Toward Improved Optical Transparency of Polyimide Aerogels

Highly translucent polyimide aerogels were prepared by combining equimolar amounts of pyromellitic dianhydride, 4,4'-hexafluoroisopropylidene di(phthalic anhydride) (6FDA), and 2,2'-dimethylbenzidine and cross-linking with 1,3,5-benzenetricarbonyl trichloride. A multivariable statistical design of experiments was used to perform a comparison study between three variables used to fabricate the aerogels: formulated repeat unit (n) of polyimide oligomers, 6FDA fraction of total dianhydride (0-50 mol %), and total polymer concentration in solution (7-10 wt %). Polymers with 25 mol % 6FDA in the backbone structure were found to produce polyimide aerogels with high optical transmission and low haze. These aerogels also possessed higher surface areas and very narrow nanoscale pore size distribution. Because of the decreased thermal conductivity with increasing amount of 6FDA in the backbone, these aerogels may find use where the combination of high optical transparency and thermal impedance is desired, such as insulated window panes. To this end, future efforts will focus on reducing the yellow color of the polyimide aerogels.