Ionic cross-linked polyvinyl alcohol tunes vitrification and cold-crystallization of sugar alcohol for long-term thermal energy storage

A new sustainable material for storing heat and releasing it on demand has been demonstrated for long-term latent heat storage (LLHS). The material consists of a high-latent-heat sugar alcohol phase change material (PCM) dispersed within ionic cross-linked matrices of polyvinyl alcohol (PVA). This material's unique property is the inhibition of undesired crystallization of the PCM during cooling due to the strong intermolecular interactions of the polymeric matrices, which leads to vitrification instead of crystallization. The release of latent heat can be controlled due to the PCM's stability below its cold-crystallization, which is triggered by reheating, as demonstrated by differential scanning calorimetry (DSC), optical microscopy (OM) and in situ X-ray diffraction (XRD). The addition of an ionic citrate cross-linker further tunes the vitrification and cold-crystallization properties of the PCM. Homogeneity and the presence of hydrogen bonding of the cold-crystalizing PCM (CC-PCM) were studied by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), Fourier transform infrared spectroscopy (FTIR) and XRD. Thermal stability was confirmed by thermogravimetric analysis (TGA) and 100 consecutive DSC heating-cooling cycles. The CC-PCM demonstrated high latent heat of fusion, up to 266 J g(-1), depending on the composition. As a super-adsorbent, PVA was able to swell and hold the liquid PCM resulting in form-stability and leakage-preventive properties above the melting temperature. Taken together, these results confirm that PVA matrices are promising for the thermal and structural stabilization of sugar alcohol PCMs, overcoming unexpected heat release and phase separation, and withstanding repeated melting-cooling cycles for LLHS.