Hydrocarbon side-chain liquid-crystalline polymers

Hydrocarbon Side-Chain Liquid Crystalline Polymers... 

Chang-Chien, G., Terminally carboxyl oligo(ethylene oxide) monomethyl ether-substituted side chain liquid crystalline polysiloxane polymer as stationary phase in capillary gas chromatography for the separation of polynuclear aromatic hydrocarbons, J. Chromatogr. A, 808, 201-209, 1998. 

Polymer A showed a crystalline X-ray diffraction pattern and an endothermic peak at 31-42 C in differential scanning thermograms. The polymer had birefringence below 90 C under a polarizing microscope. It is of interest to speculate as follows. Both of the hydrocarbon side chains and the polysaccharide main chains are crystalline and the polymer A exhibits a unique two-stage melting process of both crystalline regions. The polymer possibly forms a liquid-crystalline mesophase between these two transition temperatures. 

Cho et al. described the synthesis and polymerization of 4,8-cyclododeca-dien-l-yl-(4 -methoxy-4-biphenyl) terephthalate VIII [54,55]. Polymerization was carried out with WCl4(OAr)2/PbEt4. The double bonds in the polymer backbone were subsequently hydrogenated with H2/Pd(C), leading to a SCLCP with a fully saturated hydrocarbon backbone. This polymer system had a very flexible polymer backbone but a stiff connection between the main chain and the mesogenic unit. The distance between two adjacent side chains was about 12 methylene units. This very flexible main chain allowed the polymer to organize into a LC mesophase. Both polymers - the unsaturated and the saturated -showed smectic liquid crystalline mesophases with almost the same transition temperatures (see Table 5). 

To summarize our results on side-chain all-hydrocarbon LC polymers [I], we found that they displayed Smectic E phases at lower temperatures and Smectic B phases at higher temperatures. The corresponding LC monomers exhibited only Smectic B phases. The presence of tails seems to enhance and stabilize liquid crystallinity. The monomers and polymers that do not have tails are not liquid crystalline. All of the substituted biphenyl hydrocarbon liquid crystals found in the literature are disubstituted. Examples of monosubstituted biphenyl hydrocarbon liquid crystals have not been found. The results obtained from this work support the idea that disubstitution of the biphenyl moiety is necessary to obtain a liquid crystalline phase. 

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