Xenon NMR Study Of Nematic Liquid Crystals Confined In Submicron Cavities.pdf

This study investigates the nematic to isotropic phase transition of a liquid crystal (ZLI 1132) confined within submicron cylindrical cavities using Xenon-129 (129Xe) and Xenon-133 (I3'Xe) NMR spectroscopy. Key findings include:

1. Chemical Shift Anisotropy: At 21°C, the 129Xe resonance line in the nematic phase exhibits a chemical shift anisotropy of 15 ppm due to random distribution of director axes perpendicular to the cylinder's long axis.

2. Comparison with Bulk Liquid Crystal: The quadrupolar splitting observed in the I3'Xe NMR spectrum is slightly greater than that found in bulk, suggesting enhanced interactions within the confined system.

3. Exchange Dynamics: Two-dimensional exchange NMR reveals xenon atoms probe different average liquid crystal directors within a cavity on a 20 ms time scale and interpore exchange occurs every 400 ms. This indicates changes in director orientation within individual cavities occur on a length scale of approximately 2 pm.

4. Surface Effects: The study leverages the ability to control the cylinder's long axis orientation relative to the field, facilitating examination of surface-induced alignment effects.

5. Methodological Advancements: Two-dimensional NMR is used to probe xenon atomic motion over time scales from milliseconds to seconds, providing detailed insights into director structure within the cavities.

Context:

- Liquid crystals exhibit long-range molecular orientational order and are crucial for technologies like LCDs and potential nonlinear optical devices.
- NMR is valuable for studying liquid crystals due to its sensitivity to orientational order.
- Xenon, with its high sensitivity to local environment and structure, has been used as a microscopic probe in various materials and liquid crystal studies.
- Confined geometry significantly impacts liquid crystal behavior, making this study relevant to understanding surface effects on molecular alignment.