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Looking at Polymers in Three Different Lights

Photonics Spectra
Oct 2007
Multiple optical techniques enable in situ monitoring of materials under stress.

Hank Hogan

The industrial use of polymers is growing, but still required are models that account for physical phenomena such as temperature dependence, thermomechanical coupling and aging. Such models ideally would allow for the reliable predictions of the mechanical behavior of polymers.

Now a group from Nancy University in France has used three complementary optical techniques — two in the visible and one in the infrared — to extract the information to build useful models. Stéphane André, an assistant professor of mechanical and thermal sciences engineering, explained, “These are all based on optical phenomena in different wavelengths. This indicates how contemporary science is really affected by the progress made in radiation detectors, cameras and imaging software.”

TWPoly_Fig1_Andre.gif

Researchers used several imaging techniques to aid the characterization of polymers under stress. In one, they performed video-based extensometry by analyzing the amount of deformation in marks placed on a sample (a, b), while simultaneously recording an infrared image from the opposite side of the sample (c). Reprinted with permission of Applied Physics Letters.


The first of the three techniques, videoextensometry, is not new but has not been applied in combination with other optical techniques for in situ characterization of materials. As the name implies, this method measures the extension of a polymer in response to stress, providing a live determination of the true strain experienced by a sample. The scientists used a 575 × 560-pixel CCD camera from i2S SA in Pessac, France, for this approach, which is being commercialized by Apollor Union SA, a startup company in Moncel-lès-Lunéville.

Using a telescopic drive and associated optics, they trained the camera on the sample, capturing the location of seven marks placed on the specimen in a cruciform pattern. The number of marks was arbitrary. They used five along the tensile, or stress, axis and three perpendicular to it, with one marker in the center position, common to both directions. They measured marker locations in real time before, during and after subjecting the sample to mechanical stress or strain and then computed the midpoint of the marks and determined the true strain on the sample from that data by using image analysis.

The second technique involved infrared imaging, which the scientists performed using a 320 × 240-pixel InSb-based device from Cedip Infrared Systems of Croissy Beaubourg, France, that operated in the 3- to 5-μm range. They mounted this sensor on a telescopic drive looking at the opposite side of the sample as it was being imaged for videoextensometry, slaving the second telescopic drive to the first. This coupling ensured that the same point was imaged by both systems, without either getting in the way of the other.

TWPoly_Fig2a_2.jpg
Backscattered light from a laser signal yields information about the microscopic scatterers in a polymer sample (top), providing information about crazes and defects that develop during tensile stress (bottom). Courtesy of Stéphane André, Nancy University.

With infrared imaging, the researchers captured information related to the thermomechanical coupling of the polymer sample. This data does not reflect the intrinsic thermal behavior because the readings depend upon heat exchanges with the sample’s surroundings via conduction, convection and, possibly, radiation. To model these exchanges, the group developed an algorithm that determined the mechanical energy converted into heat, an accomplishment that André said was a first.

“Until now, nobody really solved the problem of restoring the heat power produced by the sample from temperature field measurements.”

The final characterization step involved incoherent light transport, which is related to the microscopic properties of the scatterers within a sample and, thus, provides information about the internal makeup of a polymer. In particular, the method is sensitive to scatterers ranging in size from a few tens of nanometers to several hundred microns, enabling the determination of the appearance of crazes and other microscopic defects.

For this step, André and his colleagues focused the output of a laser diode at 635 nm onto a 50-μm-wide spot on the center of the observed IR image. With a power level of 0.1 mW, the laser did not add any appreciable heat. They detected the incoherent light transport by acquiring a backscattered image from a 2 mm2 spot away from the laser point. For this, they used a CCD camera from Adimec Advanced Image Systems of Eindhoven, the Netherlands.

They applied these three techniques to characterizing high-density polyethylene during tensile stress performed with a hydraulic testing machine from MTS Systems of Eden Prairie, Minn. They attached the sample to the machine and stretched it while using the three optical methods to measure results at the same location on the sample. They found that the heat power was positive and constant during a plastic flowing phase, but spiked during a hardening phase, which also is the time of most stress. It also was when the transport length fell from 2 mm to a final 0.2 mm, a result of craze and cavity development.

The scientists are working to understand the mechanisms behind these effects. They note that, although the characterization methods do not involve complicated equipment, they produce useful information and could be of commercial interest. “We are studying the opportunity of creating our own company,” André said.

Applied Physics Letters, Aug. 13, 2007, Vol. 91, 071919.


GLOSSARY
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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