Preparation of Macroporous Conductive Carbon Cerogels from Pyrolysis of Isocyanate-Crosslinked Resorcinol Formaldehyde Aerogels

Carbon aerogels combine typical aerogal properties such as high surface area and low density with electrical conductivity. They are prepared by pyrolysis under N2 or Ar at 600-2100 degrees C of resorcinol formaldehyde (RF) aerogels, which in turn are prepared via aqueous sol-gel chemistry. Carbon aerogels are considered for numerous applications such as separation media in HPLC, as supercapacitors, as materials for hydrogen storage, as non-reflective materials, and as anodes in lithium-ion batteries. Meanwhile, porous carbons with high hydrophobic surface areas and large pore volumes are used as industrial adsorbents. In addition, macropores enhance mass transport for applicatoins in energy storage and lithium intercalation batteries. It is well-established with silica that monodispersed polystryrene beads can be used to introduce ordered mesoporosity or macroporosity. In the same approach, plystyrene beads have been also incorportated as templates in RF sol-gel matrices and have been removed later by dissolving in toluene. Here, we report that RF gels crosslinked with isocyanates yield macroporous, electrically conducting carbon aerogels without need for templating

Adhesion Enhancement of Polymeric Films on Glass Surfaces by a Silane Derivative of Azobisisobutyronitrile (AIBN).

Adhesion of polymeric films on surfaces can be due to a combination of van der Waals, electrostatic or covalent interactions between the two materials. The interfacial adhesion between a polymer film and glass or metal can be improved by using a broad class of silane coupling agents. Typically, silane coupling compounds used for adhesion improvement have structure similar to (R´O)3-Si-R, where R´O- is an alkoxy group and -R is an organofunctional group. Under appropriate reaction conditions alkoxy groups condense with hydroxyl groups available on the surface, resulting in surfaces decorated with organofunctional -R groups, which promote formation of covalent bonding of the coupling agent with polymeric networks. Alkoxy silanes with amino and vinyl organofunctional groups are common silane coupling agents and their adhesion-promoting abilities with polymeric films have been well-documented. In analogy to our previous work of forming conformal polymer coatings on three dimensional assemblies of silica nanoparticles (aerogels) via surface initiated polymerization (SIP), here we expand the scope of that work demonstrating the application of a new bidentate free radical initiator (Si-AIBN) as coupling agent that enhances adhesion of polystyrene (PS) and polymethylmethacrylate (PMMA). Si-AIBN was synthesized via a condensation reaction between 3-aminopropyltriethoxysilane (APTES) and azobiscyanovaleric acid. Si-AIBN is attached to the surface of glass by hydrolysis of the ethoxy groups and reaction with the hydroxyl groups of the surface. On supply of thermal energy those glass surfaces act like a macro initiator generating surface-bound radicals. In the presence of olefin monomers, surface-bound initiator starts formation of polymeric chains in analogy to a “grafting from” approach. Since each polymer chain is terminated by a molecule of the initiator, which is surface-bound, the adhesion of the resulting polymeric films on the substrate is promoted not only by mechanical interlocking of the polymeric chains but also by covalent bonding with the glass surface

Acid-catalyzed Time-efficient Synthesis of Resorcinol-Formaldehyde Aerogels and Crosslinking with Isocyanates

Aerogels are open-cell foams derived from supercritical fluid (SCF) drying of wet gels. Their large internal void space is responsible for low thermal conductivity, high surface area and high acoustic impedance. Most aerogels are based on inorganic metal or semimetal oxide frameworks. Pekala and co-workers synthesized resorcinol-formaldehyde (RF) organic aerogels by poly-condensation of resorcinol with formaldehyde in the presence of Na2CO3 as base catalyst, followed by drying with SCF CO2. Low-density RF aerogels prepared by this method exhibit high porosities (\u3e80%), high surface areas (400-900 m2g-1), ultrafine cell-size (\u3c500 \u3e Å) and densities as low as 0.03 g cm-3. The major drawback though, has been the length of the preparation procedure that typically spans several days. Looking at the mechanism of the process, the RF gel formation has been associated with two major reactions: (1) formation of hydroxymethyl derivatives of resorcinol; and, (2) condensation of those derivatives to methylene (-CH2-) and methylene ether (-CH2-O-CH2-) bridges. The effect of the resorcinol to catalyst (R/C) ratio on the final aerogel structure has been studied extensively. That ratio was typically varied in the range between 50- 300. Formation of particles connected with large necks was reported for low and for very high (~1500) R/C ratios. The final pore structure and the gelation time depend strongly on the sol pH; at low pHs, precipitation rather than gelation was reported. The extensive literature on base-catalyzed RF aerogels has obscured attempts towards acid-catalyzed processing. Recently, Brandt and Fricke reported an aqueous acetic acid catalyzed route for RF gel synthesis, where they still allowed a two-day period for gelation and aging. Reasoning that not only hydroxy methylation of resorcinol, but also subsequent condensation to methylene and methylene ether bridges should be all acid-catalyzed processes, we undertook a systematic look at the reaction of resorcinol with formaldehyde in CH3CN, developing a time-efficient method that yields within a few hours (as opposed to weeks) gels indistinguishable from those reported in the literature. The -OH groups of resorcinol in the resulting gels are reactive with di- and tri-isocyanate crosslinkers in analogy to silica, leading to isocyanate-derived polymer crosslinked RF aerogels, which are more robust, and more resistive to shrinkage than their native (noncrosslinked) counterparts

