2019-10

5th KCST Duhok University

The intriguing twist-bend nematic phase (NTB) is formed, primarily, by liquid crystal dimers having odd spacers. Typically the phase is preceded by a nematic phase (N) via a weak first-order transition. Our aim is to obtain dimers where the NTB phase is formed directly from the isotropic phase via a strong first-order phase transition. The analogy between such behaviour and that of the smectic A (SmA)-NI sequence suggests that this new dimer will require a short spacer. This expectation is consistent with the prediction of a molecular field theory since the decrease in the spacer length results in an increase in the molecular curvature. A vector of odd dimers based on benzoyloxybenzylidene mesogenic groups with terminal ethoxy groups has been synthesised with spacers composed of odd numbers of methylene groups. Spacers having 5, 7, 9, and 11 methylene groups are found to possess the conventional phase sequence NTB-NI; surprisingly for the propane spacer the NTB phase is formed directly from the isotropic phase. The properties of these dimers have been studied with care to ensure that the identification of the NTB phase is reliable.

2016-04

Twist - Bend Nematics and Beyond Workshop, University of Southampton, UK.

In 2011 it was suggested that the second nematic phase formed by the odd liquid crystal dimer CB7CB was the twist-bend nematic phase [1]; here the molecular origin of the phase is associated with the molecular curvature created by the methylene link between the spacer and the cyanobiphenyl group. The phase sequence for this and most other twist-bend nematics is NTB-N-I and for CB7CB the NTB-N phase transition is clearly first order [1] in contrast to the predictions of several theories that the transition should be second order [2,3]. For homologous series, as the spacer length increases so the strength of the transition weakens and the nematic range grows [4]. Conversely, the transition strengthens with decreasing spacer length and for some series it was found that the NTB phase is formed directly from the isotropic phase [5] in keeping with a molecular field theory [3]. The dimers (mO.n.Om) found to show this intriguing behaviour are

2014-07

25th International Liquid Crystal Conference, Trinity College, Dublin-Ireland.

The twist -bend nematic phase, NTB, differs from a conventional chiral nematic in that the director is tilted with respect to the helix axis. In some sense the chiral smectic C phase is the smectic analogue of the twist-bend nematic phase. The existence of the NTB phase was predicted by Dozov for bent-core molecules with the bent shape being an important element in the driving force for its formation [1]. The first example of the twist-bend nematic was observed for the liquid crystal dimer, CB7CB, in which two cyanobiphenyl groups are linked by a heptane spacer [2]. Although the spacer is flexible allowing the dimer to undergo significant changes in this spacer, on average its shape is highly bent [3]. Although this qualitative feature is important in the formation of the twist-bend nematic phase the extent of the bend, that is a quantitative feature, is also central [1]. This angle in the average structure is controlled to a great extent by the nature of the link between the rod-like mesogenic group and the spacer. It appears that a methylene linking group is especially good at stabilising the twist-bend nematic phase with respect to the conventional nematic phase [2, 4]. This behaviour is also consistent with the calculation of the bend elastic constant which tends to be very small and can be negative [2, 5]. Although the influence of the linker on the formation of the twist-bend nematic phase seems clear and reliable there are other features of the molecular structure which can surely contribute to the stability of the NTB phase. To explore some of these features we have studied a series of liquid crystal dimers in which there is a terminal chain instead of a nitrile group on each mesogenic group. The terminal chains have a length m and the spacer length is n so that we use the acronym mO.n.Om for these symmetric dimers. We have characterised the mesophases formed by the dimers using polarised optical microscopy, deuterium NMR spectroscopy and calorimetry. The spacer has a methylene link to the mesogenic group and the number of the methylene groups, n, is odd. The length of the terminal group changes from 1 to 6. The phases exhibited by some of these dimers include the expected nematic and twist-bend nematic. In addition, and importantly, phases with a layered structure appear as the length of the terminal chains grow; as might be expected. The transitional properties of the dimers exhibit odd-even behaviour with variation of the terminal chain length. We have also extended our exploration of structure-property relations for these dimers by studying binary mixtures of them as well as with CB7CB [4,6].

2014-04

British Liquid Crystal Society, Annual Conference, Durham, University of Durham, UK.

