Carboxylated ethylene copolymers, metal salts

Carboxylated ethylene copolymers, metal salts (ionomers)... 

Carboxylated ethylene copolymers, metal salts (ionomers) Ethylene-propylene-diene monomer elastomers Ethylene-yinyl acetate copolymer Ethylene-yinyl alcohol copolymer... 

Neutralization of ethylene copolymers containing up to 5%-10% acrylic or methacrylic acid copolymer with a metal salt such as the acetate or oxide of zinc, magnesium, and barium yields products referred to as ionomers. (Commercial products may contain univalent as well as divalent metal salts.) lonomers are marked by Du Pont under the trade name Surlyn. These have interesting properties compared with the nonionized copolymer. Introduction of ions causes disordering of the semicrystalline structure, which makes the polymer transparent. lonomers act like reversibly cross-linked thermoplastics as a result of microphase separation between ionic metal carboxylate and nonpolar hydrocarbon segments. The... 

It is not out of place to give brief mention to the ionomers introduced by Du Pont in the mid-1960s. To produce these polymers an alkene, usually ethylene, is copolymerized with a few per cent of a second monomer such as an -carboxylic acid in order to introduce a few carboxylic acid groups into the chain. The copolymer is then blended with a metal salt which ionizes the acid group. Heat fugitive ionic cross-links form (between carboxylic groups via the metal... 

Ethylene—Dicarboxylic Acid Copolymers. Partial neutralization of copolymers containing carboxyls in pairs on adjacent carbons, eg, ethylene—maleic acid, has been described (11). Surprisingly, there is no increase in stiffness related to neutralization. Salts with divalent metal cations are not melt processible. The close spacing of the paired carboxyl groups has resulted in ionic cluster morphology which is distinct from that of the commercial ionomer family. 

Polymers with a sizable number of ionic groups and a relatively nonpolar backbone are known as ionomers. The term was first used for copolymers of ethylene with carboxylated monomers (such as methacrylic acid) present as salts, and cross-linked thermoreversibly by divalent metal ions. Such polymers are useful as transparent packaging and coating materials. Their fluorinated forms have been made into very interesting ion-exchange membranes (considered further below). 

Ionomers consist of statistical copolymers of a non-polar monomer, such as ethylene, with (usually) a small proportion of ioniz-able units, like methacrylic acid. Ethylene-co-methacrylic acid copolymers (-5% methacrylic acid) are used to make cut-proof golf balls (see Fascinating Polymers opposite). The protons on the carboxylic acid groups are exchanged with metal ions to form salts. These ionic species phase-separate into microdomains or clusters which act as crosslinks, or, more accurately, junction zones (Figure 6-4). (We discuss interactions in a little more detail in Chapter 8.)... 

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. 

In addition to being a synthetic route to unusual graft copolymers, the metalation technique offers a way to add functional groups to the chain by reactions characteristic of organolithium compounds. Hydroxyl or carboxyl groups, for instance, can be added by treating the metalated polyisoprene or polybutadiene (22) solution with ethylene oxide or C02, respectively. The lithium alkoxide and carboxylic salt obtained (23) in... 

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. 

Copolymers of ethylene and methacrylic acid are also the basis of interesting materials which have been called ionomers. These are prepared by copolymerizing ethylene with 1-10% methacrylic acid using a high pressure process. The copolymer is then treated with a derivative of a metal (generally sodium or zinc) and some of the carboxylic groups are ionized. The sodium salt, for example, may be represented as follows ... 

---