RO membranes are a polyamide film no more than 200 nanometers thick backed by a thicker, porous support layer. Water holding dissolved salts or other contaminants is forced against one side of the membrane at substantial pressures up to about a thousand psi (roughly 7 megapascal), and comes out the other side leaving most of the impurities behind. At the moment, engineers lack a good way to measure the strength and breaking point, under stress, of these extremely thin films, the researchers suggest. The NIST technique involves measuring stiffness or elasticity by bonding film to a piece of silicon rubber, and stretching it in one direction until the film starts developing minute cracks crosswise to the tension. These occur in regular patterns, and the spacing can be analyzed to determine both the fracture strength and the onset fracture strain, or the failure point, of the film. When the technique was used to study the effect of chlorine on reverse-osmosis membranes, the team discovered that rather than causing a progressive deterioration in membrane performance, chemical damage from the chlorine happened in the first few hours. Tests using the wrinkle-crack method, however, showed that the mechanical properties degrade continuously—the material becoming more and more stiff, brittle and weak—up to the longest duration tested, 10 days. "It may be an aging effect in polymers," says NIST researcher Chris Stafford. "We're continuing to study that to figure out what's going on in there, because it's a real measurement challenge to get in on that length scale to follow the structure over time." The project is part of a broader NIST program to study materials issues related to sustainable technologies such as water purification.
