Liquid crystalline polymers physical structure

The development is reviewed of liquid-crystalline polymers whose mesophase formation derives from the nature of the chemical units in the main chain. The emphasis lies primarily on highly aromatic condensation polymers and their applications. The general properties of nematic phases formed by such polymers are surveyed and some chemical structures capable of producing nematic phases are classified in relation to their ability to form lyotropic and thermotropic systems. The synthesis, properties, physical structure and applications of two of the most important lyotropic systems and of a range of potentially important thermotropic polymers are discussed with particular reference to the production and use of fibres, films and anisotropic mouldings. 

There has been a great deal of interest in thermotropic, liquid crystalline polymers in the past twenty years or so since the discovery of useful materials based on them. Many critical factors such as structure of mesogenic units, presence and structure of flexible spacers or rigid kinks, molecular weight and its distribution, and thermal history influence thermal, physical and thermotropic properties of liquid crystalline polymers(1-13). 

This paper presents some of our results on the synthesis and structure of thermotropic main-chain liquid crystalline polyethers based on bis(4-hydroxy-phenoxy)-p-xylene. It also deals with two areas in the field of liquid crystalline polymers that have received only little attention, namely the dielectric relaxation (5-10 and Gedde, U.W. Liu, F. Hult, A. Gustafsson, A. Jonsson, H. Boyd, R.H. Polymer submitted) and the kinetics of isotropic-mesomorphic state transitions (11-14. 32). They are both very important for the understanding of the nature of the mesomorphic state in polymers and for the understanding of similarities and differences of physical phenomena between liquid crystalline and semi-crystalline polymers. 

Some polymers manifest liquid crystalline ordering, which does not have the full long-range three-dimensional periodicity of crystallinity but is far more ordered than amorphicity. Since many excellent books and articles have been published on such polymers and the author does not have much that is new to add to this background information, very little will be said about polymer liquid crystallinity in this book. Van Krevelen [3] has reviewed liquid crystallinity in polymers in a readable manner and discussed its effects on properties for which quantitative structure-property relationships are available. Adams et al [41] have published a valuable compendium of articles covering the theory, synthesis, physical chemistry, processing and properties of liquid crystalline polymers. Woodward [42] has discussed and illustrated liquid crystallinity in polymers with many beautiful micrographs. 

The fourth design feature is the formation of a liquid crystalline structure. During the last fifteen years, liquid crystalline polymers (LCP) have become one of the most exciting polymer families synthesized by chemists. Recently, the syntheses of LCPs were reviewed by Griffin (J ). Interesting physical properties of liquid crystalline polyesters were described by Huynh-ba and Cluff (13) To the interest of tribologists, some of LCPs are rather wear resistant. 

Her current research interests include studies of miscibility and physical aging in blends, nanophase separation in polymers with long side-chains, polymer dynamics, liquid crystalline polymers, composites, and systems containing nanoparticles. A common feature of these studies is the use of scattering techniques, especially neutron scattering, to study the local structure, conformation, and dynamics in polymers. She has written various reviews and book chapters in this area and has served on selection panels to allocate beam time at neutron facilities. 

What we have attempted to do here is to present rheological tests for identifying the development and relaxation of orientation and structure in liquid crystalline polymers. Because these fluids are typically quite turbid, it is difficult to use rheo-optical techniques. The interpretation of the rheological tests must then come partly from studies on quenched solid specimens. In summary, it is believed that a detailed set of rheological tests based on the transient response of LCP can be used to evaluate various liquid crystalline polymers and identify processing conditions which will lead to the optimum physical properties. 

Al-Itavi Kh, I., Frenkin, E. I., Kotova, E. V, Bondarenko, G. N., Shklyaruk, B. F., Kuleznev, V. N., Dreval, V. E., Antipov, E. M. (2000). Influence of high pressure on structure and thermophysical properties of mixes of polyethyleneterephthalate with liquid-crystalline polymer. In Abstracts of the 2-nd Russian Kargin Symposium Chemistry and physics ofpolymers in the beginning of the 21 century Chemogolovka, Part 1-1/13 [in Russian]. 

Phases in thermodynamic systems are then macroscopic homogeneous parts with distinct physical properties. For example, densities of extensive thermodynamical variables, such as particle number N of the fth species, enthalpy U, volume V, entropy S, and possible order parameters, such as the nematic order parameter for a liquid crystalline polymer etc, differ in such coexisting phases. In equilibrium, intensive thermodynamic variables, namely T,p, and the chemical potentials pi have to be the same in all phases. Coexisting phases are separated by well-defined interfaces (the width and internal structure of such interfaces play an important role in the kinetics of the phase transformation (1) and in other... 

Beside trade names also generic names are used for polymer materials. Generic names catch features of the structure and physical properties of the material, for example liquid crystalline polymer or conducting polymer. 

Aromatic polyamide fibers are produced by spinning liquid crystalline polymer solutions of PPTA-sulfuric acid dopes into a water coagulation bath [414], resulting in the formation of a crystalline fiber with a surface skin. Variations in the structure produced by annealing at elevated temperature are known to increase the fiber modulus due to a more perfect alignment of the molecules [472]. The chemistry and physics of the aromatic polyamide fibers have been reviewed [419]. 

The author s group chose polyacrylate [102, 103], polyoxyethylene [104, 105], polysiloxane [103], and a polyacrylate/polysiloxane hybrid polymer [106] as the main chain according to the design in Fig. 1.26b-e. We attached various kinds of side-chain structures onto polymer main chains described above and checked the physical properties of these side-chain-type ferroelectric liquid crystalline polymers (FLCPs). 

Two approaches to the attainment of the oriented states of polymer solutions and melts can be distinguished. The first one consists in the orientational crystallization of flexible-chain polymers based on the fixation by subsequent crystallization of the chains obtained as a result of melt extension. This procedure ensures the formation of a highly oriented supramolecular structure in the crystallized material. The second approach is based on the use of solutions of rigid-chain polymers in which the transition to the liquid crystalline state occurs, due to a high anisometry of the macromolecules. This state is characterized by high one-dimensional chain orientation and, as a result, by the anisotropy of the main physical properties of the material. Only slight extensions are required to obtain highly oriented films and fibers from such solutions. 

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