Justin W. Diehi St. Bonaventure University, St. Bonaventure, NY 14778 The scope of ionomers as ion-containing polymers is rather extensive, and this topic has been reviewed in depth by several authors (14).This article will introduce ionomers and contrast them with the properties of the two major classes of polymers, thermoplastic and thermosetting. It should be noted that reference 4, in particular, gives an excellent introductory overview plus numerous literature citations for additional reading on ionomers. Ionomers are generally defined as polymers that contain pendant acid groups, which are subsequently partially neutralized (3070%)to form salts. Thermoplasllc
Thermoplastic materials are macromolecules that can soften when heated and harden on cooling. A recent article (7)describes a rapid identification of thermoplastic polymers and a means of distinguishing between thermosets and thermoplastics. The structure of the polymer in thermoplastics is a series of linear strands with few, if any, cross-links between the strands. See Figure 1.
The individual macromolecular strands can be intertwined by chain entanglement, thus creating a polymer with some enhanced mechanical properties. Weak, secondary bonding forces can also lead to association of these individual strands. The glass transition temperature, T g ,may be a relative measure of the extent of these forces. The T , is a temperature range at which the properties of a polymer change from a brittle solid (or glass) to a rubbery elastother. In fact, the T g is a indication of possible end uses for a particular polymer. Some engineering and processing conditions are generally dictated by and dependent upon the T,. Thus thermoplastic polymers can be processed in conventional molding equipment at a convenient temperature somewhat above their T g(8). Introducing stronger secondary forces, such as hydrogen bonds in polyamides, generally increases the T , value cousiderably (9).See Figure 2. Thermosets
Thermosets have definite chemical covalent bonds or cross-links between the polymeric strands. See Figure 3. The cross-linked product or network polymer is permanently hardened and will not reversibly soften on heating. Crosslinkine of the linear and inde~endentstrands occurs when a funrt&nalgroupor bond chekcally spansor links twoseparaw chains. This is the essential difference between a tbermoplastic material (no cross-links) and a thermoset (some cross-links) macromolecule. The extent of cross-linking in the final macromolecule helps dictate some properties. Network polymers generally have no discernible T,s' and, once formed, cannot be remelted, dissolved, or reshaped. They are typically "hard", although many elastomers are lightly'
Figure 1. mermDplaSnc polymer fmmatlon.
Thermoset: A netwrh mocmrnolecule, has crosslinks Examples include concrete, Phenol-formaldehyde resins, etc. Figure 2. Hydmgen bonds In polyarnldes.
Figure 3. Thermoset polymer.
Volume 66 Number 11 November 1989
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0-0
+
0-9 C W
/
c=c
H
MER A
MER B
I
Ethylene
\
H
+
,
C
/
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=C
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Melhocr~lic Acid
Copolymer
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Figure 4. lonomer famation.
The original ionomers, introduced in the early '60's, were copolymers of ethylene and methacrylic acid. See Figure 6. Sodium and zinc cations were employed for the neutralization step. The repeating units are mixtures of neutralized copolymer and unneutralized copolymer. The ionic portion of an ionomer is generally a carboxylic acid or sulfonic acid group. Synthetic variations in the backbone copolymer, types of ionizable groups, cations used for neutralization, and the extent of neutralization are all heing actively investigated.
Figure 5. l o m m ionic cross-linking.
cross-linked and are not "hard". Hardness due to a lack of molecular flexihility makes the processing of an engineering polymer an arduous task. homers Ionomers represent a convenient middle ground between thermoplastic and thermosets in which the cross-links are not covalent bonds hut rather ionic crosslinks. The unique feature of ionomers, or ion-containing polymers, is that they can he processed in conventional molding equipment since the cross-links are thermally reversible. A typical example of an ionomer would be a random copolymer containing a carhoxylic acid pendant group. This carboxyl group can he partially neutralized by a monovalent or a divalent cation as shown in Figure 4. The interchain forces resulting from this ionic bond produces properties normally associated with a cross-linked thermoset polymer. Figure 5 shows this ionic cross-linking creates a strong ionic-type hond between the various linear ionomer strands. However, the ionic-type hond is not a covalent bond, as in thermosets. Thus raising the temperature minimizes the ionomeric hond, and the material may he reprocessed. A traditional thermoset does not have this thermal reversihility property. 902
Journal of Chemical Education
Use of lonomers It appears that ionomers can he tailored to a very wide variety of end uses. Most of the ionomeric resin is converted into packaging film for sharp items because an ionomer has exceptional puncture resistance. The resin also lends itself to skin packaging since it has a high melt strength in processing. The resin is also injection molded for such items as golf balls covers, auto bumper guards, bowling pins, and shoe components. Employing pendant sulfonate groups for the ionic association creates particularly strong physical crosslinking (10). The introduction of polymer chemistry into the undergraduate core curriculum appears a necessity in order to train future scientists properly. This subject has been covered numerous times in the recent literature and this Journal devoted acomplete issue to it (11).Moreinformation can also be found invarious texthooks on general polymer chemistry (12). 1. Holllday, L., Ed. Ionic Polymerr: Halsfead: New York, 1975. 2. Eisenbelg, A,; King. M. Ion-Contoinin8 Polymers; Academic New York. 1977. 3. Bazuin,C. G.; Eknberg,A.Ind.Eng Chem.ProdurlRes.DoveIopmrnf 1981.20,271-
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4. 5. 6. 7. 8.
Bszuin, C. G.: Eisenberg, A.J Chom. Educ. 1981.58.93%943. MacKnieht. W. J.: Earneat.T. R. J. Polvm. Sei. Macmmol. Reu. 1981.164147 0foeka.E.P. J . Moeromo~.'sei.them. i971.5.275-279. Cloutier, H.; Pmd'homme. R.E. J. Chem. Educ. 1985.62.815-819. Csrraher. C. E.: Seymour. R. B. J. Chem. Edue. 1986.63.418-419. Carraher, C. E.: Seymour,R. B. J . Chem.Educ. 1987,64,866867. Carraher, C, E.;Seymour, R.B. J , Chem. Educ. 1988.65,31&318. 9. Beck, K. R.: Karsmeyer, R.: Kunz, R J. Chem. Edue. 1984,61,668670. 10. Bl~nnerD.; Lundbem, R. D. J . Chemtech 1977,748-752. 11. J. Chem. Educ. 1981.58.831-958 12. Seymour, R.B.: Csrrsher,C. E. Polymer Chemistrr An Introduction,2nd ed.; Dekker: New York, 1987.
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