The twist-bend nematic phase, NTB, differs from a conventional chiral nematic in that the director is tilted with respect to the helix axis. In some sense the chiral smectic C phase is the smectic analogue of the twist-bend nematic phase. The existence of the NTB phase was predicted by Dozov for achiral, bent-core molecules with the bent shape being an important element in the driving force for its formation [1]. The first example of the twist-bend nematic was observed for the liquid crystal dimer, CB7CB, in which two cyanobiphenyl groups are linked by a heptane spacer [2]. The spacer is flexible, allowing the dimer to undergo significant changes in its shape, indeed on average its shape is highly bent [3]. Although this qualitative feature is important in the formation of the twist-bend nematic phase the extent of the bend, that is a quantitative feature, is also central [1]. This angle in the average structure is controlled to a great extent by the nature of the link between the rod-like mesogenic group and the spacer. It appears that a methylene linking group is especially good at stabilising the twist-bend nematic phase with respect to the conventional nematic phase [2, 4]. This behaviour is also consistent with the calculation of the bend elastic constant which tends to be very small and can be negative [2, 5]. In order to help understand the phase behaviour of such nematogens we have explored the phase diagram of binary mixtures of liquid crystal dimers with odd spacers. The dimers contain terminal chains of length m as well as the flexible spacer of length n. So we use the acronym mO.n.Om for these symmetric dimers. We have characterised the mesophases formed by the odd dimers using polarised optical microscopy, deuterium NMR spectroscopy and calorimetry. The spacer has a methylene link to the effective mesogenic group and the number of methylene groups, n, is necessarily odd. The length of the terminal group varies from 1 to 6. The phases exhibited by some of these dimers include the expected nematic and novel twist-bend nematic. In addition, and importantly, phases with a layered structure appear as the length of the terminal chains grow; as might have been anticipated. The binary mixtures exhibit a range of phase diagrams whose form depends in part on the commonality of the phases exhibited by the compounds. For example, when both exhibit nematic and twist-bend nematic phases the nematic-isotropic and the twist-bend nematic-nematic transitions vary continuously across the phase diagram. This shows a curious deviation from linearity. The phase diagram is more interesting when there is a not a commonality in the phases for the two components. Then phases are injected into the phase diagram as the composition changes.

2013-09

2. 12th European Conference on Liquid Crystals, Rhodes, Greece.

Thechiral nematic phase has a helical structure with the direct ororthogonal to the helicalaxis. This is clearly the limiting case of a more general chiral structure in which the director is tilted with respect to the helicalaxis. Indeed such a twist-bend structure was predicted for bent-core molecules having a highly bent form by Dozov in 2001 [1]. Some years earlier, as part of a parallel thread, it had been recognised that the conformation alaverage shape of a liquid crystal dimer with an odd spacer would also be bent [2]. In addition it was realised that, significantly, thed egree of bend would be larger for odd dimers with methylenerather than etherlinks [3]. Subsequently a detailed investigation [4] of the liquid crystal dimer with a heptane spacer, CB7CB, suggested that this exhibited not only a conventional nematic butal so thet wist-bend nematic phase predicted by Dozov, an identification supported by other studies of this mesogen. One of the techniques which had indicated an unusual structure for the second nematic phase was ESR spectroscopy. This revealed that although the director is uniformly aligned in the conventional nematic by the magnetic field of the spectrometer, this is not the case for thet wist-bend nematic phase [4]. Here, the form of the director distribution could be explored by the angular dependence of the ESR spectrum [5]. This contrasts with the behaviour oft he DNMR spectrum where the intrinsic magnetic field of the NMR spectrometer is sufficiently large to produce a monodomain. However, what does emerge is that the twist-bend phase is chiral[4], although remarkably the constituent molecules are achiral, and that there are domains of opposite chirality [6], in accord with Dozov’s predictions [1] Here we describe an investigation of a liquid crystal dimer having an odd spacer but with an unusual linkage. The techniques that have been employed are ESR and DNMR spectroscopies for both nematic phases and an underlying smectic phase. Analysis of the spectra, measured as a function of temperature, reveal the nature of the liquid crystal phases at a macroscopic level.

2013-04

1st International Scientific Conference – UoZ 2013

In this talk the investigations which led to the experimental confirmation of the existence of a predicted twist-bend nematic phase will be given. The work on the V shaped methylene linked dimer 1",7"-bis(4-cyanobiphenyl-4'-yl) heptane (CB7CB) - involving the use of a range of experimental techniques and in particular deuterium NMR spectroscopy-showed the presence of a nematic phase on cooling from the isotropic phase(~1150C). On further cooling below 1030C another first order transition was observed to occur leading to another liquid-crystal phase1. This lower temperature phase was initially identified as a smectic phase2. However, the final and correct identification of the phase was very recently achieved: as a "twist-bend" nematic phase1 and assigned the symbol NTB. This confirms the prediction made by Dozov3 in 2001 that such a phase could be formed when the bend elastic constant K3 becomes negative. The NTB nematic behaves as a chiral phase although the component molecules themselves are not chiral. The optical texture of this phase has rope-like stripes with each stripe having the opposite handedness from its neighbours. Further recent work using deuterium NMR has shown that non-chiral molecules dissolved in the NTB phase (like CB7CB) display chiral behaviour4. The chiral nature of the NTB phase was further demonstrated in a mixture of V-shaped molecules with one having methylene linkages5. The final part of the talk will touch briefly on the current investigation of a V shaped compound6 in which an un-identified phase is reported to occur below a normal nematic phase. It is believed that the unknown phase will prove to be a twist-bend NTB nematic phase. This work is being carried out at the University Southampton by Miss A Dawood, a postgraduate student from the University of Zakho, in partial fulfilment of the PhD at the University of Zakho.