Incorporation of flexible spacers

Some of the kinked units discussed in the previous section, such as —0— and — S—, are simple examples of flexible spacers that space the meso-genic units into separate structural sequences of smaller length-to-diameter ratios. Other segments often used as flexible spacers include oligomeric polymethylenes, polyoxyethylenes, polysiloxanes, and so forth. The following series of molecules, (3.5) with n = 1-8, make up one of the oldest examples of the flexible spacer concept in the molecular design of liquid crystals (Vorlander, 1927)  

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Incorporation of flexible spacer units, e.g., —(CH2)s-, —(CH2-CH2-0-, —S—R—S—, and —(SiR2-04r in chain to separate the rigid backbone groups (mesogens), which are responsible for the mesophases (Figure 5.7a). 

An other important concept for the modification of LCPs is the incorporation of flexible spacers between mesogenic units. These semiflexible LCPs have been extensively investigated and have to be distinguished from the above systems. As a consequence, a separate contribution in this handbook is devoted to this class of LCPs (Sect. 2 of this Chapter). 

Incorporation of flexible spacers, to decrease rigidity. This permits the development of bends or elbows in the chain. 

The wholly aromatic processable PAs is achieved by modifying diamines, diacids, or both structures. The modification of the monomers can be broadly categorized in the four pathways to get the processable aromatic PA (i) incorporation of flexible spacers (ii) incorporation of bulky substructures as side substituent, (iii) incorporation of non-linear rigid alicyclic structures and cardo moieties and (iv) incorporation of fluorine as trifluoromethyl or a mixture of thereof. A short discussion on the monomers for the processable PAs has been described below. 

Incorporation of flexible siloxane spacers into side chain or main chain liquid crystalline polymers have been shown to drastically reduce the transition temperatures 255,267,271,272,277) anc[ aiso increase the response time of the resultant systems to the applied thermal, optical or electrical fields 350-353>. In addition, siloxanes also provided elastomeric properties and improved the processibility (solution or melt) of the resulting liquid crystalline copolymers. 

In this example, we have attempted to reveal the tme nature of nematic conformation characteristic of flexible spacers incorporated in the LC state. The nematic conformation predominates in the individual -[Ms-X-(CH2) -X] c- units constituting a given polymeric sequence. In an independent work [26], PVT studies oti the mainchain LCs carrying OE-t5q>e spacers have been carried out It is interesting that the expansivity of the nematic LC phase was found to be larger than that of the isotropic melt. According to the conventional thermodynamic theories of polymeric fluids, the expansivity is closely related to the free volume of the liquid state [46 9]. 

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

When 7.48 reacts with copper(i), which usually forms four-co-ordinate tetrahedral complexes, a double-helical species, 7.49, is indeed formed (Fig. 7-30). This is a genuine self-assembly process - simply treating the ligand with the appropriate metal ion leads to the desired structure. A wide variety of other spacer groups have been incorporated between didentate domains. In practice, some consideration needs to be given to the nature of the spacer group that is selected. If it is too long, or too flexible, other co-ordination possibilities can occur. 

Figure 2 Some molecular architectures of liquid crystalline polymers incorporating rodlike mesogens and flexible spacers. ...

In the last few years, it has become fully appreciated that polymeric antioxidants are effective in retarding the thermal and autooxidation. Such polymer-bound stabilizers are similar in efficiency to the low molecular weight stabilizer incorporated into the polymer by blending. The polymer-bound stabilizer should have a flexible spacer between the point of attachment to the polymer and the functional group of the phenolic antioxidants. 

By far the largest group of liquid crystalline polymers containing imide groups are copoly(esterimide)s (PEIs). These have been sub-divided into flexible copolymers containing aliphatic spacers and wholly aromatic copolymers. There are several reports of copoly(amide-imide)s and copoly(ether-imide)s (both thermotropic and lyotropic) and these are treated separately. There are only a few examples of wholly aromatic imide containing LCPs, as polyimides frequently melt well above their decomposition temperature. Successful attempts have been made to incorporate flexible spacers, but in most instances the spacers are based on ether units, and the polymer is more strictly classified as a poly(etherimide). 

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