Structures of Mesophases

Godovsky, Y. K. Makarova, N. N. Matukhina, E. V. Mesophase Behavior and Structure of Mesophases in Cyclolinear Polyorganosiloxanes. In Silicones and Silicone-Modified Materials, Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Eds. ACS Symposium Series 729 American Chemical Society Washington, DC, 2000 pp 98-114. 

Beginning with a discussion of the necessary basic physics and chemistry, the book proceeds to a description of the main topics of current research in this field. The Langmuir-Blodgett technique, self-assembly, and methods of film deposition exploiting the ordered structure of mesophases are discussed. Separate chapters are devoted to the properties and computer modelling of both liquid crystals and films at the air/water interface. Order in biomemebranes is also discussed. 

Other factors that affect the structure of mesophases include the concentration ratio between surfactants and inorganic precursors. For example, MCM-41 was prepared at a Ci6H33(CH3)3N+/Si ratio of less than 1. As the Ci6H33(CH3)3N+ /Si ratio increases beyond 1, the cubic phase (Ia3d) can be produced. Similarly, Nb-TMSl (/ 6m), Nb-TMS2 (E63/WWC), Nb-TMS3 (Pm-3n), and Nb-TMS4 (p2) were prepared at various surfactant to Nb ratios. ... 

M. Endo, Structure of mesophase pitch-based carbon fibres, J. Mater. Sci., 23,598-605 (1988). 

X. M. Bourrat, E. J. Roche and J. G. Lavin, The role of discllnations in the structure of mesophase pitch-based carbon fibers, Proc. Int. Symp. Carbon, Tsukuba, Tanso ed., 320-323, Nov. 4-8,1990. 

Quite often, room-temperature X-ray scattering data are used to characterize packing structures of mesophases by assuming that the mesophase structures can be frozen when the polymers are cooled down from mesophase temperatures to room temperature. However, liquid crystalline polymers normally crystallize at room temperature. The X-ray scatterings from liquid crystalline polymers at... 

Cross Section. The cross-section structure of mesophase-pitch carbon fibers is one of the four types shown in Fig. 8.12 and is determined by the spinning method, the temperature of stabilization, and the partial pressure of oxygen.The formation of a skin-core structure or skin effect is often observed. This structure is similar to that of the PAN-based carbon fiber shown In Fig. 8.8 above. 

Mesophase Behavior and Structure of Mesophases in Cyclolinear Polyorganosiloxanes... 

As the surfactant concentration is further increased, intermicellar forces come into play and give rise to liquid crystalline phases, also called lyotropic phases or mesophases. The structure of mesophases can be extremely varied (see below). 

NMR spectroscopy can provide valuable information on the structure of mesophases. The most commonly used method for this is the analysis of line shapes in spectra of various nuclei, and, in particular, of H. For a large number of mesophases, NMR spectra are motionally averaged and the spectral line shape reflects the phase symmetry. Line shape analysis of NMR spectra has been employed successfully to investigate the molecular organization and tilt angle of smectic C phases, as well as the structure of columnar discotic phases and it has been shown to be more sensitive than optical techniques to some aspect of biaxial ordering. Moreover, this technique has been shown to be extremely helpful in the discrimination of lyotropic phase symmetry. As examples, NMR spectra of some lyotropic phases are reported in Figure 4. 

The spinning temperature and the type of spinning process also determine the structure of the final carbon fibre. For example, the radial structure of mesophase-pitch fibres is caused by a laminar flow of mesophase pitch through a spinneret and can be suppressed by changing the laminar flow to turbulent flow. Carbon fibres with different structures have been produced using spinnerets of different shapes (Figure 6). 

The structure of mesophases has been investigated by polarizing microscopy, light scattering, x-ray scattering, and SANS (see Chapter 9). The methods used in the past have been insufficient for the study of surfactant phases [154]. Recent studies have significantly benefited from NMR spectrography. 

In liquid crystalline mesophases, there also exist domain boundaries in polydomain samples, in addition to domain disclinations. However, judging from the fact that neither the mobility nor the p,x-product depend on the size of the domains in a polydomain sample, these structural defects hardly affect the carrier transport properties of smectic mesophases [48-50]. Until now, the exact reason why structural defects in smectic mesophases are less harmful to carrier transport has not been explained. It is possible that the flexibility of the molecular orientation in mesophases, or the soft structure of mesophases, makes local carrier transport possible at defect sites. This is another outstanding feature of carrier transport in the mesophases, which distinguishes mesophases from crystalline materials. It provides... 

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