Sulfonated ionomers preparation

Typically, carboxylate ionomers are prepared by direct copolymerization of acrylic or methacrylic acid with ethylene, styrene or similar comonomers by free radical copolymerization (65). More recently, a number of copolymerizations involving sulfonated monomers have been described. For example, Weiss et al. (66-69) prepared ionomers by a free-radical, emulsion copolymerization of sodium sulfonated styrene with butadiene or styrene. Similarly, Allen et al. (70) copolymerized n-butyl acrylate with salts of sulfonated styrene. The ionomers prepared by this route, however, were reported to be "blocky" with regard to the incorporation of the sulfonated styrene monomer. Salamone et al. (71-76) prepared ionomers based on the copolymerization of a neutral monomer, such as styrene, methyl methacrylate, or n-butyl acrylate, with a cationic-anionic monomer pair, 3-methacrylamidopropyl-trimethylammonium 2-acrylamlde-2-methylpropane sulfonate. 

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. 

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. 

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. 

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. 

Moore, R.B. and Martin, C.R. (1986) Procedures for preparing solution-cast perfluoro-sulfonate ionomer films and membranes. Anal. Chem., 58, 2569-2570. 

Pentenamer Ionomers. Unsaturated polypentenamer elastomers have been derivatized by post-synthesis reactions (72—74). Phosphonate, thioglycolate, sulfonate, and carboxylate derivatives have been prepared and converted into ionomers. 

Mixed anionic (sulfonated - carboxylated) ionomers [81] were prepared by sulfonation of maleated block-copoly (styrene/ethylene-butylene/styrene) (m-SEBS) by acetyl sulfate, followed by neutralisation of the sulfonated maleated product, leading to the formation of a new block copolymer ionomer based on both carboxylate and sulfonate anions according to Scheme 4.6. FT-IR spectra confirm the presence of both carboxylated and sulfonate ions (Figure 4.9). 

The ionic aggregates present in an ionomer act as physical crosslinks and drastically change the polymer properties. The blending of two ionomers enhances the compatibility via ion-ion interaction. The compatibilisation of polymer blends by specific ion-dipole and ion-ion interactions has recently received wide attention [93-96]. FT-IR spectroscopy is a powerful technique for investigating such specific interactions [97-99] in an ionic blend made from the acid form of sulfonated polystyrene and poly[(ethyl acrylate - CO (4, vinyl pyridine)]. Datta and co-workers [98] characterised blends of zinc oxide-neutralised maleated EPDM (m-EPDM) and zinc salt of an ethylene-methacrylic acid copolymer (Zn-EMA), wherein Zn-EMA content does not exceed 50% by weight. The blend behaves as an ionic thermoplastic elastomer (ITPE). Blends (Z0, Z5 and Z10) were prepared according to the following formulations [98] ... 

Ionomers are used to prepare membranes for a variety of applications including dialysis, reverse osmosis, and in electrolytic cells for the chlor-alkali industry. This latter application needs materials that show good chemical resistance and ionomers based on perfluorinated backbones with minor amounts of sulfonic or carboxylic acids are ideal. They also show good ion-exchange properties. 

Solid-state NMR also allows for the study of sample preparation effects on the local cation environment. Na-SPS with 1.7% styrene sulfonate groups cast from dimethyl formamide or tetrahydrofuran (THF)/ water mixtures shows more aggregated ions, i.e., a more intense peak at 20 ppm, than ionomers cast from less... 

Ionomers have been prepared by two general routes (1) copolymerization of a low level of functionalized monomer with an oleflnlcally unsaturated comonomer or (2) direct functionalization of a preformed polymer. Almost all ionomers of practical Interest have contained either carboxylate or sulfonate groups as the ionic species. Other salts, such as phosphonates, sulfates, thloglycolates, ammonium, and pyridinium salts have been studied, but nowhere to the extent of the carboxylate and sulfonate anlonomers. (An anlomer is defined as an lonomer In which the anion is bonded to the polymer. Conversely, ionomers that have the cation bonded to the polymer are termed cationomers). Relatively little information is available on the structure and properties of these types of ionomers. 

A special class of lonomer in which the salt groups are only at the chain ends, i.e., telechellc ionomers, have recently received considerable attention as model lonomer systems. Kennedy and coworkers (86-88) prepared linear and tri-arm star telechellc sulfonated polyisobutylene by heterogeneous sulfonation of an olefin-terminated polyisobutylene with acetyl sulfate. Omels et... 

A supramolecular approach is usually employed in the preparation of such materials. Commonly used base polymers are ionomers containing sulfonate groups, such as poly(styrenesulfonate). Azo-containing molecules are frequently employed for the side-chain mesogen [95]. 

An interesting group of new ionomers have been prepared from liquid olefin telechelic polyisobutylenes. Syntheses involved the quantitative sulfonation (by acetyl sulfate) of linear and/or tri-arm star polyisobutylenes carrying exactly two or three -CH2-C(CH3)=CH2 termini, respectively, followed by neutralization with various bases ... 

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 second route to ionomers Involves modification of a preformed polymer. Sulfonation of EPDM, for example, permits the preparation of sulfonated-EPDM with a level of sulfonate groups in proportion to the amount of sulfonating agent(27). These reactions are conducted in homogeneous solutions permitting the direct neutralization of the acid functionality to the desired level. Isolation of the neutralized ionomer is effected by conventional polymer isolation techniques, such as coagulation in a nonsolvent or solvent flashing. These procedures are detailed in several patents and publications(28-31). 

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. 

Until recently, perfluorinated ionomrs with high equivalent weights were believed to be insoluble. Covitch(50), however, has identified a number of solvents and dissolution procedures for the sulfonyl fluoride precursor and sulfonate and carboxyl ate Nafion ionomers with 1100 to 1200 equivalent weight. This development has great potential for the preparation of sulfonate and carboxyl ate ionomer blends, the... 

We are currently exploring new routes to the synthesis of ionomers with controlled architecture, i.e. with control over the amount and location of ionic groups in the polymer backbone. One of our main interests is the synthesis of ion containing block copolymers. The applicability of anionic polymerization in the synthesis of block copolymers and other well defined model systems is well documented (22-24) Not as well appreciated, however, is the blocky nature that certain emulsion copolymerizations may provide. Thus, we have utilized both anionic and free radical emulsion polymerization in the preparation of model ionomers of controlled architecture. In this paper, the synthesis and characteristics of sulfonated and carboxylated block ionomers by both free radical emulsion and anionic polymerization followed by hydrolysis will be discussed. 

---