Generation of frustrated liquid crystal phases by mixing an achiral n–smc mesogen with an antiferroelectric chiral smectic liquid crystal

in Journal of Chemical Physics (2005), 122(14), 144906

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric ... [more ▼]

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric SmCa phase, at least seven different mesophases have been induced which neither component forms on its own: a twist-grain-boundary (TGB) phase, two or three blue phases, the untilted SmA phase, as well as all three chiral smectic-C-type ``subphases,'' SmCalpha, SmCbeta, and SmCgamma. The nature of the induced phases and the transitions between them were determined by means of optical and electro-optical investigations, dielectric spectroscopy, and differential scanning calorimetry. The induced phases can to a large extent be understood as a result of frustration, TGB at the border between nematic and smectic, the subphases between syn and anticlinic tilted smectic organization. X ray scattering experiments reveal that the smectic layer spacing as well as the degree of smectic order is relatively constant in the whole mixture composition range in which AFLC behavior prevails, whereas both these parameters rapidly decrease as the amount of HOAB is increased to such an extent that no other smectic-C-type phase than SmC/SmC exists. By tailoring the composition we are able to produce liquid crystal mixtures exhibiting unusual phase sequences, e.g., with a direct isotropic-SmCa transition or a temperature range of the SmCbeta subphase of about 50 K. (C) 2005 American Institute of Physics. [less ▲]

(-)-isopinocampheol substituted mesogens: An investigation of the effect of bulky terminal groups in chiral smectic liquid crystals

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 ▲]