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-strain-tolerant 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(OCH2CH2)nOH (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 (CH3CN/acetone mixtures). Those parameters were varied systematically using statistical design-of-experiments methods. The skeletal frameworks of the resulting poly(isocyanurate-urethane) (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 (Tg) varied from about 30 (n = 4) to 70 °C (n = 1). At and above Tg, 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(CH2)2). Below the Tg 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 Tg + 10 °C and Tg – 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.

Polymeric aerogels (PA-xx) were synthesized via room-temperature reaction of an aromatic triisocyanate (tris(4-isocyanatophenyl) methane) with pyromellitic acid. Using solid-state CPMAS 13C and 15N NMR, it was found that the skeletal framework of PA-xx was a statistical copolymer of polyamide, polyurea, polyimide, and of the primary condensation product of the two reactants, a carbamic–anhydride adduct. Stepwise pyrolytic decomposition of those components yielded carbon aerogels with both open and closed microporosity. The open micropore surface area increased from <15 m2 g–1 in PA-xx to 340 m2 g–1 in the carbons. Next, reactive etching at 1,000 °C with CO2 opened access to the closed pores and the micropore area increased by almost 4× to 1150 m2 g–1 (out of 1750 m2 g–1 of a total BET surface area). At 0 °C, etched carbon aerogels demonstrated a good balance of adsorption capacity for CO2 (up to 4.9 mmol g–1), and selectivity toward other gases (via Henry’s law). The selectivity for CO2 versus H2 (up to 928:1) is suitable for precombustion fuel purification. Relevant to postcombustion CO2 capture and sequestration (CCS), the selectivity for CO2 versus N2 was in the 17:1 to 31:1 range. In addition to typical factors involved in gas sorption (kinetic diameters, quadrupole moments and polarizabilities of the adsorbates), it is also suggested that CO2 is preferentially engaged by surface pyridinic and pyridonic N on carbon (identified with XPS) in an energy-neutral surface reaction. Relatively high uptake of CH4 (2.16 mmol g–1 at 0 °C/1 bar) was attributed to its low polarizability, and that finding paves the way for further studies on adsorption of higher (i.e., more polarizable) hydrocarbons. Overall, high CO2 selectivities, in combination with attractive CO2 adsorption capacities, low monomer cost, and the innate physicochemical stability of carbon render the materials of this study reasonable candidates for further practical consideration.Keywords: aerogels; carbon aerogels; carboxylic acid; CO2 sequestration; copolymer; polyamide; polyimide; polyurea; pryomellitic acid;

This paper is a thorough investigation of the chemical transformations during pyrolytic conversion of phenolic resins to carbons, and reports that all carbons obtained from main-stream phenolic resins including phloroglucinol–formaldehyde (FPOL), phloroglucinol–terephthalaldehyde (TPOL), resorcinol–formaldehyde (RF), and phenol–formaldehyde (PF) contain fused pyrylium rings and charge-compensating phenoxides. Those four phenolic resins were prepared via a fast HCl-catalyzed process as low-density nanostructured solids classified as aerogels, which, owing to their open porosity, allowed air circulation through their bulk. In that regard, the first step of this study was the air-oxidation of those phenolic resin aerogels at 240 °C. In FPOL and TPOL aerogels, that air-oxidation step kicked off a cascade of reactions leading to ring-fusion aromatization and formation of pyrylium O+-heteroaromatic rings in every repeat unit of the polymeric backbone. Despite the complexity of the process, those structural forms were well-defined, and were retained through pyrolytic carbonization (800 °C). Under the same conditions (240 °C/air), RF and PF aerogels did not undergo aromatization; instead, they just went through an autooxidation-like process that converted the –CH2– bridges between phenolic moieties into carbonyls (CO). Importantly, however, upon further stepwise pyrolysis under Ar, by 600 °C all four systems (TPOL, FPOL, RF and PF), irrespective of whether they had been previously oxidized or not, converged to a common chemical composition. Thereby, carbon produced by pyrolysis of phenolic resins at 800 °C always contains fused pyrylium rings. All chemical analysis relied on FTIR, solid-state 13C NMR, XPS and CHN analysis. The only and significant difference made by the low-temperature (240 °C) air-oxidation step was identified with the surface areas of carbons from aromatizable systems (TPOL and FPOL), which were higher than those from direct pyrolysis of as-prepared aerogels. Upon further reactive etching with CO2, those surface areas went as high as 2778 ± 209 m2 g−1. Those findings are directly relevant to high surface area carbons for gas sorption (e.g., capture and sequestration of CO2) and ion exchange materials.

