Effect of Polymer Structure on Ion Transport in an Anhydrous Proton Conducting Electrolyte

We have studied the effect of polymer structure of a proton-conducting polymer electrolyte on proton conductivity. Proton conducting electrolytes composed of mixtures of a MePEG(7)SO(3)H acid, our MePEG(n) polymer, and a PEG containing cross-linker have been prepared. We have independently varied two structural parameters in our polymer electrolyte, viscosity and free volume, and studied the effects of these changes on the H+ conductivity. The copolymerization of a PEG(13) cross-linker increases the viscosity and the molecular weight distribution (i.e., PDI) of our MePEG(n) polymers. In addition, the fractional free volume of our MePEG(n) polymers increases as the length of the PEG side chains in the polymer increases from n = 7 to n = 12. Under certain conditions, high-viscosity, cross-linked MePEG(n) polymers show almost the same ionic conductivity as less viscous, uncross-linked MePEG(n) polymers. The effect of this change in viscosity is compensated by a Simultaneous change in the free volume of the MePEG(n) polymer. These results indicate that both free volume and viscosity are important factors in predicting the ionic conductivity in our polymer electrolyte. When shown as a Walden plot, the data appear below the ideal Walden line, indicating that the MePEG(7)SO(3)H acid is behaving as a weak acid in our anhydrous MePEG(n) polymer.