Phase Change Behaviorand Mechanical Transformationof Eutectic Hydrated-Salt Phase-Change Gels Tuned by Water

Hydrated-salt phase-change gels (PCGs), composed of polymer and hydrated-salt phase change materials (PCMs), are dynamic materials with switchable mechanical states mediated by the solid–liquid phase change behavior of PCMs. Water is an important factor affecting the phase change behavior of salt hydrates and expected to be used in tuning the nanomechanical performance of PCGs. In this work, eutectic MgCl₂·6H₂O–Mg(NO₃)₂·6H₂O PCMs (Mg-PCMs) and their corresponding phase-change gels (Mg-PCGs) were prepared. Effects of the water content on the phase change behavior and nanomechanical properties of Mg-PCMs and Mg-PCGs were investigated via differential scanning calorimetry (DSC) and atomic force microscopy (AFM). First of all, a precise eutectic mass percent range of 40.05% ≤ <i>x</i> ≤ 41.30% [<i>x</i>MgCl₂·6H₂O–(1 – <i>x</i>)Mg(NO₃)₂·6H₂O] was determined by DSC at a low ramp rate (0.2 °C/min). Extra (<i>n</i> > 6.000, 41.30% MgCl₂–58.70% Mg(NO₃)₂–<i>n</i>H₂O) or less (<i>n</i> < 6.000) water could lead to changes in phase transition behavior due to the formation of ice or tetrahydrate salt. A quantitative relationship between the characteristic phase change enthalpies and water contents was calibrated and fitted. With an increase in water content and the resulting changes in phase change behavior, the Young’s modulus of solid Mg-PCMs decreases from about 72.52 to 2.02 GPa. In addition, Mg-PCGs were prepared and could switched from a clear soft solid to a white rigid solid with up to 10⁴-times change in modulus (490 kPa vs 7.79 GPa), exhibiting excellent tensile and load-bearing properties. As the water content changes, upper and lower limits of the switchable modulus can be tuned in the ranges of 13.90–3.32 GPa (rigid) and 0.91–0.48 MPa (soft), respectively. In addition, Mg-PCGs show stable cycling performance after one hundred DSC tests. The temperature-responsive PCGs switching between two stable solids with upper and lower limits of modulus quantitatively tuned by water show great application prospects in bionic and automation fields.