This paper is a thorough investigation of the chemical transformations during pyrolytic conversion of phenolic resins to carbons, and reports that all carbons obtained from main-stream phenolic resins including phloroglucinol–formaldehyde (FPOL), phloroglucinol–terephthalaldehyde (TPOL), resorcinol–formaldehyde (RF), and phenol–formaldehyde (PF) contain fused pyrylium rings and charge-compensating phenoxides. Those four phenolic resins were prepared via a fast HCl-catalyzed process as low-density nanostructured solids classified as aerogels, which, owing to their open porosity, allowed air circulation through their bulk. In that regard, the first step of this study was the air-oxidation of those phenolic resin aerogels at 240 °C. In FPOL and TPOL aerogels, that air-oxidation step kicked off a cascade of reactions leading to ring-fusion aromatization and formation of pyrylium O+-heteroaromatic rings in every repeat unit of the polymeric backbone. Despite the complexity of the process, those structural forms were well-defined, and were retained through pyrolytic carbonization (800 °C). Under the same conditions (240 °C/air), RF and PF aerogels did not undergo aromatization; instead, they just went through an autooxidation-like process that converted the –CH2– bridges between phenolic moieties into carbonyls (CO). Importantly, however, upon further stepwise pyrolysis under Ar, by 600 °C all four systems (TPOL, FPOL, RF and PF), irrespective of whether they had been previously oxidized or not, converged to a common chemical composition. Thereby, carbon produced by pyrolysis of phenolic resins at 800 °C always contains fused pyrylium rings. All chemical analysis relied on FTIR, solid-state 13C NMR, XPS and CHN analysis. The only and significant difference made by the low-temperature (240 °C) air-oxidation step was identified with the surface areas of carbons from aromatizable systems (TPOL and FPOL), which were higher than those from direct pyrolysis of as-prepared aerogels. Upon further reactive etching with CO2, those surface areas went as high as 2778 ± 209 m2 g−1. Those findings are directly relevant to high surface area carbons for gas sorption (e.g., capture and sequestration of CO2) and ion exchange materials.

Isocyanates react with carboxylic acids and yield amides. As reported herewith, however, transferring that reaction to a range of mineral acids (anhydrous H3BO3, H3PO4, H3PO3, H2SeO3, H6TeO6, H5IO6, and H3AuO3) yields urea. The model system for this study was a triisocyanate, tris(4-isocyanatophenyl)methane (TIPM), reacting with boric acid in DMF at room temperature, yielding nanoporous polyurea networks that were dried with supercritical fluid CO2 to robust aerogels (referred to as BPUA-xx). BPUA-xx were structurally (CHN, solid-state 13C NMR) and nanoscopically (SEM, SAXS, N2-sorption) identical to the reaction product of the same triisocyanate (TIPM) and water (referred to as PUA-yy). Minute differences were detected in the primary particle radius (6.2–7.5 nm for BPUA-xx versus 7.0–9.0 nm for PUA-yy), the micropore size within primary particles (6.0–8.5 Å for BPUA-xx versus 8.0–10 Å for PUA-yy), and the solid-state 15N NMR whereas PUA-yy showed some dangling −NH2. All data together were consistent with exhaustive reaction in the BPUA-xx case, bringing polymeric strands closer together. Residual boron in BPUA-xx aerogels was quantified with prompt gamma neutron activation analysis (PGNAA). It was found very low (≤0.05% w/w) and was shown to be primarily from B2O3 (by 11B NMR). Thus, any mechanism for systematic incorporation of boric acid in the polymeric chain, by analogy to carboxylic acids, was ruled out. (In fact, it is shown mathematically that boron-terminated star polyurea from TIPM should contain ≥3.3% w/w B, irrespective of size.) Retrospectively, it was fortuitous that this work was conducted with aerogels, and the model system used H3BO3, whereas the byproduct, B2O3, could be removed easily from the porous network, leaving behind pure polyurea. With other mineral acids, results could have been misleading, because the corresponding oxides are insoluble and remain within the polymer (via skeletal density determinations and EDS). On the positive side, the latter is a convenient method for in situ doping robust porous polymeric networks with oxide or pure metal nanoparticles (Au in the case of H3AuO3) for possible applications in catalysis.

Polyamide aerogels with ferrocene as a monomer repeat unit were prepared in one step from ferrocene dicarboxylic acid and tris(4-isocyanatophenyl)methane. Pyrolysis at ≥800 °C yielded nanoporous carbons doped throughout with crystallites of α-Fe (about 50 nm in diameter), which in turn were shrouded in graphitic ribbons (<30 graphene layers thick). Transmetalation was carried out with aqueous solutions of Au, Pt, Pd, Rh, and Ni salts, via a path akin to galvanic corrosion, whereas graphitic ribbons separated anodes (α-Fe particles) from cathodes (defects along the ribbons). The new metallic phases formed clusters of smaller crystallites (10–20 nm in diameter) on the graphitic ribbons, leaving behind empty cage-like formations previously occupied by the Fe(0) nanoparticles. All metal-doped carbons were monolithic and over 85% porous. Catalytic activity was demonstrated with the oxidation of benzyl alcohol to benzaldehyde catalyzed with carbon-supported Au or Pt, the reduction of nitrobenzene by hydrazine to aniline catalyzed with carbon-supported Fe, and two Heck coupling reactions of iodobenzene with styrene or butyl acrylate, catalyzed with carbon-supported Pd. The distinguishing feature of those catalysts was that they could be just picked up, for example, with a pair of tweezers, and redeployed in a new reaction mixture immediately, thus bypassing less efficient recovery processes like filtration.

