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 m 2 g −1 ), and lower thermal conductivities (down to 0.071 W m −1 K −1 ) than their thermally polymerized counterparts (corresponding best values: 64 m 2 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 −CH 2 − 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 HClcatalyzed PBOs, can be as high as 520 m 2 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.

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 ).

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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 13 C NMR and several liquid 1 H 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 linearpolynorbornene (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).

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 nanotopology of the two components. SEM and N 2 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 (D f $ 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 noncovalent 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.