Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Improving Mechanoluminescent Sensitivity of 1,2-DioxetaneContaining Thermoplastic Polyurethanes by Controlling Energy Transfer across Polymer Chains Wei Yuan,†,‡ Yuan Yuan,†,‡ Fan Yang,†,‡ Mengjiao Wu,†,‡ and Yulan Chen*,†,‡ †
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Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, Tianjin University, Tianjin 300354, P. R. China ‡ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, P. R. China S Supporting Information *
ABSTRACT: 1,2-Dioxetane was found as a mechanoluminescent probe when it was covalently linked into a polymer backbone. To bring about a further leap in sensitivity without sacrificing thermal stability of this mechanophore is highly desirable while a great challenge. Herein, we presented a new strategy to improve the mechanoluminescent sensitivity of 1,2-dioxetane-containing polyurethanes (PUs) by incorporating fluorescent dyes as the repeating units into the main chains of PUs. This approach provides simplified procedures to obtain mechanoluminescent films with high quality and increased amount of acceptor units. Compared to the same mechanophore-containing polymer with physically incorporated fluorophores, the two kinds of PUs not only exhibited enhanced light intensity and tunable emission colors but also displayed much lower force threshold upon deformation due to the efficient energy transfer. By virtue of these advantages, the current work will promise an invaluable tool to study failure mechanisms of thermoplastic elastomers at molecular level with unprecedented resolution.
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INTRODUCTION Thermoplastic polyurethanes (PUs) have been utilized in various demanding applications due to their excellent characters such as high elasticity and mechanical strength, melt processability, and so forth.1−3 However, their mechanical degradation is a frustrating but inevitable problem. Therefore, studying the failure mechanisms of PUs is a necessity for the creation of strong and durable polymeric materials. Despite the complicated nature of the underlying failure mechanisms mainly derived from the heterogeneous structures of these segmented elastomers,4 studies have already indicated that stress-induced bond scission, a microscopic chemistry reaction, was the initial and key event that eventually caused macroscopic damage of the polymers. Thus, if these rare failure initiation events could be sensitively probed and characterized, rational molecular strategies for advanced PUs will result. Conventional investigations of PUs failure were performed with techniques such as ESR, XRD, and NMR spectroscopy.5,6 However, they are highly dependent on mesoscopic characterization parameters and high-tech instruments, whose applications and resolutions are limited. Recently, researchers have demonstrated that polymer mechanochemistry could offer great opportunities to overcome the above-mentioned challenges and obtain sensitive 2D spatio- and temporal information between macroscopic mechanical forces and chemical reactivity.7−18 One of the © XXXX American Chemical Society
outstanding examples was discovered by Sijbesma and coworkers, whereby the stress-induced chemical transformations in polymers can be visualized by monitoring the position and intensity of the force-induced luminescence.19,20 Their findings as the so-called mechanochemiluminescent polymers were based on a mechanophore from the chemiluminescent 1,2dioxetane unit (Scheme S1). Upon applying mechanical force to the polymers, the covalently linked weakest dioxetane bonds centered along the polymer chains experienced the highest stress and broke first:21 the four-membered ring decomposed into two ketone moieties, one of which is in an electronically excited state.22 When the excited ketone relaxed into the ground state, blue light emitted. This way, these autoluminescent molecular probes with photons emitting directly from bond scission make it possible to sensitively distinguish between different mechanisms of polymer failure. However, the previously reported mechanoluminescent thermoplastic elastomer materials emitted relatively faint blue light (λem = 420 nm) under a high threshold force which was not easily recognized by the naked eye. To further improve the sensitivity of the luminescent probe for the convenience of their failure events investigation, fluorescent dyes were Received: August 3, 2018 Revised: October 25, 2018
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DOI: 10.1021/acs.macromol.8b01668 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. (a) Synthetic Scheme of PU and One Possible Sequence of Blocks in the Multiblock Polymers; (b) Mechanically Induced Chemiluminescence from 1,2-Dioxetane to Acceptor through Energy Transfer across the Polymer Chain
compound 2, compound 3 (A or B), and DBTDL (5 μL) in CHCl3, a CHCl3 solution of HDI was added dropwise. The solution was stirred at 30 °C for 30 min, and then a solution of PTMG (Mn = 650 g/mol) in CHCl3 was added to the mixture. The solution was stirred at 65 °C, resulting in a viscous solution. The completion of the reaction is determined by GPC. Upon completion, the mixture was cooled to room temperature, opened to air, and precipitated into stirring nhexane. The n-hexane was decanted, and the polymer was washed three times by adding fresh n-hexane and decanting the solvent. The polymer was collected and dried under vacuum.