Monolithic nanoporous iron was prepared via carbothermal reduction of interpenetrating networks of polybenzoxazine and iron oxide nanoparticles. Excess carbon was burned off at 600 °C in air, and oxides produced from partial oxidation of the Fe(0) network were reduced back to Fe(0) with H2 at different temperatures (temp) ranging from 300 to 1300 °C. Samples were carbon-free, for temp > 400 °C also oxide-free, and are referred to according to the final H2-reduction temperature as Fe-temp. Fe-temp monoliths were infiltrated with perchlorates, dried exhaustively and were ignited with a flame in open air. Most experimentation was conducted with LiClO4. Depending on temp, monoliths fizzled out (≤400 °C), exploded violently (500–900 °C) or behaved as thermites (≥950 °C). Samples sealed in evacuated tubes did not explode, while if sealed under N2 the explosive effect was intensified. Thus, explosive behavior was attributed to rapid heating and expansion of gas filling nanoporous space. However, although that condition was necessary for explosive behavior, it was not sufficient. Based on SEM, particle sizes via N2 sorption, electrical conductivity measurements and mechanical strength data under quasi-static compression, it was concluded that the boundaries between the three types of behavior after ignition were associated with (a) mild sintering (fizzling/explosive boundary at around 500 °C); and, (b) melting-like fusion of skeletal nanoparticles (explosive/thermite boundary at around 950 °C). Overall, mechanically weaker networks fizzled out; too strong behaved as thermites; networks of intermediate strength exploded. For thermite behavior in particular, other factors may be also at play, such as a combination of reduced porosity, a substoichiometric amount of LiClO4 and a slower heat release rate. The latter was supported by TGA data in O2 and was attributed to a slower rate of oxidation of progressively thicker nanostructures as the H2-reduction temperature increased.

Mechanically strong polymer-crosslinked templated silica aerogel (CTSA) monoliths with ordered tubular mesopores were synthesized through an acid-catalyzed, surfactant-templated sol–gel process followed by covalent crosslinking of the elementary building blocks with polyurea. Specifically, a structure-directing reagent (triblock copolymer, Pluronic P123) was used in combination with variable amounts of a micelle-swelling reagent (1,3,5-trimethylbenzene) to regulate the size, shape, morphology of the elementary building blocks, as well as the pore size distribution of acid-catalyzed silica. The structure was subsequently treated with variable concentrations of a diisocyanate that reacts with surface –OH groups as well as residual gelation water adsorbed on the surface of silica. The developing polymer (polyurea) adheres to the walls of the mesoporous tubes and leaves macropores open. Rather than using a typical supercritical fluid (typically from CO2) drying protocol, the polymer-crosslinked materials of this study are strong enough to withstand stresses imposed by evaporating solvents and were dried from pentane under ambient pressure. The morphostructural properties of CTSAs were characterized before and after compression testing using a battery of methods including SEM, TEM and small-angle X-ray scattering. Mechanical properties were investigated using quasi-static compression tests, tensile, high-strain-rate dynamic tests as well as shear creep measurements. In addition, dynamic mechanical analysis as well as heat transfer tests was conducted. The Young’s modulus was found to be about 800 MPa while the specific energy absorption was as high as 123 J/g, making this material a prime candidate for ballistic protection.

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 13C NMR. The porous network was probed with N2-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 N2-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 H2 or CO, yielded coarse powders. Turning to smelting reduction, we used the acid environment of gelling [Fe(H2O)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, Mössbauer, XRD, and CHN analysis, we found out that iron(III) oxide undergoes pre-reduction to Fe3O4 and participates in the oxidation of PBO, which is a prerequisite for robust carbons suitable as structure-directing templates. Subsequently, interconnected submicrometer-size Fe3O4 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 (Fe3O4) process. Carbothermal reduction, oxidative removal of residual carbon (air), and re-reduction (H2) of α-Fe2O3 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 LiClO4 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).

We describe a new room-temperature HCl-catalyzed method for the synthesis of polybenzoxazine (PBO) aerogels from bisphenol A, formaldehyde, and aniline that cuts the typical multiday high-temperature (≥130 °C) route to a few hours. The new materials are studied comparatively to those from heat-induced polymerization, and both types are evaluated as precursors of carbon (C-) aerogels. In addition to the ortho-phenolic position of bisphenol A, the HCl-catalyzed process engages the para position of the aniline moieties leading to a higher degree of cross-linking. Thereby, the resulting aerogels consist of smaller particles with higher mesoporosity, higher surface areas (up to 72 m2 g–1), and lower thermal conductivities (down to 0.071 W m–1 K–1) than their thermally polymerized counterparts (corresponding best values: 64 m2 g–1 and 0.091 W m–1 K–1, respectively). It is also reported that the carbonization efficiency (up to 61% w/w), the nanomorphology, and the pore structure of the resulting C-aerogels depend critically on a prior curing step of as-prepared PBO aerogels at 200 °C in the air. According to spectroscopic evidence and CHN analysis, curing at 200 °C in air oxidizes the −CH2– bridges along the polymeric backbone and subsequently fuses aromatic rings (see Abstract Graphic) in analogy to transformations during carbonization processing of polyacrylonitrile. C-aerogels from cured PBO aerogels are microscopically similar to their respective parent aerogels; however, they have greatly enhanced surface areas, which, for C-aerogels from HCl-catalyzed PBOs, can be as high as 520 m2 g–1 with up to 83% of that attributed to newly created micropores. The acid-catalyzed route is used in the next article for the synthesis of iron oxide/PBO interpenetrating networks as precursors of iron(0) aerogels.

