samples of film

The visual and thermal properties of polyethylene can be tweaked to produce colorful films with a wide range of heat-radiating capabilities. [Image: Felice Frankel]

Temperature and color are intrinsically intertwined—as anyone who has made the uncomfortable mistake of wearing a black shirt under the sweltering sun can attest—and tuning one property tends to affect the other. Thus these properties are usually optimized separately. Now, a U.S. team has engineered strong, colorful polymer films that can be tailored to reflect or trap infrared (IR) radiation, regardless of their response to visible light (Opt. Mater. Exp., doi: 10.1364/OME.9.001990).

These flexible, lightweight films can be designed to stay cool when exposed to sunlight, or even to have tunable transparency and haze characteristics. The ability to simultaneously yet independently control the colorful film’s optical and thermal properties, the team believes, could give it applications in everything from architecture to apparel—regulating the temperature of buildings and people, without requiring power.

Stretch it out

Team leader and OSA Senior Member Svetlana Boriskina, Masschusetts Institute of Technology (MIT), found her inspiration for this work in stained-glass windows. People have been adding particles and pigments to glass to create colorful effects for centuries, but manipulating glass’s response to heat is not so easily done. Boriskina and her team wanted to create a sustainable solution that could control both visible light and infrared radiation—visual appeal and thermal comfort.

To modify these properties together, the team turned to a lightweight polymer with thermal conducting properties as a base material for its film. For several years, MIT co-author Gang Chen had been working on a technique to manipulate flexible polymer materials to conduct instead of insulate against heat. By stretching polyethylene, he found that the material’s internal structure changed so as to be thermally conductive.

Making plastic more crystalline

Boriskina and her team took this technique and added a colorful flair. The researchers fabricated flexible films with high tensile strength out of a mixture of polyethylene powder, a chemical solvent and nanoparticles, which varied based on the desired color. The team then uniaxially stretched the material on a warmed-up roll-to-roll apparatus.

Polymers are typically amorphous, but they may exhibit some crystalline domains depending on how they’re made. As the polymer composite films were stretched, the amorphous areas became increasingly aligned, boosting crystallinity and transparency while decreasing thermal emittance in the mid-IR spectral range. The team found that the more a film was stretched, the more heat was able to dissipate as phonons (quanta of vibrational energy) traveled along the parallel polymer chains. Alternatively, the less the material was stretched, the more it was able to insulate heat as the phonons became trapped in the tangled polymer chains.

Adding color

Various nanoparticles, dyes and phosphorescent pigments embedded in the plastic allowed the researchers to imbue the films with optical properties, including color. The researchers selected specific organic and inorganic particles to provide spectral selectivity in both the visible and IR parts of the electromagnetic spectrum—spectral selectivity in the visible range allowed them to control color while low absorptance in the near IR allowed for thermal control under direct sunlight.

For example, the team engineered a dark gray film embedded with silica nanoparticles and a black film embedded with copper(II) oxide. These films showed 20–65 percent absorptance in the visible part of the spectrum and became 40–50 percent transparent and 40 percent reflective in the near IR. A normal piece of paper painted black, in contrast, exhibited uniformly high absorptance and low reflectance across both spectral ranges.

Turn up the heat

The team used a solar simulator to test its films, finding, for example, that a film engineered not to absorb heat from sunlight was 20° C cooler than a reference material under the simulator. The team also tested the samples under localized laser beam illumination. Infrared imaging revealed that heat spread laterally along a stretched sample after being illuminated by a laser beam, promoting cooling as the heat spread away from the initial hot spot.

The team expects to eventually commercialize the films, but the next step is to test the materials in natural sunlight. The promising results from this study have inspired the team to begin developing polyethylene fibers and fabrics for constructing wearable technologies.

In addition to the MIT scientists, the team included researchers from the Combat Capabilities Development Command Soldier Center, USA. The study was funded by the U.S. Army.