Polymeric liquid crystals—macromesogens

Ethylene (ethene) (1) is a gas composed of small molecules however, on polymerisation a sohd, polymer material (2) is produced. Poly(ethylene) (2) is a very simple polymer yet it is probably the most common polymer. The monomer unit is a gas and yet when polymerised the material can be soft and pliable or hard and strong depending upon the degree of polymerisation (determined by the manner of synthesis, see Chapter 8). Poly(metl9rl methacrylate) (4) is, in principle, a similar polymer to poly(ethylene) because the monomer stmctures are both alkenes. However, poly(methyl methacrylate) is a polar derivative of poly(ethylene) and the properties of the two materials are distinctly different. Poly(metltyl methaciylate) is hard and transparent and is known as Plexiglas or Perspex. Polymers 2 and 4 are both called addition polymers (see Chapter 8). 

Polymers can also be prepared by a condensation approach where, for example, two monomer units are both bifunctional. In such cases a small molecule e.g., water) is eliminated at each reaction site and long polymer chains result. Adipic acid (5) and hexametliylenediamine (6) can be polymerised by the elimination of water to produce nylon (7). Nylon is an example of a polyamide and parallels the natural protein analogues. Polyesters (e.g., Dacron or Terylene, 10) are also prepared by a condensation 

Overall, the properties of all polymers are different and highly dependent upon the manner of synthesis (see Chapter 8) some are soft and pliable whereas others are very hard and strong. Matty polymers are modified to give them better physical properties, e.g., plasticisers make polymers less brittle and UV absoibers enhance photochemical stability. 

So how do these common, useful polymers relate to liquid ciystal polymers (LCPs) Well if the alkene monomer (1) is substituted by a moiety conducive to the generation of liquid crystal phases, then the resulting polymer may exhibit liquid crystalline phases (see below). Similarly, if the methyl ester unit in monomer 3 is replaced by a long, rod-like moiety similar to the liquid crystal stractures shown in Chapter 3, then on polymerisation a liquid crystalline methacrylate polymer may result. In each case the mesogenic units are attached to the side of the polymer backbone and such systems are denoted side chain polymers. If either or both of the bifunctional units involved in a condensation polymerisation e.g., 8) are of a mesogenic nature e.g., a biphenyl or terphenyl unit), then a liquid crystalline polymer may result. In this case the mesogenic units are a part of the backbone stmcture and such systems are denoted main chain polymers. 

Liquid crystal polymers exhibit the same liquid crystalline phases and mesophases exhibited by low molar mass mesogens. However, the identification of the mesophases generated by polymers is usually far more difficult than for low molar mass materials (see Chapter 9). Usually, the nematic phase is readily characterised but smectic phases, especially the highly ordered analogues, are often uncharacterised and simply denoted S Many liquid crystal polymers, like conventional polymers, exhibit a glass 

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Polymeric Liquid Crystals—Macromesogens 103 Table 5.2. The influence of terminal chain on mesomorphic properties. 

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