Thermotropic LCPs

Four companies (trade name in parentheses), Amoco (Xydar), Hoechst-Celanese (Vectra), Du Pont, and Granmont (Granlar), make thermotropic LCPs for various types of extmsion and mol ding processes. Six companies have discontinued TCP materials that were either commercial or under development. These companies include ICl, BASF, Eastman, Bayer, General Electric, and Monsanto. 

Other properties of thermoplastics as well. Owing to the relatively low-melt viscosity of thermotropic LCPs, often a small amount of LCP decreases the blend viscosity significantly and renders the matrix thermoplastics easier to process [9-211. 

The blends of thermotropic LCPs and thermoplastics are generally two-phase systems where the dispersed LCP phase exists as small spheres or fibers within the thermoplastic matrix. Often a skin/core morphology is created with well-fibrillated and oriented LCP phases in the skin region and less-oriented or spherical LCP domains in the core. 

Industrial state business climate, rating of, 19 523, 524t, 525t Industrial textiles, 24 623 Industrial thermotropic LCPs, 13 381-382 Industrial used oil, contaminant levels in, 21 424t... 

Thermostats, mercury-switch, 16 42 Thermosyphon, 13 225, 226 Thermosyphon reboiler, 11 774 Thermotropic LCPs, 13 381-382 Thermotropic liquid crystalline compounds, 15 87-89t... 

Research on liquid crystalline polymers(LCP) is a fashionable subject with the goal of developing speciality polymers of superior mechanical and thermal properties. Besides these properties, other interesting properties of LCP have not been fully utilized. We are trying to use thermotropic LCP for photon-mode image recording material. 

Since thermotropic LCPs have recently been used in applications requiring high modulus and strength, much effort has been expended to improve the processability... 

Working with a solution is needed for polymers which above their melting point would degrade (example aromatic polyamide fibres such as Kevlar and Twaron). For fibres the removal of the solvent is not too problematic. In e.g. injection moulding applications solvents caimot be used here thermotropic LCP s have to be used. Since these would degrade during processing, they are diluted by copolymerisation (example poly-hydroxy-benzoic acid - co - PETP)... 

Perhaps the most widely utilized (and studied) lyotropic LCP is poly j -phenylene terephthalamide (PPTA), more commonly known as Kevlar (see Figure 1.70). Kevlar belongs to the class of aramids that are well known for their LCP properties. Because these polymers are crystalline in solution, they are often spun into filaments, from which the solvent is subsequently removed in order to retain the aligned polymer structure. The result is a highly oriented, strong filament that can be used for a wide variety of structural applications. Most thermotropic LCPs are polyesters or copolymers that can be melted and molded into strong, durable objects. 

Two types of liquid crystal polymers (LCP s) can be distinguished lyotropic and thermotropic. The first type is formed in a solution this is done when the polymer has such a high melting point (low AS, see 4.2), that it cannot be handled in the molten condition without being degraded. If this limitation is not present, then the orientation can be brought about in the melt in such a case we have a thermotropic LCP. 

An example of a thermotropic LCP is a copolymer of polyhydroxy benzoic acid and polyethylene terephtalate ... 

TABLE 6.18 Ratios of characteristic transition temperatures of thermotropic LCPs... 

FIG. 15.42 Schematic representation of the viscosity (A) as a function of temperature for thermotropic LCPs and (B) as a function of concentration for lytropic LCPs. 

For lyotropic LCPs, there is a hiphasic window of concentrations over which nematic and isotropic phases coexist, t he polymer concentrations in the coexisting isotropic and nematic phases are designated by C (or = Trd Lv /A) and Cj (or (pj = jrd Lv2/A), respectively. There is also a theoretical concentration C, at which the isotropic phase becomes unstable to orientational fluctuations. According to the Onsager theory, 02/0 = 1.047 and 02/0i = 1.27 (see Section 2.2.2). Thermotropic LCPs often have a biphasic window of temperatures over which isotropic and nematic phases coexist. This biphasic window exists in nominally single-component thermotropics because of polydispersity the nematic phase is typically enriched in the longer molecules relative to the coexisting isotropic phase (D Allest et al. 1986). 

Steady-state negative N values have been reported for some thermotropic LCPs, but... 

To avoid these problems, model thermotropic LCPs are often studied whose melting temperatures are low enough to access both the nematic and isotropic states without chemical reactions, and which have side groups that suppress crystallite formation (Kim and Han 1993 Chang and Han 1997). 

The first thermotropic LCPs were reported in the mid-1970 s by Roviello and Sirigu (15) and Jackson and Kuhfuss (16). Since then, a large number of LCPs have been reported an excellent, though now dated, review of main-chain (i.e., the mesogen is in the polymer backbone) thermotropic LCPs was published by Ober et al. (17). In the 1980 s, several thermotropic aromatic copolyester LCPs were commercialized. 

Thermotropic LCPs have high melt elasticity, but exhibit little extrudate swell. The latter has been attributed to a yield stress and to long relaxation times (60). The relaxation times for LCPs are normally much longer than for conventional polymers. Anomalous behavior such as negative first normal stress differences, shear-thickening behavior and time-dependent effects have also been observed in the. rheology of LCPs (56). Several of these phenomena are discussed for poly(benzylglutamate) solutions in the chapter by Moldenaers et al. 

Processing. Relatively little has been published on the processing of thermotropic LCPs. The morphology of melt-processed articles is dependent on the deformation and thermal histories. Extensional flows produce fibrillar structures with high orientation in the machine direction. Flows with complicated stress distributions and temperature gradients, such as encountered in injection molding, yield complicated morphologies. 

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