Alkoxysilane modification

Scheme 1. Alkoxysilane modification of organic polymers. Left silylation with IPTES Right silylation with TESSA,...

Anotlier important modification metliod is tire passivation of tire external crystallite surface, which may improve perfonnance in shape selective catalysis (see C2.12.7). Treatment of zeolites witli alkoxysilanes, SiCl or silane, and subsequent hydrolysis or poisoning witli bulky bases, organophosphoms compounds and arylsilanes have been used for tliis purjDose [39]. In some cases, tire improved perfonnance was, however, not related to tire masking of unselective active sites on tire outer surface but ratlier to a narrowing of tire pore diameters due to silica deposits. 

Ohtsuki, C., Miyazaki, T., Kyomoto, M., Tanihara, M. and Osaka, A. (2001) Development of bioactive PMMA-based cement by modification with alkoxysilane and calcium salt. Journal of Materials Science-Materials in Medicine, 12, 895-899. 

Donath, S., Militz, H. and Mai, C. (2004). Wood modification with alkoxysilanes. Wood Science and Technology, 38(7), 555-566. 

Schneider, M.H. and Brebner, K.I. (1985). Wood-polymer combinations the chemical modification of wood by alkoxysilane coupling agents. Wood Science and Technology, 19(1), 67-73. 

The second method of catalyst incorporation in mesoporous materials involves, generally, multi-step syntheses where organic moieties (i.e. amines, phosphines, thiols) are immobilized followed by post modification toward the final product. Alternatively, the catalyst may be synthesized as the corresponding alkoxysilane complex followed by immobilization into the mesostructured materials. For instance, Kiihn and co-workers demonstrated covalent... 

Figure 3. Reaction schemes for modification of the glass surface by the azidofunctional alkoxysilane (reaction 1) and reaction of the modified glass surface with the polyethylene matrix (reaction 2).

The introduction of organofunctional groups, i.e. the use of alkoxysilanes of the type (R O Si—X—A, where A is a functional organic group and X is a chemically inert spacer permanently linking Si and A, results in a more extensive chemical modification of the materials76. The properties of the organic functions A supplement the properties of the polysiloxane matrix formed by hydrolysis and condensation of the Si(OR )3 and Si(OR )4 units. 

Chlorosilanes and alkoxysilanes commonly used in the modification of silica, with formula, acronym and common application with silica, (res. research, block. blocking agent, chrom. chromatography, comp. composites, enz. enzyme immobilization, IC integrated circuits, cat. catalyst, met preconcentration of metals). 

The incorporation of organofunctional groups on the silica surface may be effectuated during the synthesis of the silica material. The addition of organofunctional alkoxysilanes to the TEOS solution in the sol-gel process, produces functionalized silica gels. This procedure does not allow a careful control of the obtained surface morphology. Since the relative amounts of silane and TEOS is the only variable parameter, neither layer thickness, nor modification density can be precisely tuned. This results in an irreproducible functionalization of the surface. 

The preparation of organofunctional silica gels on industrial scale is performed by liquid phase reaction. As a solvent, water, a water/ethanol or water/acetone mixture is used. Chlorosilanes or alkoxysilanes are used for this type of modification. 

Finally, the modification with alkoxysilanes and alkylsiloxanes is briefly discussed. 

Morrall2 used a HPLC system with two columns. The first column was loaded with the controlled pore glass (CPG) to be modified. The second column was used for separation of the reaction effluents. This column was coupled to a refractive index detector, allowing for quantitative detection of the effluents. The reaction was initiated by injecting an APTS/toluene mixture and stopped by injection of pure toluene. With this so-called stop-flow mechanism reaction times down to 18 seconds could be used. From these analyses it became evident that upon mixing of the aminosilane with the silica, a very rapid physisorption occurs. The initial adsorption of the APTS (from toluene solution on dried CPG) occurred before the 18 second minimum time delay of the stop-flow apparatus. For non-aminated silanes the adsorption proved to be much slower. This study also revealed the pivotal role of surface water in the modification of siliceous surfaces with alkoxysilanes, as discussed in the previous chapter. 

In a number of cases, (e.g. modification of a silica surface, containing N-fiinctions), alkoxysilanes are more useful than the corresponding chlorosilanes for reacting with the silica surface. 

A variety of methacryloyloxymethyl alkoxysilanes have been synthesized. The compounds show exceptionally high reactivity rates with regard to hydrolysis and condensation compared to the parent 3-methacryloyloxypropyltrimethoxysilane 3e. This high reactivity renders compounds 3a-d highly interesting for the modification of inorganic surfaces and makes them very attractive as... 

Modifications of alkoxysilanes with diols or polyols such as ethylene glycol, glycerol, etc. have been well known since the middle of the last century however, their application was hampered by their hydrolytic instability [7, 8]. Only in recent years is interest in this type of glycol-modified silanes increasing again because of some obvious advantages ... 

A cosolvent is usually present in fairly large amount and miscible with the main solution solvent. A cosolvent will alter micellar behavior and the reactivity of the inorganic components. A cosolvent could lead to increase surfactant solubility and modification of the critical micellar concentration (increased in most cases). The cosolvent is normally not located within the micellar aggregates. However, specific interaction with the polar head groups is possible. Furthermore, the kinetics of hydrolysis and condensation of inorganic precursors (e.g. alkoxysilane) may be affected. Methanol and ethanol are typical examples, acting as cosolvent in aqueous syntheses of mesoporous silica. 

Adsorption of alkoxysilanes onto cellulose fibers also holds promise for modification of the cellulose surface. The basic formula of the silane-coupling agents used has an organofunctional group on one side of the chain and an alkoxy group on the other. Abdelmouleh studied the adsorption of several prehydrolyzed alkox-... 

HMS have been chemically modified using (3-aminopropyl)triethoxysilane (APTES). A solution (30 ml) of HMS (0.1 g) and APTES (2 ml) was prepared in toluene. In fact, APTES, an aminosilane, can be used in the silanisation process to functionalise the HMS surfaces with alkoxysilane molecules. APTES can also be used to covalently bond thermoplastics to polydimethylsiloxane (PDMS). APTES-functionalised HMS surfaces have been shown to be non-toxic to embryonic rat cardiomyocytes in vitro. In this study, the resulting magnetite/silicon particles were washed with ethanol and then dried under vacuum at 60 C for 24 h [57]. The obtained amine-functionalised HMS (HMS-NHi) were dispersed by ultrasound in toluene (30 ml). Another modification involved D,L-lactide (0.4 g) and PEG (0.1 g), which were placed into a three-neck round-bottom flask containing a magnetic stirrer. The catalyst, Sn(Oct)2, was added to the flask at a concentration of 0.1% of the solution the flask was put into an oil bath to react for 24 h at 110 C in nitrogen under ambient conditions [58]. 

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