**2. Molecular shape of nematic mesogens**

Materials that exhibit liquid crystalline phases are called mesogens. The relationship between the microscopic shape of mesogens that form the nematic phase and the macroscopic symmetry of the phase affect their physical properties. Hence, when designing a molecular structure, the aim is to influence and particularly enhance the molecular polarization.

The uniaxial nematic phase is found in simplest low molar mass liquid crystals or mesophases represented by compounds made up of long cylindrical-shaped molecules (calamitic mesogens) (**Figure 1a**) or discs (discotic mesogens) (**Figure 1b**); the biaxial nematic phase is found to be prevalent in bent-core compounds (banana mesogens) (**Figure 1c**).

The general structure of nematic liquid crystal compounds is composed of aromatic cycles which are planar, rigid, and polarizable, connected by conjugated double bonds along the long axis of the molecule. The rigidity of calamitic (rodshaped) molecules provides the orientational order of the molecular axes, while their functionalization with one or two flexible hydrocarbon tails provides mobility by preventing these systems from crystallizing. Overall, an essential condition that must be met is the length of the molecule that should be greater than its diameter. The mesomorphism is generally enhanced by the presence of strong polar groups near the center of the molecule and by the weak polar groups at the extremities of the molecule.

**Figure 1.** *Typical structures in thermotropic nematic liquid crystals: (a) calamitic, (b) discotic, and (c) bent-core.*

If molecular units from nematic mesogens are chiral molecules or mixtures of achiral molecules with optically active compounds, the molecular orientation of a nematic phase is distinguished by a helical modulation, a characteristic to the cholesteric phase, closely related to the nematic phase. They are of great technical importance for appliances with displays.

The length of bent-core molecules and the bending angle of the central unit are most important for the formation of polar ordered mesophases. Hence, a minimum number of rings between four and five are required (for three rings the system is stabilized by intramolecular H-bonding). Symmetric bent-core units (BU) comprising 2,7-disubstituted naphthalene derivatives show mesophases with highest thermal stability, while less symmetric biphenyl derivatives exhibit wide mesomorphic domains. Generally, increasing the size of the rigid bent unit increases the transition temperatures, so in order to obtain polar phases, longer terminal chains are required. The nature, position, and direction of the linking groups between the rings influence as well the ordering in mesophases; the most commonly used are ester and a combination of ester and imine groups. The obtaining of polar mesophases depends as well on the bending angle, which can vary between 105 and 140°.

In contrast to classical nematics formed by calamitic mesogens, nematic mesophases formed by bent-core mesogens exhibit unique properties, due to high packing density in a uniform direction and a polar order in the layers [15, 16]. As a result, they exhibit mesophases with a permanent polarization in the absence of an electric field (spontaneous symmetry breaking of achiral molecules and super helical structures).

For most rigid bent-core mesogens, nematic phases are rarely observed because of high tendency to associate into mesophases with positional long-range order. By contrast, flexible bent-core molecules form nanostructured nematic phases, including the twist-bend nematic phase discovered very recently [17]. Accordingly, the bent core nematic phase contains uniaxial, biaxial, and polar nematics and phases with tetrahedratic symmetry [18].

## **3. Characterization of nematic mesophases**

The most common techniques for characterization of nematic mesophases are polarized optical microscopy (POM), differential scanning calorimetry (DSC), X-ray scattering at wide and small angles (WAXS, SAXS), solid-state NMR, IR, and Raman spectroscopy [19].

Nematic phase is best highlighted in polarized optical microscopy, through which its fluidity and Schlieren texture are revealed (**Figure 2**). The method consists in observing the texture defects for which it was concluded that the presence of only the so-called two-brush disclinations is specific to the biaxial nematic mesophase and the presence of four-brush disclinations is characteristic of the uniaxial nematic mesophase. However, texture alone is not sufficient to determine whether the phase is uniaxial or biaxial. The microscopic method is considered less eloquent due to the influence of the sample preparation on the obtained image, surface effects, and the interaction between the liquid crystal sample molecules with the walls of the microscopic plate. It is used as an early method in identifying biaxial mesophase, but it must be accompanied by other methods in which no disruptive external factors are involved (e.g., solid-state NMR).

Conoscopy with and without a circular polarizer represents a valuable method to confirming biaxiality. This consists in looking at the interference image of a sample in the focal plane. The sample must be well aligned with the director n (homeotropic alignment) and oriented parallel to the incident light. The conoscopic

**7**

related to biaxiality.

**Figure 2.**

*Introductory Chapter: Nematic Liquid Crystals DOI: http://dx.doi.org/10.5772/intechopen.92726*

image is characterized by the presence of so-called conoscopic isogyres that provide information about the axiality of the sample [8]. If the mesophase is uniaxial, the Maltese isogyre is observed, while if it is biaxial, a separation of the isogyres is

*(a) Nematic droplets, (b) Schlieren texture of a presumably uniaxial nematic (four-brush disclinations), and* 

Texture defects, singularities, or disclinations appear in the topology (arrangement of molecules) of a mesophase and are quantified by the factor S. The factor S is defined macroscopically, for the nematic mesophase, by the number of curves without birefringence (dark brushes) that meet in a point (S = no. of curves/4) and can have the values ±1 (for uniaxial nematic) or ±1/2 (biaxial nematic) (**Figure 2**). Differential scanning calorimetry signals, associated transition enthalpies and entropies, represent the first indications that a phase might be of special interest. Hence, the first-order nature of the transitions between crystalline, liquid crystalline, and disordered phase is evidenced. However, the second-order transitions of conventional uniaxial nematic phases Nu or polar biaxial Nb phases are difficult to detect only using the DSC technique, which gives only a clue of the existence of a

X-ray diffraction represents the most important method of identifying mesophases. The analysis of the position and intensity of X-ray reflections allows the complete description of the mesophase structure. For nematic phases, postulates is the proof of the presence of cybotactic clusters in the diffractogram of phases as

Additional analytical methods for characterizing mesophases include miscibility studies with known liquid crystals, electro-optical measurements, or light scattering to test for cluster formation in uniaxial nematics or ESR spectroscopy, dielectric

observed, obtaining a characteristic conoscopic image.

