Liquid Crystalline Polymers
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Figure 1. Molecular arrangement in (a) a liquid crystalline melt compared with that of (b) a conventional molten polymer.
director. The quality of orientation can be defined by the function: S = Vi(3 - 1)
where 9 is the angle between the chain axis of a molecule and the director, and the angular brackets denote an average over all chains. Perfect alignment, where all values of 0 are zero, corresponds to S = 1; for random orientation S = O.S is usually referred to as the order parameter by those working with small molecule liquid crystals, although the function is also well known in the polymer field where it is more usually called P2, and the higher even-order harmonic coefficients P4, P6 etc. are sometimes used to give a fuller description of the actual orientation distribution. Preferred orientation in a liquid crystalline melt or solution can be measured by techniques such as x-ray diffraction, NMR, and IR spectroscopy. However, it is most readily apparent as optical birefringence, which means that the quiescent polymer liquid appears bright between crossed polars. It is important to realize, however, that the director orientation may not remain constant throughout the material. In some cases, individual domains form which are separated by boundaries to give a structural analogue of grains in a polycrystalline metal. An example of such a structure in a liquid crystalline polymer is shown in Figure 2. Alternatively, there are structures in which the director changes gradually and continuously with position. Between crossed polars they appear as in Figure 3, where the dark bands correspond to regions where the director is parallel to either the polarizer or the analyzer. The points from which the bands radiate are singularities called disclinations, which are
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Figure 2. Microstructure of a liquid crystal polymer showing domains of uniform orientation separated by well-defined boundaries. Viewed in circularly polarized light. (Photo courtesy of T.J. Lemmon.)
Figure 3. Schieren texture in a liquid crystalline polymer seen between crossed polars. (Photo courtesy of T.J. Lemmon.) a natural consequence of a wandering director in three dimensions. The markings of a human fingerprint form a good two-
dimensional analogy. The combination of optical anisotropy and its variation with position gives the polymer melt an opalescent appearance, which is one characteristic of the liquid crystalline state.
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Applications of Liquid Crystalline Polymers Liquid crystalline polymers represent an important extension to the science and (a) (b) technology of polymers on the one hand and liquid crystals on the other. Polymers processed in the liquid crystalline phase show the development of enhanced anisotropy, and thus unique properties in the solid state. The inherent stiffness of the molecules necessary for liquid crystallinity gives other advantages, such as good solvent resistance. By way of contrast, the importance of polymers to the established liquid crystal device indu
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