Thermotropic polymeric liquid crystals

Polymeric liquid crystals can be classified into either of two types, thermotropic and lyotropic, according to their formation conditions, or three types, main-chain, side-chain and complex liquid crystalline polymer (MCLCP, SCLCP and CLCP, 

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The contemporary period is characterized by a rapid accumulation of information about thermotropic polymeric liquid crystals. It is yet too diversified and, as a rule, gives only a qualitative description of the observed phenomena. The next step in LC polymer investigations should be aimed at a quantitative description of their behaviour. This approach should necessarily lead to progress in the comprehension of the quite peculiar mesomorphic state of matter as well as in the introduction of LC polymers into technological practice. 

Thermotropic Polymeric Liquid Crystals (PLCs) which are formed by regularly alternating mesogenic elements and flexible spacer groups in the main chain are currently the focus of intensive investigation The standard meso-gens, such as biphenyl, stilbene, azo or azoxybenzene derivatives, which form the core of low molecular mass liquid crystals (LMLCs), are used in synthesis of PLCs as well. 

The last volume is devoted to polymeric liquid crystals (with chapters about main-chain and side-group thermotropic liquid crystal polymers), amphiphilic liquid crystals, and natural polymers with liquid-crystalline properties. 

Boehm, R. E., Martire, D. E., and Madhusudana, N. V., A statistical thermodynamic theory of thermotropic linear main chain polymeric liquid crystals, Macwmolecules, 19, 2329-2341 (1986). 

C. Noel and J. Virlet, "DSC, Miscibility and X-ray Studies of the Thermotropic Liquid Crystalline Polyesters with Aromatic Moieties and Flexible Spacers in the Main Chain", in "Liquid Crystals and Ordered Fluid, A. Griffin and A. Johnson eds., Plenvim Press, New-York, vol. 1+, 1+01 (198I+) Finkelmann, "Synthesis, Structure, and Properties of Liquid Crystalline Side Chain Polymers", in "Polymeric Liquid Crystals", A. Ciferri, W.R. Krigbaum and R.B. Meyer eds. Academic Press, New-York (I982)... 

The papers presented in this symposium give some indication of the wide variety of polymers which are now known to form liquid crystalline phases Polymeric liquid crystals are usually classified according to the mesophase structure e g., nematic, cholesteric, smectic A, etc ). However, these classes are quite broad For example, the cholesteric lyotropic phases formed by synthetic polypeptides in suitable solvents differ markedly from the cholesteric thermotropic phases formed from silicone polymers with cho-lesteryl ester side chains. In particular, the driving forces behind the formation of the mesophases are quite different for these two examples, being essentially due to chain stiffness in the first case and to anisotropic dispersion force interactions in the second case It may therefore be useful to classify polymeric liquid crystals according to the polymer chain structure ... 

The molecular approach which we will see eventually proved to be most successful in treating negative is based on the work of Doi [23]. Doi noted that the well established phenomenological theories for thermotropes (which he termed TLP for Ericksen, Leslie and Parodi [68]) which is successful in describing many dynamic phenomena in MLC nematics, is limited for polymeric liquid crystals in that it does not predict nonlinear viscoelasticity. Doi s approach determines the phenomenological coefficients from molecular parameters, so that the effects of, for example, molecular weight and concentration can be treated. He considers a single molecule (the test rod ) and notes that as concentration increases, constraints on its motion are imposed by collisions with other rods. This constraint can be modeled as a tube... 

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... 

Non-polymeric, liquid crystals are divided into thermotropic and lyotropic liquid crystals. Compounds which have liquid crystalline behavior in solution are called lyotropic liquid crystals. The amount of solvent is then the most important variable. Mainly thermotropic liquid crystals will be discussed here. 

Price, G.J. Shillcock, I.M. Inverse gas chromatography study of the thermodynamic behaviour of thermotropic low molar mass and polymeric liquid crystals. Phys. Chem. Chem. Phys. 2002, 4, 5307-5316. 

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 backbone types of polymeric liquid crystals may be lyotropic, requiring a solvent for entrance into the liquid crystalline phase, or thermotropic, requiring heat for entrance into the hquid crystalline phase. However, when dry and at service temperature, both types ordinarily exhibit three-dimensional crystalline order. 

This section pertains to reports on oriented molecules in which phases other than the usual thermotropic nematics have been used. Studies in chiral, smectic, columnar, lyotropic and polymeric liquid crystals as well as other unusual phases have been presented. The use of carbon-proton heteronuclear selective refocusing 2D NMR experiment designed for the spectral analysis of enantiomers dissolved in weakly ordering chiral liquid crystal solvents has been proposed." The method permits the extraction of carbon-proton residual dipolar couplings for each enantiomer from a complex or unresolved proton-coupled... 

Joseph, E. G., Wilkes, G. L., and Baird, D. G., Preliminary thermal and structural studies of blends based on a thermotropic liquid crystalline copolyester and poly(ethylene terephtalate), Polymeric Liquid Crystals (A. Blumstein, ed.. Plenum Press, New York, 1985, p. 197. 

Thermotropic liquid crystals may be classified as either low molar mass (i.e. non-polymeric), or high molar mass (i.e. polymeric) and within each of these broad classifications, there are several sub-classifications. 

A review of the literature demonstrates some trends concerning the effect of the polymer backbone on the thermotropic behavior of side-chain liquid crystalline polymers. In comparison to low molar mass liquid crystals, the thermal stability of the mesophase increases upon polymerization (3,5,18). However, due to increasing viscosity as the degree of polymerization increases, structural rearrangements are slowed down. Perhaps this is why the isotropization temperature increases up to a critical value as the degree of polymerization increases (18). 

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