Mesophase stability

Derivatives of aliphatic alkynes (14 and 15) are more thermally unstable than 12, but they show SmA and N phases at low temperatures (below 130 °C). The type of phase and the mesophase stability depend on the length of both the terminal and the lateral chains. When both chains are elongated, the mesomorphism becomes metastable and compounds 14 display monotropic N and SmA transitions. Complexes IS, which contains an ester group with an opposite direction to that of complexes 14, display less stable nematic mesophases. 

There has been an increasing interest in liquid crystals containing fluorine atoms because fluorination produces important changes in the melting temperatures, viscosity, birefringence, dielectric anisotropy and mesophase stability. This why structure-mesomorphism relationship studies were carried out in mono- and... 

Whereas the mesophase morphologies of compounds 35-37 depend on the size ratio of polar and apolar segments, the mesophase stabilities are mainly influenced... 

It should also be noted that the stability of the distinct mesophases can be quite different. It seems that there is a significant effect of molecular shape and topology, stabilizing SmA phases in the system 41/43 and Colhex phases in the system 35/37. In addition, the mesophase stability is often reduced close to the transition to another mesophase (see Fig. 15). Hence, the order-disorder temperatures can only be roughly estimated based on segmental solubility parameters [24, 25]. 

Besides the simple effects of mesophase stabilization by increased polar-apolar incompatibility and modification of mesophase morphology due to size effects, there are additional specific effects based on the lipophobicity (R[ -RH incompatibility) and rigidity of perfluorinated segments. In this respect it is important to... 

In all cases of these rod-like molecules the fluorination of terminal chains leads to mesophase stabilization and to stabilization of smectic phases with respect to nematic phases. In most cases an increase of the melting temperatures11 and a stabilization... 

Hydrogen bonded dimers of 4-RF-substituted benzoic acids [119, 120, 122, 126], /rcyclohexane carboxylic acids [120], and hydrogen bonded complexes of benzoic acids with 4,4 -bipyridine and stilbazole [166, 243-246] can be regarded as supramolecular rod-like mesogens with two fluorinated chains. Figures 33 and 34 show comparisons of selected benzoic acid dimers and their bipyridine and stilbazole complexes. Comparison of the benzoic acid dimers 122 and 123 with -alkyl chains and F-terminated -alkyl chains, respectively, indicates that a single F-atom at the end of the alkyl chain reduces the mesophase stability... 

LC phases usually corresponds to at least the number of attached chains [258]. Moreover, no SmA phases were observed for non-fluorinated polycatenar mesogens, whereas chain fluorination obviously enables the formation of SmA phases, which are most probably of the de Vries type. It seems that non-fluorinated polycatenars are mainly stabilized by core-core interactions, whereas the mesophase stabilization by the fluorophobic effect retains the layer structure even if core-core interactions are relatively weak. 

Fig. 74 Examples for mesophase induction and mesophase stabilization by perfluorinated aromatics (77° C) [414 -18] for the mixtures 235 + 236 there is a coexistence of nematic and isotropic phase (N+Iso)...

Percec and co-workers not only developed monomers suitable for radical polymerization but also presented systems suitable for the introduction into poly (methylsiloxane)s [34] via hydrosilylation (Scheme 12). Monomer 23 with a terminal double bond was added to the poly(methylsiloxane) using Cp2PtCl2 as a catalyst to give poly (23). The trends observed for poly (23) were also found for poly(17) as polymerization improves the mesophase stability significantly. 

A variety of other liquid crystalline crown ether containing polymers were synthesized, all possessing the same mesophase-stabilizing effect of polymerization (polymer effect) [38, 39]. 

Kato, T., Frechet, J. M. J., New approach to mesophase stabilization through hydrogen-bonding molecular-interactions in binary-mixtures. J. Am. Chem. Soc. 1989, 111, 8533-8534. 

The effect of molecular mass on occurrence of the mesomorphous phase in POCS-6 polymers is de-tected. Results displayed for POCS-6 polymers, in which configuration sequences of units alternate irregularly along the backbone, indicate that the ability to crystallization in them is disturbed. For the mesomorphous phase, strict regularity of units is not the necessary condition of its occurrence. In connection with the difference between studied POCS-6 polymers by the type of tacticity and mole-cular mass, the authors have separated effects induced by these two factors and detected more accu-rate, which type of configuration sequences creates conditions for mesophase stabilization. 

Fig. 51 Diagram showing the evolution of the mesophase stability from tolane-rich (left side) to stilbene rich (right side) octopus LCDs (R = = OC12H25, R = H)...

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