Nanostructures of liquid crystal phases in mixtures of bent-core and rod-shaped molecules

We report small angle x-ray scattering (SAXS) studies of isotropic, nematic, and smectic mesophases formed by binary mixtures of bent-core (BC) and rod-shaped (RS) molecules. While optical studies indicate that the components are fully miscible, SAXS reveals fascinating structures that are consistent with segregation on a nanoscopic scale. We find that tilted smectic clusters, which have been previously reported in both the nematic and isotropic states of the pure BC materials, are also present in mixtures with up to 50 wt% of the RS compound; this is consistent with previous dielectric and flexoelectric studies on such mixtures. Unexpectedly in this concentration range the clusters are present in the isotropic and in the induced smectic phase range, as well as throughout the nematic phase. The results in the smectic phase also reveal complex layering phenomena, providing important insight into the interaction between bent and rod-shaped molecules. These studies will be crucial in the design of promising new functional nanomaterials.

Figure 1

Representative SAXS patterns and their proposed director and layer structures. (a) The mixture with16.3 wt% of RS molecules; (b) 32.3 wt% of RS in BC compound at selected temperatures.

Figure 2

2D small angle x-ray patterns in the rod-shaped molecules dominated concentration ranges. (a) 48.3 wt%; (b) 65.6 wt%; (c) 83.6 wt% of rod-shaped molecules. Note that in the low temperatures of (c) the illumination time is much shorter than in all other pictures, because the smectic peaks are so intense they would saturate the detector.

Figure 3

(a) Molecular structures and their notation. (b) and (c) Representative 2D SAXS patterns and their corresponding proposed nanostructures in the nematic and simple smectic ranges. Dotted lines indicate the layering. A thicker dotted line indicates a shorter correlation length.

Figure 4

2D SAXS profile and its model cartoons at the low temperature range (5 °C above the crystal transition).