Reorientation mechanisms in smectic A liquid crystals

• Published in: Liquid crystals Vol. 39; no. 10; pp. 1261 - 1275
• Main Authors: Siemianowski, S.D., Brimicombe, P.D., Jaradat, S., Thompson, P., Bras, W., Gleeson, H.F.
• Format: Journal Article
• Language: English
• Published: Abingdon Taylor & Francis 01.10.2012; Taylor & Francis Ltd
• Subjects: dynamics; layer reorientation mechanism; Liquid crystal polymers; Phase transitions; smectic-A; Temperature; time-resolved x-ray scattering
• ISSN: 0267-8292; 1366-5855
• DOI: 10.1080/02678292.2012.714486

The reorientation mechanism of smectic A (SmA) layers in a field is non-trivial; at least two different mechanisms have been reported. In thin samples reorientation is achieved through complete disordering and reordering of the smectic layers, while in thick samples layers can also appear to rotate continuously as a monodomain. We explore the transition between these regimes, showing qualitatively that the mechanism adopted depends on the relative importance of the sample-cell wall interaction. We observe reorientation of the smectic layers of 4′-n-octyl-4-cyanobiphenyl (8CB) in a magnetic field of flux density 4 T within 4 K of the nematic to Sm-A transition. Using time-resolved small angle X-ray scattering on samples of thickness 170 μm, 270 μm or 340 μm, we determine the transition between the thickness regimes, demonstrating a new multi-step process that is adopted in specific cases. Importantly, the time-resolved experiments allow the dynamics of the reorientation to be probed, and we measure temperature-independent angular speeds of 0.038 ± 0.002° s −1 for the continuous rotation process. Finally, we report evidence of apparent random changes in layer spacing (∼0.5 Å) as a precursor to the rotation of the Sm-A layers in some cases and discuss the possible origin of this phenomenon.