Introduction - Physical Gels from Biological and Synthetic Polymers - page 8

1.3.3 Inorganic gels

' '

The synthesis of solid materials via soft chemistry has been widely developed over the last

-

two decades. These syntheses involve sol gel chemistry based on inorganic polymerization

of molecular precursors. The sol gel process is a wet-chemical technique for the fabrication

of metal oxide materials, starting either from a chemical solution (

-

' sol ' is short for ' solution ' )

or from colloidal particles, to produce an integrated network (a ). Typical precursors

are alkoxides M(OR) z , where M is a metal with valency z (Si, Ti, Zr, Al, Sn,

' gel '

)andORis
an alkoxide corresponding to a deprotonated alcohol which undergoes hydrolysis and
polycondensation (step-addition) reactions to form a system composed of solid particles.
The sol evolves towards a continuous inorganic network, composed of particles with sizes
ranging from 1 nm to 1

...

μ m, dispersed in a solvent containing a liquid phase.

gel syntheses developed mainly from 1980 but are nowadays very widely used.
The approach is interesting in that it is a cheap, low-temperature technique that allows for

Sol -

s chemical composition. The process can be used for producing
monolithic ceramics, glasses,

fine control of a product '

fibres, membranes, aerogels or powders (e.g. microspheres
or nanospheres), and it can be fabricated as very thin

films of metal oxides for various

purposes. Making thin films requires an advanced knowledge of the rheology of the sol -

gel transition in order to control the thickness and regularity of the film with precision.

Several rheological studies can be found involving the sol gel transition in silica gels

from tetraethoxysilane (TEOS) (Devreux et al., 1993 ) or from tetramethoxysilane
(TMOS) (Martin et al., 1987 ). Sol

-

gel transitions exhibiting similarities with the poly-
merization of organic molecules have been explored in the context of a uni

-

ed descrip-
tion of gelation phenomena (see Chapter 3 ). Other than this, such inorganic gels will not
be discussed further in this topic.
However, physical gels of a wide variety of other types will be covered. As outlined
above, these gels can be classi

ed according to their mechanism of network formation.
Recent experimental studies not only allow a much better understanding of gel structures
at a very local scale (~1 nm) but enable rheological measurements of gels to be carried out
under de ned small- and large-deformation regimes. Because the number of gelling

systems has increased following a series of innovations, it is necessary to both compare
and categorize results for current systems. However, it is now possible to reveal the
unique of some of these systems to rationalize the origin of their properties.

This remains the overall objective of this topic.

' 'fingerprint' '

1.4 Physical gels

Throughout this topic we identify several mechanisms leading to physical gelation,
depending on the polymer and on the solvent. In volume terms, the major component
in physical gels is the solvent, and in most of the physical gels presented in this topic the
solvent is aqueous. However, in a few other systems the choice of appropriate organic
solvent plays a very important role in gelation and often helps modify the structure of
polymer aggregates ( Chapter 8 ).