By Josh Perry, Editor
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Researchers at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass. demonstrated that the color and the thermal properties of a novel tissue-like polymer material can be tuned separately, according to a report from the university.

The visual and thermal properties of polyethylene can be tweaked to produce colorful films with a wide range of heat-radiating capabilities. (Felice Frankel/MIT)

Typically, the color of a material indicates its thermal properties because its response to visible and infrared radiation are linked, but MIT researchers created samples where this connection is removed. They created thin black films that reflect heat and stay cool and a “rainbow of other colors, each made to reflect or absorb infrared radiation regardless of the way they respond to visible light.”

With this new process, MIT scientists can design materials for a variety of applications from building facades to covers for solar panels to wearables and clothing.

The researchers started by trying to develop polymers that conducted heat rather than acting as insulators. They discovered that stretching polyethylene changed its internal structure and its thermal conductivity. Researchers decided to take that breakthrough beyond the electronics applications it was intended for and to see if the color of the material could be altered as well.

“To fabricate the colorful films, the team started with a mixture of polyethylene powder and a chemical solvent, to which they added certain nanoparticles to give the film a desired color,” the article explained. “For instance, to make black film, they added particles of silicon; other red, blue, green, and yellow films were made with the addition of various commercial dyes.”

A roll-to-roll device was used to soften the film and make it more pliable. When stretched, the polymer became more transparent and its internal structure became more organized, with the polymer chains forming parallel fibers.

“When the researchers placed each sample under a solar simulator — a lamp that mimics the visible and thermal radiation of the sun — they found the more stretched out a film, the more heat it was able to dissipate,” the article continued. “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.”

Researchers hope to build a website with algorithms for determining the material’s color and thermal properties based on dimensions and internal structures.

The research was recently published in Optical Materials Express. The abstract stated:

“Sustainable architecture requires development of new materials with tailored optical, mechanical, and thermal properties to provide both aesthetic appeal and energy-saving functionalities. Polymers and polymer-based composites emerge as promising lightweight and conformable materials whose optical spectra can be engineered to achieve both goals.

“Here, we report on the development of new types of organic-inorganic films composed of ultrahigh molecular weight polyethylene with a variety of organic and inorganic nano- and micro-scale inclusions. The films simultaneously provide ultra-light weight, conformability, either visual coloring or transparency on demand, and passive thermal management via both conduction and radiation.

“The lightweight semi-crystalline polymer matrix yields thermal conductivity exceeding that of many metals, allowing for the lateral heat spreading and hot spots mitigation in the cases of partial illumination of films by sunlight. It also yields excellent broadband transparency, allowing for the opportunities to shape the spectral response of composite materials via targeted addition of inclusions with tailored optical spectra.

“We demonstrate a variety of dark- and bright-colored composite samples that exhibit reduced temperatures under direct illumination by sunlight, and outline strategies for materials design to further improve material performance.”