Liquid Crystals (LC)

Liquid Crystals (LC)

Liquid crystals (LC) are a special state of matter that exists between isotropic liquids and solid crystals. Within a specific temperature range, they exhibit the birefringence typical of anisotropic crystals while maintaining the fluidity, viscosity, and elasticity of liquids.

 

Structure of Liquid Crystals

 

Liquid crystal molecules are polymers, most of which are elongated and rod-like. The ring structures within these molecules are rigid, contributing to the linearity and optical anisotropy of liquid crystals. The side chains add flexibility, affecting viscosity and phase transition temperature. The terminal groups are usually polar, with their polarity impacting the overall polarity of the liquid crystal molecules.

 

Types of Liquid Crystals

 

Based on the arrangement and order of the molecules, liquid crystals are categorized into three main types:

 

1. Smectic Phase Liquid Crystals: Molecules are arranged in layers, with both the long and short axes of the molecules aligned within each layer. They are less commonly used in LCDs.

   

2. Nematic Phase Liquid Crystals: Molecules are rod-like and aligned along their long axes. This type is the most widely used in LCDs due to its lower viscosity and ease of movement.

 

3. Cholesteric Phase Liquid Crystals: Molecules are flat and layered, with the direction of the molecules in each layer staggered, creating a helical structure. This type is used to adjust the pitch in nematic liquid crystals or as compensation films in displays.

 

Characteristics of Liquid Crystals

 

1. Electrical Anisotropy

   Liquid crystals exhibit electrical anisotropy due to their rod-like structure. The dielectric constant varies along the long and short axes of the molecules. If the dielectric constant along the long axis (a) is greater than that along the short axis (b), the liquid crystal is called positive; if a < b, it is negative. Under an electric field, positive liquid crystal molecules align parallel to the electric field, while negative ones align perpendicular. The degree of rotation depends on the field strength and the intrinsic molecular interactions and viscosity.

 

2. Optical Anisotropy

   Liquid crystals also exhibit optical anisotropy. The refractive index differs along the long and short axes. In nematic liquid crystals, molecules act as uniaxial crystals with their optical axis along the long axis. Incident light undergoes birefringence, splitting into ordinary and extraordinary rays, which propagate at different speeds, creating a phase difference. This phase difference affects the polarization and intensity of the transmitted light, influencing the display’s transmittance, contrast, and viewing angle.

 

3. Mechanical Properties

   The mechanical properties of liquid crystals are represented by their elastic constants: splay, twist, and bend. The magnitude of these constants (bend > splay > twist) affects the driving voltage and response time of the display. Different display modes, which involve different molecular rotations, are influenced by these constants to varying degrees.

 

4. Resistivity

   The resistivity of liquid crystals typically ranges from 10^8 to 10^12 Ω·cm, approaching that of insulators. Higher resistivity indicates better stability. The conductivity, inversely related to resistivity, affects image retention. Higher conductivity indicates lower purity and more ionic impurities.

 

5. Viscosity

   Viscosity in liquid crystals is due to intermolecular forces, represented by fluid viscosity and rotational viscosity. Fluid viscosity affects flow, while rotational viscosity impacts response time and clearing point temperature. In displays, rotational viscosity is more commonly referenced, with higher values leading to longer response times.

 

6. Phase Transition Temperature

   The phase transition temperature includes the clearing point and melting point. The clearing point is the temperature at which liquid crystals transition to an isotropic liquid state, while the melting point is the transition to a solid crystalline state. The operational temperature range, from the smectic-nematic transition to the clearing point, is typically -20°C to 75°C.

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