Jerzy Małecki

Nonlinear dielectric study of pretransitional effects in liquids above the phase transition temperature

Abstract Variations in dielectric permittivity in an electric field (NDE) rae measured for two solutions in the single-phase precritical region and for the isotropic phase of MBBA above the clearing temperature T c . The magnitude of the effect measured in the critical solutions is proportional to ( T - T c ) −ζ , where ζ ≈ 1 3 , pointing to a relationship with fluctuations in density and concentrations. In MBBA, the effect is proportional to ( T - T *) −ζ , where ζ ≈ 1 and T * = T c - 0.7°. The results and their discussion show that NDE is an efficient method for the study of phase transitions in liquids, especially pretransitional effects at temperatures even 10–20° above the transition point.

Determination of Orientational Order Parameter in Various Liquid-Crystalline Phases

Many methods for the determination of the order parameter in nematic liquid crystals are known. However, most of them are not applicable to cholesteric and smectic phases. Therefore, experimental data on the order parameter in cholesteric and smectic phases are hardly available. To determine the orientational order parameter S we exploited the de Gennes observation, that any anisotropic physical quantity may be a measure of orientational ordering in nematic mesophase. The order parameter can be calculated directly from a macroscopic quantity measured in the experiment. From this idea we developed a simple method for the determination of the order parameter based on birefringence measurements, which can be applied for nematic, cholesteric, and smectic phases. A precise and simple method for measurement of the optical birefringence is described. An approximate empirical formula is applied for normalization of the optical birefringence similar to that used by Haller for polarizabilities. The results are in excellent agreement with the literature data. We proved that in the case of optical anisotropy the order parameter is equal within ±1% to the one evaluated from polarizability anisotropy. Thus, the discussion concerning the local field can be omitted. Using the described method we determined the orientational order parameter in various mesophases.

A hard-disc system, an experimental model

Abstract An experimental model of a hard-disc system (the two-dimensional analogue of a hard-sphere system) is described. The melting transition is studied qualitatively by a photographic technique. The half-width of the singlet distribution function is measured and compared with results obtained by the method of molecular dynamics. Images of the pair distribution function are presented.

The determination of dielectric anisotropy in ferroelectric smectic C

The method of determination of the three main components of the electric permittivity tensor, e, in chiral smectics C was proposed. To illustrate the method we measured the electric permittivity for the ferroelectric smectic DOBAMBC in the SmC and SmA phases and calculated the components of ∊-tensor as a function of temperature.

Phase Transitions in a Liquid Crystal with Long-Range Dipole Order

The synthesis and physical properties of a liquid-crystalline material exhibiting the ferrielectric C n *-phase in a broad temperature range are described. The contradictory literature data on this substance are critically reviewed. Dielectric, optical, electrooptic, calorimetric and miscibility studies allowed us to verify the existing data and to establish the complicated phase sequence of the studied substance. The substance possesses many phases specific for materials called antiferroelectric without having the classical antiferroelectric C a *-phase. The range of existence of the ferrielectric C n *-phase is very broad (about 20°C). This feature can be useful in structural investigations of the C n * phase.

Intermolecular interactions in benzene solutions of 4-heptyl-3′-cyano-biphenyl studied with non-linear dielectric effects

Abstract Static linear and nonlinear dielectric effects have been used for studies of molecular association of solutions of 4-heptyl-3′-cyano-biphenyl (7CB3) in benzene in the temperature range from 283 K to 353 K, within the full experimentally accessible concentration range. For 7CB3 none of the mesogenic properties was found. The energies of intermolecular interactions have been determined from the experimental linear and nonlinear polarization data. To achieve agreement between the experimental and theoretical data one has to consider simple dimerization of 7CB3 molecules, only. The determined value of dipole moment of dimer suggests that molecules are almost antiparallel oriented. The most unusuael and amazing result of these investigations comes from the comparison with previously obtained results of similar study for mesogenic isomer 4-heptyl-4′-cyano-biphenyl (7CB4). In this isomer not only the nematic phase was observed in the pure compound, but also in diluted solutions the self-association is considerably stronger and includes at least tetramers and characteristic equilibria between low polar and high polar multimers, which is not observed for 7CB3. The discussion and comparison of molecular properties of these compounds, and reasons of so different behavior will be the object of the separate work.

Non-linear electrooptic effect in antiferroelectric liquid crystal

Electrooptic phenomena caused by weak electric fields, much lower than those needed for the helix unwinding, in helical smectic liquid crystals were studied in thin planar samples. The investigations were performed in chiral liquid crystal 4-(1-methyl-heptyloxycarbonyl) phenyl 4′-(3-butanoyloxy propyl-1-oxy) biphenyl-4-carboxylate which exhibits antiferro-electric properties. We have found that electric field applied to a helical smectic liquid crystal caused two effects. First, the helix was deformed and the position of effective optic axis changed by an angle proportional to the field strength. The second effect, quadratic in field, causes the change in the shape of the indicatrix. As a consequence, the relative changes in the light intensity caused by external electric field consist of two components. The first component represents the modulation with the fundamental frequency and the second one with the doubled frequency (second harmonic of the electrooptic effect). The ab- solute values of the first- and second-order electrooptic coefficients have been determined and their temperature dependence discussed.

Study of self-association of 2-methyl-2-butanol based on non-linear dielectric effect

Abstract Non-linear electrical polarisation was applied to studies of self-association of solutions of 2-methyl-2-butanol in cyclohexane in the whole concentration range at temperatures from 287 to 318 K. The free energy, enthalpy and entropy of hydrogen bonds in chain multimers and low polar cyclic tetramers were determined. The angle β between dipole moments of two neighbouring molecules was found to be close to tetrahedral. The mean association number (χ) amounts to 2.5 for pure alcohol. The fraction of cyclic species rapidly increases in diluted solutions, reaching almost a constant value of 80% for concentrated solutions and pure alcohol.

Molecular equilibria investigations in 3-acetylpyridine using non-linear dielectric effect and dipole moment measurements

Abstract The dipole moment and non-linear dielectric effect for diluted solutions of 3-acetylpyridine were studied within the temperature range 288–323 K in six solvents of different polarity. Taking into account the trans-cis equilibrium and dimerization the enthalpy and entropy were calculated directly from the experimental data and under the assumption that the polarity of the environment is independent of temperature. Significant differences in the values of thermodynamical functions were found to be strongly dependent on the method used. The results obtained showed a higher stability of the polar cis conformer in more-polar environments. It was also shown that the relatively stable dimers of 3-acetylpyridine appearing in non-polar solvents when transferred to more-polar environments become unstable and their structure is no longer well defined.

Dielectric Relaxation in Liquid Crystal Phases with Polar Order

Dielectric properties of a chiral smectic liquid crystal exhibiting ferroelectric and antiferroelectric forms of dipole order have been studied in the frequency range 10 Hz ÷ 1 MHz. Three relaxation bands were found in the hexatic ferroelectric phase and in the antiferroelectric phase, and six bands in the apparently ferrielectric state. The number of relaxation processes observed in the studied frequency range reduces to one in the smectic Cα* and A phases. The results of measurement suggest that phases exhibiting ferrielectric properties might be in fact a mixture of ferroelectric and antiferroelectric clusters. This mixed state is stabilized by surface interactions.