The color of a material can often tell how it handles heat. With clothing, for example, the darker the pigment, the warmer you're likely to feel on a hot day. Likewise, the more transparent a glass window, the more heat it can let through. A material's responses to visible and infrared radiation are often naturally linked.
Samples of strong, tissue-like polymer material were created, the color and heat properties of which can be tailored independently of the other. For instance, samples of very thin black film were fabricated to reflect heat and stay cool. Films exhibiting a rainbow of other colors also were created, each made to reflect or absorb infrared radiation regardless of the way they respond to visible light.
The color and heat properties of this new material can be specifically tuned to fit the requirements for wide-ranging applications including colorful, heat-reflecting building facades, windows, and roofs; light-absorbing, heat-dissipating covers for solar panels; and lightweight fabric for clothing, outerwear, tents, and backpacks — all designed to either trap or reflect heat, depending on the environments in which they would be used.
While it's relatively simple to tailor the color of glass, the material's response to heat is difficult to tune. For instance, glass panels reflect room-temperature heat and trap it inside the room. If colored glass is exposed to incoming sunlight from a particular direction, the heat from the Sun can create a hotspot, which is difficult to dissipate in glass. If a material like glass can't conduct or dissipate heat well, that heat could damage the material. The same can be said for most plastics, which can be engineered in any color but for the most part are thermal absorbers and insulators, concentrating and trapping heat rather than reflecting it away.
By carefully stretching polymers like polyethylene, the material's internal structure could be changed in a way that also changed its heat-conducting properties. The polymer-fabrication technique was adapted by adding a twist of color.
To fabricate the colorful films, researchers started with a mixture of polyethylene powder and a chemical solvent, to which they added certain nanoparticles to give the film a desired color; to make black film, they added particles of silicon and other red, blue, green, and yellow films were made with the addition of various commercial dyes. Each nanoparticle-embedded film was attached onto a roll-to-roll apparatus that was heated up to soften the film, making it more pliable as the material was carefully stretched.
As they stretched each film, they found that the material became more transparent and polyethylene's microscopic structure changed as it stretched. Where normally the material's polymer chains resemble a disorganized tangle, when stretched, these chains straighten out, forming parallel fibers.
When each sample was placed under a solar simulator — a lamp that mimics the visible and thermal radiation of the Sun — the more stretched out a film was, 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 then shoot away from its source, in a “ballistic” fashion, avoiding the formation of hotspots. The less the material was stretched, the more insulating it was, trapping heat and forming hotspots within polymer tangles.
By controlling the degree to which the material is stretched, polyethylene's heat-conducting properties could be controlled, regardless of the material's color. The nanoparticles were chosen not just by their visual color, but also by their interactions with invisible radiative heat. This technique could be used to produce thin, flexible, colorful polymer films that can conduct or insulate heat, depending on the application. In addition to films, the technique can be used to fabricate nanoparticle-embedded polyethylene thread that can be stitched together to form lightweight apparel, designed to be either insulating or cooling.