Shape memory polymers (SMPs) remember and return to an original shape when triggered by a suitable stimulus, typically a change in temperature. They are pursued as cost-effective, low-density, higher-straintolerant alternatives to shape memory alloys. Arguably, porous SMPs may offer the near-ultimate refinement in terms of density reduction. To that end, shape memory polymeric aerogels (SMPAs) may offer a viable approach. The necessary condition for SMPs is rubber-like superelasticity, which is introduced via cross-linking. Cross-linking is also a necessary condition for inducing phase separation during solution-phase polymerization of suitable monomers into 3D nanoparticle networks. Such networks form the skeletal frameworks of polymeric aerogels. Those principles were explored here with rigid trifunctional isocyanurate cross-linking nodes between flexible urethane tethers from four short oligomeric derivatives of ethylene glycol: H(OCH 2 CH 2 ) n OH (1 ≤ n ≤ 4). Formation of self-supporting 3D particle networks depended on the solubility of the developing polymer, which translated into specific combinations of the diol, monomer concentration, and composition of the solvent (CH 3 CN/acetone mixtures). Those parameters were varied systematically using statistical design-of-experiments methods. The skeletal frameworks of the resulting poly(isocyanurateurethane) (PIR-PUR) aerogels consisted of micrometer-size particles. Bulk densities were in the 0.2−0.4 g cm −3 range, and typical porosities were between 70% and 80% v/v. Glass transition temperatures (T g ) varied from about 30 (n = 4) to 70 °C (n = 1). At and above T g , all SMPAs showed rubber-like elasticity. They also became stiffer after the first stretching cycle, which was traced to maximization of H-bonding interactions (NH•••OC and NH••• O(CH 2 ) 2 ). Below the T g zone, the elastic modulus of all formulations decreased by about 1000 fold. That property gave rise to a robust shape memory effect (SME), the quality of which was evaluated via several figures of merit that were calculated from tensile stretching data over five temperature cycles between T g + 10 °C and T g − 40 °C. All thermomechanical testing was carried out with dynamic mechanical analysis (DMA). The strain fixity was always >98%, pointing to very low creep. After the first cycle, strain recovery (a measure of fatigue) improved from 80−90% to about 100%, and the fill factor, a cumulative index of performance, reached 0.7, which is in the range of fast elastomers. The robust shape memory effect was demonstrated with deployable panels and bionic hands capable of mimicking coordinated muscle function.

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Phenolic aerogels containing oxygen and other polymeric aerogels containing both oxygen and nitrogen (polybenzoxazine and a polyamide‐polyimide‐polyurea co‐polymer) are converted to carbon aerogels (800 °C/Ar), and are etched with CO2 (1000 °C). Etching opens closed pores and increases micropore volumes and size. Heteroatoms are retained in the etched samples. All carbon aerogels are evaluated as CO2 absorbers in terms of their capacity and selectivity toward CH4, H2, and N2. CO2 adsorption capacity is linked to microporosity. In most cases, monolayer coverage of micropore walls is enough to explain CO2 uptake quantitatively. The interaction of CO2 with micropore walls is evaluated via isosteric heats of adsorption, and is stronger with carbons containing only oxygen heteroatoms. The adsorption capacity of those carbons (5–6 mmol g−1) is on par with the best carbon and polymeric CO2 adsorbers known in the literature, with one exception however: etched carbon aerogels from low‐density resorcinol‐formaldehyde aerogels show a very high CO2 uptake (14.8 ± 3.9 mmol g−1 at 273 K, 1 bar) attributed to a pore‐filling process that proceeds beyond monolayer coverage, whereas surface phenoxides engage in a thermally neutral carbonate forming reaction (surface‐O– + CO2 → surface‐O–(CO)–O–) that continues until micropores are filled.

We report the carbothermal synthesis of sturdy, highly porous (>85%) SiC and Si 3 N 4 monolithic aerogels from compressed polyurea-cross-linked silica xerogel powders. The high porosity in those articles was created via reaction of core silica nanoparticles with their carbonized polymer coating toward the new ceramic framework and CO that escaped. Sol−gel silica powder was obtained by disrupting gelation of a silica sol with vigorous agitation. The grains of the powder were about 50 μm in size and irregular in shape and consisted of 3D assemblies of silica nanoparticles as in any typical silica gel. The individual elementary silica nanoparticles within the grains of the powder were coated conformally with a nanothin layer of carbonizable polyurea derived from the reaction of an aromatic triisocyanate (TIPM: triisocyanatophenyl methane) with the innate −OH, deliberately added −NH 2 groups, and adsorbed water on the surface of silica nanoparticles. The wet-gel powder was dried at ambient temperature under vacuum. The resulting free-flowing silica/polyurea xerogel powder was vibrationsettled in suitable dies and was compressed to convenient shapes (discs, cylinders, donut-like objects), which in turn were converted to same-shape SiC or Si 3 N 4 artifacts by pyrolysis at 1500 °C under Ar or N 2 , respectively. The overall synthesis was time-, energy-, and materials-efficient: (a) solvent exchanges within grains of powder took seconds, (b) drying did not require high-pressure vessels and supercritical fluids, and (c) due to the xerogel compactness, the utilization of the carbonizable polymer was at almost the stoichiometric ratio. Chemical and materials characterization of all intermediates and final products included solid-state 13 C and 29 Si NMR, XRD, SEM, N 2 -sorption, and Hg intrusion porosimetry. Analysis for residual carbon was carried out with TGA. The final ceramic objects were chemically pure, sturdy, with compressive moduli at 37 ± 7 and 59 ± 7 MPa for SiC and Si 3 N 4 , respectively, and thermal conductivities (using the laser flash method) at 0.16 3 ± 0.010 and 0.070 ± 0.001 W m −1 K −1 , respectively. The synthetic methodology of this report can be extended to other sol−gel derived oxide networks and is not limited to ceramic aerogels. A work in progress includes metallic Fe(0) aerogels.