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

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, R t(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, R t(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 R t(N) with E was traced to a relationship between the activation barrier for shape recovery, ΔA #, and the specific energy of deformation, (reorganization energy, λ), which in turn is proportional to the elastic modulus. Data were fitted well (R 2 = 0.92) by the derived equations. The inverse correlation between R t(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.

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