LC polymers

FIGURE 3.28 Schematic visualization of possible mesomorphic transitions as temperature increases in an LC system showing the organization of mesogens (represented as cylinders) in the smectic and nematic states. 

Most LC polymers in the market are linear, highly aromatic thermotropic polyesters. They combine chemical stability with chain rigidity and retain dimensional stability (remain as glasses) up to 200°C or 300°C and are also very chemically stable (resistant to oxidation). These types of PLCs, such as the commercial Vectra and Xydar, are usually processed in the melt state with conventional fabrication techniques such as extrusion and molding (see Chapter 14). A major advantage of such polymers is that they can be melt processed and form extranely precise molded structures that do not shrink on cooling. They have been used in molded parts for microelectronics and in mounting brackets for optical communications. 

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Cardanol could also be converted to an azophenyl group containing LC polymer. This was possible by per-... 

The steric frustrations have also been detected in LC polymers [66-68]. For example, the smectic A phase with a local two-dimensional lattice was found by Endres et al. [67] for combined main chain/side chain polymers containing no terminal dipoles, but with repeating units of laterally branched mesogens. A frustrated bilayer smectic phase was observed by Watanabe et al. [68] in main-chain polymers with two odd numbered spacers sufficiently differing in their length (Fig. 7). 

A novel second-order nonlinear optical medium which should offer considerable fabrication flexibility has been described. The physics of alignment of the highly nonlinearly polarizable moiety was discussed. However, observation of complex dynamical and thermal behavior indicates that an important role is played by the polymer liquid crystalline host. Additional properties of modified members of this family of lc polymers were consequently investigated. The explanations of guest alignment stabilization and thermal dependence of the alignability remain unresolved issues. 

Liquid Crystal Displays (LCD). Liquid crystal displays, once limited to small devices such as calculators, are now displacing color CRT (cathode ray tube) displays in commercial quantities. The ability to fabricate these display devices at high quality and at low cost is partially due to the wider spread use of photopolymer-based materials. Photopolymer technology is being used for the alignment of liquid crystal (LC) elements (49), the orientation of ferroelectric materials (50), the synthesis of LC polymers (57) and the manufacture of color filters for liquid crystal display applications (52). 

FIGURE 5.7 Schematic Representation of typical, (partially) electroluminescent LC polymer architectures. (a) Rodlike structure, (b) Hairy-rod structure, (c) Combined main-chain-side-chain system, (d) Semiflexible segmented structure, (e) Semiflexible segmented structure with disklike mesogen. (After Weder, C. and Smith, P., Main-chain liquid-crystalline polymers for optical and electronic devices, in Encyclopedia of Materials Science and Technology, Buschow, K.H., Cahn, R.W., Flemings, M.C., Ilschner, B., Kramer, E.J., and Mahajan, S., Eds., Elsevier Science, New York, 2001.)... 

Most of the LC polymer systems discussed so far contain the LC moiety incorporated in the polymer main chain. One of the few examples of polarized emission of light using a side-chain LC system was reported by Chang et al. [107]. A polyacrylate was used as the polymer backbone. The nematic LC side-chains consisted of ethyl- and propyl-substituted bis-tolan units. When deposited on a rubbed PEDOT film, polarized EL dichroic ratios were reported to be around 6. 

There are several very important advantages we obtain from this novel class of LC polymers. They can be prepared with well defined chain ends and narrow molecular weight distribution. Therefore, we could provide important information concerning the influence of the nature of the polymer chain ends on their mesomorphic... 

The mesophases formed by liquid crystalline (LC) polymers are well known to impart strength, toughness, and thermal stability to plastics and fibers (1-5). While LC polymers have been widely studied, their potential utility as coatings binders seems to have been overlooked. Among the very few reports that may describe LC polymers in coatings are patents claiming that -hydroxybenzoic acid (PHBA), a monomer commonly used in LC polymers, enhances the properties of polyester powder coatings (6-9). 

Studies of crosslinked networks of LC polymers have been proposed ( ), and prepared without disruption of the mesophase (11-14). Crosslinked elastomers were shown to retain the mesophase up to certain crosslink densities (15,16). 

The thermal behavior of la-lg observed by DSC (Fig. 1) confirms the presence of mesophases and is typical of low molecular weight thermotropic LC materials (M). The lower T , for lb and Id are consistent with the higher entropy of activation for crystallization of odd-n spacers, demonstrated in several main chain LC polymers (23). The apparent absence of nematic-smectic transitions in the DSC... 

Liquid-crystal (LC) polymers are the subject of worldwide research and development. (1-5). Commercial films and plastics such as Kevlar (du Pont), Xydar (Dartco) and Ekonol (Sumitomo) utilize the remarkable ability of LC behavior to enhance physical properties. 

In the field of coatings, however, crystal formation in the film is usually avoided few reports ( ) of LC polymers in coatings binders are known to the authors. 

We have been seeking a method for synthesizing oligomeric LC polyols that could be adapted to economical production. Reactions of p-hydroxybenzoic acid (PHBA) were investigated. PHBA is commonly used as a monomer in LC polymers (3-A), and its use in binders for powder coatings has been described. (8-11). 

Poly(l,4-oxybenzoyl) (LII) [IUPAC poly(oxy-l,4-phenylenecarbonyl), obtained by selfreaction of p-hydroxybenzoic acid, and the various aramids (Sec. 2-8f) were among the first LC polymers studied. The experience in commercializing poly(l,4-oxybenzoyl) and the... 

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