Imidazolium Ionomer Derivatives of Isobutylene-Rich Elastomers: Thermosets, Emulsions, Filler Composites and Clay Nanocomposites
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Ionomers are valued for their exceptional physical properties, antimicrobial activity and superior adhesion to high surface energy solids and polymer blend components. Carboxylate, or sulfonate derivatives, of ethylene-rich thermoplastics are the most commercially available ionomers. Elastomeric ionomers bearing quaternary ammonium and phosphonium halide functionality have a literary standing in both scientific and patent-based publications. Currently cationic ionomers have shown great prominence in their inactivity to a wide range of bacteria and fungi. The specific focus of this research is in the derivatives of isobutylene-rich elastomers due to their exceptional impermeability, oxidative stability and vibration dampening characteristics. Imidazole-derived ionomers support a wider range of ionomer chemistry compared to ammonium and phosphonium analogues. N alkylation of nucleophiles including butyl imidazole, vinyl imidazole and 1,1’(1,4-butanediyl)bis(imidazole) by brominated poly(isobutylene-co-isoprene) yield thermally stable imidazolium bromide salts capable of supporting free radical cures and/or siliceous filler dispersions through further chemical modifications. The versatility of imidazole chemistry extends to the synthesis of isobutylene rich thermoset ionomers. This derives material properties from both a network of covalent crosslinks and a network of ion pair aggregates. Un-crosslinked elastomers are prone to creep and stress relaxation, hence a need for thermoset ionomer chemistry. Ion pairs are poorly solvated by the low dielectric constant of the polymer backbone and favoured thermodynamically by way of self assembly of the ionic functionality. The aggregation of ion pairs establishes a non covalent network of polymer chains whose dynamic mechanical properties approach those of conventional covalent thermosets comprised of carbon-carbon and/or sulfide crosslinks. However, the lability of this ionic network leads to a poor response to static loads leaving the thermoformable ionomers unqualified for engineering applications. A direct route is a more desirable method in preparing thermoset ionomers comprised of covalent crosslink networks and ionic functionality. In all, these reactive imidazolium ionomers look promising in supporting new value added applications for isobutylene rich derivatives, which include yet are not limited to elastomer thermosets, emulsions, filler composites and clay nanocomposites.