Origin of weak layer contraction in de Vries smectic liquid crystals

Dena M. Agra-Kooijman, HyungGuen Yoon, Sonal Dey, and Satyendra Kumar
Phys. Rev. E 89, 032506 – Published 21 March 2014

Abstract

Structural investigations of the de Vries smectic-A (SmA) and smectic-C (SmC) phases of four mesogens containing a trisiloxane end segment reveal a linear molecular conformation in the SmA phase and a bent conformation resembling a hockey stick in the SmC phase. The siloxane and the hydrocarbon parts of the molecule tilt at different angles relative to the smectic layer normal and are oriented along different directions. For the compounds investigated, the shape of orientational distribution function (ODF) is found to be sugarloaf shaped and not the widely expected volcano like with positive orientational order parameters: ⟨P2⟩ = 0.53–0.78, ⟨P4⟩ = 0.14–0.45, and ⟨P6⟩∼0.10. The increase in the effective molecular length, and consequently in the smectic layer spacing caused by reduced fluctuations and the corresponding narrowing of the ODF, counteracts the effect of molecular tilt and significantly reduces the SmC layer contraction. Maximum tilt of the hydrocarbon part of the molecule lies between approximately 18° and 25° and between 6° and 12° for the siloxane part. The critical exponent of the tilt order parameter, β∼0.25, is in agreement with tricritical behavior at the SmA–SmC transition for two compounds and has lower value for first-order transition in the other compounds with finite enthalpy of transition.

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  • Received 17 July 2013
  • Revised 27 November 2013

DOI:https://doi.org/10.1103/PhysRevE.89.032506

©2014 American Physical Society

Authors & Affiliations

Dena M. Agra-Kooijman1, HyungGuen Yoon2, Sonal Dey1, and Satyendra Kumar1,*

  • 1Department of Physics, Kent State University, Kent, Ohio 44242, USA
  • 2TV Development Team, LCD Business, Samsung Display, Chungcheongnam-Do 336-741, Korea

  • *Corresponding author: skumar@kent.edu

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Vol. 89, Iss. 3 — March 2014

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