MIT Researchers Develop Transparent Polymer that Could Offer Cheaper Alternative to Smart Windows

Researchers at the Massachusetts Institute of Technology (MIT) have developed a new theory to predict the amount of light transmitted via a material, considering its level of stretch and thickness. Based on this theory, the researchers were able to successfully predict the varying transparency of a rubber-like, transparent polymer as it was stretched and inflated.

Three colors of PDMS — a widely used rubbery, transparent polymer — are inflated like a balloon as light from a light box shines from below. The top row shows the PDMS material before being inflated. The bottom row shows the PDMS inflated, revealing the MIT logo placed underneath. Photo: Melanie Gonick/MIT

The results of the study have been reported in the journal, Advanced Optical Materials.

According to Francisco López Jiménez, a postdoc in MIT’s Department of Civil and Environmental Engineering, a better understanding of the prototype polymer structure could help in developing cheaper alternatives for smart windows - surfaces capable of adjusting the incoming light.

For buildings and windows that automatically react to light, you don’t have to spend as much on heating and air conditioning. 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.

Francisco López Jiménez, Postdoc, MIT

López Jiménez added that several layers of the polymer structure can be used to cover the window surfaces. Designers could use this theory to measure the amount of force to apply to a polymer layer to adjust the amount of incoming light.

The research group included López Jiménez; Shanmugam Kumar of the Masdar Institute of Science and Technology in Abu Dhabi; and Pedro Reis, the Gilbert W. Winslow CD Associate Professor of Civil and Environmental Engineering and Mechanical Engineering.

The current work was influenced by a similar study performed by López Jiménez, Reis, and Kumar. In this project, the team examined the light-transmitting properties of a basic block of PDMS, a type of transparent and rubber-like polymer that is commonly used for several purposes. Certain darkened regions are found in this polymer block, and the researchers wanted to find out whether the light traveling via the material could be adjusted by deforming the polymer block.

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

Francisco López Jiménez, Postdoc, MIT

The team wanted to develop a soft color composite, a type of material that changes transparency or color when exposed to external stimuli, such as mechanical, chemical, or electrical force. A thin and rectangular stack of transparent PDMS sheets were developed that were combined with a solution of black, tiny dye particles that can be mechanically stretched or deformed. In an un-stretched state, the material appears opaque, but as it is stretched and inflated like a balloon, it lets in more light.

In the first experiments, a light was shone through the polymer structure, which was injected with dye particles, and the amount of light transmitted through the material was characterized, without any deformation. The polymer was then stretched in a perpendicular way towards the direction of light, and both the polymer’s thickness and the amount of light coming through the material were determined. The measurements were then compared with the predictions made from the group’s equation based on the Beer-Lambert Law, an optics theory that depicts the way light passes through a material with specified characteristics. This theory was combined with the experimental analysis, and a basic equation was derived to predict how much light is transmitted via a PDMS structure, which is mechanically deformed.

To further validate the equation, another series of experiments were performed in which a disc-shaped PDMS structure was clamped and inflated like a balloon, while a light was shone from below. The researchers determined the amount of light transmitted through the structure and observed that upon stretching the material, more light comes through it. They also found that the intensities of the light were the same as predicted by their equation.

We can predict and characterize the evolution of light as we strain it. 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.

Francisco López Jiménez, Postdoc, MIT

López Jiménez added that in the future, this equation would be used to adjust the materials optical transmittance and transparency with more complicated textures and surfaces.

Soft color composites offer exciting opportunities to provide materials with switchable and tunable optical properties. 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.

Pedro Reis, Associate Professor of Civil and Environmental Engineering

According to Shengqiang Cai, assistant professor of mechanical and aerospace engineering at the University of California at San Diego, beside smart window technology, the colored polymer could even be utilized as a strain test for other types of materials.

I imagine that a thin layer of such soft composite can be potentially used as nonlinear strain gauge. Through attaching a thin layer of the soft composite to an engineering structure, we may be able to visualize its surface deformation, which is clearly important information for monitoring the structure’s safety. The idea is very simple. However, functions of the system are very robust, as demonstrated by these authors.

Shengqiang Cai, Assistant Professor of Mechanical and Aerospace Engineering, University of California

The Cooperative Agreement between MIT and Masdar funded the study.

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