Flexible Polymer Offers Tunable Transparency

An experimental, rubber-like polymer structure has been used to model predictions of material transparency based on thickness and degree of structure. The material and predictive model could enable inexpensive materials for smart windows and other visually active materials.

"For buildings and windows that automatically react to light, you don't have to spend as much on heating and air conditioning," said researcher López Jim&eacuate;nez of the Massachusetts Institute of Technology. "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."

Jiménez and colleagues from MIT and the Masdar Institute of Science and Technology in Abu Dhabi, U.A.E., had been working on a related project, analyzing 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 wanted to see how deforming the block would change the light traveling through the material.

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 forceThe team created a thin, rectangular stack of transparent PDMS sheets mixed with a solution of black, micron-sized dye particles that could be easily stretched or deformed mechanically. With no deformation, the structure appeared opaque. As it was stretched or inflated, the material let 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 passing through.

The researchers compared their measurements with predictions from their equation, 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, the team carried out 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.

The researchers hope to use the equation to tune the transparency and optical transmittance of materials with more complex surfaces and textures, while the relatively simple soft color composition mechanism could be used for indoor light control through smart windows. 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 work was published in Advanced Optical Materials (doi: 10.1002/adom.201500617).