Poor crystals

A unique method to generate poor crystals is shown in Fig. 6.71. It involves the compression of crystals of poly(4-methyl-l-pentene), P4MP1, with a glass transition Tj = 303 K, melting transition T ° = 523 K, and a heat of fusion of AHf= 10 kl moF (see also Fig. 5.124). At room temperature, the tetragonal crystals of P4MP1 have a 

Melt-crystallized Destructiofi of Cr talliiie Order by penteli f) Increa ing Hydrostatic Pressure 

Above 1.9 kb aii oonformational iong-range order has disappeared, but the chains remain largely parallel and have gained internal energy by closer packing of the disordered chains. The disorder can be reversed by removal of the pressure. 

NonequNibrIum Phase Diagram of Poly(4-methyh1-pentene) 

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Single-crystal siHcon can also be grown from various fluxes and by a combination of electrolysis and fluxes at temperatures weU below the melting point of pure siHcon (16). The main disadvantages are the inclusion of the flux in the crystal and the poor crystal quaHty. Potential advantages are a decrease in growth temperature and purification during electrolysis. 

Plastic clays composed largely of poorly crystallized kaolinite but having small amounts of iUite, and at times montmorillonite, are widely used ia bonding mol ding sands, especially ia the United States. 

If the phases present can be unambiguously identified, microscopy can be used to determine the geometry of interface initiation and advance, and to provide information about particle sizes of components of mixed reactants in a powder. Problems of interpretation arise where materials are poorly crystallized and where crystallites are small, opaque, porous or form solid solutions. With the hot-stage microscope, the progress of reactions can be followed in some instances and the occurrence of sintering and/or melting detected. 

The reactions of some aromatic metal carboxylates are on the borderline of classification as solid-state processes. While there is no evidence of liquefaction, rates of decomposition in the poorly crystallized or vitreous reactant obey kinetic expressions more characteristic of reactions proceeding in a homogeneous phase. 

If die well-crystallized PAs are heated, the two characteristic diffraction signals move together and meet at what is called the Brill temperature (Fig. 3.5).6,20 Above this Brill temperature, there is only one spacing for both directions this implies a pseudohexagonal packing, such as that observed in the poorly crystallized PA-6 at room temperature. 

Compound 15 has an ill-defined structure 70) due to poor crystal quality. It is, however, interesting to note that 15 became resolved (space group 7 2,2, Z = 8) in an attempted cocrystallization experiment in the presence of optically pure 1. The two molecules of 15 in the asymmetric unit may be seen as a sort of a self inclusion , where molecules are again linked into infinite H-bond chains analogous to 13. 

Determinations in Table 15 cover some twenty-five years, and a number of studies suffered from poor crystals, so the initial level of comparison is at the level of a few picometers. In a number of cases, disorder between E and E vitiated detailed discussion, for example of the potentially interesting and well-represented series of ArgEE molecules. For simple ligands, the E—E bond lengths fall into quite tight ranges, slightly shorter than in the elements, and the E—E values interpolate. 

Though yellow-colored, the 2-methylproline thus obtained is spectroscopically pure (according to the 1H and 1 C NMR spectra, see Note 17). It Can be further purified by dissolution in methanol with 5% w/w activated charcoal and filtration ot the resulting suspension through Celite. In spite of the poor crystallizing tendency of most amino acids, 2-methylproline can be recrystallized from... 

Using coprecipitation methods with a suitable mixture of solutions described above, the resulting LDH materials are often poorly crystallized and exhibit compositional fluctuations due mainly to the difference in the values of the pH at which the precipitation of M(II)(OH)2 and M(III)(OH)3 hydroxides occurs. Consequently, the chemical formula of the final material may not reflect the composition of the solution prior to the precipitation as noted in Chapter 1. Controlling the amount of anion incorporated under such conditions is very difficult. A "chimie douce method has been proposed by Delmas et al. in an effort to overcome this problem [181,182]. The process is illustrated schematically in Fig. 8. Since the synthesis starts from a highly crystalline layered y-oxyhydroxide precursor, it was suggested that this favored the formation of very crystalline LDHs with controllable M(1I)/M(III)... 

