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. 

Zn-EMAA ionomers, 74 464-465, 474, 482. See also EMAA ionomers Zn-EPDM elastomeric ionomers, 74 482 ZnSe system, for laser diodes, 22 179 Zn-soap, in fatty acid neutralization, 22 740... 

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. 

The ionomer which was isolated from the neutralization of sample SBD-2 was a brown-colored elastic network of moderate strength. Ionomer samples SBD-1 and SBD-2, neutralized to the stoichiometric end point using KOH, were compression molded at 140°C and examined for tensile properties. The results, as shown in Figure 16, illustrate the profound influence of crystallinity on the elastomeric inner block. The semi-crystalline material (SBD-1) behaves much like a rigid plastic, while the amorphous sample (SBD-2) is an elastomer of moderate strength. 

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. 

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. 

A number of ionic polymers exist that have a recognized elastomer as the covalent backbone and have a small ionic content, so they may be called elastomeric ionomers. The ions provide at least a part of the cross-links in these polymers. Those elastic ionomers that are cross-linked exclusively by their ions have, however, the useful feature of being thermoplastic. 

Elastomeric ionomers have also been developed from ethylene-propylene-diene ternary copolymers known as EPDM rubbers. The diene is commonly ethylidene norbomene. Du Pont made carboxylated ionomers by free-radical grafting of maleic anhydride (0.5—5%) onto the diene moiety of the polymer and neutralized the product with rosin salt. The ethylidene norbomene can also be sulfonated, thus ... 

Polymer alloys comprised 20-30 wt% PP, 25-35 wt% uncross-linked elastomeric ethylene-propylene-1,4-hexadiene, EPDM (60-80 wt% ethylene), 30-50 wt% ionomer, and 2-3 wt% ethylene/ -butyl acrylate/ glycidyl methacrylate, EBA-GMA. The blends were used in applications where a wide temperature range and abrasive conditions were encountered... 

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]. 

Thermoplastic elastomers based on blends of a silicone rubber (cross-linked during processing) with block copolymer thermoplastic elastomers have also been prodnced (36,37). Other types that have been stndied (38) include graft copol5uners and elastomeric ionomers, but these have not become commercially important. 

Introduction of ionic groups or proteins into polymers (forming ionomers) leads to physical associations at the temperature of use [110, 111]. For example, Surlyn (DuPont) is a copolymer of ethylene and methacrylic add that shows enhanced zero-shear viscosity and elastomeric green strength. Viscoelastic characteristics are also enhanced due to loss of associations at the appropriate dissociation temperature. Ion pair associations are exploited to obtain misdbUity in otherwise immiscible polymers [112],... 

We would like to make one last but important comment. With aU the new membranes, the current MEA technology that uses a Nation ionomer as an electrode binder should be optimized for new electrolyte and DMFC systems. Novel MEA processes for new electrolytes will reduce the interfacial resistance of membranes and electrodes. Although conventional membranes, such as Nafion, have an elastomeric property and low glass transition temperature, methanol-impermeable membranes have some degree of rigidity. Thus, the adhesion process has to be optimized so that those efforts of membrane development can maximize MEA performance. 

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. 

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