Biocompatible dysprosia aerogels were synthesized from DyCl3·6H2O and were reinforced mechanically with a conformal nano-thin-polyurea coating applied over their skeletal framework. The random mesoporous space of dysprosia aerogels was filled up to about 30% v/v with paracetamol, indomethacin, or insulin, and the drug release rate was monitored spectrophotometrically in phosphate buffer (pH = 7.4) or 0.1 M aqueous HCl. The drug uptake and release study was conducted comparatively with polyurea-crosslinked random silica aerogels, as well as with as-prepared (native) and polyurea-crosslinked mesoporous silica perforated with ordered 7 nm tubes in hexagonal packing. Drug uptake from random nanostructures (silica or dysprosia) was higher (30–35% w/w) and the release rate was slower (typically >20 h) relative to ordered silica (19–21% w/w, <1.5 h, respectively). Drug release data from dysprosia aerogels were fitted with a flux equation consisting of three additive terms that correspond to drug stored successively in three hierarchical pore sites on the skeletal framework. The high drug uptake and slow release from dysprosia aerogels, in combination with their low toxicity, strong paramagnetism, and the possibility for neutron activation render those materials attractive multifunctional vehicles for site-specific drug delivery.Keywords: aerogels; biocompatibility; drug delivery; dysprosium; indomethacin; insulin; paracetamol; rare earth;

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.Keywords: aerogels; Cassie−Baxter; hydrogen bonding; oil spill cleanup; polyurea; superhydrophobic;

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 13C and 15N NMR, rheometry, N2- and Hg-porosimetry, SEM, and small-angle X-ray scattering (SAXS). Macroscopic properties such as styrofoam-like thermal conductivities (∼0.030 W m–1K–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.Keywords: aerogel; flexible; fractal; macroporosity; mesoporosity; microporosity; nanomorphology; nanoparticle; polyurethane; rigid; sol−gel; structure−property relationship;

Polydicyclopentadiene (pDCPD) is a polymer of emerging technological significance from separations to armor. It is a paradigm of ring opening metathesis polymerization (ROMP) and should be an ideal candidate for strong nanoporous solids (aerogels), however, excessive swelling of pDCPD wet-gels in toluene (up to 200% v/v), followed by de-swelling and severe deformation in acetone, renders the resulting aerogels unusable. With only 4–5% of the pendant cyclopentene double bonds of pDCPD engaged in crosslinking (see previous paper of this issue), introducing additional crosslinking with polymethylmethacrylate (PMMA) was deemed appropriate. Thus, even with an uptake of PMMA as low as 13% w/w, the resulting aerogels kept the shape and dimensions of their molds. Evidence though suggests (e.g., DSC) that PMMA remains a linear polymer, hence pDCPD/PMMA networks resist deformation, not because of molecular-level crosslinking, but due to a synergism related to the nano-topology of the two components. SEM and N2 sorption on dry aerogels show that macroscopic deformation of wet-gels is accompanied by coalescence of nanoparticles. Small angle X-ray scattering (SAXS) shows that both deformed (pDCPD) and non-deformed (pDCPD/PMMA) aerogels consist of same-size primary particles (8–9 nm radius) that form non-mass-fractal secondary particles (21–27 nm radius). On the other hand, rheology shows that the pDCPD gel network is formed by mass fractal aggregates (Df ∼ 2.4). Putting this information together, it is concluded that the pDCPD network is formed by aggregates of secondary particles. It is suggested that particles coalescence is driven by non-covalent interactions that squeeze deformable secondary particles of one fractal assembly inside the empty space of another. As supported by skeletal density considerations, PMMA fills the space between primary particles; thus, secondary particles become rigid and can no longer squeeze past one another into the empty space of their higher fractal aggregates.

Polydicyclopentadiene (pDCPD) is a polymer of emerging technological significance from separations to armor. It is a paradigm of ring opening metathesis polymerization (ROMP) and some of its remarkable properties (e.g., strength) have been attributed to crosslinking of the pendant cyclopentenes. pDCPD should be an ideal candidate for strong nanoporous solids (aerogels), however, excessive swelling of the wet-gels precursors in toluene (up to 200% v/v), followed by de-swelling and severe deformation in acetone, renders the resulting aerogels unusable. Based on spectroscopic evidence (IR, solid state 13C NMR and several liquid 1H NMR controls), only 4–5% of the pendant cyclopentene double bonds of pDCPD are engaged in crosslinking, via Wagener-type olefin coupling. Deformation was rectified via free radical polymerization of methylmethacrylate (MMA) in the pores of pDCPD wet-gels. The uptake of PMMA was varied in the 13–28% w/w range by varying the concentration of MMA. Evidence (e.g., differential scanning calorimetry) though suggests that PMMA remains a linear polymer, hence the pDCPD/PMMA network resist deformation, not because of molecular-level crosslinking, but due to a synergism related to the nano-topology of the two components (see next paper of this issue). With cylindrical monoliths available, the nature of the interparticle chemical bonding in pDCPD/PMMA aerogels was probed top-down with thermal conductivity and compression testing, using linear-polynorbornene (pNB) aerogels as a control system. The latter, with no pendant cyclopentenes, has no chance for interpolymer chain crosslinking. The solid thermal conduction and stiffness of pDCPD/PMMA and pNB aerogels scale similarly, pointing to a common mechanism for interparticle bonding. That was assigned to cross-metathesis, effectively extending the polymer chains of one nanoparticle into another, and was reflected on very high polydispersities (8–13).

