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.

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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.

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This paper describes a generalizable approach to the synthesis of monolithic multiscale micro/meso/ macro-porous polymers with potential use in catalysis, gas separations and gas storage. The model system is based on hyperbranched polyimides synthesized via an unconventional route from dianhydrides and triisocyanates. Simulations reproducing experimental observables (e.g., XRD, skeletal densities) indicate that relatively small hyperbranched oligomers pack into inherently microporous primary nanoparticles. The latter phase-separate, react through their dangling surface functionality and form robust, monolithic, meso/macro-porous networks. Macroporosity is controlled mainly by the concentration of the reactants; mesoporosity by the chemical identity of the monomers that directs the way particles fill space, e.g., as strings of beads versus globular clusters. Microporosity, being an inherent property of the molecular network, survives pyrolysis and is transferred along with meso and macroporosity to multiscale nanoporous carbons. The materials described herewith are classified as polyimide and carbon aerogels.