DOI: https://doi.org/10.29363/nanoge.aohm.2019.027
Publication date: 8th January 2019
While mixed conduction polymer materials demonstrate a wide range of potential applications, understanding of the connection between morphology, structure and ion transport in these materials is quite limited. Herein we present a computationally driven study of two polythiophene derivatives with oligoethylene glycol side chains, investigating the effects of morphology and monomer structure on ionic conduction. Two repeat unit structures were synthesized, one with an oxygen atom directly conjugated to the polythiophene backbone (P3EGT), and one with a methylene spacer between the initial oxygen atom and the polythiophene backbone (P3MEGT). Using molecular dynamics (MD) simulations, we demonstrate that amorphous P3MEGT has a higher ionic conduction than P3EGT, whereas P3EGT has the higher crystalline conduction. The lower crystalline conduction in P3MEGT is due to ion caging, a feature not present in P3EGT. To investigate their structural evolution with Li+ introduction, the polymers were blended with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and studied using grazing incidence wide angle X-ray scattering. From this, it was determined that introduction of LiTFSI results in both a reduction in crystallinity and expansion of the side-chain stacking direction. Further, it is shown that LiTFSI is present in both the crystalline and amorphous regions at low loadings, though the crystalline region saturates at high loadings. The ionic conduction was measured using electrochemical impedance spectroscopy, determining that ionic conduction occurs predominantly in the amorphous domains for both polymers. Further, the measurements show that P3MEGT has a higher ionic conduction for all conditions, a result consistent with MD simulations. By using MD simulations to augment our experimental results we have deepened our understanding of the effect of monomer structure on ionic conductivity of conjugated polymers.
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