- Researchers develop a new tissue-like polymer whose thermal and optical properties can be configured separately.
- It can be used in a variety of applications, such as colorful heat-reflecting windows and facades.
The color and thermal properties of a material are connected together: bright color like yellow or pink reflect heat while dark color, especially black, absorb more heat from the environment. That’s why you are advised to wear light color shirts in hot summer days.
Similarly, a see-through glass window can let in more solar heat. It’s not actually the heat that passes through the glass, but electromagnetic radiation in the infrared range. Depending on the material’s optical characteristics, it can let in a wide range of spectrum before and after visible light frequencies. Thus, infrared and visible light are often naturally linked.
Recently, researchers at MIT developed a new tissue-like polymer material whose thermal and optical (color) properties can be configured independently of each other. A thin white film, for instance, can be tuned to absorb heat and stay warm. In fact, each color can be made to absorb or reflect infrared light irrespective of how they respond to visible light.
Both optical and thermal properties of the material can be tuned as per requirements for a variety of applications, such as heat-dissipating building facades and light absorbing covers for solar panels.
While it is relatively easy to add different colors to the glass material, altering its response to heat is a challenging task. The same is true for plastic materials that trap heat instead of deflecting it away.
Materials’ Internal Structure Is The Key To Their Thermal Properties
In one of their previous studies, researchers discovered that it’s possible to change heat-conducting properties of polymers, such as polyethylene, by carefully stretching them, which changes their internal structure.
This time they added a chemical solvent and specific nanoparticles to fabricate colorful films. To produce a black film, for example, they added silicon particles. They then attached this film onto roll-to-roll equipment to make it more flexible while stretching the material.
The polymer became more transparent as researchers stretched each film. The polymer chains straighten out, producing parallel fibers. The heat can travel (in the form of phonons) along these long parallel chains, reducing the hotspot’s temperature.
Polyethylene films, available in multiple colors, can reflect or trap heat as per needs | Credit: Felice Frankel
The material’s heat-conducting properties can be controlled by changing the extent to which polymer is stretched, irrespective of its color. Also, it’s important to select appropriate nanoparticles that interact well with radiative heat.
The team tested their sample films using artificial sunlight from a solar simulator. Films consisting of dark silicon nanoparticles exhibited 20°C less temperature than a film covered with black dyes and pigments.
The team plans to test this new strong material in the real environment (natural sunlight) before moving to commercialization plans. They will also release algorithms to measure the optical and thermal properties, based on material’s internal structure and dimensions.
They are currently working on polyethylene fibers and knitted textiles that can be used to produce lightweight apparel, designed to be either cooling or insulating.