Water insensitive and solvent-free synthesis of biodegradable solid-solid phase change materials based on poly (ethylene glycol) for thermal energy storage

Usually, polyethylene glycol (PEG)-based biodegradable solid-solid phase change materials (BSSPCMs) are synthesized with isocyanate and PEG, removing water is necessary due to the sensitivity to water of isocyanate groups. In this study, a water insensitive synthetic scheme based on carboxyl and aziridinyl was employed to prepare PEG-based BSSPCMs via solvent-free bulk polymerization: First, a carboxyl modified polyethylene glycol was synthesized through esterification reaction of citric acid and PEG with the molar ratio 2:1. Then, the BSSPCMs were prepared based on an self-curing and organic solvent-free process, using trimethylolpropane tris(1-aziridine propionate) as the cross-linking agent. The chemical structure, crystalline properties, phase change properties, thermal stability, and reliability were investigated by Fourier transform infrared spectroscopy, X-ray diffraction, polarizing optical microscopy, differential scanning calorimetry, thermogravimetric analysis, and accelerated thermal cycling testing, respectively. Solid-solid phase change behavior was testified by the leakage test. The biodegradation experiment was conducted to prove the biodegradability of solid-solid PCMs. The results indicated that BSSPCMs had a similar crystalline structure to PEG, and the crystal size was smaller than PEG. The phase change temperature of BSSPCMs was in the range of 25-65 degrees C, and the latent heat of phase change materials (PCMs) was about 103-108 J/g. Moreover, the extent of supercooling of BSSPCMs was reduced. Thermogravimetric analysis and accelerated thermal cycling test results confirmed the considerable thermal stability and reliability of BSSPCMs. The prepared PCMs also exhibited a good biodegradability in the natural environment.