Aliphatic—aromatic liquid-crystalline

Aliphatic-aromatic liquid-crystalline polymers, synthesis, 16-17 Aliphatic glycol polyesters experimental procedures, 17-18 properties, 17... 

Aharoni S M (1988), Hydrogen-bonded highly regular strictly alternating aliphatic aromatic liquid-crystalline poly(ester amides) . Macromolecules, 21 (7), 1941-1961. 

Figure 1 demonstrates the drastic influence on the stability region of a lamellar liquid crystalline phase when an aromatic hydrocarbon is substituted by an aliphatic one. The lamellar phase formed by water and emulsifier is stable between 20 and 60 wt % water. Addition of an aromatic hydrocarbon (p-xylene) to the liquid crystalline phase increased the maximum amount of water from 45 to 85% (w/w) (Figure 1 left). Inclusion of an aliphatic hydrocarbon (n-hexadecane) gave the opposite result the maximum water content in the liquid crystalline state was reduced (right). Some of the factors which govern the association behavior of these surfactants and cause effects such as the one above are treated below. 

Figure I. Difference in the phase region of the lamellar liquid crystal (black) when an aromatic hydrocarbon (left) is replaced by an aliphatic one (right) demonstrates the sensitivity of the lyotropic liquid crystalline structure to weak intermodular forces. The emulsifier is a polyoxyethylene (9) nonyl phenol ether.

The source of all carbon relevant to the present context is the feedstock of hydrocarbon molecules (aliphatic, aromatic, with and without heteroatoms). Figure 10 summarizes the possibilities for their conversion into black carbon. The chemical route comprises polymerization into aromatic hydrocarbons with final thermal dehydrogenation. This process often includes a liquid crystalline phase immediately before final solidification. In this phase large aromatic molecules can sclf-organizc into parallel stacks and form well-ordered precursors for graphitic structures with large planar graphene layers. This phase is referred to... 

The lowest cost process for preparing all-arcmatic liquid crystalline polyesters involves the reaction of aromatic carboxylic acids with acetates of aromatic hydroxy ocnpounds a recent history (2) describes the development of these ICP s. Because aoetic acid is evolved in the process and reaction temperatures are above 300°C, expensive corrosion-resistant reactors must be installed for ccranercial production. In cur latest paper (2) of this ICP series, we described a number of aliphatic-aromatic ICP s which can be produced in conventional polyester reactors and injection molded to give plastics with very high mechanical properties, heat-deflection temperatures (HDT s), and solvent resistance. These ICP s (la) were prepared by the reaction of the dimethyl ester of... 

Weiss and coworkers have conducted thorough investigations of the Norrish n reaction of several aromatic (1,4-7) and aliphatic (2,3) ketones in the isotropic, liquid crystalline, and solid phases of BS (27-29) and CCH-4 (30,49). Their experiments using BS as solvent demonstrate particularly elegantly the remarkable correspondence between solute/mesogen structural similarities and the ability of smectic (and solid) phases to control solute conformational motions. 

By far the largest group of liquid crystalline polymers containing imide groups are copoly(esterimide)s (PEIs). These have been sub-divided into flexible copolymers containing aliphatic spacers and wholly aromatic copolymers. There are several reports of copoly(amide-imide)s and copoly(ether-imide)s (both thermotropic and lyotropic) and these are treated separately. There are only a few examples of wholly aromatic imide containing LCPs, as polyimides frequently melt well above their decomposition temperature. Successful attempts have been made to incorporate flexible spacers, but in most instances the spacers are based on ether units, and the polymer is more strictly classified as a poly(etherimide). 

In (3.65) there is more of an aliphatic component than an aromatic one. Phenylene rings are each isolated by aliphatic components. No mesogenic units of rigid-rods are present in the molecule. However, the thermotropic liquid crystalline phase was observed for many of the polymers although... 

Friberg et al. (17) demonstrated that a sudden increase in emulsion stability arose in the concentration range where a liquid-crystalline phase could be separated from the emulsion. They postulated, with good reason, that this liquid-crystalline phase was the viscous layer around the emulsion droplets that Davies had been seeking. Further, the liquid-crystal stabilization hypothesis explained the difference in stability between emulsions of aromatic and aliphatic hydrocarbons, because aromatic hydrocarbons, because of their large polarizability, are more prone to form lyotropic liquid-crystal structures than aliphatic hydrocarbons. 

The development of H-bonded complexes is now considered. An influence of UV irradiation on optical properties of low-molecular-weight azobenzene-con-taining material (Fig. 2.4b) has been investigated (Aoki et ah, 2000) on the basis of such interactions. The first observation of photoinduced optical anisotropy in H-bonded complexes of azobenzene dyes and copolymers (Fig. 2.4b) has been recently demonstrated (Medvedev et ah, 2005). In this case, the induced anisotropy was stable, and the maximum dichroic ratio of 2 has been observed. A kinetics of the induction of birefringence (maximum value of ca. 0.01) in one of these complexes is shown in Fig. 2.5. An influence of H-bonding on the mesomorphic and photoorientation properties was recently demonstrated (Cui and Zhao, 2004). In this approach, the amorphous azopyridine side-chain polymer was converted into liquid crystalline polymers through self-assembly with a series of commercially available, aliphatic, and aromatic carboxylic acids (Fig. 2.4d). 

In another approach to introduce liquid crystallinity, aliphatic tails were connected to the convex side of a glycoluril-based host (23. Fig. 6a).It was reasoned that in the solid state, this host would form dimers, in a similar way as the parent compound 3b. As can be seen in Fig. 6c. such a dimer resembles the structure of a typical rod-like mesogen. Although the material formed by 23 exhibited the typical features of a liquid crystal, i.e., birefringent optical textures and a high degree of malleability, calorimetric and powder diffraction measurements showed that it was a plastic crystal composed of lamellae of dimers (Fig. 6c). Strong Ti-n interactions between the aromatic surfaces of adjacent molecules are the reason the material is not liquid crystalline. The... 

X = an aromatic or aliphatic bridging group referred to in the structure shown at the beginning of this entry. Thermal properties of liquid crystalline, bismaleimide-based ester monomers ... 

J.D. Sudha, Synthesis and characterization of hydrogen-bonded thermotropic liquid crystalline aromatic-aliphatic poly(ester-amide)s from amido diol, J. Polym. Sci. A Polym. Chem. 38 (2000) 2469-2486. 

A final example of induced mesogenicity in a multicomponent system is the well studied, but less well understood, carbonaceous mesophases which are comprised of a myriad of unidentified molecules which are created in situ as petroleum pitches are heated to temperatures where chemical transformations occur [163]. The processes leading to a mesophase involve decreases in both the elemental weight fraction of hydrogen and the group fraction of aliphatic carbon atoms [164]. Model studies have demonstrated that the component molecules of these phases are fused, polycyclic aromatic molecules with disk-like shapes the exact structures of the components depend upon the natures of the precursor molecules which are heated [164-167]. All of the carbonaceous mesophases somewhat resemble discotic nematic phases [168]. At least some of them probably represent another example of liquid crystallinity induced by mixing molecular components which, when separated, are not mesogenic. 

This family of noncovalently bonded PLCs may also include polyelectrolyte + surfactant complexes which, as will be seen, can also give rise to liquid crystalline mesophases. In these complexes there is only a flexible alkyl chain attached to the ionic head group in the small molecule constituent, with no rigid aromatic core present. Since surfactants themselves are frequently thermotropic liquid crystals, it is not surprising that their complexes with polyelectrolytes may produce PLCs, in both cases driven by the incompatibility between the ionic and aliphatic parts leading to amphitropic systems [27]. 

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