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In materials science, polymorphism is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements. The complete morphology of a material is described by polymorphism and other variables such as crystal habit, amorphous fraction or crystallographic defects. Polymorphism is relevant to the fields of pharmaceuticals, agrochemicals, pigments, dyestuffs, foods, and explosives. When polymorphism exists as a result of difference in crystal packing, it is called packing polymorphism. Polymorphism can also result from the existence of different conformers of the same molecule in conformational polymorphism. In pseudopolymorphism the different crystal types are the result of hydration or solvation. This is more correctly referred to as solvomorphism as different solvates have different chemical formulae. An example of an organic polymorph is glycine, which is able to form monoclinic and hexagonal crystals. Silica is known to form many polymorphs, the most important of which are; α-quartz, β-quartz, tridymite, cristobalite, coesite, and stishovite. A classical example is the pair of minerals, calcite and aragonite, both forms of calcium carbonate. An analogous phenomenon for amorphous materials is polyamorphism, when a substance can take on several different amorphous modifications. ==Background== In terms of thermodynamics, there are two types of polymorphic behaviour. For a monotropic system, a plot of the free energy of the various polymorphs against temperature do not cross before all polymorphs melt—in other words, any transition from one polymorph to another below melting point will be irreversible. For an enantiotropic system, a plot of the free energy against temperature shows a crossing point threshold before the various melting points.〔''Solid-state investigation of polymorphism and tautomerism of phenylthiazole-thione: a combined crystallographic, calorimetric and theoretical survey'' A. Carletta, C. Meinguet, J. Wouters, A. Tilborg, Cryst. Growth Des. 2015. 〕 It may also be possible to revert interchangeably between the two polymorphs by heating or cooling, or through physical contact with a lower energy polymorph. ] The first observation of polymorphism in organic materials is attributed to Friedrich Wöhler and Justus von Liebig when in 1832 they examined〔F. Wöhler, J. Liebig, Ann. Pharm. 1832, 3, 249 – 282. 〕 a boiling solution of benzamide: upon cooling, the benzamide initially crystallised as silky needles, but when standing these were slowly replaced by rhombic crystals. Present-day analysis〔''Polymorphism in Benzamide: Solving a 175-Year-Old Riddle'' Juergen Thun, Lena Seyfarth, Juergen Senker, Robert E. Dinnebier, and Josef Breu Angew. Chem. Int. Ed. 2007, 46, 6729 –6731〕 identifies three polymorphs for benzamide: the least stable one, formed by flash cooling is the orthorhombic form II. This type is followed by the monoclinic form III (observed by Wöhler/Liebig). The most stable form is monoclinic form I. The hydrogen bonding mechanisms are the same for all three phases, however they differ strongly in their pi-pi interactions. Polymorphs have different stabilities and may spontaneously convert from a metastable form (unstable form) to the stable form at a particular temperature. Most polymorphs of organic molecules only differ by a few kJ/mol in lattice energy. Approximately 50% of known polymorph pairs differ by less than 2 kJ/mol and stability differences of more than 10 kJ/mol are rare. They also exhibit different melting points, solubilities (which affect the dissolution rate of drug and consequently its bioavailability in the body), X-ray crystal and diffraction patterns. Various conditions in the crystallisation process is the main reason responsible for the development of different polymorphic forms. These conditions include: * Solvent effects (the packing of crystal may be different in polar and nonpolar solvents) * Certain impurities inhibiting growth pattern and favour the growth of a metastable polymorphs * The level of supersaturation from which material is crystallised (in which generally the higher the concentration above the solubility, the more likelihood of metastable formation) * Temperature at which crystallisation is carried out * Geometry of covalent bonds (differences leading to conformational polymorphism) * Change in stirring conditions Despite the potential implications, polymorphism is not always well understood.〔Crystal Engineering: The Design and Application of Functional Solids, Volume 539, Kenneth Richard Seddon, Michael Zaworotk 1999〕 In 2006 a new crystal form of maleic acid was discovered 124 years after the first crystal form was studied. Maleic acid is a chemical manufactured on a very large scale in the chemical industry and is a salt forming component in medicine. The new crystal type is produced when a co-crystal of caffeine and maleic acid (2:1) is dissolved in chloroform and when the solvent is allowed to evaporate slowly. Whereas form I has monoclinic space group ''P''21/''c'', the new form has space group ''Pc''. Both polymorphs consist of sheets of molecules connected through hydrogen bonding of the carboxylic acid groups; but, in form I, the sheets alternate with respect of the net dipole moment, whereas, in form II, the sheets are oriented in the same direction. 1,3,5-Trinitrobenzene is more than 125 years old and was used as an explosive before the arrival of the safer 2,4,6-trinitrotoluene. Only one crystal form of 1,3,5-trinitrobenzene was known in the space group ''Pbca''. In 2004, a second polymorph was obtained in the space group ''Pca''21 when the compound was crystallised in the presence of an additive, trisindane. This experiment shows that additives can induce the appearance of polymorphic forms. Walter McCrone has stated that "every compound has different polymorphic forms, and that, in general, the number of forms known for a given compound is proportional to the time and money spent in research on that compound." 〔〔W. C. McCrone, in Physics and Chemistry of the Organic Solid State, eds. D. Fox, M. M. Labes and A. Weissberger, Interscience Publishers, London, 1965, vol. 2, pp. 725-767.〕〔''Pharmaceutical Stress Testing: Predicting Drug Degradation'', Second Edition Steven W. Baertschi, Karen M. Alsante, Robert A. Reed 2011 CRC Press〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Polymorphism (materials science)」の詳細全文を読む スポンサード リンク
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