Pendant Organic Groups

The key function of the organic (methyl) groups is to provide the intrinsic surface activity of the silicones. The order of increasing surface energy for single carbon based groups is —CF3 > —CF2— > —CH3 > —CH2—. Liquid surface tension measurements show that, as expected, the order of increasing surface activity is hydrocarbon, followed by silicone, and then by fluorocarbon. Interfacial tension measurements against water, however, show the order of increasing interfacial activity to be fluorocarbon, hydrocarbon, silicone. Silicones do not fit the simple pattern that a reduction in surface energy means an increase in hydrophobicity and interfacial tension because of their backbone flexibility, which allows them to adopt various orientations at different interfaces. The interfacial tension of silicone is also independent of chain length indicating high molecular chain freedom. In addition, critical surface tension of wetting values for silicones have been found to be higher than their liquid surface tension values, meaning that they are able to spread over their own absorbed film. This has an advantage in achieving complete, uniform surface coverage, facilitates the efficient spreading of other materials and results in smooth, lubricating films. In addition, due to the organic groups, the solubility parameters of sili-cones are significantly lower than those of water and many organic materials making them useful in forming barriers to wash-off or wear and increasing the substantivity of formulations. The introduction of functional groups such as phenyl, alkyl, polyether, amino etc. onto the backbone expands the properties and benefits of silicones further [1-3].

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