due to the filling of the micropores (<2 nm) with the silica aerogel.
This also results in the surface area reduction observed as micropores
have a higher contribution to the surface area.
In conclusion, we have demonstrated the controlled growth of
a grafted polymer with polydispersities between 1.2 and 1.8 from the
surface of a silica gel. Once the composites were SCD, we established
the first relationship of a silica aerogel composites bulk mechanical
properties to those of a polymers molecular weight used to reinforce
the composite. It was found that as the molecular weight increases, so
does the composite mechanical properties while retaining many of the
aerogel like physical properties.
Acknowledgements
We thank the Energy Materials Corp, the NSF (DMR-0645618) and
IBM for support of this work. We also thank the University of
Arizona, University Spectroscopy and Imaging Facility and Mass
Spectroscopy Facility.
Fig. 3 Flexural strength as a function of density for PMMA–silica
aerogel composites. The triangle line is for polymer free silica aerogels,
and diamond line is for PMMA–silica aerogel composites.
Notes and references
reduction in strength is observed in other silica aerogels prepared with
trialkoxysilane co-monomers.16 The strength of the PMMA–silica
aerogel composites increases with the density of the composite aer-
ogel and the molecular weight of the PMMA. PMMA–silica aerogel
composite prepared from a polymerization time of 6 h with a grafted
molecular weight of ꢂ20 kg molꢀ1 increased the flexural strength to
154 ꢁ 4.9 kPa. This is approximately 5ꢃ stronger than the initiator-
modified aerogels with only a 25% increase in the density (r ¼ 0.116 g
cmꢀ3). The strongest PMMA–silica aerogel composite had a density
of 0.327 g cmꢀ3, a molecular weight (Mw) of 63 kg molꢀ1 and a flex-
ural strength of 635 ꢁ 1.7 kPa. This is 18ꢃ stronger than the initiator-
modified silica aerogel. From these results, the strengths observed are
higher than what can be attributed due to mass addition, thus the
molecular weight of the grafted polymer plays an important role in
the composites mechanical properties.
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In addition to increasing density, most approaches to strengthen
aerogels using the addition of mass lead to reduction in surface area.
Nitrogen sorption surface area analyses of the composite aerogels
reveal that the smaller pores fill first. As the polymerization time is
extended a pore volume reduction is noticed accompanied by a shift
in the composites average pore size and an attenuation of the
composites surface area. The surface area of a polymer-free, initiator-
modified silica aerogel was 1483 m2 gꢀ1 with an average pore size of
ꢀ3
˚
114 A, a PMMA composite with a density of 0.236 g cm had
a surface area of 221 m2 gꢀ1 and an average pore size of 270 A. The
˚
average pore size shift to larger sizes and the lower surface areas are
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 6863–6865 | 6865