Mesogenic pitch

Huttinger and Christ [58] based their studies on liquid phase impregnation of a fiber preform with pitches of different contents of mesophase spherulites. The matrix formed was mainly mesophase because the isotropic mesogenic pitch was pressed out. The composites were stabilized by treatment with air before carbonization of the matrix in order to avoid swelling. After graphitization at 2100°C, flexural strengths up to 650 MPa were measured. 

MESOPHASE PITCH BASED CARBON FIBERS (MPP BASED CARBON FIBERS) are CARBON FIBERS obtained from MESOGENIC PITCH after it has been transformed into MESOPHASE PITCH (MPP) at least during the process of spinning, after the spun MESOPHASE PITCH fibers have been made non-fusible (stabilized) and carbonized. 

Mesogenic pitch is a pitch with a complex mixture of numerous essentially aromatic hydrocarbons. It does not contain anisotropic particles detectable by optical microscopy. Mesogenic pitch is low in quinoline-insoluble fractions and capable of transforming into MPP during continuous heat treatment above 750 K by the formation of optically detectable carbonaceous mesophase. 

See bulk mesophase, carbonaceous mesophase, mesogenic pitch, pitch... 

The carbonaceous mesophase particles are formed from the aromatics of high molecular mass in mesogenic pitch, which have not yet been aggregated to particles detectable by optical microscopy within the apparently isotropic pitch matrix. The carbonaceous mesophase is insoluble in quinoline and pyridine, but the amount of I mesophase measured from microscopical observation appears somewhat higher because parts of the carbonaceous mesophase can be extracted by the solvents. 

See carbon fibers, mesogenic pitch, MPP, pitch, pitch-based carbon fibers... 

Chiral nematic Hquid crystals are sometimes referred to as spontaneously twisted nematics, and hence a special case of the nematic phase. The essential requirement for the chiral nematic stmcture is a chiral center that acts to bias the director of the Hquid crystal with a spontaneous cumulative twist. An ordinary nematic Hquid crystal can be converted into a chiral nematic by adding an optically active compound (4). In many cases the inverse of the pitch is directiy proportional to the molar concentration of the optically active compound. Racemic mixtures (1 1 mixtures of both isomers) of optically active mesogens form nematic rather than chiral nematic phases. Because of their twist encumbrance, chiral nematic Hquid crystals generally are more viscous than nematics (6). 

The introduction of a second chiral atom in the system leads to a reduction in the mesogenic properties and only a monotropic chiral nematic transition is observed for compound 23. However, when this compound is cooled down from the isotropic liquid state at a cooling rate of 0.5 °Cmin , very unusual blue phases BP-III, BL-II and BP-I are observed in the range 103-88 °C. Blue phases usually require pitch values below 500 nm. Hence the pitch value of the cholesteric phase for 23 must be very short, suggesting that the packing of two chiral carbons forces a faster helical shift for successive molecules packed along the perpendicular to the director. 

Mesitylene, production from acetone, 1 164 Mesityl oxide, 14 589-590 characteristics of, 16 337 hydrogenation, 16 337-338 hydrogen peroxide treatment of, 16 338 Z-menthol from, 24 520 production of, 16 336-337 production from acetone, 1 164, 174 Mesogenic diols, 25 460 Mesogenic molecules, solids of, 15 82 Mesogens, 24 53, 54 Mesomixing, 16 683 Mesomorphic behavior, 24 53-54 Mesomorphic phase transitions, 15 102 Mesomorphism, 15 81. See also Liquid crystalline materials Mesophase pitch-based carbon fiber, 26 734-735... 

The chiral pitch refers to the distance (along the director) over which the mesogens undergo a full 360° twist (the structure repeats itself every half pitch since the positive and negative directions along the director are equivalent). The pitch may be varied by adjusting temperature or adding other molecules to the LC fluid. For many types... 

The terminus of chirality induction is used for processes, in which the structural information of a chiral molecule is transferred to an initially achiral collective which then will form a superstructural chiral phase. One of the most prominent examples can be found in the field of liquid crystals The doping of a nematic LC phase with chiral mesogenes (dopants) can lead to a twisted, helical cholesteric phase. Noteworthy is the fact that the length scales of the chiral information that characterizes the involved species can differ by several orders of magnitude a few Angstrpms in a single chiral molecule, but the pitch of a helical cholesteric phase amounts typically a few microns. 

Cholesteric liquid crystals are similar to smectic liquid crystals in that mesogenic molecules form layers. However, in the latter case molecules lie in two-dimensional layers with the long axes parallel to one another and perpendicular or at a uniform tilt angle to the plane of the layer. In the former molecules lie in a layer with one-dimensional nematic order and the direction of orientation of the molecules rotates by a small constant angle from one layer to the next. The displacement occurs about an axis of torsion, Z, which is normal to the planes. The distance between the two layers with molecular orientation differing by 360° is called the cholesteric pitch or simply the pitch. This model for the supermolecular structure in cholesteric liquid crystals was proposed by de Vries in 1951 long after cholesteric liquid crystals had been discovered. All of the optical features of the cholesteric liquid crystals can be explained with the structure proposed by de Vries and are described below. 

Recently Blumstein and coworkers reported on the thermotropic properties of a series of main chain copolyesters with different azoxybenzene mesogenic units and flexible spacers consisting of varying ratios of (-I-) 3-methyl adipic acid and 1,12-dodecanedioic acid. Melting temperature of the copolyesters showed minimum values for either the 50/50 compositions or the 25/75 combination of the two spacer components, depending on the nature of the mesogenic units. However, Tj tended to decrease linearly with the content of the 1,12-dodecanedioic acid unit, except in one case. They also observed that the cholesteric pitch of the copolyesters seemed to increase as the concentration of the achiral unit, dodecanedioic acid, increased. Similar observations were reported earlier by Strzelecki s group... 

In main chain cholesteric liquid crystalline polymers, the mesogenic groups and flexible spacers are linked alternatively. The flexible units contain asymmetrical carbon atoms which enable the polymers to possess chirality and thus form cholesteric liquid crystals. By varying the ratio of chiral to non-chiral parts, the cholesteric temperature range and pitch can be changed. The cholesteric range depends on the mol fraction of the polymers. A typical main chain cholesteric liquid crystalline polymer is shown in Figure 6.27. 

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