Crystallinity liquid-crystal polymers

Crystalline polymers primarily of interest as fibres, including some grades which may be considered as liquid crystal polymers. 

Figure 25.24. Difference in behaviour between liquid crystal polymers and conventional crystalline polymers in the melt at rest, during shear and when cooled after shearing...

Liquid crystal polymers (LCP) are a recent arrival on the plastics materials scene. They have outstanding dimensional stability, high strength, stiffness, toughness and chemical resistance all combined with ease of processing. LCPs are based on thermoplastic aromatic polyesters and they have a highly ordered structure even in the molten state. When these materials are subjected to stress the molecular chains slide over one another but the ordered structure is retained. It is the retention of the highly crystalline structure which imparts the exceptional properties to LCPs. 

Liquid crystalline solutions as such have not yet found any commercial uses, but highly orientated liquid crystal polymer films are used to store information. The liquid crystal melt is held between two conductive glass plates and the side chains are oriented by an electric field to produce a transparent film. The electric field is turned off and the information inscribed on to the film using a laser. The laser has the effect of heating selected areas of the film above the nematic-isotropic transition temperature. These areas thus become isotropic and scatter light when the film is viewed. Such images remain stable below the glass transition temperature of the polymer. 

To produce novel LC phase behavior and properties, a variety of polymer/LC composites have been developed. These include systems which employ liquid crystal polymers (5), phase separation of LC droplets in polymer dispersed liquid crystals (PDLCs) (4), incorporating both nematic (5,6) and ferroelectric liquid crystals (6-10). Polymer/LC gels have also been studied which are formed by the polymerization of small amounts of monomer solutes in a liquid crystalline solvent (11). The polymer/LC gel systems are of particular interest, rendering bistable chiral nematic devices (12) and polymer stabilized ferroelectric liquid crystals (PSFLCs) (1,13), which combine fast electro-optic response (14) with the increased mechanical stabilization imparted by the polymer (75). 

Liquid crystalline polymers (LCPs), 10 374, 517-518 13 370-372. See also Liquid-crystal polyesters (LCPs) Liquid-crystal polymers (LCPs) thermotropic, 13 381-382 Liquid-crystalline thermotropic polyesters, 20 34... 

It is the intent of this doeument to define the terms most commonly encountered in the field of polymer blends and eomposites. The scope has been limited to mixtures in which the eomponents differ in ehemical composition or molar mass or both and in which the continuous phase is polymeric. Many of the materials described by the term multiphase are two-phase systems that may show a multitude of finely dispersed phase domains. Hence, incidental thermodynamic descriptions are mainly limited to binary mixtures, although they can be and, in the scientific literature, have been generalized to multicomponent mixtures. Crystalline polymers and liquid-crystal polymers have been considered in other documents [1,2] and are not discussed here. 

Some polymers undego other thermal transitions in addition to Ts and Tm. These include crystal-crystal transitions (i.e., transition from one crystalline form to another and crystalline-liquid crystal transitions. 

Liquid crystal polymers (LCP) are polymers that exhibit liquid crystal characteristics either in solution (lyotropic liquid crystal) or in the melt (thermotropic liquid crystal) [Ballauf, 1989 Finkelmann, 1987 Morgan et al., 1987]. We need to define the liquid crystal state before proceeding. Crystalline solids have three-dimensional, long-range ordering of molecules. The molecules are said to be ordered or oriented with respect to their centers of mass and their molecular axes. The physical properties (e.g., refractive index, electrical conductivity, coefficient of thermal expansion) of a wide variety of crystalline substances vary in different directions. Such substances are referred to as anisotropic substances. Substances that have the same properties in all directions are referred to as isotropic substances. For example, liquids that possess no long-range molecular order in any dimension are described as isotropic. 

