Ionic conduction of lithium and magnesium salts within laminar arrays in a smectic liquid-crystal polymer electrolyte

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Abstract
Three polymers having the structure shown below where R =–C16H33 or –C12H25 and n= 5 or 6 (coded C16O5, C16O6 and C12O6), have been synthesised and their complexes with LiClO4, LiBF4, LiCF3SO3, LiBr and Mg(ClO4)2 have been prepared. [graphic omitted] Using thermal analysis, polarised light microscopy, small-angle (SAXS) and wide-angle X-ray scattering techniques (WAXS) techniques and energy-minimised modelling it is shown that the complexes having 0.5 mol salt per polymer repeating unit form smectic liquid-crystal structures above the side-chain melting temperatures [e.g. 43.5 °C for C16O5-LiClO4(1 : 1)]. The salt is complexed to the two-dimensional helical polyether backbone layers which are separated by hydrocarbon layers. In contrast to expectation from conventional polymer electrolyte behaviour, the ionic conductivities of the lithium salt complexes in the liquid-crystal phases are greatest for systems having the most extensive two-dimensional organisation and follow the sequence C16O5 > C16O6 > C12O6. Mg(ClO4)2 complexes with C16O5 and C16O6 are similarly organised in two dimensions. The results suggests that ionic mobility is most extensive within the plane of the polyether layers and there is no evidence for ‘tunnelling’ through the hydrocarbon material.

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