Cybotactic nematics

Fig. 1.1.3. X-ray diffraction patterns from a cybotactic nematic bis-(4 -n-octyloxybenzal)-2-chloro-l,4-phenylenediamine a) an unaligned (though not quite randomly oriented) sample (b) a well aligned sample. (De Vries. )...

Smectic and cybotactic nematic systems may be expected to show an oscillatory density profile near the surface. That this is the case has been strikingly demonstrated by the very fine X-ray reflectivity measurements of Pershan et (figs. 2.7.3 and 2.7.4). Surface-induced smectic... 

Francescangeli, O. Samulski, E. T. Insights into the cybotactic nematic phase of bent-core molecules. Soft Matter IdKl, 6, 2413-2420. 

As in the case of linear polymers (see Qiapter 5), a comparative analysis of comb-shaped LC polymers should be conducted by comparing samples with DP above the critical value, i.e., using samples in which is not dependent on DP. In addition, the effect of DP on the type of mesophase which they can form was recently found on the example of acrylic polymers with cyanobiphenyl groups. The low-molecular-weight fractions of the polymer (DP = 20-70) only produce a nematic or cybotactic nematic type of mesophase, while the formation of a smectic mesophase is characteristic of fractions with a higher molecular weight (cf. Fig. 6.7a). 

The formation of die fan-shaped textures typical of low-molecular-weight and polymer smectics is characteristic of some nematic polymers [40]. Such polymers should apparently have some elements of lay ed ordering which diffra from those known for low-molecular-weight cybotactic nematics. 

Cybotactic nematic phases show a short-range, but very definite, smecticlike ordering of the molecular centres in planes. They are the following two types ... 

Secondary nematic structure was found by Blumstein and cowork-gj.gi29,243 during the X-ray investigation of polyester prepared from 4,4 -hydroxy(2,2 -methyl)azoxybenzene and dodecanedioic acid. The diffraction pattern of the nematic shows the development of enhanced order characteristic of smectic C phase the first of the meridional arcs splits up into four spots. This phenomenon is incompatible with the classical definition of the nematic phase and suggests an additional order of macromolecules within cybotactic groups for which a structural model was proposed. Cybotactic nematic mesophases were also found for the copolyesters of the same series containing spacers with an even number of methylene units, but ordinary nematic phases were identified for the members with odd-numbered spacers. ... 

Cybotactic nematic phases were also reported for SCLCPs. " 246-248,255 jhggg re polymers in which pronounced smectic order parameter fluctuations occur. This is especially true of polymers which exhibit an Sa or a Sc phase at lower temperatures. 

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