A-crystal phase

Fig. 15 1224 cm-1 band assigned to the crystalline chain packing of the a-crystal phase of sPS as a function of annealing time at 120 °C, which appears only after the induction period of crystallization 30 min [12]... 

Apart from its conventional a-crystal phase P.R.53 1 was also found in a considerably more yellowish-red (3-crystal modification [3],... 

There are two approaches to the solution of the phase problem that have remained in favor. The first is based on the tremendously important discovery or Patterson in the 1930s ihal the Fourier summation of Eq. 3. with (he experimentally known quantities F2 (htl> replacing F(hkl) leads nol to a map of scattering density, but to a map of all interatomic vectors. The second approach involves the use of so-called direct methods developed principally by Karie and Hauptman of the U.S. Naval Research Laboratory and which led to the award of the 1985 Nobel Prize in Chemistry. Building upon earlier proposals that (he relative intensities of the spots in a diffraction pattern contain information about a crystal phase. Hauptman and Karie developed a mathematical means of extracting the information. A fundamental proposition of (heir direct method is that if thrice intense spots in the pattern have positions whose coordinates add up to zero, their relative phases will cancel out. Compulations done with many triads of spots yield probable phases for a significant number of diffracted waves and further mathematical analysis leads lo a likely solution for the structure of the molecule as a whole. 

Fig. 5.25 Phase diagrams for PS-PEO diblocks in diethyl phthalate (Ciervais and Gallot 1973a), a selective solvent for PS. SEO-3 M = 34.7kgmol 59% PEO.This sample forms a non-crystalline lamellar phase (lam) and a lamellar crystalline phase (crystal). SEO-9 Ma - 75 kg mol-1, 70.5% PEO. This sample forms a non-crystalline hexagonal-packed cylinder phase (hex) and a crystal phase.

Although most of the initial polymers and copolymers are no longer available, these systems should help us to investigate the details and structural consequences of a pre-ordering of the polymer melt, or the formation of a precursor crystal in polymer crystallization. From the present and as yet still sketchy results, it appears however that (a) if a crystal phase results from the transformation of a liquid crystal phase (usually of hexagonal symmetry), different orientations of the crystal unit cell are likely, which is not observed in conventional polymer crystallization (for example, in polyethy-... 

The consequences of product accumulation on the structural and chemical properties of a reacting crystal have been examined by several authors. These can be analyzed from a perspective that focuses on molecular changes or from a crystal phase point of view. A molecular analysis focuses on local structural properties that affect the reaction mechanism [55,57,58]. A solid-phase analysis focuses on how a crystalline ensemble reacts to the changes in composition that occur as the reaction proceeds [59-61]. However, as suggested in Scheme 5, a separation... 

The phase transition temperature of the Step 7 product was also examined using a polarizing microscope. When the temperature was initially elevated, the crystal phase reverted to isotropic liquid phase in the vicinity of 80°C. Gradually lowering the temperature from 90°C resulted in an N phase at approximately 60°C. When the temperature was finally lowered to ambient temperature, the formation of a crystal phase was observed. As a result this agent was judged to be a biaxial liquid crystal. 

Thus, it is possible that all of the three controls are acting on the hafnium contribution to seawater. Despite the low concentrations of neodymium in seawater, the high Nd/Hf ratios in Fe-Mn crusts indicate relatively lower hafnium abundances. Because zircon is not a crystallizing phase in basalts, hafnium is not sequestered in zircon in the oceanic cmst and it may be more available for dissolution due to hydrothermal processes compared to continental rocks. As a result the flux of hafnium from the oceanic crust into seawater, relative to the continental flux, may be higher than for neodymium, making it more visible. 

X-ray diffraction was used as a complementary technique and evidenced the formation of a crystallized phase for the regeneration at higher temperatures. The peaks can be attributed to a mixed oxide of molybdenum and nickel with the following formula x (NiO), (M0O3), z (H2O). So the metals partially sinter during regeneration into a bulky phase, which is no longer active. The appearance of this nickel - molybdate type phase, expressed as the area of... 

DTA and XRD. After the drying step at 120°C, all the samples correspond to complex mixtures containing major m " (precursor oxidation state) and minor oxides and hydroxides (M = Ce or Pr) and no well-defined phase could be evidenced from the XRD results. Fig. 1 shows the thermograms obtained under air of all series 1 samples and of one series 2 sample (x = 1). All the thermograms consist in endothermic peaks due mainly to the elimination of a small quantity of the residual water molecules and to the transformation of hydroxide into oxide ions. Therefore, the possible exothermic reactions such as the formation of a crystallized phase or the oxidation of Ce " or Pr " atoms are hidden by the endothermal events. 

The most cotmnon solntion to this problem consists of introducing in the sample we wish to analyze a known amoimt of a crystal phase which is not foimd in the initial sample. The intensities are then measnred for the various characteristic peaks of the phases we want to qnantitatively analyze. The intensities of the control sample s diffraction peaks are affected by the absorption in the same way all the others are. The same study is then done on a control sample containing all of the crystalline phases foimd in the studied sample, but in known amounts and with the addition of the same amount of control powder. Let 1 be the intensity of a peak obtained in the unknown sample for a given phase i and Ii(T) the one obtained in the test sample then, for any phase i, we have ... 

Figure 3.4.24 Vapor sorption isotherms for benzene(open circle) and n-hexane(solid circle) at 20 °C (a) and the pressure selective on-off adsorption (b). The difficulty of fitting these large guests into the a-crystal phase is illustrated by the negligible adsorption before crystal phase transition into the (3-crystal phase.

A very small contact angle tends to be formed also if the solid is a crystal, phase a is air, and (3 is the liquid phase of the crystal material. This implies that the crystal acts as an effective catalytic impurity for the formation of its own melt. This explains why very little overheating generally suffices to induce the phase transition crystal —> melt, as was mentioned in Section 14.1. 

We first show that the crystal phase is not ubiquitous (2,5). There exist certain molecules that cannot possibly have a crystal phase. For these materials the liquid phase which everyone admits exists at high temperatures extends to all low temperatures. Thus, certain materials must of necessity have the amorphous phase as the low temperature equilibrium phase. See the Appendix. One is thus forced to face squarely the problem of determining the equilibrium low temperature properties of those materials that form glasses. 

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