Liquid crystal polymers textures

In microscale models the explicit chain nature has generally been integrated out completely. Polymers are often described by variants of models, which were primarily developed for small molecular weight materials. Examples include the Avrami model of crystallization,- and the director model for liquid crystal polymer texture. Polymeric characteristics appear via the values of certain constants, i.e. different Frank elastic constant for liquid crystal polymers rather than via explicit chain simulations. While models such as the liquid crystal director model are based on continuum theory, they typically capture spatiotemporal interactions, which demand modelling on a very fine scale to capture the essential effects. It is not always clearly defined over which range of scales this approach can be applied. 

Demus, D. and L. Richter, Textures of Liquid Crystal Polymers, Verlag Chemie, Weinheim, 1978. 

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. 

Walker, L. Wagner, N. Rheology of region I flow in a lyotropic liquid-crystal polymer the effects of defect texture. J. Rheol. 1994, 38 (5), 1525-1547. 

Nevertheless in polymeric liquid crystals the same types of orientational defects and thus the same types of textures as present in the low mass counterparts have been observed. The textures often formed by polymers are the threaded texture, the schlieren texture and the focal conic texture of smectics. As is for low mass liquid crystals, the texture is a consequence of defects (disclinations and dislocations, refer to Chapter 1) present in the liquid crystal and is characteristic of a specific type of the phase. The texture examination has become a very useful tool in the determination of the type and nature of the polymeric liquid crystals. 

Electro-Optic Properties of Polymer Stabilized Liquid Crystals. Polymer networks have been used to stabilize many of the liquid crystal display states in various types of displays quite advantageously. In this section, we present some recent work on correlating the material properties of the liquid crystal/polymer network composite to the electro-optic properties of the flat-panel displays specifically cholesteric texture displays (75) and simple nematic birefringent type displays (7(5). 

Fig. 7. A texture of a main-chain liquid crystal polymer, X, as seen in transmitted light between crossed polars. By analogy with small molecule liquid crystal parlance, it is known as a Schlieren texture. The fine granular nature of the background is caused by crystallization on cooling otherwise the microstructure is unchanged from that in the liquid crystal state. (Photograph courtesy of T. J. Lemmon, Department of Materials Science and Metallurgy, University of Cambridge.)...

One of the most intriguing orientation phenomena seen in rigid-chain liquid crystal polymers is that of banding. The texture appears in samples which have been sheared and oriented in extensional flow, as contrasting bands normal to the direction of extensional flow. The phenomenon was observed in Kevlar fibres by Dobb et al It has since been reported in other lyotropic liquid crystal polymers " and in thermotropic random copolymers. In the latter studies a... 

McArdle, C. B., ed.. Side Chain Liquid Crystal Polymers, Blackie, Glasgow, 1989. Ciferri, A ed.. Liquid Crystallinity in Polymers, VCH Publishers, New York, 1991. Gray, G. W., and Goodby, J. W., Smectic Liquid Crystals—Textures and Structures, Heyden Son, Philadelphia, 1984. 

Typical textures of liquid crystal polymer phases... 

FIGURE 5.2 The main phases of liquid crystal polymers and the corresponding textures. 

The self-organizing nature of liquid crystal polymers is reflected in their complex flow behavior. The relaxation phenomenon of lyotropic polymer solution after shear cessation leads to band texture morphology that can be further induced to isotropic materials. Future research should focus on solvents influence on band size implicitly related to the induced pattern in polymers with different structures. Another aspect that could be explored is the imidization of patterned polyimide precursors and those conditions in which the texture is still maintained. 

Wang J., and Labes M.M. Control of the anisotropic mechanical properties of liquid crystal polymer films by variations in their banded texture. Macromolecules. 25 no. 21 (1992) 5790-5793. 

L. M. Walker, W. A. Kernick, and N. J. Wagner, In situ analysis of the defect texture in liquid crystal polymer solutions under shear. Macromolecules 30,508-514 (1997). 

An example of an experiment involving continuous sample rotation with synchronized data acquisition is shown in Fig. 10 [123]. A thin liquid crystal cell filled with a side chain liquid crystal polymer was continuously rotated about an axis perpendicular to the magnetic field. The director behavior was followed by deuteron NMR as well as by polarizing microscopy. The optical texture and the orientation-dependent... 

A wide variety of pressure distribution is caused due to the flow of a liquid crystal polymer melt or solution. Graziano and Mackley (1984) were the first to analyse the development of textures during the shearing of a cmiventional nematic liquid crystal polymer and also thermotropic copolymer. Windle et al. (1985) studied the effect of shear deformation of a thermotropic Uquid and observed fine-scale texture (Windle et al. 1985). 

Alderman N, Mackley M (1985) Optical textures observed during the shearing of thermotropic liquid-crystal polymers. Faraday Discuss Chem Soc 79 149-160 Antoun S, Lenz RW, Jin I (1981) Liquid crystal polymers. IV. Thermotropic polyesters with flexible spacers in the main chain. J Polym Sci Polym Chem Ed 19 1901-1920 Baek SG, Magda JJ, Cementwala S (1993) Normal stress differences in liquid crystalline hydroxypropyl cellulose solutions. J Rheol 37 935-945 Barnes HA (2003) A review of the rheology of filled viscoelastic systems. In The British Society of Rheology, pp 1-36... 

In the field of conventional engineering thermoplastics we have a detailed understanding of the isotropic state, we appreciate the stress history deployed in a moulding process, we can measure relaxation phenomena and so predict residual orientation, and so we can deduce the property spectrum of a final product. If the time-scale between recognition of liquid crystalline phenomena in melts and its commercial exploitation appears protracted we need only note the observation of Professor J. L. White summing up at a recent conference in Kyoto, that, for liquid crystal polymers, stress history, optical anisotropy and texture are independent variables. In fact, to make the connection from basic material property to performance in the final product, industrial technologists have had to learn a new science. 

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