Mesogenic groups with fluorinated

Properties have been determined for a series of block copolymers based on poly[3,3-bis(ethoxymethyl)oxetane] and poly [3,3-bis(methoxymethyl)oxetane]- (9-tetrahydrofuran. The block copolymers had properties suggestive of a thermoplastic elastomer (308). POX was a good main chain for a weU-developed smectic Hquid crystalline state when cyano- or fluorine-substituted biphenyls were used as mesogenic groups attached through a four-methylene spacer (309,310). Other side-chain Hquid crystalline polyoxetanes were observed with a spacer-separated azo moiety (311) and with laterally attached mesogenic groups (312). 

The difference between the hydrogenated and fluorinated compounds lies in the fact that the former give only N and SmC phases (see Sec. 4.1.4. of this chapter) while the latter display SmA and SmC phases with a perfluorinated (31a) chain and SmC or SmC phases for a semi-perfluorinated chain (31b). First discovered in 1982 with polar rod-like molecules [39], the ribbon SmC phase was then found in low molecular weight mesogens devoid of a CN or NO2 group. With their perfluorinated stiff rod, the compounds 31a are similar to polar... 

It is noteworthy that the formation of a mesophase was sustained in the copolymers even incorporating low proportions of mesogenic fluorinated side groups. Moreover, the introduction of repeat co-units with hexyl substituents lowered Tg by internal plasticization and resulted in a somewhat broader mesophase range for the copolymers relative to the fluorinated homopoymer (e.g. T -Tg = 164°C for copolymer). 

For a typical phenylcyclohexane-based mesogenic core structure, this results in a Ae of ca 4. The dielectric anisotropy is increased if one or two additional fluorine substituents are introduced ortho to the terminal group (Table 4.6). For larger systems with three or more phenylene moieties in their mesogenic core structure a further increase of the dielectric anisotropy is achieved by using the additional rings as a scaffold for attachment of further lateral fluorine substituents. 

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