According to a study published in Carbohydrate Polymers by Aalto researchers, pectin has a new use as a photothermal material. This opens the door for creating innovative, sustainable bio-based photothermal materials.
The representative infrared thermal images under/turning off irradiation for 2 mm PEG/PMMA composite and Pectin/PEG/PMMA composite (pectin concentration of 4.5wt%). Image Credit: Fangxin Zou and Yujiao Dong/Aalto University
For the study, a group of investigators resumed their investigation on pectin materials that began in 2020. Pectin is a biodegradable and abundant polymer derived from plant cell walls, particularly fruits. Previously, the researchers created an anisotropic structure in pectin cryogel, which implies that it has different structures in both vertical and horizontal directions. The substance is considered a matrix and is susceptible to infiltration attempts.
Imagine a sponge that has many pores in it – and we try to fill the pores with some material. 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.
Yujiao Dong, Researcher, Multifunctional Materials Design Group, Aalto University
To advance the research, the scientists used phase-change materials (PCMs) in their tests. PCMs are materials that can transition between two fundamental states of matter (solid and liquid) when heated or cooled. Polyethylene glycol (PEG), the phase-change material utilized for this study, is a widely used polymer whose molecular weight can be determined.
If the weight is low enough, the material can change phases even at room temperature; for example, it can be solid at normal temperature and become liquid when the temperature rises slightly.
“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 added.
One of the key conclusions of this study is that pectin can have photothermal capabilities, which means that it can absorb sunlight and turn it into heat. This allows the infiltrant PEG material to be somewhat heated within, causing a phase transition.
The researchers illustrate it in the study by placing a “window” beneath a solar simulator, which is a device that mimics sunlight. It is evident that the material undergoes a phase change and becomes transparent under mild heating.
The PEG phase-change material was included in the porous material as part of the study’s technique. One of the problems was that the scientists had to carry out the infiltration under a vacuum because of the porous material's small pore size.
They poured PEG into the pores and added the PMMA.
Yujiao stated, “When temperature goes up, PEG becomes liquid and can leak out of the pores, therefore, we use PMMA as a kind of encapsulation that helps to stabilize it and prevent PEG from leaking out of the porous material.”
He further added, “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 demonstrated in the article that they were able to repeatedly heat and cool this material, and the efficiency remained high even after 100 cycles.
Speaking about possible practical uses, Yujiao cited a prior study in which the authors showed the optical characteristics of its composite (with PMMA) and employed pectin as a material for sound absorption. Phase-change materials were used in the current investigation to examine the properties.
When the phase-change material is solid, the composite appears hazy or not transparent. It also has a relatively high optical haze value, which means that sunlight will not pass through it straight and could instead be dispersed.
“Moreover, pectin itself can absorb 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 elaborated.
Interestingly, pectin/PEG composites have the ability to store energy because of their photo-thermal characteristics, and if they were used as windows, the temperature inside a home would not fluctuate significantly.
“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,” Yujiao noted.
Considering how other researchers might expand on these findings, Yujiao believes that since the directional structure has been thoroughly examined, one approach might be to attempt to combine the material created in this work with other materials. However, the issue of how to scale up the finished product will always exist.
Yujiao stated, “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.”
The photo-thermal properties of pectin, which had not been previously documented, are one of the study’s primary findings. Yujiao thinks that these features could inspire researchers to identify new applications for pectin in a variety of sectors.
Journal References:
Dong, Y. et. al. (2024) Variable-transmittance bio-based phase change composites based on the photothermal property of pectin. Carbohydrate Polymers. doi.org/10.1016/j.carbpol.2024.122416
Zou, F. et. al. (2023) Maximizing sound absorption, thermal insulation, and mechanical strength of anisotropic pectin cryogels. Chemical Engineering Journal. doi.org/10.1016/j.cej.2023.142236
Zou, F. et. al. (2023) Optically transparent pectin/poly(methyl methacrylate) composite with thermal insulation and UV blocking properties based on anisotropic pectin cryogel. Chemical Engineering Journal. doi.org/10.1016/j.cej.2022.135738