Amplifying Colossal Thermal Expansion of a Martensitic Molecular Crystal through Interlayer Shear-Induced Side-Chain Liberation

Abstract

Molecular crystals capable of colossal thermal expansion (TE) are fascinating owing to their substantial and continuous volume changes and reasonably linear responses to temperature. This makes them promising candidates for micromachine applications. Macroscopic motion is driven by subtle yet cooperative movements of molecules that respond to the thermal motions of dynamic functional units. The study of p-TIPS-DSB presented here offers a compelling case highlighting the relationship between the degree of dynamicity of functional units and TE behavior. In its α-phase, the p-TIPS-DSB crystal undergoes an irreversible martensitic transition to the β-phase, accompanied by significant cooperative interlayer shear. This process substantially enhances the mobility of the side-chains driven by the increased free volume surrounding them. This nearly doubles the volumetric TE coefficient from 255.3 (10) to 444.9？(32)？MK？1, particularly in the actuation direction from 175.0？(7) to 291.7？(20)？MK？1, enabling about 4.5？% elongation/contraction. As demonstrated here, p-TIPS-DSB exhibits a decent force density (>1.4×107？N？m？3) and precise motion control capabilities due to its hysteresis-free and non-abrupt TE nature. Furthermore, we demonstrated the limited operating distance of colossal TE materials can be amplified by utilizing levers, highlighting the high potential of these materials for use in micromachines.