Chris J. Welch

Do the short helices exist in the nematic TB phase

Dimeric compounds forming twist-bend nematic, Ntb, phase show unusual optical textures related to the formation of arrays of focal conic defects (FCDs). Some of the focal conics exhibit submicron internal structure with 8 nm periodicity, which is very close to that found in the crystalline phase of the material, that might suggest surface freezing.

The design and investigation of the self-assembly of dimers with two nematic phases

A series of non-symmetric dimers were synthesised containing either cyanobiphenyl or difluoroterphenyl moieties on one side and a range of long, short, bent, polar or apolar mesogens on the other side of the molecules. The dielectric anisotropy of the mesogens was varied systematically. The systems were characterised by differential scanning calorimetry (DSC), optical polarizing microscopy (OPM) and detailed X-ray diffraction (XRD) studies, both in the nematic and the Nx phase. The results are compared and structure–property relationships are discussed. A model for the assembly in the Nx phase is developed discussing Ntb structures, coaxial helices, swiss roll structures and chiral domain formation.

Mesophase structure and behaviour in bulk and restricted geometry of a dimeric compound exhibiting a nematic–nematic transition

We present structural studies of a dimeric compound composed of a central heptyl spacer linking two mesogens consisting of terphenyl units at which two adjacent fluoro groups are attached to each central ring. The terminal rings are linked to pentyl chains as terminal groups. The material exhibits a nematic-nematic transition and a low temperature modulated phase. The higher temperature nematic phase was found to exhibit an anomaly of the bend elastic constant similar to that of the dimers with N-Ntb phase sequence, and the physical properties of the low-temperature nematic phase are similar to those of the known Ntb materials. The structure of the low-temperature modulated smectic/columnar phase is described together with its ability to form freely suspended films and fibres. The relation of the modulated structure to the fibre formation and to the appearance of the labyrinthine instability in freely-suspended films is discussed.

Mixtures of disc-shaped and rod-shaped mesogens with chiral components

A novel mesogen has been synthesised, that not only combines two one disc-shaped and rod-shaped mesogens, but also includes a chiral group in the centre of the molecule. A protection and selective deprotection procedure allows the introduction of the chiral group in the central linking group of the mesogen. However, using this versatile procedure, a variety of other groups can be introduced selectively into the mesogen. The chiral group induces the formation of a cholesteric phase. Interestingly, the N* mesophase is miscible with the nematic phases of the precursor discs and rods. This study provides further evidence for the miscibility of discs and rods in a (chiral) nematic phase.

Comparative analysis of anisotropic material properties of uniaxial nematics formed by flexible dimers and rod-like monomers

ABSTRACT We report temperature dependencies of material properties such as dielectric anisotropy, birefringence, splay (), twist (), and bend () elastic constants of the uniaxial nematic (N) phase formed by flexible dimers of DTC5C9 and compare their behaviour to that of a corresponding monomer MCT5. DTC5C9 forms a twist-bend nematic (Ntb) at temperatures below the N phase. Anisotropic properties of MCT5 are typical of the rod-like mesogens. In particular, birefringence increases as the temperature is reduced, following the classic behaviour, described by Haller. The elastic constants also follow the standard behaviour, with their ratios being practically temperature independent. In contrast, DTC5C9 shows a dramatic departure from the standard case. Birefringence changes non-monotonously with temperature, decreasing on approaching the N–Ntb phase transition. decreases strongly to 0.4 pN near the N–Ntb transition, although remains finite. The ratios of the elastic constants in DTC5C9 show a strong temperature dependence that can be associated with the bend-induced changes in the orientational distribution function. The measured elastic properties are consistent with the tendency of the dimeric molecules to adopt bent configurations that give rise to the Ntb phase. Graphical Abstract

NMR Study of a Bimesogenic Liquid Crystal with Two Nematic Phases

Recent interest in bimesogenic liquid crystals showing two nematic phases has led us to investigate the nematic mean-field interactions in these nematic phases by using rigid solutes as probes. The nematic potential that is modelled by two independent Maier-Saupe terms is successful in fitting the observed dipolar couplings (order parameters) of para-, meta- and ortho-dichlorobenzene solutes in both the nematic phases of 39 wt% of 4-n-pentyl-4′-cyanobiphenyl (5CB) in α,ω-bis(4-4′-cyanobiphenyl)nonane (CB_C9_CB) to better than the 5% level. The derived liquid-crystal potential parameters G1 and G2 for each solute in the N and Ntb phases will be discussed. The most interesting observation is that G1 (associated with size and shape interactions) is almost constant in the Ntb phase, whereas G2 (associated with longer-range electrostatic interactions) has large variation, even changing sign.

The induction of the Ntb phase in mixtures

ABSTRACT We report the induction of the Ntb phase over a wide temperature and concentration range in a binary system. This was achieved by addition of a flexible dopant without LC properties to a flexible dimer exhibiting only a nematic LC phase. The Ntb phase was identified by Polarising Optical Microscopy (POM), Differential Scanning Calorimetry (DSC) and X-ray Diffraction (XRD) techniques. Graphical Abstract

Characterization of large tilt-angle flexoelectro-optic switching in chiral nematic liquid crystal devices

ABSTRACT The ‘flexoelastic ratio’ is commonly used to characterise the electro-optic behaviour of chiral nematic liquid crystal (LC) devices that exhibit flexoelectro-optic switching. There has been renewed interest in this electro-optic effect of late as new LC materials and mixtures have been developed that exhibit large tilt angles, Ø, of the optic axis (Ø ≥ 45°) whilst maintaining a fast response time (< 1 ms). In this study, we compare the different flexoelastic ratios that are obtained for fixed and variable-pitch chiral nematic LCs for materials with a tilt of the optic axis as large as Ø = ± 45°. We show that for large tilt angles of the optic axis the values for the two different flexoelastic ratios measurably diverge. Of the two ratios, we propose that for large tilt-angle mixtures it is more appropriate to use the flexoelastic ratio that is derived for the case when the pitch of the helix is constrained. In addition, a simple way of determining the ‘pitch-constrained’ flexoelastic ratio is presented that involves identifying the electric field amplitude at the point for which the transmission levels are the same for both positive and negative electric field polarities. GRAPHICAL ABSTRACT