Polyurea (PUA) develops H-bonding with water and is inherently hydrophilic. The water contact angle on smooth dense PUA derived from an aliphatic triisocyanate and water was measured at θ=69.1±0.2°. Nevertheless, texture-related superhydrophobic PUA aerogels (θ'=150.2°) were prepared from the same monomer in one step with no additives, templates, or surfactants via sol-gel polymerization carried out in polar, weakly H-bonding acetonitrile. Those materials display a unique nanostructure consisting of micrometer-size spheres distributed randomly and trapped in a nanofiber web of the same polymer. Morphostructurally, as well as in terms of their hydrophobic properties, those PUA aerogels are analogous to well-studied electrospun fiber mats incorporating particle-like defects. PUA aerogels have the advantage of easily scalable synthesis and low cost of the raw materials. Despite large contact angles and small contact areas, water droplets (5 μL) stick to the aerogels surface when the substrate is turned upside-down. That so-called Petal effect is traced to H-bonding at the points of contact between the water droplet and the apexes of the roughness of the aerogel surface. Monoliths are flexible and display oleophilicity in inverse order to their hydrophobicity; oil fills all the available open porosity (94% v/v) of cocoon-in-web like aerogels with bulk density ρb=0.073 g cm(-3); that capacity for oil absorption is >10:1 w/w and translates into ∼6:1 w/v relative to state-of-the-art materials (e.g., graphene-derived aerogels). Oil soaked monoliths float on water and can be harvested off.

Flexible and foldable aerogels have commercial value for applications in thermal insulation. This study investigates the molecular connection of macroscopic flexibility using polymeric aerogels based on star-shaped polyurethane-acrylate versus urethane-norbornene monomers. The core of those monomers is based either on a rigid/ aromatic, or a flexible/aliphatic triisocyanate. Terminal acrylates or norbornenes at the tips of the star branches were polymerized with free radical chemistry, or with ROMP, respectively. At the molecular level, aerogels were characterized with FTIR and solid-state 13 C NMR. The porous network was probed with N 2 -sorption and Hg-intrusion porosimetry, SEM and SAXS. The interparticle connectivity was assessed in a top-down fashion with thermal conductivity measurements and compression testing. All aerogels of this study consist of aggregates of nanoparticles, whose size depends on the aliphatic/ aromatic content of the monomer, the rigidity/flexibility of the polymeric backbone, and generally varies with density. At higher densities (0.3−0.7 g cm −3 ), all materials were stiff, strong, and tough. Aerogels based on urethane-acrylates built around a rigid/aromatic core exhibited a rapid decrease of their elastic modulus with density (slopes of the log−log plots >5.0), and at about 0.14 g cm −3 , they were foldable. Data support that molecular properties of the monomer affect macroscopic flexibility indirectly, not so through the particle size, but rather through the growth mechanism and consequently through the interparticle contact area. Thus, flexible aerogels of this study showed no indication for polymer accumulation onto the primary nanostructure (particle sizes via N 2 -sorption and SAXS were comparable), and their interparticle contact area was comparatively lower. Because for flexibility purposes, interparticle contact area is related to interparticle bonding, it is speculated that if the latter is controlled properly (e.g., through adjustment of the monomer functional group density) it might lead to superelasticity and shape-memory effects.

There is a specific need for nanoporous monolithic pyrophoric metals as energetic materials and catalysts. Adapting modern-day blast furnace methodology, namely, direct reduction of highly porous iron oxide aerogels with H 2 or CO, yielded coarse powders. Turning to smelting reduction, we used the acid environment of gelling [Fe(H 2 O) 6 ] 3+ sols to catalyze co-gelation of a second, extremely sturdy, carbonizable in high yield polybenzoxazine (PBO) network that plays the dual role of a reactive template. Formation of two independent gel networks was confirmed with rheology/dynamic mechanical analysis performed in tandem with the same sol and its gel, and results were correlated with data from microscopy (SEM, STEM) and small-angle X-ray scattering (SAXS) for the elucidation of the exact topological association of the two components. By probing the chemical interaction of the two networks with infrared, Mossbauer, XRD, and CHN analysis, we found out that iron(III) oxide undergoes pre-reduction to Fe 3 O 4 and participates in the oxidation of PBO, which is a prerequisite for robust carbons suitable as structure-directing templates. Subsequently, interconnected submicrometer-size Fe 3 O 4 nanoparticles undergo annealing at more than 800 °C below the melting point of the bulk oxide and are reduced to iron(0) at 800 °C, presumably via a solid (C)/liquid (Fe 3 O 4 ) process. Carbothermal reduction, oxidative removal of residual carbon (air), and re-reduction (H 2 ) of α-Fe 2 O 3 formed in the previous step were all carried out as a single process by switching gases. The resulting pure iron(0) monoliths had a density of 0.54 ± 0.07 g cm −3 and were 93% porous. Infiltration with LiClO 4 and ignition led to a new type of explosive behavior due to rapid heating and expansion of gases filling nanoporous space; annealing at 1200 °C reduced porosity to 66%, and those materials behaved as thermites. Ignition in a bomb calorimeter released 59 ± 9 kcal mol −1 of iron(0) reacted and is associated with oxidation to FeO (theoretical, 66.64 kcal mol −1 ).

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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