Thermotropic liquid crystal polymers processing

Both conventional and liquid-crystal polymers can be processed by melt spinning. The principal requirement is that the polymer must not degrade before becoming molten. Examples are nylon-6, nylon-6,6 and poly(ethylene terephthalate) ( polyester , terylene , or dacron ) for conventional polymers and the Vectra series (see section 12.4.3) among the thermotropic liquid-crystal polymers. 

Wang L, Allan PS, Bevis MJ (1995) Enhancement of internal weld line strength in thermotropic liquid crystal polymer moldings. Blast Rubber Compos Process Appl 23 139-150... 

The miscibility limit between thermotropic liquid crystal polymers and flexible chain polymers can be predicted by calculations corresponding to the spinodal curve at constant temperature. This miscibility is increased with the increase of the degree of disorder (y/jc,) of the liquid crystal polymer and with the decrease of the degree of polymerization. Two quantitative parameters from the Hory s lattice theory can be used to estimate the phase behavior of this kind of blend at melt processing temperature the polymer-polymer interaction parameter and the degree of disorder (y/Xj). In binary polymer blends phase separation may occur for any value of degree of disorder (y/Xj)... 

It has also been found that the properties of ordinary engineering polymers can be greatly improved by blending them with thermotropic liquid crystal polymers. Usually the processing properties are also improved. The fact that the materials are biphasic is not a disadvantage. The predominantly flexible phase contains some liquid-crystalline sequences which provide a certain amount of reinforcement in these regions. Costs are lower than for pure liquid-crystalline materials. 

Much of the current research on polymeric liquid crystals is directed towards thermotropic liquid crystals which are formed when certain polymer melts are cooled. Many of those polymers includes chemical groups similar to low-molecular-weight nematics either along the main chain or in the side chains. These materials are technologically interesting because of their ease of processing. Theoretically, these materials are expected to show curious behaviours which are a mixture of their... 

These thermotropic liquid-crystalhne polymers have high melting points but can be melt-processed like other thermoplastics. The macroscopic orientation of the extended-chain crystals depends on the orientation imparted by flow during processing (molding, extrusion, etc.). Because of the fibrous nature of the extended-chain crystals, these plastics behave as self-reinforced composites, with excellent mechanical properties, at least in the chain direction. This is illustrated in Table 4.3 for molded specimens of a hquid-crystalline copolyester of ethylene glycol, terephthalic acid, and / -hydroxybenzoic acid [14]. In the direction parallel to the flow, the properties listed in Table 4.3 favorably compare with ordinary crystalline thermoplastics (nylons, polyesters) reinforced with up to 30% glass fibers. 

Fibers with very high strength and modulus can be fabricated from polymers that have a molecular structure in which the chains are packed in small cross-sectional areas with strong bonds and low elongation. However, some of these aromatic polymers have melting points that are higher than their decomposition temperatures due to the rigidity of their molecules. It is therefore impossible to process them in thermotropic liquid crystal form. 

Aciemo D, Nobile MR (1993) Processing and properties of thermotropic liquid crystalline polymers. In Mantia FPL (ed) Thermoplastic liquid crystal polymer blends. Technomic, New York, p 76... 

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