Introduction to Physical Polymer Science || Polymers in the Liquid Crystalline State

✍ Scribed by Sperling, L.H.

• Publisher: Wiley
• Year: 2005
• Tongue: English
• Weight: 299 KB
• Edition: 4
• Category: Article
• ISBN: 0471757128
• DOI: 10.1002/0471757128.ch7

✦ Synopsis

POLYMERS IN THE LIQUID

CRYSTALLINE STATE 325

7.1 DEFINITION OF A LIQUID CRYSTAL

Liquid crystals, LCs, are substances that exhibit long-range order in one or two dimensions, but not all three. Both small molecules and polymers may exist in the liquid crystalline state, but generally special spatial structures are required. Thus, while amorphous substances are more or less entirely disordered (see Chapter 5) and crystalline materials are ordered in all three dimensions (see Chapter 6), the LCs lie in-between in properties. Liquid crystals are ordered in one or two dimensions only. Liquid crystals all exhibit some degree of fluidity. LC polymers are a relatively new discovery dating from about 1950 (1), with the field growing extremely rapidly. Engineering advances utilizing polymers that go through an LC stage include new classes of high-modulus fibers, high-temperature plastics, and a host of new electronic and data storage materials.

The formation of liquid crystals is a direct consequence of molecular asymmetry. It arises because two molecules cannot occupy the same space at the same time and is largely entropically derived.

A simple example of a "liquid crystal" may be considered using a sink filled with water. Onto the water surface, toothpicks are slowly added without overlap. At first, the toothpicks are random in order. However, as the surface becomes more concentrated, the toothpicks begin to align into small groups more or less side by side. Now there are two phases present: the dilute, disordered phase, and the LC phase. The reason why the toothpicks order them-