Light Scattering Study of Ionomer Solutions. 3. Dynamic Scattering from Sulfonated Polystyrene Ionomers in a Polar Solvent (Dimethylformamide)

Dynamic light scattering experiments were conducted on samples of a random ionomer, partially sulfonated polystyrene (SPS) (Na salt), which had small amounts of ionic groups (less than 4 mol %), dissolved in an aprotic, polar solvent, dimethylformamide (DMF). For high molecular weight ionomers (MW: 200 000 and 400 000), the (effective) diffusion coefficient, D_eff, obtained from the methods of cumulants, increases significantly with polymer (ionomer) concentration and bends over at high ionomer concentration. The D_eff increases with ion content of the ionomers at fixed polymer concentration. These results are consistent with those found for the same ionomers by low-angle light scattering: i.e., the reciprocal reduced scattered intensity increases significantly with ionomer concentration, becomes almost horizontal at higher concentration, and, at fixed ionomer concentration, increases with ion content. Those results reflect a significant increase in inverse osmotic compressibility with polymer concentration, which arises from long-range electrostatic interactions among ionomer molecules in salt-free solution. For low molecular weight ionomer (MW: 50 000), quite contrasting dynamic behavior is observed: the D_eff is insensitive to polymer (ionomer) concentration, and, at fixed polymer concentration, the D_eff decreases with ion content. The effect of added salts (LiCl) on D_eff of ionomer solution was also studied: the D_eff increases linearly with polymer (ionomer) concentration over the salt concentration range studied (0.0001–0.1 M). The slope of the line in the diffusion coefficient vs concentration plot increases significantly with decreasing salt concentration. This is due to increased interactions between ionomer molecules caused by less screening on ionomer charges by added salts. The D_eff at infinite dilution is almost independent of added salt concentration, which suggests little change in ionomer size over the salt concentration range studied (0.0001–0.1 M).