Liquid crystal polymer

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The term liquid crystal polymer ( LCP ) or liquid crystal polymer (engl. Liquid Crystal Polymer , LCP ) denotes polymers which in the melt ( thermotropic ) or dissolved ( lyotropic liquid crystalline) properties, and thus a certain degree of order to show. For a polymer to have liquid crystalline properties, mesogens must be present in the polymer. These can be in the main chain as well as in side chains.

The arrangement of the mesogens in backbone LCP leads to a rod-shaped molecular shape. Such molecules are not very flexible. This results in extraordinary mechanical and chemical properties. Parallel to the molecular axis, LCPs have extremely high tensile strength and a high modulus of elasticity , which makes main chain LCPs ideal for use as high-performance fibers (e.g. protective clothing, sports equipment, space technology). In addition, the strongly anisotropic geometry ensures a strong intermolecular cohesion, which means that the melting points (if any) are correspondingly high and the solubility is generally poor. Precision components (e.g. scales, medical devices) can therefore be formed which, in addition to the mechanical properties mentioned above, also retain their shape in the presence of water or organic solvents.

Side chain LCPs combine the properties of liquid crystals and those of polymers with flexible main chains. This arrangement leads to a fixation of the liquid-crystalline properties of the mesogens, which with the polymer main chain via z. B. ester bonds are linked. This prevents any flow of conventional liquid crystals, e.g. B. in liquid crystal screens can be prevented.

Liquid crystalline polymers also occur in nature. So DNA, just like poly amino acids , such as. B. spider silk , lyotropic phases in water in low concentration. In addition, some polysaccharides can also be referred to as LCP.

However, synthetic LCPs such as aramid (developed in 1965 by Stephanie Kwolek under the brand name Kevlar ) and Vectran , which are used in ropes and for ballistic protection, are known above all .

These are backbone LCPs. From a chemical point of view, the first two are aromatic polyamides ; Vectra and Vectran are aromatic polyesters . Vectran is available as a fiber material, while Vectra is the starting polymer for these fibers.

If liquid-crystalline main chain polymers or side-chain polymers are linked with one another, liquid-crystalline elastomers are obtained .

Liquid-crystalline starting polymers, which can then be further processed by injection molding or extrusion, include:

literature

  • M. Ballauff: Liquid-crystalline polymers. In: Chemistry in Our Time. Volume 22, 1988, No. 2, p. 63, ISSN  0009-2851 .
  • AM Donald , AH Windle: Liquid Crystalline Polymer. Cambridge University Press, 1992, ISBN 0-521-30666-3 .