Mesogenic core

Binger and Hanna [76] have considered the alignment of 4-n-octyl-2-flurophenyl-4-n-octyloxybiphenyl-4 -carboxylate (MBF) and 8CB at room temperature on the (110) surface of polyethylene and the (100) surface of nylon. They find that alignment of both molecules is dominated by the behaviour of the flexible tails that prefer to align parallel between two polymer chains. This forces the mesogenic core to straddle one or more chains. This is illustrated in Fig. 4 for the specific case of MBF on the (110) surface of... 

ID and 2D 13C NMR were carried out in a series of novel nematogens by Bayle and co-workers to study the effects on the conformation and order due to the addition of lateral and/or terminal substituents.249 251 For example, lateral flexible substituents are found to adopt a mean conformation more or less parallel to the mesogenic core. As a result, lateral chains are less disordered than terminal chains. Nematogens containing polyoxyethylene ether (POE) chain as a lateral crown ether and terminal chain(s) have been... 

The magnetic susceptibilities of dimer liquid crystals such as NC-Ph—Ph—O—(CH2>n—Ph—Ph—CN(n 9, 10) are measured by a SQUID magnetometer. The results obtained are interpreted within the framework of the RIS approximation, the effect arising from the conformational anisotropy of the flexible spacer being strictly taken into account. The order parameters of the mesogenic core axis thus estimated are found to be consistent with those directly observed at just below 7N) by the ZH NMR technique using mesogen-deuterated samples. 

Any increase of polarity of the hydrocarbon, for example by substitution with chlorine or by replacing CH2 groups by ether oxygens, also increases incompatibility with perfluoroalkanes as polar interactions become more important [89, 96]. Polar groups are fundamental constituents of the mesogenic cores, especially as linking units and as substituents at aromatic moieties, and these groups increase the incompatibility between the aromatic cores and Rp-chains. 

Polycatenars form a broken-layer type 2D density modulated phases because of the mismatch between the cross-section areas required for the terminal chains and for the mesogenic core. For such compounds layers become locally bent to provide more space for tails and, since the bending of layers is finite, this leads to... 

Powder diffraction patterns of the two fractions displayed characteristics nearly identical to those of BP6L, that is, a sharp peak at 12.6 A and a broad peak centered at 3.8 A. The powder diffraction patterns of BP6L, BP6Ls and BP6Li have been reported previously (2). Only the THF insoluble fraction showed any indication of significant crystallinity with additional peaks centered at 3.9 A and 6 A. The presence of the same 12.6 A spacing in both patterns is due to the fact that all fractions have the same bromine-to-bromine distance in the mesogenic core. 

In LC polymers possessing mesogenic cores linked by ester groups, it is possible for transesterification to occur at elevated temperatures. In... 

The strong effect on the mesophase sequence of lateral mono- and difluorination at different sites of an aromatic mesogenic core structure is illustrated by the data in Table 4.5 [40]. The relatively moderate mesogenic properties of material 14 were dramatically improved by introduction of two lateral fluorine substituents in the liquid crystal 22. Unfortunately, this target was achieved only by a purely empirical trial-and-error process requiring a tremendous amount of synthetic work. 

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. 

If the molecular dipole moment is oriented perpendicularly to the long molecular axis, a material with negative Ae is obtained [31b]. For steric reasons simple lateral monofluorination of an aromatic moiety within the mesogenic core structure does not result in a perpendicular orientation of the molecular dipole moment. This can be achieved only by pair-wise lateral difluorination, which results in mutual cancellation of the respective longitudinal components of the dipole moment vector leaving only the perpendicular contribution. Examples of the most commonly used dielectrically negative materials based on this concept are listed in Table 4.10. 

Table 4.12 Comparison of liquid crystals containing a tetrafluoroethylene bridge in their mesogenic core structure (52, S3) with their ethylene-bridged (54, 55) and directly linked (9) analogs [41].

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