Page 60 - Plastics News March 2019

TECHNOLOGY

Controlling thermal conductivity of polymers with light

n a new study, researchers have designed and group, modulate interchain bonding strength and drive
Idemonstrated a novel type of polymer demonstrating a reversible transition between crystal and liquid," said
a switchable thermal conductivity controlled by light. Jaeuk Sung, a MatSE Ph.D. student at Illinois. To capture the
The material has the potential to route the conduction thermal conductivity transitions of azobenzene polymers
of heat on-demand and enable new, smarter, ways to under light illumination, the Illinois research team used
manage heat. The findings are reported in Proceedings a technique called time-domain thermoreflectance
of the National Academy of Sciences. "Polymers are used (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
extensively in engineered systems, but these materials properties. We used synchrotron-based x-ray scattering to
have almost always been considered thermally static. elucidate the structure associated with each state during
Discovery of polymers that can be optically triggered the transformation, closing the synthesis-characterization-
to quickly switch between thermally conducting and function loop for this sophisticated polymer," added Cecilia
insulating states will open up entirely new opportunities Leal, a MatSE professor at Illinois. "Keeping an electrical
in thermal engineering," explained Paul Braun, a materials device warm is as equally important as keeping it cold.
science and engineering (MatSE) professor and director Materials with such switchable thermal conductivity would
of the Illinois Materials Research Laboratory. "To the enable ways to keep electrified systems safe, reliable and
best of our knowledge, this is the first observation of a efficient even under extreme condition." explained Andrew
light-triggered reversible crystal-liquid transition in any Alleyne, the director of the National Science Foundation
polymeric material. The particularly notable finding in (NSF) sponsored Engineering Research Center for Power
this study is the fast, reversible 3-fold change in thermal Optimization of Electro-Thermal Systems (POETS) which
conductivity associated with the phase transition," supported this work, and a professor in Mechanical Science
explained Jungwoo Shin, a MatSE Ph.D. student at Illinois. and Engineering at Illinois. "The ability to rapidly switch
The thermal switching polymer developed by the Illinois the thermal properties of a polymer by exposure to light
research team demonstrates a powerful control of the opens up exciting new routes for control of thermal
thermophysical properties of a polymer in response to transport and energy conversion at the molecular level,"
light. This ability originates from a photo-responsive added Nancy Sottos, a MatSE professor at Illinois.This
molecule,azobenzene, which can be optically excited finding provides a striking example of how light can be
by ultraviolet (UV) and visible light. "We synthesized a used to control the thermal conductivity of polymers. A
complex polymer functionalized with light-responsive better understanding of the physical relationship between
azobenzene groups. By illuminating with UV and visible thermal conductivity and macromolecular ordering would
light, we could change the shape of the azobenzene also help push the limits of traditional polymers.

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