Biaxial Nematics from Ribbon-like Chains

Abstract The performance of conjugated polymers (CPs) in optoelectronic devices is strongly influenced by nematic ordering, which enhances charge transport through improved molecular alignment. Traditional worm-like chain models predict only uniaxial nematic phases due to rotational averaging about the chain backbone, and cannot capture the biaxial nematic behavior observed in ribbon-like molecules, such as CPs with π-conjugation backbones. We recently developed the ribbon-like chain (RLC) model, which includes anisotropic bending and twist stiffness, and provides a framework for studying both uniaxial and biaxial nematic ordering. Here, we present a self-consistent field theory for biaxial nematic phases in solutions of RLCs, by adopting Maier–Saupe-type interactions. The existing work for rigid rods, rigid ribbons, and twistable ribbons are shown to correspond to the variety of limits of our model. The phase diagrams evaluated for RLCs with finite stiffness values reveal that twist stiffness plays a disproportionately important role compared to bending stiffness, while molecular weight effects reflect a competition between energetics and conformational entropy. The results provide a foundation for coarse-grained simulations that can predict nematic ordering in specific CP chemistries and guide the rational design of high-performance organic electronic materials.