Design, Synthesis, and Characterization of New Imidazolium-Based Ionic Liquid Crystals and Ionic Liquid Crystal Monomers
Ionic liquid crystals (ILCs) are molecules that combine the useful properties of ionic liquids (ILs) with the nanoscale self-assembled order of liquid crystals (LCs). This Ph.D. research focuses on two primary efforts: (a) to increase the IL-like character of new ILC molecules via incorporation of multiple imidazolium units, and (b) to enable the use of nanostructured materials over a range of environments via incorporation of multiple polymerizable groups to stabilize the formed LC phases via cross-linking.
Bicontinuous cubic LC phases have a distinctive 3D ordered nanostructure composed of two incompatible, intertwined domains with overall cubic symmetry. This feature makes these phases particularly useful and desirable for transport and uptake/release applications. A novel amphiphilic ILC platform based on a hexyl-bridged bis(imidazolium) headgroup connected to a single alkyl tail was developed that exhibited both bicontinuous cubic thermotropic LC (TLC) and lyotropic LC (LLC) phases. A total of fifteen compounds (CnX) with varying even tail lengths (Cn) and anions X = Br–, BF4–, or Tf2N– were synthesized. Many of these compounds exhibited other TLC and LLC phases.
To obtain cross-linkable CnX analogs, a systematic study of the effect of different polymerizable groups on TLC phase formation was performed. A series of non-symmetric, monoimidazolium- based amphiphilic salts possessing two radically polymerizable groups at each end was synthesized. The structural features that favored TLC formation were investigated via the use of different polymerizable groups (e.g., n-alkyl-2,4-hexadiene, acrylate, and vinyl) and alkyl spacer lengths. Long alkyl tail spacers and less polar polymerizable groups favored TLC behavior. The unprecedented n-alkyl-2,4-hexadiene polymerizable group presented the best TLC phase behavior, similar to the alkyl-1,3-diene tails previously pioneered by our group. Successful TLC phase retention upon in situ cross-linking was also demonstrated. Finally, initial ionic conductivity measurements on the cross-linked nanostructures formed by these new ionic TLC monomers were performed.