Multifunctional silanes

Relative hydrolysis and condensation rate studies of multifunctional silanes, Si(OR), under acidic and basic catalysis showed that the first (OR) group hydroly2es much more readily than subsequent groups (195). Sdanol—sdanol condensation is much slower than sdanol—alkoxysilane condensation, even if the alkoxysilane is monofunctional, thus suggesting that chain extension is insignificant ia the presence of a cross-linker (196—199). 

Xerogels Ag, Au Two steps (1) gamma radiation initiation of metal-particle growth in a solution containing metal ions and multifunctional silanes (2) xerogels with metal clusters grafted on an oxide network prepared via hydrolysis and condensation 76... 

While the surface coverage obtainable with monofunctional silanes is limited by steric hindrance, the reaction of multifunctional silanes can be carried out under conditions that result in a significantly higher content of bonded phase than is possible with monofunctional silanes. The addition of water to the reaction results in the formation of additional silanols, which then can react further and add more bonded phase. Traditionally, phases with a... 

The majority of the commercially available bonded phases are today based on monofunctional silanes. The main reason for this is the increased reproducibility of the preparation. With di- and trifunctional silanes, even small amounts of water present in the reagents, either on the surface of the silica or in the reaction solvent, can increase the amount of bonded phase attached to the surface substantially. This problem does not exist with monofunctional silanes. Also, hindered mass transfer has been observed when multifunctional silanes were reacted with small-pore silicas to a high coverage level. The latter issue should, however, not be a fundamental problem, since the pore size can be matched to the coverage level to avoid this phenomenon. 

Silane cross-linking Multifunctional silanes Silieone robbers, PE... 

Kawase, T., Sawada, H. End-capped fluoroafkyl-functional silanes. Part II modification of polymers and possibility of multifunctional silanes. J. Adhes. Sci. Technol. 16, 1121-1140... 

Fig. 1.18. A typical reaction in which vinyl-terminated PDMS chains are end linked with a multifunctional silane.

A more thorough investigation of the effects of cross-link functionality requires use of the more versatile chemical reaction illustrated in Fig. 1.18. Specifically, vinyl-terminated PDMS chains were end linked using a multifunctional silane [78]. This reaction was used to prepare PDMS model networks having functionalities ranging from three to 11, with a relatively unsuccessful attempt to achieve a functionality of 37. The modulus 2C increased with increasing functionality, as expected from the increase in constraints on the cross-links, and as predicted in Eqs. (1.17) and (1.18). Similarly, 2C2 and its value relative to 2C both decreased, for reasons that have already been mentioned. 

---