Nanomechanical measurements (model system and microimage of typical specimen). a) thin rigid film on elastic substrate b) initial strain induces surface wrinkles parallel to stress c) additional strain induces regular pattern of cracks in the film d) typical specimen imaged with optical profilometer (280 X 210 micrometers.) Image: Chung, Lee/NIST |
Researchers at NIST have demonstrated a measurement
technique that reliably determines three fundamental mechanical properties of
near-nanoscale films. The technique, which highlights the challenge of making
mechanical measurements on an object with at least one dimension comparable to
the size of a virus, should enable better design and engineering for a variety
of thin-film technologies, particularly reverse-osmosis membranes for water
purification.
Reverse-osmosis membranes, explains NIST researcher Chris
Stafford, are an interesting challenge for the materials scientist. The
membranes are used in water purification systems—a polyamide film no more than
200 nm 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, and comes out the other side
leaving most of the impurities behind. The mechanical integrity of the membrane
is obviously essential—it can’t tear or develop pinhole leaks under the
pressure—but engineers lacked a good way to measure the strength and breaking
point, under stress, of these extremely thin films.
The NIST technique builds on earlier work by the team that
demonstrated that you can reliably determine Young’s modulus—a measure of
stiffness or elasticity—for thin and ultrathin films by bonding it to a piece
of silicon rubber, and then carefully stretching it in one direction. The film
will develop a regularly spaced pattern of wrinkles (try it with a piece of
plastic wrap), and the spacing of the wrinkles, the amount of stretch and some
math gives you the modulus. In the new work, they basically pull harder until
the film starts developing minute cracks crosswise to the tension. These too,
it turns out, 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.
Applying their technique to study the effect of chlorine on
reverse-osmosis membranes, the team uncovered a puzzle. Chlorine in the water
is known to cause a progressive deterioration in membrane performance,
generally thought to be the result of prolonged chemical attack by the
chlorine. Not so, according to the NIST team. “Chemically the chlorine
attack is pretty quick,” says Stafford.
Spectroscopic chemical analysis showed that all the chemical damage from
chlorine exposure happens in the first few hours. Tests using the wrinkle-crack
method, however, show 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 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.
