Researchers at the Massachusetts Institute of Technology (MIT) have optically engineered a polymer material to allow its color and thermal properties to be tailored independently of each other. The new films simultaneously provide light weight, conformability, either visual coloring or transparency on demand, and passive thermal management through conduction and radiation. To fabricate the films, the team started with a mixture of polyethylene powder and a chemical solvent. Nanoparticles were added to the mixture to give the film the desired color. For example, to make black film, the researchers added silicon particles. Red, blue, green, and yellow films were made by adding particles of commercial dyes. The choice of which nanoparticles to use was made based not only on color, but also on how the particles interacted with invisible radiative heat. The visual and thermal properties of polyethylene can be tweaked to produce colorful films with a wide range of heat-radiating capabilities. Courtesy of Felice Frankel. The researchers then attached each nanoparticle-embedded film onto a roll-to-roll apparatus, which they heated up to soften the film, making it more pliable and easier to stretch. The material became more transparent as it was stretched. The researchers observed that the polyethylene’s microscopic structure also changed as it was stretched. When stretched, the material’s polymer chains straightened, forming parallel fibers. When the researchers placed each sample under a solar simulator, they found that the more a film was stretched, the more heat it was able to dissipate. The long, parallel polymer chains essentially provided a direct route along which heat could travel. Along these chains, heat in the form of phonons could shoot away from its source, in a “ballistic” fashion, avoiding the formation of hot spots. The researchers also found that the less they stretched the material, the more insulating it was, trapping heat and forming hot spots within tangles of polymer chains. By controlling the degree to which the material was stretched, the researchers could control the polyethylene’s heat-conducting properties, regardless of the material’s color. The team demonstrated samples of thin black film that were designed to reflect heat and stay cool. They created films exhibiting a range of colors, designing each sample to reflect or absorb IR radiation regardless of the way it responded to visible light. They demonstrated a variety of dark- and bright-colored composite samples that exhibited reduced temperatures under direct illumination by sunlight, According to the researchers, the lightweight semicrystalline polymer matrix yields thermal conductivity exceeding that of many metals. It also yields excellent broadband transparency, allowing for opportunities to shape the spectral response of composite materials through the targeted addition of inclusions with tailored optical spectra. Researcher Svetlana Boriskina said that the technique could potentially be used to produce thin, flexible, colorful polymer films that could conduct or insulate heat, depending on the application. Going forward, Boriskina plans to launch a website that offers algorithms to calculate a material’s color and thermal properties, based on its dimensions and internal structure. In addition to films, her group is now working on fabricating nanoparticle-embedded polyethylene thread that could be stitched together to form lightweight apparel, designed to be either insulating or cooling. “This is in film factor now, but we’re working it into fibers and fabrics,” Boriskina said. “Polyethylene is produced by the billions of tons and could be recycled, too. I don’t see any significant impediments to large-scale production.” The research was published in Optical Materials Express (https://doi.org/10.1364/OME.9.001990).