Abstract
ABSTRACTThe splay, twist and bend elastic constants (K11, K22 and K33) have been measured as a function of temperature in bent-core/calamitic mixtures based on three different calamitic materials (5CB, 8CB and ZLI1132) and two bent-core dopants. The behaviour of the splay and bend constants are as expected; a reduction in K33 of ~20%, in line with predictions from mixing rules and other observations. Interestingly, no change is seen in the splay constant, K11 of the calamitic hosts. Surprisingly though, the twist elastic constant exhibits a reduction of 30 – 40% in all mixtures across the nematic range, an effect not previously reported and much larger than mixing rules can explain. The elastic behaviour is universal in our mixtures. We explain part of the reduction in the twist deformation by considering the influence of the chiral conformer fluctuations of the bent-core molecules on the twist elastic constants of the mixtures. However, the dramatic reduction can only be fully explained by also including contributions from chiral conformer fluctuations of the calamitic host, a form of chiral amplification.
Highlights
Nematic liquid crystals (NLCs) are fluids with longrange orientational ordering that exhibit anisotropy in their optical, electrical and elastic properties
Most NLCs that are used in devices tend to be rod-like in shape but more recently nematic phases formed from bent-core (BC) molecules have garnered interest, due to properties including increased flexoelectricity [1,2] as well as a reversal of the relationship between the splay (K11) and bend (K33) elastic constants, ie
We report a reduction in the twist and bend elastic constants when NLCs are doped with 10% of bent-core mesogens, whilst the splay elastic constant remains unaffected
Summary
Nematic liquid crystals (NLCs) are fluids with longrange orientational ordering that exhibit anisotropy in their optical, electrical and elastic properties. The anisotropy, together with their electro-optical responsiveness has led to NLCs being used widely in devices. Most NLCs that are used in devices tend to be rod-like (calamitic) in shape but more recently nematic phases formed from bent-core (BC) molecules have garnered interest, due to properties including increased flexoelectricity [1,2] as well as a reversal of the relationship between the splay (K11) and bend (K33) elastic constants, ie. Elastic constants are important features of liquid crystals as they are fundamental physical properties that influence both the threshold voltage and the steepness of the electro-optical response in NLC devices. The twist (K22) constant in particular is significant as it is directly related to the Fréedericksz transition threshold voltage in twisted nematic (TN) [10] and in-plane switching (IPS) devices [11]. Whilst TN devices are used less nowadays, IPS devices are still widely employed in display devices such as TVs and lower
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