A Comparison of Short-Range Molecular Order in Bent-Core and Rod-Like Nematic Liquid Crystals

Thermotropic liquid crystals exhibit strongly temperature-dependent positional correlations among molecules in the nematic phase above the transition to a smectic-A or C phase. However, even in the absence of a lower temperature smectic phase, nematics composed of reduced symmetry molecules (bent-core mesogens, for example) reveal small angle X-ray scattering (SAXS) patterns with features similar to those associated with pretransitional smectic order. We report on a quantitative analysis and comparison of two-dimensional SAXS data on a bent-core nematic compound (lacking a smectic phase) and a rod-like nematic (exhibiting a nematic to smectic-C transition). This analysis demonstrates that a nanostructure based on distinct, smectic-C ordered molecular clusters provides a more accurate account of the diffraction data from the bent-core nematic than the standard models used to describe fluctuating smectic order in rod-like nematics. The average dimensions of the clusters are weakly temperature dependent, while the interlayer spacing and layer tilt angle with respect to the average molecular long axis are temperature independent. Our results may help to explain interesting and unusual macroscopic properties recently discovered in the fluid phases of bent-core molecules.