Diselenide as a Dual Functional Mechanophore Capable of Stress Self-Reporting and Self-Strengthening in Polyurethane Elastomers

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE20 May 2022Diselenide as a Dual Functional Mechanophore Capable of Stress Self-Reporting and Self-Strengthening in Polyurethane Elastomers Xiaopei Li, Fan Yang, Yiran Li, Cheng Liu, Peng Zhao, Yi Cao, Huaping Xu and Yulan Chen Xiaopei Li Department of Chemistry, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 Google Scholar More articles by this author , Fan Yang Department of Chemistry, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 Google Scholar More articles by this author , Yiran Li Department of Physics, Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructure, Nanjing University, Nanjing, Jiangsu 210093 Google Scholar More articles by this author , Cheng Liu Department of Chemistry, Key Laboratory of Organic Optoelectronics and Molecular Engineering, Tsinghua University, Beijing 100084 Google Scholar More articles by this author , Peng Zhao Department of Chemistry, Key Laboratory of Organic Optoelectronics and Molecular Engineering, Tsinghua University, Beijing 100084 Google Scholar More articles by this author , Yi Cao Department of Physics, Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructure, Nanjing University, Nanjing, Jiangsu 210093 Google Scholar More articles by this author , Huaping Xu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Key Laboratory of Organic Optoelectronics and Molecular Engineering, Tsinghua University, Beijing 100084 Google Scholar More articles by this author and Yulan Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Chemistry, State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202201874 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Unlike biological materials that can sense mechanical force and actively remodel locally, synthetic polymers typically break down under stress. Molecular-level responses to damage with both stress-reporting and self-strengthening functions are significant yet difficult to realize for synthetic polymers. To overcome this challenge, chemo-mechanical coupling into polymers that can simultaneously ameliorate mechanical, optical, or other functional properties of a polymer combined with mechanical treatment will offer a new principle for materials design. Here, we report a kind of elastomer in which destructive forces are channelled into productive and bond-forming reactions by using diselenide (Se–Se) as a mechanophore. Polyurethane has been functionalized with labile Se–Se bonds, whose mechanical activation generates seleno radicals that trigger radical transfer and cross-linking reactions in situ. These reactions are activated efficiently in a mechanical way by compression in bulk materials. The resulting covalent networks possess turn-on mechano-fluorescence and increased moduli, which provide the functions of stress reporting, mechano-healing, and mechano-remodeling for the deformed film. This study not only illuminates the mechano-responsive nature of Se–Se bonds in the bulk state but also paves the way for the development of new stress-responsive materials. Download figure Download PowerPoint Introduction Early in the 1930s, Staudinger and coworkers observed the molecular response of synthetic polymeric materials to applied mechanical force,1–4 which was later on illuminated as the mechano-degradation via hemolytic scission of C–C covalent bonds along the polymer backbone. This historical knowledge laid the foundation of polymer mechano-chemistry, yet at the same time, it supported the bias that mechanical force was destructive and associated primarily with polymer degradation and mechanical failure.5–7 In recent years, such a view of mechano-chemical reactions has been redirected, benefiting enormously from the success in design and synthesis of versatile mechanically activated functional groups (“mechanophores”). Up to now, the productive use of mechanical force in polymer science has included mechanically induced color/fluorescence change,8–14 mechano-chemiluminescence,15–19 mechanical release of small molecules,20–24 mechano-catalysts,25–27 and so on. These advances have set the stage for the development of stress-reporting and self-repairing materials using mechano-responsive polymers. Elastomers are the functional material of choice for organic electronics, biological scaffolds, sensors, and so on. In such scenarios, developing durable elastomers that can sensitively detect mechanical damage and at the same strengthen their mechanical properties under mechanical loading is of fundamental importance, yet remains a challenge. For this purpose, stress must activate a bond-forming reaction, for example, creating a new cross-linking prior to, or immediately following