A light-sensitive polymer changes how it conducts heat and lithium ions by switching molecular order, revealing a path toward self-regulating materials for batteries and electronics.
(Nanowerk Spotlight) In almost every energy device, from a smartphone battery to a solar cell, heat and charged particles move through materials. How efficiently they do so determine performance and safety. A battery that stores lithium ions must let those ions move quickly through an electrolyte, yet it must also manage heat to avoid thermal runaway. A film that carries heat away from an electronic chip should conduct thermal energy but not allow ions to drift.
These needs often conflict because the same structural features that speed heat flow can slow ion motion, and vice versa. Scientists have long sought a single material that could balance both properties and adjust them when needed.
The challenge comes from how molecular order affects transport. In an ordered structure where polymer chains pack tightly and align in layers, vibrations travel smoothly, carrying heat with little resistance. But those same ordered regions restrict the flexibility required for ions to find pathways through the material. When the structure becomes disordered, the chains move more freely and ions can pass, but the vibrations scatter, reducing thermal conductivity. The ability to switch between these two structural states on command would offer a powerful way to control both heat and charge in energy systems.
Light is an appealing trigger for such switching because it can act without direct contact, can reach specific regions, and can be reversed simply by changing the wavelength. A Advanced Functional Materials paper (“Photo‐Switching Thermal and Lithium‐Ion Conductivity in Azobenzene Polymers”) by scientists at University of Illinois Urbana-Champaign, investigates this approach in detail. It reports a polymer that changes how it moves both heat and lithium ions when illuminated with ultraviolet or visible light. The response occurs within seconds and repeats reliably across many cycles.
The work focuses on a methacrylate polymer named pPPHM that contains azobenzene side groups connected by six carbon spacers. Azobenzene is a molecule that changes shape under light. In visible light, it stays in a straight form called trans. Under ultraviolet light, it bends into a cis form. That small molecular change alters how neighboring groups pack together. When the azobenzene is in the trans state, the side chains align and form ordered, lamellar layers. When the light changes to ultraviolet, the molecules bend, the layers lose alignment, and the structure becomes disordered and fluid-like at room temperature.
This structural change produces measurable effects. In the ordered trans state, the polymer conducts heat at about 0.45 watts per meter kelvin. In the disordered cis state, thermal conductivity drops to about 0.15. At the same time, lithium-ion diffusivity increases from roughly ten to the minus seven to ten to the minus five square centimeters per second. Both heat and ion transport are therefore switched by light, but in opposite directions: one decreases while the other increases.
Photo-switching azobenzene polymer. a) Chemical structure and 𝜋–𝜋 stacking geometry of light-responsive azobenzene polymer (pPPHM) during reversible cis-trans photoisomerization under UV and visible light illuminations. b) Schematic illustration of the phase transition of pPPHM during cis-trans photoisomerization. TranspPPHM (crystalline, left) exhibits high thermal conductivity owing to the extended vibrational modes’ mean free path along the stretched sidechains supported by the interchain 𝜋–𝜋 network. However, the dense polymer chain network obstructs lithium-ion diffusion (ambient condition or under visible light); Cis-pPPHM (liquid, right) exhibits low thermal conductivity due to the highly disordered chains, but lithium-ions can diffuse through the mobile polymer chains (under UV light). (Image: Reprinted from DOI:10.1002/adfm.202519774, CC BY)
The researchers show that this behavior depends on how the polymer chains are made. They compare reversible addition fragmentation chain transfer (RAFT) polymerization with conventional free radical polymerization. RAFT produces chains of uniform length and higher stereoregularity, which favors the formation of ordered crystalline regions. Films prepared by RAFT show about eighty percent crystalline coverage under polarized optical microscopy, whereas those made by the conventional method show only about twenty seven percent and appear more disordered. This difference strongly affects how well the material responds to light.
Under visible light, RAFT films display bright, ordered regions that disappear when illuminated with ultraviolet light. When visible light is restored, the ordered regions return. These reversible transitions confirm that the azobenzene groups in the RAFT polymer switch efficiently between the trans and cis forms. The conventional films show much weaker changes, indicating that chain irregularity prevents large structural rearrangements.
X ray scattering experiments provide direct evidence of the structural transition. In the ordered trans state, the scattering pattern shows sharp peaks with a spacing of about 27 angstroms, characteristic of lamellar layers. Ultraviolet light removes these peaks within about forty seconds, and visible light restores them in a similar time. The process is fast, fully reversible, and stable through repeated cycles.
