Empowering self-reporting polymer blends with orthogonal optical properties responsive in a broader force range

Self-reporting polymers, which can indicate damage with perceptible optical signals in a tailored force range, are useful as stress-sensitive sensors. We demonstrate a simple approach to realize this function by embedding two distinct mechanophores — rhodamine (Rh) and bis(adamantyl)-1,2-dioxetane (Ad), in polyurethane/polylactic acid blends. The deformed blends generate red coloration and red chemiluminescence. Such a unique dual-responsive behavior was evaluated by solid-state UV-vis spectroscopy, macroscopic tensile tests with in situ RGB and light intensity analyses, which supported a stress-correlated occurrence of the ring-opening of Rh, the scission of Ad and the fluorescence resonance energy transfer process between the respective mechanochemical species. Complementarity stemming from the difference in properties and manifestations of the two mechanophores is essential. That is, the more labile Rh allows shifting the appreciable optical changes to a much lower force threshold; the transient nature and high dynamic range of mechanochemiluminescence from Ad map in real time where and when many of the covalently incorporated dioxetane bonds break; besides, the disrupted yet non-scissile structure of Rh acts as a fluorescent acceptor to effectively harvest chemiluminescence from ruptured Ad. The current strategy is thus empowering multi-functional mechano-responsive polymers with greatly improved sensitivity and resolution for multimodal stress reporting.


General Experimental Details
Materials. Unless otherwise stated, all regents were purchased from commercial sources and used without further purification. All reactions were performed under argon atmosphere unless otherwise specified, and all glass wares were oven dried before use.
5,5′/7′-(2-Hydroxyethylenoxy)adamantylideneadamantane 1,2-dioxetane (Ad), 3'- Optomechanical testing. Tensile experiments were carried out on a TA Rheometrics, DHR-2 equipped with an Xpansion Instruments, SER3, extensional fixture. The two rotating drums of the fixture are colored black by permanent marker to eliminate reflecting light. The pco.edge 5.5 camera equipped with a Nikon AF NIKKOR 50 mm 1:1.4D lens was used to record videos in darkness ( Figure S15b). All the videos were recorded in the rolling shutter color mode with a shooting rate of 200 fps and exposure time of 5.00 ms.
The frames of the resulting video were exported as separate monochrome TIF-files and light intensity was analyzed with a homemade program in MATLAB as literature. S3 The total intensity for a dark image as the noisy signal was subtracted from all film intensities.
The dimension of the strips used for tensile tests was 30 × 5.3 × (0.20 ± 0.03) mm.

RGB color analysis
The RGB value was obtained by analyzing images in MATLAB software. RGB ratios S5 of each image was calculated using Grassmann's law.
= ; = ; = r, g, and b are the red, green and blue component of the RGB ratio, respectively. R, G and B are the average intensity of red, green and blue channels in the region of interest.
The dibutyltin dilaurate (17 μL Figure S1). The chemical compositions of the Rh-Ad-PU were determined by 1 H NMR ( Figure S2). Rh and Ad were successfully coupled into polyurethane chains in a covalent way, as confirmed by the appearance of characteristic peaks at 8.00 and 2.82 ppm, respectively.
PLLA was prepared via one-step ring-open polymerization. L-lactide (1.66 g, 11.5 mmol) and 1,4-butanediol (45.06 mg, 0.5 mmol) were added into a two-necked flask under argon atmosphere. After heating to 125 ℃, the mixture was melted. Then, Sn(Oct)2 (13.98 mg, 0.035 mmol) with tetrafluoride tablets was added and the reaction was kept at 160 ℃ for 2 hours. The reaction mixture was cooled down to room temperature, followed by adding dichloromethane. The mixture was then deposited in cold methanol. The precipitate was collected and dried in vacuo at room temperature to afford PLLA as a white solid powder (1.0 g). The molecular weight of PLLA (Mn = 4.3 kDa) was characterized by 1 H-NMR as shown in Figure S5 and calculated through formula. S4 = + 1 * 72 * 2 + 90 (1) Where I1 and I2 are the areas of peaks 1 and 2, respectively, 72 is the molar mass of one lactic acid repeat unit, and 90 is the total molar mass of the rest part of the molecule.
General preparation method of PU/PLLA blend polymer films.
The as-prepared Rh-Ad-PU, Rh-PU, Ad-PU or L-Blank-PU respectively, was added to tetrahydrofuran (THF) and stirred until a uniform solution was obtained. Meanwhile, different amount of PLLA (Table S1) were added into THF respectively and stirred until the powders were fully dissolved. Then, the two polymer solutions were mixed together and stirred for 3 hours before poured into a Teflon mould (50 × 30 × 10 mm). THF was S8 evaporated under ambient conditions followed by vacuum drying of the films at room temperature for 12 hours. The removal of THF was confirmed by DSC analyses ( Figure   S9). Table   Table S1. Feed Ratios of Polymer Films in This Study.