Organo-Siloxane Tetrapodes

An accepted experimental proof for the existence of a biaxial nematic phase in a thermotropic liquid crystal remained missing for a very long time. However, in recent years, biaxial nematic phases have been found in liquid crystalline polymers as well as in liquid crystals made of rod-disc mesogens, banana-shaped (bent-core) molecules, and organo-siloxane tetrapodes. Here, some characteristics of these systems and the corresponding experimental procedure for the investigation of phase biaxiality will be introduced. Further details for the individual systems can be found in the cited literature. 

In a recent publication, these findings have been nicely confirmed by deuterium NMR investigations of a spin probe dissolved in the organo-siloxane tetrapodes [40], where the same sample-flip technique was applied as in [4, 11], In their study, the authors point out that the mode of stabilization of the biaxial nematic phase in tetrapodes and liquid-crystalline side-chain polymers, i.e., the lateral fixation of the mesogenic group, may in fact be similar. 

Finally, polarized IR spectroscopy is another sensitive method for the study of phase symmetry in liquid crystals. It was recently applied to prove phase biaxiality in organo-siloxane tetrapodes [37] - a supermolecular system forming quasi-flat platelets. Planar homogenously and homeotropically oriented samples were studied in order to derive all relevant order parameters from the three components of the IR absorbance. The presence of both a uniaxial and a biaxial nematic phase was detected, and again, optical textures and conoscopic observations supported these findings. 

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