Bromide and Hydroxide Conductivity-Morphology Relationships in Polymerized Ionic Liquid Block Copolymers
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2015 American Chemical Society. A polymerized ionic liquid (PIL) diblock copolymer, poly(MMA-b-MEBIm-Br), was synthesized at various compositions from an ionic liquid monomer, (1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium bromide) (MEBIm-Br), and a nonionic monomer, methyl methacrylate (MMA), via the reverse addition-fragmentation chain transfer (RAFT) polymerization technique. A hydroxide-conducting PIL diblock copolymer, poly(MMA-b-MEBIm-OH), was also prepared via anion exchange metathesis of the bromide-conducting block copolymer. In a former study, the conductivity and morphology of the bromide- and hydroxide-conducting PIL diblock copolymer were examined at one fixed PIL composition: 17.3 mol %. In this study, additional PIL compositions of (6.6, 11.9, and 26.5 mol %) were explored to fully understand the previous unusual conductivity results. Both bromide and hydroxide conductivities were higher in the PIL block copolymer at PIL compositions of 11.9, 17.3, and 26.5 mol % compared to the PIL homopolymer under the same experimental conditions, even though the homopolymer possessed a higher water and ionic content compared to the block copolymers. These unusual results suggest that the confinement of the PIL microdomain within the block copolymer morphology enhances ion transport compared to its predicted value. Morphology factors (or normalized ionic conductivity, f) were as high as >3 at some conditions, which is much higher than the maximum theoretical limit for randomly oriented lamellar domains (f = 2/3). Application of percolation theory revealed a 3-4-fold enhancement of conductivity when comparing the inherent conductivity to the measured PIL homopolymer conductivity. Both morphology factor analysis and percolation theory corroborate with the absolute conductivity results and the hypothesis that PIL domain confinement in PIL block copolymers enhances conductivity over its bulk properties.