Crosslinking 3D Assemblies of Silica Nanoparticles (Aerogels) by Surface-Initiated Free Radical Polymerization of Styrene and Methylmethacrylate

Quasi-stable, ultra-low density, three-dimensional assemblies of nanoparticles are referred to as aerogels. Aerogels are open-cell foams derived from supercritical fluid (SCF) drying of wet gels. Their large internal void space is responsible for low dielectric constants, low thermal conductivities and high acoustic impedance. At the same time though those materials are fragile and impractical for high load applications. The fragility problem has been addressed by casting a thin conformal polymer coating over the entire internal porous surface of the nanostructure.1 That process is referred to as crosslinking. The coating connects chemically skeletal nanoparticles and renders interparticle necks wider. Thus, the internal void space is not compromised significantly, while the flexural strength of a typical monolith is increased by 300Ã for a nominal increase in density only by a factor of three. A major issue, however, is the fact that current preparation procedures for crosslinked aerogels involve several solvent exchange steps, which are expensive and must be eliminated before crosslinked aerogels become commercially viable. In order to eliminate solvent exchange steps, the reagents for the crosslinking process should be included in the sol, which in turn means that the crosslinking chemistry should be deconvoluted from the gelation chemistry (an ionic process). In this context, there are several chemistries involving nanoparticle surface modification for making core-shell structures. These methods span the entire range from layer-by-layer electrostatic assembly of oppositely charged materials,2-6 to atom transfer radical polymerization,7 and direct free radical polymerization of olefins from surface-bound initiators such as peroxides,8 and AIBN.9,10 by comparison, free-radical initiators have received the least attention as silica surface modifiers, while all literature examples concern asymmetric peroxide and AIBN derivatives, which are attached on silica only from one side. Such monodentate free-radical initiators would not work for our purposes, because upon homolytic cleavage they would produce one surface-bound radical, which is the desirable outcome, but they would also release a second radical in the mesopores. The polymer formed in the solution filling the mesopores will have to be removed, and that introduces more solvent exchange steps. For our purposes, we need bidentate free-radical initiators that will attach themselves on silica from both sides. In that regard, we report here the synthesis of AIBN analogue as well as its incorporation into silica, and the surface initiated polymerization of styrene and methylmethacrylate to yield conformal polymer coatings on the mesoporous surfaces of typical base-catalyzed aerogels

Novel Porous Polymer Compositions for the Synthesis of Monolithic Bimodal Microporous/Macroporous Carbon Compositions Useful for Selective CO₂ Sequestration

The present invention discloses novel porous polymeric compositions comprising random copolymers of amides, imides, ureas, and carbamic-anhydrides, useful for the synthesis of monolithic bimodal microporous/macroporous carbon aerogels. It also discloses methods for producing said microporous/macroporous carbon aerogels by the reaction of a polyisocyanate compound and a polycarboxylic acid compound, followed by pyrolytic carbonization, and by reactive etching with CO2 at elevated temperatures. Also disclosed are methods for using the microporous/macroporous carbon aerogels in the selective capture and sequestration of carbon dioxide

