Spirolactam and ESIPT in a Double-Network Elastomer: Multicolor Mechanochromism and Dual-Ratiometric Strain Sensing in Low and High Tensile Stress

Monitoring strain/stress in the bulk polymer material by mechanochromic mechanophores (MMs) with multimodal analyses can provide valuable information under loading. Herein, multicolor polymer mechanochromism from cyan to blue and further to purplish red and dual ratiometric strain sensing in low and high tensile stress were realized by incorporating noncovalent bond-type excited-state intramolecular proton transfer MM PhMz-4OH and covalent bond-type rhodamine MM Rh6G-2OH into the first network of a PU-PMA (polyurethane-polymethacrylate) double-network elastomer. In in situ fluorescence spectra during tensile tests, the normalized intensity of the enol emission increased linearly in low tensile stress (<275% or <3.49 MPa) and that of the rhodol emission increased linearly in high tensile stress (>275% or >3.49 MPa), which attributed to the higher activation force of spirolactam than that of PhMz-4OH disaggregation. The growth of R/G due to the ring opening of Rh6G-2OH in RGB analyses could also provide linear strain sensing under high stress. The solvent effect on polymer mechanochromism was further investigated. This strategy might provide a detailed tracking of the whole loading experience in elastomers.