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2.3 Lyotropic Main Chain Liquid Crystalline Polymers

2.3 Lyotropic Main Chain Liquid Crystalline Polymers. Formed by dissolution of amphiphilic polymer molecules in appropriate solvents. Factors affect the development of lyotropic solutions Structure of the polymer, which should be quite rigid Molar mass of the molecules

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2.3 Lyotropic Main Chain Liquid Crystalline Polymers

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  1. 2.3 Lyotropic Main Chain Liquid Crystalline Polymers Formed by dissolution of amphiphilic polymer molecules in appropriate solvents. • Factors affect the development of lyotropic solutions • Structure of the polymer, which should be quite rigid • Molar mass of the molecules • Solvent, temperature and most importantly • One of the most important groups of synthetic polymers is the aromatic polyamides • Rigid structure from the ring systems coupled by the amide link • Extended conjugation between the phenyl rings and the coupling unit with a trans conformation

  2. Solvent Systems For poly(L-glutamate)s: dioxane, methylene chloride For aromatic polyamides: protonating acids H2SO4, CF3SO3H, CH3SO3H; strong aprotic solvents hexamethylene phosphoramide (HMPA), dimethylacetamide (DMAC), or N-mehtyl pyrrolidone (NMP) with a small amount of LiCl or CaCl2 • Characteristic viscosity behavior • Viscosity increases with the increase in polymer concentration • Sharp decrease in the viscosity at a critical polymer concentration because of the formation of the oriented nematic domains. The additional chain orientation in the direction of the fiber long axis leads to a dramatic enhancement of the polymer properties. Alternatives to metal or carbon fibers.

  3. 2.4. Thermotropic Main Chain Liquid Crystalline Polymers • Many of thermotropic main chain liquid crystal polymers are polyesters that are synthesized by condensation reactions • Interfacial polymerization • High temperature solution polymerization • Ester interchange reaction in the melt • Commonly used monomer units • Hydroxybenzoic acid • p-Terephthalic acid • 2,6-Naphthalene dicarboxylic acid • 4,4’-Biphenol

  4. Polymer prepared by condensation polymerization tends to be • Very insoluble • High melting point • Mesomorphic transitions at high temperatures • Difficult to process • The melting points of the main chain liquid crystal polymers can be reduced in a number of different ways: • Incorporation of flexible spacer units • Copolymerization of several mesogenic monomers of different sizes to give a random and more irregular structure • Introduction of lateral substituents to disrupt the chain symmetry • Synthesis of chains with kinks, such as unsymmetrically linked aromatic units

  5. The use of flexible spacers is a popular approach • Flexible units of varying length space the mesogens along the chain and reduce the overall rigidity. • The bridging groups in the repeat units must be rigid to maintain the overall stiffness of mesogens, and they are unusually multiple bond units. • Ester groups also serve this purpose, particularly when in conjunction with aryl rings where the conjugation leads to stiffening of the overall structure. • Small variation in structure can lead to formation of different mesophase. • When the number of methylene unit (n) in the spacer is odd, a nematic phase is observed. • When n is even, a smectic mesophase results.

  6. For polyesters with multiple rings but different orientation of the ester units, the phase can also be changed. The introduction of a flexible spacer can lower the melting point and increase the temperature rang in which the mesophase is stable. Both Tm (melting point) and Ti (isotropization temperature) decrease as n increases. The polymers with spacers having an even number of CH2 units usually have higher Tm and Ti than those with an odd number. The long rang ordering will tend to try and maintain the orientation of mesogen parallel to the director axis and this may be easier for even numbered CH2 unit spacers if they are in the all trans zig-zag conformation.

  7. Modification of poly(ethylene terephthalate) by reacting the preformed polymer with p-acetoxybenzoic acid.

  8. Introducing a mesogenic unit to the structure at the points where the two units combine, producing a thermotropic liquid crystal polymer with a flexible spacer. At ~30 mol % incorporation, a nematic phase appears in the melt. The optimum mechanical properties are obtained when 60 to 70% of the oxybenzoate is present in the chain (a large increase in the tensile strength accompanied by a corresponding decrease in the melt viscosity). Introduction of a lateral substituent into the mesogen will also lower Tm and Ti, as the bulky side groups will tend to force the chains apart thereby reducing the intermolecular forces of attraction.

  9. The introduction of kinks by using meta substituted monomer or a crankshaft monomer such as 6-hydroxy-2-naphthoic acid (HNA) can be equally effective. Other strategies for disrupting chain symmetry include the use of cross-shaped molecules or discotic mesogens. Plasticization of polyester with a small liquid crystal molecule to lower the processing temperature. Bridging groups other than esters are also used.

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