Synthesis and Electropolymerization of 2-(3-Thienylethyl)-3-thiopheneacetate

Conducting polymers offer great promise in the development of electronic devices, displays and sensors. In that regard, biomolecules can be attached to these polymers via functional groups, but the presence of such groups oftentimes hinders polymerization.1-3 One strategy is to polymerize a functionalized monomer precursor of a conducting polymer, followed by reaction of the dangling functional groups of the polymer with appropriately functionalized labels. An attractive functionalized thiophene for that purpose is 3-thienylethanol. Unfortunately, however, this monomer is not polymerizable.1,2 Thus, 3-thienylethanol has been esterified, followed by co-polymerization with 3-methylthiophene (3-MT), and subsequent hydrolysis of the ester to the free alcohol.1,2 Here, we present a viable alternative, whereas we have synthesized monomer 1 from commercially available 3-thiopheneacetic acid and 2-(3-thienylethanol). Monomer 1 was electropolymerized both by itself and as a copolymer with 3- methylthiophene (3-MT). The ester linkage was reduced with LiAlH4, and the resulting hydroxyl groups were re-esterified by reaction with acetyl chloride. The functional group transformations were followed by FT-IR

Protection of 2-(3-Thienyl)ethanol with 3-Thienylacetic Acid and Hard Cross- Linked Conducting Films by Electropolymerization of the Ester

The ester (compound 1) of 2-(3-thienyl)ethanol (T-etOH) with 3-thienylacetic acid was synthesized as a monomer whose two thiophene groups could be electropolymerized independently, becoming members of different polymer chains in a highly-crosslinked highly-insoluble polymer. Indeed, 1 was electropolymerized successfully alone and together with 3-methylthiophene (3MeT). Films of poly(1) are hard (3H, as opposed to less than 6B for poly(3MeT)), and the close proximity of the polymeric strands creates pi-stacking interactions. The behavior of 1 suggests that by: (a) limiting the potential used for the oxidation of monomeric esters of T-etOH at the foot of their oxidation waves (less than 1.8 V vs. Ag/AgCl); and, (b) compensating for the decrease in the electrogenerated radical concentration by increasing the monomer concentration, practically all esters of T-etOH should be electropolymerizable. This was confirmed by durable film formation from the archetypical ester of T-etOH, the 2-(3-thienyl)ethyl acetate (T-etOAc), whose homo-electropolymerization is reported for the first time

Conformal Internal Coating of Macroporous Co-Continuous MCF-silicas with Isocyanate Derived Polymers

Since their discovery in 1992,1,2 templating sol-gel silica with structuredirecting agents has resulted in new materials with ordered mesoporous (2-50 nm) structure that is investigated for application in areas as diverse as high surface area supports for caralysts or as nanostructured drug storage and release platforms.3 Structure-directing agents are typically surfactants ranging from cationic detergents such as alkyl-trimethylammonium salts with C8-C18 hydrocarbon chains to non-ionic tertiary amines, polyethylene oxide or amphiphilic triblock copolymers such as poly(ethyleneoxide)-blockpoly( propyleneoxide)-block-poly(ethyleneoxide) (e.g., Pluronic P123). Depending on the surfactant and the conditions (e.g., kind and concentration of catalyst) the structure of the mesopores may vary (e.g., from cubic to hexagonal etc.) the surfactant forms templating micells, which can be enlarged by adding organic swelling agents as for example mesitylene to P123. The resulting materials are referred to as Mesoporous Cellular Foams (MCFs) and they contain a co-continuous three-dimensional macroporous (\u3e50 nm) pore system consisting of interconnected spherical compartments with mesoporous walls.4 Owing to a combination of a low resistance to hydraulic flow with short diffusion path lengths within the mesoporous skeletal walls surrounding the macropores, such systems attract attention as chromatographic stationary phases,5 and in fact certain versions of this technology are already incorporated in monolithic separation columns marketed by Merck Co. under the trade name ChromolithTM.6 Typical preparation conditions for those materials involve phase separation induced by the templating agent, followed by gelation of an alkoxysilane. Ambient pressure drying removes the gelation solvent and a final high-temperature calcination removes the templating agent. These conditions induce shrinkage that not only makes reproducible in-place-of-use preparation of the monolith problematic but also causes chemical alternation of the silica surface, densification of the skeletal framework and partial collapse of the macropores. Here we explore a different approach for minimizing shrinkage, reduce cracking, increase mechanical strength and reproducibility of templated silica type of materials. For this we employ a method we developed recently by which we use the native -OH group surface functionality of silica in order to direct the polymerization of a di-isocyanate.7 Bi-continuous macro- /mesoporous monolithic silicas are prepared by Nakanishi\u27s modification of Stucky\u27s method,8 in which P123 (molecular weight~5,800) is used as templating agent while 1,3,5-trimethylbenzene (TMB) is employed as an expanding agent.3 the templating agent was washed off by Soxhlet extraction and the resulting wet gels were exposed to a solution of a di-isocyanate in acetone; unreacted di-isocyanate was washed off and the samples were dried with CO2 taken out supercritically. Comparative characterization was conducted for isocyanate-treated and non-treated samples. Isocyanate-treated monoliths undergo minimal shrinkage, they are much more robust than native samples while they maintain the surface area of the untreated monoliths

