Synthesis and Properties of Step-Growth Polyamide Aerogels Cross-linked with Triacid Chlorides

We report the first synthesis of step-growth aromatic polyamide (PA) aerogels made using amine end-capped polyamide oligomers cross-linked with 1,3,5-benzenetricarbonyl trichloride (BTC). Isophthaloyl chloride (IPC) or terephthaloyl chloride (TPC) were combined with <i>m</i>-phenylenediamine (mPDA) in <i>N</i>-methylpyrrolidinone (NMP) to give amine-capped polyamide oligomers formulated with up to 40 repeat units. Addition of the cross-linker, BTC, typically induces gelation in under 5 min. Solvent exchange of the resulting gels into ethanol followed by supercritical CO₂ drying gives colorless aerogels with densities ranging from 0.06 to 0.33 g/cm³, compressive moduli between 5 and 312 MPa, and surface areas as high as 385 m²/g. Dielectric properties were also measured in the X-band frequency range. It was found that relative dielectric constant decreased with density as seen with other aerogels with the lowest relative dielectric constant being 1.15 for aerogels with densities of 0.06 g/cm³. Because of their superior mechanical properties, these aerogels can be utilized in a number of aerospace related applications, such as insulation for rovers, habitats, deployable structures, and extravehicular activity suits, as well as low dielectric substrates for antennas and other electronics. Because of potentially lower cost relative to polyimide and other polymer aerogels, they also have potential for use in more terrestrial applications as well, such as insulation for refrigeration, building and construction, and protective clothing

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, <i>R</i>_t(<i>N</i>), and the elastic modulus, <i>E</i>, 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, <i>R</i>_t(<i>N</i>), which in turn were related to the microstructure. Materials with bicontinuous microstructures were stiffer and showed slower recovery rates. Thereby, using the elastic modulus, <i>E</i>, as a proxy for microstructure, the correlation of <i>R</i>_t(<i>N</i>) with <i>E</i> was traced to a relationship between the activation barrier for shape recovery, Δ<i>A</i>^#, and the specific energy of deformation, (reorganization energy, λ), which in turn is proportional to the elastic modulus. Data were fitted well (<i>R</i>² = 0.92) by the derived equations. The inverse correlation between <i>R</i>_t(<i>N</i>) and the elastic modulus, <i>E</i>, provides a means for qualitative predictability of the shape recovery rates, the fill factors, and the overall quality of the shape memory effect

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, <i>R</i>_t(<i>N</i>), and the elastic modulus, <i>E</i>, 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, <i>R</i>_t(<i>N</i>), which in turn were related to the microstructure. Materials with bicontinuous microstructures were stiffer and showed slower recovery rates. Thereby, using the elastic modulus, <i>E</i>, as a proxy for microstructure, the correlation of <i>R</i>_t(<i>N</i>) with <i>E</i> was traced to a relationship between the activation barrier for shape recovery, Δ<i>A</i>^#, and the specific energy of deformation, (reorganization energy, λ), which in turn is proportional to the elastic modulus. Data were fitted well (<i>R</i>² = 0.92) by the derived equations. The inverse correlation between <i>R</i>_t(<i>N</i>) and the elastic modulus, <i>E</i>, provides a means for qualitative predictability of the shape recovery rates, the fill factors, and the overall quality of the shape memory effect

Thermoresponsive Shape-Memory Aerogels from Thiol–Ene Networks

Thermoresponsive shape-memory polymer aerogels have been produced from thiol–ene networks of 1,6-hexanedithiol, pentaerythritol tetrakis­(3-mercaptopropionate), and triallyl-1,3,5-triazine-2,4,6-trione. The thiol–ene networks form organogels with either acetonitrile or acetone as the solvent, which can be subsequently removed using supercritical CO₂ extraction. The resulting aerogels have nearly quantitative shape fixing and shape recovery with a glass transition temperature ranging from 42 to 64 °C, which serves as the thermal transition trigger for the shape-memory effect. The aerogels have a porosity of 72% to 81% but surface areas of only 5–10 m²/g