Fully-Zwitterionic Polymer-Supported Ionogel Electrolytes Featuring a Hydrophobic Ionic Liquid

Abstract

In this report, fully-zwitterionic (ZI) copolymer scaffolds for ionogel electrolytes have been synthesized via in situ photopolymerization using various molar ratios of 2-methacryloyloxyethyl phosphorylcholine (MPC) and sulfobetaine vinylimidazole (SBVI) within the hydrophobic ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI TFSI). Depending on the chemical composition of the ZI scaffold, ionogel room temperature ionic conductivities are found to vary between 2.5 and 6.7 mS cm at a fixed 20 mol % total polymer content. Compressive elastic moduli also exhibit a strong dependence on the co-monomer ratio, with values between 23 kPa and 11 MPa observed because of different degrees of ZI physical cross-linking. These results, together with NMR chemical shift analysis, suggest that the phosphorylcholine ZI group of MPC interacts more strongly with EMI TFSI, while SBVI prefers to self-aggregate and form dipole-dipole cross-links in the ionic liquid (IL). Self-diffusivity measurements of the EMI cations and TFSI anions in both ionogel and ZI solution samples confirm that slower ion diffusion in MPC-containing systems is due to attractive zwitterion/IL interactions, and not merely reduced mobility in the presence of a polymeric scaffold. This work highlights the importance of relative zwitterion/IL and ZI dipole-dipole interactions on the properties of a novel class of fully-ZI polymer-supported ionogel electrolytes containing a hydrophobic IL suitable for future electrical energy storage applications.

Zwitterionic Copolymer‐Supported Ionogel Electrolytes: Impacts of Varying the Zwitterionic Group and Ionic Liquid Identities

“…This is due to an increase in the density of noncovalent ZI dipole-dipole cross-links formed as the number of ZI groups in the scaffold is increased. While the greater solubility of MPC compared to SBVI in EMI TFSI was also found in our recent investigation of fully-zwitterionic copolymer-supported ionogels, [43] the mirroring of this trend in EMI TCB along with a higher solubility of SBVI in this more hydrophilic ionic liquid are important new observations. Second, as only ionogels having a homogeneous appearance are represented in Figure 3, it can be seen that the upper solubility limit of SBVI in the two ionic liquids was observed to differ (i. e. maximum 7 mol % loading in EMI TFSI gels vs. 10 mol % in EMI TCB gels), while MPC was much more soluble in both ionic liquids and yielded transparent ionogels up to at least 18.5 mol % (the largest ZI content examined in this study).…”

“…All gel precursor solutions contained 80 mol % ionic liquid; in order to ensure gelation and obtain visually homogeneous samples, the copolymer scaffold-forming components (totaling 20 mol %) consisted of 1.5 mol % pentaerythritol tetraacrylate (PETA-4), with the remaining 18.5 mol % comprised of varying amounts of the ZI monomer of interest (SBVI or MPC) and the non-zwitterionic monomer 2,2,2-trifluoroethyl methacrylate (TFEMA). [41,43] Figure 1c displays an illustrative cartoon representation of a zwitterion-containing poly(TFEMAco-MPC-co-PETA-4) scaffold that provides structural support to the surrounding ionic liquid, forming a composite ionogel electrolyte. Consistent with the highly efficient nature of free radical photopolymerization in ionic liquids, [53] we do not find any detectable evidence of unreacted monomers in these types of ionogels after UV curing.…”

“…Our group was the first to report the creation of zwitterionic copolymer-supported ionogel electrolytes featuring a hydrophobic ionic liquid with a wide electrochemical stability window, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI TFSI), which were readily synthesized via in situ UV-initiated free radical copolymerization. [43] One of the important findings of that study was the large difference in solubility within the ionic liquid exhibited by different zwitterionic monomers. [42] Recently, we demonstrated the ability of fullyzwitterionic copolymer-supported EMI TFSI ionogels to promote high ionic conductivity while tuning gel stiffness by controlling the ZI co-monomer ratio.…”

“…[35,36] Thoughtful design of specific ion-polymer interactions, however, has been successfully shown as one potential strategy to improve ion pair dissociation within ionogel electrolytes. [41][42][43][44][45][46][47] While several studies have elaborated the use of ZI polymers for hydrogel materials, primarily selected because of their antifouling character, biocompatibility, or self-healing attributes, [46,48,49] there are relatively few reports on ZI polymersupported nonaqueous gel electrolytes. [41][42][43][44][45][46][47] While several studies have elaborated the use of ZI polymers for hydrogel materials, primarily selected because of their antifouling character, biocompatibility, or self-healing attributes, [46,48,49] there are relatively few reports on ZI polymersupported nonaqueous gel electrolytes.…”

“…[41] Sun et al described the formation of ZI homopolymer-based ionogels within the halogen anion-based, water-miscible ionic liquid 1-butyl-3-methylimidazolium chloride. [43] In this report, we describe the synthesis and characterization of ZI copolymer scaffold-supported ionogel electrolytes formed in situ within two different aprotic ionic liquids, and we compare the behaviors of two distinct ZI monomers. [43] One of the important findings of that study was the large difference in solubility within the ionic liquid exhibited by different zwitterionic monomers.…”