Monolithic polyimide aerogels (PI-ISOs) have been prepared by drying wet-gels synthesized via a rather underutilized room-temperature reaction of pyromellitic dianhydride (PMDA) with 4,4′-methylene diphenyl diisocyanate (MDI). The reaction is followed up to the gelation point by liquid ¹³C-NMR in DMSO-d₆ and it proceeds through a seven-member ring intermediate that collapses to the imide by expelling CO₂. PI-ISOs are characterized comparatively with aerogels referred to as PI-AMNs, obtained via the classic reaction of PMDA and 4,4′-methylenedianiline (MDA). The two materials are chemically identical, they show similar degrees of crystallinity (30–45%, by XRD) and they both consist of similarly sized primary particles (6.1–7.5 nm, by SANS). By N₂-sorption porosimetry they contain both meso- and macroporosity and they have similar BET surface areas (300–400 m² g⁻¹). Their major difference, however, is that PI-AMNs are particulate while PI-ISOs are fibrous. The different morphology has been attributed to the rigidity of the seven-member ring intermediate of PI-ISOs. PI-AMNs shrink significantly during processing (up to 40% in linear dimensions), but mechanically are much stronger materials than PI-ISOs of the same density. Upon pyrolysis at 800 °C both PI-ISO and PI-AMN are converted to porous carbons; PI-AMNs loose their nanomorphology and more than 2/3 of their surface area, as opposed to PI-ISOs, which retain both. Etching with CO₂ at 1000 °C increases the BET surface area of both PI-AMN (to 417 m² g⁻¹) and PI-ISO (to 1010 m² g⁻¹), and improves the electrical conductivity of the latter by a factor of 70.

This study establishes that the necessary and sufficient condition for efficient reaction between nanoparticles includes both high surface-to-volume ratios and high compactness. For this, a wide range of interpenetrating networks of resorcinol-formaldehyde (RF) and metal oxide (MOx, M: Fe, Co, Ni, Sn, Cu, Cr, Ti, Hf, Y, Dy) nanoparticles were synthesized via a simple one-pot process using the acidity of gelling solutions of hydrated metal ions to catalyze gelation of RF. The compactness of the nanoparticles in the dry composites is controlled by the drying method: supercritical fluid (SCF) CO₂ drying affords aerogels with open skeletal frameworks, while drying under ambient pressure yields much more compact xerogels. A second independent method to impart compactness is by crosslinking the framework nanoparticles with a conformal polyurea (PUA) coating followed by drying with SCF CO₂: although those materials (X-aerogels) have an open aerogel-like structure, upon heating in the 200 °C range, the conformal PUA coating melts and causes local structural collapse of the underlying framework creating macropores defined by xerogel-like walls. Depending on the chemical identity of the metal ion, pyrolysis at higher temperatures sets off carbothermal processes yielding pure metal monolithic nanostructures (up to 800 °C; cases of M; Fe, Co, Ni, Sn, Cu) or carbides (up to 1400 °C; cases of M: Cr, Ti, Hf). Irrespective of the specific chemical processes responsible for those transformations, the rate determining factor is the innate compactness of the xerogels, or the induced skeletal compactness in X-aerogels: both kind of materials react at as much as 400 °C lower temperatures than their corresponding native aerogels. By comparison, bulk (micron size) mixtures of the corresponding oxides and carbon black remained practically unreacted in the entire temperature range used for the nanoparticle networks. In addition to the significance of the RF-MOx interpenetrating networks in the design of new materials (mesoporous and macroporous monolithic metals and carbides), the effect of compactness on the activation of the carbothermal processes has important implications for process-design engineering.

Smelting of interpenetrating networks of resorcinol-formaldehyde (RF) and iron oxide (FeOx) aerogels yields porous ferromagnetic and superparamagnetic materials in monolithic form with compositions closely resembling that of pig iron.