Global structure, crystalline phase

The Rietveld Fit of the Global Diffraction Pattern. The philosophy of the Rietveld method is to obtain the information relative to the crystalline phases by fitting the whole diffraction powder pattern with constraints imposed by crystallographic symmetry and cell composition. Differently from the non-structural least squared fitting methods, the Rietveld analysis uses the structural information and constraints to evaluate the diffraction pattern of the different phases constituting the diffraction experimental data. 

X-Ray diffraction data give the atomic coordinates and thereby the conformations of molecules in the crystalline phase. If many structures of a given type are known, which unfortunately is rarely the case for medium rings, it is likely that an excellent picture of the global conformational energy minimum will be obtained as lattice effects should be more or less random. 

The global structural features of the liquid, glassy and crystalline phases of a Ionic Liquid are often similar, but the local ion configurations of these phases sensitively depend on packing effects and are often not identical. 

The structure of semicrystalline polymers can best be described as a nanophase aggregate of two or more phases such as crystalline, amorphous and intermediate, to be described later. Within this global, metastable phase structure, local areas may be contained which may have a melting and crystallisation equilibrium that can be detected by MTDSC, as will be described in Section 4.5. 

The phase transition from disordered states of polymer melt or solutions to ordered crystals is called crystallization-, while the opposite process is called melting. Nowadays, more than two thirds of the global product volumes of synthetic polymer materials are crystallizable, mainly constituted by those large species, such as high density polyethylene (HOPE), isotactic polypropylene (iPP), linear low density polyethylene (LLDPE), PET and Nylon. Natural polymers such as cellulose, starch, silks and chitins are also semi-crystalUne materials. The crystalline state of polymers provides the necessary mechanical strength to the materials, and thus in nature it not only props up the towering trees, but also protects fragile lives. Therefore, polymer crystallization is a physical process of phase transition with important practical relevance. It controls the assembly of ordered crystalline structures from polymer chains, which determines the basic physical properties of crystalline polymer materials. 

A vast number of reports show that small molecules or amphiphilic peptides may assemble into nanofibers, vesicles, and gels. Organic foldamers which are able to do so are very attractive for their potential applications in materials science. Motivated by the observation of the liquid crystalhnity of globally amphiphilic )8-peptides, Gellman and coworkers designed j8-peptide iso-A which possesses a globally amphipathic structure when folded, and -peptide A which does not. It was anticipated that iso-A but not A would form a liquid-crystalline (LC) phase... 

The analysis of the simulations of crystalline Si 2 has focused on a study of the lattice parameters and their temperatures dependence, on the elastic constants (which can be gotten from the Parrinello-Rahman fluctuation relations [45]), and quantities characterizing the local aspects of the structure (probability distributions to find an atom a distance r away from its equilibrium position, partial radial pair distributions, average bond angles and their distributions, etc.). Of particular interest is also the global order parameter ()) of the phase transition between a-quartz and 5-quartz, which measures the rotation of the (distorted) tetrahedra about the [100] axis,... 

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