Showing papers in "Materials World in 1993"

Liquid crystalline polymers

Abstract: Not much more than a decade ago, the plastics industry viewed itself as a mature branch of the heavy chemical industry. Its raison d'etre was the mass production of four or five main-line polymers, and profits were equated to tonnage output, plant efficiency, and clever downstream processing such as film blowing. The chemistry was essentially simple and the monomer, of course, cheap. There was, however, a spark of new thinking. A trend was developing toward the design and manufacture of more complex, more expensive polymers, with special properties which could command a special price. Such products would sell advanced scientific know-how, not just engineering expertise which could all too easily be exported to the major oil producers in the form of a polymer plant. Designing particular molecules to achieve desired properties is now a major theme of polymer producers. There is a move toward increasing the aromatic content of polymer backbones to achieve greater levels of chemical and thermal stability, while the development of new cross-linking systems remains as chemically intensive as ever. It is, however, the introduction of liquid crystalline polymers which, above all, has exploited the principles of molecular design, while at the same time challenging our understanding in a new area of polymer science. A polymer is “liquid crystalline” where the chains are sufficiently rigid to remain mutually aligned in the liquid phase although the perfect positional periodicity of a crystal is no longer present. In other words there is a long-range orientational order without long-range positional order (Figure 1). Structurally, therefore, the phase is intermediate between a crystal and a liquid leading to the use of the term mesophase . Where the liquid crystalline phase forms on melting the polymer, it is known as thermotropic , but where it is achieved by solvent addition it is called Inotropic. Increasing temperature, or solvent concentration, will eventually lead to the reversion of the liquid crystal phase to the normal isotropic polymer melt.