Reversible color-changing and thermal-energy storing nanocapsules of three-component thermochromic dyes

Abstract

Three component thermochromic dyes including fluoran type leuco dye, n-tetradecanol, and phenolphthaleinwere prepared and encapsulated in polymethylmethacrylate in this study. Nanoencapsulated system was prepared via oil in water emulsion polymerization technique. According to the morphological investigation by SEM, nanocapsules having spherical shape, compact morphology and lower average diameter around 200 nm were prepared successfully. UV–Visible Spectroscopy analysis and the photographs taken by a digital camera indicated that thermochromic systems and their nanocapsules exhibited reversible color change with thermal stimuli. They were green color at room temperature and lost their color at temperatures upon melting point of the thermochromic systems. In the study, 2,4-dihydroxybenzophenone and 2-hydroxy-4-methoxybenzophenone were added to the thermochromic systems to investigate UV resistance of the colorants in the systems. The effect of the UV absorbers on UV resistance of the thermochromic systems was investigated by color measurements based on CIELab system after UV aging of the nanoencapsulated systems. The results proved that especially 2,4-dihydroxybenzophenone was an effective material for increasing UV resistance of the systems and had color developer property. Besides, both thermochromic systems and their nanoencapsulated derivatives exhibited thermal energy storage/releasing property. While thermochromic systems had a heat storage capacity of over 200 J/g, the heat storage capacity of their nanocapsulated derivatives had reached up to 180 J/g. Melting temperature of the solvent n-tetradecanol decreased slightly in the thermochromic systems and further in nanoencapsulated form due to the cryoscopic effect of the non-volatile solutes in the system.