Hydration-Modulated Glass Transition and Dynamics in Amorphous Porous Organic Cages

Hydration plays a crucial role in modulating the thermal and dynamic behavior of soft porous materials, yet its microscopic impact on molecular-scale nanoporous systems such as porous organic cages (POCs) remains poorly understood. In this work, we employ molecular dynamics simulations to systematically investigate the influence of water content on the glass transition, structure, and mobility of amorphous POC systems. Five model systems with H₂O:POC molar ratios ranging from 0 to 40 were constructed, and temperature-dependent simulations were carried out from 203 to 373 K. Our results show that an increase in hydration systematically depresses the glass transition temperature, with T_g decreasing from 325.43 K in the dry system to 279.18 K at a H₂O:POC ratio of 40, confirming the strong plasticizing effect of water. Structural analyses show that water initially expands partially collapsed POC assemblies at low hydration, but this effect is saturated at higher water contents because of limited adsorption capacity. Consistently, the number of POC-water hydrogen bonds per water molecule decreases with an increase in hydration, indicating weaker average water-framework affinity. Dynamic analyses reveal a clear crossover from strongly subdiffusive water motion at low hydration, with diffusion exponents (α) as low as ∼0.2, to near-Fickian diffusion at high hydration and increased temperatures, where α approaches 1. Together, these results establish a molecular-level picture of hydration-driven plasticization in amorphous POC systems and provide guidance for designing responsive nanoporous materials with tunable thermomechanical and transport properties.