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Edited by Timothy M. Swager, Massachusetts Institute of Technology, Cambridge, MA, and approved February 12, 2019 (received for review October 19, 2018)
Significance
Heat is carried as diffusion of vibrational modes in insulating polymers, a process that is highly dependent on the macromolecular ordering of a polymer. As a result, changes in macromolecular ordering have potential to significantly change the thermal transport property of a polymer. Here, we design and synthesize a thermally switchable azobenzene polymer that exhibits a reversible crystal-to-liquid transition in response to UV and visible light. By driving a transition between the planar (trans) and nonplanar (cis) conformational states of azobenzene moieties attached to the polymer, we modulate interchain π-π bonding, resulting in fast and reversible thermal and structural transitions. This work unravels the pathway of crystal-to-liquid transitions of the azobenzene polymer and the resulting thermal and physical property changes.
Abstract
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.
Footnotes
↵1J. Shin and J. Sung contributed equally to this work.
- ↵2To whom correspondence may be addressed. Email: pbraun{at}illinois.edu or d-cahill{at}illinois.edu.
Author contributions: J. Shin, J. Sung, K.M.L., and T.J.W. designed research; J. Shin, J. Sung, M.K., and B.L. performed research; J. Shin, X.X., and D.G.C. contributed new reagents/analytic tools; J. Shin, J. Sung, M.K., B.L., C.L., N.R.S., P.V.B., and D.G.C. analyzed data; and J. Shin, J. Sung, P.V.B., and D.G.C. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: The raw synchrotron X-ray scattering datasets of the 2D diffraction images reported in this paper have been deposited in Globus MDF Open, dx.doi.org/doi:10.18126/M2VH2X.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1817082116/-/DCSupplemental.
Published under the PNAS license.
Light-triggered thermal conductivity switching in azobenzene polymers
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