Light-triggered thermal conductivity switching in azobenzene polymers

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−1K−1) and low thermal conductivity (0.10 W m−1K−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.

High-contrast and reversible polymer thermal regulator by structural phase transition

“…The associated references are as follows. nCB ( 31 ); VO 2 ( 26 ); K, Zn, Cd, and Hg ( 32 ); c-Se ( 33 ); Ni-Mn-In alloy ( 13 ); C 6 H 14 ( 34 ); Sn, In, and LiNO 3 ( 15 ); C/C 16 H 34 composite ( 17 ); Ge 2 Sb 2 Te 5 ( 27 ); azobenzene polymers ( 14 ). ( D ) Multiple on/off thermal cycles of nanofiber #2 around the phase transition temperature of 440 K.…”

“…Through thermally induced martensitic transition, a ~1.5× thermal switching ratio was observed in Ni-Mn-In alloys ( 13 ). Recently, a 3.5× thermal switching ratio has been achieved in light-responsive azobenzene polymers by modulation of chain alignment, which results from the conformational transition between nonplanar (cis) and planar (trans) azobenzene groups under green and ultraviolet light illumination ( 14 ).…”

Solid‐State Thermal Control Devices

“…As an example, Shin et al. [ 162 ] reported for azobenzene polymer a change of structure from a planar ( trans ) to a nonplanar ( cis ) in response to UV light stimuli. The whole switching process is illustrated in Figure .…”

Photoinduced Strain‐Assisted Synthesis of a Stiff‐Stilbene Polymer by Ring‐Opening Metathesis Polymerization

“…Various types of molecular photoswitches, such as stilbene, [8] azobenzene, [9] spiropyran, [10] diarylethene, [2d, 11] coumarin [12] or fulgide, [12] have been incorporated into the main or side chain of the polymer to achieve reversible photoresponsive polymers [13] . Due to the light‐induced molecular changes, these molecular switches can cause a structural reconfiguration or change in the properties of polymer chains, which leads to fast, precise and remote control of various macroscopic properties, such as shape, [2d] wettability, [10b] optical properties, [14] adhesion, [15] solubility, [16] conductivity, [14, 17] and so on. Compared to the side chain polymer, the main‐chain polymer made up of photoswitchable repeating units has improved thermal and mechanical properties that provide better photomechanical response [18] .…”