A ferroelectric liquid crystal conglomerate composed of racemic molecules

Science

Volumn 288, Issue 5474, 2000, Pages 2181-2184

  Walba, David M ^a   Körblova, Eva ^a   Shao, Renfan ^a   Maclennan, Joseph E ^a   Link, Darren R ^a   Glaser, Matthew A ^a   Clark, Noel A ^a  

^a UNIVERSITY OF COLORADO   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

BIREFRINGENCE; CRYSTAL STRUCTURE; CRYSTALLIZATION; IMAGE ANALYSIS; LIQUID CRYSTAL; PHASE TRANSITION; POLARIZATION; PRIORITY JOURNAL; RACEMIC MIXTURE; REACTION ANALYSIS; REVIEW; STRUCTURE ANALYSIS; SYNTHESIS; THERMOSTABILITY;

EID: 0343431504     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.288.5474.2181     Document Type: Review

References (23)

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11. There is, a priori, no electrostatic free energy difference between the macroscopic FE and AFE structures, assuming that the layers are infinite homogeneous dipolar sheets. At the molecular level, one or the other structure could be favored electrostatically, depending on the details of molecule-pair correlations across layers. In the SmCP system, the magnitude of this effect, or indeed which structure should be favored electrostatically, is not known.
12. A SmCP phase with anticlinic methyl-terminated layer interfaces may be considered a smectic bilayer with all anticlinic layer interfaces (one of these layer interfaces cuts the molecules in half and is rigidly anticlinic because of the covalent structure). The polar symmetry of a bilayer smectic with all anticlinic layer interfaces is discussed in P. E. Cladis and H. R. Brand, Liq. Cryst. 14, 1327 (1993).
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14. 1 symmetry [H. R. Brand, P. E. Cladis, H. Pleiner, Eur. Phys. J. B 6, 347 (1998)]. There is no compelling evidence to date, however, that such symmetry breaking occurs in the bow-phases. Thus, in the smectic phases of triester 3, molecules are oriented randomly about the axis normal to the director, and the twofold symmetrical symbolic structures shown in Fig. 1 capture the symmetry of the phase rather than of the molecules. The presence of twofold symmetry normal to the director (about P), such that properties of the phase are invariant with sign of the director, is the only "symmetry rule" remaining intact in the smectic LC field.
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17. All attempts to prepare freely suspended films of triester 3 failed. Instead, stable freely suspended liquid crystal fibers of ∼10-μm diameter are produced. X-ray diffraction from these fibers shows them to consist of smectic layers in the form of concentric cylinders. Although this observation provides additional confirmation of the fluid smectic nature of the B7 phase, such fibers have not, to date, proven useful for determining the FE or AFE nature of the phase. On the basis of a very small number of examples, it appears that the tendency to form fibers instead of films is general for B7 materials. Details of the x-ray scattering observed for triester 3, and apparently for all B7 materials, are quite unusual and complex. In bulk unaligned samples, there appear to be multiple coexisting phases with slightly different layer spacings that are all close to the extended length of the mesogen. This could result from metastable structures forming from the isotropic melt along with the thermodynamic phase. Additional scattering peaks suggest some kind of periodic order in the plane of the layers, on a very large length scale. A full understanding of the B7 structure requires an explanation of this unusual x-ray scattering behavior. The observed layer spacings, however, prove that no unusual interdigitation occurs in the B7 phase of compound 3.
18. The LC cells used in the present work are of the same type as those used for characterization of the classic bow-phase materials (7). These cells, which are commercially available from Displaytech (www.displaytech.com), have glass substrates with a 3.5-to 4.5-μm cell gap, coated with low-pretilt polyimide alignment layers. The rubbing directions on the two substrates are parallel.
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22. Supplemental Web material is available at Science Online at www.sciencemag.org/feature/data/1049224.shl.
23. Supported by the Ferroelectric Liquid Crystal Materials Research Center (NSF MRSEC award no. DMR-9809555).