【Basic Knowledge of Glossary Explanations for “Polymers and Materials”-024】Glossary explanations related to polymers
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This post was last edited by ljtjbx on 2015-8-14 08:06. 1. Viscous flow: In linear polymers with no chemical bonds between molecules, relative sliding of the molecules occurs, a phenomenon known as viscous flow. 2. Secondary relaxation: Below Tg (for amorphous and low-crystalline polymers) or Tm (for highly crystalline polymers), the process in which small-scale motion units change from being in motion to being frozen, or from frozen to in motion, is called secondary relaxation. 3. Boltzmann superposition principle: The mechanical relaxation behavior of polymers is characterized by the linear addition of various relaxation processes that have occurred over time. 4. Thermotropic liquid crystals: Substances that enter a liquid crystal state when the temperature is increased within a certain range. 5. Solvated liquid crystals: They are dispersed by a solvent and become in a liquid crystal state within a certain concentration range.
What are the substances of these two types of LCDs, 4 and 5?
I checked some information and would like to share it: Based on the conditions under which liquid crystals are formed, they can be divided into two categories: thermotropic liquid crystals and lyotropic liquid crystals. Thermotropic liquid crystals refer to liquid crystals formed from a single compound or a homogeneous mixture of a few compounds. Substances that typically exhibit a liquid crystal phase only within a certain temperature range. The molecular weight of typical rod-shaped thermotropic liquid crystals is generally around 200–500 g/mol, with a molecular length ratio of approximately 4 to 8. Thermotropic crystals are divided into three types based on the arrangement of their rod-shaped molecules: nematic liquid crystals, smectic liquid crystals, and cholesteric liquid crystals. (1) Nematic liquid crystals: Their molecules are rod-shaped, with molecules in certain areas tending to align in the same direction. The short-range interactions between molecules are relatively weak, allowing them to arrange and move freely; this arrangement results in low viscosity and high fluidity for the molecules. The main characteristic of nematic liquid crystals is that they possess the optical properties of uniaxial crystals and are highly sensitive to external influences; they are the primary material used in liquid crystal display devices. (2) Smectic liquid crystals: The molecules in smectic liquid crystals are also rod-shaped, and they are arranged in layers; the long axes of the molecules in each layer are aligned in the same direction, but these long axes form a certain angle with the plane of the layer. The thickness of each layer is approximately equal to the length of the molecules, and the distance between layers can vary. Due to the strong bonding forces between molecules within a molecular layer and weak bonding forces between layers, this type of liquid crystal is fluid, but its viscosity is higher than that of nematic liquid crystals. Smectic liquid crystals exhibit positive birefringence; therefore, smectic liquid crystal display devices have superior properties compared to nematic liquid crystal display devices. (3) Cholesteric liquid crystals: Their molecules are arranged in flat layers, with the long axes of the molecules parallel to the plane of the layer. Within each layer, the long axes of the molecules are parallel to one another; there is a slight twist angle between the long axes of molecules in adjacent layers. The direction vectors of the molecules in each layer rotate continuously and uniformly along the normal direction of the layer, resulting in a helical structure for the entire liquid crystal. The distance between two layers that are twisted by 360° forms what is known as the pitch, denoted by L. Typically, L is on the order of 10² nm. This special spiral structure endows the crystal with distinct optical properties such as significant optical rotation, circularly polarized light dichroism, and selective light scattering. Therefore, cholesteric liquid crystals are often used as additives to control the arrangement of liquid crystal molecules or directly as color-changing liquid crystal films.
Can you specify which substances?
We don’t come into contact with liquid crystal polymers very often, but there are indeed many items around us that use them. Liquid crystal polymers are used in fields such as electronics, electrical engineering, optical fibers, automotive industry, and aerospace due to their excellent properties including high strength, high rigidity, heat resistance, and electrical insulation. Fibers made from liquid crystals can be used to create fishing nets, bulletproof vests, sports equipment, brake pads, optical fibers and display materials, etc. They can also be turned into films for use in flexible printed circuits, food packaging, and more. Thermotropic liquid crystal polymers can also be used to create polymer blends with various plastics; in these blends, the liquid crystal polymers act as fiber reinforcements, which helps to **improve the strength, stiffness, and heat resistance of the material.