Sulfonated polystyrene ionomers studies

Chu, B. Wu, D.Q. Lundberg, R.D. MacKnight, W.J. Small-angle x-ray scattering (SAXS) studies of sulfonated polystyrene ionomers. 1. Anomalous SAXS. Macromolecules 1993, 26, 994. 

Several SANS studies of ionomers have appeared on both deuterium labeled and unlabeled systems(8,12,18-20). The earlier work(14) showed that an ionic peak, similar to that observed by x-rays, could be discerned in some cases, especially when the sample was "decorated" by the incorporation of D20. It was also tentatively concluded(19) that the radius of gyration, R, of the individual chains is not altered when the acid 1s converted to the salt in the case of poly-styrene-methacrylic acid copolymers. Subsequent SANS experiments were performed on sulfonated polystyrene ionomers with up to 8.5% sul-fonation(12). The results of this study indicated that aggregation of the ionic groups is accompanied by considerable chain expansion, which is consistent with the theory of Forsman(ll). 

Bodycomb J, Hara M. Light scattering study of ionomers in solutions. 5. CONTIN analysis of dynamic scattering data from sulfonated polystyrene ionomer in a polar solvent (dimethylformamide). Macromolecules 1995 28 8190-8196. 

Hara M, Wu J. Light scattering study of ionomers in solution. 2. Low-angle scattering from sulfonated polystyrene ionomers. Macromolecules 1988 21 402-407. 

The melt behavior of sulfonated polystyrene ionomers was studied by Lundberg and coworkers41. As was shown in the case of a sulfonated elastomer30, sulfonation of polystyrene leads to an increase in the melt viscosity (measured at 250 °C) of the SPS upon neutralization, indicating increased association of the sodium poly-(salt). A sudden jump of the melt viscosity occurs at the point of complete neutralization, where a critical concentration of Na polystyrenesulfonate is reached, apparently resulting in a sharp phase separation between the ionic and hydrophobic domains (Figure 5). 

Sulfonate vs Methacrylate vs Carboxylate. A comparison study (19) of the melt viscosity values of polystyrene ionomers, either randomly carboxylated or sulfonated in the para position of a benzene ring, showed that the melt viscosity values at 220°C of the two ionomers were significantly different. For example, at 2 mol% of ions, the melt viscosity of sodium sulfonated polystyrene ionomer was more than 2 orders of magnitude higher than that of the sodimn carboxylated polystyrene ionomer. 

So far most studies of aggregation behavior have been conducted with low polarity solvents such as THF [4,9,15]. Recently there have been a few reports with nonpolar solvents such as xylene [16-18]. While these systems require low ion contents to form a solution, the ionomer electrical environment is more similar to that found in the solid state (i.e. low dielectric constant). In this work we use static and dynamic light scattering techniques to examine the aggregation behavior of sulfonated polystyrene ionomers in a nonpolar solvent, toluene. 

The use of ionomers such as lightly sulfonated polystyrene as an oil-based fracturing fluid viscosifier has been studied (70). 

Tn the case of nonpolar diluents, very few studies have appeared in the literature. The Zn sulfonated polystyrene plasticized by 3.4 wt. % dodecane shows no change in either position, intensity or shape of the ionomer peak compared to that of the bulk polymer (27) this was taken to indicate that the ionic microphase is unaffected by this diluent. The Mn sulfonated polystyrene plasticized b,v up to 10 wt % DOP (Figure 11) shows a decrease in intensity of the peak, but no change in its shape or position (28). This was explained as a dilution effect of the ionic phase due to swelling of the matrix by the plasticizer. 

Reference 7 reviews a number of electron microscopy studies of ionomer morphology in the period up to 1979. None of these studies makes a convincing case for the direct imaging of ionic clusters. This is because of the small size of the clusters (less than 5 nm based on scattering studies) and difficulties encountered in sample preparation. The entire problem was reexamined in 1980(21). In this study ionomers based on ethylene-methacrylic acid copolymers, sulfonated polypentenamer, sulfonated polystyrene and sulfonated ethylene-propylene-diene rubber (EPDM) were examined. The transfer theory of imaging was used to interpret the results. Solvent casting was found to produce no useful information about ionic clusters, and microtomed sections showed no distinct domain structure even in ionomers neutralized with cesium. Microtomed sections of sulfonated EPDM, however,... 

The work described in the present paper concerns the Influence of water and organic solvents on the ionic interactions in lightly sulfonated polystyrene (SFS) ionomers. The focus will be specifically directed towards the Influence of the solvent environment on the cation-anion and cation-cation interactions. Fourier transform Infrared spectroscopy (FTIR) was used to probe the former while electron spin resonance spectroscopy (ESR) was used to study the latter. Experiments were carried out with dissolved, swollen, and bulk ionomers. 

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. 

Recent studies in our laboratories have been concerned with the physical properties of sulfonated ionomers such as sulfonate ethylene/propylene/ethylidene norbornene terpolymers (4, or lightly sulfonated polystyrene (S-PS) (11). These ionomers exhibit pronounced ion pair association (at sulfonate levels > 15 milli-equivalents/100 g polymer) to a degree that they appear crosslinked covalently. These interactions can be dissipated by the addition of a polar additive, thereby showing that such associations are indeed physical and do not arise due to covalent crosslinking. 

Few studies have been conducted heretofore on sulfonated ionomers in solvents which can be considered relatively polar, as defined by a high dielectric constant. A recent study (13) on acrylonitrile-methallyl sulfonate copolymers in dimethyl-formamide is a notable exception. S-PS is readily soluble in a wide variety of solvents, some of them exhibiting rather high values of dielectric constant, such as dimethylformamide (DMF) or dimethylsulfoxide (DMSO). The reduced viscosity-concentration behavior of sulfonated polystyrene is markedly different in polar solvents from that in nonpolar-solvent systems. Typically there is a marked upsweep in reduced viscosity at low polymer concentrations and clearly a manifestation of classic polyelectrolyte behavior. ( 7)... 

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