Multifunctional Polyurea Aerogels from Isocyanates and Water. A Structure−Property Case Study

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 (Et3N) 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 (13C NMR, IR, XRD), the elementary nanoparticle level (SANS/USANS), and the microscopic level (SEM). Their porous structure has been probed with N2-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.