By far the most common chemistry for ion exchange polymers is based on styrene. The monomer and the polymer polystyrene are shown in the figure. It is possible to introduce functional groups after the polymer is formed, or styrene derivatives with the appropriate functional groups can be copolymerized with styrene. Very seldom is a simple linear polymer used because it is too oily and it changes dimensions markedly as one ion exchanges for another. Almost all ion exchange resins have some degree of crosslinking that comes from adding divinyl benzene to the reaction mixture. The vinyl groups participate in different chains and link them together as shown.
Typical crosslinking is from 0.5 to 15 per cent as determined by the proportions of styrene and divinyl benzene. The above figure is somewhat misleading, and the next Java applet may clarify things. Use the choice box to try different amounts of divinyl benzene, but do each one several times to get reasonable statistics. Note that there is not very much divinyl benzene. Note also that this applet does not complete the crosslinking and merely shows in red where there is potential for crosslinking. Having the benzene rings above and below the line of CH groups is better than having them lined up as in the first sketch, but the single bonds to a carbon atom are free to rotate and put the polymer into three dimensions.
The particularly significant effects as crosslinking increases are a prolonged time for equilibration and an accompanying decrease in capacity. Another property that depends on manufacturing technique is particle size. Small particles provide great surface but cause large head losses. There are many permutations of charge, strength, particle size, crosslinking, temperature, pH, and the like that allow tailoring ion exchange to a particular task.
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Mar 29 11:27:30 EST 1995