High Ion Content Siloxane Phosphonium Ionomers with Very Low Tg

Polysiloxane phosphonium single-ion conductors grafted with oligomeric PEO and with ion contents ranging from 5 to 22 mol % were synthesized via hydrosilylation reaction. The parent Br- anion was exchanged to F- or bis-(trifluoromethanesulfonyl)imide (TFSI-). X-ray scattering data suggest ion aggregation is absent in these phosphonium ionomers, which contributes to low glass transition temperatures (below -70 degrees C) with only a weak dependence on both ion content and counteranion type. Conductivities weakly increase with ion content but exhibit a strong dependence on anion type. The highest conductivity at 30 degrees C is 20 mu S/cm for dry neat ionomer, with the TFSI- anion, consistent with its relatively delocalized negative charge and large size that weaken interactions between TFSI- and the phosphonium cation.