Extruded polymer liquid crystals

Applications. The polyamides have important applications. The very high degree of polymer orientation that is achieved when liquid crystalline solutions are extruded imparts exceptionally high strengths and moduli to polyamide libers and lilms. DuPont markets such polymers, e g. Kevlar, and Monsanto has a similar product, e.g.. X-500. which consists of polyamide and hydrazide-lype polymers. Liquid-crystal polymers arc also used in olccirnnpnc displays. 

Liquid polymers (at ambient temperature) are in general macromolecules with a relatively low molecular weight, many of them being in fact oligomers. Some liquid polymers are utilized as synthetic oils. Certain polymers can form liquid crystals in other words they can have an ordered structure while being in liquid state (either melted or in a solution). The orientation of certain polymeric molecules in liquid state such that the properties of the material are anisotropic is possible. Polymer liquid crystals have practical applications, and solution of liquid crystal polymers can be used for extruding fibers that have a highly crystalline structure after solvent elimination. 

Kent S, Rybnikar F, Geil PH Carter JD (1994) Morphology of a thermotropic random terpolymer liquid crystal polymer crystallized in the bulk compression mouldings, extrudates and fibres. Polymer 35 1869... 

T300, T50 and PI00 are carbon fibers Spectra 1000 is a gel-spun polyethylene and SSE PE is a solid state extruded polyethylene. Other polymers on die plot have stiff backbones and a liquid crystal character],... 

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... 

Kevlar is a lyotropic liquid crystal which can be obtained from a sulfuric acid solution when the concentration reaches a critical value, e.g., 10%. However, this polymer like other LCPs is also anisotropic and its mechanical properties is directional, but less so in fibers than in the extruded plaques. The fiber properties in Table I are compared with other organic fibers and steel ( 15). On an equal weight basis, Kevlar has a strength several times that of steel. Perhaps, Kevlar is the first polymer at least comparable to metals. 

Lyotropin poiymer Polymers that decompose before melting but that form liquid crystals in solution under appropriate condition. They can be extruded from high concentration dopes to give fibers of high modulus and orientation for use in advanced composites, tire cord, ballistic protective devices, etc. 

The strength of polymers in general is derived from the orientation of the polymer chains when the polymer is extmded. The polymer chains of a liquid crystal polymer are inherently ordered and so when extruded in the liquid crystal phase, liquid crystal polymers acquire extremely high strength. For example, compared with nylon, which is extruded in the liquid phase, Kevlar is thirty times stronger despite being only slightly... 

---