While the Curiosity rover wheels around Mars examining the chemical composition of the soil there, materials scientists here on Earth are already preparing for more advanced missions to the Red Planet. According to National Aeronautics & Space Administration scientist Mary Ann B. Meador, before sending people or larger vehicles to Mars, scientists must develop insulating materials to counter the planet’s exotic environment.
Unlike the moon, Mars has a low-pressure atmosphere, said Meador when speaking last month at the American Chemical Society national meeting in Philadelphia. For this reason, space suits worn by astronauts for the past 40 years, with their simple, heat-reflecting metallized layers of insulation, just won’t cut it on the Red Planet: The layers would collapse on each other and permit heat transfer.
To insulate the space suits, NASA is eyeing aerogels, high-surface-area porous solids prized for their light weight and low heat conduction. The materials could also find use in the parachute-like decelerators NASA is developing to prevent large payloads from disintegrating while descending to Mars’s surface.
But aerogels now on the market aren’t up to the task, Meador noted. Those products, made by removing the solvent from a silica gel, are good insulators, Meador said, “but they will flake apart and produce a lot of dust,” reducing their performance. Plus, they’re not sufficiently flexible for use in decelerators; the materials must fold and store inside small hatches before they deploy.
So Meador of Ohio’s NASA Glenn Research Center, Haiquan (Heidi) Guo of Ohio Aerospace Institute, and colleagues have developed all-polymer aerogels as replacements for the silica-based ones. Not only can the new aerogels be fabricated as thin flexible films, but they are also 500 times as strong as their silica counterparts, Meador said in a session sponsored by the Division of Polymer Chemistry. A thick piece, she added, can even support the weight of a car without collapsing.
The researchers make their aerogels out of polyimides, which are high-temperature polymers typically found in aircraft engine parts. To fabricate the new aerogels, Meador and the group mix together diamines and dianhydrides to form long polyimide strands. Then they add one of two multiamine compounds—octa(aminophenyl)silsesquioxane (OAPS) or 1,3,5-triaminophenoxybenzene—that cross-link the strands to form a network structure. Finally, the team removes the solvent from the polymer gel with supercritical CO2 extraction.
Although their exact properties vary depending on the building blocks used, the resulting polymer aerogels are generally yellowish, opaque, and about 90% porous. They also stand up to temperatures of 400 °C without breaking down. When buried beneath a heat-resistant ceramic fabric in a decelerator, layers of this aerogel should survive entry into Mars’s atmosphere, the researchers believe.
One of the most promising aerogels made by the team so far, Meador said, is composed of OAPS, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and a 50-50 mole % mixture of 2,2'-dimethylbenzidine and 4,4'-oxydianiline. Using the half-and-half combo of amines, she explained, allows the team to make an aerogel that has good moisture resistance and is still flexible (ACS Appl. Mater. Interfaces, DOI: 10.1021/am301347a).
The researchers will soon join with aerospace firm Boeing to test their optimized material at the high heat fluxes it would experience when descending through Mars’s atmosphere, Meador said.
But polymer aerogels aren’t limited to applications far, far away. The materials could be used on Earth too. “They don’t create dust, and they’re flexible,” Meador said, so polymer aerogels might find use as wraparound insulation for oil pipelines or as liners for sleeping bags.
“These aren’t going to compete with insulation that you have in your attic, though,” Meador said. In an attic, there’s room for a thick layer of less costly fiberglass insulation. Instead, polymer aerogels might make a difference in applications where space is at a premium, she explained. For instance, in the future the refrigerator insulation of today might be replaced with ultrathin polymer aerogels to increase storage space for food.
“The aerogel research at NASA has evolved from silica-based materials, which are highly interesting but difficult to produce,” said David Schiraldi, a polymer scientist at Case Western Reserve University. Fabrication of the new polymer aerogels, he added, produces much less chemical waste than routes to the old versions. “Over the past couple of years, the NASA program has really moved the ball forward with their materials.”