Liquid crystal is a state of matter combining properties of conventional liquids (isotropic liquids) and solid crystals. Liquid crystals flow like isotropic liquids but they exhibit long–range order of molecules, typical for crystalline solids. This group of materials is probably best known for their use in displays (LCD, LED). As liquid crystals research is largely driven by the display industry, many papers focus on properties connected with the performance of these devices, including threshold voltage (minimum amount of voltage that is necessary to produce any molecular movement) or direct current (DC) electric conductivity (its source are ions, which cause image sticking). In our work we chose a more fundamental approach, we show the temperature evolution of static and dynamic properties and describe them using theoretical models. We put special stress on a topic that is often neglected in literature: the influence of pretransitional fluctuations on tested properties.
In the paper, we compared the properties of a pure liquid crystal (8OCB) with its nanocolloids doped with nanoparticles (BaTiO3). Measurements were carried out in a wide range of temperatures and covers isotropic liquid, two different liquid crystalline, and solid phases. We investigated such parameters as dielectric constant (which describes the ability to polarize a material), relaxation time (time that takes a perturbed molecule to return to its initial state), or parameters describing the shape of the relaxational curve (e.g. its maximum).
It was shown that the addition of nanoparticles leads to the permanent orientation of molecules, approximately parallel to the external electric field. This caused even up to a 16% increase in dielectric constant value in comparison to the pure sample.
The fractional Debye-Stokes-Einstein equation was used to assess coupling/decoupling between orientational (rotational) and translational motions. In the isotropic phase in each sample, orientational processes were faster than translational. The small concentrations of nanoparticles (lower than 1 weight percent) made this effect even stronger. In the low-temperature liquid crystal phase (smectic A) addition of nanoparticles lead to a slowdown of orientational motions, which was not observed before. The decoupling is especially visible in near phase-transition temperature regions. It results from pretransitional fluctuations. They are also evidenced in temperature evolutions of relaxation peak maximum and dielectric constant. To describe the latter, a new equation, proposed by one of the authors (ADR) in her previous work, was applied.
New evidence for a premelting effect in the solid phase is also shown.