Rewritable Papers Based on Azobenzene-Containing Polymer Particles for Security Marking and Self-Indication

Rewritable papers based on switchable molecules have attracted much attention for different applications. The performance of hydrochromic rewritable papers is very important in terms of being environmentally friendly and nontoxic. In this study, azobenzene compounds with two different hydroxyl and carboxylic acid functionalities were doped on poly(methyl methacrylate) (PMMA) and poly(methyl methacrylate-co-glycidyl methacrylate) (PMMA-GMA) particles prepared by one-step emulsifier-free emulsion polymerization. Ultraviolet–visible (UV–vis) spectra of the AZO1 in CH3CN solution at 560 nm are more bathochromic than the absorption peak of AZO2 at 515 nm. By adding water to AZO1, a new peak at 410 nm is observed with a color change from purple to yellow, which is the result of the formation of the hydrochromic form. In AZO2, the hydrochromic absorption wavelength is 445 nm with a color change from pink to yellow/orange. The difference in absorption wavelengths of the two stable and hydrochromic states of AZO1 and AZO2 is 150 and 70 nm, respectively. The hydrochromic rewritable papers were prepared by physical incorporation of azobenzene-containing polymer particles on cellulosic papers. The samples containing PMMA particles go through the pattern with more clarity, more vivid color, faster return, and erasing. The sample containing hydroxyl-functional azobenzene is more suitable for rewritable papers due to the presence of donating exochromic group and providing more clear patterns from fingerprints, handwriting, and stamping. The rewritable papers showed applicability in 10 cycles of writing by water and changing the color from purple to yellow and back during the time and water evaporation. These rewritable papers were used in human sweat pore mapping applications, security marking of different documents, camouflage, and also indication of latent fingerprints, diapers wetting, and fever. Such hydrochromic rewritable papers are highly important in sustainability insights and also reducing the huge consumption of cellulosic papers produced from natural resources.