Liquid crystalline polymer calamitic

Liquid crystallinity can be attained in polymers of various polymer architectures, allowing the chemist to combine properties of macromolecules with the anisotropic properties of LC-phases. Mesogenic imits can be introduced into a polymer chain in different ways, as outhned in Fig. 1. For thermotropic LC systems, the LC-active units can be connected directly to each other in a condensation-type polymer to form the main chain ( main chain liquid crystalline polymers , MCLCPs) or they can be attached to the main chain as side chains ( side chain liquid crystalline polymers , SCLCPs). Calamitic (rod-Uke) as well as discotic mesogens have successfully been incorporated into polymers. Lyotropic LC-systems can also be formed by macromolecides. Amphiphihc block copolymers show this behavior when they have well-defined block structures with narrow molecular weight distributions. 

Main ehain liquid crystalline polymers can be constructed from rod-like (calamitic) and disc-like (discotic) units by a condensation process (see Chapter 8). Seen here in racemic form, polymer 11 is a poly ether with repeating mesogenic-like core units separated by flexible alternating hydrocarbon spacers. This typical architecture ensmes a sufficiently low melting point for liquid crystalline phases to be exhibited. Polymer 11 has a molecular weight of 17,000 and a polydispersity of 1.7. 

In many cases, these polymer chains take on a rod-like (calamitic LCPs) or even disc-like (discotic LCPs) conformation, but this does not affect the overall structural classification scheme. There are many organic compounds, though not polymeric in nature, that exhibit liquid crystallinity and play important roles in biological processes. For example, arteriosclerosis is possibly caused by the formation of a cholesterol containing liquid crystal in the arteries of the heart. Similarly, cell wall membranes are generally considered to have liquid crystalline properties. As interesting as these examples of liquid crystallinity in small, organic compounds are, we must limit the current discussion to polymers only. 

Liu X H, Bruce DW, Manners I. 1997. Novel calamitic side chain metallomesogenic polymers with ferrocene in the backbone S5mthesis and properties of thermotropic liquid crystalline poly(ferrocenylsilanes). Chem Commun (3) 289 290. 

Nevertheless, we were not able to carry out TOP measurements with calamitic monomers, because the dark currents in the liquid crystalline phases were too high. As mentioned before, such measurements are possible with the cyclic tetramers. The transient photocurrents of the tetrasiloxanes 19, 21, 22 illustrate that the carrier transport is totally dispersive, i. e. dominated by deep traps in which the charge carriers are captured. This is a typical behaviour of amorphous polymers. No transit time could be detected and no statements about the carrier mobility can be made for the rod-like mesogens. 

Percec V, Kawasumi M (1992) Synthesis and characterization of a thermotropic nematic liquid crystalline dendrimeric polymer. Macromolecules 25 3843-3850 Percec V, Chu P, Ungar G, Zhou J (1995) Rational design of the first nonspherical dendrimer which displays calamitic nematic and smectic thermotropic liquid crystalline phases. J Am Chem Soc 117 11441-11454... 

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