One-Step Polyimide Aerogels from Anhydrides and Isocyanates
After the 1989 synthesis of resorcinol-formaldehyde (RF) aerogels, for a number of years the terms “organic aerogel” and “RF-aerogel” were almost synonymous. This is slowly changing as other classes of organic aerogels show up in the literature, including polyurethane, polyurea and more recently polystyrene, polybenzoxazine, polydicylcopentadiene and polyimide aerogels. The latter are pursued for the good chemical resistance, excellent mechanical properties and high thermal stability of their polyimide skeletal framework. Typically, polyimides are synthesized from an anhydride and a diamine by the classic DuPont two-step process via polyamic acids, which are converted to polyimides chemically with the use of dehydrating agents (e.g., acetic anhydride) along with base catalysts (e.g., pyridine). Kinetically formed isoimides are converted to thermodynamically stable imides by high temperature treatment. The first polyimide aerogels were described in 2006, and they were synthesized by that route. Here we report polyimide aerogels via an alternative one-step room-temperature route that involves reaction of the same di-anhydride with the corresponding di-isocyanate. The final aerogels are chemically indistinguishable (by IR and 13C CPMAS NMR) from those synthesized via the polyamic acid route. However, in terms of properties, the two materials are vastly different. Aerogels synthesized via the isocyanate route are nanofibrous as opposed to nanoparticulate. Although the surface areas are comparable (300-400 m2 g-1), isocyanate-route polyimides shrink less resulting in lower density materials (0.04 g cm-3). Upon pyrolytic conversion to carbon aerogels, isocyanate-derived polyimide materials retain their fibrous nanostructure.
C. Chidambareswarapattar et al., "One-Step Polyimide Aerogels from Anhydrides and Isocyanates," Abstracts of Papers of the American Chemical Society, American Chemical Society (ACS), Aug 2010.
240th ACS National Meeting and Exposition (2010: Aug. 22-26, Boston, MA)
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