Thermal-Induced Chromism and Fluorescence Emission of Rhodamine B-Containing Polymers with Different Polarities: Tracking Temperature and Development of Thermochromic Labels

Thermochromic polymers, as a highly applicable intelligent category of innovative materials, are capable of changing their color with temperature variations. Different characteristics of the matrix and specially its polarity can be influential factors in the responsivity of thermochromic polymers. In this study, polystyrene, poly(methyl methacrylate), and poly(dimethylaminoethyl methacrylate) with different polarities were synthesized via reverse atom transfer radical polymerization. Thermochromic polymers were prepared by physical incorporation of the N-(rhodamine-B)lactam-ethylenediamine leuco dye and HCl color developer into the three polymers. The effect of polarity of the polymers on their thermochromism and fluorescence properties was investigated. The thermochromic polymers changed their color from colorless to a colored state with increasing temperature due to the intermolecular proton transfer in the system. UV–vis and fluorescence spectroscopies were used to evaluate the thermochromism and fluorescence properties of the polymer systems, respectively, at different temperatures. The intensity of absorption and emission of the thermochromic polymers increases with increasing temperature, where increasing polarity of the polymer matrix has an opposite effect on the intensity of absorption and emission. The change in color with temperature variation was detectable with the naked eye for the three polymer samples. In addition, investigation of the fatigue resistance and also response and recovery times of the samples showed temperature-sensing ability of the samples. These results showed that all samples have high responsivity and coloration for at least 10 heating–cooling cycles. Biocompatible thermochromic cellulosic papers were also fabricated, which showed irreversible coloration at different temperatures due to the removing solvent. Chromic labels with the ability of irreversible thermal-induced chromism was used as preservation taggants, which indicate that the food materials were kept at the right or wrong temperature. Therefore, they have a high potential for applications in various fields, such as temperature sensors, condition preservation taggant or labels for food or cosmetic, and anticounterfeiting inks.