Comb-shaped spiropyran-functionalized polyurethane coatings for controllable color-changing applications

Abstract

Photochromic polymers convert optical inputs into readable changes in color or absorbance, making them highly attractive for information security, visual sensing, rewritable displays, and wearable warning devices. In this study, a series of side-chain spiropyran-bearing polyurethanes (SPBPUs) were synthesized by incorporating a difunctional spiropyran-containing diol (SP-OH) into a poly (tetramethylene ether glycol) (PTMG)-based polyurethane backbone. A series of side-chain spiropyran-functionalized polyurethanes was prepared using a difunctional SP-containing diol. The films showed UV-induced coloration and thermally accelerated recovery, with MC-associated absorption at approximately 560–570 nm. SPBPU-3 reached a tensile strength of 12.39 MPa and retained 95.7% of its colored-state absorbance and 91.1% of its switching contrast after ten UV/thermal cycles, compared with 38.7% contrast retention for the physically blended SP/PU control. Room-temperature observations confirmed spontaneous dark recovery at 25 °C and a longer visual color-persistence window for SPBPU-3. Carbonyl-band analysis was consistent with formulation-dependent association, although it did not establish a quantitative hydrogen-bond–kinetics relationship. The time-dependent color response enabled textile authentication, fixed-time visual UV indication, and reversible transmittance modulation on lens substrates. These applications should therefore be regarded as laboratory demonstrations. Direct leachate analysis, normalized time-resolved recovery measurements, calibrated exposure-response testing, and standardized coating durability assessments remain necessary. The present results establish the reversible optical behavior and its practical boundaries while leaving detailed segmental dynamics and nanoscale organization for further study. These results support side-chain covalent functionalization as a route to reusable photochromic coatings for dynamic authentication and visual UV indication, while solvent sensitivity and wear-related response losses identify limitations that require further optimization.