Helicity Control in a Chiral Twist-Bend Nematic Phase of an Achiral Conjugated Polymer Using Cosolvency Effect

The chiral twist-bend nematic (N_tb) phase composed of heliconical structures formed by an ensemble of achiral bent-core-like mesogens is the newest nematic phase, having only been identified experimentally in 2011. To date, there are still many unknowns regarding the nature of its phase behavior and material universality. For example, such a phase has rarely been reported in a lyotropic or polymeric system. Herein, we show the formation of N_tb phase of an isoindigo-bithiophene-based conjugated polymer in the mixture of two seemingly poor yet selective solvents, which is governed by the balancing of the polymer backbone and side chain interactions. More importantly, by varying solvent mixing ratio, we show a fine control of liquid crystalline phase behaviors, including the aspect ratio of nematic tactoids and their critical size for homogeneous-to-bipolar transition in the coexistence region of the isotropic + nematic phase, and the helical pitch at multi-length scales in the N_tb phase. The underlying mechanism involves the rigidity modulation of the polymer fibers serving as building blocks for the liquid crystalline phases. This study opens up perspectives on assemblies of achiral polymeric materials into chiral N_tb phase with controllable helicity and phase behavior.