Poorly Crystallizing Proteins Contain Substantial Disordered Regions... 

There are few strictly comparable structurally characterized alkynyl/diynyl/poly-ynyl systems, which make detailed comparisons between the various unsaturated ligands difficult. For diynyl and poly-ynyl complexes, this is no doubt due to the poor packing qualities of the rod-like mono-metallic systems and resulting poor crystal quality. Structural features of poly-ynyl complexes are summarized in Table II. 

Polymers such as polystyrene, poly(vinyl chloride), and poly(methyl methacrylate) show very poor crystallization tendencies. Loss of structural simplicity (compared to polyethylene) results in a marked decrease in the tendency toward crystallization. Fluorocarbon polymers such as poly(vinyl fluoride), poly(vinylidene fluoride), and polytetrafluoroethylene are exceptions. These polymers show considerable crystallinity since the small size of fluorine does not preclude packing into a crystal lattice. Crystallization is also aided by the high secondary attractive forces. High secondary attractive forces coupled with symmetry account for the presence of significant crystallinity in poly(vinylidene chloride). Symmetry alone without significant polarity, as in polyisobutylene, is insufficient for the development of crystallinity. (The effect of stereoregularity of polymer structure on crystallinity is postponed to Sec. 8-2a.)... 

A well crystallized lepidocrocite with smooth edges (surface area 32.5 m g ) was nonporous and displayed a reversible type II N2 adsorption isotherm (Gomez-Villa-cieros et ah, 1984). Poorly crystallized, high surface area material also has a type II adsorption isotherm, but with B type hysteresis this material contained mesopores 2-20 nm across (Crosby et al., 1983 Madrid and De Arambarri, 1985). Lepidocrocite crystals with highly serrated terminals had a surface area of 67 m g of which 13% could be attributed to micropores ca. 1.5 nm across (Weidler, 1995). 

Air oxidation at room temperature (RT) of a 0.0454 M FeCl2 solution (buffered with NaHC03 and previously outgassed with N2) until the greenish colour of the suspension is replaced by a yellow precipitate (some hours). The product is rather poorly crystallized (Schwertmann and Cornell, 2000). 

Oxidation of a hot FeS04 solution with a mixed Na0Cl/Na2C03 solution. This gives a poorly crystallized material (Duvigneaud and Derie, 1980). 

Schwertmann, U. Taylor, R.M. (1979) Natural and synthetic poorly crystallized lepidocrocite. Clay Min. 14 285-293... 

The form of nickel in particles from different industries varies. The mineralogical composition, chemical content, and form of dusts from nine industries in Cracow, Poland, were examined (Rybicka 1989). The chemical form of a particle-associated heavy metal that was assessed by a five-step extraction scheme classified the metal as exchangeable, easily reducible (manganese oxides, partly amorphous iron oxyhydrates and carbonates), moderately reducible (amorphous and poorly crystallized iron oxyhydrates), organically bound or sulfidic, and residual. Dusts from power plants had a silicate characteristic with quartz and mullite predominant. Approximately 90% of the nickel from these... 

The double bond in bicyclopropylidene (144) is shorter than in 142 and in ethene. Bond distances (A) from XD (-40 °C) have large uncertainties because of poor crystal quality,... 

How quickly equilibrium is approached is another question. Hemley made his measurements (11) at temperatures of 200°C. and higher, and at 25°C. one will obtain poorly crystallized products. Garrels (5), who has tested similar systems, states that after adding acid or base, pH may return to close to its original value in a matter of hours, even if the solid reaction products are too fine-grained to be identified by x-rays. 

The crystal to liquid crystal transition has been measured frequently, but not studied in detail. From the generally poor crystallization of the polyester types which make up the liquid crystals3) one expects typical samples to have low crystal-... 

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