Introduction
1.12 Liquid crystals characterization techniques
Page | 26 Figure 1.30: Schematic representation of (a) homeotropic and (b) planar alignment
configurations of discotic columnar phases.
Page | 27 phase. The LC phase's symmetry-dependent elasticity causes the textures in combination with defects and surface conditions. However, the birefringent is absent in homeotropically aligned samples, leaving only a dark field. Crystals are also optically anisotropic and birefringence under POM, but due to the fluid nature of the mesophase, they can be distinguished from LCs.
When liquid crystals with low viscosity melt from the crystalline phase, they begin to flow.
When they cool from the isotropic phase, they form textures, which change the texture of the mesophase when pressed. On the other hand, crystals do not change in texture or they form cracks with defined edges when the glass slide is pressed. Schlieren and thread-like textures are frequently associated with nematic liquid crystals, whereas fan-shaped structures, mosaic, and dendritic textures are common in columnar mesophase. Straight linear defects are common in the textures of the ordered columnar mesophase.
Differential scanning calorimetry (DSC).
Differential scanning calorimetry (DSC) is a thermoanalytical technique that measures the difference in heat required to raise the temperature of a sample and a reference as a function of temperature. The sample and reference are kept at nearly the same temperature throughout the experiment. The basic idea behind this technique is that when the sample undergoes a physical transformation, such as phase transitions, more or less heat must flow to it than to the reference in order to keep both at the same temperature. The amount of heat that must be transferred to the sample depends on whether the process is exothermic or endothermic. For example, as a solid sample melts to a liquid, more heat must be applied to the sample in order to raise its temperature at the same rate as the reference. This is due to the sample absorbing heat during the endothermic phase transition from solid to liquid. Similarly, as the sample goes through exothermic processes (such as crystallization), less heat is required to raise the sample temperature. Differential scanning calorimeters can measure the amount of heat absorbed or released during such transitions by observing the difference in heat flow between the sample and the reference. DSC can also detect more minor physical changes, such as glass transitions.
Because of its utility in evaluating sample purity and studying polymer curing is widely used in industrial settings as a quality control instrument. DSC is a technique used to investigate liquid crystals. Some types of matter pass through a third state as they transition from solid to liquid, displaying properties from both phases. This anisotropic liquid is classified as crystalline liquid or mesomorphous liquid. DSC can detect the small energy changes that occur
Page | 28 when matter transitions from a solid to a liquid crystal and from a liquid crystal to an isotropic liquid. The typical DSC thermogram is plotted as heat flow (mW) versus temperature (°C), with the peaks corresponding to phase transition temperatures and the area under the curve corresponding to the associated enthalpy change. The transition from crystal to LC phase usually results in a significant enthalpy change (20-100 kJ/mol) as it moves from a highly ordered crystalline state to a less ordered (or short-range ordered) LC phase. The LC phase to the isotropic phase transition, on the other hand, involves much lower enthalpy values (below 10 kJ/mol). The DSC thermogram reveals the number of mesophase transitions and the structural ordering of the LC phases. Although DSC does not provide any concrete evidence of the liquid crystallinity of the material, it acts as a complementary technique to POM.
Powder X-ray diffraction
X-ray diffraction is a powerful tool for the structural characterization of most materials. It is possible to determine an exact and usually unique structure for atomic or molecular arrays with three-dimensional periodicity. In contrast, only a time-averaged or statistical distribution can be established for liquids, where the constituent atoms are in constant motion. This technique can be used to study the supramolecular organization of liquid crystalline phases into various lattices and corresponding packing parameters in each phase. The XRD technique works on the principle of Bragg’s law, and the equation is nλ = 2d sinθ. For practical reasons, the Bragg’s law will be used to convert all diffraction angles (2θ) into interplanar spacing distances. Where λ is the X-ray wavelength, d is the interplanar spacing generating the diffraction, θ is the diffraction angle, h, k, l are the Miller indices of the associated reflection, and n is an integer.
Hereafter, all diffractions will be considered as their distance equivalents instead of their diffraction angles. The mesophase of the XRD pattern clearly shows two different angle regions small-angle region ( from 2θ = 0 to 12-15°) and a wide-angle region (from 2θ > 12- 15°). The latter is distinguished by a broad halo produced by the slow motion of the flexible molten alkyl chains. The columnar phase in the wide-angle region exhibits the two peaks:
molten alkyl chain and π- π core stacking (core-core distance) that differentiates the mesophase from “true” crystals. The core-core separation is usually in the order of 3-4 Å. In the small- angle region, sharp reflection peaks are observable relative to the intercolumnar dhkldistances.
The small-angle area usually shows a few sharp peaks that are indexed to a specific mesophase structure, such as Colh, Colr, and Colob. The XRD profile of a mesophase provides structural
Page | 29 symmetry information and data on (i) the ordering of the peripheral hydrocarbon chains and the central core, (ii) the core-core correlation length along the columnar axis, and (iii) the intercolumnar distance and lattice parameters.
Solid-state NMR
The solid-state NMR helps in the investigation of the molecular dynamics in the liquid crystalline mesophase. This technique can investigate the rotation of the core in relation to its neighbors or the peripheral mobility of alkyl chains. The various electronic environments of the aromatic protons in the intracolumnar packing and the tilted arrangement of the discs in the solid phase can be determined. All of these complementary experimental methods must be used in order to obtain a clear, comprehensive, and unambiguous picture of the self-assembling behavior of discotic mesogens.
A mesogen transition from crystalline to liquid crystalline phase is accompanied by an increase in molecular dynamics, such as axial and lateral displacement of discs and rotation of discs around the columnar axis, among other things. The centers of gravity of the mesogens in the columnar mesophases are located along the column axis, and the peripheral chains induce fluidity and allow the column to slide concerning each other, resulting in self-healing behavior.