Polymers are regularly used as thermal insulators for everything from keeping beverages hot to keeping sensitive electronics cool. In some cases, polymers can even be used as thermal conductors to enable efficient heating or cooling.
In a new study, researchers at the University of Illinois at Urbana-Champaign have designed and demonstrated a novel type of polymer demonstrating a switchable thermal conductivity controlled by light.
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. 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.
Credit: University of Illinois Materials Research Lab
The material has the potential to route the conduction of heat on-demand and enable new, smarter, ways to manage heat.
The findings are reported in Proceedings of the National Academy of Sciences.
"Polymers are used extensively in engineered systems, but these materials have almost always been considered thermally static. Discovery of polymers that can be optically triggered to quickly switch between thermally conducting and insulating states will open up entirely new opportunities in thermal engineering," explained Paul Braun, a materials science and engineering (MatSE) professor and director of the Illinois Materials Research Laboratory.
"To the best of our knowledge, this is the first observation of a light-triggered reversible crystal-liquid transition in any polymeric material. The particularly notable finding in this study is the fast, reversible 3-fold change in thermal conductivity associated with the phase transition," explained Jungwoo Shin, a MatSE Ph.D. student at Illinois.
The thermal switching polymer developed by the Illinois research team demonstrates a powerful control of the thermophysical properties of a polymer in response to light. This ability originates from a photo-responsive molecule,azobenzene, which can be optically excited by ultraviolet (UV) and visible light.
"We synthesized a complex polymer functionalized with light-responsive azobenzene groups. By illuminating with UV and visible light, we could change the shape of the azobenzene group, modulate interchain bonding strength and drive a reversible transition between crystal and liquid," said Jaeuk Sung, a MatSE Ph.D. student at Illinois.
To capture the thermal conductivity transitions of azobenzene polymers under light illumination, the Illinois research team used a technique called time-domain thermoreflectance (TDTR) developed by David Cahill, a MatSE professor at Illinois.
"The way heat is carried in 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," David Cahill said.
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.
"We could observe that, upon light exposure, this material quickly switches from one state to another with dramatically distinct heat transport properties. We used synchrotron-based x-ray scattering to elucidate the structure associated with each state during the transformation, closing the synthesis-characterization-function loop for this sophisticated polymer," added Cecilia Leal, a MatSE professor at Illinois.
"Keeping an electrical device warm is as equally important as keeping it cold. Materials with such switchable thermal conductivity would enable ways to keep electrified systems safe, reliable and efficient even under extreme condition." explained Andrew Alleyne, the director of the National Science Foundation (NSF) sponsored Engineering Research Center for Power Optimization of Electro-Thermal Systems (POETS) which supported this work, and a professor in Mechanical Science and Engineering at Illinois.
"The ability to rapidly switch the thermal properties of a polymer by exposure to light opens up exciting new routes for control of thermal transport and energy conversion at the molecular level," added Nancy Sottos, a MatSE professor at Illinois.
This finding provides a striking example of how light can be used to control the thermal conductivity of polymers. A better understanding of the physical relationship between thermal conductivity and macromolecular ordering would also help push the limits of traditional polymers.
Additional co-authors of this article include Minjee Kang and Xu Xie (Illinois), Byeongdu Lee (Argonne National Laboratory), and Kyung Min Lee and Timothy White (Air Force Research Laboratory).
Paul Braun | EurekAlert!
From foam to bone: Plant cellulose can pave the way for healthy bone implants
19.03.2019 | University of British Columbia
Additive printing processes for flexible touchscreens: increased materials and cost efficiency
19.03.2019 | INM - Leibniz-Institut für Neue Materialien gGmbH
Due to the special arrangement of its molecules, a new coating made of corn starch is able to repair small scratches by itself through heat: The cross-linking via ring-shaped molecules makes the material mobile, so that it compensates for the scratches and these disappear again.
Superficial micro-scratches on the car body or on other high-gloss surfaces are harmless, but annoying. Especially in the luxury segment such surfaces are...
The Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT) in Arizona released its first image of the surface magnetic field of another star. In a paper in the European journal Astronomy & Astrophysics, the PEPSI team presents a Zeeman- Doppler-Image of the surface of the magnetically active star II Pegasi.
A special technique allows astronomers to resolve the surfaces of faraway stars. Those are otherwise only seen as point sources, even in the largest telescopes...
Researchers at Chalmers University of Technology and the University of Gothenburg, Sweden, have proposed a way to create a completely new source of radiation. Ultra-intense light pulses consist of the motion of a single wave and can be described as a tsunami of light. The strong wave can be used to study interactions between matter and light in a unique way. Their research is now published in the scientific journal Physical Review Letters.
"This source of radiation lets us look at reality through a new angle - it is like twisting a mirror and discovering something completely different," says...
New research group at the University of Jena combines theory and experiment to demonstrate for the first time certain physical processes in a quantum vacuum
For most people, a vacuum is an empty space. Quantum physics, on the other hand, assumes that even in this lowest-energy state, particles and antiparticles...
Physicists in the EPic Lab at University of Sussex make crucial development in global race to develop a portable atomic clock
Scientists in the Emergent Photonics Lab (EPic Lab) at the University of Sussex have made a breakthrough to a crucial element of an atomic clock - devices...
11.03.2019 | Event News
01.03.2019 | Event News
28.02.2019 | Event News
20.03.2019 | Health and Medicine
20.03.2019 | Trade Fair News
20.03.2019 | Trade Fair News