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John O | March 2019

Light is used as a switch to control the thermal conductivity of novel polymers

By Josh Perry, Editor


Scientists at the University of Illinois at Urbana-Champaign designed and demonstrated a polymer material that can be switched from thermally insulating to thermally conductive using light, according to a report from the university.


Under ambient conditions or visible light (left side), the polymer is crystalline and has a high thermal conductivity. Once exposed to ultraviolet (UV) light (right side) it transforms to a low thermal conductivity liquid. (University of Illinois at Urbana-Champaign)


The thermal switch stems from a photo-responsive molecule, azobenzene, which is excited by ultraviolet (UV) and visible light. The molecule was added to a complex polymer structure and when exposed to light changed the shape of the azobenzene group, leading to a phase-change between crystal and liquid states.


Thermal conductivity transition was captured using a novel time-domain thermoreflectance system developed at the university. Synchrotron-based X-ray scattering techniques were used to viaulize the structural changes.


“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,” the article noted. “Thus, unravelling the mechanism of the light-triggered phase transition was critical to understand the polymer’s unique behavior.”


Researchers believe that this breakthrough could lead to polymers being used in a range of applications for both heating and cooling.


The research was recently published in Proceedings of the National Academy of Sciences (PNAS). The abstract read:


“Materials that can be switched between low and high thermal conductivity states would advance the control and conversion of thermal energy. Employing in situ time-domain thermoreflectance (TDTR) and in situ synchrotron X-ray scattering, we report a reversible, light-responsive azobenzene polymer that switches between high (0.35 W m−1 K−1) and low thermal conductivity (0.10 W m−1 K−1) states.


“This threefold change in the thermal conductivity is achieved by modulation of chain alignment resulted from the conformational transition between planar (trans) and nonplanar (cis) azobenzene groups under UV and green light illumination. This conformational transition leads to changes in the π-π stacking geometry and drives the crystal-to-liquid transition, which is fully reversible and occurs on a time scale of tens of seconds at room temperature.


“This result demonstrates an effective control of the thermophysical properties of polymers by modulating interchain π-π networks by light.”

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