Ion and polymer transport in polymerized ionic liquids

Polymerized ionic liquids (PILs) is a novel type of ionic polymers in which small molecule ionic liquids (ILs) are covalently attached to a polymer backbone. The advantages of ILs include high thermal and electrochemical stability, high ionic conductivity, negligible volatility and board design space, which enabled a wide variety of engineering applications. By incorporating IL moieties into a polymer structure to form a PIL, it’s possible to overcome the liquid nature of an IL with a mechanically more robust material by leveraging the ion conduction properties of ILs and the viscoelasticity of the polymer. The overarching theme of this PhD thesis is to understand the fundamental mass transport and material dynamics in PILs. The first model system is a series of backbone PILs where the IL moieties are directly placed on the polymer backbone. The tethered ammonium (Am) cations and mobile bis(trifluoro-methane)sulfonamide (TFSI) anions were fixed throughout this system. The linear and network PILs with either dodecyl (C11) or tetraethylene oxide (4EO) linkers were synthesized via step-growth polymerization and compared. The glass transition temperature (Tg), the extent of ion aggregation increased while the ionic conductivity (σ) at a fixed temperature decreased upon crosslinking and switching from polar 4EO to non-polar C11 linker. The network C11 PIL showed enhanced ion conductivity close to Tg when σ(T) data was normalized by Tg and such decoupling between ion and polymer dynamics was attributed to the aggregates structure providing percolated pathways for ion conduction when the segmental motion slows down significantly. In addition, by fixing the network structure and the hydrocarbon linker chemistry, the linker length and mobile ion type were systematically tuned. Odd-even effects in Tg and σ were observed when the linker length changed from butyl (C4) to decyl (C12) in the TFSI PILs while it was not observed in the BF4 PILs. The odd linker TFSI PILs had consistently lower Tg and higher conductivity than the even linker PILs and the effect diminished as linker length increased. The extent of ion aggregation monotonically increased with linker length and was not responsible for the odd-even effects. The network C4 TFSI PIL showed 1 order of magnitude higher conductivity close to Tg compared to the other even linker TFSI PILs. The second model system is a series of pendant PILs where the IL groups are affixed to the polymer side chain. The studied ions are tethered imidazolium (Im) cations and mobile TFSI anions. Here linear acrylate PILs with increasing molecular weight (MW) were synthesized by living radical polymerization and dynamic properties including zero-shear viscosity (η0), storage (G’) and loss (G”) modulus were investigated. The degree of polymerization (N) of the intermediate bromide (Br) PILs was accurately characterized by size exclusion chromatography (SEC) with light scattering (LS). Low dispersity (Đ <1.23) and a good match of MW between SEC and end group analysis were found. Viscosity-MW scaling of linear Im TFSI PILs showed η0 ~ N1.0 (N=15-92, unentangled) and η0 ~ N2.3 (N=92-254, transition), which resolved the conflicting results in previous literature and indicated the electrostatic friction doesn’t increase the viscosity scaling factors. The ionic conductivity slightly decreased with increasing MW and they overlapped once normalized by Tg. To probe the polymer self-diffusion, fluorescent labels was copolymerized into these linear PILs and polymer diffusion coefficient (D) was measured by fluorescence recovery after photobleaching (FRAP). The experimental diffusion-MW scaling closely followed the expected D~N-1 relationship in the unentangled regime, which proved the long time polymer self-diffusion is also not affected by the charge interaction. In addition, the cation and anion types were varied in these linear pendant PILs and the ionic conductivity of all PILs decreased as polymer thickness reduced below 100nm, indicating significant confinement effect.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Materials Science & Engr

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Zhao, Qiujie

Contributors dc:contributor

• Evans, Christopher M
• Schroeder, Charles M
• Schweizer, Kenneth S
• Braun, Paul V

• polymerized ionic liquids
• poly(ionic liquids)
• ionic polymer
• ion transport
• polymer dynamics

Statement dc:rights

• Copyright 2021 Qiujie Zhao

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/113181

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/113181