Hierarchical lyotropic liquid crystalline behaviors of supramolecular polymers influenced by alkyl chain branching

Abstract

Lyotropic liquid crystals can be hierarchically assembled from rigid–flexible molecules via one-dimensional supramolecular polymerization, which provides a convenient way to form complex soft matter and advanced materials from simple building blocks. Although the relationships between molecular structures and supramolecular polymerization behaviors have been well established via the elucidation of self-assembly mechanisms and pathways, the clarification on the impact of structural parameter changes on the hierarchical liquid crystalline properties remains unexplored. In the current study, we have demonstrated the great influence of alkyl chain branching on the lyotropic liquid crystalline behaviours. Two flexible–rigid–flexible monomers have been designed with the same amide core, which differs in the wedge-shaped alkyl chains in both peripheries [six unbranched n-dodecyl units versus branched (S)-3,7-dimethyloctyl units]. For both compounds, one-dimensional supramolecular polymerization occurs in dilute decane as driven by intermolecular hydrogen bonds. Although hierarchical supramolecular assembly takes place for both monomers in concentrated decane solutions, it is found that monomers with the unbranched alkyl chains form lyotropic nematic liquid crystals, different from the hexagonal columnar liquid crystals of the branched counterpart. This distinction originates from the steric repulsion effect caused by the branched (S)-3,7-dimethyloctyl units, which in turn affects monomer arrangement modes in liquid crystals (perpendicular orientation for the unbranched monomer along the one-dimensional supramolecular polymers versus tilted orientation for the branched one). Overall, the current study provides new insights into the understanding of lyotropic liquid crystalline behaviors influenced by the peripheral chain branching in the monomeric structures.