Key RO membrane properties measured with new test

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July 21, 2011 -- A new measurement technique for determining important mechanical properties of near-nanoscale films could enable better design and engineering of reverse osmosis membranes for water purification.

Traditionally, engineers have lacked a good way to measure the strength and breaking point, under stress, of the extremely thin films used in RO membranes. Researchers at the National Institute of Standards and Technology (NIST) have developed a technique based on the concept that Young's modulus -- a measure of stiffness or elasticity -- for thin and ultrathin films can be reliably determined by bonding it to a piece of silicon rubber, and then carefully stretching it in one direction.

This will cause a regularly spaced pattern of wrinkles to develop in the film. The modulus can be determined by the spacing of the wrinkles, the amount of stretch and a mathematical formula.

In their latest work, the researchers pulled the silicon rubber harder until the film developed minute cracks crosswise to the tension. They found that these cracks also 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.

They applied the technique to study the effect of chlorine on reverse osmosis membranes. Chlorine is known to cause a progressive deterioration in membrane performance, generally thought to be the result of prolonged chemical attack by the chlorine. The NIST team, however, found that chemical damage from chlorine exposure happens within the first few hours. Testing with the "wrinkle-crack method" showed that the mechanical properties degrade continuously up to the longest duration tested (10 days).

"It may be an aging effect in polymers," said 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 like water purification, but the research team notes that the wrinkle-crack method itself would be broadly applicable to mechanical studies of almost any nanoscale thin film in fields as diverse as artificial skin, flexible electronics, thin-film sensors, fuel cells and photovoltaics.

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