Sulfonated ionomers

A variety of ionomers have been described in the research literature, including copolymers of a) styrene with acrylic acid, b) ethyl acrylate with methacrylic acid, and (c) ethylene with methacrylic acid. A relatively recent development has been that of fluorinated sulfonate ionomers known as Nafions, a trade name of the Du Pont company. These ionomers have the general structure illustrated (10.1) and are used commercially as membranes. These ionomers are made by copolymerisation of the hydrocarbon or fluorocarbon monomers with minor amounts of the appropriate acid or ester. Copolymerisation is followed by either neutralisation or hydrolysis with a base, a process that may be carried out either in solution or in the melt. 

Fitzgerald J.J. and Weiss R.A., Synthesis, properties and structure of sulfonate ionomers, J. Macromol. Sci. Rev. Macromol. Chem. Phys. C, 28, 99, 1988. 

Ionomer formation 150 to 400 Depending on cations in sulfonated ionomers [83-85] [87-90]... 

Sulfonated ionomers are also characterised by IR spectroscopy [75-77]. Agarwal and coworkers [76] analysed the Zn+2 salt of sulfonated EPDM. The peak at about 1200 cm 1 is due to the asymmetric stretching of the sulfonate group. The band at 1020 cm 1 is ascribed to the symmetric stretching of the -S03 group. The position and nature of the absorption band depend on the nature of the cation [76]. The band at 610-615 cm 1 is due to C-S stretching of the polymer -S03 band. 

Figure 1. Linear and three-arm star telechelic polyisobutylene-based metal sulfonate ionomers.

Figure 2. Stress vs. strain for telechelic polyisobutylene-based metal sulfonate ionomers. Numbers at each curve indicate sample molecular weight and mole % ion content (in parenthesis).

Also, for the counterion effect on the effective diameters it is found (18) that Do decreases with Increasing ionic radius of the counterions 1800 A for Na Jon (0.96 X), 1600 X for K ion (1.33 A), and 1500 A for Cs ion (1.69 A). Original Kc/Rq vs. c curves were reported elsewhere (16). We also pointed out by viscosity measurements (li) that for sulfonated ionomers, the effect of counterion (i.e., counterion binding) Increased in the order of Li < Na < K < Cs, since the solvated ion size decreased in the order of Li > Na > K > Cs. Light scattering results show this tendency more quant i tat ively. 

LANTMAN ETAL. Sulfonate Ionomers in a Nonionizing Solvent... 

The existence of ion clustering in perfluorinated sulfonate ionomers was first reported by Yeo and Eisenberg in 1975. This phenomenon has been subsequently studied for perfluorinated sulfonate and carboxylate ionomers by many Experimental evidence to support the conclusion that ion clustering occurs in these materials includes thermorheological behavior/ X-ray diffraction results/" " " IR data/ " NMR data,"" " ESR data/ Mossbauer spectroscopic fluores-... 

Ionomers of practical interest have been prepared by two synthetic routes (a) copolymerization of a low level of functionalized monomer with an olefinically unsaturated monomer or (b) direct functionalization of a preformed polymer. Typically, carboxyl containing ionomers are obtained by direct copolymerization of acrylic or methacrylic acid with ethylene, styrene and similar comonomers by free radical copoly-merization. Rees (22) has described the preparation of a number of such copolymers. The resulting copolymer is generally available as the free acid which can be neutralized to the degree desired with metal hydroxides, acetates and similar salts. Recently, Weiss et al.(23-26) have described the preparation of sulfonated ionomers by copolymerization of sodium styrene sulfonate with butadiene or styrene. 

Telechelic polyisobutylene sulfonate Ionomers Model ionomer... 

A specialty class of carboxyl containing elastomers are the telechelic ionomers. In these systems the carboxyl functionality terminates both ends of the polymer chain. Such polymers range in molecular weight from 1500 to about 6000. These materials can be prepared via several synthetic routes involving anionic or free radical initiated polymeri-zation(32-34). Recently, telechelic sulfonate ionomers of poly-isobutylene have been synthesized(35). These systems offer an unusual opportunity to assess the influence of chain length, chain architecture, cation type, and the Influence of polar additives on ionomer properties. 

