Liquid crystals lie at the heart of display technology. They are also the focus of a growing number of new applications, due to their large susceptibility to external stimuli: electric or magnetic fields, light and temperature. To best utilize this kind of material, we need to know how molecules pack into functional structures. The packing often involves either positional ordering (ordinary crystalline materials), orientational ordering (e.g. nematic liquid crystals, where identical molecules align their long axes but are otherwise free to move), or both. While positional ordering is easily probed by x-ray diffraction, orientational ordering is challenging.
The “blue phase” of liquid crystal (so called because it appeared blue when first observed under an optical microscope) is a nanoscale cubic phase formed of crisscrossing double-twist cylinders. As with the nematic phase, blue phase structure cannot be resolved using nonresonant x-ray diffraction. In this work, researchers used resonant soft x-ray scattering (RSoXS) at ALS Beamline 188.8.131.52 in combination with theoretical modeling to unambiguously resolve the 3D structure of chiral liquid crystals in the blue phase and twist-bend nematic phase, as well as to study the phase transition between them. They used polarized light tuned to the absorption K-edge of carbon.
Not only were the researchers able to determine the symmetry and size of the blue phase crystallographic unit cell, they found evidence of a new structural model for the twist-bend nematic phase, composed of two interlocked and shifted helices. These and other results show that RSoXS, combined with polarization analysis, is a powerful tool for the precise determination of molecular orientational order in materials lacking positional order.
Work performed at ALS Beamline 11.0.1.
M. Salamończyk, N. Vaupotič, D. Pociecha, C. Wang, C. Zhu, and E. Gorecka, “Structure of nanoscale-pitch helical phases: blue phase and twist-bend nematic phase resolved by resonant soft X-ray scattering,” Soft Matter 13, 6694 (2017), doi:10.1039/C7SM00967D.