The team measures thermal conductivity using time domain thermoreflectance, which tracks how a short heat pulse passes through the material. The data reveal that thermal conductivity rises with the fraction of ordered lamellae. The authors describe the film as a stack of crystalline and amorphous layers, where heat must pass through both. The overall conductivity follows a harmonic mean relation, meaning that the slower layer dominates the total resistance. As the film becomes more ordered, vibrations move more coherently along the azobenzene side chains, and heat flows more easily. When ultraviolet light breaks this alignment, the paths for vibration become irregular and the material conducts heat less effectively.
To show how light can control structure in space as well as time, the researchers used an optical mask to write a pattern on a film. By exposing part of the film to visible light through the mask, they created a bright crystalline letter I that appeared under polarized light. When the sample was flooded with ultraviolet light, the pattern vanished. This demonstration suggests that future devices could locally adjust heat and ion transport simply by projecting light patterns.
For ionic transport, the team added lithium bis trifluoromethanesulfonimide, a common lithium salt also known as Li-TFSI, to the polymer. They placed the doped film between transparent electrodes and measured electrical impedance while illuminating it with either ultraviolet or visible light. From these measurements, they calculated lithium-ion diffusivity under different conditions.
At low temperature under visible light, the polymer remains solid in the trans form and ion motion is slow. When illuminated with ultraviolet light, the film becomes liquid-like and diffusivity increases by two orders of magnitude. As temperature rises, diffusivity first increases due to thermal activation, then decreases above about 97 degrees Celsius as the polymer relaxes toward the trans structure. Above about one hundred fifty degrees, the curves for ultraviolet and visible illumination merge, showing that thermal effects dominate over photo switching.
Time-dependent tests show that at thirty degrees, ultraviolet light raises lithium-ion diffusivity from nearly zero to about 1.7 times ten to the minus six square centimeters per second within one hundred seconds. Visible light brings it back down to about 0.1 times ten to the minus six in fifty seconds.
Turning off ultraviolet light without applying visible light leads to a slow return over tens of minutes, indicating that optical control is far more efficient than passive relaxation. At one hundred degrees, the switching is faster, and the maximum diffusivity in the cis state rises to about 13 times ten to the minus six. The polymer maintains stable behavior through repeated cycles and after six months of storage in air.
The mechanism connecting molecular geometry and transport is direct. In the trans state, azobenzene groups lie flat and promote tight stacking of side chains. The dense packing allows heat-carrying vibrations to propagate efficiently but restricts segmental motion, limiting ion flow. Ultraviolet light bends the azobenzene units, breaks the stacks, and creates a disordered structure where vibrations scatter but ions can move more freely. Visible light restores the flat trans configuration and the original balance.
The study highlights how structural control at the molecular level can lead to coupled changes in thermal and ionic transport. It also shows that controlled polymerization through RAFT is essential. Without uniform chains, the material cannot form the ordered regions needed for high thermal conductivity or show the large reversible changes under light. The authors note that molecular weight and orientation of side chains further influence performance, providing design parameters for future materials.
Previous studies cited in the paper investigated azobenzene systems that altered optical or mechanical properties, and some reports demonstrated changes in either thermal or ionic conductivity. The present work brings both effects together in a single solid polymer and connects them through a clear structural mechanism. By linking molecular motion, crystallinity, and transport properties, the paper establishes quantitative evidence that light can tune both heat and ion flow within one material.
The implications reach beyond basic science. A light-responsive polymer could manage both ion conduction and heat dissipation in lithium batteries or serve as an adaptive thermal interface in flexible electronics. The patterning experiment suggests that these effects could be localized, allowing fine control over thermal and electrical pathways through simple optical inputs. The material’s ability to maintain performance over repeated cycles indicates that such applications could be practical if integrated into device architectures.
This study demonstrates that precise molecular design can convert light-driven conformational changes into functional control over two fundamental transport processes. It provides a clear framework for how structural order, molecular geometry, and synthesis methods determine the dual modulation of heat and ion movement. The result is a polymer that responds predictably, reversibly, and rapidly to light, pointing toward new ways to create materials that adapt their behavior to changing conditions.
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David G. Cahill (University of Illinois Urbana-Champaign)
, 0000-0001-5969-3460 corresponding author
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