Water-free proton-conducting polysiloxanes: A study on the effect of heterocycle structure

Proton-conducting, thermally and electrochemically stable, heterocycle-grafted polysiloxanes have been synthesized via hydrosilylation of vinyl or ally functionalized weakly basic heterocyclic motifs with a polymethylhydrosiloxane precursor. The basicity of the amphoteric heterocycles was tuned by introducing electron-withdrawing groups, whose presence also produced a decrease in the polymers' glass transition temperature. The proton conductivity depended strongly on the T-g of the polymer matrix and the volume fraction of proton carriers, while the effect of reducing the pK(a) of the heterocycle was less pronounced. The resulting polymers showed the highest reported proton conductivities of up to 0.1 mS/cm at temperatures below 80 degrees C and up to 5 mS/cm at 180 degrees C when doped with trifluoroacetic acid.