Nav: Home

Material may offer cheaper alternative to smart windows

January 21, 2016

If you've ever blown up a balloon or pulled at a pair of pantyhose, you may have noticed that the more the material stretches, the more transparent it becomes. It's a simple enough observation: the thinner a material, the more light shines through.

Now MIT scientists have come up with a theory to predict exactly how much light is transmitted through a material, given its thickness and degree of stretch. Using this theory, they accurately predicted the changing transparency of a rubber-like polymer structure as it was stretched like a spring and inflated like a balloon.

Francisco López Jiménez, a postdoc in MIT's Department of Civil and Environmental Engineering, says the researchers' experimental polymer structure and their predictive understanding of it may be useful in the design of cheaper materials for smart windows -- surfaces that automatically adjust the amount of incoming light.

"For buildings and windows that automatically react to light, you don't have to spend as much on heating and air conditioning," López Jiménez says. "The problem is, these materials are too expensive to produce for every window in a building. Our idea was to look for a simpler and cheaper way to let through more or less light, by stretching a very simple material: a transparent polymer that is readily available."

López Jiménez envisions covering window surfaces with several layers of the polymer structure. He says designers could use the group's equation to determine the amount of force to apply to a polymer layer to effectively tune the amount of incoming light.

The research team -- which includes López Jiménez; Pedro Reis, the Gilbert W. Winslow CD Associate Professor of Civil and Environmental Engineering and Mechanical Engineering; and Shanmugam Kumar of the Masdar Institute of Science and Technology in Abu Dhabi -- has published its results this week in the journal Advanced Optical Materials.

Stacking the polymer deck

The current work arose from a related project by Reis, López Jiménez, and Kumar, in which they analyzed the light-transmitting properties of a simple block of PDMS -- a widely used rubbery, transparent polymer. The polymer block contained some darkened regions, and the team was looking to see how deforming the block would change the light traveling through the material.

"It was a happy accident," López Jiménez says. "We were just playing with the material, and we soon got interested in how we can predict this and get the numbers right."

The researchers set out to fabricate a type of soft color composite -- a material that changes color or transparency in response to external stimuli, such as electrical, chemical, or mechanical force. Reis and López Jiménez created a thin, rectangular stack of transparent PDMS sheets, mixed with a solution of black, micron-sized dye particles, that may be easily stretched, or deformed mechanically. With no deformation, the structure appears opaque. As it is stretched or inflated, the material lets in more light.

In initial experiments, the researchers shone a light through the polymer structure infused with dye particles and characterized the amount of light transmitted through the material, without any deformation. They then stretched the polymer perpendicular to the direction of light and measured both the thickness of the polymer and the light coming through.

A theory on light

They compared their measurements with predictions from their equation, which they devised using the Beer-Lambert Law, a classical optics theory that describes the way light travels through a material with given properties. The team combined this theory with their experimental analysis, and derived a simple equation to predict the amount of light transmitted through a mechanically deformed PDMS structure.

To verify their equation, Reis and López Jiménez carried out one more set of experiments, in which they clamped the PDMS structure in the shape of a disc, then inflated the material like a balloon, as they shone a light from below. They measured the amount of light coming through and found that as the material was stretched and thinned, more light came through, at exactly the same intensities that were predicted by their equation.

"We can predict and characterize the evolution of light as we strain it," López Jiménez says. "If you give me the initial material properties and measure the incoming light intensity, we know exactly how much light will go through with deformation."

He adds that going forward, he hopes to use the equation to help tune the transparency and optical transmittance of materials with more complex surfaces and textures.

"Soft color composites offer exciting opportunities to provide materials with switchable and tunable optical properties," Reis says. "Applying this relatively simple but both robust and predictable mechanism is an exciting challenge worth pursuing for concrete engineering applications such as indoor light control through smart windows."
-end-
This work was supported by the Cooperative Agreement between the Masdar and MIT.

Additional background

ARCHIVE: Untangling the mechanics of knots

http://news.mit.edu/2015/untangling-mechanics-knots-0908

ARCHIVE: Wrinkle predictions

http://news.mit.edu/2015/predicting-wrinkles-fingerprints-curved-surfaces-0202

ARCHIVE: Untangling how cables coil

http://news.mit.edu/2014/predict-cable-coiling-1003

Massachusetts Institute of Technology

Related Polymer Articles:

New polymer material may help batteries become self-healing, recyclable
Lithium-ion batteries are notorious for developing internal electrical shorts that can ignite a battery's liquid electrolytes, leading to explosions and fires.
Unique polymer fibres: Light, strong, and tough
Strong and tough yet as light as a feather - materials with this exceptional combination of properties are urgently needed in many industrial sectors and in medicine, as well as being of great interest for scientific research.
Researchers add order to polymer gels
Gel-like materials have a wide range of applications, especially in chemistry and medicine.
Bundlemers (new polymer units) could transform industries
From tires to clothes to shampoo, many ubiquitous products are made with polymers, large chain-like molecules made of smaller sub-units, called monomers, bonded together.
New synthetic polymer degradable under very mild acidic conditions
A new type of degradable synthetic polymer was prepared by Rh-catalyzed three-component polymerization of a bis(diazocarbonyl) compound, bis(1,3-diketone), and tetrahydrofuran.
New polymer tackles PFAS pollution
toxic polyfluorinated alkyl substances (PFAS) pollution -- commonly used in non-stick and protective coatings, lubricants and aviation fire-fighting foams -- can now be removed from the environment thanks to a new low-cost, safe and environmentally friendly polymer.
New polymer films conduct heat instead of trapping it
MIT engineers have flipped the picture of the standard polymer insulator, by fabricating thin polymer films that conduct heat -- an ability normally associated with metals.
Polymer reversibly glows white when stretched
Polymers that change their appearance in response to mechanical forces can warn of damage developing in a material before the stress causes structural failure.
New materials: Growing polymer pelts
Polymer pelts made of the finest of fibers are suitable for many different applications, from coatings that adhere well and are easy to remove to highly sensitive biological detectors.
A record-long polymer DNA negative
A fragment of a single strand of DNA, built of the nucleobases cytosine and guanine, can be imprinted in a polymer - this has been shown by chemists from Warsaw, Denton and Milan.
More Polymer News and Polymer Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Clint Smith
The killing of George Floyd by a police officer has sparked massive protests nationwide. This hour, writer and scholar Clint Smith reflects on this moment, through conversation, letters, and poetry.
Now Playing: Science for the People

#562 Superbug to Bedside
By now we're all good and scared about antibiotic resistance, one of the many things coming to get us all. But there's good news, sort of. News antibiotics are coming out! How do they get tested? What does that kind of a trial look like and how does it happen? Host Bethany Brookeshire talks with Matt McCarthy, author of "Superbugs: The Race to Stop an Epidemic", about the ins and outs of testing a new antibiotic in the hospital.
Now Playing: Radiolab

Dispatch 6: Strange Times
Covid has disrupted the most basic routines of our days and nights. But in the middle of a conversation about how to fight the virus, we find a place impervious to the stalled plans and frenetic demands of the outside world. It's a very different kind of front line, where urgent work means moving slow, and time is marked out in tiny pre-planned steps. Then, on a walk through the woods, we consider how the tempo of our lives affects our minds and discover how the beats of biology shape our bodies. This episode was produced with help from Molly Webster and Tracie Hunte. Support Radiolab today at Radiolab.org/donate.