One Pot Synthesis of Multifunctional Aramid Aerogels
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One Pot Synthesis of Multifunctional Aramid Aerogels Chakkaravarthy Chidambareswarapattar1, Dhairyashil P. Mohite1, Zachary J. Larimore2, Hongbing Lu3, Chariklia Sotiriou-Leventis1,* and Nicholas Leventis1,* 1 Department of Chemistry, Missouri University of Science and Technology, Rolla, MO 65409, U.S.A. ([email protected][email protected]) 2 Department of Mechanical Engineering, Missouri University of Science and Technology, Rolla, MO 65409, U.S.A. 3 Department of Mechanical Engineering, University of Texas at Dallas, Richardson, TX 75080, U.S.A.
ABSTRACT Aerogels are quasi-stable, low-density, three-dimensional assemblies of nanoparticles, but they are commonly associated with poor mechanical properties. The most successful efforts to improve their mechanical properties involve cross-linking of the skeletal nanoparticles with polymers. However, post gelation cross-linking is time-consuming. Hence, it is reasonable to seek robust all-polymer aerogels among polymers known for their high mechanical strength. As a result, here we report the facile one-pot synthesis of a new class of Kevlar-like aerogels based on the rather underutilized reaction of multifunctional isocyanates and carboxylic acids. The resulting materials are up to 84% v/v porous with surface areas as high as 380 m2 g-1. The ultimate compressive strength per unit density is within 10% equal to that of Kevlar 49. The high specific energy absorption (37 J g-1) and Styrofoam-like thermal conductivity (0.028 W m-1 K-1) combined with thermal stability up to 350 oC render aramid aerogels multifunctional materials suitable for defense, civil and transportation related applications. Upon pyrolysis at 800 oC they can be converted to 80% (v/v) porous, electrically conducting carbons with surface areas as high as 474 m2 g-1. INTRODUCTION Aerogels are quasi-stable, low-density, three-dimensional assemblies of nanoparticles which are pursued because of their low thermal conductivity, high acoustic impedance, low dielectric constant and high surface areas [1]. Organic aerogels were invented by Kistler in 1931 and were reported along with their inorganic (oxide) counterparts. Fragility is the main issue with oxide aerogels and Kistler himself reported that the organic variety is more robust [2]. It is therefore reasonable to seek aerogel among polymers known specifically for their mechanical strength. In that regard, aramids are pursued as advanced materials, because of their thermal stability, chain rigidity and inherent chemical inertness of the aromatic moiety [3]. Better known commercial aramids come with familiar names as Kevlar fiber and Nomex honeycomb. They can replace steel and glass in many applications, particularly in the aerospace industry where their relatively low density is an additional advantage [4]. If we combine the properties of aramids with the inherent properties of aerogels, we should realize similar mechanical properties along with thermal and acoustic insulation, which are attractive for aerospace applications. Realizing that classical meth
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