Reference : On the origin of high optical director tilt in a partially fluorinated orthoconic ant...
Scientific journals : Article
Physical, chemical, mathematical & earth Sciences : Chemistry
http://hdl.handle.net/10993/20555
On the origin of high optical director tilt in a partially fluorinated orthoconic antiferroelectric liquid crystal
English
Lagerwall, Jan mailto [Universit├Ąt Stuttgart]
Giesselmann, Frank [> >]
Saipa, Alexander [> >]
Dabrowski, Roman [> >]
2004
Liquid Crystals
Taylor & Francis Ltd
31
9
1175-1184
Yes (verified by ORBilu)
International
0267-8292
1366-5855
[en] Mesomorphic properties ; phase transitions ; homologous series
[en] We have investigated the orthoconic antiferroelectric liquid crystal mixture W107 by means of optical, X-ray and calorimetry measurements in order to assess the origin of the unusally high tilt angle between the optic axis and the smectic layer normal in this material. The optical birefringence increases strongly below the transition to the tilted phases, showing that the onset of tilt is coupled with a considerable increase in orientational order. The layer spacing in the smectic A (SmA) phase is notably smaller than the extended length of the molecules constituting the mixture, and the shrinkage in smectic C (SmC) and smectic C-a (SmCa) is much less than the optical tilt angle would predict. These observations indicate that the tilting transition in W107 to a large extent follows the asymmetric de Vries diffuse cone model. The molecules are on average considerably tilted with respect to the layer normal already in the SmA phase but the tilting directions are there randomly distributed, giving the phase its uniaxial characteristics. At the transition to the SmC phase, the distribution is biased such that the molecular tilt already present in SmA now gives a contribution to the macroscopic tilt angle. In addition, there is a certain increase of the average tilt angle, leading to a slightly smaller layer thickness in the tilted phases. Analysis of the wide angle scattering data show that the molecular tilt in SmCa is about 20degrees larger than in SmA. The large optical tilt (45degrees) in the SmCa phase thus results from a combination of an increased average molecule tilt and a biasing of tilt direction fluctuations.
Researchers
http://hdl.handle.net/10993/20555
10.1080/02678290410001732279
Univ Stuttgart, Inst Phys Chem, D-70569 Stuttgart, Germany. Mil Univ Technol, PL-00908 Warsaw, Poland. Lagerwall, JPF, Univ Stuttgart, Inst Phys Chem, Pfaffenwaldring 55, D-70569 Stuttgart, Germany.

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