•Correlation of structure and thermal transport properties in a micro-porous solid.•Model system is based on polyurea aerogels.•Microstructure changes from a network of fibers to a string-of-pearls-like structure.•An exceptionally low exponent α = 1 was found for λsolid∝ρα.•Particle connectivity has a significant impact on λsolid.This study correlates microstructure with thermal transport properties in nanoporous solids. The model system is based on polyurea (PUA) aerogels. Those aerogels demonstrate a dramatic change in microstructure with density. Low density aerogels consist of entangled nano-fibers changing into interconnected nanoparticles as the density increases. The nanostructure was probed in terms of both particle size and network interconnectivity with scanning electron microscopy and small angle X-ray scattering. Thermal conductivity values between 0.027 and 0.066 W/mK were obtained with the hot-wire method for PUA samples with densities between 0.04 and 0.53 g/cm3. Both, pressure and temperature dependent experiments were performed for the deconvolution of total thermal conductivity into gaseous, radiative, and transport-through-the-solid-framework contributions. Subsequently, thermal conductivity along the solid framework was considered as a function of microstructure. That leads to a quantitative evaluation of the impact of primary particle characteristics and network interconnectivity on the solid thermal conductivity.

1H NMR, ESI-MS, and DFT calculations with the M062X/6-31G* method show that, in water, the bistetrafluoroborate salt of N,N′-dimethyl-2,6-diaza-9,10-anthraquinonediium dication (DAAQ·2BF4–) exists in equilibrium with both its gem-diol and several aggregates (from dimers to at least octamers). With high concentrations of HCl (e.g., 1.2–1.5 M), all aggregates break up and the keto-to-gem-diol equilibrium is shifted quantitatively toward the quinone form. The same effect is observed with 1.5 mol equiv of cucurbit[7]uril, CB[7], with which all equilibria are shifted toward the quinone form, which undergoes slow exchange with the CB[7] cavity as both the free and the CB[7]-intercalated quinone (DAAQ@CB[7]) are observed simultaneously by 1H NMR. The affinity of DAAQ for the CB[7] cavity (Keq = 4 × 106 M–1) is in the range found for tricyclic dyes (0.4–5.4 × 106 M–1), and among the highest observed to date. A computational comparative study of the corresponding CB[7] complex of the N,N′-dimethyl-4,4′-bipyridinium dication (N,N′-dimethyl viologen, MeV) suggests that the higher binding constant for intercalation of DAAQ may be partially attributed to a lesser distortion of CB[7] (required to maximize favorable nonbonding interactions) as a result of the flat geometry of DAAQ.

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-of-initiator 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, 13C NMR, XRD, Raman, and HRTEM. Materials properties are monitored by SEM and N2-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.Keywords: aerogel; carbon; emulsion polymerization; graphite; polyacrylonitrile;

Monolithic hierarchical fractal assemblies of silica nanoparticles are referred to as aerogels, and despite an impressive collection of attractive macroscopic properties, fragility has been the primary drawback to applications. In that regard, polymer-cross-linked silica aerogels have emerged as strong lightweight nanostructured alternatives rendering new applications unrelated to aerogels before, as in ballistic protection, possible. In polymer-cross-linked aerogels skeletal nanoparticles are connected covalently with a polymer. However, the exact location of the polymer on the elementary structure of silica and, therefore, critical issues, such as how much is enough, have remained ambiguous. To address those issues, the internal nanoporous surfaces of silica wet-gels were modified with norbornene (NB) by cogelation of tetramethyl orthosilicate (TMOS) with a newly synthesized derivative of nadic acid (Si-NAD: N-(3-triethoxysilylpropyl)-5-norbornene-2,3-dicarboximide). As inferred by both rheological and liquid 29Si NMR data, Si-NAD reacts more slowly than TMOS, yielding a TMOS-derived skeletal silica network surface-derivatized with NB via monomer-cluster aggregation. Then, ring-opening metathesis polymerization (ROMP) of free NB in the nanopores engages surface-bound NB moieties and bridges skeletal nanoparticles either through cross-metathesis or a newly described stitching mechanism. After solvent exchange and drying with supercritical fluid CO2 into aerogels (bulk densities in the range 0.27–0.63 g cm–3, versus 0.20 g cm–3 of the native network), the bridging nature of the polymer is inferred by a >10-fold increase in mechanical strength and a 4-fold increase in the energy absorption capability relative to the native samples. The cross-linking polymer was freed from silica by treatment with HF, and it was found by GPC that it consists of a long and a short component, with around 400 and 10 monomer units, respectively. No evidence (by SAXS) was found for the polymer coiling up into particles, consistent with the microscopic similarity (by SEM) of both native and cross-linked samples. Most importantly, the polymer does not need to spill over higher aggregates for greatly improved mechanical strength; mechanical properties begin improving after the polymer coats primary particles. Extremely robust materials are obtained when the polymer fills most of the fractal space within secondary particles.Keywords: aerogel; cross-linking; hierarchical; mechanism; norbornene; polymer; primary particles; rheology; ROMP; SANS; SAXS; secondary particles; silica;

