Temperature- and creep-resistant Diels–Alder salogels for shape stabilization of salt hydrate phase change materials

Abstract

Shape stabilization of inorganic salt hydrate phase change materials (PCMs) above their melting point is required to overcome the leakage problem in the molten state. However, previous strategies in designing PCM-stabilizing polymer networks, e.g. salogels, resulted in a limited range of temperature stability (up to 50 °C) and thus could not be applied to PCMs with widely ranging melting temperatures. In this work, we introduce a polymer salogel system that was not only able to retain PCMs over a wide temperature range (up to 120 °C) but also provide robust mechanical stabilization allowing creation of leakage-free, creep-resistant thermal energy storage materials. This salogel system consisted of a polymer gel formed by the Diels–Alder (DA) reaction between furan-modified polyvinyl alcohol (PVA) and a bismaleimide crosslinker with an entrapped salt hydrate PCM – either lithium nitrate trihydrate (LiNO₃·3H₂O, LNH) or magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O, MgNH), which were chosen for their widely different melting temperatures of 29 and 89 °C, respectively. Rheological studies of DA salogels were compared side-by-side with the salogels stabilized by another type of dynamic covalent chemistry based on boronate ester bonds. Compared to the viscoelastic boronate ester salogels which showed poor and dramatically decreasing strain recovery with temperature in creep experiments, DA salogels exhibited high elasticity, high gel-to-sol transition temperature (T_gel > 120 °C) and superior creep resistance at elevated temperatures (≥80% strain recovery at T ≤ 95 °C). Moreover, the DA crosslinking preserved the temperature response of gelation above 120 °C, important for end-of-life material removal in thermal management applications. Finally, DA salogels demonstrated high retention of latent heat characteristics of neat PCMs due to the high (>90%) content of inorganic salt hydrates and preserved their thermal properties over 50 melting/crystallization cycles.