Polymers are regularly used as thermal insulators and can even be used as thermal conductors to enable efficient heating or cooling. A new type of polymer was created that demonstrates a switchable thermal conductivity controlled by light.

Under ambient conditions or visible light (left), the polymer is crystalline and has a high thermal conductivity. Once exposed to UV light (right), it transforms to a low-thermal-conductivity liquid. In imaging performed using polarized optical microscopy, crystals appear bright and liquid dark. Inset images show schematic illustration of crystalline and liquid polymer in each state.

Polymers are used extensively in engineered systems but have almost always been considered thermally static. The new thermal switching polymer demonstrates powerful control of the thermophysical properties of a polymer in response to light. This ability originates from a photo-responsive molecule (azobenzene) that can be optically excited by ultraviolet (UV) and visible light.

Researchers synthesized a complex polymer functionalized with light-responsive azobenzene groups. By illuminating with UV and visible light, they could change the shape of the azobenzene group, modulate interchain bonding strength, and drive a reversible transition between crystal and liquid.

To capture the thermal conductivity transitions of azobenzene polymers under light illumination, time-domain thermoreflectance (TDTR) was used. The way heat is carried in a polymer is related to the diffusion of vibrational modes. In ordered crystals, these vibrational modes travel much further than what is observed in disordered liquids. As a result, an extreme change in molecular ordering of the polymer can significantly alter the thermal conductivity.

This extreme change in macromolecular ordering, e.g., crystal-to-liquid, is rare in nature and has not been reported previously for any polymer system in response to light. Thus, unravelling the mechanism of the light-triggered phase transition was critical to understand the polymer's unique behavior. Synchrotron-based x-ray scattering as used to elucidate the structure associated with each state during the transformation, closing the synthesis-characterization-function loop for the polymer.

For more information, contact Paul Braun at This email address is being protected from spambots. You need JavaScript enabled to view it..

Tech Briefs Magazine

This article first appeared in the August, 2019 issue of Tech Briefs Magazine.

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