Liquid crystal polymers molecular structures

Brostow, W. 1990. Properties of polymer liquid crystals-choosing molecular structures and blending. Polymer 31 979. 

This chapter is concerned with the definition and evaluation of molecular anisotropy in liquid crystal polymers. Molecular orientation is a central theme of the liquid crystal state however, its definition requires considerable care. The basic mathematical relationships for the description of molecular anisotropy are reviewed together with an assessment of the various orienting units ranging from the molecular envelope to particular covalent bonds. The different methods of measuring orientation parameters for liquid crystal polymer samples are compared. Particular emphasis is placed upon X-ray and electron scattering procedures which are able to provide structural as well as orientational information. The experimental methods are highlighted through the use of several examples. 

Liquid crystal polymers (LCP) are a recent arrival on the plastics materials scene. They have outstanding dimensional stability, high strength, stiffness, toughness and chemical resistance all combined with ease of processing. LCPs are based on thermoplastic aromatic polyesters and they have a highly ordered structure even in the molten state. When these materials are subjected to stress the molecular chains slide over one another but the ordered structure is retained. It is the retention of the highly crystalline structure which imparts the exceptional properties to LCPs. 

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

Liquid polymers (at ambient temperature) are in general macromolecules with a relatively low molecular weight, many of them being in fact oligomers. Some liquid polymers are utilized as synthetic oils. Certain polymers can form liquid crystals in other words they can have an ordered structure while being in liquid state (either melted or in a solution). The orientation of certain polymeric molecules in liquid state such that the properties of the material are anisotropic is possible. Polymer liquid crystals have practical applications, and solution of liquid crystal polymers can be used for extruding fibers that have a highly crystalline structure after solvent elimination. 

We are all familiar with the three states of matter gases, liquids and solids. In the 19th century the liquid crystal state was discovered [1 and 2] this can be considered as the fourth state of matter [3]. The essential features and properties of liquid crystal phases and their relation to molecular structure are discussed here. Liquid crystals are encoimtered in liquid crystal displays (LCDs) in digital watches and other electronic equipment. Such applications are also considered later in this section. Surfactants and lipids form various types of liquid crystal phase but this is discussed in section C2.3. This section focuses on low-molecular-weight liquid crystals, polymer liquid crystals being discussed in the previous section. 

Shibaev, V.P. Freidzon, Ya.S. Kostromin, S.G. Molecular Architecture and Structure of Thermotropic Liquid Crystal Polymers with Mesogenic Side Groups Shibaev, V.P. Freidzon, Ya.S. Kostromin, S.G., Ed. Springer-Verlag New York, 1994, pp 77-120. 

Fig. 13. Molecular structures of the liquid crystal systems studied Liquid crystal side chain polynKrs (1,2), liquid crystal main chain polymers (3,4) and low molecular wdght analogues (5-8). The Greek letters, a, P, 8, ip and ip, refer to five different derivatives of the same liquid crystal, deuteriated at different sites of the mesogen or alkyl chain, as indicated in the formula of liquid crystal polymer 4...

A new class of liquid crystal/polymer network composite with very small amounts of polymer network (3 Wt%) is described. These composites are formed by photopolymerization of the monomers in-situ from a solution of monomer dissolved in low-molar-mass liquid crystals. Several techniques have proven useful to characterize these polymer networks. This review describes polymer network structure and its influence on electro-optic behavior of liquid crystals. Structural formation in these composites begins with the phase separation of polymer micronetworks, which aggregate initially by reaction-limited, and then by diffusion-limited modes. The morphology can be manipulated advantageously by controlling the crossover condition between such modes, the order of the monomer solution prior to photopolymerization, and the molecular structure of monomers or comonomers. 

It was also shown that it is possible to synthesize polymethacrylate liquid crystal polymers with mesomorphic properties that contain ferrocenes with two flexible chains at the 1,1 -positions [59]. Based on dilatometric measurements, a head-to-tail molecular arrangement of the monomeric units occurs within the smectic A phase. Because of special electrochemical properties of ferrocene, these materials are of interest for developing electroactive mesomorphic polymers. The structure of the polymer can be shown as follows [59] ... 

Shibaev, V.P., Freidzon, S., Kostromin, S.G. Molecular architecture and structure of thermotropic liquid crystal polymers with mesogenic side chain groups. In Shibaev, V.P., Lam, L. (eds.) Liquid Crystalline and Mesomorphic Polymers, pp. 77-120. Springer-Verlag, New York (1993)... 

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