Surfaces

Because of the great importance of the surface properties of the polysiloxanes, this topic is treated separately in this chapter. Hydrophobic polysiloxanes having simple aliphatic or aromatic side groups have surfaces that show essentially no attraction to water. In fact, polysiloxanes can serve as water repellants. This property is very useful for applications such as protective coatings on historical monuments and for controlling the surfaces of other polymers, sensors, and quantum dots. Hydrophobic surfaces can be readily regenerated if the surface becomes damaged. Regeneration occurs by rearrangements of the polysiloxane chains so that the hydrophobic methyl groups are once again covering the surface. The flexibility of the siloxane chain backbone facilitates this process. It is also possible to prepare hydrophobic films using methyl-modified siloxane melting gels. Glass surfaces or wool fibers can be coated with polydimethylsiloxane (PDMS) to make them more hydrophobic. In some cases, it is necessary to modify a polysiloxane surface to make it hydrophilic or hydrophobic. Hydrophobization is one aspect of the general topic of modifying and managing the properties of polymer surfaces. An important example involves soft contact lenses that contain PDMS, which is often used because of its very high permeability to oxygen, which is required for metabolic processes within the eye. Such lenses do not feel comfortable however because they do not float properly on the aqueous tears that coat the eye. There are a number of ways to modify the surfaces. There is even a way to make “unreactive” silicones react with inorganic surfaces. In some applications it is useful to have hydrophilicity in the bulk of the polymer instead of just at the surface. One way of doing this is by simultaneously end linking hydrophilic poly(ethylene glycol) (PEG) chains and hydrophobic PDMS chains. Another way is to make a PDMS network with a trifunctional organosilane R’Si(OR) end linker that contains a hydrophilic R’ side chain, such as a polyoxide. Treating only the surfaces is another possibility, for example, by adding hydrophilic brushes by vapor deposition/hydrolysis cycles. Such hydrophilic polysiloxanes can also serve as surfactants.