A number of other characteristics are required in order to ensure a viable polymeric conductor. Chain orientation is needed to enhance the conducting properties of a polymeric material, especially the intermolecular conduction (i.e., conduction of current from one polymer molecule to another). This is a problem with many of the polymers that are amorphous and show poor orientation. For moderately crystalline or oriented polymers, there is the possibility of achieving the required orientation by mechanical stretching. Liquid crystal polymers would be especially advantageous for electrical conduction because of the high degree of chain orientation that can be achieved. A problem encountered with some doped polymers is a lack of stability. These materials are either oxidants or reductants relative to other compounds, especially water and oxygen. 

No coherent threadline could be maintained and the extmdate flew off the windup as short, brittle, crystalline lengths. Not until many years later did other workers show that this polymer on cooling exhibits a mesophase transition directly from the isotropic melt to a smectic A phase. Good sources of information on liquid crystals and liquid crystal polymers are available (212—216). 

Similady, liquid-crystal polymers exhibit considerable order in the liquid state, either in solution (lyotropic) or melt (thermotropic). When crystallized from solution or melt, they have a high degree of extended-chain crystallinity, and thus have superior mechanical properties. Kevlar (Du Pont) is an aromatic polyamide (a ram id) with the repeating unit designated as (2). It is spun into... 

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. 

McFarlane, F. E., Nicely, V. A., Davis, T. G. Liquid crystal polymers. II. Preparation and properties of polyester exhibiting liquid-crystalline melts, in Contemporary Topics in Polymer Science, Vol. 2 (ed.) Pearse, E. M., Schaefgen, J. R., p. 109, London, Plenum 1977... 

A special category is formed by oriented polymers, which have considerably higher stiffnesses. The most obvious example is the textile fibre the orientation, frozen-in in a crystalline structure, raises E by a factor of 3 to 5. Extremely high orientations, as met in liquid-crystal polymers (LCP s) result in even higher E-values, namely 60 to 120 GPa ... 

In addition to the opportunities for new materials synthesis and characterization along these lines, transport properties, rheology, and processing techniques for liquid crystal polymers are essentially unexplored. Experiences with synthesis of polymer structure based on these liquid crystal templates may open up other creative avenues for template synthesis, for example, inside other crystalline structures, chlathrates, or zeolites, or on surfaces [4], Composites, alloys, or mixtures of liquid crystalline and flexible polymers may produce new materials. 

Chapoy LL (Ed), "Recent Advances in Liquid Crystalline Polymers", Elsevier Appl Science Publisher, London, 1985. Ciferri A, Krigbaum WR and Meyer RB, (Eds) "Polymer Liquid Crystals", Academic Press, New York, 1982. Gordon M and Plate NA (Eds), "Liquid Crystal Polymers", Advances in Polymer Science 59, 60 and 61, Springer, Berlin and New York, 1984, (with important contributions of Flory PJ Uematsu I and Y Papkow SP Ober CK, Jin JJ and Lenz RW Wunderlich B and Grebowicz J Dobb M and McIntyre JE Finkelmann H and Rehage G and Shibaev VP and Plate NA). 

Most polymers fall in the class of translucent resins. These include acetal, polyamide, polybutylene terephthalate (PBT), polyethylene, and polypropylene as examples. There are very few neat polymers that are truly opaque (this depends on thickness as well). Liquid crystal polymer (LCP) is an example of a typically opaque polymer. It is theorized that these semicrystalline and crystalline resins will scatter some portion of incident light due to spherulitic crystal structure and the amorphous-crystalline region interfaces themselves. 

The data in Table 23.2 show that for these crystalline resins, the contrast ratio generally increases with increased degree of crystallinity. Nylon 6,6 has the lowest contrast ratio (less opaque) compared to liquid crystal polymer, which has an... 

Liquid crystal polymers (LCPs) have been a source of considerable interest for some time, as they have been shown to offer particular advantages in terms of their processability and physical properties which make them attractive in a wide range of engineering applications.346 Serrano and his colleagues have reviewed metallomesogenic polymers, including the liquid crystalline properties of several of the platinum poly-yndiyl polymers described above.85,86... 

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 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. 

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