On the nematic phases of achiral dimers and strongly polar mesogens : dielectric and optical investigations

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Ζάββου, Ευαγγελία

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Liquid crystals represent an intermediate state of matter between the crystalline solids and the isotropic liquids. The simplest and most widely studied liquid crystalline phase is the uniaxial, apolar nematic phase (N) composed by achiral mesogens. Nematic liquid crystals are particularly intriguing due to the facile manipulation of the average molecular orientation with external stimuli, a feature widely exploited in technological applications, the most notable being the liquid crystal displays (LCDs). While liquid crystalline phases with partial positional order (e.g. smectics) are known to exhibit rich phase polymorphism, that was not the case for the nematic phase, despite the theoretical predictions. In fact, until recently the only experimentally evidenced nematic phase composed by achiral mesogens was the uniaxial, apolar N phase. During the last decade, the realisation of two novel nematic phases with lower symmetry has attracted major scientific interest. Quite surprisingly, a class of symmetric bent-shaped achiral dimers was found to exhibit two distinct nematic phases connected with a weak first order transition. The high temperature phase is the conventional uniaxial nematic phase (N), while the low temperature phase is a 1D modulated and structurally chiral nematic, where the modulation is exclusively referred to the helical variation of the (local) orientational order with a 10 nm period. This phase was initially termed as Nx. Later, some authors identified this Nx with the twist-bend nematic (Ntb) proposed by R.B. Meyer in the 1970s, while others argue that the Nx represents a novel polar and twisted nematic (NPT) phase, where the direction of the polarity roto-translates along the axis of modulation. The second mesophase is the recently discovered ferroelectric nematic phase (NF), which was found in highly electrostatically polar rod-like mesogens and exhibits remarkably high spontaneous polarisation values in the order of several μC/cm^2. In the NF phase the inversion symmetry is broken and as a result macroscopic domains of opposite polarisation are formed with separating domain walls, in analogy to ferromagnetic systems. Several questions, both fundamental and technologically oriented, regarding the structure, symmetries, domain formation, alignment, and response to external stimuli of NF phase remain open and intense research is conducted in these directions. The aim of this dissertation is the study of the optical and dielectric properties of compounds exhibiting these novel nematic mesophases. The first part of the thesis is dedicated to the dielectric and optical characterisation of liquid crystalline systems that exhibit the uniaxial N phase and at lower temperatures the structurally chiral Nx phase. These systems are symmetric liquid crystal dimers and an oligomer with odd number of carbon atoms in their flexible spacer. The temperature dependence of birefringence was investigated through Polarising Optical Microscopy using a Berek compensator and monochromatic light, while dielectric measurements were carried out on magnetically/electrically aligned samples. The studied systems are divided into two categories with respect to the sign of their dielectric anisotropy (Δ𝜀 ). In the category of systems with positive dielectric anisotropy (Δ𝜀 > 0) fall members of the symmetric cyanobiphenyl methylene-linked CBnCB (n = 9, 11) dimers and their mixtures with the corresponding monomer (CB9CB-5CB). The experimental findings, supported by a simple mean-field theory for flexible bent-core molecules, allow for the calculation of the temperature dependence of the orientational order parameter, as well as of the intra-molecular orientational dipolar correlations. The role and the relative magnitude of the intra- and inter-molecular dipolar orientational correlations is analysed in the framework of Kirkwood-Fröhlich theory for dielectric permittivity of anisotropic polar fluids, providing an excellent representation of the measured dielectric permittivity in both N and Nx phases. Moreover, the odd-membered ether-linked cyanobiphenyl CBOnOCB dimers (n = 5, 7, 9 ,11) are studied in terms of their optical anisotropy and dielectric response and the effect of the spacer length on the studied physical properties is discussed. CBOnOCB dimers do not exhibit the Nx phase, despite their chemical affinity with their methylene-linked counterparts. Thus, a comparative analysis of the experimental findings with dimers of the CBnCB series with same number of atoms in the spacer, along with a detailed study of their conformational properties give new insights into the key factors preventing the formation of the low temperature nematic phase in ether-linked dimers. Oligomeric systems with negative dielectric anisotropy (Δ𝜀 < 0 ), namely the laterally fluorinated liquid crystal dimer DTC9C5 and its homologous C9trimer, are also investigated. Results for the corresponding monomer MCT5 are also presented in order to clarify the dependence of the optical and dielectric anisotropy on the number of connected mesogenic units. The role of dipole-dipole correlations on the static dielectric permittivity in the nematic phase of the dimer and the trimer is also discussed. The last part of the dissertation is devoted to the study of the recently discovered ferroelectric nematic phase (NF). The study is focused on the investigation of the mechanical and non-linear optical properties of a strongly polar mesogen, exhibiting a room-temperature ferroelectric nematic phase directly on cooling from the isotropic liquid. The non-linear optical response within the NF phase is studied through Second Harmonic Generation by means of Confocal Laser Scanning Microscopy. The director reorientation in a magnetic field is explored through optical transmission measurements and dielectric spectroscopy. The combination of the experimental findings along with a developed model capable to describe the director reorientation in a magnetic field suggests that the strong splay rigidity of the NF phase is connected to electrostatic self-interaction of polarisation, avoiding the polarisation splay.



Liquid crystals, Nematic phase, Liquid crystal dimers, Dielectric spectroscopy, Dielectric anisotropy, Twist-bend nematic, Optical polarising microscopy, Birefringence, Phase transitions, Structural chirality, Ferroelectric nematic phase, Ferroelectric switching, Magneto-optical properties, Nonlinear-optical response