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

Abstract

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-to-backbone 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. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012