Poly(3,4-ethylenedioxythiophene), PEDOT, films are used as antistatic coatings on electrically insulating substrates such as plastic and glass. A novel method for the synthesis of conducting PEDOT films on insulators relies on sol–gel chemistry to attach a di-Si(OEt)3 functionalized free radical initiator (AIBN) on oxidized surfaces, followed by: (a) attachment of 3,4-(vinylenedioxy)thiophene (VDOT: an analogue to EDOT susceptible to radical addition through its vinylenedioxy group); and, (b) oxidative (with FeCl3) co-polymerization of surface-confined VDOT with 3,4-ethylenedioxythiophene (EDOT). In conjunction with classical photolithography, the method yields thin (∼150 nm) yet dense, pinhole-free (confirmed electrochemically), hard (>6H), extremely adhesive (5B), patterned, highly conducting (52 mho cm−1) films. The process is applied mainly on glass but it works equally well on oxidized metal surfaces (aluminum, steel, Pt). Control studies related to “grafting from” with surface-confined AIBN were conducted by growing inexpensive poly(styrene) and poly(methylmethacrylate) films.

According to recent reports, supramolecular complexes of the pyrylium cation with cucurbit[x]urils (CB[x], x = 7, 8) show promising photoluminescence suitable for electroluminescent devices. In turn, photoluminescence seems to be related to the stereochemistry of the complexes; however, that has been controversial. Here, we report that in H2O, 2,6-disubsituted-4-phenyl pyryliums (Pylm) form dimers quantitatively (equilibrium constants >104 M–1), but they enter as such only in the larger CB[8]. In terms of orientation, 1H NMR shows that Me-Pylm, Ph-Pylm, and t-Bu-Pylm insert their 4-phenyl groups in either the CB[7] or CB[8] cavity. The orientation of iPr-Pylm in the iPr-Pylm@CB[7] complex is similar. Experimental conclusions are supported by DFT calculations using the M062X functional and the 6-31G(d) basis set. In the case of (iPr-Pylm)2@CB[8], 1H NMR of both the guest and the host indicates that both guests might enter CB[8] from the same side with their iPr groups in the cavity, but DFT calculations leave room for ambiguity. In addition to the size and hydrophobicity of the 2,6-substituents of the guests, as well as the size and flexibility of the hosts, theory reveals the importance of explicit solvation (H2O) and finite temperature effects (particularly for 1H NMR shielding calculations) in the determination of the stereochemistry of those complexes.

Polyimide aerogel monoliths are prepared by ring-opening metathesis polymerization (ROMP) of a norbornene end-capped diimide, bis-NAD, obtained as the condensation product of nadic anhydride with 4,4′-methylenedianiline. The density of the material was varied in the range of 0.13−0.66 g cm−3 by varying the concentration of bis-NAD in the sol. Wet gels experience significant shrinkage, relative to their molds (28%−39% in linear dimensions), but the final aerogels retain high porosities (50%−90% v/v), high surface areas (210−632 m2 g−1, of which up to 25% is traced to micropores), and pore size distributions in the mesoporous range (20−33 nm). The skeletal framework consists of primary particles 16−17 nm in diameter, assembling to form secondary aggregates (by SANS and SEM) 60−85 nm in diameter. At lower densities (e.g., 0.26 g cm−3), secondary particles are mass fractals (Dm = 2.34 ± 0.03) turning to closed-packed surface fractal objects (DS = 3.0) as the bulk density increases (≥0.34 g cm−3), suggesting a change in the network-forming mechanism from diffusion-limited aggregation of primary particles to a space-filling bond percolation model. The new materials combine facile one-step synthesis with heat resistance up to 200 °C, high mechanical compressive strength and specific energy absorption (168 MPa and 50 J g−1, respectively, at 0.39 g cm−3 and 88% ultimate strain), low speed of sound (351 m s−1 at 0.39 g cm−3) and styrofoam-like thermal conductivity (0.031 W m−1 K−1 at 0.34 g cm−3 and 25 °C); hence, they are reasonable multifunctional candidate materials for further exploration as thermal/acoustic insulation at elevated temperatures.Keywords: aerogels; end-capped; norbornene; polyimides; ROMP;

Polymerization of trifunctional polyaromatic carboxylic acids and isocyanates in dilute DMF solutions using the rather underutilized reaction of the carboxylic acid group (–COOH) with isocyanates (–NCO) towards amides (–NH(CO)–) induces early phase separation of surface-active aramid nanoparticles that form a solvent-filled 3D network stabilized against collapse by the chemical energy of the interparticle covalent bridges (crosslinks). Those wet-gels can be dried with liquid CO2 taken out at the end as a supercritical fluid into lightweight (bulk density ∼0.3 g cm−3) highly porous (77% v/v) multifunctional materials classified as aerogels with high specific energy absorption (37 J g−1), open-air speed of sound (338 m s−1) and Styrofoam-like thermal conductivity (0.028 W m−1 K−1).

