Polymorphism crystallinity

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. 

Riboflavin forms fine yellow to orange-yeUow needles with a bitter taste from 2 N acetic acid, alcohol, water, or pyridine. It melts with decomposition at 278—279°C (darkens at ca 240°C). The solubihty of riboflavin in water is 10—13 mg/100 mL at 25—27.5°C, and in absolute ethanol 4.5 mg/100 mL at 27.5°C it is slightly soluble in amyl alcohol, cyclohexanol, benzyl alcohol, amyl acetate, and phenol, but insoluble in ether, chloroform, acetone, and benzene. It is very soluble in dilute alkah, but these solutions are unstable. Various polymorphic crystalline forms of riboflavin exhibit variations in physical properties. In aqueous nicotinamide solution at pH 5, solubihty increases from 0.1 to 2.5% as the nicotinamide concentration increases from 5 to 50% (9). 

Alkenoic acids also have polymorphic crystalline forms. For example, both oleic and elaidic acids are dimorphic with melting points of 13.6 and 16.3°C for oleic, and 43.7 and 44.8°C for elaidic acid (14). 

Sihcon dioxide exists in several structural forms polymorphic crystalline sihca, synthetic quartz crystals, amorphous silica, and vitreous silica. This classification is not complete as there are other forms of silica synthesized for speciahzed apphcations. Various forms of sihca are mentioned briefly below. 

When milk is cooled, lipid crystals form and grow. These crystals may have different polymorphic forms, only one of which is stable under a particular set of conditions. Other polymorphic crystalline forms are unstable and are transformed into the stable form (Mulder and Walstra 1974). This phenomenon, known as polymorphism, has been extensively studied (Mulder and Walstra 1974 Brunner 1974), and the... 

The structure of 2 has since been redetermined on two polymorphic crystalline modifications.27 One crystal was grown from hot benzene and the other was obtained from a solution of dichloromethane-hexane by slow evaporation. The former crystal contains one molecule whereas the latter crystal contains two independent molecules. The structure of one of these molecules is depicted in Fig. 1 and key structural parameters for 1 and 2 are listed in Table I. 

In this paper we report for the first time our studies of hydrogen bonding in the imidazole-carboxylate system, a common motif in enzyme active sites. The H solid state NMR data of polymorphic crystalline histidines were taken on a Chemagnetic... 

Whereas atactic PS is an amorphous polymer with a Tg of 100 CC, syndio-tactic PS is semicrystalline with a Tg similar to aPS and a Tm in the range 255-275 °C. The crystallization rate of sPS is comparable to that of polyethylene terephthalate). sPS exhibits a polymorphic crystalline behavior which is relevant for blend properties. In fact, it can crystallize in four main forms, a, (3, -y and 8. Several studies [8] based on FTIR, Raman and solid-state NMR spectroscopy and WAXD, led the a and (3 forms to be assigned to a trans-planar zig-zag molecular chain having a (TTTT) conformation, whereas the y and 8 forms contain a helical chain with (TTG G )2 or (G+G+TT)2 conformations. In turn, on the basis of WAXD results, the a form is said to comply with a unitary hexagonal cell [9] or with a rhombohedral cell [10]. Furthermore, two distinct modifications called a and a" were devised, and assigned to two limiting disordered and ordered forms, respectively [10]. 

Prusiner, P. and Sundaralingam, M. (1976). The crystal and molecular structures of two polymorphic crystalline forms of virazole (l-JS-D-ribofuranosyl-l,2,4-triazole-3-carboxamide). A new synthetic broad spectrum antiviral agent. Acta Crystallogr. B, 32, 419-26. [159]... 

Commercial fats solidify in several crystalline polymorphic forms. Two desirable stable forms are commonly designated by the Greek letters beta ((3) and beta-prime (p )- Table 2 (7) lists many common fats and oils and their most commonly exhibited polymorphic crystalline form. 

Polymorphism Crystallinity. One of the earliest reported uses of NIR for polymorphism was for the polymorphs of caffeine in 1985.In 1987, Gimet and Luong used NIR to ascertain changes in crystallinity during processing. 

Three polymorphous crystalline ferric hydroxides, a-, /k and y-FeOOH are known as the major components of naturally occurring oxidized products of iron, which, together with Fe3Oj. and various amounts of amorphous hydrated oxides, constitute rust on steel surfaces. The properties of each component and the state of the composite formed from the small crystal... 

The TTP molecules are chiral because of the angle of about 55° between the planes of the tetrachlorophthalimide and p-tolyl parts of the molecule. As the space group C2 is chiral, spontaneous resolution has taken place on crystallisation but the absolute configuration of the crystal used in the structure determination could not be determined. It would be interesting to see if the chirality of the channel is adequate to resolve enantiomers but this was not attempted. The a- and y-polymorphs crystalline in centrosymmetrical space groups and thus are racemates (in a-TTP the molecules lie on crystallographic mirror planes while in y-TTP their conformation is similar to that found in the inclusion compounds). Of course the twisted TTP molecules will racemise rapidly in solution. 

