Isocyanate-Derived Organic Aerogels: Polyureas, Polyimides, Polyamides
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Isocyanate-Derived Organic Aerogels: Polyureas, Polyimides, Polyamides Nicholas Leventis1,*, Chariklia Sotiriou-Leventis1, Naveen Chandrasekaran1, Sudhir Mulik1, Chakkaravarthy Chidambareswarapattar1, Anand Sadekar1, Dhairyashil Mohite1, Shruti S. Mahadik1, Zachary J. Larimore1, Hongbing Lu2, Gitogo Churu2 and Joseph T. Mang3 1
Department of Chemistry, Missouri University of Science and Technology, Rolla, MO 65409, U.S.A. ([email protected]) 2 Department of Mechanical Engineering, University of Texas at Dallas, Richardson, TX 75080, U.S.A. 3 Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A.
ABSTRACT We describe organic aerogels derived from multifunctional isocyanates through reaction with water (polyureas), acid andydrides (polyimides) and carboxylic acids (polyamides). All processes are invariably single-step, one-pot and take place at room or slightly elevated temperatures. The resulting materials are robust, their density may vary over a very wide range and their nanomorphology can be either particulate or fibrous, but in all cases they all consist of similarly sized primary particles. INTRODUCTION Organic (polymeric) aerogels were invented by S. Kistler in the early 1930s and were reported together with their inorganic counterparts (silica, alumina etc.) [1]. However, subsequent development focused mainly on the latter, and polymer aerogels remained dormant till the late 1980s when R. Pekala started publishing on aerogels whose framework consisted of a phenolic-type resin, resorcinol-formaldehyde (RF) [2]. In RF aerogels, Pekala was looking not only for a robust alternative to silica in thermal insulation [3], but also for a precursor to electrically conducting carbon aerogels [4]. In that regard, the RF chemistry was probably the one of choice as in the late 1980s glassy carbon, the pyrolysis product of phenolic resins, was already a commercial success. Pekala’s paradigm created a momentum that resulted in other type of phenolic aerogels (melamine-formaldehyde, resorcinol-propanal etc.) [5], in polyurethane and polyurea aerogels [6], while the area is undergoing an almost explosive growth in the last five years with numerous other types of organic aerogels including polystyrene (2005) [7], polyimides (2006) [8], polybenzoxazine (2009) [9], and polydicyclopentadiene aerogels synthesized via ring opening metathesis polymerization (ROMP-2007) [10]. Typically, the synthesis of organic aerogels has been along the same lines for the synthesis of the corresponding bulk polymers, and therefore not necessarily the most economic, efficient or yielding the best material properties. For instance, even recently, polyurea (PUA) aerogels have been prepared via the classic reaction of multifunctional isocyanates with amines [11]. Similarly, polyimide aerogels have been described through the classic DuPont process that
involves reaction of anhydrides with amines yielding a polyamic acid that is dehydrated with sacrificial agents like acetic anhydride/pyridine (as a catalyst), followed by an energy-intensive high-
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