Showing papers by "Jan Jadżyn published in 2008"
TL;DR: The experimental results indicated soft anchoring in the case of spherical magnetic particles and rigid anchoring for rodlike and chainlike magnetic particles, with parallel initial orientation between the magnetic moments of the magnetic particle and director.
Abstract: In this work the 4-(trans- 4'-n -hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) liquid crystal was doped with differently shaped magnetite nanoparticles. The structural changes were observed by capacitance measurements and showed significant influence of the shape and size of the magnetic particles on the magnetic Freedericksz transition. For the volume concentration phi= 2 x 10(-4) of the magnetic particles, the critical magnetic field was established for the pure liquid crystal, and for liquid crystals doped with spherical, chainlike, and rodlike magnetic particles. The influence of the magnetic field depends on the type of anchoring, which is characterized by the density of anchoring energy and by the initial orientation between the liquid crystal molecules and the magnetic moment of the magnetic particles. The experimental results indicated soft anchoring in the case of spherical magnetic particles and rigid anchoring in the case of rodlike and chainlike magnetic particles, with parallel initial orientation between the magnetic moments of the magnetic particles and director.
126 citations
TL;DR: In this article, the authors describe the observations of structural transitions in ferronematics based on the thermotropic nematics 6CHBT (4-trans-4'-n-hexyl-cyclohexyl]-isothiocyanato-benzene).
Abstract: In this work we describe the observations of structural transitions in ferronematics based on the thermotropic nematics 6CHBT (4-trans-4'-n-hexyl-cyclohexyl-isothiocyanato-benzene). The ferronematic droplets were observed in solutions of nematogenic 6CHBT dissolved in phenyl isocyanate and doped with fine magnetic particles. The phase diagram of the transitions from the isotropic phase to the nematic phase via a droplet state was found. Magneto-dielectric measurements of various structural transitions in this new system enabled us to estimate the type of anchoring of the nematic molecules on the magnetic particle surfaces in the droplets.
29 citations
TL;DR: It is found that an ability to the dipolar aggregation of the molecules depends on the alkyl tail length, and, in particular, the ability is strongly reinforced when the number n changes from 7 to 8.
Abstract: Based on the temperature behavior of the entropy increment induced by the probing electric field in the isotropic phase of mesogenic compounds belonging to the homologous series C(n)H(2n+1)PhCOOPhCN ( n = 4-10), it is found that an ability to the dipolar aggregation of the molecules depends on the alkyl tail length, and, in particular, the ability is strongly reinforced when the number n changes from 7 to 8. The role of the molecular structure in the self-assembling process is discussed.
23 citations
TL;DR: The linear and nonlinear dielectric relaxation spectra, recorded in the isotropic phase of mesogenic 4-(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene (6CHBT), reveal an equilibrium between the molecules that are involved and non-involved in the pseudo-nematic domains, which are spontaneously formed in the prenematics region.
Abstract: The linear and nonlinear dielectric relaxation spectra, recorded in the isotropic phase of mesogenic 4-(trans-4‘-n-hexylcyclohexyl)isothiocyanatobenzene (6CHBT), reveal an equilibrium between the molecules that are involved and non-involved in the pseudo-nematic domains, which are spontaneously formed in the prenematic region. A perturbation of the equilibrium by a static electric field of high strength (E0 ≈ 107 V/m) is followed by a nonlinear dielectric relaxation process that, measured with the probing electric field E(ω) of small amplitude and variable high frequency (up to 100 MHz), reflects the rate of the domains formation. The fraction of the mesogenic molecules involved in the prenematic domains at different temperatures is estimated.
19 citations
TL;DR: The singularities in dependence of the entropy increment on the temperature and on the mixtures composition are discussed in terms of the prenematic molecular self-organization extent in mesogenic liquids of different density of dipoles.
Abstract: The paper concerns two aspects of the entropy in mesogenic systems: (i) the entropy jump (ΔS0NI) at the phase transition from the isotropic liquid (I) to the nematic liquid crystalline state (N), and (ii) the entropy increment (ΔS) caused by the ordering action of the probing electric field applied to the dipolar system. The system studied are the mixtures of strongly polar mesogenic solvent n-hexylcyanobiphenyl (C6H13PhPhCN, 6CB) and the nonpolar nonmesogenic admixture 4-ethylcyclohexyl-4′-n-nonylphenyl (C2H5CyHxPhC9H19, 2CyPh9). The entropy jump at the I−N phase transition in pure 6CB [ΔS0NI= 1.52 J/(mol K)] was evaluated from the analysis of the phase diagram of the mixture 6CB + 2CyPh9 with use of the Landau−Lifshitz theory; the resulting value of the transition enthalpy (ΔH0NI = TNIΔS0NI = 0.50 kJ/mol) agrees well to that obtained with the calorimetric methods. The field-induced entropy increment (ΔS) was calculated, at the given temperature, from the static dielectric permittivity derivative value (...
7 citations
TL;DR: In this article, a review of recent results on static dielectric behavior in vicinity of weakly first order transition from isotropic liquid to a liquid crystalline nematic phase of compounds used in opto-electronic devices is presented.
Abstract: The paper presents a review of recent results on static dielectric behaviour in vicinity of weakly first order transition from isotropic liquid to a liquid crystalline nematic phase of compounds used in opto-electronic devices. Temperature behaviour of static permittivity and its derivative are interpreted in terms of a basic thermodynamic quantities increment (internal energy, entropy and Helmholtz free energy) induced by a probing electric field.
7 citations
TL;DR: The results obtained provide new relaxation features, which are presented in the form of analytic expressions and illustrated by Cole-Cole-like diagrams and three-dimensional dispersion and absorption plots in order to show the influence of the coupling parameter a measuring the importance of the rotational motion over the translational one.
Abstract: The problem of the nonlinear ac dielectric response in polyelectrolytes is carried out by considering these electrically charged systems acted on by a strong dc bias electric field superimposed on a weak ac field. An infinite set of coupled (translation-rotation) kinetic equations is derived, and by using a perturbation procedure, we establish a new expression for the complex dielectric increment of an assembly of nonpolar rodlike macroions surrounded by small counterions. By considering the stationary regime, we show that the electric polarization of the nonpolar macroion is given by calculating the expectation value of P1(cos ϑ)H1(x), where P1 and H1 stand for the first Legendre and Hermite polynomials, respectively. The results so obtained provide new relaxation features, which are presented in the form of analytic expressions and illustrated by Cole-Cole-like diagrams and three-dimensional dispersion and absorption plots in order to show the influence of the coupling parameter a measuring the importan...
2 citations
TL;DR: In this article, the magnetic Freedericksz transition in HAB-based (4,4′-di-n-heptylazoxybenzene) ferronematics was described.
Abstract: In this work we describe the magnetic Freedericksz transition in HABbased (4,4′-di-n-heptylazoxybenzene) ferronematics. The ferronematic samples were prepared with two different volume concentrations of magnetic particles φ1 = 2 × 10−5 and φ2 = 2 × 10−4. The liquid crystal HAB is nematic at the temperatures from ≈ 53◦C to 70◦C and smectic A at the temperatures under 53◦C. The anisotropy of dielectric permitivity of HAB liquid crystal is positive in nematic phase and negative in smectic A phase. The magnetic Fredericksz transition was studied in nematic phase at temperature 60◦C.
2 citations