Ethylene based ionomers

This article focuses on the commercial, ethylene-based ionomers and includes information on industrial uses and manufacture. The fluorinated polymers used as membranes are frequently included in ionomer reviews. Owing to the high concentration of polar groups, these polymers are generally not melt processible and are specially designed for specific membrane uses (see Fluorine compounds, organic—perfluoroalkane sulfonic acids Membrane technology). 

The semicrystalline, ethylene-based ionomers of commerce are flexible, transparent polymers notable for high strength and elasticity in both soUd and molten states. The ionic bonding is completely reversible (8) and has a strong influence on properties, even at temperatures well above the melting point. 

Ethylene-based ionomers, 14 482 Ethylene-based (C-2) process, for methyl methacrylate production, 16 244, 252-254... 

On the basis of these measurements and related physicochemical studies, it is possible to process a structure for ethylene-based ionomers. The un-ionized acid copolymer is thought to consist of two phases a crystalline polyethylene phase and an amorphous phase consisting of polyethylene crosslinked by hydrogen-bonded carboxylic dimers. The ionized copolymers exhibit three distinct phases a crystalline polyethylene phase, an amorphous polyethylene phase, and a dispersed ionic phase consisting of ionic domains. 

This is evidenced by the amount of literature on ionomers and by the appearance of two monographs devoted to the subject (J, ). Most of the research effort on the ionomers has focused on only a small number of materials, notably ethylenes (3-9 ), styrenes (10,11), rubbers (12-16) and recently aromatic (17) and fluorocarbon-based ionomers (18). The last material is known for its high water permeability and cation permselectivity. Because of its unique properties, it has been employed as an ion-exchange membrane in chlor-alkali cell operations in electrochemical industries. Perfluorinated ion-exchange membranes are the subject of the present chapter. 

Wouters, M. E. L., Litvinov, V. M., Goossens, J. G. R, Binsbergen, F. L., van Duin, M., and Dikland, H. G. 2003. Morphology of ethylene-propylene copolymer based ionomers as studied by solid state NMR and small angle x-ray scattering in relation to some mechanical properties. Macromolecules 36 1147-1156. 

In general, the preparation of ionomers is a straightforward procedure. The particular acid group of interest can be introduced onto the hydrocarbon backbone either by direct copolymerization or post-synthesis reaction. The following five important groups of ionomers illustrate the various methods of preparation. These ionomer families are ethylene-based materials, ionic elastomers, modified polystyrenes, perfluorinated resins and halato-telechelic polymers. 

The term ionomer is used to describe a variety of copolymers consisting primarily of hydrocarbon backbones to which are attached a relatively small number of ionizable branches. This family of polymers includes two groups that are ethylene-based, one based on ethylene-co-methaciylic acid copolymer and the other on sulfonated ethylene-propylene-diene terpolymer. The latter is an essentially noncrystalline mbber that is beyond the scope of the semicrystalline materials covered in this work (the synthesis, properties, and structure of sulfonate ionomers are reviewed in Ref 90). lonomers based on partially or completely... 

Within the scope of the original definition, a very wide variety of ionomers can be obtained by the introduction of acidic groups at molar concentrations below 10% into the important addition polymer families, followed by partial neutralization with metal cations or amines. Extensive studies have been reported, and useful reviews of the polymers have appeared (3—8). Despite the broad scope of the field and the unusual property combinations obtainable, commercial exploitation has been confined mainly to the original family based on ethylene copolymers. The reasons for this situation have been discussed (9). Within certain industries, such as flexible packaging, the word ionomer is understood to mean a copolymer of ethylene with methacrylic or acryhc acid, partly neutralized with sodium or zinc. 

Ionomer resins consisting of ethylene—methacrylic acid copolymers partially neutralized with sodium or zinc were commercially introduced in 1964 by Du Pont under the Sudyn trademark (1). More recently, a similar line of products, sold as Hi-Mdan resins, has been commercialized by Mitsui—Du Pont in Japan. lolon ionomeric resins, based on ethylene—acrylic acid, are produced by Exxon in Belgium. Ionomers containing about 1 mol % of carboxylate groups are offered by BP in Europe as Novex resins. Low molecular weight, waxy Aclyn ionomers are produced and sold by AHiedSignal. 

A process based on saponification of ethylene—acrylate ester copolymers has been practiced commercially in Japan (29). The saponification naturally produces fully neutralized polymer, and it is then necessary to acidify in order to obtain a pardy neutralized, melt-processible product. Technology is described to convert the sodium ionomer produced by this process to the zinc type by soaking pellets in zinc acetate solution, followed by drying (29). 

In most ionomers, it is customary to fully convert to the metal salt form but, in some instances, particularly for ionomers based on a partially crystalline homopolymer, a partial degree of conversion may provide the best mechanical properties. For example, as shown in Fig. 4, a significant increase in modulus occurs with increasing percent conversion for both Na and Ca salts of a poly(-ethylene-co-methacrylic acid) ionomer and in both cases, at a partial conversion of 30-50%, a maximum value, some 5-6 times higher than that of the acid copolymer, is obtained and this is followed by a subsequent decrease in the property [12]. The tensile strength of these ionomers also increases significantly with increasing conversion but values tend to level off at about 60% conversion. 

For partially crystalline ionomers, such as those based on copolymers of ethylene and methacrylic acid, even time or aging at room temperature can have an effect on mechanical properties. For example, upon aging at 23°C, the modulus of the acid form of the copolymer increased 28%, while in the ionomer form, the increase ranged up to 130%, with the specific gain in modulus depending on the degree of conversion and on the counterion that was present [17]. 

Since 1960, gutta-percha or synthetic balata has been replaced by ionomer for use as a cover material in golf balls. It is based on a combination of ethylene and a carboxylic acid. It can be processed at a reasonable temperature to produce good, tough ball covers. Because of higher processing temperature of the ionomer... 

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. 

Ionomer. Ionomer is the generic name for polymers based on sodium or zinc salts of ethylene-methacrylic acid copolymers in which interchain ionic bonding, occurring randomly between the long-chain polymer molecules, produces solid-state properties. 

Sodium ionomers are commercially recognized as the most effective nucleating agents for PET compounds. The typical use rate of sodium ionomer-based nucleating agents is 3-4 wt%. The sodium salt of poly(ethylene-co-methacrylic acid) is a particularly effective nucleating agent for PET. 

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

Mani S, Weiss RA, Williams CE, Hahn SF. Microstructure of ionomers based on sulfonated block copolymers of polystyrene and poly(ethylene-afe-propylene). Macromolecules 1999 32 3663-3670. 

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