There are only few materials, which have shown long temperature range twisted grain boundary (TGB) phases. One such material is the chiral binary mixture of 7OCB and $5^{*}$ CBB (mole ratio 0.8 and 0.2) which shows unique phase sequence of cholesteric $\left(N^{*}\right)$, a wide temperature range TGBA $(\sim 31°\mathrm{C})$ and reentrant cholesteric $\left({N^{*}}_{\mathrm{re}}\right)$ phases. In the present work we are reporting the dielectric spectroscopy of the above mixture with a chiral analog of earlier reported nematic $\left(N\right)$, smectic-$A$ $\left(\mathrm{Sm}A\right)$, and reentrant nematic $\left(N_{\mathrm{re}}\right)$ phase sequences [Phys. Rev. A 46, 7733 (1992)] for different conditions of molecular orientations. Two modes of dielectric relaxations have been detected in a homeotropically aligned sample with unusually low relaxation frequencies for one of them. Planar oriented molecules in the TGBA phase show a soft mode of relaxation and support the recently proposed theory for the soft mode relaxation of the TGBA phase [Phys. Rev. E 65, 11701 (2001)]. By applying the dc electric field on planar oriented molecules in the TGBA phase, it has been possible to obtain a helix free homeotropically aligned TGBA phase.

• Received 2 February 2005

DOI:https://doi.org/10.1103/PhysRevE.72.021703