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Monolithic Hierarchical Fractal Assemblies of Silica Nanoparticles Cross-Linked with Polynorbornene via ROMP: A Structure–Property Correlation from Molecular to Bulk through Nano
journal contribution
posted on 2016-02-20, 12:05 authored by Dhairyashil
P. Mohite, Zachary J. Larimore, H. Lu, Joseph T. Mang, Chariklia Sotiriou-Leventis, Nicholas LeventisMonolithic hierarchical fractal assemblies of silica
nanoparticles are referred to as aerogels, and despite an impressive
collection of attractive macroscopic properties, fragility has been
the primary drawback to applications. In that regard, polymer-cross-linked
silica aerogels have emerged as strong lightweight nanostructured
alternatives rendering new applications unrelated to aerogels before,
as in ballistic protection, possible. In polymer-cross-linked aerogels
skeletal nanoparticles are connected covalently with a polymer. However,
the exact location of the polymer on the elementary structure of silica
and, therefore, critical issues, such as how much is enough, have
remained ambiguous. To address those issues, the internal nanoporous
surfaces of silica wet-gels were modified with norbornene (NB) by
cogelation of tetramethyl orthosilicate (TMOS) with a newly synthesized
derivative of nadic acid (Si-NAD: N-(3-triethoxysilylpropyl)-5-norbornene-2,3-dicarboximide).
As inferred by both rheological and liquid 29Si NMR data, Si-NAD reacts more slowly than TMOS, yielding a TMOS-derived
skeletal silica network surface-derivatized with NB via monomer-cluster
aggregation. Then, ring-opening metathesis polymerization (ROMP) of
free NB in the nanopores engages surface-bound NB moieties and bridges
skeletal nanoparticles either through cross-metathesis or a newly
described stitching mechanism. After solvent exchange and drying with
supercritical fluid CO2 into aerogels (bulk densities in
the range 0.27–0.63 g cm–3, versus 0.20 g
cm–3 of the native network), the bridging nature
of the polymer is inferred by a >10-fold increase in mechanical
strength and a 4-fold increase in the energy absorption capability
relative to the native samples. The cross-linking polymer was freed
from silica by treatment with HF, and it was found by GPC that it
consists of a long and a short component, with around 400 and 10 monomer
units, respectively. No evidence (by SAXS) was found for the polymer
coiling up into particles, consistent with the microscopic similarity
(by SEM) of both native and cross-linked samples. Most importantly,
the polymer does not need to spill over higher aggregates for greatly
improved mechanical strength; mechanical properties begin improving
after the polymer coats primary particles. Extremely robust materials
are obtained when the polymer fills most of the fractal space within
secondary particles.
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TMOSstitching mechanismGPCmacroscopic propertiestetramethyl orthosilicateHFbulk densitiesROMPMonolithic Hierarchical Fractal Assembliesnanoporous surfacesfractal space29 Si NMR datananostructured alternativesNBSEMenergy absorption capabilitynadic acidaerogelfractal assembliessilica nanoparticlespolymer coats10 monomer unitsfluid CO 2SAXS
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