It is well-known that isocyanates and water yield polyureas; however, that reaction is not generally associated with the synthesis of the latter, being used instead for environmental curing of films baring free NCO groups or for foaming polyurethanes. Here we report that careful control of the relative isocyanate/water/catalyst (Et 3 N) ratio in acetone, acetonitrile, or DMSO prevents precipitation, yielding instead polyurea (PUA) gels convertible to highly porous (up to 98.6% v/v) aerogels over a very wide density range (0.016-0.55 g cm -3 ). The method has been implemented successfully with several aliphatic and aromatic di and triisocyanates. PUA aerogels have been studied at the molecular level ( 13 C NMR, IR, XRD), the elementary nanoparticle level (SANS/USANS), and the microscopic level (SEM). Their porous structure has been probed with N 2 -sorption porosimetry. Despite that the nanomorphology varies with density from fibrous at the low density end to particulate at the high density end, all samples consist of similarly sized primary particles assembled differently, probably via a reaction-limited cluster-cluster aggregation mechanism at the low density end, which changes into diffusion-limited aggregation as the isocyanate concentration increases. Higher density PUA aerogels (>0.3 g cm -3 ) are mechanically strong enough to tolerate the capillary forces of evaporating low surface tension solvents (e.g., pentane) and can be dried under ambient pressure; under compression, they can absorb energy (up to 90 J g -1 at 0.55 g cm -3 ) at levels observed only with polyurea-cross-linked silica and vanadia aerogels (50-190 J g -1 at similar densities). At cryogenic temperatures (-173 °C) PUA aerogels remain relatively ductile, a fact attributed to sintering effects and their entangled fibrous nanomorphology. Upon pyrolysis (>500 °C, Ar), PUA aerogels from aromatic isocyanates are converted to carbon aerogels in high yields (∼60% w/w). Those properties, considered together with the simple synthetic protocol, render PUA aerogels attractive multifunctional materials.

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Polyimide aerogel monoliths are prepared by ring-opening metathesis polymerization (ROMP) of a norbornene end-capped diimide, bis-NAD, obtained as the condensation product of nadic anhydride with 4,4 0 -methylenedianiline. The density of the material was varied in the range of 0.13À0.66 g cm À3 by varying the concentration of bis-NAD in the sol. Wet gels experience significant shrinkage, relative to their molds (28%À39% in linear dimensions), but the final aerogels retain high porosities (50%À90% v/v), high surface areas (210À632 m 2 g À1 , of which up to 25% is traced to micropores), and pore size distributions in the mesoporous range (20À33 nm). The skeletal framework consists of primary particles 16À17 nm in diameter, assembling to form secondary aggregates (by SANS and SEM) 60À85 nm in diameter. At lower densities (e.g., 0.26 g cm À3 ), secondary particles are mass fractals (D m = 2.34 ( 0.03) turning to closed-packed surface fractal objects (D S = 3.0) as the bulk density increases (g0.34 g cm À3 ), suggesting a change in the network-forming mechanism from diffusion-limited aggregation of primary particles to a space-filling bond percolation model. The new materials combine facile one-step synthesis with heat resistance up to 200 °C, high mechanical compressive strength and specific energy absorption (168 MPa and 50 J g À1 , respectively, at 0.39 g cm À3 and 88% ultimate strain), low speed of sound (351 m s À1 at 0.39 g cm À3 ) and styrofoam-like thermal conductivity (0.031 W m À1 K À1 at 0.34 g cm À3 and 25 °C); hence, they are reasonable multifunctional candidate materials for further exploration as thermal/acoustic insulation at elevated temperatures.

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Monolithic 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 29 Si NMR data, Si-NAD reacts more slowly than TMOS, yielding a TMOS-derived skeletal silica network surfacederivatized 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 CO 2 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 crosslinking 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.

Porous carbons, including carbon (C-) aerogels, are technologically important materials, while polyacrylonitrile (PAN) is the main industrial source of graphite fiber. Graphite aerogels are synthesized herewith pyrolytically from PAN aerogels, which in turn are prepared first by solution copolymerization in toluene of acrylonitrile (AN) with ethylene glycol dimethacrylate (EGDMA) or 1,6-hexanediol diacrylate (HDDA). Gelation is induced photochemically and involves phase-separation of "live" nanoparticles that get linked covalently into a robust 3D network. The goal of this work was to transfer that process into aqueous systems and obtain similar nanostructures in terms of particle sizes, porosity, and surface areas. That was accomplished by forcing the monomers into (micro)emulsions, in essence inducing phase-separation of virtual primary particles before polymerization. Small angle neutron scattering (SANS) in combination with location-ofinitiator control experiments support that monomer reservoir droplets feed polymerization in ∼3 nm radius micelles yielding eventually large (∼60 nm) primary particles. The latter form gels that are dried into macro-/mesoporous aerogels under ambient pressure from water. PAN aerogels by either solution or emulsion gelation are aromatized (240 °C, air), carbonized (800 °C, Ar), and graphitized (2300 °C, He) into porous structures (49−64% v/v empty space) with electrical conductivities >5× higher than those reported for other C-aerogels at similar densities. Despite a significant pyrolytic loss of matter (up to 50−70% w/w), samples shrink conformally (31−57%) and remain monolithic. Chemical transformations are followed with CHN analysis, 13 C NMR, XRD, Raman, and HRTEM. Materials properties are monitored by SEM and N 2 -sorption. The extent and effectiveness of interparticle connectivity is evaluated by quasi-static compression. Overall, irrespective of the gelation method, PAN aerogels and the resulting carbons are identical materials in terms of their chemical composition and microstructure. Although cross-linkers EGDMA and HDDA decompose completely by 800 °C, surprisingly their signature in terms of different surface areas, crystallinity, and electrical conductivities is traced in all the pyrolytic products.

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