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.

Resorcinol (R)-formaldehyde (F) aerogels are pursued as precursors of carbon aerogels, which are electrically conducting. They are usually prepared via a week-long base-catalyzed gelation process from an aqueous sol. For this work, we reasoned that because both the reaction of R with F and the condensation of the resulting hydroxymethyl resorcinol with R are electrophilic aromatic substitutions, they should proceed easily by acid catalysis in one pot. Thereby, we have developed and reported an HCl-catalyzed gelation process in CH 3 CN, which is completed in about 2 h at room temperature or in 10 min at 80°C. The final aerogels are chemically indistinguishable (by IR and 13 C CPMAS NMR) from typical basecatalyzed samples. In analogy to phenol-formaldehyde resin formation, the mechanism may involve o-quinone methide intermediates (hence the red color prevailing throughout the process). The effect of aging is discussed in terms of shrinkage and is attributed to further reaction and incorporation of more formaldehyde into wet gels, followed by syneresis (reaction with one another of dangling oligomeric appendices on the skeletal framework).

Skeletal nanoparticles of porous low-density materials formally classified as aerogels are cross-linked by surface-initiated polymerization (SIP) using a new surface-confined bidentate free-radical initiator structurally related to azobisisobutyronitrile (AIBN). Methylmethacrylate, styrene, and divinylbenzene are introduced in the mesopores, and upon heating at 70°C, all mesoporous surfaces throughout the entire skeletal framework are coated conformally with a 10-12 nm thick polymer layer indistinguishable spectroscopically from the respective commercial bulk materials. The amount of polymer incorporated in the structure is controlled by the concentration of the monomer in the mesopores, and albeit an up to a 3-fold increase in bulk density (up to 0.6-0.8 g cm -3 ) and a decrease in the porosity even down to 40%, the materials remain mesoporous with average pore diameters increasing from 20 nm in the native samples to 41 and 62 nm in PMMA and polystyrene cross-linked samples, respectively. The new materials combine hydrophobicity with vastly improved mechanical properties in terms of strength, modulus, and toughness relative to their native (non-cross-linked) counterparts. The effect of polymer accumulation on the modulus has been also simulated numerically. Being able to use SIP for cross-linking 3D assemblies of nanoparticles comprising the skeletal framework of typical aerogels paves the way for the deconvolution of cross-linking from gelation (a free-radical versus an ionic process, respectively), so that ultimately all gelation and cross-linking reagents can be included together in one pot, leading to great process simplification. The mechanical properties of the new materials render them appropriate for anti-ballistic applications (e.g., armor).

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Carbon (C) aerogels are made by pyrolysis of resorcinol-formaldehyde (RF) aerogels under Ar, and they combine electrical conductivity with a high open mesoporosity. However, because macropores are known to facilitate mass transfer, macroporous C-aerogels could be useful for application in separations or as fuel cell and battery electrodes. Macropores are typically incorporated in C-aerogels during gelation of the RF precursors by using either "hard" templating with silica or polystyrene beads, or "soft" templating with surfactants. Here, we report an alternative method, where open macroporosity is introduced by pyrolyzing RF aerogels whose skeletal nanoparticles have been cross-linked covalently with an isocyanatederived polymer that coats conformally the entire RF framework. The structural, physical, and chemical evolution of the X-RF network was monitored at various stages during pyrolysis by DSC, TGA, SEM, N 2 adsorption porosimetry, and 13 C NMR. The accumulated evidence shows that the cross-linker first loses its chemical bonding with the skeletal nanoparticles and then melts, exerting surface tension forces on the RF framework, which cause a partial structural collapse that creates macropores. The xerogel-like internal texture of the macroporous walls is responsible for close contact of the carbon skeletal nanoparticles, resulting in an about 7× lower bulk electrical resistivity of the macroporous material relative to the corresponding mesoporous network, which is obtained by pyrolysis of native (i.e., non-crosslinked) RF aerogels. The new macroporous material was evaluated electrochemically for possible application as an electrode in batteries and fuel cells.

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