Correlating backbone-to-backbone distance to ionic conductivity in amorphous polymerized ionic liquids

The morphology and ionic conductivity of poly(1- n-alkyl-3- vinylimidazolium)-based homopolymers polymerized from ionic liquids were investigated as a function of the alkyl chain length and counterion type. In general, X-ray scattering showed three features: (i) backbone-to-backbone, (ii) anion-to-anion, and (iii) pendant-to-pendant characteristic distances. As the alkyl chain length increases, the backbone-tobackbone separation increases. As the size of counterion increases, the anion-to-anion scattering peak becomes apparent and its correlation length increases. The X-ray scattering features shift to lower angles as the temperature increases due to thermal expansion. The ionic conductivity results show that the glass transition temperature (T _g) is a dominant, but not exclusive, parameter in determining ion transport. The T_g-independent ionic conductivity decreases as the backbone- to-backbone spacing increases. Further interpretation of the ionic conductivity using the Vogel-Fulcher-Tammann equation enabled the correlation between polymer morphology and ionic conductivity, which highlights the importance of anion hoping between adjacent polymer backbones.