Crosslinking elastomeric ionomers

It may also be argued that plasticised PVC may be considered as a thermoplastic elastomer, with the polymer being fugitively cross-linked by hydrogen bonding via the plasticiser molecules. These materials were, however, dealt with extensively in Chapter 12 and will not be considered further here. The ionomers are also sometimes considered as thermoplastic elastomers but the commercial materials are considered in this book as thermoplastics. It should, however, be kept in mind that ionic cross-linking can, and has, been used to fugitively crosslink elastomeric materials. 

Although much of early work on ionomers had focused on non-elastomeric materials, attention has recently been shifted to elastomeric ionomers as potential thermoplastic elastomers(TPE), i.e. elastomers which flow at high temperatures yet retain their network structure at ambient temperatures. For a materid to fiinction as a useful elastomer, the polymer chains must be interconnected in a three-dimensional network. Classically, such crosslinked elastomers cannot flow readily. However, if an elastomer is physically crosslinked via strong ionic bonds, this may lead to a potential TPE. The ionic bonds form physical crosslinks between the polymer chains and thus promote good elastomeric character, yet at higher temperatures they become sufficiently labile to allow the material to flow and be processed as a TPE. 

Another approach of physically crosslinked SMP networks was demonstrated by the melt blending of an elastomeric ionomer based on the zinc salt of sulfonated poly[ethylene-ran-propylene-ran-(5-ethylidene-2-norbornene)] and low molecular mass fatty acids. In such a polymer network the nanophase separated ionomer provided the permanent network physically crosslinked by the zinc salt, while the fatty acids are located in nanophases, whose melting is triggering the shape recovery [61]. 

Elastomeric ionomers based on the sulfonation of chlorinated PE were introduced by DuPont during the early 1950s. Curing of these materials with various metal oxides gives rise to a combination of ionic and covalent crosslinks and these elastomers are commercially available under the trade name Hypalon. 

With the exception of elastomeric hydrocarbon polymers, other crosslinked polymers besides of ionomers have found little commercial success as compared to the uncrosslinked hydrocarbon polymers. 

Since the ions in ionic polymers are held by chemical bonds within a low dielectric medium consisting of a covalent polymer backbone material with which they are incompatible, the polymer backbone is forced into conformations that allow the ions to associate with each other. Because these ionic associations involve ions from different chains they behave as crosslinks, but because they are thermally labile they reversibly break down on heating. lonomers therefore behave as cross-Unked, yet melt-processable, thermoplastic materials, or if the backbone is elastomeric, as thermoplastic rubbers. It should be noted that it is with the slightly ionic polymers, the ionomers, where the effect of ion aggregation is exploited to produce meltprocessable, specialist thermoplastic materials. With highly ionic polymers, the polyelectrolytes, the ionic cross-linking is so extreme that the polymers decompose on melting or are too viscous for use as thermoplastics. 

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