Characteristics of mesophase

The polyamides are soluble in high strength sulfuric acid or in mixtures of hexamethylphosphoramide, /V, /V- dim ethyl acetam i de and LiCl. In the latter, compHcated relationships exist between solvent composition and the temperature at which the Hquid crystal phase forms. The polyamide solutions show an abmpt decrease in viscosity which is characteristic of mesophase formation when a critical volume fraction of polymer ( ) is exceeded. The viscosity may decrease, however, in the Hquid crystal phase if the molecular ordering allows the rod-shaped entities to gHde past one another more easily despite the higher concentration. The Hquid crystal phase is optically anisotropic and the texture is nematic. The nematic texture can be transformed to a chiral nematic texture by adding chiral species as a dopant or incorporating a chiral unit in the main chain as a copolymer (30). 

A very unusual characteristic of mesophase pitch is the extreme dependency of its viscosity on temperature [19,34,35]. This factor has a profound influence on the melt-spinning process (described above), as a mesophase pitch fiber will achieve its final diameter within several millimeters of the face of the spinnerette, in sharp contrast to most polymeric fibers. 

Yoon, S.-H., Korai, Y. and Mochida, I., Spinning characteristics of mesophase pitches derived from naphthalene and methylnaphthalene with HF/BF3, Carbon, 1993, 31(6), 849 856. 

Riggs, D. M. Dlefendorf, R. J., "Effects of Heat-Treatment on the Molecular Characteristics of Mesophase Forming Materials", Extended Abstracts, 14th Biennial Conference on Carbon, 1979, pp. 413-414. 

N. Imanishi, H. Kashiwagi, T. Ichikawa, Y. Takeda, 0. Yamamoto, and M. Inagaki, Charge-discharge characteristics of mesophase-pitch-based carbon fibers for lithium cells, J. Electrochem. Soc., 140 [2], 315-320 (1993). 

Biggs, D. M. and R. J. Diefendorf, "Elongational Characteristics of Mesophase-Containing Pitches," paper presented at the 31st Pacific Coast Regional Meeting of the American Ceramic Society, October 1978. Anon., Japan Economic Journal, October 16, 1984, p. 16. 

Table III. Characteristics of Mesophase Oligomers (p=5) with General Formula ...

GaUi, G., MartineUi, E., Chiellini, E. et al. (2005) Low surface energy characteristics of mesophase-forming ABC and ACB triblock copolymers with fluorinated B. blocks. Molecular Crystals arul Liquid Crystals, 441,211-226. Genzer, J., Sivaniah, E., Kramer, E.J. et al. (2000a) Temperature dependence of molecular orientation on the surfaces of semifluorinated polymer thin films. Langmuir, 16,1993-1997. 

Pitches can be transformed to a mesophase state by further chemical and physical operations. Heat treatment of conventional pitches results in additional aromatic polymeriza tion and the distillation of low molecular weight components. This results in an increase in size and concentration of large planar aromatic molecular species whereupon the precursor pitch is transformed to a mesophase state exhibiting the characteristics of nematic Hquid crystals (1). Additional heat treatment converts the mesophase pitch to an infusible aromatic hydrocarbon polymer designated as coke. 

Many cellulose derivatives form Hquid crystalline phases, both in solution (lyotropic mesophases) and in the melt (thermotropic mesophases). The first report (96) showed that aqueous solutions of 30% hydroxypropylceUulose [9004-64-2] (HPC) form lyotropic mesophases that display iridescent colors characteristic of the chiral nematic (cholesteric) state. The field has grown rapidly and has been reviewed from different perspectives (97—101). 

The phase behavior of several polybibenzoates with oxyalkylene spacers has been reported [11,14,15,20-27]. These spacers include the dimer of trimethylene glycol and different ethylene oxide oligomers. The most noticeable characteristic of these polybibenzoates with ether groups in the spacer is the considerable decrease of the rate of the mesophase-crystal transformation. Thus, Fig. 8 shows the DSC curves corresponding to a sample of poly[oxybis(trimethylene)p,p -bibenzoate], PDTMB, with a structure similar to that of P7MB but with the... 

PTEB-Q) to the annealed ones, owing to the presence of the crystalline phase. Moreover, the temperature of the peak increases with the annealing, as well as the broadness of the relaxation. These results suggest that the liquid crystalline phase gives raise to an a relaxation similar to that of amorphous polymers despite the existence of the two-dimensional order characteristic of smectic mesophases, and it changes following the same trend than that of semicrystalline polymers. 

Then, the three-layer model provides an easy method for evaluating the characteristics of the mesophase, by introducing a significant flexibility in the study of the physical behaviour of particulates. The drawback of the model is its instability to the values of the thermal expansions and the moduli of the composite, which must be evaluated with very high accuracy, fact which is a difficult task. Small deviations in measuring the a s and the E s may vary considerably the balance of characteristic values of the composite. However, the introduction of the influence of the mesophase to the physical behaviour of the composite, made in this model, is a certain advancement in the knowledge of the behaviour of these complicated substances. 

Liquid crystals have a degree of order characteristic of solid crystals, but they can flow like viscous liquids. They are mesophases, intermediate between solids and liquids their properties can be modified by electric fields and changes in temperature. 

Most solid materials produce isotropic liquids directly upon melting. However, in some cases one or more intermediate phases are formed (called mesophases), where the material retains some ordered structure but already shows the mobility characteristic of a liquid. These materials are liquid crystal (LCs)(or mesogens) of the thermotropic type, and can display several transitions between phases at different temperatures crystal-crystal transition (between solid phases), melting point (solid to first mesophase transition), mesophase-mesophase transition (when several mesophases exist), and clearing point (last mesophase to isotropic liquid transition) [1]. Often the transitions are observed both upon heating and on cooling (enantiotropic transitions), but sometimes they appear only upon cooling (monotropic transitions). 

Lyotropic LCs can also be described by a simple model. Such molecules usually possess the amphiphilic nature characteristic of surfactant, consisting of a polar head and one or several aliphatic chains. A representative example is sodium stearate (soap), which forms mesophases in aqueous solutions (Figure 8.4a). In lyotropic mesophases, not only does temperature play an important role, but also the solvent, the number of components in the solution and their concentration. Depending on these factors, different types of micelles can be formed. Three representative types of micelles are presented in Figure 8.4b-d. 

Given the complex process to produce mesophase carbon (graphitized microbeads and fibers), natural graphite can be very competitive in terms of its manufacturing costs [18]. The physical characteristics of certain SLC type materials are extremely close to the characteristics of state-of-the-art MCMB grades. 

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... 

In addition to the cubic and/or inverse cubic forms described above, further transitional forms exist between the lamellar phase and the hexagonal mesophase (cubic, type II) or inverse hexagonal mesophase (cubic, type III) [6]. In contrast to the discontinuous phases of types I and IV, cubic mesophases of type II and III belong to the bieontinuous phases (Fig. 4f). A range of lyotropic mesophases are possible, depending on the mesogen concentration, the lipophilic or hydrophilic characteristics of the solvent, and the molecule itself [6]. 

Note Nematic droplets display a texture characteristic of a nematic mesophase since they occur nowhere else. 

Collective segmental rotation is considered to be the natural form gaining conformational entropy as the essential factor for stabilizing mesophases or liquid-crystalline structures. Despite the approximate character of our conception it should be possible to identify the significant characteristics of the intermolecular segmental arrangement in mesophases, in liquid crystals or in a polymer melt. 

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