and outpacing chain scission with detectable output.28 Pioneering work on mechanochemical cross-linking was reported by Craig et al.,29 based on the ultrasound-induced ring-opening reaction of gem-dibromocyclopropane and subsequent nucleophilic substitution with a bifunctional carboxylate. Later on, a progression on constructive bond formation, particularly shifting from solution to polymer blends and gels, was rapidly achieved by Sijbesma, Gong and others.27,30,31 Very recently, Otsuka et al. developed mechano-responsive polyurethanes with the radical generated from difluorenylsuccinonitrile as the mechanophore, which exhibited mechano-chromic and self-strengthening functions.32 Up to now, mechanophores that are powerful enough for both stress-reporting and self-strengthening have been limited. More labile, dual-functional mechanophores applicable in the bulk state are highly desirable. The diselenide bond (Se–Se) is an important dynamic covalent bond that can undergo the metathesis reaction with the generation of seleno radicals under stimuli such as pH, temperature, light, and so on.33–40 For a long time, small organic selenium compounds have been well known as useful reagents in free radical reactions.41–43 Recently, we also demonstrated that in the solution state, the diselenide-centered polymers were mechano-responsive. Both osmotic pressure and shear force under sonication could trigger the breakage of the Se–Se bond to generate polymeric seleno radicals.44,45 Given the relatively low bond energy of Se–Se bond (172 kJ mol−1) and the high reaction efficiency of many seleno radicals,46 in this contribution, we explored the mechano-chemical behaviours of the Se–Se bond containing polymers and the resulting seleno radicals in the bulk state. A new type of mechano-sensitive linear and cross-linked polyurethanes that contained Se–Se moieties in the main chain and methyl methacryloyl groups in the side chains were prepared. These elastomers were able to damage-report and self-strengthen synergistically under mechanical loading. The mechanical nature of the dual functions was uncovered because it was based on the radical transfer reaction and cross-linking of methyl methacrylate monomers initiated by the mechanically dissociated Se radicals (Scheme 1). These findings can promote the practical usage of Se–Se containing elastomers as structural materials. Scheme 1 | Schematic illustration of the mechanical nature of damage-reporting and self-strengthening functions in this work. Download figure Download PowerPoint Experimental Methods General All solvents and reagents were purchased from Sigma-Aldrich (St. Louis, MO, United States), TCI (Tokyo, Japan) , or Adamas (Shanghai, China) and used without further purification, unless otherwise noted. Di-(1-hydroxylundecyl) diselenide (DSe-diol) was synthesized according to previously published methods,1 and 5,6-dihydroxyhexyl methacrylate (DHMA) was also synthesized according to previously published methods.2,3 1,4-Butanediol (BDO) was purified by dehydration over anhydrous magnesium sulfate and by distillation under reduced pressure. Polytetramethylene glycol (PTMG: Mn = 1000 g/mol) was dried at 80 °C under vacuum for 2 h before use. Dimethylformamide (DMF) was dried with CaH2 and purified by vacuum distillation, then stored with 4A molecular sieves. All reactions were performed under an argon atmosphere unless otherwise specified, and all glassware was oven-dried before use. Representative polymerize procedure Taking PU-1 for example: A solution of DSe-diol (127 mg, 0.253 mmol), methylenediphenyl diisocyanate (MDI) (930 mg, 3.720 mmol) and dibutyltin dilaurate (5 μL) in anhydrous DMF (10 mL) was stirred at 35 °C for 1 h. Dry PTMG (Mn = 1000 g/mol, 510 mg, 0.510 mmol) and DHMA (209 mg, 1.035 mmol) in anhydrous DMF (10 mL) was added to this mixture and further stirred at room temperature for 16 h. Then, BDO (175 mg, 1.940 mmol) was added to the mixture under N2, and the mixture was stirred for 24 h at room temperature. The reaction was stopped by adding 0.5 mL of methanol. The crude product was purified by precipitation in methanol three times, washing with hexane and drying in vacuo. The product was dissolved in tetrahydrofuran (THF), and the solution was cast to give PU-1 film. Compression test Films were compressed by using Powder press PC-15 (Tianguang, Tianjin, China). All processing was done at room temperature in ambient air conditions. Rheometry The rheological properties were measured using an oscillatory rheometer (TA Rheometrics, DHR-2; TA Instruments, United States) equipped with an 8 mm parallel plate-plate geometry. Prior to the experiments, ca. 800–2000 μm thick film samples were prepared, and each film sample was placed between the parallel plates. Storage moduli were determined at 10 Hz. Electron