Molecular dynamics analysis of plastic crystals as solid-solid phase-change materials for thermal energy storage

Solid-solid phase-change materials (PCMs) represent a promising class of thermal energy storage materials, offering high energy density while eliminating leakage risks associated with solid-liquid PCMs. These materials undergo a transition from an ordered to a disordered crystalline structure upon exceeding the solid-solid phase transition temperature. Their intrinsic stability also facilitates the development of durable composite PCMs. However, the molecular mechanisms governing their thermophysical properties during solid-solid phase transitions remain poorly understood, limiting their broader application. This study employs molecular dynamics (MD) simulations to investigate the thermophysical behavior of plastic crystals - pentaerythritol, pentaglycerine, neopentyl glycol, and tris(hydroxymethyl)aminomethane - as promising solid-solid PCMs. The results reveal a linear decrease in density and a corresponding increase in specific volume with rising temperature, with abrupt volumetric changes occurring during phase transitions. These transitions exhibit latent heat values of up to 216 kJ/kg, in agreement with experimental data, highlighting their significant energy storage capacity. Molecular-level analyses show the critical role of hydrogen bonding networks in determining these properties. Specifically, during the transition from the low-temperature alpha phase to the high-temperature gamma phase, strong hydrogen bonds are replaced by weaker ones, as evidenced by radial distribution function analyses. This structural reorganization correlates with a significant decrease in thermal conductivity across the phase transition. These findings provide valuable insights into the relationship between molecular structure and thermophysical properties in solid-solid PCMs. By clarifying these interactions, this work guides the rational design of advanced thermal energy storage materials with optimized performance for engineering applications.