Date on Honors Thesis

Spring 4-28-2021

Major

Chemistry and Music Performance (Piano)

Minor

Mathematics

Examining Committee Member

Dr. Kevin Miller, Advisor

Examining Committee Member

Dr. R. Daniel Johnson, Committee Member

Examining Committee Member

Dr. J. Ricky Cox, Committee Member

Examining Committee Member

Dr. Warren Edminster, Committee Member

Abstract/Description

Ionic liquids (ILs), besides being well known as environmentally friendly solvents, have attracted attention for a wide variety of other applications, including gas separation, 3D-printing and electrochemical actuators. However, for many such applications, incorporation of the IL moiety into a polymeric material eliminates the use of mechanically unstable free ILs while maintaining many of their electrochemical benefits. Such polymers can take two basic forms: poly(ionic liquid)s are ion-containing polymers in which the IL group is attached pendant to the polymeric backbone, while ionenes are ion-containing polymers in which the IL group is anchored directly into the backbone of the repeating unit. While incorporation into a polymer network inherently reduces ion mobility and thus leads to a decrease in conductivity, particularly in a covalently crosslinked polymer network, polymer architecture can be modified to maximize conductivity. The following experiments attempt to determine specific nanostructural parameters that govern the conductivities of imidazolium-containing ionene networks.

In this thesis, different imidazolium-containing monomers were synthesized by adding alkenyl linkers to an imidazole ring in order to correlate structural variation with polymer properties anion exchange to incorporate the bulky bistriflimide anion, which has been demonstrated in the literature to yield high conductivities relative to other anions. These monomers were incorporated into a covalently crosslinked ionene framework using thiol-ene photopolymerization. The crosslink density was controlled using two synthetic variations: the length of the alkenyl group on the original imidazolium-containing ene monomer, and the thiol:ene functional group ratio. In general, networks in which a stoichiometric thiol:ene functional group ratio was employed provided the highest gel fractions, thermal stabilities, glass transition temperatures (Tg’s), and crosslink densities. They also exhibited the lowest anhydrous ionic conductivities.

A combination of increasing the chain length of the ene monomer and a thiol:ene functional group ratio in which excess ene was utilized provided the highest ionic conductivities (~10-5 S/m at 30 °C). This trend of higher conductivity based on increased chain length of the ene monomer did not hold at high temperatures, though; above 50°C, the trend inverts, and the ionene networks with shorter chain lengths had higher conductivities. Moreover, incorporation of a second imidazolium ring in the ene monomer resulted in the highest observed conductivity.

These data suggest two conclusions. The first is that lower Tg values and high ion densities both contribute to an increase in conductivity. The second is that at low temperatures, Tg (polymer flexibility) is the controlling factor, whereas at high temperatures, at which presumably the thermal energy and thus polymer flexibility is high enough to support sufficient conduction for all networks, the ion density is the controlling factor (spatial ion clustering). The unlocking of this fascinating set of competing trends will lend guidance to future studies of related ionene networks. Overall, this thesis will demonstrate that, with precise control of the network architecture, that the thermal, mechanical and conductive properties can be appropriately tuned.

Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

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