Temperature-Responsive Poly(Vinyl Alcohol)-Borax Salogels for Shape Stabilization of an Inorganic Phase Change Material

Abstract

A temperature-responsive gel has been introduced to achieve shape stabilization of phase change materials (PCMs). Inorganic salt hydrates are promising PCMs, but their low viscosity at temperatures above their melting point leads to leakage and corrosion in thermal storage modules. Temperature-responsive salogels can trap inorganic salt hydrates to achieve shape stabilization when the temperature is lower than the gel-sol transition temperature (Tgel) to overcome the leakage problem. Increasing the temperature above Tgel transforms the system to the liquid state enabling easy removal of the PCM material from the thermal storage units. This work focuses on the salogels system consisting of poly(vinyl alcohol) (PVA) dissolved in calcium nitrate tetrahydrate (CNH). Through variations in molecular weight, degree of hydrolysis, and concentration of PVA, precise control of Tgel of the salogel was achieved. Importantly, at the matched PVA concentrations, gelation occurred in CNH but not in water. This difference is rationalized by the unique features of CNH as a solvent, such as an extremely high salt content and scarcity of water. Interactions of PVA and solvent were investigated using attenuated total reflection Fourier Transform Infrared spectroscopy (ATR-FTIR), and a suggested mechanism of gelation of PVA in CNH is discussed. To further enhance the salogel strength and improve control over the gelation temperature, borax as a crosslinker that forms dynamic covalent bonds with PVA was introduced. PVA/borax/CNH salogels could achieve Tgel as high as 70°C using 3 wt% PVA and as low as ~0.3 wt% concentration of borax. The crosslinking mechanism of borax and PVA in CNH involves formation of dynamic covalent borate ester bonds, which lead to salogel strengthening. Compared to traditional PVA-borax hydrogels in water, PVA-borax salogels in CNH showed the unique feature of highly repeatable temperature reversibility. In addition, the reported salogels not only provide shape stabilization of CNH but also exhibit robust self-healing properties.