Two series of monomers, 4-[4-(allyloxy)benzoyloxy]phenyl-3-(perfluoro-n-alkyl)ethyl propanoates and 2-(perfluoro-n-alkyl)ethyl-4-[4-(undec-10-en-1-oyloxy)-benzoyloxy] benzoates have been synthesised. These compounds contain a fluorinated chain obtained from 2-(perfluoro-n-alkyl)ethanol or from 2-(perfluoro-n-alkyl)ethyliodide, and an aliphatic chain containing a double bond attached to a mesogenic moiety. Their hydrosilylation with polymethylhydrogenosiloxane gives the corresponding polysiloxanes with a spacer with three or ten methylene units. These new fluorinated compounds are characterized by a combination of techniques consisting of differential scanning calorimetry (DSC) and thermal optical polarized microscopy. All the monomers exhibited smectogenic properties of type A but for the monomers with a long spacer between the unsaturated function and the mesogenic core (compounds B), the mesophase range decreases dramatically. All of the corresponding polysiloxanes exhibited a liquid crystalline behaviour over a wide temperature range. The influence of the fluorinated chain lengthening leads for the monomers and the polymers to an increasing of the transition temperatures.
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Bracon et al. (1999) studied this question.
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