The morphology and ion conductivity of two polymerized ionic liquid (PIL) diblock copolymers of similar chemistry, but differing alkyl side chain lengths, were investigated. The two PIL diblock copolymers in this study consist of a non-ionic component (MMA) and an ionic liquid (IL) component (1-[(2-methacrloyl)X)-3-butylimidazolium bromide) (MXBIm-Br), where X represents ethyl (E) or undecyl (U), i.e., poly(MMA-<i>b</i>-MEBIm-Br) and poly(MMA-<i>b</i>-MUBIm-Br) for short and long alkyl side chain lengths, respectively. These two copolymers were compared at a similar ion exchange capacity (IEC = 1.4 meq/g) and similar MXBIm-Br volume fraction (~40 vol%). Transmission electron microscopy, small and intermediate-angle X-ray scattering, and small-angle neutron scattering were used to examine differences in morphology between the two polymers under dry, humidified, and fully hydrated conditions. The impact of these morphological differences on bromide ion conductivity was explored. Specifically, the block copolymer with a longer alkyl side chain length has a higher conductivity with lower water uptake at similar IEC and MXBIm-Br volume fraction, which correlates to differences in morphology. Only in the longer alkyl side chain length copolymer was an additional scattering peak observed in both X-ray and neutron scattering under fully saturated conditions. These results suggest the formation of an ionomer/water channel within the MUBIm-Br microdomain under saturated conditions, which results in accelerated ion transport. This study suggests that the location of the imidazolium moiety within the PIL block copolymer can play a role in the morphology and subsequently the ion conductivity, which may have an impact on various energy and separations applications.

Impact of alkyl chain length on ion conduction and morphology in polymerized ionic liquid diblock copolymers

249th National Meeting (2015)

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