Coupling agents principles

Spectroscopic techniques are extremely useful for the characterization of filler surfaces treated with surfactants or coupling agents in order to modify interactions in composites. Such an analysis makes possible the study of the chemical composition of the interlayer, the determination of surface coverage and possible coupling of the filler and the polymer. This is especially important in the case of reactive coupling, since, for example, the application of organofunctional silanes may lead to a complicated polysiloxane interlayer of chemically and physically bonded molecules [65]. The description of the principles of the techniques can be found elsewhere [15,66-68], only their application possibilities are discussed here. 

Scheme 198 Principle of use of UNCA s as coupling agents in peptides synthesis.

For NR nanocomposite filled with silica, it has always been known that the hydrophilicity-hydrophobicity issue is a challenge since silica is hydrophilic and NR is hydrophobic. The usual method to overcome this issue is by adding coupling agent. In 1987 Wu and coworkers introduced admicellar polymerization where a thin polymeric film will be formed on the silica s surface. This process yields a thin film of polymer on the silica which can further enhance the adhesion between the surfaces of silica and rubber. The steps involved in admicellar polymerization are outlined in Scheme 7.7. In principle, a bilayer of surfactant, i.e. the admicelle, is first formed on the surface of the silica. Monomer will then penetrate the admicelle, i.e. the adsolubilization of monomer. Upon addition of initiator to the reaction system, in situ polymerization occurs in the admicelles. Finally, the surfactant is removed by washing with water and an ultrathin polymer layer is formed on the surface of the silica. The polymerization of the monomer in the admicelles can be induced by thermal process, chemical initiators or radiation. ... 

Principle Silane Coupling Agent Types and their Use... 

Consider the Merrifield method described above and the use of an N-protected serine (Ser, S) reactant that is to be reacted by coupling the carboxylate of this N-protected serine (Ser, S) at the amine of an amino acid with a protected ester group. It should be clear, based on the chemistry discussed in Chapter 8 (Alcohols) that both of the coupling agents discussed in conjunction with the Merrifield method above, viz., dicyclohexylcarbodiimide (DCCD) and carbonyldiimidazole (CDI) (Scheme 12.76) cannot be used because they both are known to react with alcohols. Of course, in principle, the alcohol can be protected too, but a separate step is then required and the standard Merrifield system as outlined above will need modification. 

One of the most common questions is how much coupling agent to use. There is no quick answer to this, but there are some simple guiding principles. 

The same principles used in selecting a silane for reinforced plastics can be used to select a silane for paints, inks, coatings, and adhesives. The chemical nature of the organic material that is used in the application is the major criterion that governs the effectiveness of a particular coupling agent as an adhesion promoter in any application. 

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