physically mixed with the polymers to increase the light intensity through energy transfer. But this strategy encountered several issues: (1) the loading of fluorophore was limited due to the ease of crystallization and aggregation for most conjugated dyes at concentration higher than 0.5 wt %; (2) tedious mixing and film-forming procedures were required, with inevitable microbubbles or -cavities existing in the films and partial degradation of PU backbones during solvent evaporation, which severely influenced the transduction of mechanical energy to the mechanophore. Hence, an efficient film-forming process with improved energy transfer is greatly needed for the development of highly sensitive mechanoluminescent PUs. In the current contribution, controlling the energy transfer across the polymer chains offers a further leap in sensitivity: greatly intense mechanoluminescence was achieved from PU containing 1,2-dioxetane, whereby fluorescent acceptors were covalently linked into the PU backbone, rather than using the physically mixing way (Scheme 1). Based on this strategy, polymeric dioxetane films can be facilely fabricated by a onestep compression procedure and with increased acceptor moieties integrated. As a result, prominently enhanced autoluminescence intensity with tunable emission colors and decreased activation threshold from these PU films facilitate systematical investigations of their mechanoluminescent behaviors, regarding the varied dioxetane/acceptor unit ratio, molecular weight of the material, and the strain rate to which it was subjected.
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RESULTS AND DISCUSSION Synthesis and Characterization of Mechanoluminescent Polymers. Segmented polyurethanes (PU-A′-X and PUB′-X) containing 1,2-dioxetane as the mechanophore (cleavage site) and different conjugated moieties (photoaccepted luminophore) were synthesized. The key role of the conjugated blocks is to harvest blue luminescence from the opening ring of 1,2-dioxetanes and emit intense fluorescence with tunable colors. For this purpose, three necessary factors were taken into consideration:26,27 (1) the emission spectrum of the excited bis(adamantyl) ketone and the absorption spectrum of the acceptor should overlap to a certain degree, as is a prerequisite for the occurrence of energy transfer; (2) high fluorescence quantum efficiency of the acceptor is required; (3) the distance of donor and acceptor is ca. 10−100 Å, which was not easy to be controlled by physical blending; (4) the conjugated dye should be readily functionalized and polymerized into the polymer main chain. Herein, we selected 9,10-bis(phenylethynyl)anthracene (acceptor A) and 4,7diphenylbenzo[c][1,2,5]thiadiazole (acceptor B) as the fluorescent monomer, both of which can be functionalized with bis-hydroxyl groups. PU-A′-X and PU-B′-X were then prepared using a step-growth polymerization approach under mild conditions by combining 1,6-diisocyanatohexane (HDI), bis-hydroxyl-terminated poly(tetramethylene glycol) (PTMG, Mn = 650 g/mol), bis-hydroxyl-functionalized bis(adamantyl)-
EXPERIMENTAL SECTION
Procedure for Synthesis of PUs. 5,5′/7′-(2Hydroxyethylenoxy)adamantylideneadamantane 1,2-dioxetane,17 4,4′-(anthracene-9,10-diylbis(ethyne-2,1-diyl))diphenol (acceptor A),23 4,4′-(benzo[c][1,2,5]-thiadiazole-4,7-diyl)diphenol (acceptor B),24 and 9,10-bis(phenylethynyl)anthracene (acceptor M)25 were prepared according to the literature. The general synthetic procedure of PU-A′-X and PU-B′-X is as follows: to a mixture of compound 1, B
DOI: 10.1021/acs.macromol.8b01668 Macromolecules XXXX, XXX, XXX−XXX
