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In physical and organic chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have the same size, shape, or mass. A sample of objects that have an inconsistent size, shape and mass distribution is called non-uniform. The objects can be in any form of chemical dispersion, such as particles in a colloid, droplets in a cloud, crystals in a rock, or polymer molecules in a solvent. Polymers can possess a distribution of molecular mass; particles often possess a wide distribution of size, surface area and mass; and thin films can possess a varied distribution of film thickness. IUPAC has deprecated the use of the term ''polydispersity index'' having replaced it with the term ''dispersity'', represented by the symbol ''Đ'' (pronounced D-stroke〔) which can refer to either molecular mass or degree of polymerization. It can be calculated using the equation ''Đ''M = ''M''w/''M''n, where ''M''w is the weight-average molar mass and ''M''n is the number-average molar mass. It can also be calculated according to degree of polymerization, where ''Đ''X = ''X''w/''X''n, where ''X''w is the weight-average degree of polymerization and ''X''n is the number-average degree of polymerization. In certain limiting cases where ''Đ''M = ''Đ''X, it is simply referred to as ''Đ''. IUPAC has also deprecated the terms ''monodisperse'', which is considered to be self-contradictory, and ''polydisperse'', which is considered redundant, preferring the terms ''uniform'' and ''non-uniform'' instead.〔Stepto, R. F. T.; Gilbert, R. G.; Hess, M.; Jenkins, A. D.; Jones, R. G.; Kratochvíl P. (2009). "(Dispersity in Polymer Science )" ''Pure Appl. Chem.'' 81 (2): 351–353. DOI:10.1351/PAC-REC-08-05-02.〕 ==Overview== A monodisperse, or uniform, polymer is composed of molecules of the same mass. Natural polymers are typically monodisperse. Synthetic monodisperse polymer chains can be made by processes such as anionic polymerization, a method using an anionic catalyst to produce chains that are similar in length. This technique is also known as living polymerization. It is used commercially for the production of block copolymers. Monodisperse collections can be easily created through the use of template-based synthesis, a common method of synthesis in nanotechnology. A polymer material is denoted by the term polydisperse, or non-uniform, if its chain lengths vary over a wide range of molecular masses. This is characteristic of man-made polymers.(). Natural organic matter produced by the decomposition of plants and wood debris in soils (humic substances) also has a pronounced polydispersed character. It is the case of humic acids and fulvic acids, natural polyelectrolyte substances having respectively higher and lower molecular weights. Another interpretation of polydispersity index is explained in the article Dynamic light scattering (cumulant method subheading). In this sense, the PDI values are in the range from 0 to 1. The polydispersity index (PDI) or heterogeneity index, or simply dispersity (Đ), is a measure of the distribution of molecular mass in a given polymer sample. Đ calculated is the weight average molecular weight () divided by the number average molecular weight (). It indicates the distribution of individual molecular masses in a batch of polymers. Đ has a value equal to or greater than 1, but as the polymer chains approach uniform chain length, Đ approaches unity (1).〔Peter Atkins and Julio De Paula, ''Atkins' Physical Chemistry'', 9th edition (Oxford University Press, 2010, ISBN 978-0-19-954337-3)〕 For some natural polymers Đ is almost taken as unity. Đ (PDI) from polymerization is often denoted as: , where is the weight average molecular weight and is the number average molecular weight. is more sensitive to molecules of low molecular mass, while is more sensitive to molecules of high molecular mass. 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「Dispersity」の詳細全文を読む スポンサード リンク
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