Page 458 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Inorganic Polymers 421

by drawing or shearing. The gel is dried by evaporation forming a xerogel or by supercritical fl uid
extraction giving an aerogel. Consolidation to dense glasses or ceramics is carried out by thermal
treatment and sintering.
Since both aerogels and xerogels have high surface areas and small pore diameters, they are used

as ultrafiltration media, antireflective coatings, and catalysts supports. Final densification is carried

out by viscous sintering.
The rate of silicate sol and gel formation is pH and water–alcohol sensitive as is the solubility
−4
of the amorphous silica that is formed. Silica networks are based on (SiO ) tetrahedra modifi ed
4
−
+
by (O Si-O , M ) units and often addition of boron oxide, aluminum oxide, titanium IV oxide, or
3
zirconium IV oxide.
The nature of the reactants can be varied giving various silicate-like products. The formation of
borosilicate glasses using the sol-gel approach is described below:
NaOR + B(OR) + Si(OR) + H O → NaOH + B(OH) + Si(OH) + ROH (12.17)
3 4 2 3 4

NaOH + B(OH) + Si(OH) → Na O• B O • SiO • H O → Na O• B O • SiO + H O (12.18)
3 4 2 2 3 2 2 2 2 3 2 2
The use of organically modified silicates (ceramers) gives a wide variety of products with a

variety of structures and properties. Such ceramers have been used as adhesives for glass surfaces,
protective coating for medieval stained glass, and as scratch-resistant coatings for plastic eyeglass
lenses. They have also been used in the reinforcement of plastics and elastomers, and their nano-
scale pores allow their use as porous supports, and as selective absorbents.

Sol-gel preparations of tetraethoxysilane can be spun into fibers once the appropriate viscosity
has been achieved. These fibers are only slightly weaker than silica-glass fi bers.

Hybrid materials have been made by incorporating end-capped poly(tetramethylene oxide) blocks
to tetramethoxysilane sol-gel glasses. These materials have high extensibility with interdispersed
organic and inorganic regions.
12.10 AEROGELS
Aerogels are highly porous materials where the pore sizes are truly on a molecular level, less than 50
nm in diameter. This gives a material with the highest known internal surface area per unit weight.

One ounce can have a surface area equal to 10 football fields, more than 1,000 square meters in one
gram.
Porous materials can be either open pored such as a common sponge, or closed pored such as the
bubble-wrap packaging. Aerogels are open-pored materials such that unbonded material can move
from one pore to another.

While in the gel state, the preaerogel has some flexibility, but as a solid aerogels behave as a frag-
ile glass. It may be very strong in comparison to its weight, but remember it is very light. Aerogels
are more durable when under compression. Compression can be simple such as sealing the aerogel
sample in a typical food sealer packing. Aerogels are best cut using a diamond coated saw similar
to that used by rock cutters to slice rocks.
When handled, aerogel samples will initially appear to exhibit some flexibility but then burst

into millions of pieces. For large arrays of atoms, such as solid metals and polymers below their

glass transition temperature, energy can be absorbed through bond flexing or bending. For polymers
between their glass transition and melting points, kinetic energy can also be absorbed through seg-
mental movement. Silica aerogels, being an inorganic polymer in the glassy state at room temper-

ature, is a brittle material. As force is applied, there is very little bond flexing, so that the applied
kinetic energy results in the collapse of the network with the force of impact spread over a large
part of the aerogel and over a time because of the time required to transfer this energy from one cell
to another within the aerogel matrix. Because the aerogel is open pored, gas contained within the

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