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ABSTRACT: Polyimide gels are produced by cross-linking anhydride capped polyamic acid oligomers with aromatic triamine in solution and chemically imidizing. The gels are then supercritically dried to form nanoporous polyimide aerogels with densities as low as 0.14 g/cm(3) and surface areas as high as 512 m(2)/g. To understand the effect of the polyimide backbone on properties, aerogels from several combinations of diamine and dianhydride, and formulated oligomer chain length are examined. Formulations made from 2,2'-dimethylbenzidine as the diamine shrink the least but have among the highest compressive modulus. Formulations made using 4,4'-oxydianiline or 2,2'dimethylbenzidine can be fabricated into continuous thin films using a roll to roll casting process. The films are flexible enough to be rolled or folded back on themselves and recover completely without cracking or flaking, and have tensile strengths of 4-9 MPa. Finally, the highest onset of decomposition (above 600 °C) of the polyimide aerogels was obtained using p-phenylene diamine as the backbone diamine with either dianhydride studied. All of the aerogels are suitable candidates for high-temperature insulation with glass transition temperatures ranging from 270-340 °C and onsets of decomposition from 460-610 °C.
ACS Applied Materials & Interfaces 02/2012; 4(2):536-44. · 4.53 Impact Factor
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ABSTRACT: Reinforcing silica aerogels by conformally coating the nanoskeleton with a polymer has been demonstrated to be an effective way to improve mechanical properties. In addition, the mesoporosity and low thermal conductivity is maintained, making this robust form of aerogel an enabling material for a variety of aerospace applications. However, the process for making the aerogels can be quite long, involving production of the gel, solvent exchanges, and diffusion of monomer, followed by more solvent exchanges and supercritical fluid extraction. This paper for the first time compares a synthetic scheme that shortens the process to make epoxy reinforced aerogels by eliminating monomer diffusion and half of the solvent washes to the previously described diffusion-controlled process. The ethanol-soluble epoxy monomers are included in the initial step of the sol-gel process without interfering with gelation of the starting silanes. Notably, properties of aerogels made using a low amount of amine reactive sites have properties similar to those previously reported that used the longer diffusion controlled process, whereas higher amounts of amine sites produce less desirable monoliths with much higher density and lower surface areas.
ACS Applied Materials & Interfaces 06/2010; · 4.53 Impact Factor
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ABSTRACT: Cross-linking silica aerogels with organic groups has been shown to improve the strength over un-cross-linked aerogels by as much as 2 orders of magnitude. Previous cross-linking chemistry has been developed using solvents specifically chosen to dissolve the monomers and accommodate the reaction temperature. Because the process of making the aerogels requires so much solvent, it is of interest to consider less toxic solvents such as ethanol to increase safety and enhance scale up. To this end, two different epoxy precursors with suitable solubility in ethanol were evaluated as cross-linkers for silica gels prepared from (3-aminopropyl)triethoxysilane and tetraethylorthosilicate. In addition, 1,6-bis(trimethoxysilyl)hexane (BTMSH) was used as an additive in the underlying silica structure to add flexibility to the aerogels. It was found that the ethanol-derived aerogels exhibited more shrinkage than those prepared from other solvents but that including BTMSH in the aerogels significantly reduced this shrinkage. Inclusion of BTMSH also imparted the ability of the aerogel monoliths to recover elastically when compressed up to 50% strain. In addition, optimized cross-linked aerogels prepared in this study have mechanical properties comparable to those using other more undesirable solvents and cross-linkers.
ACS Applied Materials & Interfaces 04/2009; 1(4):894-906. · 4.53 Impact Factor
