Reversible thermochromic modulation for polydiacetylene derivatives with amide and aromatic groups targeting thermal monitor

Temperature monitoring is essential for many natural and engineering processes in our daily life. Reversible and irreversible temperature sensors for different applications or environments have been devised to accommodate these processes. In this study, two polydiacetylenes (PDA-1 and PDA-2) functionalized with amide groups are prepared by topochemical polymerization, which exhibit reversible and irreversible thermochromic behavior at different temperatures. The thermochromic properties of these PDAs are evaluated by UV–vis absorption spectrum. Both samples can undergo multi-step thermochromic processes, which turn red at 80 and 90 °C, respectively. Their thermal stabilities are characterized by thermogravimetric analyses, which demonstrate that both PDAs are sufficiently stable when used up to 200 °C. The changes in the unsaturated structure before and after heating the resulting PDAs are revealed by Raman spectroscopy. In-situ FTIR spectroscopy captures the dynamic changes of molecular bonds and conjugated structures under thermal stimulation. Compared with PDA-1, strong hydrogen-bonding networks and aromatic interactions in headgroup provide PDA-2 with the capability of recovering its initial molecular organization and average π-electron conjugation length upon removal of thermal stimuli. PDAs present aggregated sheet-like structure before and after heating upon 100 °C. XRD results reveal that the blue-to-red color transition originated from the slight increase of the d-spacing value of the macromolecular backbones. The CR value of PDA-2 reaches 0.65 at 100 °C, indicating superior color transfer performance. Once cooled to 20 °C, the CR value sharply drops back to its initial level, demonstrating that its thermochromic reversibility is quite high. In particular, PDA-2 presents a reversible thermochromic feature upon heating at 100 °C, making it an efficient and reusable temperature-sensing material.