Your future home could feature windows that adapt to the changing world outside. During the day, these ‘smart windows’ would let in sunlight to brighten your living room, while in the cool evening, they would morph to an opaque state, offering privacy.
At the A*STAR Institute of Sustainability for Chemicals, Energy and Environment (A*STAR ISCE2), Senior Principal Scientist Jianwei Xu and colleagues have been exploring the potential of thermochromic materials, which change colour depending on temperature, to power these adaptive windows.
“Beyond privacy, smart windows like these would be useful in colder climates, where they would help trap solar heat during the day and reduce heat loss at night,” said Xu, noting that this technology presents an elegant solution to the challenges of creating sustainable architecture.
But there’s a catch: thermochromic materials often aren’t clear enough to make effective windows. Those that have the best transparency, like hydrogels and thermogels, are too soft to be free-standing: they have to be laid as films over other supporting layers, making the overall window design more bulky and less efficient.
Xu’s team collaborated with Qiang Zhu, Head of the Advanced Characterisation and Instrumentation Department at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE), and IMRE colleagues to develop a new material that overcomes these limitations.
Changes in the molecular structure and optical qualities of (poly)alkyl acrylate polymers at different temperatures.
The material was discovered partly thanks to a lucky coincidence; while studying solid-solid phase-changing materials for heat storage in general, the researchers noticed a particular group of plastic-like polymers—poly(alkyl acrylates)—turned extremely clear when heated. “This prompted us to further look into their thermochromic properties, which turned out to be exceptional,” said Xu.
The researchers created poly(alkyl acrylates) through a clever bit of chemistry involving experiments with tiny molecular ‘arms’ that hold the polymers’ structures together. By adjusting the chemical environment and varying the number of arms—between two to four per molecule—they could finely control how the polymers responded to heat. This precise structural control was key to producing clear, robust and free-standing thermochromic materials.
“The specific crosslinkers used in this study are commonly employed with acrylate-type polymers,” Xu explained.
Using advanced imaging techniques, such as X-ray diffraction, the team confirmed how the materials changed at a microscopic level, ensuring they remained clear and stable through repeated heating and cooling.
“What particularly struck us was the significantly high optical contrast and clarity achievable when heated,” Xu noted. In their published report, the team reported that their polymers achieved an optical transparency of up to 99 percent, and a solar modulation ability of up to 87 percent. This matched and even outperformed some of the other promising materials in the literature, which attained around 70 to 99 percent and up to 70 percent in those two respective parameters.
Xu sees broad potential applications for their novel thermochromic polymers. Beyond windows, they could be used in greenhouse panels to help regulate internal temperatures or in thermoelectric devices to enhance energy efficiency. The team is currently working to enable a reverse transition with these polymers, improving their versatility as part of a new generation of adaptive materials.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Sustainability for Chemicals, Energy and Environment (A*STAR ISCE2) and A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
