Local molecular orientation concept

The Concept of Local Molecular Orientation (LMO). When observed by the naked eye a piece of semi-coke has a macroporous texture. When broken into fragments the pore walls appear as lamellae. After polishing, the pore walls appear as isochromatic areas when observed by optical microscopy. 

The capabilities of Oberlin and Bonnamy to see the big picture, and thus synthesize the advancements in seemingly unrelated carbon research fields, both old and new, have been amply demonstrated in previous volumes of this series. Here they bring order to the often confusing issue of disordered carbons, whose relevance has increased tremendously as a consequence of the often frustrated (and certainly incomplete) conversion of a wide variety of carbon precursors to nanotubes, fullerenes, or graphene. As they reviewed all the relevant information, unexpected new concepts emerged related to the carbonization and graphitization processes, and they were able to define more precisely the domain of existence of the various substances such as amorphous carbon and [basic structural units], as well as [local molecular orientations and] turbostratic carbons. .. by using diffraction techniques or their associations. ... 

Modern theoretical developments in the molecular thermodynamics of liquid-solution behavior are based on the concept of local composition. Within a liquid solution, local compositions, different from the overall mixture composition, are presumed to account for the short-range order and nonrandom molecular orientations that result from differences in molecular size and intermolecular forces. The concept was introduced by G. M. Wilson in 1964 with the publication of a model of solution behavior since known as the Wilson equation. The success of this equation in the correlation of VLE data prompted the development of alternative local-composition models, most notably the NRTL (Non-Random-Two Liquid) equation of Renon and Prausnitz and the UNIQUAC (UNIversal QUAsi-Chemical) equation of Abrams and Prausnitz. A further significant development, based on the UNIQUAC equation, is the UNIFAC method,tt in which activity coefficients are calculated from contributions of the various groups making up the molecules of a solution. 

The greater stability of the crystalline state of networks formed from unoriented but crystalline chains compared with networks formed from amorphous polymers, can be explained in the same way as for networks formed from axially oriented natural rubber. Although prior to network formation the crystallites are randomly arranged relative to one another, portions of chains are still constrained to lie in parallel array. The cross-linking of the predominantly crystalline polymer cannot, therefore, involve the random selection of pairs of units. The units that can be paired are limited by the local chain orientation imposed by the crystalline structure. An increase in the isotropic melting temperature of such networks would therefore be expected. It can be concluded that orientation on a macroscopic scale is not required for partial order in the liquid state to develop. Concomitantly a decrease in the entropy of fusion will result, which reflects the increase in molecular order in the melt. This is an important concept that must be kept in mind when studying the properties of networks formed in this manner. This conclusion has important implications in studying the properties of networks formed from unoriented crystalline polymers. 

The reorientation of the local chain axis gives rise to the partial narrowing of the amorphous REV-8 spectra and is of a random nature. In a short period of time, a particular segment of the macromolecular chain assumes a distribution of directions which deviate from its orientation at rest. In analogy to the description of molecular ordering in liquid crystals—, we use the concept of a local order parameter for the quantitative characterization of the extent of these fluctuations. 

Another concept in synthesis is epitaxy. Epitaxy is the continuation of the crystal orientation of the monocrystalline substrate in the deposited crystalline product, which may be the same compound as the substrate or a different solid that has the same crystal orientation as the monocrystalline solid. Epitaxial layers are essential for microlithography in the electronic industry carefully formed epitaxial layers do not have localized electronic interface states, which are deleterious for the functioning of the device. The process conditions for epitaxy by molecular beam epitaxy (MBE) are very low process pressure, comparatively high temperatures, and a low growth rate. MBE is a form of CVD, which was described in Chapter 6. Liquid phase epitaxy (LPE) is a form of growth of single crystals from a melt. 

Solid-state NMR methods allow the investigation of local orientation, dynamics and conformational order of polymer chain segments. Si chemical shifts contain information about molecular moieties and about the conformation of chain segments. The line width of a wideline spectrum characterizes the strength of the dipolar couplings among protons and, therefore, the molecular mobility. H spin diffusion, mediated by the homonuclear dipolar couplings, is a powerful technique to obtain information about the spatial proximity of molecular moieties. These concepts have recently been combined in a two-dimensional Wideline Separation Experiment (WISE-NMR spectroscopy)... 

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