Hydrogen-Bond Density Drives the Order-Disorder Phase Transition from Orientational to Vibrational in Polyalcohol Plastic Crystals

Abstract

Neopentane-derived plastic crystals undergo order-disorder phase transitions accompanied by exceptionally large entropy changes, making them attractive solid-state materials for thermal energy storage and barocaloric cooling. Yet it remains unclear whether the microscopic nature of this entropy change is an intrinsic property of each compound or can be understood generally and tuned systematically through molecular design. Here, we use classical molecular dynamics simulations of neopentyl glycol (NPG), pentaglycerine (PG), and pentaerythritol (PE), validated against experimental transition temperatures, enthalpy changes, and phase diagrams, to show that the character of the phase-transition entropy undergoes a complete inversion as the hydrogen-bond connectivity of the molecule increases: orientational disordering dominates in NPG, whereas lattice vibrations dominate in PE and become remarkably robust against pressure. PG occupies a balanced intermediate regime in which the orientational, conformational, and vibrational channels contribute comparably. This inversion reflects two coupled microscopic mechanisms: a growing low-frequency gap between the phonon densities of states of the ordered and disordered phases as the hydrogen-bond network densifies, and an increase in molecular symmetry that progressively suppresses orientational disordering. Because the vibrational channel is substantially less sensitive to pressure than the orientational one, hydrogen-bond density emerges as a practical, chemically accessible design handle for tailoring not only the magnitude but also the pressure robustness of the caloric response in hydrogen-bonded plastic crystals, with direct implications for the search for new barocaloric and thermal-energy-storage materials.