On polymerized liquid crystals

Unfortunately, at present, the relevant literature not only lacks classification of experimental data on polymeric liquid crystals, but also criteria determining the existence of the liquid crystalline state in polymers are nowhere to be found. More often than not, certain authors refer to polymer systems under study or specific states of these systems as liquid-crystal ones without sufficient grounds. 

Symposium on Polymeric Liquid Crystals (2nd 1983 Washington, D.C.) Polymeric liquid crystals. 

Proceedings of the Second Symposium on Polymeric Liquid Crystals, Division of Polymer Chemistry, held as part of the American Chemical Society meeting, August 28-31, 1983, in Washington, D.C. 

Much of the current research on polymeric liquid crystals is directed towards thermotropic liquid crystals which are formed when certain polymer melts are cooled. Many of those polymers includes chemical groups similar to low-molecular-weight nematics either along the main chain or in the side chains. These materials are technologically interesting because of their ease of processing. Theoretically, these materials are expected to show curious behaviours which are a mixture of their... 

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. ... 

The synthesis, characterization and mesomorphic behavior of diacrylates based on polymerized liquid crystals are reported. Several types of polymer stabilized liquid crystal display devices were prepared from the dispersions of low concentration of diacrylates in liquid crystals and subsequently the prepolymers were polymerizaton by ultraviolet radation toorm polymer networks. The morphology stupes show that the orientation of polymer networks induced by the surface treatment of the substrate has led to the preferential liquid crystal alignment... 

V.S. Papkov, Yu.K. Godovskii, V.S. Svistunov, et al.. First All-Union Symposium on Polymeric Liquid Crystals [in Russian], Suzdal (1982). 

Note 1 The rotational viscosity coefficients are of the order of lO -lO" Pa s for low-molar-mass liquid crystals for polymeric liquid-crystals their values depend strongly on the molar mass of the polymer. 

Most polymeric liquid crystals are based on stiff rod-like molecular units which are called calamitic mesogens. There are some unusual polymers (which are not discussed here) that contain flat disk-like molecular units called discotic mesogens in which the disks form columnar arrays like stacks of poker chips. 

Blumstein, R. B. and Stickles, E. Influence of molecular weight on some properties of polymeric liquid crystals. Proc. 28th Macromol. Symp. IUPAC, U. Mass., Amherst, MA, p. 799, 1982... 

In a majority of works on LC polymers, the main attention was paid to the synthesis and structural studies of such polymers. Significantly less information is available on physical properties of LC polymers, especially, when compared to low-molecular liquid crystals. In this chapter some rheological and dielectric properties of polymeric liquid crystals, characteristics of their dynamic properties and intramolecular mobility, are considered. 

The macromolecular nature provides an interesting feature of LC polymeric cholesterics, namely the possibility of obtaining monochromic films. Thus for polymeric liquid crystals the helix pitch is practically not altered with temperature below Tg, when a cholesteric phase is frozen in a glassy matrix (Fig. 23a). This implies that fast cooling of polymeric films from a mesomorphic state (shown with arrows) fixes their optical properties, which makes it possible to use them at ordinary temperatures as selective monochromic reflectors. On the other hand, such polymeric films display the extraordinary polarizing properties of cholesterics, i.e. the different absorption... 

All electrooptical effects known to the present time for polymeric liquid crystals may be divided into two groups. First of all there are so called orientational effects, which are due solely to the effect of the electric field (field effect) on LC polymers, but are not a result of a current flowing. The second group of electrooptical effects is attributed to the phenomena ascribed to the anisotropy of electrical conductivity (Act) of liquid crystals. These are called electrohydrodynamic effects. 

Comparing the effect of an electric field on low-molecular and polymeric liquid crystals it is necessary to stress the following ... 

The discovered dependence of kinetic parameters of orientation processes on the degree of polymerization 44) is a consequence of the duplex nature of LC polymers — that is the presence of the main chain and of mesogenic side groups. This is why a correct juxtaposition of the kinetic characteristics of orientational processes of low-molecular and polymeric liquid crystals requires an explicit knowledge of the degree of polymerization of a corresponding polymer. 

One of the outlets of electrooptical phenomena in LC polymers is the construction of devices for the recording and optical duplication of information. This was first described in the works of Soviet scientists from Moscow State University8. Figure 27 illustrates the principle of information recording on oriented layer of a polymeric liquid crystal. 

The brief data presented in this chapter concerning the initial steps of structure formation in LC polymer solutions, are significant from two viewpoints. On the one hand, the study of these processes provides quantitative information about the molecular parameters and IMM of LC polymers, which is the basis for the understanding and prediction of physico-chemical behaviour of polymeric liquid crystals in bulk. On the other hand, understanding of the features of intramolecular structure formation in dilute solution, reveals broad prospects for the investigation of the formation of lyotropic LC systems of polymers with mesogenic side groups, which is in its infancy 195). 

The rapid growth of the number of publications concerning polymeric liquid crystals indicates that we should expect the appearance of new fundamental studies on the transition of rigid- and semirigid-chain polymers into this state. The range of moderately concentrated solutions for these polymers is studied sufficiently well, while the development of the methods of establishing the liquid. crystalline state in superconcentrated systems and in pure polymers with semirigid chains, as well as the analysis of kinetics of phase transitions, are the subject for further theoretical and experimental works. 

Another technique which uses microscopy is based on the miscibility of compounds with identical mesophases and was developed by the Halle liquid crystal group for model liquid crystals. Noel has applied this method to mixtures composed of well-known model liquid crystals with polymeric liquid crystals 3 - ). Assuming that the method is applicable to mixtures of polymers and low molecular weight compounds, the type of mesophase can be positively identified if the polymer and model are miscible. 

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 effect of polymer-solvent interactions on the mesophase can be derived from the rigidity of the polymer chain, the critical concentration to form liquid crystalline phase, and relaxation studies. After shearing a rigid polymeric liquid crystal, a banded texture is formed in which the direction of the bands... 

Side-chain type polymeric liquid crystal with side-on attachment Fig. 4.8 Basic types of liquid crystalline polymers... 

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