Meador Mary Ann B, McMillon Emily, Sandberg Anna, Barrios Elizabeth, Wilmoth Nathan G, Mueller Carl H, Miranda Félix A
NASA Glenn Research Center , 21000 Brookpark Road, Cleveland, Ohio 44135, United States.
ACS Appl Mater Interfaces. 2014 May 14;6(9):6062-8. doi: 10.1021/am405106h. Epub 2014 Jan 31.
The dielectric and other properties of a series of low-density polyimide block copolymer aerogels have been characterized. Two different anhydride-capped polyimide oligomers were synthesized: one from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 4,4'-oxidianiline (ODA) and the other from biphenyl-3,3',4,4'-tetracarboxylic dianhydride and ODA. The oligomers were combined with 1,3,5-triaminophenoxybenzene to form a block copolymer networked structure that gelled in under 1 h. The polyimide gels were supercritically dried to give aerogels with relative dielectric constants as low as 1.08. Increasing the amount of 6FDA blocks by up to 50% of the total dianhydride decreased the density of the aerogels, presumably by increasing the free volume and also by decreasing the amount of shrinkage seen upon processing, resulting in a concomitant decrease in the dielectric properties. In this study, we have also altered the density independent of fluorine substitution by changing the polymer concentration in the gelation reactions and showed that the change in dielectric due to density is the same with and without fluorine substitution. The aerogels with the lowest dielectric properties and lowest densities still had compressive moduli of 4-8 MPa (40 times higher than silica aerogels at the same density), making them suitable as low dielectric substrates for lightweight antennas for aeronautic and space applications.
一系列低密度聚酰亚胺嵌段共聚物气凝胶的介电性能和其他性能已得到表征。合成了两种不同的酸酐封端的聚酰亚胺低聚物：一种由2,2-双(3,4-二羧基苯基)六氟丙烷二酐(6FDA)和4,4'-氧二苯胺(ODA)合成，另一种由联苯-3,3',4,4'-四羧酸二酐和ODA合成。这些低聚物与1,3,5-三氨基苯氧基苯结合形成一种嵌段共聚物网络结构，该结构在1小时内凝胶化。聚酰亚胺凝胶经过超临界干燥得到相对介电常数低至1.08的气凝胶。将6FDA嵌段的量增加至总二酐量的50%，降低了气凝胶的密度，推测这是通过增加自由体积以及减少加工过程中的收缩量实现的，从而导致介电性能随之降低。在本研究中，我们还通过改变凝胶化反应中的聚合物浓度来改变与氟取代无关的密度，并表明有无氟取代时，因密度变化引起的介电变化是相同的。具有最低介电性能和最低密度的气凝胶的压缩模量仍为4 - 8兆帕(在相同密度下比二氧化硅气凝胶高40倍)，这使其适合作为航空和航天应用中轻型天线的低介电基板。
