Tuesday, June 25, 2013

1306.5504 (Dong Chen et al.)

A Twist-Bend Chiral Helix of 8nm Pitch in a Nematic Liquid Crystal of
Achiral Molecular Dimers

Dong Chen, Jan H. Porada, Justin B. Hooper, Arthur Klittnick, Yongqiang Shen, Eva Korblova, Dmitry Bedrov, David M. Walba, Matthew A. Glaser, Joseph E. Maclennan, Noel A. Clark
Freeze Fracture Transmission Electron Microscopy (FFTEM) study of the nanoscale structure of the so-called "twist-bend" nematic (NX) phase of the cyanobiphenyl (CB) dimer molecule CB(CH2)7CB reveals a stripe texture of fluid layers periodically arrayed with a bulk spacing of d ~ 8.3 nm. Fluidity and a rigorously maintained spacing produce long-range-ordered fluid layered focal conic domains. Absence of a lamellar x-ray reflection at wavevector q ~ 2{\pi}/8 nm-1 or its harmonics in synchrotron-based scattering experiments indicates that this periodic structure is achieved with no detectable associated modulation of the electron density, and thus has nematic molecular ordering. A search for periodic ordering with d ~ 8nm in CB(CH2)7CB using atomistic molecular dynamic computer simulation yielded equilibration of a conical twist-bend helixed nematic ground state, of the sort first proposed by Meyer, and envisioned in systems of bent molecules by Dozov and Memmer, We identify {\theta} ~ 33 degree as the cone angle, and p ~ 8nm as the full pitch of the helix, the shortest ever found in a nematic fluid.
View original: http://arxiv.org/abs/1306.5504

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