(6el) Tuning the Ionic Conductivity of Polymerized Ionic Liquid Homo-, Random, and Block Copolymers

Polymerized ionic liquids (PILs) are an emerging class of ion-conducting polymers based on the familiar chemistries of ionic liquids. In contrast to ionic liquids, either the cation or anion is tethered to the polymer backbone in a PIL which minimizes leakage of the charged species into neighboring materials (for example in a thin film transistor). A major advantage of PILs over conventional ion conducting membranes is that they can operate in the absence of water allowing for high temperature operation. In this project, we seek a better understanding of the conductivity mechanisms in PILs, particularly in materials which conduct protons. One of the main structural features of ion-conducting polymers is the aggregation of ionic groups due to the non-polar, low dielectric constant polymer backbone. These aggregates have a pronounced influence on the conductivity, viscosity, and dielectric constant of the material and we seek to tune these properties via two distinct routes. The first is through the incorporation of polar, non-ionic groups into the backbone of a random copolymer PIL. The second is via nanostructuring of PILs within self-assembled block copolymers containing a non-conductive, mechanically robust block.

A variety of comonomers that are either polar or non-polar and either bulky or flexible have been incorporated into random copolymers with the PIL to tune the dielectric constant, mechanical properties, and conductivity. We have found that up to ~ 20 wt% of a non-conductive species can be added into a random copolymer without substantially altering the Tg-normalized conductivity. This leads to improvements in processability and solubility of such materials and the ability to tune the overall material properties by the judicious choice of comonomers.

The role of nanoconfinement on the conductivity of imidazolium based PILs, where the imidazolium can act as a proton donor/acceptor and conduct protons, has also been investigated. In polystyrene-PIL block copolymers, where the PIL is confined by a hard phase, an order of magnitude increase in the conductivity is observed in the block copolymer relative to the pure PIL. We attribute this to a change in the packing of the imidazolium PIL upon confinement which leads to more efficient pathways for proton transport through the phase separated domain. Substantial differences in the hydrogen bond network in the block copolymer are also observed via FTIR experiments. The effect of confinement is substantially reduced if a softer confining block, methyl acrylate, is employed. Finally, the conductivity in PS-PIL block copolymers is shown to scale with the domain size of the PIL.