The interplay between spatial and heliconical orientational order in twist-bend nematic materials

Area of Science:

• Materials Science
• Condensed Matter Physics
• Organic Chemistry

Background:

• Helical structures in organic molecules, like protein alpha-helices, typically rely on hydrogen bonding and positional order.
• A twist-bend nematic (NTB) phase with nanoscale heliconical structure has been observed in liquid crystal oligomers, notably without hydrogen bonding or molecular chirality.

Purpose of the Study:

• To investigate the fundamental nature of the heliconical structure in novel sulfur-containing dimer liquid crystals.
• To elucidate the relationship between molecular structure, phase transitions, and the formation of the twist-bend nematic (NTB) phase.

Main Methods:

• Utilized hard and resonant tender X-ray scattering techniques to study two novel sulfur-containing dimer materials.
• Performed simultaneous temperature-dependent measurements of helical pitch and correlation lengths for helical and positional order.

Main Results:

• Observed an unexpected strong dependence of helical pitch on the spacer length connecting monomer units.
• Found a significant drop in positional correlation length at the transition to the NTB phase.
• Detected helical structure not only in the NTB phase but also in an adjacent smectic phase, indicating a novel layered SmATB phase.

Conclusions:

• The study reveals that nanoscale heliconical structures can form in liquid crystals without traditional requirements like hydrogen bonding.
• The findings suggest a new layered phase (SmATB) where smectic layering coexists with heliconical order, driven by bent dimer conformations.
• X-ray scattering provides crucial insights into the complex ordering and phase behavior of these unique materials.