Impact of Side Chain Chemistry on Long and Short Range Order in Mixed Transport Polymers

Abstract

Conjugated polymers have been well optimised for transport of electronic charge, however it has recently been shown that the addition of hydrophilic side chains can enable them to operate as 'mixed transport' materials when in use with an aqueous electrolyte. This class of polymers has been successfully applied in a variety of electronic devices, such as batteries and organic electrochemical transistors. However, efficient movement of electronic and ionic charges through the bulk relies critically on the packing and microstructure of the polymer films, which is significantly altered upon the exchange of traditional alkyl side chains to hydrophilic ones. By using Molecular Dynamics, we seek to understand the impact of the side chain chemistry on the
polymer packing. We develop a tailored force field for the modelling of polythiophene mixed conductors and validate it by successfully reproducing a series of bithiophene molecular crystals. Subsequently, we use the force field and experimental data to predict the crystalline packing behaviours of polythiophenes before and after functionalising them with hydrophilic side chains. Molecular Dynamics further allows us to study the amorphous topologies of mixed conducting polymers. Through simulation of amorphous films in water we study the formation of amorphous alkyl networks in the polymer films, which play a critical role in inhibiting swelling and increasing the materials electrochemical stability. Finally, we use Metadynamics to characterise the formation of side chain-cation chelates. We show that the strength of the chelate formation is strongly dependant on the length of the side chains, with implications for the optimisation of chemical design in this class of materials.