In the quest for strong lightweight materials, silica aerogels would be very attractive, if they were not fragile. The strength of silica aerogel monoliths has been improved by a factor of over 100 through cross-linking the nanoparticle building blocks of preformed silica hydrogels with poly(hexamethylene diisocyanate). Composite monoliths are much less hygroscopic than native silica, and they do not collapse when in contact with liquids.

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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A large array of easily available small-molecule (as opposed to industrial oligomeric) triisocyanates and aromatic polyols render polyurethanes a suitable model system for a trend-based systematic study of structure−property relationships in nanoporous matter as a function of the monomer structure. Molecular parameters of interest include rigidity, number of functional groups per monomer (n), and functional group density (number of functional groups per phenyl ring, r). All systems were characterized from gelation to the bulk properties of the final aerogels. Molecular and nanoscopic features of interest, including skeletal composition, porous structure, nanoparticle size, and assembly, were probed with a combination of liquid-and solid-state 13 C and 15 N NMR, rheometry, N 2 -and Hg-porosimetry, SEM, and small-angle Xray scattering (SAXS). Macroscopic properties such as styrofoam-like thermal conductivities (∼0.030 W m −1 K −1 ), foam-like flexibility, or armor-grade energy absorption under compression (up to 100 J g −1 ) were correlated with one another and serve as a top-down probe of the interparticle connectivity, which was again related to the monomer structure. Overall, both molecular rigidity and multifunctionality control phase-separation, hence, particle size and by association porosity (e.g., meso versus macro) and internal surface area. With sufficiently rigid monomers, skeletal frameworks include intrinsic microporosity, rendering the resulting materials hierarchically nanoporous over the entire porosity regime (micro to meso to macro). Most importantly, however, clear roles have been identified not only for the absolute number of functional groups per monomer, but also for parameter r. The latter is expressed onto the surface of the skeletal nanoparticles (controls the surface functional group density per unit mass) and becomes the dominant structure-directing as well as property-determining parameter. By relating the molecular functional group density with the functional group density on the nanoparticle surfaces, these results establish that for three-dimensional (3D) assemblies of nanoparticles to form rigid nanoporous frameworks, they have first and foremost to be able to develop strong covalent bonding with one another. These findings are relevant to the rational design of 3D nanostructured matter, not limited to organic aerogels.

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).