An ionomer (/ˌaɪˈɑːnəmər/) (iono- + -mer) is a polymer that comprises repeat units of both electrically neutral repeating units and a fraction of ionized units (usually no more than 15 mole percent) covalently bonded to the polymer backbone as pendant group moieties. This means that most ionomers are copolymers of the neutral segments and the ionized units, which usually consist of carboxylic acid groups.
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The classification of a polymer as an ionomer depends on the level of substitution of ionic groups as well as how the ionic groups are incorporated into the polymer structure. For example, polyelectrolytes also have ionic groups covalently bonded to the polymer backbone, but have a much higher ionic group molar substitution level (usually greater than 80%); ionenes are polymers where ionic groups are part of the actual polymer backbone. These two classes of ionic-group-containing polymers have vastly different morphological and physical properties and are therefore not considered ionomers.
Ionomers have unique physical properties including electrical conductivity and viscosity—increase in ionomer solution viscosity with increasing temperatures (see conducting polymer). Ionomers also have unique morphological properties as the non-polar polymer backbone is energetically incompatible with the polar ionic groups. As a result, the ionic groups will microphase separate into ionic-rich domains in most ionomers.
Some commercial applications for ionomers are golf ball covers, semipermeable membranes, sealing tape and thermoplastic elastomers. Common examples include polystyrene sulfonate, Nafion and Hycar.
Synthesis
Usually ionomer synthesis consists of two steps – the introduction of acid groups to the polymer backbone and the neutralization of some of the acid groups by a metal cation. In very rare cases, the groups introduced are already neutralized by a metal cation. The first step (introduction of acid groups) can be done in two ways; a neutral non-ionic monomer can be copolymerized with a monomer that contains pendant acid groups or acid groups can be added to a non-ionic polymer through post-reaction modifications. For example, ethylene-methacrylic acid and sulfonated perfluorocarbon (Nafion) are synthesized through copolymerization while polystyrene sulfonate is synthesized through post-reaction modifications.
In most cases, the acid form of the copolymer is synthesized (i.e. 100% of the carboxylic acid groups are neutralized by hydrogen cations) and the ionomer is formed through subsequent neutralization by the appropriate metal cation. The identity of the neutralizing metal cation has an effect on the physical properties of the ionomer; the most commonly used metal cations (at least in academic research) are zinc, sodium, and magnesium. Neutralization or ionomerization, can also be accomplished in two ways: the acid copolymer can be melt-mixed with a basic metal or neutralization can be achieved through solution processes. The former method is preferred commercially. However, as commercial manufacturers are reluctant to share their procedures, little is known about the exact conditions of the melt-mixing neutralization process other than that hydroxides are generally used to provide the metal cation. The latter solution neutralization process is generally used in academic settings. The acid copolymer is dissolved and a basic salt with the appropriate metal cation is added to this solution. Where dissolution of the acid copolymer is difficult, simply swelling the polymer in the solvent is sufficient, though dissolving is always preferred. Because basic salts are polar and are not soluble in the non-polar solvents used to dissolve most polymers, mixed solvents (e.g. 90:10 toluene/alcohol) are often used.
Neutralization level must be determined after an ionomer is synthesized as varying the neutralization level varies the morphological and physical properties of the ionomer. One method used to do this is to examine the peak heights of infrared vibrations of the acid form. However, there may be substantial error in determining peak height, especially since small amounts of water appear in the same wavenumber range. Titration of the acid groups is another method that can be used, though this is not possible in some systems.
Surlyn
For example, DuPont's created the coating and packaging material Surlyn, a copolymer of ethylene and methacrylic acid. Ethylene polymerized with a small amount of methacrylic acid results in the copolymer poly(ethylene-co-methacrylic acid), which is polyethylene, with occasional repeat units of methacrylic acid. DuPont neutralizes the acid with NaOH, yielding the sodium salt. Crystals of ethylene-methacrylic acid ionomers exhibit dual melting behavior.