Polyphilic Liquid Crystals

Molecules composed of more than two distinct incompatible units can be regarded as polyphilic if polyphilicity is manifested by formation of a periodic LC structure where the distinct molecular segments are organized in at least three distinct nanocompartments. Some examples of polyphilic LCs were already included in the 

Most of the amphiphiles discussed hitherto have only two distinct parts in their molecules which are different in their polarity. Polyphilic compounds possess more than two of them (see Fig. 33) depicting the cal-amitic biphenyl derivative 50 with three different parts a polyfluorinated one, an alkoxy chain, and the aromatic (biphenyl) section. The mesomorphic properties of this triphilic examples are compiled in Table 15. 

The polyphilic quality leads to a further ordering of the molecules in their meso-phase Fig. 34 gives a simple schematic drawing of a directed lamellar phase. The molecules have a polar orientation within the layers and long range correlations between them. This special mesophase structure causes macroscopically polar properties. With regard to this kind of molecular arrangement some polyphilic compounds have been successfully studied in respect of ferroelectric properties [161-165]. 

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Polyphilic Liquid Crystals - Structure and Phase Behaviour. . . 223... 

Keywords Smectic liquid crystals, Molecular conformations, polar and steric frustrations, Polyphilic and perfluorinated mesogens. X-ray diffraction... 

We start with some elementary information about anisotropic intermolec-ular interactions in liquid crystals and molecular factors that influence the smectic behaviour. The various types of molecular models and commonly accepted concepts reproducing the smectic behaviour are evaluated. Then we discuss in more detail the breaking of head-to-tail inversion symmetry in smectic layers formed by polar and (or) sterically asymmetric molecules and formation of particular phases with one and two dimensional periodicity. We then proceed with the description of the structure and phase behaviour of terminally fluorinated and polyphilic mesogens and specific polar properties of the achiral chevron structures. Finally, different possibilities for bridging the gap between smectic and columnar phases are considered. 

Keywords Columnar mesophase Cubic mesophase Dendrimer Liquid crystal Metallomesogen Micro-segregation Organic semiconductor Perfluorinated molecule Polyphilic molecule Self-assembly... 

Fig. 14 (a) Chemical structures of the polyphilic dispersion-promoter molecules, (b) Tailor-designed polyphilic molecules promoting CNT dispersion in the nematic host. Pyrene anchoring group (blue), mesogenic CB unit (dark red), flexible hydrocarbon or ethylene oxide spacer (green), and liquid crystal host (light red) [464]. (Reproduced by permission of The Royal Society of Chemistry)... 

In principle, there is no reason to exclude the polar ordering of dipoles in the smectic A phase (Fig. 2b) along the molecular axis. No examples of such phases have been observed for low-molar-mass liquid crystals or for LC polymers. Tour-nilhac et al. [101] suggested an approach to ferroelectricity in the phase of polyphilic molecules consisting of aromatic, aliphatic, and perfluorinated parts they realized their approach successfully for an achiral smectic C phase only. An attempt to extend that approach to polyphilic LC copolymers [102] indicated antiferroelec-tric behavior of the synthesized materials. 

In conclusion, electric field effects in liquid crystals is a well-developed branch of condensed matter physics. The field behavior of nematic liquid crystals in the bulk is well understood. To a certain extent the same is true for the cholesteric mesophase, although the discovery of bistability phenomena and field effects in blue phases opened up new fundamental problems to be solved. Ferroelectric and antiferroelectric mesophases in chiral compounds are a subject of current study. The other ferroelectric substances, such as discotic and lyotropic chiral systems and some achiral (like polyphilic) meso-genes, should attract more attention in the near future. The same is true for a variety of polymer ferroelectric substances, including elastomers. 

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