Structures of liquid-crystalline polymers

Figure 1.68 The structure of liquid crystalline polymers (a) nematic, (b) smectic and (c) cholesteric. Reprinted, by permission, from J. L. Fergason, Scientific American, 211(2), pp. 78, 80. Copyright 1964 by Scientific American, Inc.

FIG. 6.14 Schematic representation of some common structures of liquid crystalline polymers (From Jansen, 1996). 

FIG. 6.16 Schematic structure of liquid crystalline polymer with mesogenic group in the main chain. 

Fig. 12.25. Structure of liquid crystalline polymers showing (a) nematic, (b) smectic, and (c) cholesteric orders.

M Ballauff. Structure of liquid crystalline polymers. In RW Cahn, P Haasen, EJ Kramer, eds. Materials Science and Technology, Vol 12. New York VCH, 1993. 

The light microscope can be used to identify liquid crystalline phases, characterize molecular order, and quantify the distribution of defects. As such, the technique is seldom exploited to maximum advantage, particularly if studies are restricted to observations between crossed polars in transmitted light. Also, there is much scope for misinterpreting the observed contrast. TTiis chapter offers a systematic description of the options (and pitfalls) that are relevant to a research microscopist studying the structure of liquid crystalline polymers. 

In recent years considerable effort has been directed toward the study of the structures of liquid-crystalline polymers and copolymers. Using x-ray diffraction techniques developed for the examination of fibers (1,2). detailed structural information can be obtained provided the polymers are aligned in a strong magnetic field or by mechanical means. As reported more recently (3,), quite similar x-ray intensity distributions have been obtained from nematic monodomains of dimer molecules aligned in relatively weak magnetic fields. 

Investigations into the Structure of Liquid-Crystalline Polymer—Liquid-Crystalline... 

The texture of polymeric smectic. As discussed in Chapter 1 there are many subcategories of smectic phases. The identification of a smectic phase by POM becomes more difficult with less certainty when the order of molecular packing in the phase is increased. Other techniques such as WAXS are often used together with POM for conclusive identification of smectics (Gray and Goodby, 1984). Because of the complexity in the molecular structure of liquid crystalline polymers there are fewer types of polymeric smectics. The polymeric smectics most often observed are the less ordered types A and C. 

Figure 1. Structures of liquid crystalline polymers (LCPs) (A) rigid rod LCP [25, 65] (B) main chain LCP with flexible spacer [26] (C) side group LCP with flexible spacer [27] (D) combined main/side group LCP [28] (E) side group LCP without flexible spacer or mesogen jacketed LCP [29], (F) well-defined three-arm star [30] (G) LC dendrimer [33].

The common view of the structure of liquid crystalline polymers at rest is that of randomly oriented domains of highly ordered regions embedded a matrix of lesser order. We have shown this idea in the schematic drawing in Figure 4. The domain concept is substantiated... 

X-ray scattering is a powerful tool for the study of structures of liquid crystalline polymers. Theoretically, X-ray scattering from a given material can be treated as the Fourier transform of electron density of the material. For a nematic liquid crystalline polymer, the Fourier transform can be considered as the convolution of electron density of a long, hard rod and a layer of randomly... 

In the past two decades, a large number of novel liquid crystalline polymers have been created by polymer chemists around the world. X-ray scattering technique has been extensively used and proven to be a powerful tool in studying the structures of liquid crystalline polymers, while interpretation of X-ray diffraction patterns of liquid crystalline polymers remains a difficult task for many chemists. It is hoped that this chapter will help to fill the gap between polymer chemists and physicists. 

Figure 1. The chemical structures of liquid crystalline polymer and low molecular weight liquid crystals.

The molecular structure of liquid crystalline polymers is one of their characteristic features. It comprises rigid, rod-like macromodules which align in the melt to produce liquid structures. Although more expensive than some competing polymers, LCP possess better flow characteristics to fill the thin walls of modern connectors, for example. 

LC film series demonstrates a variety of optical functions by orienting the molecules in the liquid crystalline polymer layer. Three structures of liquid crystalline polymer layers are shown in Fig. 4.18. Stretching techniques cannot be used to change the orientation of the molecules in the vertical direction of the film, i.e., in the hybrid alignment and twisted alignment, which is the source of the advanced features of the LC film series. ... 

The results reported here should allow some insight into the microscopic structure of liquid crystalline polymers and its relation to their properties,... 

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