Liquid crystal history

Figure 24. Biomesogenic structures a) (Bio)meso-gens displaying order-disorder distributions in CPK-presentation (left to right and top to bottom) hexa-n-alkanoyl-oxybenzene discoid - Chandrasekar s first non-rodlike liquid crystal [28 a, 51c] enantiomeric cholesteric estradiol- and estrone-derivatives [ 17 a, c, d, 26 f, 51 a, s, u] Reinitzer s cholesterolbenzoate [21, 22] - together with the acetate the foundation stones of liquid crystal history [21, 22] Kelker s MBBA -first liquid crystal fluid at ambient temperature [ 13 f, g] Gray s cyanobiphenyl nematics for electrooptic displays [25 a, 51 e] lyotropic lecithin membrane component [7 a, 14, 27 d, 52 a] and valinomycin-K -membrane carrier [7 a, 35] thermotropic cholesteryl-side-chain-modifiedpolysiloxanes with the combination of flexible main-chain and side-chain spacers [51 a, h] thermotropic azoxybenzene polymers with flexible main-chain spacers [51a] thermotropic cya-...

One is inclined to think of materials as being solids when editing an encyclopedia of materials some years ago, I found it required an effort of imagination to include articles on various aspects of water, and on inks. Yet one of the most important families of materials in the general area of consumer electronics are liquid crystals, used in inexpensive displays, for instance in digital watches and calculators. They have a fascinating history as well as deep physics. 

It is perhaps not too fanciful to compare the stormy history of liquid crystals to that of colour centres in ionic crystals resolute empiricism followed by fierce strife between rival theoretical schools, until at last a systematic theoretical approach led to understanding and then to widespread practical application. In neither of these domains would it be true to say that the empirical approach sufficed to generate practical uses such uses in fact had to await the advent of good theory. 

The history of liquid crystals started with the pioneer works of Reinitzer and Lehmann (the latter constructed a heating stage for his microscope) at the end of the nineteenth century. Reinitzer was studying cholesteryl benzoate and found that this compound has two different melting points and undergoes some unexpected color changes when it passes from one phase to another [1]. In fact, he was observing a chiral nematic liquid crystal. 

Reinitzer, F. Beitrage zur Kenntnis des Cholesterins , Monatsh. 9, 421, 1888. The term liquid crystals was first used by O. Lehmann in Fluessige Kristalle . Engelmann, Leipzig, 1904. See also H. Kelker, History of liquid crystals. Mol. Cryst. Liq, Cryst. 21, 1 (1973). For a history of the discovery and recognition of plastic crystals see J. Timmermanns, Plastic crystals, a historical review. J. Phys. Chem. Solids 18,1 (1961)... 

As can be seen in H, Kelkers l) excellent review on the history of liquid crystals, investigations on liquid crystalline polymers already exist before F. Reinitzer in 1888 gave the very first description of a low molar mass liquid crystal (1-l.c.). While, however, 1-l.c. s have become an extensive field of research and application during the past decades, these activities on l.c. polymers have come rather late. The research on l.c. polymers during the last years is mainly joined with activities in material science and tries to realize polymers with exceptional properties. These exceptional properties are expected because of the combination of the physical anisotropic behavior of l.c. and the specific properties of macromolecular material. 

The induction of chirality in liquid crystals (LCs) has a long history [100-104]. The supramolecular induction can be used to assign absolute configurations [105-108], conformations of molecules [109,110] and the interplay between inter- and intra-molecular interactions [111], and models can be developed to justify the sense of the inductions that are observed. Twisting powers of dopants—the twist per mole—can be pushed to extraordinary values [112]. Given the history and vast body of work, we will focus here on the more contemporary aspects of work in this area. 

Cogswell (1985) expressed it in the following words "To make the connection from the basic material properties to the performance in the final product, industrial technologists had to learn a new science". It is more or less so, that - for liquid crystal polymers -properties like stress history, optical and mechanical anisotropy, and texture seem to be independent variables this in contradistinction to the situation with conventional polymers. 

The big difference between normal isotropic liquids and nematic liquids is the effect of anisotropy on the viscous and elastic properties of the material. Liquid crystals of low molecular weight can be Newtonian anisotropic fluids, whereas liquid crystalline polymers can be rate and strain dependent anisotropic non-Newtonian fluids. The anisotropy gives rise to 5 viscosities and 3 elastic constants. In addition, the effective flow properties are determined by the flow dependent and history dependent texture. This all makes the rheology of LCPs extremely complicated. 

The surfactant association structures have a long history of research ranging from the McBaln introduction of the aqueous micellar concept(1.) over the interpretation of mlcelllzatlon as a critical phenomenon — — to the analysis of the structure of lyotropic liquid crystals(A) and the comprehensive picture of the phase relations in water/surfactant/amphlphile systems.These studies have emphasized the relation between the association structures in isotropic liquid solutions and the liquid crystalline phases. Parallel extensive investigations in crystalline/ liquid crystalline lipid structureshave provided important insight in the mechanisms of the associations. 

While it was assumed above that only G. is affected by thermal history, in the case of main chain polymeric liquid crystals pronounced time dependent variability in G c has recently also been observed (7,8). It was shown that the lack of equilibrium perfection in the nematic phase can lead to substantial depression of the isotropization temperature T c =T. Thus non-equilibrium mesomorphic states can also, in principle, affect the phase sequence-(enantiotropic, monotropic) in the case of polymeric liquid crystals. 

Effect of Shear History on the Rheological Behavior of Lyotropic liquid Crystals... 

The history of our knowledge of the electrical conductivity of organic solids has been discussed by several authors who were involved in the work on the electrical conductivity of polycyclic aromatic hydrocarbons in the years after 1950 in a special volume of Molecular Crystals, liquid Crystals [6] edited by H. Inokuchi. At the beginning of this newer period of research, there were at least five milestones Stimulated by the measurements of Eley on phthalocyanine, Akamatu and Inokuchi in the year 1950 discovered a thermally-activated specific conductivity with an activation energy of E= 0.39 eV in violanthrone, and obtained similar values for related aromatic soUds [7]. The interpretation given at the time for this value in terms of the model of an intrinsic semiconductor with a band gap AEg = 2E is, to be sure, obsolete today (see below), but the results clearly showed that no conductivity exists at T = 0 and thus no intrinsic charge carriers are present in the crystal. 

Some reviews focusing on amphotropy have been published they take the development of liquid crystal research in this field into account [1-12]. With regards to the history of amphotropic liquid crystals it seems that in 1854 (about 35 years before O. Lehmann and F. Reinitzer established liquid... 

A. S. Sonin, 100 Years - The History of Discovery and Research on Liquid Crystals (russ.) (Ed. B. K. Vainshtein), Nauka Publishers, Moscow, Russia, 1988. 

The dualities of supramolecular biomeso-genic organizations [7, 17, 18] reemerge in the process of their scientific discovery. The history of liquid crystals , lovingly retraced by Kelker [13] (Fig. 5), parallels the development of molecular biology [5 - 8]. Liquid crystals are beautiful and mysterious, I am fond of them for both reasons , states de Gennes [19]. And, indeed, how can we not feel attracted to what in the inanimate world seems most closely related to us ... 

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