Lyotropic nematic solvents

A most interesting lyotropic nematic solvent which is suitable as anisotropic solvent for very polar molecules has been proposed by Lawson and Flautt [12], and Black et al. [13]. A typical nematic phase of this type is composed of 35 wt% sodium decyl sulfate, 7 wt% n-decyl alcohol, 7 wt% sodium sulfate and 51 wt% water (D2O in nmr experiments). This solvent is stable between 10°C and 75°C. Moreover, these mixtures can be easily oriented homogeneously by magnetic fields. 

A lyotropic, nematic solution of cellulose was formed in a NH3/NH4SCN solvent in what are presumably good solvent compositions. Evidence strongly suggests that the twisted nematic or cholesteric structure that results when solutions of chiral cellulose chains interact may be repressed or compensated so that interactions among chiral centers are minimized. Our reasoning is based a body of experimental evidence which includes ... 

However, more remarkable was the discovery that in alkane solvents, large, orrfiometallated macrocyclic complexes of palladium shown in Figure 92 would form lyotropic columnar phases [171], These remarkable materials have been shown to form columnar hexagonal phases and, in suitable solvents, lyotropic nematic phases derived from columnar organization. Further, in certain non-mesomorphic examples, mesophases can be induced by the addition of an electron acceptor such as trinitrofluorenone chiral phases are introduced when the acceptor is resolved 2 -(2,4,5,7-tetranitro-9-fluorenylideneaminoxy)propionic acid (know as TAPA). 

The precipitation process is assumed to lead either from the lyotropic, nematic liquid crystalline state via predpitation with water under maintenance of ordered water-polymer layers to the crystal form II, or with other solvents (and also from low polymer concentrations with water) through disordered solvent-polymer structures to crystal form I In both crystal structures the ultimate polymer crystal layers are H-bonded in the aystallographic bc-plane (100), as shown in Fig. 6.2. In crystal form 1 the second molecular chain goes through the center of the unit cell (Pn or P2j/n space group, 2 chains per unit cell, monoclinic, pseudo-orthorhombic)... 

In addition, these complexes, except 49a and 50a, form lyotropic columnar (oblique) and nematic phases when dissolved in linear, apolar organic solvents (alkanes) over wide temperature and concentration ranges. Interestingly, for some of them, 49b-c, an unexpected transition between two lyotropic nematic phases has been observed, for which a model has recently be proposed [93]. As for 48, formation of lyotropic nematic and columnar mesophases is also extended by n-n interactions with electron-acceptors, such as TNF, in apolar solvents (pentadecane). Induction of chiral nematic phases by charge transfer interactions, in a ternary mixture (49b/alkane/TAPA TAPA is 2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)-propionic acid and is used (and is available commercially) enantiomerically pure), has recently been demonstrated for the first time [94], and opens new perspective for producing chiral nematic phase of disc-like compounds. 

Analogous ochre-yellow polymers 4.26 that contain thiophene in the main chain (Mw= 11,300-35,300 PDI = 1.5-2.7) have also been prepared [61]. Upon slow concentration of their solutions in chlorinated solvents, such as chloroform, lyotropic nematic liquid-crystalline phases form. In one case, a lyotropic smectic state was identified. 

Recently, there are several reportsA.) discussing the twist mode of the binary system of the thermotropic liquid crystals composed of a nematic solvent and a cholesteric solute. Another interest is in attempting to pursue the structural similarity between thermotropic liquid crystals and lyotropic one, especially for the dependence of cholesteric pitch on the temperature. 

The extrapolated line of log S-log C crossed each other at a critical concentration Cq at which S stays constant and independent of temperature. These results suggest that the temperature dependence of the cholesteric pitch would inflect at the concentration higher than Cq This is analogous to the behavior of thermotropic liquid crystals composed of cholesteric solute and nematic solvent, where the sign of dS/dT reverses at a critical concentration. It is understood that the behavior of both thermotropic and lyotropic liquid crystals is comparable provided that the nematic substances of the former are substituted with the solvents of the latter. The critical concentration Cq is about 0.41 vol/vol and this value is very close to the concentration at which the side chains on neighboring molecules of the polymer come to contact each other ( refer to fig.5 ). From these results, it is expected that the origin or mechanism of twist would change at this concentration Cq. The... 

Finally it shall be mentioned that until now no inverse nematic phases could be detected. Also the existence of nematic phases in non-polar solvents could not be proven for any studied system. Thus it is not yet possible to replace the thermotropic nematic phases in displays by lyotropic systems because lyotropic nematics caimot be orientated in an electric field with low voltages due to the rather high conductivity of the water solvent compared to non-polar organic compounds. 

New compounds containing two or even four palladium atoms and eight or twelve flexible side chains have been synthesized by Praefcke et al. [138]. The compound, 58, shows an enantiotropic discotic hexagonal columnar mesophase over a wide range of temperature and a lyotropic nematic phase in various organic solvents. X-ray diffraction studies reveal an oblique arrangement of disordered columns. 

Lyotropic nematic phases (see Section A) can also be produced by preparing, for instance, binary or ternary mixtures of organic disc-like compounds in suitable solvents such as hydrocarbons [20]. In linear saturated [20,21] or, as found recently [21], even better in cyclic saturated hydrocarbons, preferably cyclohexane [21], almie or in such a solvent plus an achiral or a chiral electron acceptor compound, induction of lyotropic Ncd or N coi phases, respectively, can occur. Sometimes, an Ncd phase can be formed in addition to a columnar phase [21]. Furthermore, it has also been observed that even two different No>i phases can be induced in diat way in the same system [22,23] showing a nematic-nematic phase transition [22-24] due to a diffa ence in the construction of their columns. In one of these Ncoi phases the constituent discs of the columns spontaneously formed are tilted with respect to the column axis, but in the second, parallel Ncoi phase they are untilted [22,23]. However, reliable data about the length of the columns in Ncoi phases do not yet seem to exist... 

