Ionomers carboxylate

C=0 stretching in carboxylate ionomers (depends on nature cation and ionic structure) ... 

Carboxylate ionomers have been characterised with Fourier transform-infrared (FT-IR) in the region of antisymmetric stretching vibration of carboxylate anions. Figure 4.8 shows carboxylate ionomer [89] of ethylene methacrylic (4%) copolymer). 

Figure 4.8 FT-IR spectra of carboxylate ionomers ionised with zinc salt in the range of...

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). 

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. 

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-... 

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. 

Weiss at al.(11) observed similar effects of solvent polarity on the solution behavior of copolymers of styrene and sodium styrene sulfonate and the Na-salt of an SPS with a lower backbone molecular weight than that studied by Lundberg and Phillips. Weiss and co-workers observed no such behavior with a mixed solvent of toluene (e 2.4) and methanol (c 32 6). Insight of the differences in the solution behavior of Na and Mg salts of the carboxylate ionomers in DMF reported by Schade and Gartner, it is worth noting that neither Weiss et al. nor Lundberg and hillips reported data in polar solvents for cations other than Na. ... 

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. 

Figure 5. TMA Penetration Behavior of PS/PiBM Diblock Copolymer and Carboxylated Ionomer (5 mole percent-COC K ) Obtained after hydrolysis.

If the precursor forms of the sulfonate and carboxylate ionomers are available, alloys may also be prepared from solutions in Halocarbon Oil (Halocarbon Products Corporation). 

To be informative, it is desirable that the comparisons of these two different technologies be based on identical polymer backbones, having identical molecular weights, and having comparable levels of ionic functionality present. In addition, it is the purpose of these studies to make such comparisons with the same metal cation and thereby quantify, insofar as possible, the nature of the ionic interactions that exist. To do this, ionomers were prepared based on a polystyrene (PS) hydrocarbon backbone into which the ionic functionality was incorporated. PS was selected as the backbone because of the relative ease of functionalization and the relative freedom of side reactions during the sulfonation or carboxylation reactions. The polymers prepared were designed to come as close as possible in terms of ionic functionality for both sulfonate and carboxylate ionomers over a range of ionic contents. 

Under the conditions of this qualitative measurement the following observations can be made. The carboxylate ionomer displays a modest increase in the glass transition (Tg) compared with PS, but more significantly, it exhibits a plateau to temperatures greater than 150°C before a gradual softening is observed. This plateau is clearly attributable to the ionic associations persisting above the backbone Tg in this system. 

However, in the case of the sulfonated ionomer it is seen that while the Tg is again increased above that of PS, the plateau attributable to ionic associations persists to temperatures some 75° to 100 °C higher than that for the carboxylate ionomer. Similar but less dramatic differences between S-PS and C-PS are seen at lower functional levels (12). 

Elastomeric ionomers have also been developed from ethylene-propylene-diene ternary copolymers known as EPDM rubbers. The diene is commonly ethylidene norbomene. Du Pont made carboxylated ionomers by free-radical grafting of maleic anhydride (0.5—5%) onto the diene moiety of the polymer and neutralized the product with rosin salt. The ethylidene norbomene can also be sulfonated, thus ... 

Ames RL, Way D, Bluhm EA. 2005. Dehydration of nitric acid using perfluoro carboxylate ionomer membranes. J. Membr. Sci. 249 65-73. 

Asahi Kasei has developed [13-16] a modified membrane by treating the cathode side surface of the sulfonate ionomer to convert the sulfonate functional groups at and near the surface to carboxylate functional groups. It was found that the carboxyl ionomer of this structure was not sufficiently long-lived in the chlor-alkali environment and more stable carboxyl ionomer structures were developed. 

Comparison of the dynamic mechanical properties of the p-carboxylated polystyrene ionomers and those of the P(S-co-MANa) ionomers shows a more subtle difference. For the p-carboxylate ionomers the ion pairs are farther from the poljuner backbone than for the P(S-co-MANa), and thus form larger multiplets (102,103). Again, since the size of multiplets is smaller for the P(S-co-MANa), the total number of multiplets is larger, which results in the smaller amount of reduced mobility regions for the carboxylated polystyrene system, at a comparable ion content. As a result, the ionic modulus should be lower in the p-carboxylate system than in the methacrylate system. 

In the case of the p-sulfonated polystyrene ionomer, the ion pairs are at the same distance from the backbone as in thep-carboxylated polystyrene ionomer. As was mentioned above, however, the ionic interaction in the sulfonated ionomer is stronger than that in the carboxylated ionomer. Thns, at the identical ion content, the modulus-temperature curves show longer ionic platean, and the volume fraction of reduced mobility regions for the p-sulfonated ionomer is smaller than for the p-carboxylated ionomer (103), leading to less clnstering for the p-snlfonated ionomers. 

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