Inorganic salt hydrates are of interest as phase change materials (PCMs) because of their high latent heat of fusion, high volumetric energy storage density, enhanced thermal conductivity, and non-flammability compared to their organic counterparts. However, their low viscosity and flowability above their melting point leads to problems such as phase separation, leakage, and corrosion in thermal management applications. Our group previously demonstrated the ability of poly(vinyl alcohol) (PVA) to form thermo-responsive physical gels in the high salinity environment of salt hydrates due to dehydration of polymer chains. In addition, salogels formed from hydrogen bonding between PVA and polyamidoamine dendrimers were shown to shape stabilize lithium nitrate trihydrate (LNH). Here we expand our work to another PCM- calcium nitrate tetrahydrate (CNH), in which PVA formed strong gels containing &gt;95% PCM. ATR-FTIR spectroscopy was applied to reveal the gelation mechanism. The salogels were further strengthened by the dynamic covalent borate ester linkages between PVA and borax to prevent leakage of CNH above its melting point (42 °C). In contrast to the gels in water, PVA-borax salogels were found to be thermo-responsive and could be disassembled reversibly and repeatably allowing for easy filling and removal of PCM from a heat exchanger module. The gel-sol transition temperature, determined from rheology, was highly tunable over a wide temperature range (7-70 °C) and could be controlled by adjusting PVA molecular weight, degree of hydrolysis, borax amount, and pH. Thermal behavior over multiple melting/freezing cycles was found to be repeatable and bulk salogels showed shape stabilization of PCM above its melting point without leakage. The dynamic covalent bonding confers self-healing capability to the salogels, which was demonstrated using both rheology experiments and in bulk salogel samples.

Thermo-responsive salogels with dynamic covalent bonds for shape stabilization of an inorganic phase change material

ACS Spring 2022

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