Liquid Crystals Chapter 1

Chapter 1: Introduction to Liquid Crystals

• Liquid crystals (LCs) demonstrate, display, or show characteristics both mobility and order at molecular and supramolecular levels.
• LCs have intermediate order compared to solid and thus exhibit distinct properties that distinguish them from both traditional solids and simple liquids. The unique characteristics of liquid crystals make them valuable in various applications, particularly in the field of display technology, such as in LCD screens. In liquid crystals, the molecules maintain a certain degree of freedom to move, akin to that found in liquid states. However, unlike regular liquids, the molecular arrangements are not completely random; there's a specific organization that allows them to align in a particular direction. This order is crucial in determining the optical properties of liquid crystals, such as birefringence, which is the double refraction of light observed in certain materials. crystals and disordered liquids.
• Molecules in crystalline state possess a unique set of characteristics that distinguish them from both solid crystals and ordinary liquids. These characteristics include specific ways in which the molecules are arranged and behave, contributing to the exceptional properties of liquid crystals. Unlike in solid crystals where both positional and orientational orders are rigidly fixed, liquid crystals exhibit a flexible arrangement that enables them to transition between different phases while maintaining some level of organization. This distinctive behavior allows liquid crystal molecules to align in response to external fields, thereby influencing their optical behavior and functionality in various applications such as displays and sensors. orientational and three-dimensional positional order.
• Liquid crystals have orientational order (tendency of molecules to point along a common direction) and in some cases positional order in one or two-dimensional liquid crystals can exhibit a unique property where they possess a level of order not only in the orientation of their molecules but also in how these molecules are arranged spatially across two dimensions. This enhances their capability to respond to various stimuli, such as electric fields or temperature changes, thereby causing shifts in their physical state and, consequently, their optical characteristics. dimensions.
• In the liquid state, molecules, which are groups of atoms bonded together, play a crucial role in determining the physical and chemical properties of substances. In the context of liquid crystals, the unique arrangement and orientation of molecules give rise to their distinctive behavior. Unlike molecules in solids, which are tightly packed and vibrate around fixed positions, or those in simple liquids that move freely with no long-range order, liquid crystal molecules exhibit a level of organization that allows for both flexibility and order. This specific molecular alignment is what enables liquid crystals to react sensitively to external stimuli such as electric fields or temperature changes, thereby making them essential for applications in advanced technologies like screens, sensors, and optical devices. move randomly and rotate freely.
• LCs combine The properties of liquid crystals are fascinating and multifaceted, primarily arising from their unique molecular arrangement. One of the key characteristics is their anisotropy, which means that their physical properties vary depending on the direction of measurement. This anisotropic behavior is crucial for their application in technologies such as displays, where the alignment of liquid crystal molecules can affect light transmission. Additionally, liquid crystals exhibit a range of phases, including nematic, smectic, and cholesteric, each with distinct ordering and properties, which can be influenced by temperature, electric fields, and magnetic fields. These phases enable liquid crystals to respond dynamically to external stimuli, making them tunable materials suited for various electronic and optical applications. Their ability to switch states quickly and retain order under certain conditions is what allows for high-resolution display technologies and other applications in modern electronics. of crystalline (optical and electrical anisotropy) and liquid (molecular mobility and fluidity) states.

Classification of Liquid Crystals

• LCs can be obtained by heating mesogens or by dissolving amphiphilic systems in a solvent.
• Thermotropic: LC phases formed by temperature variation.
  • Enantiotropic: phases appear on heating and cooling.
  • Monotropic: phases appear only on cooling.
• Lyotropic: LC phases formed by dissolving the compound in a solvent.
• Amphotropic: LC phases formed by both heat and solvent.

Lyotropic Liquid Crystals

• Typically consist of a flexible lipophilic tail and a polar head group.
• The tail is usually an alkyl chain with 6 to 20 methylene groups.
• The head can be ionic, zwitterionic, or non-ionic.
• Formation is driven by the separation of incompatible hydrophilic and hydrophobic parts.
• Micelles form at a critical concentration, with polar heads at the interface and hydrophobic tails inside.
• Depending on concentration, micelles can form different shapes (disc-like, cylindrical, plate-like aggregates) leading to different lyotropic mesophases.

Thermotropic Liquid Crystals

• Transition from solid to liquid involves a cascade of transitions through LC phases.
• Often, organic or metal-organic compounds.
• Usually consist of rigid and flexible parts (aromatic cores and paraffinic chains).
• Mobility explained by large-amplitude molecular or molecular part motions.
• Orientational order arises from parallel alignment of anisometric molecules.
• Positional order is a consequence of attractive forces and amphiphilicity.

Conventional Liquid Crystals

• Rod-like and disc-shaped mesogens.
• Popularly known as calamitics and discotics respectively.
• Exhibit thermotropic mesomorphism.
• Previously thought molecules needed to be rod-like shape, but disc-like molecules (e.g. benzene-1, 2, 3, 4, 5, 6-hexayl hexaoctanoate) can also display mesomorphism.

Non-Conventional Liquid Crystals

• Characterised by a structural contrast within a molecule (incompatible parts).
• Examples include oligomers, polycatenars, bent-core molecules, polyhydroxy amphiphiles, octahedral complexes, and star-shaped molecules

Mesophase Morphologies of Thermotropic Liquid Crystals

• Three main types of calamitic mesophases: nematic, cholesteric, and smectic, classified by order of positional and orientational order.
• Nematic: the simplest LC phase, molecules are on average oriented, but not ordered in space, exhibiting a constant director (n)
• Cholesteric: a chiral variant of nematic, exhibiting a helical arrangement of molecules, exhibiting selective reflection of light.
• Smectic: layers of molecules ordered along the layer normal as well as within a layer, with a variety of types (A, B, C, etc.) based on the degree of layer alignment.
• Chiral smectic phases are a result of chiral molecules.

Sequence rule and Reentrant behavior

• Thermotropic LCs can exhibit a series of LC phases between crystal and isotropic liquid states.
• Systematic observation led to sequence rules for phase changes, often featuring a characteristic order of smectic, then nematic phase prior to melting.
• Reentrant transitions refer to phases which undergo more than one phase transition upon changing temperature or pressure, ultimately returning to an initial state.

Discotic Liquid Crystals

• Discotic LCs comprise of disc-shaped molecules, with the core often aromatic.
• Common mesophases include Columnar mesophases, which consist of stacks ordered along the core axis.
• Columnar LCs have varied morphologies, which depend on molecular arrangement.

Applications of Liquid Crystals

• Displays (e.g., LCDs, TN, STN, LC devices for wide viewing angles)
• Temperature sensors (cholesterics)
• Gas sensors (columnar phases)
• OLEDs (columnar phases)
• Controlled drug release
• Chromatography