Lyotropic liquid crystalline polymer

Note 2 A liquid-crystalline polymer can exhibit one or more liquid state(s) with one- or two-dimensional, long-range orientational order over certain ranges of temperatures either in the melt (thermotropic liquid-crystalline polymer) or in solution (lyotropic liquid-crystalline polymer). 

Figures 7 and 8 display such plots for various lyotropic liquid-crystalline polymer systems, which range in q from 5.3 to 200 nm. As expected, most data points come close to the theoretical curve. This finding suggests that liquid crystallinity of stiff-chain or semiflexible polymer solutions has its main origin in the hard-core repulsion of the polymers.

Introduction of ring-opening metathesis as a versatile polymerisation technique (ROMP) by Chauvin and Herisson Nobel Prize Chemistry to Paul J. Flory for his fundamental achievements, both theoretical and experimental, in the physical chemistry of the macromolecules Fully aromatic polyamides developed Aramids, being lyotropic liquid crystalline polymers of high strength, due to extended molecular chains (Morgan and Kwolek)... 

It is now generally accepted that folding is universal for spontaneous, free crystallisation of flexible polymer chains. It was first of all found in crystallisation from very dilute solutions, but it is beyond doubt now, that also spherulites, the normal mode of crystallisation from the melt, are aggregates of platelike crystallites with folded chains, pervaded with amorphous material. "Extended chain crystallisation" only occurs under very special conditions in the case of flexible chains for rigid polymer chains it is the natural mode ("rigid rod-crystallisation" from the melt in case of thermotropic polymers, and from solution in case of the lyotropic liquid-crystalline polymers both of them show nematic ordering in the liquid state). 

The rigid rod chains in para-aramids tend to form so-called liquid crystals when they are dissolved in polar solvents or heated to certain temperatures. The polymers showing liquid crystalline behavior in melts are called the thermotropic liquid crystalline polymers, and those showing similar properties in solution are called the lyotropic liquid crystalline polymers. These liquid crystals exist in three distinct phases according to their specific structures (Fig. 12.25). 

In this chapter lyotropic liquid crystalline polymers are considered, where micelle-like organizations of the macromolecules cause the formation of mesophases in defined concentration and temperature regions. For these polymers it has to be assumed that the polymer backbone or the monomer units must contain an amphiphilic character. 

Because biopolymers may have properties uncommon in synthetic systems, they can be very attractive as model systems to test specific ideas. An early example of this can be seen in the work on PBLG, a synthetic polypeptide. Although the motivation for its original synthesis failed, it provided a firm basis for many of the early studies on lyotropic liquid crystalline polymers. It was one of the first systems to have its phase diagram characterised, for comparison with Flory s predictions, and a study of its viscosity demonstrated that there is a non-monotonic increase in viscosity with concentration as the liquid crystalline phase is entered. 

Burghardt, W.R. Molecular orientation and rheology in sheared lyotropic liquid crystalline polymers. Macromol. Chem. Phys. 1998, 199 (4), 471-488. 

Ugaz, V.M. Cinader, D.K. Burghardt, W.R. Origins of region I shear thinning in model lyotropic liquid crystalline polymers. Macromolecules 1997, 30 (5), 1527-1530. 

A typical example of a thermotropic liquid crystalline polymers is the polyesters and the mesogen substituted polysiloxane. The aromatic amide, the super strength fiber known commercially as Kevlar belongs to the lyotropic liquid crystalline polymers. The other important lyotropic liquid crystalline polymers are poly(7-benzyl-L-glutamate), abbreviated as PBLG, cellulose derivatives, the tobacco mosaic virus, etc. 

The most important development in lyotropic liquid crystalline polymers after Kevlar is probably the synthesis of poly [benzo(l,2-d 4,5-d ) bisthiazole-2,6-diyl]-l,4-phenylene, or for short, poly(p-phenylene benzo-bisthiazole) ( PBT Wolfe and Loo, 1980 Wolfe et al., 1981), and the closely related poly [benzo(l,2-d 5,4-d )bisoxazole-2,6-diyl]-l,4-phenylene or poly(p-phenylene benzobisoxazole) ( PBO Helminiak and Arnold, 1977 Wolfe and Arnold, 1981). Both PBT and PBO are lyotropic liquid crystalline and can be spun into fibers with mechanical properties even superior to that of Kevlar fibers. The molecular structures of these polymers are shown in Figure 5.2. 

Figure 6.13. Order parameter vs. reduced concentration of lyotropic liquid crystalline polymers. (From Doi Edwards, 1986.)...

When compared with lyotropic liquid crystalline polymers, the viscosity theory associated with thermotropic liquid crystalline polymers is far less studied except for a very few works done in theory (de Gennes, 1982) and experiment (Martins et al, 1986). More work should be carried out in this respect. 

The recently published reviews by Papkov on lyotropic liquid crystalline polymer systems (12 ) and by Shibaev and Plate on the liquid crystalline states in polymers (13) should be regarded as the first attempts to systematize the great body of available experimental data with a view to elaborating adequate techniques of producing liquid crystalline polymer systems. 

A famous and technologically significant lyotropic liquid crystal is a lyotropic liquid crystalline polymer called Kevlar. Kevlar (10) is a synthetic polyamide of rather simple structure. However, when Kevlar is dissolved in high concentrations in sulfuric acid, a liquid crystalline phase is generated. 

Industrial Lyotropic Liquid-Crystalline Polymers (Aramid Fibers). 

There are several commercial thermotropic polyesters that exhibit outstanding high-temperature capabilities. These include (14,15) an increasing number of fibers and high temperature plastics. Similar to the lyotropic liquid crystalline polymers, the thermotropics exhibit unusually low viscosities because of orientation and lack of entanglement. Of course, the orientation serves to improve their mechanical properties. The chemical structure can be varied significantly. 

The question of how to spin a lyotropic liquid crystalline polymer which has high thermal stability and degrades before it melts presented a major challenge. Fortunately for the makers of PBO this question had already been answered during the development of Kevlar. PBO is spun directly from the PPA solution used... 

Lyotropic liquid crystalline polymers (LLCPs) show its liquid crystaUinity in solutions. The Uquid crystallinity can be modulated by the type and the nature of the solvent used. The temperature of the solvent at which the polymer dissolves and the concentratirMi of the polymer also plays a significant role in determining the crystallinity of LLCPs in solutions. 

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