In this age of looming bioterrorism, keeping chemical warfare vapors off of soldiers is a primary military concern.
A group of Colorado scientists may have just come up with a solution that can keep troops safe while giving them the comfort of breathability.
In the past, military personnel had two choices: they could wear a breathing apparatus along with a full-body suit of cross-linked butyl rubber, or a garment fortified with activated carbon. Although the latter could adsorb any harmful aerosols floating their way, it was heavy--imagine wearing an outfit with charcoal stuffed in every pocket--and the carbon needed to be periodically replaced. As for the garment made of rubber, the same material used in hockey pucks and most waterproof clothing, it kept most agents out, but it also locked the soldier's sweat in--leading to heatstroke or even death, in some extreme cases.
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Butyl rubber, a linear polymer, "is a really good barrier," says University of Colorado at Boulder chemist Douglas L. Gin, "but it's really good both ways."
Gin, along with Brian Elliott, a researcher at TDA Research in Wheat Ridge, Colo., led a team that sought to tweak butyl rubber for greater water transport through the material. In order to get butyl rubber into a state useful for a suit, the linear polymer must be cross-linked to make a three-dimensional network, which, according to Gin, "makes it more chemically, mechanically and thermally robust."
The researchers blended butyl rubber with liquid crystals that could be organized to form hydrophilic, 1.2-nanometer-wide pores. These holes would allow water to pass through, but they would block chemical agents that were either too big to fit or were hydrophobic and therefore repelled by the pores. Unfortunately, when testing the new material with water vapor and 2-chloroethyl ethyl sulfide (CEES or "half mustard" gas), Gin and Elliott discovered that water vapor moved across the membrane, but not at a rate that would be acceptable for military use.
So the team went back to the drawing board, using a new liquid crystal that created, rather than pores, a "bicontinuous cubic" made up of interconnected, three-dimensional sheets.
"The first version was just pipes," explains Gin. "It's really easy to block a pipe--all you have to do is plug it somewhere in the middle and transport is completely stopped." With the second version, "it now is more like intercrisscrossing layers of water in three dimensions. So you get easy transport no matter which way water wants to go in or out."
Sure enough, when the scientists tested the water vapor transport rate for a four-micrometer-thick film of their new material, it performed at four times the minimum acceptable rate by the military and 300 times that of plain, cross-linked butyl rubber. It is also about 30 percent less permeable to CEES, making it more effective for protection against a chemical agent.
Gin and Elliott are now working to make their new, improved butyl rubber as effective at thinner swaths. They also believe they may be able to use the material not only in suits, but as filters for solutions from brine to contaminated water.
"The crux of our whole technology is we can take conventional commercial butyl rubber and, by blending it with our polymerizable liquid crystal, we can actually make these water conduits in there," Gin remarks. "And depending on the type of liquid crystals, we can control the dimensionality from one-dimensional cylinders or tubes to the much more effective interconnected 3-D water manifold system."
