The field of ferroelectric liquid crystals (FLCs) is one of the most fascinating aspects of the Science of Materials for their
interesting electro-optic applications. Among different chemical and physical properties those related to the molecular
dynamics are very stimulating due to the relationship between molecular motions and macroscopic response to external
fields, such as electric and magnetic ones. In this work, the molecular dynamics of a ferroelectric smectogen, namely
the (S)-2-methylbutyl-[4'-(4"-heptyloxyphenyl)-benzoyl-4-oxy-(S)-2-((S)-2')-benzoyl)-propionyl)]-propionate (ZLL 7/*)
has been investigated by means of 2H NMR and dielectric relaxation techniques. The first method allows us to get
information on the molecular motions in the fast motion regime, in particular the diffusion reorientational motions of the
whole molecule, as well as of the internal motions affecting the phenyl and biphenyl fragments.
The second technique, which covers the slow motion regime, has been used to detect collective motions and fluctuation
modes. This study allows us to have a complete idea of molecular motions in the different smectic phases formed by the
ZLL 7/* mesogens, in particular the paraelectric smectic A (SmA), the ferroelectric smectic C* (SmC*), the
antiferroelectric smectic C*A (SmC*A) and the re-entrant ferroelectric smectic C* (SmC*r) phases. This last phase has
been investigated for the first time in this work, from the point of view of the dynamic behaviour. All results will be
discussed in the framework of the molecular dynamics of ferroelectric liquid crystals, reported in the literature so far.
New series of chiral liquid crystalline materials with the same rigid core with keto group and a various number of (S)-lactate groups in the chiral part of the moelcule have been studid by DSC, X-ray, electro-optics and dielectric spectroscopy. All compounds contain branched terminal alkyl chain derived from (S)-2-methylbutanol. The influence of the lactate number on mesogenic behavior of those compounds has been studied. All compounds show the paraelectric SmA phase and the ferroelectric SmC* phase in a rather wide temperature range. The antiferroelectric SmC*A phase was detected for the compound possessing two lactate groups. A polar field-induced smectic phase have been observed by electro-optic studies.
The influence of the wavelength of light on values of polarisability tensor components αparallel, αperpendicular, polarisability anisotropy (αparallel - αperpendicular) and the order parameter S of liquid crystal molecules was verified. The refractive indices are functions of temperature and light wavelength. The values αparallel and αperpendicular of a liquid crystal molecule can be calculated from refractive indices, density, and coefficient in the Valentova equation. Here, it was found that there is a relation between the wavelengths of the light and values of (αparallel - αperpendicular) and αparallel, αperpendicular of p-(p-ethoxyphenylazo)phenyl heptanoate molecule. The values of the order parameter S obtained for the studied material are identical for all wavlengths of light.
Two series of ferroelectric liquid crystalline materials, non-substituted and containing a methoxy lateral group at the aromatic ring of the molecular core have been investigated by dielectric relaxation spectroscopy in the range from 1 Hz to 1 MHz. All compounds exhibit chiral SmC* phase. A low temperature ferroelectric phase, consisting of the ordinary SmC* phase and a crystalline phase, was detected in three of the investigated compounds. The frequency dispersion reveals two modes (denoted as mode 1 and mode 2) detected in both ferroelectric phases. The analysis of the frequency dispersion of the complex permittivity provides relaxation frequency and dielectric strength of these modes. In the SmC* phase, high frequency mode 1 is the Goldstone mode. Low frequency mode 2 is attributed to a non-homogenous distribution of ionic space charge, which is localized near the surfaces. The values of Ps in the low temperature ferroelectric phase are lower than in the ordinary SmC* phase for the methoxy substituted compounds but shows no anomaly for the non-substituted material.
Dielectric dispersion has been studied in the frequency range of 10Hz divided by 1MHz in liquid crystals with 2- alkoxypropriate chiral group. Two of them exhibit the spontaneous helix twist inversion in the SmC* phase. The dispersion reveals two modes, dielectric strengths as well as relaxation frequencies of which exhibit a strong dependence on the sample thickness. Mode 1 with the relaxation in the kHz range is the Goldstone mode, which is combined with a thickness mode, but it becomes the pure thickness mode in the temperature range near the helix twist inversion temperature, where the helicoidal structure is unwound. This situation is reflected in the temperature dependence of characteristics parameters of mode 1. The lower frequency mode 2 is attributed to a mode connected with a non-homogeneous distribution of ionic space charge, which is localized near the sample surface.
New series of ferroelectric liquid crystals derived form alkoxycinnamic acid containing chiral alkyllactate group was synthesized. All compounds have a wide temperature range SmC* phase at temperatures above 100 degrees C and high spontaneous polarization. Some of homologues exhibit spontaneous helix twist unwinding in the SmC* phase accompanied with the helix twist inversion.
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