Carboxylate ionomers obtained

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 4. FT-IR spectra of (A) polystyrene/poly(isobutyl methacrylate) diblock copolymer and (B) carboxylated block ionomer obtained after hydrolysis with K0-.

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

EPDM-Derived Ionomers. Another type of ionomer containing sulfonate, as opposed to carboxyl anions, has been obtained by sulfonating ethylene—propjlene—diene (EPDM) mbbers (59,60). Due to the strength of the cross-link, these polymers are not inherently melt-processible, but the addition of other metal salts such as zinc stearate introduces thermoplastic behavior (61,62). These interesting polymers are classified as thermoplastic elastomers (see ELASTOLffiRS,SYNTHETIC-THERMOPLASTICELASTOLffiRS). 

Ionomer resins — Modified polymers obtained by heating and pressing certain polymers containing carboxylic groups in the presence of metallic ions. 

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. 

The preparation of ionomers involves either the copolymerization of a functionalized monomer with an olefinic unsaturated monomer or direct functionalization of a preformed polymer. Typically, free-radical copolymerization of ethylene, styrene, or other a-olefins with acrylic acid or methacrylic acid results in carboxyl-containing ionomers. The copolymer, available as a free acid, is then neutralized partially to a desired degree with metal hydroxides, acetates, or similar salts. The second route for the preparation of ionomers involves modification of a preformed polymer. For example, sulfonated polystyrene is obtained by direct sulfonation of polystyrene in a homogeneous solution followed by neutralization of the acid to the desired level. Some commercially available ionomers are listed in Table 15.17. 

This procedure can also be used for prepolymers possessing other reactive groups, such as mercapto, chloromethyl, bromomethyl epoxy, amino, aziridine, vinyl, carboxyl, hydroxy or alkoxymethyl groups. Oligomer emulsions can also be obtained by mixing water into the dissolved or liquid ionomer, because these products are inert with respect to water. 

There may be several reasons for this improvement caused by additives. One is the anchoring effect of additives against reorientation of polymer network at the Pt-ionomer interface. Second, the carboxylic acid group formed an ion pair with the impurity cations, and decreased the effective concentration of the latter ions in the polymer. The fact that CVs obtained with additives did not alter significantly... 

In addition to the perfluorosulfonic acid derivatives, carboxylic acid forms have been developed. These may be obtained by modification of Nafion films or by direct synthesis. In the case of modification, the ionomer is converted from a thionyl chloride to a carboxylic acid. Typical reducing agents are hydriodic acid, hydrobromic acid and hydrophosphonic acid. The carboxylic acid form can be made directly by copolymerization of tetrafluoroethylene and a carboxylated perfluorovinyl ether. The synthesis of the ether is a difficult task, however, as cyclization can occur. One synthetic route to the ether is shown in Scheme 5. After copolymerization with tetrafluoroethylene, the ester group can be quantitatively hydrolyzed to yield the sodium salt. 

Also semiconductor nanoparticles can be prepared in the presence of their polymeric matrix. Polymers with functional groups such as hydroxyl, carboxyl, or amines can form complexes with different metal salts. Subsequent treatment with, for example, H2S gas results in sulfide nanoparticles. Only recently Shen et al. used cadmium acrylate ionomers to form ion cluster, copolymerized with methyl methacrylate, and obtained highly transparent CdS/PMMA nanocomposites... 

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