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Synopsis Because the field of plastics is one of the fastest changing areas today, the need arises to offer relevant, comprehensive material on polymers. Buy New Learn more about this copy. International Edition. Customers who bought this item also bought. New Quantity Available: 2. Seller Rating:. New Quantity Available: 5.
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This is shown in Figure 5. Both the decrease in viscosity and appearance of opaqueness arise from the onset of liquid crystallinemorphology, which in turn is due to increased backbone rigidity. Liquid crystal melts orsolutions have, in consequence, lower viscosities than melts or solutions of random-coil polymers. An equally important observation for the above copolyester LCPs is that the ordered arrangement ofpolymeric mesophases in the melt is retained upon cooling, which is manifested in greatly improvedmechanical properties see Figure 5. The liquid crystalline behavior is therefore advantageous fromthe standpoint of both processing and properties.
Thermotropic liquid crystal copolyesters of structuressimilar to I are now available commercially. Ordered behavior is also observed in solutions of some liquid crystal polymers lyotropic LCPs. After Lenze, R. News, 11, These form domains that are anisotropic and within which there is nematic order of thechains.
There is, however, no directional correlation between the domains themselves unlessthe solutions are shared. Lyotropic LCPs exhibit quite characteristic viscosity behavior in solution as the polymerconcentration in solution is changed.
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Typically the viscosity follows the trend shown in Figure 5. As more and more polymer is added to the solvent, the viscosity increases while the solution remainsisotropic and clear. At a critical concentration which depends on the polymer and the solvent thesolution becomes opaque and anisotropic and there occurs a sharp fall in viscosity with furtherincrease in the polymer concentration. After Morgan, P. Tensile strength of Kevlar see Table 4.
Light-weight bulletproof vests aremade containing up to 18 layers of woven Kevlar cloth. In fact, major drawbacks to the type of rigid polymers that exhibit liquid crystallinebehavior are that they have a very high melting point—e. The melting points of LCPs can be reduced in a number of different ways schematically represented inFigure 5. Copolymerization of several mesogenic monomers of different sizes to give a random and more irregular structure Figure 5. Introduction of kinks in the main chain, such as by using meta substituted monomers Figure 5.
The mesogenic moiety consists of two cyclic units, normally joined by a short rigid bridginggroup. The schematic chemical constitution ofthese main chain LCPs, together with example of the various types of groups that have been used areshown in Table 5. The bridging groups are usually multiple bond units, because they must be rigid to maintain theoverall stiffness of the mesogens.
Ester groups also serve this purpose, particularly when the cyclic unitsare aryl rings where the conjugation leads to a stiffening of the overall structure II : OC CO O O ll Many of the examples of thermotropic main-chain LCPs are polyesters that are synthesized bycondensation reactions including interfacial polymerizations, or by high-temperature solution polymer-izations using diols and diacid chlorides.
However, the preferred method is often an ester interchangereaction in the melt. Among the commonly used monomer units are p-hydroxybenzoic acid, terephthalicacid, 2,6-naphthalene dicarboxylic acid, 2-hydroxynaphthoic acid, and 4,biphenol. The spacer units are usually introduced by acopolymerization reaction and the proportion of the spacer units relative to the mesogens can be variedresulting in alteration of the melting point and the temperature range of mesophase.
The majority of the main chain LCPs having group arrangements as shown in Table 5. Thus for polyesters with the following structures: O O C CH2 n C OO X A nematic phase is observed when the number of methylene units n in the spacer is odd, but asmectic mesophase results when n is even. Similarly, for polyesters with multiple rings, the phases can benematic or smectic depending on the orientation of the ester units, e.
As shown schematically in Figure 5. After Krigbaum, W. Macromolecules, 16, mesogens attached as side chains can exhibit Thisis illustrated in Table 5. A typical side-chain LCPstructure would thus be that shown at the top of Table 5. Structures of this type can be synthesized in anumber of ways. One such scheme is shown in Figure 5.
It is generally observed that as longer spacer units are introduced, the Tg of the polymer is lowered byinternal plasticization and the tendency for the more ordered smectic phase to develop is increased. Asimilar ordering effect is also encouraged by lengthening the alkyl tail unit see Table 5. The ordered state of the mesophase in the aforesaid crystalline polymers is readily frozen and lockedinto a glassy state if the temperature is rapidly brought down below the Tg and remain stable until heatedabove Tg again. The phenomenon offers several interesting application possibilities in optoelectronicsand information storage discussed latter.
Whereas the relaxation time for this transition in low-molecular-weight mesogens is of the order ofseconds, this may be several orders of magnitude larger in polymer systems due to viscosity effects. Though this makes the use of polymeric liquid crystals less attractive in rapid-response display devices,the additional stability that can be gained in polymeric systems can be distinctly advantageous for someapplications as thermorecording in optical storage systems.
This structure can often be detected in mesogenic systems containing a chiralcenter and hence is also called the chiral nematic state. The structure, as shown in Figure 5. Polymers: Chemistry and Physics of Modern Materials. Chiralnematic LCPs have thus been synthesized as side-chain polymers by introducing a chiral unit in the tailmoiety of mesogens or by copolymerizing cholesterol-containing monomers with another potentialmesogenic monomer. After Folmer, B. With permission. For VI , theincrease in cholesterol content apparently causes the helix twist to become tighter, i.
The pitch is also sensitive to temperature. When the temperature israised, the helix tends to unwind i. Like the other side-chain LCPs described in the previous section, these materials also offer thepossibility of locking the chiral nematic phase into the glassy state by rapid cooling to temperatures belowTg. Xydar injection-molding resins are aromatic polyesters based on terephthalic acid, p-hydroxybenzoic acid, and p,p0-dihydroxybiphenyl. Blackie, Glasgow and London.
An advantage of Xydar, however, is that it offers high retention of mechanical propertieseven at temperatures in excess of C. Vectra isan aromatic polyester condensate derived from 2,6-dihydroxynaphthalene. Its main advantages are itsfast cycling and ease of processing, the melt temperature for both injection and extrusion grades beingtypically about C. Cycle time of Vectra can be less than half that of other engineering plastics, i. This is due to a number of factors. Since the melt is already ordered, partsrequire little or no crystallization time.
LCPs moreover have an exceptionally low heat of transition; solittle heat of crystallization must be removed, thereby allowing for fast cooling. And because compoundshave excellent stiffness at high temperatures, parts are stiff enough to eject without cooling. Excellent physical properties, low combustibility andsmoke generation, resistance to chemicals and solvents, and stability towards radiation are some of theimportant properties of LCPs.
These LCPs also retain theirexcellent mechanical properties when used for long periods of time at elevated temperatures. The addition of bulk lubricants reduces the melt viscosity. For example, LCPs have a modulus from 1.