Polyurethane Aerogels Based on Cyclodextrins: High-Capacity Desiccants Regenerated at Room Temperature by Reducing the Relative Humidity of the Environment.

Polyurethane aerogels were prepared from a rigid aromatic triisocyanate (tris(4-isocyanatophenyl)methane) and cage-shaped α- and ß-cyclodextrins as rigid polyols. Gelation was carried out in DMF using dibutyltin dilaurate as catalyst. Wet-gels were dried to aerogels (abbreviated as α- or ß-CDPU-xx) with supercritical fluid CO2. "xx" stands for the percent weight of the two monomers in the sol and was varied at two levels for each cyclodextrin: 2.5% and 15%. All aerogels were characterized with solid-state 13C and 15N NMR, CHN analysis, FTIR, XPS, SEM, and gas (N2 and CO2) sorption porosimetry. α- and ß-CDPU-xx aerogels were investigated as desiccants at room temperature. All materials had relatively higher capacities for water adsorption from high-humidity environments (99%) than typical commercial desiccants like silica or Drierite. However, α-CDPU-2.5 aerogels did stand out with a water uptake capacity reaching 1 g of H2O per gram of material. Most importantly though, adsorbed water could be released quantitatively without heating, by just reducing the relative humidity of the environment to 10%. All α- and ß-CDPU-xx aerogel samples were cycled between humid and dry environments 10 times. Their unusual behavior was traced to filling smaller mesopores with water and was attributed to a delicate balance of enthalpic (H-bonding) and entropic factors, whereas the latter are a function of pore sizes.

K-Index: A Descriptor, Predictor, and Correlator of Complex Nanomorphology to Other Material Properties.

Morphology is a qualitative property of nanostructured matter and is articulated by visual inspection of micrographs. For deterministic procedures that relate nanomorphology to synthetic conditions, it is necessary to express nano- and microstructures numerically. Selecting polyurea aerogels as a model system with demonstrated potential for rich nanomorphology and guided by a statistical design-of-experiments model, we prepared a large array of materials (208) with identical chemical composition but quite different nanostructures. By reflecting on SEM imaging, it was realized that our first preverbal impression about a nanostructure is related to its openness and texture; the former is quantified by porosity ( Π), and the latter is oftentimes related to hydrophobicity, which, in turn, is quantified by the contact angle (θ) of water droplets resting on the material. Herewith, the θ-to-Π ratio is referred to as the K-index, and it was noticed that all polyurea samples of this study could be put in eight K-index groups with separate nanomorphologies ranging from caterpillar-like assemblies of nanoparticles, to thin nanofibers, to cocoon-like structures, to large bald microspheres. A first validation of the K-index as a morphology descriptor was based on compressing samples to different strains: it was observed that as the porosity decreases, the water-contact angle decreases proportionally, and thereby the K-index remains constant. The predictive power of the K-index was demonstrated with 20 polyurea aerogels prepared in 8 binary solvent systems. Subsequently, several material properties were correlated to nanomorphology through the K-index and that, in turn, provided insight about the root cause of the diversity of the nanostructure in polyurea aerogels. Finally, using response surface methodology, K-indexes and other material properties of practical interest were correlated to the monomer, water, and catalyst concentrations as well as the three Hansen solubility parameters of the sol. That enabled the synthesis of materials with up to six prescribed properties at a time, including nanomorphology, bulk density, BET surface area, elastic modulus, ultimate compressive strength, and thermal conductivity.

Nanostructure-Dependent Marcus-Type Correlation of the Shape Recovery Rate and the Young's Modulus in Shape Memory Polymer Aerogels.

Thermodynamic-kinetic relationships are not uncommon, but rigorous correlations are rare. On the basis of the parabolic free-energy profiles of elastic deformation, a generalized Marcus-type thermodynamic-kinetic relationship was identified between the shape recovery rate, Rt( N), and the elastic modulus, E, in poly(isocyanurate-urethane) shape memory aerogels. The latter were prepared with mixtures of diethylene, triethylene, and tetraethylene glycol and an aliphatic triisocyanate. Synthetic conditions were selected using a statistical design of experiments method. Microstructures obtained in each formulation could be put into two groups, one consisting of micron-size particles connected with large necks and a second one classified as bicontinuous. The two types of microstructures could be explained consistently by spinodal decomposition involving early versus late phase separation relative to the gel point. Irrespective of microstructure, all samples showed a shape memory effect with shape fixity and shape recovery ratios close to 100%. Larger variations (0.35-0.71) in the overall figure of merit, the fill factor, were traced to a variability in the shape recovery rates, Rt( N), which in turn were related to the microstructure. Materials with bicontinuous microstructures were stiffer and showed slower recovery rates. Thereby, using the elastic modulus, E, as a proxy for microstructure, the correlation of Rt( N) with E was traced to a relationship between the activation barrier for shape recovery, Δ A#, and the specific energy of deformation, (reorganization energy, λ), which in turn is proportional to the elastic modulus. Data were fitted well ( R2 = 0.92) by the derived equations. The inverse correlation between Rt( N) and the elastic modulus, E, provides a means for qualitative predictability of the shape recovery rates, the fill factors, and the overall quality of the shape memory effect.

Selective CO2 Sequestration with Monolithic Bimodal Micro/Macroporous Carbon Aerogels Derived from Stepwise Pyrolytic Decomposition of Polyamide-Polyimide-Polyurea Random Copolymers.

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.