Mesophase nature

Davis S. A., Burkett S. L., Mendelson N. FI. and Marm S., Bacterial templating of ordered macrostructures in silica and silica-surfactant mesophases. Nature 385 (1997) pp. 420-423. 

S.A. Davis, S.L. Burkett, N.H. Mendelson, and S. Mann, Bacterial Templating of Ordered Macrostructures in Silica and Silica-Surfactant Mesophases. Nature (London), 1997, 385, 420-423. 

Lu Y, Fan H, Stump A, Ward TL, Reiker T, Brinker CJ. 1999. Aerosol assisted self assembly of spherical, silica nanoparticles exhibiting hexagonal, cubic, and vesicular mesophases. Nature 398 223 226. 

Caffrey, M., Cherezov, V. (2009). Crystallizing membrane proteins using lipidic mesophases. Nature Protocols, 4, 706—731. 

Mesophase formation in coal-tar pitch is encouraged by a reduction of the natural quinoline-insoluble matter content, which resembles carbon black but is not optically anisotropic and is characterized by an atomic carbon hydrogen ratio of 4 1. In contrast, the atomic carbon hydrogen ratio of mesophase is about 2 1. 

Another method which might avoid the problems associated with natural pitch feeds involves producing mesophase from a synthetic precursor. Recently, Mochida et al. [30] developed a process in which mesophase is produced by the polymerization of naphthalene or methyl naphthalene, with the aid of a HF/BF3... 

To date, there has been relatively little work reported on the mesophase pitch rheology which takes into account its liquid crystalline nature. However, several researchers have performed classical viscometric studies on pitch samples during and after their transformation to mesophase. While these results provide no information pertaining to the development of texture in mesophase pitch-based carbon fibers, this information is of empirical value in comparing pitches and predicting their spinnability, as well as predicting the approximate temperature at which an untested pitch may be melt-spun. 

The rigid nature of the mesophase pitch molecules creates a strong relationship between flow and orientation. In this regard, mesophase pitch may be considered to be a discotic nematic liquid crystal. The flow behavior of liquid crystals of the nematic type has been described by a continuum theory proposed by Leslie [36] and Ericksen [37]. 

Since PAN-based carbon fibers tend to be fibrillar in texture, they are unable to develop any extended graphitic structure. Hence, the modulus of a PAN-based fiber is considerably less than the theoretical value (a limit which is nearly achieved by mesophase fibers), as shown in Fig. 9. On the other hand, most commercial PAN-based fibers exhibit higher tensile strengths than mesophase-based fibers. This can be attributed to the fact that the tensile strength of a brittle material is eontrolled by struetural flaws [58]. Their extended graphitic structure makes mesophase fibers more prone to this type of flaw. The impure nature of the pitch preciusor also contributes to their lower strengths. 

Several works have been published about the phase behavior of polybibenzoates, showing the ability of the biphenyl group to produce mesophase structures. Different spacers have been used, and the results show that the structure of the spacer influences very much the transition temperatures and the nature and stability of the mesophases, as well as the ability to generate three-dimensional crystals. 

Yazami et al. [128, 129] studied the mechanism of electrolyte reduction on the carbon electrode in polymer electrolytes. Carbonaceous materials, such as cokes from coal pitch and spherical mesophase and synthetic and natural graphites, were used. The change in with composi-... 

A decisive factor for the physical behaviour of a composite is the adhesion efficiency at the boundaries between phases. In all theoretical models this adhesion is considered as perfect, assuming that the interfaces ensure continuity of stresses and displacements between phases, which should be different because of the proper nature of the constituents of composites. However, such conditions are hardly fulfilled in reality, leading to imperfect bonding between phases and variable adhesion between them. The introduction of the mesophase layer has as function to reconcile in a smooth way the differences on both sides of interfaces. 

In this way, both boundaries of the mesophase layer with the inclusion and the matrix are automatically satisfied and, therefore, Equation (29) is a convenient relationship, yielding the variable Ej(r)-modulus accomodating, in a natural way, the smooth transition from a large Ef-modulus to a reduced Em-modulus for the matrix and vice versa. 

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. 

The interest in the structures of simple R2Si(OH)2 compounds lies in the fact that one of them, Bu 2Si(OH)2, forms a discotic liquid crystalline phase (308,309). Despite many attempts, it has not proved possible to obtain crystals of Bu 2Si(OH)2 suitable for a crystallographic study, the material obtained from various solvents usually being of a fine fibrous nature. The discotic phase of Bu 2Si(OH)2 has been proposed (309) to be due to the formation of dimeric disks of molecules which remain on breaking the interdimer hydrogen bonds in a structure of type 65 at the transition between crystal and mesophase. As has been described, structure type 65 is found for several diols similar to Bu 2Si(OH)2, and it is thus quite likely that Bu 2Si(OH)2 does indeed have the proposed structure. 

Complex 17-8 melted at 92 to 93 °C - higher than the melting point of di-bromotetrafluorobenzene at 78-81 °C - yet no mesophase was seen [24]. It is believed that this is due to the much weaker nature of the N"-Br interaction, an observation consistent with those made for complexes between alkoxystil-bazoles and bromopentafluorobenzene [20]. 

Type 1 gels are mesophases that are so highly ordered that they resist disruption of their structure and are thus extraordinarily viscous, to the point of appearing solid-like, even though no high molecular weight species need be present in the system. Surfactants, both synthetic (e.g., sodium dodecylsulfate) and natural (e.g., phospholipids), and clays are typical representatives of this class. 

The following natural precursors have been selected for KOH activation coal (C), coal semi-coke (CS), pitch semi-coke (PS) and pitch mesophase (PM). An industrial activated carbon (AC) was also used. Activation was performed at 800°C in KOH with 4 1 (C KOH) weight ratio, for 5 hours, followed by a careful washing of the samples with 10% HC1 and distilled water. The activation process supplied highly microporous carbons with BET specific surface areas from 1900 to 3150 m2/g. The BET surface area together with the micro and the total pore volume of the KOH-activated carbons are presented in Table 1. The mean micropore width calculated from the Dubinin equation is designed as LD. 

High porosity carbons ranging from typically microporous solids of narrow pore size distribution to materials with over 30% of mesopore contribution were produced by the treatment of various polymeric-type (coal) and carbonaceous (mesophase, semi-cokes, commercial active carbon) precursors with an excess of KOH. The effects related to parent material nature, KOH/precursor ratio and reaction temperature and time on the porosity characteristics and surface chemistry is described. The results are discussed in terms of suitability of produced carbons as an electrode material in electric double-layer capacitors. 

The future remains bright for the use of carbon materials in batteries. In the past several years, several new carbon materials have appeared mesophase pitch fibers, expanded graphite and carbon nanotubes. New electrolyte additives for Li-Ion permit the use of low cost PC based electrolytes with natural graphite anodes. Carbon nanotubes are attractive new materials and it appears that they will be available in quantity in the near future. They have a high ratio of the base plane to edge plain found in HOPG. The ultracapacitor application to deposit an electronically conductive polymer on the surface of a carbon nanotube may be the wave of the future. 

Synthetic carbonaceous materials are widely used in these applications. Several types of synthetic materials (e.g. graphitized mesophase carbon microbeads (MCMB), graphitized milled carbon fiber, and even, initially, hard carbons) became the materials of choice at the time of commercialization of first successful lithium-ion batteries in late 1980s. New trends, mainly driven by cost reduction and need for improved performance, currently shift focus towards application of natural graphite. 

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