*(c) Schlieren texture of a presumably biaxial nematic phase (two-brush disclinations).*

mesophase; therefore a combination with other methods is required.

spectroscopy, atomic force microscopy (AFM), and rheology.

**Figure 2.**

*Liquid Crystals and Display Technology*

importance for appliances with displays.

helical structures).

with tetrahedratic symmetry [18].

Raman spectroscopy [19].

**3. Characterization of nematic mesophases**

external factors are involved (e.g., solid-state NMR).

If molecular units from nematic mesogens are chiral molecules or mixtures of achiral molecules with optically active compounds, the molecular orientation of a nematic phase is distinguished by a helical modulation, a characteristic to the cholesteric phase, closely related to the nematic phase. They are of great technical

The length of bent-core molecules and the bending angle of the central unit are most important for the formation of polar ordered mesophases. Hence, a minimum number of rings between four and five are required (for three rings the system is stabilized by intramolecular H-bonding). Symmetric bent-core units (BU) comprising 2,7-disubstituted naphthalene derivatives show mesophases with highest thermal stability, while less symmetric biphenyl derivatives exhibit wide mesomorphic domains. Generally, increasing the size of the rigid bent unit increases the transition temperatures, so in order to obtain polar phases, longer terminal chains are required. The nature, position, and direction of the linking groups between the rings influence as well the ordering in mesophases; the most commonly used are ester and a combination of ester and imine groups. The obtaining of polar mesophases depends as well on the bending angle, which can vary between 105 and 140°. In contrast to classical nematics formed by calamitic mesogens, nematic mesophases formed by bent-core mesogens exhibit unique properties, due to high packing density in a uniform direction and a polar order in the layers [15, 16]. As a result, they exhibit mesophases with a permanent polarization in the absence of an electric field (spontaneous symmetry breaking of achiral molecules and super

For most rigid bent-core mesogens, nematic phases are rarely observed because of high tendency to associate into mesophases with positional long-range order. By contrast, flexible bent-core molecules form nanostructured nematic phases, including the twist-bend nematic phase discovered very recently [17]. Accordingly, the bent core nematic phase contains uniaxial, biaxial, and polar nematics and phases

The most common techniques for characterization of nematic mesophases are polarized optical microscopy (POM), differential scanning calorimetry (DSC), X-ray scattering at wide and small angles (WAXS, SAXS), solid-state NMR, IR, and

Nematic phase is best highlighted in polarized optical microscopy, through which its fluidity and Schlieren texture are revealed (**Figure 2**). The method consists in observing the texture defects for which it was concluded that the presence of only the so-called two-brush disclinations is specific to the biaxial nematic mesophase and the presence of four-brush disclinations is characteristic of the uniaxial nematic mesophase. However, texture alone is not sufficient to determine whether the phase is uniaxial or biaxial. The microscopic method is considered less eloquent due to the influence of the sample preparation on the obtained image, surface effects, and the interaction between the liquid crystal sample molecules with the walls of the microscopic plate. It is used as an early method in identifying biaxial mesophase, but it must be accompanied by other methods in which no disruptive

Conoscopy with and without a circular polarizer represents a valuable method to confirming biaxiality. This consists in looking at the interference image of a sample in the focal plane. The sample must be well aligned with the director n (homeotropic alignment) and oriented parallel to the incident light. The conoscopic

**6**

*(a) Nematic droplets, (b) Schlieren texture of a presumably uniaxial nematic (four-brush disclinations), and (c) Schlieren texture of a presumably biaxial nematic phase (two-brush disclinations).*

image is characterized by the presence of so-called conoscopic isogyres that provide information about the axiality of the sample [8]. If the mesophase is uniaxial, the Maltese isogyre is observed, while if it is biaxial, a separation of the isogyres is observed, obtaining a characteristic conoscopic image.

Texture defects, singularities, or disclinations appear in the topology (arrangement of molecules) of a mesophase and are quantified by the factor S. The factor S is defined macroscopically, for the nematic mesophase, by the number of curves without birefringence (dark brushes) that meet in a point (S = no. of curves/4) and can have the values ±1 (for uniaxial nematic) or ±1/2 (biaxial nematic) (**Figure 2**).

Differential scanning calorimetry signals, associated transition enthalpies and entropies, represent the first indications that a phase might be of special interest. Hence, the first-order nature of the transitions between crystalline, liquid crystalline, and disordered phase is evidenced. However, the second-order transitions of conventional uniaxial nematic phases Nu or polar biaxial Nb phases are difficult to detect only using the DSC technique, which gives only a clue of the existence of a mesophase; therefore a combination with other methods is required.

X-ray diffraction represents the most important method of identifying mesophases. The analysis of the position and intensity of X-ray reflections allows the complete description of the mesophase structure. For nematic phases, postulates is the proof of the presence of cybotactic clusters in the diffractogram of phases as related to biaxiality.

Additional analytical methods for characterizing mesophases include miscibility studies with known liquid crystals, electro-optical measurements, or light scattering to test for cluster formation in uniaxial nematics or ESR spectroscopy, dielectric spectroscopy, atomic force microscopy (AFM), and rheology.

*Liquid Crystals and Display Technology*