Compressive Behavior of Crosslinked Mesoporous Silica Aerogels at High Strain Rates

Aerogels are low-density mesoporous materials with exceptionally low dielectric constants, low thermal conductivities (up to 40 times better thermal insulators than the best fiberglass) and high acoustic impedance. Their practical applications, however, have been slow due to their hydrophilicity and brittleness. The fragility problem was resolved by nanocasting a ~2-4 nm thick polymer layer on the skeletal silica nanoparticles that strengthens the weak inter-nanopartical necks. The thin polymer coats conformally the skeletal framework without clogging the mesopores and reinforces the structure by chemically crosslinking the nanoparticles. With a density increase by only three times, crosslinking aerogels have the flexural strength increased by 300 times. The method has been applied for crosslinking aerogels consisting of oxides of more than 30 different elements from the periodic table. The mechanical properties of crosslinked silica aerogels with different polymers were improved to a similar level since the polymer bond energy was similar among polymers.Therefore, to improve the mechanical properties further, we turn to the network morphology. For this study we turned to certain micelle-templated aerogels known to have a worm-like microstructure with a macro/mesoporous skeletal framework. For comparison, macroporous monolithic silica aerogels consisting of both random and ordered mesoporous walls have been synthesized via an acid-catalyzed sol-gel process from tetramethoxysilane (TMOS) using a tri-block copolymer (Pluronic P123) as a structure-directing agent and 1,3,5-trimethylbenzene (TMB) as a micelle-swelling reagent. Although those monoliths are more robust than base-catalyzed silica aerogels of similar density, the mechanical properties can be improved dramatically by letting di-isocyanate react with the silanols on the mesoporous surfaces. The compressive behavior of both crosslinked templated silica aerogels with/without ordered mesostructure and non-templated silica aerogels was characterized under high strain rates using a long split Hopkinson pressure bar (SHPB). Their mechanical properties at different strain rates are compared with those of engineering plastics polymethyl methacrylate (PMMA) and polycarbonate (PC). The effect of water absorption and of low temperatures on the compressive behavior was also investigated

Novel Highly Porous Ceramic and Metal Aerogels from Xerogel Powder Precursors, and Methods for their Production and Use

The present invention discloses novel methods for producing highly porous ceramic and/or metal aerogel monolithic objects that are hard, sturdy, and resistant to high temperatures. These methods comprise preparing nanoparticulate oxides of metals and/or metalloids via a step of vigorous stirring to prevent gelation, preparing polymer-modified xerogel powder compositions by reacting said nanoparticulate oxides with one or more polyfunctional monomers, compressing said polymer-modified xerogel powder compositions into shaped compacts, and carbothermal conversion of the shaped xerogel compacts via pyrolysis to provide the highly porous ceramic and/or metal aerogel monolithic objects that have the same shapes as to their corresponding xerogel compact precursors. Representative of the highly porous ceramic and/or metal aerogel monolithic objects of the invention are ceramic and/or metal aerogels of Si, Zr, Hf, Ti, Cr, Fe, Co, Ni, Cu, Ru, Au, and the like. Examples include sturdy, shaped, highly porous silicon carbide (SiC), silicon nitride (Si3 N4), zirconium carbide (ZrC), hafnium carbide (HfC), chromium carbide (Cr3 C2 ), titanium carbide (TiC), zirconium boride (ZrB2 ), hafnium boride (HfB2 ), and metallic aerogels of iron (Fe), nickel (Ni), cobalt (Co), copper (Cu), ruthenium (Ru), gold (Au), and the like. Said aerogel monolithic objects have utility in various applications such as, illustratively, in abrasives, in cutting tools, as catalyst support materials such as in reformers and converters, as filters such as for molten metals and hot gasses, in bio-medical tissue engineering such as bone replacement materials, in applications requiring strong lightweight materials such as in automotive and aircraft structural components, in ultra-high temperature ceramics, and the like