Large Modulation of Conjugated Polymer Liquid Crystal Structure and Supramolecular Helicity via Cosolvency Effect

Abstract

Lyotropic liquid crystalline mesophases are commonly observed during solution processing of conjugated polymer systems and play a crucial role in defining their optical, electronic and mechanical properties. These mesophases arise during evaporation assembly and imprint their solution-state structures into thin films, significantly impacting the final film morphology and device properties. However, the detailed microstructures of these mesophases and methods to control their phase behavior are not well understood. In this work, we demonstrate a cosolvency effect, where a mixture of two seemingly poor solvents acts as a good solvent, promoting mesophase formation within a specific mixing ratio window. This cosolvency effect is driven by the selective affinities of the solvents for the polymer backbone and side chains. By adjusting the mixing ratio, we can control mesophase formation and tune the rigidity of the polymer fiber aggregates, which are the key structural elements of these mesophases. The rigidity of these fibers has a significant effect on mesophase behavior. In the biphasic phase, increased fiber rigidity leads to a higher aspect ratio of tactoids and an increase in the critical volume required for the homogeneous-to-bipolar transition. In the striped twist-bend nematic phase, where fibers assemble helically, the pitch at both micron and nanoscale levels increases with fiber rigidity. Such tuning of helical structures is rarely demonstrated before by nonsynthetic approaches, which offers an easy handle to control chiraloptical and spin transport properties in a plethora of emerging applications ranging from spintronics, optoelectronics and bioelectronics.