Surfactant-templated mesoporous silica aerogels (or nanofoams) with their entire skeletal framework nanoencapsulated conformally by a thin polyurea layer are emerging as materials with high specific strength and high energy absorption. In this paper a modified split Hopkinson pressure bar was used to investigate their mechanical behavior under dynamic compression at high strain rates. The evolution of the mesoporous structure under such dynamic impact conditions was simulated using the Material Point Method (MPM). The material point model was generated from X-ray micro-computed tomography whereas each voxel was converted to a material point corresponding to the local skeletal density of the material. Simulation results agree well with the experimental data, indicating that the MPM can effectively model the compression of complex mesoporous structures. Simulations indicate a nearly uniform deformation at all three stages of compression: the elastic region, compaction and the final densification due to the low ratio of pore size to wall thickness and random distribution of the pores. Simulations have also indentified the function of the conformal polymer coating as a reinforcing factor, showing that different porosities, obtained by varying the skeletal wall thickness, affect the local stress distribution. Eventually, simulations confirm that the stress–strain behavior of aerogels under compression follows a power-law relationship with the initial bulk density, consistent with experimental results.Research highlights► The evolution of mesoporous structures in templated aerogels is simulated using MPM. ► MPM captured nonlinear and complex surface contacts in deformations of aerogels. ► The compressive behavior of template aerogels was determined at a high strain rate. ► The templated aerogels absorb 108 J/g energy up to 70% compressive strain. ► Simulations show uniform deformation due to low pore size/thickness and random porosity.

SiC retains high mechanical strength and oxidation stability at temperatures above 1500 °C, representing a viable alternative to silica, alumina, and carbon, which have been in use as catalyst supports for more than 60 years. Preparation of monolithic porous SiC is usually elaborate and porosities around 30% v/v are typically considered high. This report describes the synthesis of monolithic highly porous (70% v/v) SiC by carbothermal reduction (1200−1600 °C) of 3D sol−gel silica nanostructures (aerogels) conformally coated and cross-linked with polyacrylonitrile (PAN). Synthesis of PAN-cross-linked silica aerogels is carried out in one pot by simple mixing of the monomers, whereas conversion to SiC is carried out in a tube reactor by programmed heating. Intermediates after aromatization (225 °C in air) and carbonization (800 °C under Ar) were isolated and characterized for their chemical composition and materials properties. Data are interpreted mechanistically and were used iteratively for process optimization. Solids 29Si NMR validates use of skeletal densities (by He pycnometry) for the quantification of the conversion of silica to SiC. Consistent with the topology of the carbothermal process, data support complete conversion of SiO2 to SiC requiring a C:SiO2 ratio higher than the stoichiometric one (=3). The morphology of the SiC network is invariant of the processing temperature between 1300 and 1600 °C, and hence it is advantageous to carry out the carbothemal process at higher temperatures where reactions run faster. Those samples are macroporous and consist of pure polycrystalline β-SiC (skeletal density: 3.20 g cm−3) with surface areas in the range reported previously for biomorphic SiC (∼20 m2 g−1). Although the micromorphology remains constant, the crystallite size of SiC increases with processing temperature (from 7.1 nm at 1300 °C to 16.5 nm at 1600 °C). Samples processed at 1200 °C are mesoporous and amorphous (by XRD), even though they consist of ∼75% mol/mol SiC. The change in the morphology of SiC in the 1200−1300 °C range has been explained by a melting mechanism. This comprises the first report of using a polymer cross-linked aerogel for the synthesis of another porous material.

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.

N-Substituted 4-benzoylpyridinium monocations form stable host−guest complexes with cucurbit[7]uril (CB[7]) in DMSO (Keq ≈ 0.6−1.9 × 103 M−1). Observation of simultaneous reversible and quasi-reversible e-transfer processes from the free and intercalated quests, respectively, is attributed to the pre-e-transfer host−guest equilibrium. The standard rate constant for Me-BP@CB[7] (ks = 1.0 × 10−4 cm·s−1) reflects e-transfer across 5.7 Å, corresponding to the distance of the intercalated guest from the outmost perimeter of CB[7] (5.3 Å).

Skeletal nanoparticles of porous low-density materials formally classified as aerogels are cross-linked by surface-initiated polymerization (SIP) using a new surface-confined bidentate free-radical initiator structurally related to azobisisobutyronitrile (AIBN). Methylmethacrylate, styrene, and divinylbenzene are introduced in the mesopores, and upon heating at 70 °C, all mesoporous surfaces throughout the entire skeletal framework are coated conformally with a 10−12 nm thick polymer layer indistinguishable spectroscopically from the respective commercial bulk materials. The amount of polymer incorporated in the structure is controlled by the concentration of the monomer in the mesopores, and albeit an up to a 3-fold increase in bulk density (up to 0.6−0.8 g cm−3) and a decrease in the porosity even down to 40%, the materials remain mesoporous with average pore diameters increasing from 20 nm in the native samples to 41 and 62 nm in PMMA and polystyrene cross-linked samples, respectively. The new materials combine hydrophobicity with vastly improved mechanical properties in terms of strength, modulus, and toughness relative to their native (non-cross-linked) counterparts. The effect of polymer accumulation on the modulus has been also simulated numerically. Being able to use SIP for cross-linking 3D assemblies of nanoparticles comprising the skeletal framework of typical aerogels paves the way for the deconvolution of cross-linking from gelation (a free-radical versus an ionic process, respectively), so that ultimately all gelation and cross-linking reagents can be included together in one pot, leading to great process simplification. The mechanical properties of the new materials render them appropriate for anti-ballistic applications (e.g., armor).