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. 

Novel sulfonated and carboxylated ionomers having "blocky" structures were synthesized via two completely different methods. Sulfonated ionomers were prepared by a fairly complex emulsion copolymerization of n-butyl acrylate and sulfonated styrene (Na or K salt) using a water soluble initiator system. Carboxylated ionomers were obtained by the hydrolysis of styrene-isobutyl-methacrylate block copolymers which have been produced by carefully controlled living anionic polymerization. Characterization of these materials showed the formation of novel ionomeric structures with dramatic improvements in the modulus-temperature behavior and also, in some cases, the stress-strain properties. However no change was observed in the glass transition temperature (DSC) of the ionomers when compared with their non-ionic counterparts, which is a strong indication of the formation of blocky structures. 

Since the conductivity of electrolytes and the cross section and thickness of the membrane are known, a can be determined from the voltage drops across the three pairs of probe electrodes 1-2, 3-4 and 5-6. The sodium current efficiency (CE) can also be determined by titrating the amount of caustic soda generated over a given period of time. The confinement chambers around the working electrodes are used to eliminate free bubbles near the membrane. Our normalized transport data for sulfonate, carboxylate and sulfonamide ionomers are plotted In Figure 5 the universal percolative nature of perfluorinated ionomers can be clearly eeij. The prefactor sulfonate ionomers. The exponent t is 1.5 0.1 in reasonable agreement with theory and the thresholds are between 8 to 10 vol. %, which are consistent with the bimodal distribution in cluster size postulated by the cluster-network model (5.18). This theory has also been applied recently to delineate sodium selectivity of perfluorinated ionomers (20). 

Fig. 3 Variation of equilibrium cluster diameter dQ i ith EH, cation form and water content, where EQ=275 joule-cm is the tensile modulus of a dry, 1200 EH sulfonate ionomer, A=0.667 is a constant and dQ is obtained from SAXS and water sorption data. The solid line is a least square fit of Eq. 1 to the EH and cation form data.

Fig. 7 Current efficiency of carboxylate/sulfonate ionomer blends. Curves are predicted behavior of oriented oblate spheroids whose aspect ratios are 0.01 (top curve), 0.25 (middle curve) and 0.995 (bottom curve), respectively. Reproduced with permission from Ref. 15, Fig. 2. Copyright 1983, American Chemical Society.

In a previous paper (2), the author described a method to dissolve the sulfonyl fluoride precursor form of a perfluorinated sulfonate ionomer. Commercially available forms of Nafion are supplied as activated membranes (i.e., saponified from the precursor to the ionic form), and near-quantitative reconstitution of the precursor functionality (such as RSOjF) must first be performed using a chemical reagent such as SF. f4) before dissolution in perhalogenated solvents is possible. Besides adding to the cost of membrane manufacture, SF. is extremely toxic and corrosive and must be handled in nickel alloy pressure equipment. Therefore, a method for dissolving perfluorinated ionomers directly would be more desirable. 

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. 

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. 

LUNDBERG AND PHILLIPS Solution Behavior of Metal-Sulfonate Ionomers 203... 

While this equilibrium ignores the polymer backbone and the majority of the solvent system, it does predict many characteristics of metal sulfonate ionomers in hydrocarbon solution. 

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)... 

While solvents such as THF and Dioxane exhibit ion pair association with sulfonate ionomers, the addition of more polar cosolvents can have a marked effect on the reduced viscosity-concentration profiles. For example,- Figure 2 illustrates the influence of varying... 

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. 

These observations suggest that sample preparation of sulfonate ionomers could be very important in determining physical properties of such systems. There are several qualifying comments that should be made. 

Polar solvents such as dimethylformamide, dimethylsulfoxide, and tetrahydrofuran-water mixtures behave differently in that polyelectrolyte behavior is observed at extreme dilution for sulfonate ionomers therefore, the behavior described above does not apply directly to these solvent systems. 

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