(658d) Ion Transport in Nanostructured Block Copolymer/Ionic Liquid Membranes

Nanostructured membranes containing structural and ion-conducting domains are of great interest for a wide range of applications requiring high conductivity coupled with mechanical durability. Mixtures of block copolymers with ionic liquids are particularly attractive because the excellent electrochemical properties of the ionic liquid can be templated into well-defined nanochannels and the mechanical properties of the resulting membranes can be easily tuned. Understanding the relationship between morphological and conductive properties of such materials is essential for designing optimal membranes. This relationship was investigated for mixtures of poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) with the ionic liquid imidazolium bis(trifluoromethylsulfonyl)imide ([Im][TFSI]), where the ionic liquid selectively resides in the P2VP domains of the block copolymer. Scaling relationships have been developed, which describe the conductivity of block copolymer/ionic liquid membranes as a function of block copolymer composition, ionic liquid concentration, and temperature. Furthermore, the effect of confinement on the mechanism of proton conductivity in [Im][TFSI] was examined using quasi-elastic neutron scattering and NMR diffusion experiments. We have found that there are high levels of a fast proton hopping transport mechanism when [Im][TFSI] with extra neutral imidazole is confined to a lamellar block copolymer nanostructure. This contributes to a lower activation energy for macroscopic ion transport compared to that in a comparable mixture of [Im][TFSI] with P2VP homopolymer. These results portend the rational design of nanostructured membranes having improved mechanical properties and conductivity.