Thermally reversible ionomers

An early, and widely used, commercial example of side-chain functionalities inducing interactions between polymer chains are ionomers, hydrocarbon macromolecules bearing, for example, carboxylic acid groups [e.g., poly(ethylene-co-methacrylic acid)], which are partially or fuUy neutralized with metal or quaternary ammonium ions. These ionomers are thermoplastic ionic polymers boasting unique physical properties such as enhanced impact strength, toughness, and thermal reversibility. They were developed and commercialized by DuPont, and have recently attracted attention due to their self-healing properties. ... 

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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