paramagnetic resonance study 3 mm × 3 mm films of PU-1, PU-2, PU-3, and PU-4 were prepared. Each film was compressed by a Powder press PC-15. The compressed samples were transferred into an electron paramagnetic resonance (EPR) glass capillary without degassing. EPR measurements were carried out on a Bruker EMXPLUS Spectrometer (Karlsruhe, Germany). The spectra of compressed samples were measured using a microwave power of 2 mW and a field modulation of 0.1 mT with a time constant of 0.03 s and a sweep rate of 0.375 mT/s at 25 °C. Since a Mn marker was not used because it overlapped with spectra originated from methacryloyl radicals, the exact g value could not be calculated. Fluorescent spectroscopy The measurements were carried out using before and after compression of each polyurethane (PU) film. The excitation wavelength at 365 nm was selected, and the fluorescence emission peaks were observed at 570 and 605 nm. Atomic force microscopy study Atomic force microscopy (AFM) imaging experiments were carried out on a commercial AFM (JPK Nanowizard IV, Berlin, Germany) in quantitative imaging (QI) mode. Before the AFM test, the compressed part and uncompressed part of the sample were cut off and glued on with a clean glass slide via epoxy resin. The silicon AFM probe (Olympus, AC-160, Japan, stiffness coefficient 40 N/m, probe tip radius 5 nm) was used to randomly select three regions (the resolution of each region was 128*128). The image data and Young’s modulus were analyzed using JPK (JPK Instruments AG, German) data processing software. Results and Discussion Design and preparation of diselenide containing elastomer First, the reactivity of selenium radicals toward acrylate monomers was screened. According to a series of control experiments in solution triggered by photoirradiation, we found, among different diselenide derivatives, that an alkyl group-modified seleno radical generated from DSe-diol possessed good reactivity47 and was an ideal candidate as the initiator for radical polymerization of methyl methacrylate (experimental details can be found in the supplemental experimental procedures, Supporting Information Figure S1). Based on this knowledge, we then developed a kind of segmented PU elastomer (PU-1, Mn = 18.7 kDa) with bis-undecyl-substituted diselenides (DSe-diol) incorporated into the main chain and polymerizable side chains (Figure 1). The Se–Se moieties could act as the latent initiator which then afforded selenol radicals upon mechanical stimuli and initiated radical polymerization. Besides, the methyl methacryloyl units were fixed as the side groups with the expectation that an efficient cross-linking reaction could be triggered, leading to a remarkable change in the physical properties of the material. Experimentally, DSe-diol, DHMA, PTMG-1000 (Mn = 1000 g/mol), and methylenediphenyl diisocyanate (MDI) reacted first. The subsequent formation of a hard segment was achieved by adding the chain extender BDO to the prepolymer solution to afford PU-1 (experimental details can be found in the supplemental experimental procedures, Supporting Information Table S1). Bulk films were then prepared via solution-casting in Teflon molds and subsequent drying for 1 day in vacuo. Figure 1 | (a) Monomers used to synthesize the segmented polyurethanes. (b) One possible sequence of blocks in linear PU-1 and cross-linked PU-6. Download figure Download PowerPoint Mechano-activation of elastomers with self-reporting and self-strengthening characters Compression tests were conducted on the PU-1 film to investigate its mechanically responsive activity. After imposition of ca. 400 MPa pressure for 5 min, the compressed films were soaked in THF for 1 day. As shown in Figure 2a, the soluble pristine linear PU turned into an insoluble film. Later, more quantitative analyses of the mechanical properties before and after compression were performed. As shown in Figures 2a and 2b, with the increase of compression cycles, both the gel fraction (see supplemental experimental details) and storage moduli (Gʹ) increased gradually, indicating that the mechanical properties of the resulting films were dependent on the magnitude of the exerted mechanical force. After 15 cycles, ca. 4.2 folds of the enhancement of Gʹ with ca. 10% gel fraction were achieved. These results clearly demonstrated that the cross-linking reaction took place and could effectively strengthen the Se–Se bond-containing elastomers. Figure 2 | (a) Gel fraction and (b) storage modulus as a function of compression cycles of the compressed PU-1 fil (Data represent average and standard deviation from three parallel experiments for each sample. Insert: Images of PU-1 film before and after compression and subsequent soaking in THF.) (c) Fluorescence spectra of PU-1 film before and after compression (Insert: