Biomedical Engineering Reference
In-Depth Information
3.5 EVOLUTION OF NANOPOROSITY
The main difference that sets sol-gel derived glasses apart from
melt-quenched glasses is that gel-derived glasses have a fine-scale
porosity. This is particularly evident with the acid-catalysed process.
The pores are usually in the micropore (diameters below 2 nm) or
mesopore (diameters greater than 2 nm but less than 50 nm) ranges.
These classifications are the official IUPAC classifications [6].
The structure of a gel is established at the time of gelation. The
size of the sol particles and the cross-linking within the particles (i.e.
density) depend upon the pH and
R
ratio (
R
[H
2
O]/[Si(OR)], molar
ratio). The physical characteristics of the gel network depend greatly
upon the size of particles (extent of cross-linking prior to gelation).
The variables of major importance are temperature, nature and con-
centration of electrolyte (acid, base), nature of the solvent and type of
alkoxide precursor.
The gel evolves from a sol, where the small particles that formed
from the hydrolysis of an alkoxide precursor are weakly interacting
with each other (Figure 3.4). The gel forms as condensation reactions
occur between the particles, causing them to bond together. Bridging
oxygen bonds link the particles together, forming a continuous network
(Figure 3.5).
Primary particles of about 2 nm diameter agglomerate to form
secondary particles of 5-100 nm diameter [7]. At low pH, O-Si-O
bonds are formed and the condensation process resembles classical
polycondensation, resulting in a three-dimensional silica network and
depolymerisation is unlikely [3]. Gelation occurs when the secondary
=
Ageing
60
Drying
60-130
Stabilisation
600
Mixing
+
Gelation
Room Temperature
°
C
°
C
°
C
Sample
appearance
TEOS
Gelling Point
Wet Gel
Figure 3.5
Schematic showing the evolution of a nanoporous glass network through
the acid-catalysed sol-gel process. (Adapted with permission from [7] Copyright
(2009) Royal Society of Chemistry.)
