All-organic lead-free thermochromic and dielectric switchable epoxy microcomposites from singly incorporating leuco dye microcapsules for advanced encryption

In this study, an all-organic lead-free thermochromic and dielectric switchable epoxy microcomposite (TEP) is presented from singly incorporating leuco dye microcapsule materials (LDM) into the epoxy matrix. TEP can simultaneously adjust the color and permittivity in response to temperature between 30 degrees C and 70 degrees C. Moreover, the switchable dielectric behavior of TEP can be manipulated by LDMs content and voltage frequency. The switchable dielectric behavior of TEP permittivity is proposed to originate from the inner interfacial polarizations in LDMs triggered by the order-disorder phase transition. Before the phase transition, the molecules in LDMs are in ordered and compact arrays with restricted mobility, which can hardly undergo transportations and reorientations, leading to the weak interfacial polarization strength at the inner surfaces of LDMs and the low permittivity of TEP. After the phase transition, the molecules in LDMs are liberated with enhanced intermolecular distances and mobility, facilitating the inner interfacial polarization in LDMs and leading to the high permittivity of TEP. The correlation between the switchable dielectric behavior and the order-disorder phase transition is furtherly verified based on the Lichtenecker's mixing model. This study unlocks a novel function of switchable permittivity for the LDM materials and provides an innovative strategy for designing the thermochromic and dielectric switchable smart materials.