C6 Liquid Crystals

C.6.1 Describe the meaning of the term liquid crystals Solid particles have an orderly arrangement in which molecules are fixed in position and orientation Liquid particles are free to move around randomly and change their orientation. However, some show properties in between the liquid and solid states. Liquid crystals have some order and some ability to move around. For example, liquid crystals can orient the molecules like a solid, but still move to different positions.

C.6.2 Distinguish between thermotropic and lyotropic liquid crystals. Thermotropic liquid crystal materials occur as liquid crystals over a certain temperature range between the solid and liquid phases. (LCD TVs, alarm clocks, etc.) Biphenyl nitriles are commonly used as thermotropic liquid crystals.

C.6.2 Distinguish between thermotropic and lyotropic liquid crystals. Lyotropic liquid crystals show the liquid crystal state at certain concentrations. Examples are soap solutions. At higher concentrations the soap molecules group together to form micelles.

C.6.3 Describe the liquid-crystal state in terms of the arrangement of the molecules and explain thermotropic behaviour. As a solid, the rod shaped molecules (such as biphenyl nitriles) are fixed in position and arranged in layers. When these are heated, the order is disrupted to form nematic liquid crystals. The layer arrangement no longer exists. However, they still point in the same direction. With further heating, the directional order is lost as well as it moves into the liquid phase.

C.6.4 Outline the principles of the liquid-crystal display device. A polarized lens (polarizer) causes light to vibrate in only one direction. If a second polarizer was set up perpendicular to the first, then it would block the transmission of light.

C.6.4 Outline the principles of the liquid-crystal display device. A liquid crystal can rotate the plane of light so it can pass through the second polarizer. This is due to the ability of the liquid crystal molecules to orient in a certain direction.

C.6.5 Discuss the properties needed for a substance to be used in liquid-crystal displays. Chemically stable Liquid-crystal phase stable over a suitable range of temperatures Polar in order to change the orientation when a electric field is applied Rapid switching speed (Be able to orient quickly)