Sulfonated ionomers dilute solution behavior

Studies on the dilute solution behavior of sulfonated ionomers have shown these polymers to exhibit unusual viscosity behavior in solvents of low polarity. These results have been interpreted as arising from strong ion pair associations in low polarity diluents. Solvents of higher polarity, such as dimethyl sulfoxide and dimethyl formamide induce classic polyelectrolyte behavior in sulfonate ionomers even at very low sulfonate levels. To a first approximation these two behaviors, ion pair interactions or polyelectrolyte behavior, are a consequence of solvent polarity. Intramolecular association of Lightly Sulfonated Polystyrene (S-PS) results in a reduced viscosity for the ionomer less than that of polystyrene precursor at low polymer levels. Inter-association enhances the reduced viscosity of the ionomer at higher polymer concentrations. Isolation of the intra- and inter-associated species of S-PS has been attempted (via freeze drying). A comparison of selected properties reveals significant differences for these two conformations. 

Several studies (6, 13) of the solution behavior of sulfonate ionomers have provided additional insight on the nature of the ion pair aggregation. The polarity of the solvent environment has been shown to have a major influence on the dilute solution behavior of these polymers. In the course of these studies it has been observed with selected systems that both melt viscosity values and solution behavior can vary according to the history of sulfonate ionomers. This study provides some data and provides one rationale for such differences. 

Similar solution behavior was reported(9-11) for sulfonate ionomers. Rochas eit al. (9) observed a polyelectrolyte effect for acrylonitrile-methallylsulfonate copolymers in DMF. Lundberg and Phillips(10) studied the effect of solvents, with dielectric constants ranging from c 2.2 to e 46.7, on the dilute solution viscosity of the sulfonic acid and Na-salt derivatives of sul-fonated polystyrene (SPS). For highly polar solvents such as DMF and dlmethylsulfoxide (DMSO, e 46.7) they observed a polyelectrolyte effect, but for relatively non-polar solvents such as THF and dioxane (c = 2.2) no polyelectrolyte effect was observed. Like Schade and Gartner, these authors concluded that polar solvents favor ionization of the metal sulfonate group while non-polar solvents favor ion-pair interactions. 

The product isolated at dilute concentration exhibits other properties different from that of the material isolated at higher concentration. For example, conventional S-PS (1.7%) will form a homogeneous gel in xylene at concentration >3%, but will phase separate to form a gel phase in more dilute solutions, especially <1%. This behavior has been observed with a number of sulfonate ionomers. 

Effect of Temperature on the Solution Behavior of Carhoxylate and Sulfonate lonomers. Based on the results above, a substantial difference in the solution behavior of carhoxylate and sulfonate ionomers might be expected as a function of temperature. Figure 10 illustrates the effect of temperature on the solution viscosity of carboxylated and sulfonated ionomers at very low sulfonate and carhoxylate content. At low polymer concentrations it is seen that the sulfonate system manifests a higher viscosity level in 1% hexanol/xylene solution. This is consistent with the dilute solution viscosity behavior. More importantly, at high polymer concentrations it is seen from Figure 10 that the 5% S-PS curve actually goes through a maximum, while the carhoxylate system decreases mono-tonically. These results are apparently attributable to the weaker ionic association in the carhoxylate case as compared to the sulfonate system. 

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