High resolution dielectric anisotropy investigation of carbon nanotube - smectic A liquid crystal dispersion

Abstract

It is well-known that there are three common phases in condensed matter which are crystal, liquid, and gas. Crystalline solids are anisotropic, which means that many physical properties are different in direction. Besides, liquids are isotropic, which means they present the same properties in all directions. Liquid crystals (LCs) have physical features like an ordinary liquid, though their molecules are ordered, leading to anisotropy. Many liquid crytsalline materials which have one or more different mesophases exhibit properties between crystalline solid and isotropic liquid. Mesophases, which liquid crystals have, are mostly characterized by their properties of orientation. One of the mesophases is entitled nematic (N) phase, which has a long-range orientational order. Molecules which have orientational order tend to align along a director, which is the measure of the average direction of the molecules. The other phase is called has smectic phase (Sm), which has partial positional order in addition to the orientational order. Thereby, molecules are arranged in layers in smectic phases which are coded alphabetical according to the order of the discovery. Smectic A (SmA) phases have a wide range usage in a smectic phases. In the smectic A phase, molecules have a layered structure which is normal to the layers. It is worthwhile noting that knowledge of the phase transition is crucial to understanding the properties of the liquid crystals. According to de Gennes, a physical system which has a physical property can be a measure of the order. In nematic phase, molecules have a property of anisotropy which can be measured with a dielectric anisotropy. Therefore, dielectric measurements, which are an example of physical quantities, can be used for testing liquid crystalline behavior. Phase transition is a change of a phase from one to another by external effects like temperature or pressure. The point in phase transition where two phases cannot be distinguished from each other is defined as a critical point. Temperature refers to the critical point is called critical temperature Tc. If there is a latent heat and discontinuous change in entropy near the critical point, it is called a first order phase transition. Otherwise, it is called second order phase transition. Worthwhile recalling that critical behaviour of the liquid crystalline materials can be determined by the order parameter S(T) which is produced from dielectric anisotropy data. In this work, high resolution dielectric data have been obtained for 8CB (octylcyanobiphenyl) liquid crystals in addition to 8CB nanocomposites doped with both pristine multi-walled carbon nanotubes (p-MWCNT) and -carboxyl group (-COOH) functionalized MWCNTs (f-MWCNTs). Nematic order parameter for both the nematic-isotropic (N-I) and nematic smectic A (N-SmA) of the neat 8CB and 8CB+MWCNT has been derived from Maier-Meier theory, which can be used to examine the effect of dielectric anisotropy in the nematic phase. In 8CB doped with both p-MWCNT and f-MWCNT, the N-I and N-SmA transition temperatures shifted to a lower value compared to pure 8CB. N-I transitions for all 8CB+MWCNT nanocomposites manifests weakly first order, on the other hand, N-SmA transition remains continuous. Sufficiently far away from the SmA phase, the critical exponent β which determines the tricritical behavior of order parameter S(T) are obtained 0.238 ± 0.002 on average, which is in excellent agreement with the obtained from optical birefringence data. The obtained β value is compatible with the hypothesis that the N-I phase transition exhibits tricritical behavior. All investigated compounds present the temperature dependence of N-I and the nematic-crystalline phase transitions of the order parameter is quasi-tricritical. Previous studies show that, the critical behaviour at N-SmA transition is an important research area. For the first time, the upper limits for a latent heat ∆HNA for the neat 8CB and MWCNT doped 8CB have been derived from the detailed dielectric anisotropy data in the N-SmA transition. Besides, the ∆HNA values produced in this way appear to be in agreement when compared with the values obtained from optical birefringence data and ASC measurements. By using the power law analysis of the ∆ε(T) data, which is the temperature gradient of the nematic order parameter through the N-SmA transition, the effective specific heat capacity exponents α have been yielded for all samples. For the first time, high resolution dielectric measurements ∆ε enable to invetigate the N-SmA transition behaviour.