Page 459 - Carrahers_Polymer_Chemistry,_Eighth_Edition

422 Carraher’s Polymer Chemistry

solid is forced outward as collapse occurs. The frictional forces caused by the gas passing through
a restricted opening are indirectly proportional to the square of the pore diameter. Because the pore
sizes are so small, the rapidly moving gas also absorbs a lot of the energy. Thus, energy is absorbed
by the aerogel through both collapse of the solid network structure and release of the gas within the
aerogel.
Aerogels that are about 2–5 nm in diameter have large surface to volume ratios on the order of
−1
9
2

10 meters and high specific surface areas approaching 1,000 m /g. Such large surface to volume
ratios makes the surface particularly active and potential materials as catalysts, absorbents, and
catalyst substrates.
The precise chemical makeup of the surface depends on the materials used to make the aerogel
and the method of processing. Typical aerogel sequences produce products whose surfaces are rich in
hydroxyl groups. Because of the high surface area, -Si-OH groups act as weak acids and are reactive
in typical Lewis acid–base reactions. As noted before, aerogels have many hydrogen-bonding hydrox-
yls at their surface making aerogels extremely hygroscopic. Dry aerogel materials will increase their
weight by 20% through uptake of moisture from the air. This absorption is reversible and appears to
o
o
have little or no effect on the aerogel. Water is removed through heating to 100 C–120 C.
While adsorption of water vapor has little effect on aerogels, contact with liquid water has dev-
astating effects on aerogels. When water enters the nanometer-size pores, the surface tension of the

water exerts capillary forces sufficient to fracture the silica backbone, resulting is a collapse of the
complex matrix structure. This tendency to be attacked by water is overcome through conversion of
the surface polar –OH groups to nonpolar –OR groups. The “R” is typically a trimethylsilyl group
though any aliphatic group would work. Conversion can be accomplished within the wet stage
(preaerogel) or after the supercritical drying. These treatments result in an aerogel that is called
“hydrophobic” aerogel, which is stable in water.
The pore size of aerogels varies. The International Union of Pure and Applied Chemistry clas-
sifies materials with pore sizes of less that 2 nm as “micropores,” 2–50 nm are called “mesopores,”

and those greater than 50 nm in diameter are called “macropores.” While aerogels have some pores
that fall within the micropore region, the majority of pores are in the mesopore region.
Most of the aerogels produced today are described as being transparent. While it might be
assumed that since aerogels are made of the same material as window glass and quartz (SiO ) that
2
they would be transparent, this is not necessarily the case. Transparency requires a number of fac-
tors. Thus, so-called smokey and white quarts are colored because of the presence of impurities.
Mixtures of amorphous and crystalline silicon dioxide can be made that are not transparent. The
size and distribution of reflecting and refractive sites are important factors in determining if a mate-

rial is transparent with the theme of “sameness” contributing to making a material transparent.
The majority of light we see is scattered light, that is, light that reaches our eyes in an indi-
rect manner. The scattering phenomenon is what gives us blue skies, white to gray clouds, and

poor visibility in fog. This scattering is not simply reflecting but results from the interaction of
light with an inhomogeneous site. Light-scattering photometry is used to determine the size of
polymers. Scattering is most effective when the scattering particle size is about that of the wave
length of the light. For visible light, this occurs with scattering sites that are about 400–700 nm.
Scattering centers that are much less in size than the incoming light wavelength are much less
effective at scattering the light. Since the particle sizes in an aerogel are much smaller than the
individual sites they are ineffective scattering sites. Similar to classical polymer chains, where
the entire chain or segments of the entire chain act as a scattering site, clusters of individual sites
within the aerogel act as scattering sites. Most of these scattering sites are again smaller than
the wavelength of visible light, but some are within the range to scatter visible light so that a
soft reflected light results. The different-sized scattering sites and variable wavelengths present

in visible light cause a reddening of the transmitted light (red light has a longer wavelength and
is scattered less by small clusters present in the aerogel), resulting in the blue appearance of the
reflected light from the aerogel.

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