Small molecule and polymeric liquid crystals

II. ELECTRIC AND MAGNETIC FIELD EFFECTS ON SMALL-MOLECULE AND POLYMERIC LIQUID CRYSTALS... 

In this brief review, we strive to construct a coherent picture of our current theoretical understanding of the flow and rheology of small-molecule and polymeric nematic liquid crystals. Owing to space limitations, we have presented results selectively, based more on the need to tell a somewhat coherent story than the significance of the work. 

The study of liquid crystalline materials by X-ray diffraction has until recently been confined to low molecular weight compounds because of the lack of availability of suitable polymeric mesogens. Based on the observations by X-ray diffraction made with small molecules as reviewed by Azaroflf a system for identifying the type of mesophase has been developed by DeVries and this system is now being applied to polymeric liquid crystals. 

The side chain polymeric liquid crystal (as host) is doped with NLO active small molecules (as guest). In the matrix of side chain polymeric liquid crystals, the NLO active molecules are aligned along the director. The system is then poled and processed. Meredish et al. (1982) adopted a side chain polymeric liquid crystal as the host and DANS as the dopant, as shown in Figure 6.32. The pole field is 1.3KV/mm. The resultant second NLO coefficient reached 6 x 10-9 esu. The glass temperature of the polymers is low (Tg < 50 °C) thus the orientation is not good enough. 

The phase area 4 in Fig. 6.1 is less likely for liquid crystals than for condis crystals since the small increase in order that is needed to change a mobile melt into a mobile liquid crystal occurs fast and is usually completely reversible as seen in the reversible transition shown in TMDSC of Fig. 5.144 for a small molecule and the upper curves of Fig. 5.154 for a macromolecule. Once in phase area 2, the polymeric liquid crystals will, however, not crystallize completely on cooling, but rather become a semicrystalline material. In the present scheme, they will produce metastable phase areas 9, followed by 10. 

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. ... 

Fig. 4. Because these three elastic constants are usually of similar magnitude for small-molecule nematics, one often refers to a single elastic constant, K, for the material. For polymeric materials, on the other hand, the three elastic constants can be very different and are indeed found to be very different experimentally [7]. For instance, in order to have a splay distortion, there must be a net excess of tails over heads of molecules, defined by the molecular orientation along the splay direction. In a polymeric system in which the molecular length is large, the density of chain ends is small, so splay becomes more and more energetically expensive with increasing molecular length. This and many other issues associated with polymeric liquid crystals are reviewed by Meyer [6].

Finally, a few words about other liquid-crystal theories the mean-field theory of Maier and Saupe (1959, 1960) has been very successful in describing the behaviour of small-molecule liquid crystals, but it has been much less used for polymeric liquid crystals. Other important theories primarily applied to small-molecule liquid crystals are the Landau theory and its extension, the Landau--de Gennes theory. A detailed presentation of these theories, also including the Maier and Saupe theory, is found in Vertogen and de Jeu (1988). 

As briefly mentioned earlier, thermal studies have been used in conjunction with characterization by polarized light microscopy to determine the miscibility of polymeric and small molecule liquid crystals and low molecular weight mesogens, of the same or different types of liquid crystallinity, can also be used as plasticizers or diluents for polymers, as demonstrated in a study involving side chain liquid crystalline polymers... 

The rheological and flow properties of ordered block copolymers are extraordinarily complex these materials are well-deserving of the apellation complex fluids. Like the liquid-crystalline polymers described in Chapter 11, block copolymers combine the complexities of small-molecule liquid crystals with those of polymeric liquids. Hence, at low frequencies or shear rates, the rheology and flow-alignment characteristics of block copolymers are in some respects similar to those of small-molecule liquid crystals, while at high shear rates or frequencies, polymeric modes of behavior are more important. 

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