Effect of an n-Alkoxy-2,4-hexadiene Polymerizable Tail System on the Mesogenic Properties and Cross-Linking of Mono-Imidazolium-Based Ionic Liquid Crystal Monomers

Abstract

We demonstrate that an ether-based n-alkoxy-2,4-hexadiene polymerizable tail system is an effective and modular alternative to traditional ester-based polymerizable tail groups (i.e., acrylate, methacrylate, sorbate) and alkyl-1,3-diene tails for the design of radically polymerized ionic liquid crystal (ILC) monomers. Several series of nonsymmetric 1-vinylimidazoliumbromide-based ILC monomers containing these different polymerizable tail systems were synthesized and compared for their ability to form thermotropic liquid crystal (TLC) phases and to be photocross-linked with TLC phase retention. The n-alkoxy-2,4-hexadiene tail system was found to be more favorable/conducive to TLC phase formation than acrylate, methacrylate, and sorbate tails. It was more similar to the alkyl-1,3-diene tail system in terms of its more favorable effect on TLC behavior; however, it is more modular/easier to synthesize, more resistant to thermal Diels−Alder side reaction, and more isomerically pure, making it better for ILC monomer design. Also, the n-alkoxy-2,4-hexadiene tail system was found to be very amenable to radical photo-cross-linking with TLC phase retention. To demonstrate this feature, an example cross-linkable ILC monomer with this tail system was synthesized and polymerized in the smectic A TLC phase, and the monomer and polymerized material were characterized for their ionic conductivity behavior. I onic liquid crystals (ILCs) 1 are highly desirable due to their ordered nanostructures and ionic liquid (IL)-derived properties. 2 Depending on the makeup of the ILCs, their intrinsic proton 3 or ion 4 conductivity are most commonly tested; however, additional properties such as luminescence, 5 photoisomerization, 6 and magnetism 7 have also been incorporated, allowing them to be used as functional materials for many applications. 4b,8−11 Generally, stabilization of liquid crystal (LC) phases via cross-linking is a viable means to lock-in their nanostructures while imparting durability. 12,13 Cross-linked LC phases can be formed by reacting monofunctionalized LC monomers or reactive linear LC polymers with added cross-linker or multifunctional LC monomers with ≥2 chain-addition polymerizable groups per molecule. The latter approach is preferable for more densely cross-linked LC polymer materials and, in some cases, more successful LC phase retention after cross-linking. In our group, nanoporous cross-linked lyotropic (i.e., temperature-dependent and solvent-containing) ILC assemblies have been used as water filtration membranes, 14 heterogeneous catalysts, 13b and Li-ion conducting membranes in batteries. 15 In addition, ion-conductive, aligned hexagonal polymer films 16 via cross-linkable thermotropic (i.e., temperature-dependent and solvent-free) ILCs have also been formed. Literature examples of cross-linked ILC materials from other groups include lyotropic polymerized lipid bilayers, 17 crosslinked thermotropic ILCs with high anisotropic 18 and 3D 19 ionic conductivity, and nanostructured electrochromic poly-(ILC) fil...

Nanostructured Virus Filtration Membranes Based on Two-Component Columnar Liquid Crystals

“…The fluidity of the liquid crystals allows for control of their orientation and formation of defectless and no pinhole structures. LC materials with nanosegregated structures have been applied in transport materials for energy-related fields − and separation membranes for water treatment. − ,− Nanostructured membranes prepared from thermotropic ionic liquid crystals show selective salt rejection and high virus removal. − In particular, nanostructured water treatment membranes prepared from a bicontinuous cubic (Cub bi ) liquid crystal 3 (Figure c) showed >99.9999% rejection (log 10 reduction value (LRV) > 6) of the virus bacteriophage Qβ (diameter: 25 nm) . LRV is estimated by comparing the virus concentration in the feed and permeate solutions by the following equation where C p and C f are the virus concentrations in the permeate and feed solution, respectively.…”

Photopolymerization of Ionic Liquids – A Mutually Beneficial Approach for Materials Fabrication

“…In 2016 Gin and Roberston described polymers produced from several vinyl imidazolium salts with ether‐tethered cross‐linking functionalities . Photopolymerization was initiated with both 2‐hydroxy‐2‐methylpropiophenone and 2‐hydroxy4′‐(2‐hydroxyethoxy)‐2‐methylpropiophenone.…”

“…In 2016 Gin and Roberston described polymers produced from several vinyl imidazolium salts with ether-tethered crosslinking functionalities. [87] Photopolymerization was initiated with both 2-hydroxy-2-methylpropiophenone and 2-hydroxy4'-(2-hydroxyethoxy)-2-methylpropiophenone. Polymers synthesized from monomers containing n-alkoxy-2,4hexadiene achieved the best thermotropic liquid crystalline phases out of the polymers synthesized.…”