Reinitzer discovered liquid crystallinity in 1888 the so-called fourth state of matter.4 Liquid crystalline molecules combine the properties of mobility of liquids and orientational order of crystals. This phenomenon results from the anisotropy in the molecules from which the liquid crystals are built. Different factors may govern this anisotropy, for example, the presence of polar and apolar parts in the molecule, the fact that it contains flexible and rigid parts, or often a combination of both. Liquid crystals may be thermotropic, being a state of matter in between the solid and the liquid phase, or they may be lyotropic, that is, ordering induced by the solvent. In the latter case the solvent usually solvates a certain part of the molecule while the other part of the molecule helps induce aggregation, leading to mesoscopic assemblies. The first thermotropic mesophase discovered was a chiral nematic or cholesteric phase (N )4 named after the fact that it was observed in a cholesterol derivative. In hindsight, one can conclude that this was not the simplest mesophase possible. In fact, this mesophase is chiral, since the molecules are ordered in... 

Liquid crystals are classified into lyotropic and thermotropic crystals depending on the way in which the mesomorphic phase is generated. Lyotropic liquid-crystalline solvents are formed by addition of controlled amounts of polar solvents to certain amphiphilic compounds. Thermotropic liquid-crystalline solvents, simply obtained by temperature variations, can be further classified into nematic, smectic, and cholesteric solvents depending on the type of molecular order present. Liquid crystals are usually excellent solvents for other organic compounds. Nonmesomorphic solute molecules may be incorporated into liquid-crystalline solvents without destruction of the order prevailing in the liquid-crystalline matrix (Michl and Thulstrup, 1986). Ordered solvent phases such as liquid crystals have also been used as reaction media, particularly for photochemical reactions (Nakano and Hirata, 1982). 

Both lyotropic and thermotropic liquid-crystalline synthetic polymers have been widely studied. Aromatic polyamides constitute the most important class forming liquid-crystalline solutions the solvents are either powerfully protonating acids such as 100% sulphuric acid, chloro-, fluoro- or methane-sulphonic acid, and anhydrous hydrogen fluoride, or aprotic dipolar solvents such as dimethyl acetamide containing a small percentage, usually 2-5 %, of a salt such as lithium chloride or calcium chloride. Such solutions constitute a nematic phase within certain limits. Some criteria for formation of a nematic instead of an isotropic phase are ... 

B) Flory s lattice theory (1956) has received most attention. It is an extension of Onsager s model to higher concentrations. It is ideally suited for lyotropic LCPs consisting of solvent and rigid rods. It predicts that at a phase transition an isotropic and a nematic phase coexist with respective volume fractions... 

Lyotropic liquid crystalline properties were observed for poly (arylamide) monodendrons derived from 1,3,5-substituted amino acid chloride hydrochlorides according to Kim [168]. These systems aggregate readily to form high molecular weight species in the absence of complexing ions. Amide solvents containing more than 40 wt.% of the dendrons exhibit nematic phase liquid crystalline texture under a polarizing microscope, whereas, 60 wt.% solutions produce a hard gel under static conditions. 

One of the characteristic properties of rod-like polymers is that their concentrated solutions form lyotropic liquid crystals51. Such examples among synthetic polymers are polyamides52,53 and polyisocyanates54 which form cholesteric or nematic liquid crystals in selected solvents. 

While the possibility of this dispersion of micro-domains of the nematic phase in an isotropic phase cannot be dismissed, concrete evidence for morphologies of this kind in nematogenic copolymers is not prominently in evidence. The longer sequences of rigid units undoubtedly are responsible for promotion of liquid crystallinity but, as theory suggests they appear to be uniformly dispersed Sequences of units that include many flexible members and hence are not rodlike may assume a role analogous to that of the solvent in a lyotropic system The nematic copolymer should, on this basis, consist of a single phase. 

In their original theory, Maier and Saupe supposed that the molecular interactions responsible for the nematic state are anisotropic van der Waals interactions (discussed in Section 2.3), in which case mms should be temperature-independent. However, it is now recognized that shape anisotropy is also important, even for small-molecule thermotropic nematics. By making mms temperature-dependent, the Maier-Saupe potential can, in principle, accommodate both energetic and entropic effects. In fact, if the function sin(u, u) in the purely entropic Onsager potential Eq. (2-5) is approximated by the expansion 1 — V2 cos (u, u)+. . ., then to lowest order the Maier-Saupe potential (2-7) is obtained with C/ms — Uo bT/S, where we have defined the dimensionless Maier-Saupe energy constant by Uus = ums/ksT, Thus, the Maier-Saupe potential can be used as an approximation to describe orientational order in either lyotropic (solvent-based) or thermotropic nematics. For a thermotropic melt, the Maier-Saupe theory predicts a first-order transition from the isotropic to the nematic phase when mms/ bT = U s — t i.MS = 4.55, and at this transition the scalar order parameter S jumps from zero to 0.43. S increases toward unity with further increases in Uus- The spinodal point at which the isotropic phase is unstable to even small orientational perturbations occurs atU — = 5 for the Maier-... 

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