Conductivity in liquid crystals

Note added in proof. Since this article was written, a review of electrical conduction in liquid crystals has appeared P. A. Winsor, Liquid Crystals and Plastic Crystals, Vol. 2, p. 122ff, G. W. Gray and P. A. Winsor, Eds., Ellis Horwood, Chichester, 1974. 

The high conductivity in liquid crystal (1.2 x 10 S cm at 110°C) due to the viologen derivative has been interpreted in terms of the interaction of ionic conduction of the viologen dication, the cation radical, and iodine species [10]. 

Perhaps, (l.xxiii) is a good candidate for searching for the electronic conductivity in liquid crystals, in general. 

That is, the actual reason for conduction in liquid crystals, including discotic liquid crystals, is electronic conduction, but ionic conduction due to impurities is... 

Later, electroiuc conduction by electrons in a discotic liquid crystal had been observed [7] and this finding was used to establish the electronic conductivity in a low molecular nematic liquid crystal phase [8], Furthermore, these findings illustrate why until lately electiOTuc conduction had not been found in liquid crystal material and why only ionic conduction in liquid crystals had been supported by the experimental results so far. 

There were other studies of added salt to improve ionic conductivity in liquid crystals, but they also did not investigate the orientation control, which is an important characteristic of liquid crystals [149]. 

Because this is a relatively young held, there are still issues which need to be understood. In particular, the theory of electronic conduction in liquid crystals is much less well developed than that of electronic conduction in other organic materials and, although the relationship between molecular structure and conductivity is mostly understood, some issues still remain to be resolved and understood. 

Type of Crystalline Solid Particles Involved Primary Forces of Attraction Between Particles Boiling Point Electrical Conductivity in Liquid State Other Physical Properties of Crystals Conditions Necessary for Formation Examples... 

Even when composition is fixed, viscosity and other rheological properties may depend on the size and arrangement of aligned domains within a sample of liquid crystalline material. No studies of this matter seem to have been made, however. Such structural characteristics do influence electrical conduction and diffusion in liquid crystals, as discussed further below. 

Although the dominant means to create a space-charge field within the interference pattern in liquid crystals is given by Eq. (2), it has been shown that there are other mechanisms to create a space-charge field. One is derived from the conductivity anisotropy and is known as the Carr-Helfrich effect [43,49] ... 

SCLC in Liquid Crystals. The conductivity of a liquid crystal medium is generally much smaller than that of an electrolyte, because its physical properties are closer to those of an insulator or a semiconductor. The conductivities of many liquid... 

Optical quality thin films of metallic polymers are useful, therefore, as transparent electrodes [68]. For example, polyaniline [69], polypyrrole [70] and PEDOT [71] have been used as transparent hole-injecting electrodes in polymer LEDs (the initial demonstration of mechanically flexible polymer LEDs utilized PANl as the anode [69]). Transparent conducting films can be used for a variety of purposes for example, as antistatic coatings on CRT screens, as electrodes in liquid crystal display cells, or for fabricating electrochromic windows. 

In general, for side chain liquid-crystalline polymers, macroscopic molecular alignment is not easy and therefore clear evidence of electronic charge carrier transport was confirmed first in liquid crystals with low molecular weight. In the 1990s, fast electronic conduction was verified in discotic columnar phases of triphenylene derivatives [79,80] and hexabenzocoronene derivatives [81,82] as well as smectic phases of 2-phenylbenzothiazole [83, 84] and 2-phenylnaphthalene derivatives [85], as shown in Fig. 14. Carrier... 

As described in Section II.B.l above, doping causes a drastic change in the electrical properties of polyacetylene. The initial values of electrical conductivity were of the order of 10 S cm" for unoriented materials d24-i30 when doped by iodine and AsFs, were enhanced to the order of 10 S cm, which was obtained in the parallel direction of the doped films oriented by mechanical stretching 31 Improvements in polymerization methods and in the catalyst systems also enhanced the electrical conductivity. Highly oriented films prepared in liquid crystal solvents (Section II.A.l.d.iii) exhibited a conductivity higher than 10 S cm, as did also a well stretch-oriented film prepared by Ti(OBu)4-EtsAl dissolved in silicon oil and aged at 120°C. In further studies Naarmann and Theophilou and Tsukamoto and coworkers attained a conductivity of ca 10 S cm k... 

High levels of local electrical conductivity Emission-free plastics Plastic disks with scratchproof coatings Polymeric LEDs Large-surface illuminated diodes Applications in liquid crystal displays... 

The electric effects in liquid crystals are very important in the LCD industry [44]. The presence of ions in a liquid crystal sample, confined in a LC cell between plates with electrodes, is usually disturbing, since it gives rise to ionic conduction between the charged electrodes. As a consequence, power is consumed. The ions also screen the external electric field and higher voltages have to be applied to the electrodes to effectively control the LC orientation, which also leads to an increased power consumption. 

The question arises what mechanism is responsible for the EHD instability in these polymers Such effects in liquid crystals are the result of the ionic conductivity, The free charge-carrier can appear either due to ionic impurities in the substance, or due to the injection or exclusion of electrons by neutral molecules on electrodes. 

Fig. 7.19 Scheme of the set-up for measuring thomal conductivity and specific heat in liquid crystals... 

In 1963, Richard Williams observed the formation of very regular patterns or domains in a nematic liquid crystal when the material was subjected to an electric field. This report marked the beginning of a new era in research on the electro-optic properties of liquid crystals, a field which had laid dormant for nearly 30 years. During the remaining years of the 1960 s and the early 70 s, numerous studies of electro-optic effects in liquid crystals were performed, and at the same time, investigations into the synthetic and physical chemistry of these materials were conducted. As a result of these efforts, a whole new display industry evolved. 

In this chapter we will discuss possible realizations of liquid crystalline ordering in conductive polymers. Section II presents basic properties of conducting polymers with a brief description of charge transport mechanisms in those systems. Section III discusses structural types and defects in liquid crystals, and Section IV covers some experimental results of the structural analysis of mesophases in polymers with flexible side chains and polymer/surfactant systems. Sections III and IV are presented to emphasize the properties important to conducting polymers. In Section V experimental data on liquid crystallinity in conductive polymers will be reviewed. Conclusion will be given in Section VI,... 

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