Ferroelectric liquid crystals exhibiting a very high spontaneous polarisation

Cross, Simon John

Abstract

In a ferroelectric liquid crystal small changes to the molecular structure can have significant effect upon the ferroelectric properties and mesomorphic behaviour. The addition of a substituent, which is both bulky and highly polar, to the aromatic core in a position ortho to the chiral group will alter both intermolecular and intramolecular interactions. The polar character of the substituent will affect dipolar, molecular interactions whilst its steric properties will alter the rotational freedom of the chiral centre and, change the overall molecular shape and its resultant rotational volume.
Materials are synthesised with one of several electronegative substituents attached onto the core of a mesogenic material, adjacent to the chiral centre. The effect of the substituent on the ferroelectric and mesomorphic properties was investigated with respect to the physical properties of the substituent and in combination with different core structures and chiral centres.
It was found that bulky and highly electronegative core substituents in conjunction with a 1-methylheptyloxy chiral group lead to high spontaneous polarisations. This in turn leads to very large electroclinic effects with low temperature dependence and an unusually rapid saturation of the induced (electroclinic) tilt.
Increasing the relative intramolecular distance between the chiral centre and core substituent increases clearing points, diminishes smectic C mesophase occurrence and reduces spontaneous polarisations to that expected from an unsubstituted stmcture.
Decreasing the relative intramolecular distance between the chiral centre and core substituent decreases clearing points, produces low temperature smectic C mesophases and increases the spontaneous polarisation significantly. However, if the interaction is too strong, due to a large chiral centre and bulky substituent, mesomorphism is destroyed.
An undesirable side effect of the highly polar nature of these materials is their generally poor alignment properties and incompatibilities with host materials leading to smaller spontaneous polarisations than expected.
The large spontaneous polarisations and related properties are attributed to an increased intramolecular and intermolecular (librational) interaction.

Citation

Cross, S. J. (1996). Ferroelectric liquid crystals exhibiting a very high spontaneous polarisation. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4224363

• Thesis Type: Thesis
• Deposit Date: Oct 12, 2022
• Publicly Available Date: Feb 24, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4224363
• Additional Information: Department of Chemistry, The University of Hull
• Award Date: Sep 1, 1996