Crystals transformation

The phase behavior of several polybibenzoates with oxyalkylene spacers has been reported [11,14,15,20-27]. These spacers include the dimer of trimethylene glycol and different ethylene oxide oligomers. The most noticeable characteristic of these polybibenzoates with ether groups in the spacer is the considerable decrease of the rate of the mesophase-crystal transformation. Thus, Fig. 8 shows the DSC curves corresponding to a sample of poly[oxybis(trimethylene)p,p -bibenzoate], PDTMB, with a structure similar to that of P7MB but with the... 

A novel application [161] of EGA is in the study of crystal transformations by detection of the release of organic molecules occluded by the reactant solid during preparation. 

Racemic Crystal-to-Conglomerate Crystal Transformation Reactions in 2,2 -Dihydroxy-1,1 -binaphthyl-l /le4N+CI Complex... 

Accelerated aging and crystal transformation rates have also been traced to high residual moisture content. Ando et al. studied the effect of moisture content on the crystallization of anhydrous theophylline in tablets [9]. Their results also indicate that anhydrous materials convert to hydrates at high levels of relative humidity. In addition, if hygroscopic materials (e.g., polyethylene glycol 6000) are also contained in the formulation, needle-like crystals form at the tablet surface and significantly reduce the release rate of the theophylline. 

Another reported study involved the preparation of four niclosamide solvates and the determination of the stability of the crystal forms in different suspension vehicles by DSC and TG analysis. Thermal analysis showed that the niclosamide solvates were extremely unstable in a PVP-vehicle and rapidly changed to monohydrated crystals. A suspension in propylene glycol was more stable and TG analysis showed that crystal transformation was less rapid [18]. 

The two structurally similar polymorphs of (R,S)-ethambutol dihydrochloride have been shown to bear an enantiotropic relationship, and can reversibly interconvert in a single crystal transformation mode [27]. It was reported that despite the identity in space group type and similarity in unit cell constants, the two forms could be distinguished on the basis of their X-ray powder diffraction and solid-state nuclear magnetic resonance properties. Interestingly, while the (polymorphic forms, the (R,S)-diastereomer was only obtained in two different polymorphs. 

Note These data have been obtained from the complete thermodynamic data tabulated in Kubaschewski et al. (loc. cit.), with the approximation of the simple two-term equation. This should serve for calculations not requiring an accuracy of better than 2kJmol 1 02, which is normally the case for industrial applications. Solid state crystal transformations which usually only have relatively small heats of transformation, have been ignored. 

Although PBT fiber also has a plateau region in the stress-strain curve [4], the crystalline chains do not respond to external strain in the first few percent of deformation. They increased in length only when the strain is above 4% (see Figure 11.13). Therefore, initial macroscopic deformation involved viscous flow of the amorphous phase. Furthermore, PBT undergoes strain-induced crystal transformation at moderately low strains of 15-20% [75], The differences in their microscopic crystalline chain deformation explained why PTT has a better elastic recovery than PBT even though both have contracted chains and knees in their stress-strain curves [4, 69],... 

The 31P n.m.r. of phospholipids has been the subject of a number of papers.62-89 These have been primarily aimed at investigating the conformation and motion of phospholipids in bilayers, but information has also been obtained on gel-to-liquid crystal transformations of phospholipids.65-67 A P31 1H nuclear Overhauser effect indicates that there is little tendency for mixed phosphatidylcholine/phosphatidyl-ethanolamine vesicles to segregate in separate domains.68 69 A phosphonium analogue (23) of choline chloride has been prepared and converted chemically into... 

Another example of a transannular cyclization that occurs in the solid state is provided by the epoxy alcohol 31. This compound is stable when dissolved in organic solvents and in 0.25N sulfuric acid. However, the crystals transform rapidly to 32. Although the process is accompanied by partial melting, it appears to be a true solid-state one. Interestingly, the reaction is slowed down appreciably when the dry crystals are covered with ether. Hydrogen bromide is eliminated in the reaction and it may be that an acid-catalyzed process is also occurring in the presence of solvent this process may be slowed down by the dissolution of the decomposition products in the solvent (77). 

Zobel, H. F. (1988b). Starch crystal transformations and their industrial importance. Starch/... 

The surface lattice plane of Pt (lOO)-(l x 1), created by cleavage along the close-packed cubic lattice plane (100) of platinum crystals, transforms into the... 

In most cases, the solid gradually changed to amorphous according to the progress of the photoreaction. However, crystal-to-crystal transformation was observed in the photoreaction of Id. Orange prismatic crystals gradually decolorized upon irradiation finally, colorless prisms were obtained. 

The disadvantages of using AN in rocket propellants and pyrolants are its hygroscopic nature and its crystal structure transitions. The crystal transformations from phase 1 to phase V occur with decreasing temperature as follows ... 

Fig. 11.21 shows the results of TG and DTA measurements on mixtures of AP particles and catalysts. The endothermic peak observed at 513 K is caused by the crystal structure transformation of AP from orthorhombic to cubic. A two-stage exothermic decomposition occurs in the range 573-720 K. The decomposition of the AP is seen to be drastically accelerated by the addition of catocene. The exothermic peak accompanied by mass loss occurs before the AP crystal transformation. Although the AP is sensitized by the addition of carborane, no effect is seen on the AP decomposition. The results indicate that carborane acts as a fuel component in the gas phase but does not catalyze the decomposition of AP. Thus, the critical friction energy is lowered due to the increased reaction rate in the gas phase. The results imply that the initiation of ignition by friction is caused by the ignition of the gaseous products of the AP pyrolants.PI... 

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