Aramid liquid crystalline polymer

In order to make polymers behave as liquid crystals it is necessary to introduce some structural rigidity. A typical polymer which has the required rigidity is poly(phenylenetetraphthalamide) (10.7). This material belongs to a class of polymer known as the aramids. Other liquid crystalline polymers are the thermotropic polyesters derived from /7-hydroxybenzoic acid, p, p -biphenol and terephthalic acid (10.8). 

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)... 

Following the technological breakthroughs which led to the discovery of (1) the liquid crystalline behavior ofpara-oriented aramids26 and (2) a novel method for spinning anisotropic liquid crystalline polymer solutions,27 Kevlar aramid fiber was produced and commercialized by the DuPont company in 1972. Other fibers based on aromatic polyamide compositions, which were produced and commercialized by other companies, were Technora (Teijin, Japan), Teijinconex (Teijin, Japan), andTwaron (Akzo, The Netherlands). Additionally, SVM is a fiber produced in the Former Soviet Union and it was announced in 1990 that a new aramid fiber had been introduced by Hoechst, in Germany. 

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). 

Rigid-rod polymers are often liquid crystalline polymers classified as lyotropic, such as the aramid Kevlar (DuPont), or thermotropic liquid crystalline polymers, such as Vectran (Celanese) (see Polyamides, Aromatic Liquid Crystalline Polymers, Main-Chain Liquid Crystalline Thermosets). 

Industrial Lyotropic Liquid-Crystalline Polymers (Aramid Fibers). 

Aramids — Groundbreaking research by DuPont scientists in the field of liquid crystalline polymer solutions in 1965 formed the basis for this fiber. Kevlar is probably the best known brand in this fiber category. Other popular aramid brands include... 

In the 1970s, a paradigm shift occurred when polymers with more specific properties started to be produced. This included various liquid crystalline polymers leading, for example, to the production of superstrong fibers such as Aramid / Kevlar [12]. The development of functional polymers for the conduction of light and electricity and optical switches also started then [13]. In the near future this will probably lead to highly effective and flexible polymer solar cells [14]. 

Following the technological breakthroughs which led to the discovery of ( ) the liquid crystalline behavior of para-oriented aramtds and (2) a novel method for spinning anisotropic liquid crystalline polymer solutions, " Kevlar aramid fiber was produced and commercialized... 

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

Liquid crystals, due to the presence of the ordered domains, are optically anisotropic, i.e. birefringent. This can be easily verified by observing the liquid crystal, e.g. aramid and sulfuric acid solution, at rest between crossed polarizers. The parallel arrays of polymer chains in liquid crystalline state become even more ordered when these solutions are subjected to shear as, for example, in extruding through a spinneret hole. It is this inherent property of liquid crystal... 

Fig. 3.5 Structural hierarchy in liquid-crystalline fibers. The mechanical performance of highly oriented polymers can approach the ultimate theoretical properties at high degrees of elongation. Anisotropic, rod-like macromolecules, like aromatic copolyesters composed of 2,6-naphthyl and 1,4 phenyl units, often form oriented structures, which can exhibit liquid crystallinity. Extensive structural studies of fibers of these oriented copolyesters showed a hierarchical structure like the one depicted in this Figure. In aramids (Kevlar or Twaron) similar structures may exist. Adopted with permission from [17]...

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