Solutions, ionomer, properties

Theoretical studies of ionomer aggregation in solution can rationalize and predict stable configurations of ionomer bundles as a function of basic ionomer properties. Theoretical results can be highly insightful to narrow down the configuration space for molecular simulations. 

Pellegrino J, Wang D, Rabago R, Noble RD, and Koval CA. Gas transport properties of solution-cast perfluorosufibnic acid ionomer films containing ioninc surfactants. J. Membr. Sci. 1993 84 161-169. 

Verbragge and Hill [77] have compared protonic conductivities of several different Nafion and Dow samples immersed in sulfuric acid solutions at various temperatures. Conductivities on the order of 0.06-0.085 S/cm were reported for the acid-immersed Nafion samples at 22 °C. The immersed Dow membrane samples exhibited somewhat higher conductivity (0.13-0.14 S/cm) at this temperature. These data were reported for membranes in contact with a minimum concentration of 0.3 m H2SO4 and are dependent on the sulfuric acid concentration. This study [77] presents a model of water and ion distribution based on properties of pores in the ionomer. The model, which uses Poisson s equation to describe electric potential variation in the pore, is successful in describing experimental acid partitioning results. Other earlier reports of protonic conductivity in ionomeric membranes have been given by Slade et al. [72] and by Eisman [60]. 

The water distribution within a polymer electrolyte fuel cell (PEFC) has been modeled at various levels of sophistication by several groups. Verbrugge and coworkers [83-85] have carried out extensive modeling of transport properties in immersed perfluorosulfonate ionomers based on dilute-solution theory. Fales et al. [109] reported an isothermal water map based on hydraulic permeability and electro-osmotic drag data. Though the model was relatively simple, some broad conclusions concerning membrane humidification conditions were reached. Fuller and Newman [104] applied concentrated-solution theory and employed limited earlier literature data on transport properties to produce a general description of water transport in fuel cell membranes. The last contribution emphasizes water distribution within the membrane. Boundary values were set rather arbitrarily. 

The properties of ionomer solutions are sensitive to not only the degree of the ionic functionality and the polymer concentration, hut perhaps even to a greater extent, the ability of the solvent to ionize the ion-pairs (64). Thus, non-ionizing solvents, usually those with relatively low dielectric constant, favor association of the ionic groups even in dilute solutions. In contrast, ionomer solutions may exhibit polyelectrolyte behavior in polar solvents due to solvation of the ion-pair that leaves the hound ions unshielded. 

Various techniques have been used to study the solution properties of ionomers. These include viscosity (4, 1 ), static and dynamic light scattering (12.13.15-18), small-angle neutron scattering (11.14). and spectroscopy (10). Here, we use (static and... 

A thermopolastic elastomer based on sulfonated-EPDM, S-EPDM, was developed in the 1970 s by Exxon and more recently by Uniroyal. Unlike the synthesis of the carboxylate ionomers described above, S-EPDM is prepared by a post-polymerization sulfonatlon reaction(28). Compared to the metal neutralized S-EPDM, the sulfonic acid derivative is not highly associated. The free acid materials possess low strengths and are less thermally stable. The metal salts of S-EPDM have properties comparable to crosslInked elastomers, but they do exhibit viscous flow at elevated temperatures. In the absence of a polar cosolvent, such as methanol, hydrocarbon solutions of the metal salts of S-EPDM are solid gels at polymer concentrations above several percent(31). With the addition of 1 to 5% alcohol the polymer solution becomes fluid with solution viscosities of the order of 10 to 100 poise. 

Most of the published work on Ionomers has been concerned with different approaches to incorporation of ionic groups and the resulting influence of these associations on bulk or solution properties. Studies by Makowski et al.(37), Agarwal et a1.(38), Weiss(39) and... 

Surprisingly few studies have focused on the effect of solvents or diluents on the structure and properties of ionomers. Solution results are scarce due to the limited solubility of ionomers in conventional solvents, because of the strong intermolecular associations of the ionic groups(6,7). 

In this part of the paper we examine the thermodynamic properties of hydrated ionomers. By strongly hydrated we mean that we are beyond the state of solvation shells, where V, the number of water molecules per cation, is a small number ("v A to 6). In a strongly hydrated sample, the water molecules are considered to be free, and make a concentrated solution with the cations (the counter ions) and eventually with some mobile anions (the coions). This subject has already been extensively studied because of its practical importance (1-4). From the following discussion, we shall see that some of the usual classical laws are no longer valid. For instance, the variation of the chemical potential of water with the concentration of cations may no longer hold. 

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. 

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. 

So far, the classical work on polyelectrolyte nonaqueous solutions conducted during the 1950s has been described. Here we will describe more recent work on polyelectrolyte nonaqueous solutions, conducted during the 1980s and 1990s, in particular the work on the solution properties of ionomers in a polar solvent, which may be considered as weakly charged polyelectrolytes. 

Hara M, Wu J, Lee AH. Effect of intia- and intermolecular interactions on solution properties of sulfonated polystyrene ionomers. Macromolecules 1988 21 2214-2218. 

Tonic interactions in macromolecules represent simultaneously one of the oldest and one of the most recent areas of activity in modem polymer science. While on the one hand polyelectrolyte solution studies have been pursued in a host of laboratories for decades, the study of the bulk properties of ionomers dates back hardly more than ten years ago— as a matter of fact the word ionomer came into common use only about a decade ago with the advent of the first commercial material of this type based on ethylene. Since that time, that aspect of the field has grown profoundly, both in regard to bulk investigations as well as studies in nonaqueous solutions, as is evidenced by the periodic symposia in the field, the recently initiated Gordon Conference, and the appearance of two books dealing primarily with bulk properties. 

---