SPS) (Na salt: MW = 400 000) having less than 5 mol % of ionic groups, dissolved in an aprotic polar solvent, dimethylformamide (DMF) was studied by angular light scattering measurements. At low ionomer concentration, ''abnormal'' Zimm plots, showing negative slopes in reciprocal reduced scattered intensity vs scattering vector curves, are observed. These effects arise because the scattering is dominated by external interference arising from strong eletrostatic interactions between charged ionomer chains. The behavior can be explained by using general relations derived by extending the approach of Doty and Steiner. As concentration is increased, these slopes become less negative and eventually become positive with some curvatures at small angles, which suggests the existence of large-scale ''heterogeneities'' in the solution. As ion content is increased, the initial slopes become more negative at low concentration and become more positive at higher concentration. Such behavior is suppressed upon addition of simple salts, suggesting that the observed behavior is ionic in nature. The angular light scattering experiments confirm previous conclusions drawn from low-angle light scattering experiments on the salt-free polyelectrolyte behavior of ionomers in polar solvent. The results also seem to be consistent; with the speculation about the presence of large ''heterogeneities'' in salt-free polyelectrolyte solutions, suggested by some dynamic light scattering experiments.