Images of the corresponding films under 365 nm UV light). (d) Images under 365 nm UV light of the PU-1 film before and after compression using a TJU-shaped metal mold. (e) AFM Young’s modulus histograms and (f) two-dimensional distribution maps of Young’s modulus for uncompressed part and compressed part of PU-1. Download figure Download PowerPoint Apart from its mechanical property, the optical property of the film was also changed after compression. The as-prepared PU-1 film was almost nonfluorescent, whereas after compression, it exhibited orange fluorescence under 365 nm UV irradiation (Figure 2c). Such turn-on fluorescence was attractive, since the stressed region could be vividly observed. Not only at the macroscopic scale, the jointly improved mechanical and optical properties were sensitive enough to be detected in microregions. As illustrated in Figure 2d, we stamped a designed mould on the film by hand, and under UV irradiation, the corresponding orange pattern (letters of “TJU”) was observed. Namely, mechanically self-reporting to ‘at-risk’ regions was possible. We then analyzed the patterned film using AFM quantitative imaging (QI mode), which showed that the Young’s modulus of the sample presented a bimodal distribution. According to Figure 2e, the compressed part was stiffer with a larger Young’s modulus (the proportion of the second peak increased), which was different from that of the uncompressed area. The two-dimensional distribution maps of the Young’s modulus (Figure 2f) for the uncompressed and compressed parts also illustrated the same results. Mechanistic study EPR studies of the PU-1 film revealed the presence of multiple types of radicals after compression. As shown in Figure 3a, in contrast to the pristine film that was EPR silent, the selenium radical, the radical derived from methyl methacrylate48–50 and the diphenylmethyl radical51,52 were distinctly detected in the compressed film. A similar phenomenon has been reported by Flinn, Otsuka, and others.32,52 They all found that the diphenylmethyl structure had a great tendency to afford fluorescent radicals via radical transfer reactions, for example, with the benzylic hydrogen radicals extracted from the diphenylmethyl units. In the present case, owing to the appreciable lability of the Se–Se bond, PU-1 experienced mechano-chemical transition primarily initiated by the Se-centered radical mechanism. And the presence of methacrylate and diphenylmethyl radicals indicated that the cross-linking reaction proceeded smoothly through radical propagation and the transfer steps, respectively. Furthermore, it was found that the fluorescence intensity decreased gradually and was quenched completely after about 3 h. Such observation was most likely attributed to radical quenching over time. According to these analyses, we inferred the plausible mechanism as mechanical stress-induced generation of selenol radicals to initiate the cross-linking reaction of methacrylate groups, accompanied with turn-on fluorescence from the diphenylmethyl radical formed via a radical transfer pathway (Figure 3b). Figure 3 | (a) EPR spectrum of the compressed film of PU-1 (g = 2.00295). (b) Schematic illustration of the mechanism of force-induced fluorescent radical transfer and cross-linking reactions with Se–Se as the mechanophore. Download figure Download PowerPoint To shed more light on the mechanical nature of these fluorescent and self-strengthening functions, a series of control polymer films (PU-2, PU-3, PU-4, PU-5, Supporting Information Figure S2) were prepared. As summarized in Table 1, they were PU-1 analogs polymerized by changing one of the monomers, for example, using dicyclohexylmethane-4,4ʹ-diisocyanate instead of MDI (PU-2), or BDO instead of DSe-diol (PU-3), or PTMG instead of DHMA (PU-4). And PU-5 was the physically mixed film. The mechano-responsive behaviours of the four samples were different from PU-1 (Figure 4a). In particular, the force-induced cross-linking reaction took place in the compressed PU-2 film (Figure 4b). The storage modulus and gel fraction of PU-2 increased alongside the compression cycles (Figure 4c and Supporting Information Figure S3), suggesting the resemblance of its self-strengthening ability to PU-1 with increased cross-linking density under force. As for PU-3, PU-4, and the physically mixed film PU-5, no significant changes in their mechanical properties were observed. Except for PU-4, all the other deformed films were nonfluorescent, incapable of damage reporting (Figures 4d–4g and Table 1). Supporting Information Figure S4 shows the EPR spectra of these compressed control films. The signals were different from that of PU-1, exhibiting either only two of the three types of radicals or being free of radicals. Table 1 | Key Components of the PU Samples and Their