Ionic Conductivity of Nanostructured Block Copolymer/Ionic Liquid Membranes

By Hoarfrost, Megan L. & Segalman, Rachel A.
Published in Macromolecules NULL 2011

Abstract

Nanostructured mixtures of ionic liquids and polymers are of great interest for a wide variety of electrochemical applications. Understanding the relationship between composition, structure, and ionic conductivity for these mixtures is essential for designing new materials. In this work, the effect of nanostructure on ionic conductivity, σ, is investigated for model mixtures of diblock copolymers and ionic liquids that are selective for one of the polymer microphases. It is demonstrated that the concentration dependence of σ is a function of the total volume fraction of ionic liquid and described well by percolation theory. This scaling behavior encourages the design of membranes where the amount of a mechanical component in the block copolymer can be increased to improve the strength of the membrane without sacrificing conductivity. The temperature dependence of σ is a function of the amount of ionic liquid exclusively in the conducting domain. Comparing σ for mixtures of the diblock copolymer poly(styrene-block-2-vinylpyridine) (S2VP) and two different ionic liquids, imidazolium bis(trifluoromethylsulfonyl)imide ([Im][TFSI]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][TFSI]), reveals that the chemistry of the polymer/ionic liquid pair affects both the activation energy for σ and the maximum attainable σ but does not affect how σ scales with ionic liquid concentration. The effect of the morphology on σ is also examined, and it is found that as long as the conducting phase morphology is isotropic and well-connected, σ is not affected by morphology.

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