Twist-bend nematics and heliconical cholesterics: a physico-chemical analysis of phase transitions and related specific properties

ABSTRACT

Certain nematic liquid crystals, for example, those formed by banana-shaped molecules, can exhibit a low-temperature twist-bend nematic (N_tb) phase. Upon addition of chiral dopants, a chiral version of the twist-bend phase (N*_tb) can be observed below the conventional chiral nematic (N*) phase, while under electric field the N* phase is transformed into a specific state known as ‘oblique helicoidal cholesteric’. In this work, we have studied the well-known N_tb-forming system CB7CB:CB6OCB with addition of 5CB and chiral dopants (ChDs) using optical microscopy, differential scanning calorimetry (DSC), and measurements of temperature-dependent optical transmission using optical microscopy. Effects of 5CB and different chiral dopants (of steroid or non-steroid nature) upon thermal stability of N_tb phase were analysed, and selective reflection in the visible range could be observed in the N* phase with a sufficiently high ChD content. As a peculiar feature, sharp unwinding of the cholesteric helix was observed at lower temperatures when approaching the N*_tb phase, which appears to be similar to the helix unwinding close to the smectic-A transition.

Graphical abstract

KEYWORDS:

• Twist-bend nematic
• chiral twist-bend nematic
• chiral dopant
• differential scanning calorimetry
• phase transition
• Bragg diffraction

Acknowledgments

The authors thank W. Becker (Merck, Darmstadt, Germany) for his generous gift of chiral dopant R-811. Dr. A. Buchnev (Cambridge, England) who had supplied us with polymer PI2555, and Field service specialist IV V. Danylyuk (Dish LLC, USA) for his gift of some Laboratory equipment. Mesogens CB7CB and CB6OCB were generously donated by the Materials and Manufacturing Directorate of the Air Force Research Laboratory, Wright-Patterson AFB, Ohio. Our special gratitude goes to Mariacristina Rumi for useful discussions and valuable help in the manuscript preparation. This study was conducted within the Projects “Photophysics of the processes of optical radiation interaction with photorefractive, solid-state and bio-organic media” and “Effects of self-organization of functionally active nanoparticles” funded by National Academy of Sciences of Ukraine.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the National Academy of Sciences of Ukraine [Effects of self-organization of functionally active nanoparticles] and [Photophysics of the processes of optical radiation interaction with photorefractive, solid-state and bio-organic media].