Design Based on Polyethylene Glycol - New Full Solid-state Barocaloric Material

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What is Polyethylene Glycol?

Polyethylene glycol (PEG) is prepared through a stepwise addition reaction using ethylene oxide and water or ethylene glycol as raw materials. As the degree of polymerization increases, the physical appearance and properties of polyethylene glycol gradually change. From a colorless and odorless viscous liquid to a waxy solid, its hygroscopic capacity decreases accordingly. Polyethylene glycol has the chemical properties of alcohol and can react with fatty acids to form esters. Polyethylene glycol is soluble in water, ethanol and various organic solvents. Polyethylene glycol also has good hygroscopicity, adhesion and lubricity.

Polyethylene glycol has strong water absorption and can absorb moisture from the air at room temperature. The liquid can be miscible with water in any proportion. When the temperature rises, the solid polyethylene glycol can be miscible with water in any proportion. It is a nonionic polymer that is stable under normal conditions and can oxidize with oxygen in the air at 120°C or higher. When protected by inert gases such as carbon dioxide or nitrogen, it will not change at 200~240°C. When it rises to about 300°C, the molecular links will break and degrade.

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How Can Polyethylene Glycol be made into Full Solid-state Barocaloric Material?

The new refrigeration technology based on solid-state thermal effect has attracted attention due to its advantages of green environmental protection, high efficiency and energy conservation. However, solid-liquid phase change materials are prone to problems such as container leakage and phase separation, which inevitably cause corrosion damage to refrigeration devices and irreversible refrigerant performance, making them unsuitable for all solid state refrigeration applications. Therefore, designing new materials, exploring liquid phase amorphous solidification, and maintaining its high entropy properties are of great significance.

Researchers have solidified the liquid phase of polyethylene glycol with solid-liquid phase transition into an amorphous solid phase by introducing 5 wt.% polyethylene terephthalate. By utilizing the high entropy retained by the amorphous solid phase of PEG, a novel full solid-state barocaloric material has been designed and obtained.

Design Based on Polyethylene Glycol - New Full Solid-state Barocaloric Material

The elemental PEG molecule is composed of terminal hydroxyl (-OH) and flexible methylene (CH₂) repeating units. It has high rotation and vibration capabilities. Its solid-liquid phase transition process is accompanied by a huge entropy change (~543 J·kg^-1·K^-1); while PET molecules have rigid characteristics. The -OH at the end of the PEG molecular chain forms a hydrogen bond with the O atom on the rigid PET chain, and the free movement of the PEG molecular chain segments is fixed, allowing the liquid to solidify into an amorphous solid. Transmission electron microscopy (TEM) and temperature-variable X-ray diffraction (XRD) both confirmed that after curing, elemental PEG10000 (ie, PEG10000/PET15000) undergoes a solid-solid transition from an ordered crystalline state to a disordered amorphous state as the temperature increases. Moreover, the solid-solid phase change entropy change of PEG10000/PET15000 retains 83% of the solid-liquid phase change entropy change of simple PEG10000. Using variable temperature infrared spectroscopy (IR) combined with DFT calculations, it was revealed that the conformation of the PEG molecular chain changes dramatically from order to disorder, which is also the source of the huge thermal effect associated with the phase change process. At the same time, DFT calculations reasonably gave six conformations of PEG molecules in the liquid phase (i.e., TGT, TGG, GGG, TTT, GTG, and TTG), while there were only four conformations of PEG molecules after amorphous solidification (i.e., TGT, TGG, TTT and TTG). This is because the rotation of the amorphous solidified PEG molecular chain is more difficult than that in the liquid state, making it difficult to form the two conformations GGG and GTG. Therefore, these two conformations disappear when the PEG molecules are in the amorphous solid phase. This reduction in conformational order well explains the experimentally observed decrease in entropy of amorphous PEG10000/PET15000. And the solid-solid phase change process of PEG10000/PET15000 still retains huge entropy change and thermal effects.

The barocaloric effect of PEG10000/PET15000 was measured by variable pressure differential scanning calorimetry. The results showed that the solid-solid phase transition it experienced was highly sensitive to pressure. A small pressure of 0.1 GPa can achieve an entropy change of up to 416 J·kg^-1·K^-1, which exceeds that of most autoclave materials. This work provides a new idea for exploring new full solid-state barocaloric materials and provides a material basis for technology implementation.