Mesophases polymeric

The study of blends of polymeric liquid crystals with low-molecular liquid crystals of known mesophase types, aiming at identification of polymeric mesophases, is at its very beginning there are only a few works concerning polymers with mesogenic groups in the main chain 67 69) and in the side chains as well 70 74). in view of the importance of such investigations, note that the principle of miscibility is thoroughly developed for low-molecular liquid crystals, whose molecules are similar in their sizes the justifiability of its application to the blends of polymers with low-molecular liquid crystals is not equally evident, as the molecular sizes of the components differ substantially. 

Terpyridine, N N N ligands (245-249) and their N C N and N N C analogues (250-257) have been successfully coordinated to Pt(II), leading to neutral, mono- or doubly charged species, which in some cases display bright luminescence, both in degassed fluid solutions and frozen matrices. In particular, it has been shown by Che et al. that they can form supramolecular structures, such as nanowires, nanosheets, and polymeric mesophases, with interesting optical properties (214—216,258). 

As a result of their low Tg values and lack of crystallinity, many of these polymers showed liquid crystallinity at room temperature. The liquid crystalline mesophases of the monomers were identified as nematic but the polymeric mesophases were not identified, although they possessed a very broad thermal stability between their Tg and their clearing transitions. A mesophase temperature stability of up to 170 °C was observed for the polymers with bicyclohexane central mesogenic units. These polymers showed decreases in Tg and Tj with increased spacer lengths. 

The highly viscous nature of polymeric mesophases could possibly prevent the mixing of the mesophases of polymers and certain model compounds. Therefore, while the miscibility of model compounds with a polymer may be used to identify the type of mesophase, the lack of compatibility does not necessarily suggest that the mesophases are not the same. That is, some model compounds and polymers with the same mesophase may even be inherently incompatible. As a result, some judgment is required in order to make both the proper choice of the model compound and the correct assignment of the mesophase when using this technique. 

Although the technical applications of low molar mass liquid crystals (LC) and liquid crystalline polymers (LCP) are relatively recent developments, liquid crystalline behavior has been known since 1888 when Reinitzer (1) observed that cholesteryl benzoate melted to form a turbid melt that eventually cleared at a higher temperature. The term liquid crystal was coined by Lehmann (2) to describe these materials. The first reference to a polymeric mesophase was in 1937 when Bawden and Pirie (2) observed that above a critical concentration, a solution of tobacco mosaic virus formed two phases, one of which was bireffingent. A liquid crystalline phase for a solution of a synthetic polymer, poly(7-benzyl-L-glutamate), was reported by Elliot and Ambrose (4) in 1950. 

The order parameter of the polymeric mesophase plays an important role. A decrease in the order parameter resulted in an increase in the photoresponsiveness. 

An equally important observation for the above copolyester LCPs is that the ordered arrangement of polymeric mesophases in the melt is retained upon cooling, which is manifested in greatly improved mechanical properties (see Figure 5.5b). The liquid crystalline behavior is therefore advantageous from the standpoint of both processing and properties. Thermotropic liquid crystal copolyesters of structures similar to (I) are now available commercially. 

From practical considerations, two properties are of prime interest The effect of liquid crystalline behavior on viscosity and the ability of the polymer to retain the ordered arrangement in the solid state. Liquid crystalline behavior during the melt results in lower viscosity, because the rigid polymeric mesophases align themselves in the direction of the flow. As a result, the polymer is easier to process. Also, retention of the arrangement upon cooling yields a material with greatly improved mechanical properties. Several thermotropic liquid crystalline copolyesters are now available commercially. 

At the First Symposium in 1977, the literature in this field could be encompassed in a single volume. Today, that is no longer possible. The field of Polymeric Liquid Crystals grew, and continues to grow, at a very rapid pace. At present, we know of every major mesophase in its polymeric form and of polymeric glasses, elastomers and fluids in their liquid crystalline form. Every year, new polymeric mesophases are being discovered. 

The other structural feature of polymeric mesophases is their ability to form a glassy state with a liquid crystalline order frozen in. This phenomenon may be a basis for a set of new applications of polymeric liquid crystals, especially for ferroelectric ones. 

On the other hand, an electric field causes some specific transformer tions in the structure of polymeric mesophases. As was pointed out by Shibayev [229], the field considerably changes the correlation length for the... 

Identification of nematic polymeric mesophases is a more complex problem than identification of polymer smectics. The structural data are usually limited to finding the absence of small-angle reflections in the x-rays of unoriented samples. The low enthalpy of the transition from the anisotropic to the isotropic phase (Table 6.9), close to the corresponding values characteristic of low-molecular nematic liquid crystals, and the absence of layered reflections indicate the one-dimensional type of ordering, although these data are insufficient for a complete description of the structure of nematic polymers, which can be both similar to and (Afferent from low-molecular-weight nematics. 

At present there is little data on identification of polymeric mesophases by microscopy. 

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