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See detailA study of a bistereogenic mesogen for the development of orthoconic antiferroelectric liquid crystal materials
Lagerwall, Jan UL; Yates, Chris; Rauch, Sebastian et al

in Ferroelectrics (2005), 315

The first orthoconic antiferroetectric liquid crystals (OAFLCs), i.e. smectics where the optical director changes direction by 90 degrees between neighboring layers, were mixtures of partially fluorinated ... [more ▼]

The first orthoconic antiferroetectric liquid crystals (OAFLCs), i.e. smectics where the optical director changes direction by 90 degrees between neighboring layers, were mixtures of partially fluorinated monostereogenic compounds. They have successfully demonstrated the orthoconic properties (orientation-independent dark state between crossed polarizers) but suffer from too high polarization and too short helical pitch, necessitating very thin samples. Using an (S,R) bistereogenic OAFLC we have obtained the orientation-independent dark state in rather thick samples, but several other problems arise with this compound. The strongly first-order SmA-SmCa transition produces defects leading to light leakage. In order to be switchable the sample must furthermore be mixed with chiral dopants, generally reducing the tilt angle as well as a shortening the helical pitch. Finally, a SmC phase often appears in the phase sequence of the mixture, strongly dominating over the desired SmCa phase in thin cells. [less ▲]

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See detail(-)-isopinocampheol substituted mesogens: An investigation of the effect of bulky terminal groups in chiral smectic liquid crystals
Yates, Chris; Lagerwall, Jan UL; Nobili, Maurizio et al

in Ferroelectrics (2004), 311

A ferroelectric liquid crystal with an end-substituted (-)-isopinocampheol (IPC) group has been studied in isolation and in mixtures with the antiferroelectric compound EHPOCBC. The samples were studied ... [more ▼]

A ferroelectric liquid crystal with an end-substituted (-)-isopinocampheol (IPC) group has been studied in isolation and in mixtures with the antiferroelectric compound EHPOCBC. The samples were studied with respect to electroclinic coefficients, tilt angles, X-ray layer spacing, dielectric permittivity, spontaneous polarisation and relative birefringence under an applied field. In the pure isopinocampheol substituted compound, the SmA-SmC phase transition appears strongly first order and is associated with a significant increase in effective birefringence. The smectic layer spacing decreases below the transition approximately with the optical tilt. On increased EHPOCBC concentration the change in birefringence across the transition diminishes, until the point at which the mixture adopts an anticlinic SmC(A) phase, and the expected decrease in birefringence is observed. There is very little change in birefringence with field for the IPC compound, whilst EHPOCBC shows a marked increase with increasing field, for a few degrees above the phase transition. The results are analyzed with respect to two models. In one it is proposed that the bulky IPC group disorders the SmA phase by introducing undulations in the smectic layer. These undulations may disappear in the SmC phase due to an antiparallel arrangement of neighbouring molecules. In the other model the bent shape of the mesogen as a whole plays a crucial role in changing the optical properties at the SmA-SmC transition, due to the biased rotation around the molecular axis of inertia. [less ▲]

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