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.
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.