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Organogels
An organogel is a class of gel composed of a liquid organic phase within a three-dimensional, cross-linked network. Organogel networks can form in two ways. The first is classic gel network formation via polymerization. This mechanism converts a precursor solution of monomers with various reactive sites into polymeric chains that grow into a single covalently-linked network. At a critical concentration (the gel point), the polymeric network becomes large enough so that on the macroscopic scale, the solution starts to exhibit gel-like physical properties: an extensive continuous solid network, no steady-state flow, and solid-like rheological properties.〔Raghavan, S.R.; Douglas, J.F. Soft Matter. 2012, 8, 8539.〕 However, organogels that are “low molecular weight gelators” can also be designed to form gels via self-assembly. Secondary forces, such as van der Waals or hydrogen bonding, cause monomers to cluster into a non-covalently bonded network that retains organic solvent, and as the network grows, it exhibits gel-like physical properties.〔Hirst, A.R.; Coates, I.A.; Boucheteau, T.R.; Miravet, J.F.; Escuder, B.; Castelletto, V.; Hamley, I.W.; Smith, D.K. J. Am. Chem. Soc. 2008, 130, 9113-9121.〕 Both gelation mechanisms lead to gels characterized as organogels.
Gelation mechanism greatly influences the typical organogel properties. Since precursors with multiple functional groups polymerize into networks of covalent C-C bonds (on average 85 kcal/mol), networks formed by self-assembly, which relies on secondary forces (generally less than 10 kcal/mol), are less stable.〔Ege, S. N. Organic Chemistry Structure and Reactivity, 5th ed.; Cengage Learning: Mason, Ohio, 2009.〕,〔Sinnokrot, M.O.; Sherrill, C.D. J. Phys. Chem. A. 2006, 110, 10656.〕 Theorists also have difficulties predicting characteristic gelation parameters, such as gel point and gelation time, with a single and simple equation. Gel point, the transition point from a polymer solution to gel, is a function of the extent of reaction or the fraction of functional groups reacted. Gelation time is the time interval between the onset of reaction– by heating, addition of catalyst into a liquid system, etc.– and gel point. Kinetic and statistical mathematical theories have had moderate success in predicting gelation parameters; a simple, accurate, and widely applicable theory has not yet been developed.
This article will first discuss the details of organogels formation and the variables of the characteristic gelation parameters as they relate to organogels. Then, various methods used to characterize organogels will be explained. Finally, we will review the use of organogels in various industries.
==Organogel formulation==
The formulation of an accurate theory of gel formation that correctly predicts gelation parameters (such as time, rate, and structure) of a broad range of materials is highly sought after for both commercial and intellectual reasons. As noted earlier, researchers often judge gel theories based upon their ability to accurately predict gel points. The kinetic and statistical methods model gel formation with different mathematical approaches. most researchers used statistical methods, as the equations derived thereby are less cumbersome and contain variables to which specific physical meanings can be attached, thus aiding in the analysis of gel formation theory.〔Pizzi, A.; Mittal, K. L. Handbook of Adhesive Technology, 2nd ed.; Marcel Dekker, Inc.: New York, 200; Chap. 8.〕 Below, we present the classical Flory-Stockmayer (FS) statistical theory for gel formation. This theory, despite its simplicity, has found widespread use. This is due in large part to small increases in accuracy due provided by the use of more complicated methods, and to its being a general model which can be applied to many gelation systems. Other gel formation theories cased on different chemical approximations have also been derived. However, the FS model has better simplicity, wide applicability, and accuracy, and remains the most used.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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