Impact of Side Chain Hydrophilicity on Packing, Swelling and Ion Interactions in Oxy-bithiophene Semiconductors

TL;DR

This study uses molecular dynamics and X-ray diffraction to explore how hydrophilic glycol-based side chains affect packing, swelling, and ion interactions in oxy-bithiophene semiconductors, revealing new insights into their structural and ion-binding behaviors.

Contribution

The paper introduces a validated MD force field for glycolated polythiophenes and uncovers how side chain hydrophilicity influences packing motifs, water penetration, and cation binding mechanisms.

Findings

Glycolated polymers pack with a deflected stack and s-bend side chains.

Water penetrates through the π-stack and lamellar stack in different polymers.

Cations bind via single or double chelate states with side chain length dependence.

Abstract

Exchanging hydrophobic alkyl-based side chains to hydrophilic glycol-based side chains is a widely adopted method for improving mixed-transport device performance, despite the impact on solid state packing and polymer-electrolyte interactions being poorly understood. Presented here is a Molecular Dynamics (MD) force field for modelling alkoxylated and glycolated polythiophenes. The force field is validated against known packing motifs for their monomer crystals. MD simulations, coupled with X-ray Diffraction (XRD), show that alkoxylated polythiophenes will pack with a `tilted stack' and straight interdigitating side chains, whilst their glycolated counterpart will pack with a `deflected stack' and an s-bend side chain configuration. MD simulations reveal water penetration pathways into the alkoxylated and glycolated crystals - through the {\pi}-stack and through the lamellar stack respectively. Finally, the two distinct ways tri-ethylene glycol polymers can bind to cations are revealed, showing the formation of a meta-stable single bound state, or an energetically deep double bound state, both with a strong side chain length dependance. The minimum energy pathways for the formation of the chelates are identified, showing the physical process through which cations can bind to one or two side chains of a glycolated polythiophene, with consequences for ion transport in bithiophene semiconductors.