Polymorph stable

Solubility measurements should be made for the desired polymorph in 5 or 6 representative solvents that cover the range of segment types in the NRTL-SAC method, and within the polymorphs stable temperature region where enantiotropy exists. Solubility should be measured in mol or mass fractions and... 

Although the information available from synthetic studies strongly indicates a P-T control of the chlorite polymorph, natural minerals appear to exhibit both polymorph, 7 and 14 8, at low temperature. Most diagenetic chlorites correspond to a 7 8 polymorph. However, there are occurrences especially in deep ocean sediments of a 14 8 phase. The contradiction cannot be resolved with the information available at present. It is probably reasonable to assume that the 7 8 polymorph stable for all chlorite compositions (i.e., various Fe +, Mg, A1 ratios) and that the 14 8 forms are metastable at low temperatures. However, this is certainly not definitive. 

Each polymorph contains the same chemical contents of the respective unit cells. If the chemical contents differ, for example by the presence of different amounts of solvent, they are called pseudopolymorphs. Polymorphs may differ with respect to physical properties such as melting points, or solubilities, as also may pseudopolymorphs. Their existence often presents a serious problem in the pharmaceutical industries since physical properties of crystals are often used as criteria for quality control and thereby the effectivity of a given preparation. Polymorphs and pseudopolymorphs are usually obtained when crystals are grown under different conditions. For example, metastable crystals of the 7T-donor acceptor complex between biphenylene and pyromellitic dianhydride were obtained when crystals were grown by sublimation at high temperatures, whereas a different polymorph, stable at room temperature, was grown by the same method at a lower temperature. ... 

Figure 2. Schematic representation of free energy crossovers in nanoparticle systems. The polymorph stable as a bulk material (a) is stable as a nanoparticle for surface areas between zero and A. The polymorph p, metastable as a bulk material, is stable as a nanoparticle for surface areas between B and C. For surface areas greater than C, amorphous material is stable.

Structures of polymorphs stable at various temperatures and pressures... 

A complete picture of the structural chemistry of a compound would require a knowledge of its structure in the solid, liquid, and gaseous states. The amount of information obtainable about the structure of a liquid halide is very limited, and few X-ray studies have been made. (Examples include Snl4, InClg, and Cdl2. ) We are therefore obliged to make direct comparisons of crystal and vapour. Strictly, these comparisons relate only to the process of sublimation, and if the compound is polymorphic the relevant crystal structure is that of the polymorph stable at the temperature of sublimation. 

In addition to ij, and there are a number of crystalline polymorphs stable only under pressure, though the complete phase diagram is not yet established (Fig. 15.1). There are five distinct structures (differing in arrangement of O atoms) and there are low-temperature forms of two of them the numbering is now unfortunately unsystematic ... 

Sulphur is remarkable for the number of solid forms in which it can be obtained. These include at least four well-known normal polymorphs stable under atmospheric pressure, numerous high-pressure forms of which one is the fibrous form made from plastic sulphur, so-called amorphous forms characterized by small solubility in CS2, and coloured forms produced by condensing the vapour on surfaces cooled to the temperature of liquid nitrogen. Some at least of the amorphous sulphur preparations (e.g. milk of sulphur) give X-ray diffraction lines indicative of some degree of crystallinity, and it is perhaps preferable to use the term u-sulphur rather than amorphous sulphur.The term normal polymorph used above refers to the forms now known to consist of Sg or Sg molecules. In recent years cyclic molecules S (n = 7,9, 10, 12, 18, 20) have been prepared by special methods (see below). 

The structural chemistry of this oxide in the solid and liquid states is complex. In addition to a high-pressure form and a glass there are three polymorphs stable at atmospheric pressure. As noted in Chapter 3 these three crystalline fomis represent... 

Crystals from ethyl acetate/hexane, mp 103-104" (Ondetti, Cnshman). Generally regarded as polymorphic stable... 

One would like to ignore the entropy contribution S, but the existence of polymorphs and of polymorphUc transitions is an effective demonstration that this cannot be done. As the temperature is increased past some transition point, the polymorph stable at 0 K may become metastable and then transform to a different polymorph. The entropy always increases with temperature faster in the polymorphic form that becomes stable at higher temperature. 

Polymorph Stable at atmospheric pressure Metastable, involves deformation Pressure > 3 k bar, near melting temperature Orthothombic Monoclinic Hexagonal (55,56) (60) (61, 62)... 

Fig. 18 X-ray powder diffraction profiles of ethylene-norbornene copolymer samples with norbornene content close to 50% (A)-(C), and solid norbornane in f.c.c. polymorph stable a T>30.6°C, (D) redrawn from [121]. Sample (A) is a random copolymer and amorphous. Samples (B) and (C) have a prevailing alternating constitution and are both crystalline. Sample (C) is essentially atactic, whereas sample (B) has a prevailing diisotactic configuration. (Reprinted with permission from [121]. Copyright 2003 by the American Chemical Society)...

HMX, the highest density and highest energy soHd explosive produced on a large scale, primarily for military use, exists in four polymorphic forms. The beta form is the least sensitive, most stable, and the type requited for military use. The mole fraction products of detonation of HMX in a calorimetric bomb are 3.68 N2, 3.18 H2, 1.92 CO2, 1.06 CO, 0.97 C, 0.395 NH3, and 0.30 H2. 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Crystalline Silica. Sihca exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a Hquid. The Hterature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of siHca, there are three main forms at atmospheric pressure quart2, stable below about 870°C tridymite, stable from about 870—1470°C and cristobaHte, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the stmctures are based on SiO tetrahedra linked in such a way that every oxygen atom is shared between two siHcon atoms. The stmctures, however, are quite different in detail. In addition, there are other forms of siHca that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of siHcon is six rather than four. 

Fig. 3. The main crystallographic forms of siUca stable at atmospheric pressure. The vertical directions represent the facile, displacive polymorphic transitions, whereas the horizontal directions represent the sluggish reconstmctive transitions (44).

Silicon dioxide [7631-86-9] Si02, exists in both crystalline and glassy forms. In the former, the most common polymorph is a-quartz (low quartz). All commercial appHcations of crystalline quartz use a-quartz, which is stable only below ca 573°C at atmospheric pressure. Some of the properties of a-quartz are Hsted in Table 1. 

Tempering. The state, or physical stmcture, of the fat base in which sugar, cocoa, and milk soHds are suspended is critical to the overall quaHty and stabiHty of chocolate. Production of a stable fat base is compHcated because the cocoa butter in soHdified chocolate exists in several polymorphic forms. Tempering is the process of inducing satisfactory crystal nucleation of the Hquid fat in chocolate. 

For (Z)-cinnamic acid [102-94-3], three distinct polymorphic forms have been characteri2ed. The most stable form, referred to as aHocinnamic acid, has a melting point of 68°C, and the two metastable forms, isocinnamic acids, have melting points of 58°C and 42°C, respectively. (E)-Cinnamic acid can be converted to the (Z)-isomer photochemicaHy through kradiation of a solution with ultraviolet light. 

A specific polymorph may be absolutely essential for a crystalline product, for example, one polymorph may have a more desirable color or greater hardness or disperse in water more easily than another polymorph. Often, one polymorphic form is more stable than another (for example, at 80°C the orthorhombic I form of ammonium nitrate is more stable than the trigonal form) at conditions to which a product is exposed. An interesting approach to... 

Octaphenyl cyclotetrasiloxane [546-56-5] M 793.2, m 201-202°, 203-204°, b 330-340°/lmm. Recryst from AcOH, EtOAc, or It forms two stable polymorphs and both... 

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