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For the article recently published in Carbohydrate Polymers journal, a group of investigators continued their research work on pectin materials started in 2020. Pectin is a biodegradable and abundant polysaccharide extracted from the cell walls of plants, especially fruits. Previously, the researchers managed to fabricate an anisotropic structure which means that pectin cryogel has different structures in vertical and horizontal directions. The material is considered as a matrix and can undergo infiltration attempts.  

“Imagine a sponge that has many pores in it – and we try to fill the pores with some material,” says Yujiao Dong, a researcher from the Multifunctional Materials Design group who recently defended her doctoral thesis, “Our previous findings show that this directional porous material has great optical properties when it is filled with PMMA [Poly(methyl methacrylate)], and since we also confirmed that the material exhibits beneficial mechanical properties, we thought that maybe we could infiltrate it with other substances.”

To proceed with the research work, the scientists added phase-change materials (PCMs) into their experiments, i.e. materials that can demonstrate a transition between two fundamental states of matter (solid and liquid) when heated or cooled. Polyethylene glycol (PEG), a phase-change material chosen for this study, is a quite commonly used polymer, and its molecular weight can be selected. If the weight is low enough, the material can change its phase even at room temperature, for example, at room temperature it can be solid, and if the temperature slightly increases, it can become liquid.  

“The phase-change temperature of the material that we chose is not very high, approximately +33-35°C. Here we aim for the material to absorb heat from the sun and become liquid and transparent,” Yujiao clarifies.

One of the main findings of this work is showing that pectin can have photothermal properties which means that it can absorb the sunlight and convert it to heat, thus slightly heating the infiltrant PEG material inside, so that its phase change could take place. In the article, the researchers demonstrate it as a ‘window’ which they put under a solar simulator (device that simulates sunlight), and it is possible to see that the phase change happens and that the material gets transparent after it is heated a bit.

“Our ambition was for the material (or as we call it, ‘composite’) that we were working on to have great properties, which means that if we use more phase-change materials, the demonstrated results might be better, but in this case we would need to address the ‘leakage’ problem. Therefore, we had to balance between the properties and make sure that the composite can be reusable.” With the final solution, the researchers showed in the article that they were able to repeatedly heat this material up and cool it down, and the efficiency remained at a good level even after the 100-cycle test. 

Talking about potential real-world applications, Yujiao referred to the previous study where the authors used pectin as a sound absorption material and demonstrated the optical properties of its composite (with PMMA). In the current study, they tested the properties when using phase-change materials. When the phase-change material is in the solid state, the composite appears to be blurry, i.e. not transparent, and it has a rather high optical haze value, meaning that when the sunlight shines on it, it will not directly get through and can somehow be scattered. “Moreover, pectin itself can absorb the UV light (this was also reported in our previous papers), and to us it looks like a good window material. When the sun rises, temperature increases, the added phase-change material becomes transparent and the room can be lit up, so it’s like a ‘smart’ window,” Yujiao elaborates. 

Interestingly, due to their photo-thermal properties, pectin/PEG composites can actually store energy, and if used as a window, temperature inside a house would not change sharply. “In our study, we even built a miniature house to show that the temperature inside it will increase and decrease slower compared to conventional glass,” adds Yujiao.  

Reflecting on how other researchers could build on these findings, Yujiao thinks that one of the directions could be trying to combine the material fabricated in this study with other materials, since the directional structure has been studied quite extensively. However, there will always be a problem of how to scale up the final product: “The biggest sample we were able to make was about 5cm, in other words, not very big, and if we want to use the material in the real world, we need to solve the issue of producing bigger samples.” 

One of the main findings of this paper is pectin and its photo-thermal properties, as it had not been reported previously, and Yujiao hopes that maybe these properties could inspire scientists to look for new ways how pectin could be used in various fields. 

The findings were published in (Volume 343, 1 November 2024, 122416).

Previous publications of the research team: (Volume 462, 15 April 2023, 142236); (Volume 439, 1 July 2022, 135738).

Contact information: 
Dr. Yujiao Dong (yujiao.dong@outlook.com ; )