Reversible Thermochromic and Fluorescent Poly(methyl Methacrylate) Nanocapsules for Wearable Devices, Thermal Energy Regulation, and High-Security Anticounterfeiting Inks

Encapsulated phase change materials have gained significant interest in thermal energy storage in recent years. Herein, novel thermochromic and fluorescent nanoencapsulated phase change materials were developed by coencapsulation of crystal violet lactone, bisphenol A, cetyl alcohol or 1-dodecanol, and hexadecane into poly(methyl methacrylate) (PMMA) shell cross-linked by a fluorescent coumarin cross-linker through miniemulsion polymerization. Different ternary thermochromic mixture to PMMA shell ratios were selected to elucidate their effect on the final properties of the dual thermochromic and fluorescent nanocapsules. Encapsulation of the core materials and the cross-linker structure were investigated by Fourier-transform infrared spectroscopy and proton nuclear magnetic resonance spectroscopy. The nanometric size, core-shell morphology, and relatively uniform particle size distribution of the nanocapsules were confirmed by field-emission scanning electron microscopy, transmission electron microscopy, and dynamic light scattering. Ultraviolet-visible diffuse reflectance spectroscopy confirmed the thermochromic properties and thermal fatigue resistance of the nanocapsules over 10 cooling-heating cycles, and fluorescence spectroscopy illustrated the fluorescence properties of the nanocapsules. Thermal properties and encapsulation efficiencies of the nanocapsules were measured by using differential scanning calorimetry. The thermal stability of the prepared nanocapsules was investigated by using thermogravimetric analysis. The sample with a 3:1 ratio of the encapsulated ternary thermochromic mixture to the PMMA shell containing 1-dodecanol was selected as an optimal sample for different applications due to its high thermochromic stability and color change rate in -16 to 26 °C. The optimized nanocapsules were used as anticounterfeiting inks in security documents and packaging to distinguish between original documents and products and their counterfeit counterparts. In addition, they were used to prepare thermal-energy-regulating windows and coatings for buildings. The windows can be used for temperature regulation in buildings and to embellish interior spaces in architectural design. The nanocapsules were also used in wearable devices that adjust the ambient temperature around the body by absorbing, storing, or releasing a significant amount of latent heat during the phase change process.