At any one temperature and pressure, only one crystal (polymorph) form will be stable. Any other polymorph found under these condition is metastable and will eventually convert to the stable form, but the conversion may be very slow (sometime can take years). The metastable form is a higher energy form and usually has a lower melting point, greater solubility, and faster dissolution rate. Examples are chloramphenicol palmitate (Aguir etal. 1967) and sulfameter (Khalil et al. 1972). This sulfanilamide is reported to have six polymorphs. Crystalline form II is about twice as soluble as crystalline form III. Studies in normal subjects showed that the rate and extent of absorption is approximately 40% greater from form II. Table 8.3 provides a few examples of drugs that exhibit polymorphism. 

Poly(vinylidene fluoride) (PVDF) is a semicrystalline engineering polymer with very good resistance to chemicals, oxidation, and UV radiation (J. H. Yen, 2006). PVDF is known for its polymorphism crystalline structure and complicated microstructure. It is one of the most widely studied polymers due to its non-linearity, piezo- and pyro-electricity (L. T. Vo, 2007 K. Pramoda, 2005). PVDF can crystallize in at least five well-known crystalline phases (E. Giannetti, 2001, A. Lovinger, 1982 N. S. Nalwa, 1995) ... 

Free silica is the most widespread substance in nature with a fibrogenic potential for hving tissues. Free silica (sihcon dioxide) occurs in three forms polymorphic crystalline, cryptocristalline (i.e., minute crystals), and amorphous (i.e., noncrystalline). 

IV. Barytocalcite—640 °C endotherm, baryto-cacite undergoes a polymorphic transformation 815 °C endotherm, barytocalcite decomposes to CaO and BaCOj and releases COj," 890 C endotherm, BaCOj (witherite) undergoes a polymorphic transformation 960 °C endotherm, witherite transforms to another polymorphic crystalline structure. 

IV. Eulytite—735 °C endotherm, eulytite undergoes a polymorphic transformation 871 °C endotherm, eulytite is transformed to another polymorphic crystalline structure. 

Two underlying mechanisms of color change in a piezochromic material have been advanced (/). In a polymorphic crystalline solid diat has one phase at ambient pressure and another at hi pressme, the corresponding colors or optical properties of the two phases can be observed by pressme-induced phase transition. Alternatively, pressure induced changes in molecular geometry and intermolecular interactions of the molecular solid can cilitate die continuous variation of optical properties with pressure. The latter mechanism appears to... 

Nishikawa and Ono recorded the crystaUine nature of cellulose using the X-ray diffraction patterns from fiber bundles from various plants. Cellulose is known to exist in at least four polymorphic crystalline forms, of which the structure and properties of cellulose 1 (native cellulose) and ceUulose II (regenerated cellulose and mercerized cellulose) have been most extensively studied. As a first approximation, the crystal structure of cellulose I determined by X-ray diffraction can be described by monoclinic unit cell which contains two cellulose chains in a parallel orientation with a twofold screw axis (Klemm et al. 2005). Cellulose I has two polymorphs, a triclinic stmcture (la) and a monoclinic structure (IP), which coexist in various proportions depending on the cellulose source (Azizi Samir et al. 2005) (Nishiyama 2009). The la structure is the dominate polymorph for most algae (Yamamoto and Horii 1993) and bacteria (Yamamoto and Horn 1994), whereas ip is the dominant polymorph for higher plant cell wall cellulose and in tunicates. 

Syndiotactic polypropylene (s-PP) presents a very complex polymorphic behavior (the first part of this section on s-PP modifications is reported in the publication [203-212]). s-PP chains crystallize to various polymorphic crystalline phases and,... 

SM Cooper. A novel polymorphic crystalline form of fluticasone propionate. A method for its production and pharmaceutical compositions thereof. Patent application, published as WO 98/17676, 1997. 

Stretching may change the polymorphic crystalline structure, as in the case of poly(vinylidene fluoride) (PVDF) when t monoclinic, paraelectric, a phase b transformed to the orthorombic, ferroelectric, phase by stretching or hot roiling (14]. 

Enthalpy, entropy, vapor pressure SolubUity, osmotic pressure Surface tension Viscosity, rheology Condensation form at room conditions Polymorphic crystalline forms Hardness, mechanical strength Roiling and melting temperature... 

Figure 10.10 Schematic representation of the main interconversion conditions for the helical polymorphic crystalline forms of syndiotactic polystyrene. G stands for gnest molecules of 6 and e phases.

Yang J, Pan P, Hua L, Dong T, Zhu B, Inoue Y, et al. Fractionated crystallization, polymorphic crystalline structure and spherulite morphology of poly(butylenes adipate) in its miscible blend with poly(bntylenes succinate). Polymer 2011 52 3460-8. 

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