Precursor modification

The hydrolysis and subsequent condensation behaviour of a given silane can be altered by the processing parameters mentioned in table 1.4. Another way to control the reactivity of the system is to change the silane that is used, this may involve alkyl substitution of the alkoxy groups [44], the nature of the R group may be changed and chelating ligands may also be employed.

As previously mentioned, alkyl groups can be attached directly to the silicon atom as an Si -C bond which is not hydrolysable. These bonds have a significant effect on the hydrolysis behaviour of the silane. With reference to fig. 1.3, alkyl groups are more basic than alkoxy groups and so will modify the charge distribution about the silicon accordingly. Hydrolysis under acid conditions will be speeded up because of the stabilising effect of the alkyl group on the intermediate state. The final nature of the gel will also be

Chapter 1 Precursor modification significantly affected due to the modified silane functionality. As the amount of alkyl substitution increases so does the effect of these groups on the behaviour of the silane. When the precursor has a functionality of 2 the system cannot achieve gelation since an

overall connectivity of >2 is required to achieve a 3-D network, van Bommel etal [45]

found that, for a system of (l-x)TEOS.(x)MTES, where MTES is MeSi(OEt)3, the hydrolysis time required to give the minimum gelation time increased as x increases. The minimum gelation time also increases with x. They attributed this behaviour to the faster hydrolysis rate of the alkyl substituted alkoxysilane in the acid step and the reduced condensation rate in the basic step, relative to TEOS. The overall effect of this is a reduced amount of water available to TEOS in both steps. The gelation time also increased as with the ramification of the alkyl substituent and with the amount of substitution. Glaser etal [65] formed gels from precursors of different functionality. Fig 1.4 shows a ternary phase diagram of the gel types formed with different ratios of the precursors. A higher

| | functionality appears to yield a

more brittle gel although the gel is

imparts, over specific range, a level of turbidity. In a similar vein

Klemperer etal [41] also found

that gels derived from precursors that contained Si-O-Si bonds were harder, less friable and had Figure 1.4 ternary phase diagram showing the effect of

silane functionality on the resultant gel (after [65]}. smoother surfaces than those

derived from monomeric

precursors. Precursors containing more than one silicon are thus another variation available.

Other forms of substitution can be made to modify the reactivity of the basic alkoxide, these include the replacement of one o r more alkoxy groups by an acetate group. Alternatively the nature of the ligand can be changed altogether, for example by substituting a chlorine atom for an alkoxy group. Andrianov [66] shows that the rate of

Chapter 1 Chelating agents hydrolysis changes for the examples given above in the order:

Si-C l > Si- OCOR > Si-R > SiOR

1.5.1 Chelating agents

Chelating agents may be added to the system to modify the reactivity of the precursors) present. Chelated complexes are more stable than non-chelated ones and so react more slowly. This is of benefit particularly in multi-component systems where the rate of hydrolysis of one precursor is significantly faster than that of another. Typical chelating agents include glycols and organic acids [31]. The chelating reaction is given in eqn. 1.16.

MXj, + mY -> MYm + nX (1.16)

Where m multidentate Y ligands substitute n monodentate X ligands.

The action of the chelating ligand is dependent on the nature of the ligand itself and the coordination geometry it induces about the metal atom. For example Ti(OBun)4 has been modified with acetic acid to reduce its reactivity with water [53,67]. The acetate groups form bidentate ligands [48], similar to those that are formed when oligomerisation occurs (fig. 1.1), these ligands are not easily removed by water and also increase the steric hindrance to water attempting to attack an alkoxy group. Schmidt [19] says that for Al(acac) complexes the rate of condensation is slowed down sufficiently that stable homogeneous sols can be formed. The reaction of silicon alkoxides with chelating agents is modified by the inability of silicon to increase its coordination. TEOS has to be refluxed for several hours in the presence o f acetic acid for any reaction to occur, and then substitution of an alkoxy group by an acetate group occurs [48]. This will render the silane more reactive towards water, the liberated ethanol can then react with any excess acetic acid to provide an in situ way of forming water. This may be useful in removing the inevitable water concentration gradients that arise when reactant addition is from an external source.

The action of formamide in silica sol-gel is to slow down the hydrolysis rates of the silanes [68,69]. The mechanism behind this is thought to be due to the formamide forming stronger hydrogen bonds with the silanols than they do with the alcoholic medium. A more effective shield about the silanols is therefore formed hindering condensation reactions

Chapter 1 Drying and sintering [70]. One by-product of the reduction in the rate of hydrolysis is that a more uniform pore structure is formed, this then leads to smaller differential stresses being set up when the gel is dried. Formamide and other chelating agents that have this effect belong to a class of chemicals that are known as Drying Control Chemical Additives {D.C.C.As). This effect on the pore structure allows gels to be dried into monolithic samples.