Thermotropic Biaxial Nematic Phase in Liquid Crystalline Organo-Siloxane Tetrapodes

Abstract

Infrared absorbance measurements have been carried out on two liquid crystalline organo-siloxane tetrapodes. Results unambiguously show the existence of a biaxial nematic phase below a uniaxial nematic phase. The three components of IR absorbance are used to calculate the various order parameters. On cooling, a weak first-order transition from isotropic to nematic is observed, followed by a second-order phase transition to biaxial nematic where the biaxiality parameters are found to be significantly large. Results are supported by observations from conoscopy and texture. Temperature dependences of the order parameters are well explained by the mean-field model for a biaxial phase.

Biaxial order and a rotation of the minor director in the nematic phase of an organo-siloxane tetrapode by the electric field

“…If the effects of the molecular interactions and the local field are neglected, then the absorbance components along the principal laboratory axes can be written down in terms of the components of the dipole moment along the molecular axes and the four scalar order parameters defined by the Saupe ordering matrices. 7 The orientational ordering for the tetrapodes has already been investigated in a planar cell; 14 nevertheless, such a geometry is not suitable for detecting the biaxiality in its nematic phase. It is convenient, however to measure the absorbance components of a homeotropically aligned cell and to relate the anisotropy in the A X and A Y components to the phase biaxiality parameters, P and C. The sum of these components normalized by the absorbance in the isotropic phase is related to the uniaxial order parameters, S and D, and the difference leads to P and C as given below:…”

“…Permittivity so measured is ε Y . For weak fields in the range 0.1 − 0.6 V/μm, the permittivity measured is 1 2 (ε X + ε Y ). The three components of permittivity can thus be determined.…”

“…The biaxiality is determined by the anisotropy of its geometerical frame, the angle between the mesogenic groups and the axes in that frame. Biaxiality in the nematic phase of such systems with asymmetric and symmetric mesogenic units was first discovered by Merkel et al 1 using polarized infrared (IR) spectroscopy. The NMR spectroscopy of one of the two tetrapode systems showed two distinct nematic regions, one with a vanishing asymmetry parameter corresponding to the uniaxial nematic phase and second with a significant temperature dependent asymmetry parameter corresponding to the biaxial nematic phase.…”

Computer Simulations of Biaxial Nematics

“…Besides the bent-core shaped mesogens, in the same 2004 year, a novel class of N b based on organo-siloxane tetrapodes has been found by Mehl, Vij and coworkers [9]. These thermotropic systems have been thoroughly characterised by measuring the anisotropy of infrared absorbance, with the addition of conoscopic and textural imaging under polarised light [9], and the temperature dependence of the averaged quadrupolar coupling constant of a deuterated 8CB solute probe by deuterium NMR [10].…”

“…At the time of writing a few instances of stable thermotropic biaxial nematics and their characterisation [6,7,8,9,10] have been reported. The papers of Luckhurst [11] and Praefcke [12] and the book of Chandrasekhar [13] also give a critical account of the early synthetic attempts, while assessment of the more recent achievements can be found in [14].…”