Ability to Undergo Force-Induced Cross-Linking and Fluorescence Samples PU-1 PU-2 PU-3 PU-4 PU-5 Cross-linking Yes Yes No No No Fluorescence Yes No No Yes No Components PU-3 Figure 4 | (a) Schematic illustration of possibly occurring force-induced fluorescent radical transfer and/or cross-linking reactions for control polymers PU-2, PU-3, and PU-4. (b) Images of PU-2, PU-3, PU-4, and PU-5 films before and after compression and subsequent soak in THF. (c) The storage modulus of controlled PUs before and after compression. Fluorescence spectra of (d) PU-2, (e) PU-3, (f) PU-4, and (g) PU-5 before and after compression (Insert: Images of the corresponding films under 365 nm UV light). Download figure Download PowerPoint Valuable information was obtained from these control experiments: (1) Se–Se, methyl methacrylate units, and their covalent linking into polymer chains were essential factors for cross-linking reactions; (2) the covalently attached MDI moieties were another necessity for mechano-fluorescence; and (3) the inactivity of the cross-linking reaction and fluorescence for the physical counterpart on the other hand showed that the self-strengthening and self-reporting functions for PU-1 were indeed triggered by mechanical force. Overall, results from these control experiments were in good accord with the mechanism proposed in Figures 3b and 4a, where Se–Se, MDI, and methyl methacrylate units incorporated in polymer chains served as the latent initiator, stress-reporter, and monomer for self-reporting and self-strengthening abilities, respectively. Mechano-healing and mechano-remodeling of fractured elastomers More practically, this selenol radical-involved cross-linking system was then demonstrated as sensitive and powerful enough for mechano-healing and mechano-remodeling of damaged PU samples. As illustrated in Figure 5a, the broken linear polymer PU-1 was healed into a uniform film after mild compression. The resulting film was insoluble due to the stress-induced cross-linking reactions. Notably, even for cross-linked PU networks that were in principle difficult to be processed, remodeling worked well under mechanical treatment. To demonstrate, cross-linked PU-6 containing DSe-diol and DHMA in the polymer chains and triethanolamine as the cross-linker was synthesized (Figure 1). When PU-6 was first cut into small pieces followed by compression, an intact cross-linked film could be afforded (Figure 5b). The shape of the reformed film could be remolded with the aid of different shapes and sizes of metal molds (Figure 5c). Meanwhile, the compressed region exhibited orange fluorescence. In this sense, the destructive effect of mechanical force on bulk films can effectively be shifted to productive dual functions. Figure 5 | Images showing (a) mechano-healing process of PU-1; (b and c) mechano-remodeling process of PU-6 into different shapes. Download figure Download PowerPoint Conclusion To conclude, we have developed a new kind of Se–Se containing elastomers with both stress-reporting and self-strengthening characteristics. For the first time, the mechano-chemical activation of the Se–Se unit was realized in the bulk state. Benefiting from the low bond energy of the Se–Se bond, polymeric seleno radical could be produced mechanically and were reactive enough to initiate the cross-linking reaction of methyl methacrylate side chains of the PU. In this way, mechano-healing and mechano-remodeling of both linear and cross-linked PUs with increased moduli can be achieved in a green and facile way, simply by compression of the bulk materials. Moreover, the main chain incorporating diphenylmethyl moieties experienced a radical transfer reaction, affording fluorescent radicals to self-report the excessive stress imposed on the deformed film. Notably, these molecular-level responses and their impact on polymer mechanical and optical properties resembled biological materials to the extent that were intelligent enough to sense the damage and actively remodel locally. Such dual functions, not only in biological systems but also in artificial materials, are critical for their practical usage. The research presented here thus is an important step toward functional Se-containing durable elastomers. Also, our work enriches the polymer design strategy for productive usage of mechanical force. Supporting Information Supporting Information is available and includes experiments, characterization, and supplementary figures and table. Conflict of Interest There is no conflict of interest to report. Funding Information The financial support of this research by the National Natural Science Foundation of China (grant nos. 21734006 and 21975178) and the National Key Research and Development Program of China (grant nos. 2017YFA0207800 and 2017YFA0204503) is gratefully acknowledged.