Anhydrous Proton Transport in Polymerized Ionic Liquid Block Copolymers: Roles of Block Length, Ionic Content, and Confinement

Abstract

Anhydrous proton transport has been investigated in a series of proton conducting polystyrene-blockpolymerized ionic liquid (PS-b-PIL) copolymers spanning a range of molecular weights and compositions. The PIL is a macromolecular analogue of imidazolium bis(trifluoromethane sulfonimide) (ImTFSI), a well-known proton conducting ionic liquid, and consists of imidazole linked to a polymer backbone via the 5-carbon. In contrast to prior work on nitrogen-linked imidazolium PILs, carbon-linked imidazolium has two nitrogens which can both function as proton donor/acceptors and participate in Grotthus mechanism conduction. The conductivity of the PIL block is shown to be dramatically impacted upon confinement by a PS block and can exceed the conductivity of the homopolymer in the range of 30−130 °C for PIL-rich block copolymer composition. At high temperature the conductivities track with ionic content while at room temperature the conductivities are nonmonotonic. X-ray scattering reveals a suppression of the peak associated with ionic aggregation in all block copolymers relative to the homopolymer consistent with the higher conductivities observed at room temperature. The dependence of ionic conductivity on temperature, as quantified by the VFT strength parameter D, decreases with decreasing PIL block length corresponding to a change in the packing efficiency of the conductive block. These changes in packing are hypothesized to lead to the different temperature dependences of conductivity which cause the nonmonotonic block copolymer conductivities observed at room temperature. Finally, we demonstrate that the fraction of PIL in the block copolymer is the main factor governing the high temperature ionic conductivity of these materials while confinement effects become important at room temperature.

Sulfonated Polymerized Ionic Liquid Block Copolymers

“…The ion conductivity of poly(S‐ b ‐AEBIm‐TFSI) + Li‐TFSI/IL and the nonsulfonated PIL block copolymer, poly(S‐ b ‐AEBIm‐TFSI), from a previous study with a similar PIL composition to this study is also shown in Figure . Recent work by Segalman and co‐workers with a similar styrene‐based PIL block copolymer with mobile TFSI anions at a comparable PIL composition of 26 mol% PIL shows comparable anhydrous ion conductivity to poly(S‐ b ‐AEBIm‐TFSI) over the same temperature range. In comparison to the non‐sulfonated PIL block copolymer, the anhydrous ion conductivity for poly(SS‐Li‐ b ‐AEBIm‐TFSI) + Li‐TFSI/IL is over two orders of magnitude higher at moderate temperatures (60 °C).…”

“…This class of block copolymer is unique, because of the distinct set of PIL properties that are incorporated within the block copolymer nanostructure, such as high ionic conductivity, high chemical, electrochemical, and thermal stability, and widely tunable chemical properties . PIL block copolymers are of interest for numerous potential electrochemical energy applications, most notably as anion exchange membranes for alkaline fuel cells and as solid polymer electrolytes in lithium‐ion batteries …”

Influence of Anion and Crosslink Density on the Ionic Conductivity of 1,2,3‐Triazolium‐Based Poly(ionic liquid) Polyester Networks

“…In single ion conductors, metrics of fragility also relate to the decoupling of polymer and free ion motion mentioned above. Specifically, larger values of D correlate with more Arrhenius‐like behavior and more coupling between polymer and free ion motion . With the PILs under study herein, then, the most Arrhenius‐like, polymer dynamics‐dependent low‐temperature conductivity occurs with the smallest counteranion, i.e., I − , with low‐temperature behavior becoming less Arrhenius‐like and less coupled as anion size increases.…”

Review of Chitosan-Based Polymers as Proton Exchange Membranes and Roles of Chitosan-Supported Ionic Liquids

“…The first method involved adding ionic liquids directly into the casting solution or by impregnating them into polymer membrane matrix [201][202][203][204]. The second involved attaching or embedding the ionic liquids on a filler and then introducing them into the polymer matrix or grafting them directly onto the polymer [205][206][207].…”