It is well-known that isocyanates and water yield polyureas; however, that reaction is not generally associated with the synthesis of the latter, being used instead for environmental curing of films baring free NCO groups or for foaming polyurethanes. Here we report that careful control of the relative isocyanate/water/catalyst (Et 3 N) ratio in acetone, acetonitrile, or DMSO prevents precipitation, yielding instead polyurea (PUA) gels convertible to highly porous (up to 98.6% v/v) aerogels over a very wide density range (0.016-0.55 g cm -3 ). The method has been implemented successfully with several aliphatic and aromatic di and triisocyanates. PUA aerogels have been studied at the molecular level ( 13 C NMR, IR, XRD), the elementary nanoparticle level (SANS/USANS), and the microscopic level (SEM). Their porous structure has been probed with N 2 -sorption porosimetry. Despite that the nanomorphology varies with density from fibrous at the low density end to particulate at the high density end, all samples consist of similarly sized primary particles assembled differently, probably via a reaction-limited cluster-cluster aggregation mechanism at the low density end, which changes into diffusion-limited aggregation as the isocyanate concentration increases. Higher density PUA aerogels (>0.3 g cm -3 ) are mechanically strong enough to tolerate the capillary forces of evaporating low surface tension solvents (e.g., pentane) and can be dried under ambient pressure; under compression, they can absorb energy (up to 90 J g -1 at 0.55 g cm -3 ) at levels observed only with polyurea-cross-linked silica and vanadia aerogels (50-190 J g -1 at similar densities). At cryogenic temperatures (-173 °C) PUA aerogels remain relatively ductile, a fact attributed to sintering effects and their entangled fibrous nanomorphology. Upon pyrolysis (>500 °C, Ar), PUA aerogels from aromatic isocyanates are converted to carbon aerogels in high yields (∼60% w/w). Those properties, considered together with the simple synthetic protocol, render PUA aerogels attractive multifunctional materials.

No abstract

Polyimide aerogel monoliths are prepared by ring-opening metathesis polymerization (ROMP) of a norbornene end-capped diimide, bis-NAD, obtained as the condensation product of nadic anhydride with 4,4 0 -methylenedianiline. The density of the material was varied in the range of 0.13À0.66 g cm À3 by varying the concentration of bis-NAD in the sol. Wet gels experience significant shrinkage, relative to their molds (28%À39% in linear dimensions), but the final aerogels retain high porosities (50%À90% v/v), high surface areas (210À632 m 2 g À1 , of which up to 25% is traced to micropores), and pore size distributions in the mesoporous range (20À33 nm). The skeletal framework consists of primary particles 16À17 nm in diameter, assembling to form secondary aggregates (by SANS and SEM) 60À85 nm in diameter. At lower densities (e.g., 0.26 g cm À3 ), secondary particles are mass fractals (D m = 2.34 ( 0.03) turning to closed-packed surface fractal objects (D S = 3.0) as the bulk density increases (g0.34 g cm À3 ), suggesting a change in the network-forming mechanism from diffusion-limited aggregation of primary particles to a space-filling bond percolation model. The new materials combine facile one-step synthesis with heat resistance up to 200 °C, high mechanical compressive strength and specific energy absorption (168 MPa and 50 J g À1 , respectively, at 0.39 g cm À3 and 88% ultimate strain), low speed of sound (351 m s À1 at 0.39 g cm À3 ) and styrofoam-like thermal conductivity (0.031 W m À1 K À1 at 0.34 g cm À3 and 25 °C); hence, they are reasonable multifunctional candidate materials for further exploration as thermal/acoustic insulation at elevated temperatures.

No abstract

No abstract