Rajagopalan Kartik Kumar, Haney Sebastian, Shamberger Patrick J, Sukhishvili Svetlana A
Department of Materials Science & Engineering, Texas A&M University, College Station, Texas 77843, United States.
Department of Materials Science & Engineering, University of California, Berkeley, California 94720, United States.
ACS Appl Eng Mater. 2023 Dec 8;2(3):553-562. doi: 10.1021/acsaenm.3c00522. eCollection 2024 Mar 22.
One of the challenges preventing wide use of inorganic salt hydrate phase change materials (PCMs) is their low viscosity above their melting point, leading to leakage, phase segregation, and separation from heat exchanger surfaces in thermal management applications. The development of a broad strategy for using polymers that provide tunable, temperature-reversible shape stabilization of a variety of salt hydrates by using the lowest possible polymer concentrations is hindered by differences in solubility and gelation behavior of polymers with change in the type of ion. This work addressed the challenge of creating robust, temperature-responsive shape-stabilizing polymer gels (i.e., salogels) using a low cost PCM, calcium chloride hexahydrate (CaCl·6HO, CCH). Due to the extremely high (9 M) concentration of chloride ions and the tendency to salting-out polymer chains, the previous strategy of using single-polymer salogels was not successful. Thus, this work introduced a strategy of using two polymers, poly(vinyl alcohol) and ultrahigh molecular weight polyacrylamide (PVA and PAAm, respectively), along with borax as a cross-linker to achieve temperature-reversible, shape-stable salogels. This system resulted in robust salogels whose gel-to-sol transition temperature () was tunable within an application-relevant range of gelation temperature (30-80 °C). This behavior was enabled by a synergistic combination of dynamic covalent cross-links between PVA units and entanglements of PAAm chains which were combined into a single hybrid network. The hybrid salogels had <5 wt % polymer content, maintaining ∼95% of the heat of fusion of the pure PCM. Importantly, the noncovalent nature of gelation supported thermo-reversibility of gelation, shape stability, and retention of thermal properties over 50 melting/crystallization cycles.
阻碍无机盐水合物相变材料（PCM）广泛应用的挑战之一是其在熔点以上的低粘度，这会导致在热管理应用中出现泄漏、相分离以及与热交换器表面分离的问题。通过使用尽可能低的聚合物浓度来开发一种广泛的策略，以利用聚合物实现对多种水合盐的可调谐、温度可逆形状稳定，这受到聚合物在离子类型变化时溶解度和凝胶化行为差异的阻碍。这项工作解决了使用低成本PCM六水合氯化钙（CaCl·6HO，CCH）制备坚固的、温度响应性形状稳定聚合物凝胶（即盐凝胶）的挑战。由于氯离子浓度极高（9 M）且有使聚合物链盐析的趋势，之前使用单聚合物盐凝胶的策略并不成功。因此，这项工作引入了一种使用两种聚合物（分别为聚乙烯醇和超高分子量聚丙烯酰胺，即PVA和PAAm）以及硼砂作为交联剂来实现温度可逆、形状稳定盐凝胶的策略。该体系产生了坚固的盐凝胶，其凝胶 - 溶胶转变温度（）在与应用相关的凝胶化温度范围（30 - 80 °C）内是可调谐的。这种行为是由PVA单元之间的动态共价交联与PAAm链的缠结协同组合实现的，它们结合成一个单一的混合网络。混合盐凝胶的聚合物含量<5 wt%，保持了纯PCM约95%的熔化热。重要的是，凝胶化的非共价性质支持了凝胶化的热可逆性、形状稳定性以及在50个熔化/结晶循环中热性能的保持。