Carbon (C) aerogels are made by pyrolysis of resorcinol-formaldehyde (RF) aerogels under Ar, and they combine electrical conductivity with a high open mesoporosity. However, because macropores are known to facilitate mass transfer, macroporous C-aerogels could be useful for application in separations or as fuel cell and battery electrodes. Macropores are typically incorporated in C-aerogels during gelation of the RF precursors by using either “hard” templating with silica or polystyrene beads, or “soft” templating with surfactants. Here, we report an alternative method, where open macroporosity is introduced by pyrolyzing RF aerogels whose skeletal nanoparticles have been cross-linked covalently with an isocyanate-derived polymer that coats conformally the entire RF framework. The structural, physical, and chemical evolution of the X-RF network was monitored at various stages during pyrolysis by DSC, TGA, SEM, N2 adsorption porosimetry, and 13C NMR. The accumulated evidence shows that the cross-linker first loses its chemical bonding with the skeletal nanoparticles and then melts, exerting surface tension forces on the RF framework, which cause a partial structural collapse that creates macropores. The xerogel-like internal texture of the macroporous walls is responsible for close contact of the carbon skeletal nanoparticles, resulting in an about 7× lower bulk electrical resistivity of the macroporous material relative to the corresponding mesoporous network, which is obtained by pyrolysis of native (i.e., non-cross-linked) RF aerogels. The new macroporous material was evaluated electrochemically for possible application as an electrode in batteries and fuel cells.

A strong lightweight material (X-VOx) was formulated by nanocasting a conformal 4 nm thin layer of an isocyanate-derived polymer on the entangled worm-like skeletal framework of typical vanadia aerogels. The mechanical properties were characterized under both quasi-static loading conditions (dynamic mechanical analysis, compression and flexural bending testing) as well as high strain rate loading conditions using a split Hopkinson pressure bar (SHPB). The effects of mass density, moisture concentration and low temperature on the mechanical properties were determined and evaluated. Digital image correlation was used to measure the surface strains through analysis of images acquired by ultra-high speed photography, indicating nearly uniform compression at all stages of deformation during compression. The energy absorption of X-VOx was plotted as a function of the density, strain rate and temperature, and compared with that of plastic foams. X-VOx remains ductile even at −180 °C, a characteristic not found in most materials. This unusual ductility is derived from interlocking and sintering-like fusion of nanoworms during compression. X-VOx emerges as an ideal material for force protection under impact.

Room temperature vulcanizing (RTV)-based components have been used on Mars Pathfinder, the Mars rovers, Spirit and Opportunity, as well as the Phoenix Lander as a support matrix for pigmented panoramic camera calibration targets. RTV 655 has demonstrated superiority to other polymers due to its unique range of material properties namely mechanical stability between −115 and 204 °C and UV radiation tolerance. As a result, it has been the number one choice for many space-related missions. However, due to the high mass density and the natural tendency for electrostatic charging RTV materials have caused complications by attracting and retaining dust particles (Sabri et al., 2008). In the current project we have investigated the relevant properties of polymer-reinforced (crosslinked) silica aerogels with the objective of substituting RTV-based calibration targets with an aerogel based design. The lightweight, mechanical strength, ability to accept color pigments, and extremely low dust capture makes polyurea crosslinked aerogels a strong candidate as a chromatic standard for extraterrestrial missions. For this purpose, the reflection spectra, gravimetric analysis, and low temperature response of metal oxide pigmented, polyurea crosslinked silica aerogels have been investigated and reported here.

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.

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.

Polymerization of trifunctional polyaromatic carboxylic acids and isocyanates in dilute DMF solutions using the rather underutilized reaction of the carboxylic acid group (–COOH) with isocyanates (–NCO) towards amides (–NH(CO)–) induces early phase separation of surface-active aramid nanoparticles that form a solvent-filled 3D network stabilized against collapse by the chemical energy of the interparticle covalent bridges (crosslinks). Those wet-gels can be dried with liquid CO2 taken out at the end as a supercritical fluid into lightweight (bulk density ∼0.3 g cm−3) highly porous (77% v/v) multifunctional materials classified as aerogels with high specific energy absorption (37 J g−1), open-air speed of sound (338 m s−1) and Styrofoam-like thermal conductivity (0.028 W m−1 K−1).

Poly(3,4-ethylenedioxythiophene), PEDOT, films are used as antistatic coatings on electrically insulating substrates such as plastic and glass. A novel method for the synthesis of conducting PEDOT films on insulators relies on sol–gel chemistry to attach a di-Si(OEt)3 functionalized free radical initiator (AIBN) on oxidized surfaces, followed by: (a) attachment of 3,4-(vinylenedioxy)thiophene (VDOT: an analogue to EDOT susceptible to radical addition through its vinylenedioxy group); and, (b) oxidative (with FeCl3) co-polymerization of surface-confined VDOT with 3,4-ethylenedioxythiophene (EDOT). In conjunction with classical photolithography, the method yields thin (∼150 nm) yet dense, pinhole-free (confirmed electrochemically), hard (>6H), extremely adhesive (5B), patterned, highly conducting (52 mho cm−1) films. The process is applied mainly on glass but it works equally well on oxidized metal surfaces (aluminum, steel, Pt). Control studies related to “grafting from” with surface-confined AIBN were conducted by growing inexpensive poly(styrene) and poly(methylmethacrylate) films.