Polymers displaying thermotropic

This paper is primarily concerned with polymers displaying thermotropic mesomorphism. The moieties which generate the liquid crystalline organization are called the mesogenic moieties and can be found either in the main chain or in the side groups. The structural characteristics associated with liquid crystalline behavior have been reviewed by Gray (6, 7) and will not be dis-... 

It was first reported in the early 1970s that these melt processible polymers could best be described as thermotropic systems which usually display an nematic texture in the melt phase [5]. Subsequently, a number of additional phases have been reported ranging from discotic structures to highly ordered smectic E G systems with three dimensional order. In the last several years an IUPAC sponsored study on nomenclature on thermotropic LPCs has been underway. A more complete set of definitions will be available shortly as a result of Recommendation No. 199 IUPAC [6]. 

Gao et al. have recently prepared a number of thermotropic liquid crystalline main-chain fer-rocenes exhibiting fluorescent properties.182 The reaction of 1,1 -bisf functionalized)ferrocenes, 95, with 1,4-dibromobutane, 97a, or a,a -dibromo-p-xylene 97b generated polyelectrolytes 97a,b with Mn ranging from 5400-14,700 (Scheme 2.29). These polymers were highly thermally stable with the first weight loss occurring between 172 and 330°C. The liquid crystalline properties showed these materials formed smectic liquid-crystal phases and displayed batonnet textures. 

The numerous cellulose derivatives commercially available, displaying varying second order transition, molecular mass, viscosity, lyophilicity, thermotropic properties, have considerable potential. Significant advances have been made in the collection of fundamental data, but the interpretation of this information is somewhat more difficult than for synthetic polymers, mostly because of the heterogeneity of the materials, which exhibit a broad molecular mass distribution and an uneven substitution. 

In this section, we consider the case of solutions of rigid or semi-flexible polymers which display one or several liquid crystalline phases in a given range of concentration. The main control parameter is not flie temperature as is the case for thermotropic LCPs but rather the concentration of polymer in the solvent. There are many different kinds of lyotropic LCPs. Some are synthetic like Kevlar which has become a very important structural material with mechanical properties comparable to those of steel. Some are natural like the Tobacco Mosaic Virus and like DNA which shows a nematic and a hexagonal phase. Some are mineral like the vanadium pentoxide ribbons. In the next section, we shall first describe the lyotropic system which is probably best known, namely the tobacco mosaic virus. 

Figure 24. Biomesogenic structures a) (Bio)meso-gens displaying order-disorder distributions in CPK-presentation (left to right and top to bottom) hexa-n-alkanoyl-oxybenzene discoid - Chandrasekar s first non-rodlike liquid crystal [28 a, 51c] enantiomeric cholesteric estradiol- and estrone-derivatives [ 17 a, c, d, 26 f, 51 a, s, u] Reinitzer s cholesterolbenzoate [21, 22] - together with the acetate the foundation stones of liquid crystal history [21, 22] Kelker s MBBA -first liquid crystal fluid at ambient temperature [ 13 f, g] Gray s cyanobiphenyl nematics for electrooptic displays [25 a, 51 e] lyotropic lecithin membrane component [7 a, 14, 27 d, 52 a] and valinomycin-K -membrane carrier [7 a, 35] thermotropic cholesteryl-side-chain-modifiedpolysiloxanes with the combination of flexible main-chain and side-chain spacers [51 a, h] thermotropic azoxybenzene polymers with flexible main-chain spacers [51a] thermotropic cya-...

It has been shown that thermotropic N -LC materials demonstrate supramo-lecular helical ordering that leads to CPL with high dissymmetry factors [49-51], On the other hand, it would be appealing to investigate lyotropic N -LC systems as alternative circularly polarized optical materials for use in optoelectronic devices and displays. The effects of the solvent, solution concentration, and chiral dopant employed in the lyotropic N -LC system would be of particular interest, especially in relation to the helical structure of the polymer LC phase and its chiroptical properties. It was reported that di-substituted polyacetylene (di-PA) adopting a poly(diphenylacetylene) (PDPA) structure with alkyl side chains exhibits lyotropic LC behavior [18, 19]. The PDPA structure, with phenyl moieties... 

Like their low-molecular-weight analogs, LCPs display characteristic colored textures in polarized light. Thus, the opalescence of lyotropic polyaramides in solution can be viewed with the naked eye. For thermotropic polymers the birefringence of the anisotropic melt is often easily detectable in a polarizing microscope. 

The free volume of thermoplastic miscible blends has also been determined as a function of blend composition (Zhou et al. 2003 Campbell et al. 1997 Roland and Ngai 1991). Those studies have shown that the degree of blend miscibility alters the free volume behavior as a function of blend composition. On the other hand, Hsieh et al. (2000) have studied a number of blends containing only thermotropic liquid crystalline polymers TLCP s as the only components. That work showed that regardless of their various miscibilities, TLCP blends tend to display smaller, fewer free volume sites than expected from a weighted average. This observation has been ascribed to the intrinsic affinity of nematic TLCP s. 

There has been considerable interest in recent years in the study of liquid crystalline order in polymeric materials. Following on from the use of small molecules in display applications, the possibility of creating polymers with similar characteristics became attractive. Onsager and Flory predicted that rigid rod-like macromolecules should form liquid crystalline phase. However, it was not until 1975 that the first observation of a thermotropic liquid crystalline polymer was reported. Several reviews have been published on polymeric liquid crystals. ... 

Similar to the retention of LLC phases, thermotropic LC assemblies have also been covalently captured, leading to robust polymer films that retain the original nanostructures. Mdller et al. cross-linked columnar assemblies formed by amphiphile 7 bearing crown ether units inside the pores of track-etched membranes and obtained membranes containing oriented channels. Salt-diffusion experiments demonstrated that the resultant nanoporous membrane displayed... 

We are all familiar with gases, liquids and crystals. However, in the nineteenth century a new state of matter was discovered called the liquid crystal state. It can be considered as the fourth state of matter (although plasmas are also candidates for this accolade). The essential features and properties of liquid crystal phases and their relation to molecular structure are discussed in this chapter. Specifically, the focus is on thermotropic liquid crystals (defined in the next section). These are exploited in liquid crystal displays (LCDs) in digital watches and other electronic equipment. Such applications are outlined later in this chapter. Surfactants and lipids form various types of liquid crystal phase but this was discussed separately in Chapter 4. Finally, this chapter focuses on low molecular weight liquid crystals, liquid crystalline polymers being touched upon in Section 2.10. 

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