Polymorphism of Crystalline Systems

The importance, implications, and investigation of polymorphism are described in-depth in several recent monographs and edited books [4—8]. 

It is imperative to clearly distinguish the term polymorphism from the term morphology. Polymorphism refers to the crystal structure, while morphology describes the macroscopic shape of a crystal (equant (cube), lath, needle, flake, colum, and plate) [9]. The same polymorph can be easily crystallized in different morphologies however, different polymorphs can also have the same morphology. In other words, polymorphism and morphology are independent of each other. 

Crystallization Basic Concepts and Indu rial Applications, First Edition. Edited by Wol ang Beckmann. 

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The extraordinary polymorphism of the systems described above introduces the question of its relation with the molecular behavior, as it is well known that, in classical molecular crystals, the long range crystalline order is the result of the propagation of the short range order of the molecules. 

Alexandridis P, Olsson U and Lindman B 1997 Structural polymorphism of amphiphilic copolymers Six lyotropic liquid crystalline and two solution phases in a poly(oxybutylene)-poly(oxyethylene)-water-xylene system Langmuir 23-34... 

During precipitate ageing, a gradual transformation of an initially precipitated metastable phase into a final crystalline form often occurs. The metastable phase may be an amorphous precipitate, a polymorph of the final material, a hydrated species or some system-contaminated substance (Mullin, 2001). In 1896, Ostwald promulgated his rule of stages which states that an unstable... 

The isolation of crystalline products having mixed polymorphic compositions (often referred to as concomitant polymorphism) remains a topic of interest, even though the phase rule predicts that a system at equilibrium consisting two components (solvent + solute) and three phases (solution + Form I + Form II) is uni variant. Hence, for crystallizations performed at a fixed pressure (typically atmospheric) the system becomes nonvariant and genuine equilibrium can exist at only one temperature. Therefore, concomitant products must be obtained under nonequilibrium conditions. Flexibility in molecular conformation was attributed to the concomitant polymorphs of a spirobicyclic dione [34] and of 3-acetylcoumarin [35],... 

With these three different examples it has been demonstrated that the systematics observed for the polymorphism of m-l.c. s is also valid for the side chain polymers, provided that a flexible spacer connects the rigid mesogenic moieties to the polymer main chain. Deviations from this behavior are observed, when the mesogenic moieties are directly linked to the backbone. Under these conditions, normally no liquid crystalline behavior is to be expected, according to the model considerations mentioned in Chap. 2.1. Some examples, however, proved l.c. properties for such systems, which are characterized by two striking properties Very high glass transition temperatures and only smectic structures even in case of short substituents... 

Corundum is aluminum oxide, q -A1203, which has a hexagonal crystalline structure that is analogous to hematite. However, water treatment systems most often use activated alumina, which is typically produced by thermally dehydrating aluminum (oxy)(hydr)oxides to form amorphous, cubic (y), and/or other polymorphs of corundum (Clifford and Ghurye, 2002, 220 Hlavay and Poly k, 2005 Mohan and Pittman, 2007). When compared with corundum, amorphous alumina tends to have higher surface areas, greater numbers of sorption sites, and better sorption properties. 

In addition to characterizing frozen systems intended to be freeze dried, it is important to characterize the freeze-dried product. This includes determination of the physical state of the dried product that is, crystalline, partially crystalline, or amorphous. It may also include identification of the polymorph of a crystallizing component which exhibits polymorphism and determination of whether the crystal form observed is affected by changes in formulation and processing conditions. For amorphous systems, the glass transition temperature of the amorphous solid, as well as the extent to which Ts changes with residual moisture, may be a critical attribute of the product with regard to both physical and chemical stability. 

In an extraordinarily comprehensive review of polymorph screening procedures, it has been reported that during the conduct of 245 polymorph screens, about 90% of the systems studied exhibited multiple crystalline and noncrystalline forms, and about 50% exhibited polymorphism [38]. As to cocrystal screening, it was concluded that it is most efficient to use a combination of structural and physical property evaluation methods in conjunction with screening protocols similar to those used to detect new polymorphic forms. Data from 64 cocrystal screening studies were considered, and it was shown that cocrystals were found in 61% of the studied systems. 

Cellulose powders can be created by cutting fibers into small particles, perhaps with a Wiley mill (Arthur H. Thomas Company, Swedesboro, New Jersey). On a laboratory x-ray system, powder diffraction patterns take 30 min. The positions of the peaks indicate the polymorphic form (I-IV) the powder diffraction pattern is often used as a fingerprint for comparison with the known pattern for a given crystalline form [207]. The breadth of the peaks is related to the extent of crystallinity (Figure 5.17, bottom). Using the Scherrer formula [245,246] and assuming no other distortions, the crystallite size can be calculated. Values for cotton perpendicular to the molecular axis are around 40 A. That corresponds to a 6x6 array of... 

In nucleation, or the formation of the crystalline phase from the liquid, some organization of molecules is expected. In lipids, the natural ordering of the liquid phase leads to crystal formation. In fact, rapid cooling of liquid hpids results in the formation of a diffuse crystalline phase Oow-energy polymorph) because of the ordering structure in the liquid phase. Such rapid cooling of other systems, most... 

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