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only possess larger amount of acceptors (maximum ca. 1.16%) but also form uniform films without crystallization and aggregation phenomenon just by one-step thermal compression of the as-synthesized polymers, as illuminated by SEM and optical images (Figures S6 and S7). They exhibited absorption at 420 nm and emitted bright green and greenyellow luminescence, respectively (Figure S8), allowing efficient energy transfer from emissive broken 1,2-dioxetane to these acceptors. Compared to the blue luminescence from 1,2-dioxetane, the emission light was at longer wavelength region, where both human eyes and the camera are more sensitive. Considering the optical features of the prepared PU films with good quality, all these factors will favor the transduction of mechanical energy into light and facilitate the detection of bond scission. Localization of Bond Scission in Space and Time. Optomechanical tests were performed using a rheometer and camera described in Figure S9. A representative image sequence of the PU-A′-3 film (size: 25 × 5.3 × (0.24 ± 0.01) mm3) upon deformation is shown in Figure 1, demonstrating bright green light emission from the film with high localization in space and time. Meanwhile, control experiments performed using bulk film of PU without 1,2dioxetane (PU-1, Table S1) did not give light under the same conditions, confirming the mechanical nature of the chemiluminescence from the broken bis(adamantyl)dioxetane. The relationships of stress and light intensity vs strain during plastic deformation showed that low stress (at strain of 1.2) resulted in weak luminescence throughout the film. Similar viscoelastic behavior regarding the force-induced light, as compared to the stress−strain curves, is estimated, with an increase followed by concentration of light emission detected at the location of fracture (at strain of 2.29). After the film was broken, no luminescence was observed at all. It should be noted that due to the overexposure of the detector at the moment of fracture, these numbers are a lower limit, and more intense emission may occur at fracture. Similar mechanoresponsive behavior with B′ as the acceptor unit is presented in Figure S10. Concerning the optical probing of stress evolution during fracture, these new PU films are advanced over the physically
1,2-dioxetane, and bis-hydroxyl-functionalized conjugated monomer (A and B). Because the polymerization process involves different diols, the product is a complex mixture of macromolecules, each chain containing a statistical array of coupled segments. The structures of these building blocks and one possible sequence of blocks in the multiblock PU are shown in Scheme 1. A series of PU samples with different molecular weights and varied concentration of acceptors or mechanophore were synthesized by controlling the concentration of the reaction mixture, the polymerization time, the feed ratios of A and B, and the proportions of bis(adamantyl)1,2-dioxetane (by incorporating unfunctional compound 2 as the repeating unit to keep the constant ratio of soft/hard segments) (Table 1). The chemical compositions of the target Table 1. Feed Ratios and Molecular Weights of Polymers with In-Chain Fluorophore
PU-A′-1 PU-A′-2 PU-A′-3 PU-A′-4 PU-A′-5 PU-A′-6 PU-B′-1 PU-B′-2 PU-B′-3
compd 1 (equiv)a
compd 2 (equiv)b
compd 3 (equiv)
HDI (equiv)
PTMG (equiv)
Mn (kDa)
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
0.45 0.45 0.45 0.45 0.4 0.25 0.45 0.4 0.25
0.05c 0.05c 0.05c 0.05c 0.1c 0.25c 0.05d 0.1d 0.25d
11.08 11.08 11.08 11.08 11.08 11.08 11.08 11.08 11.08
10 10 10 10 10 10 10 10 10
33 60 88 99 64 62 56 62 61
a
Compound 1. bCompound 2. cIncorporating acceptor A. dIncorporating acceptor B.
PUs were determined by 1H NMR. And referred from Figures S1−S5, the acceptor units A and B were successfully coupled into polyurethane chains in a covalent way, confirmed by the appearance of aromatic peaks in the region of 6.94−8.68 and 6.96−7.92 ppm with chemical shifts relative to the hydroxyl bifunctionalized monomers A and B, respectively. Unlike our prior PU films with physically mixed dyes, to our delight, the new polymers with fluorescent monomers in a covalent way, taking PU-A′-1 and PU-B′-1 for instance, not
Figure 1. (a) Optical images and intensity analysis of a bulk film of PU-A′-3 during stretching. (b) Stress and light intensity vs strain of PU-A′-3 at a strain rate of 20 s−1. The analyzed intensity is based on the same region within the sample. C
DOI: 10.1021/acs.macromol.8b01668 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. Cumulative light intensity vs (a) strain and (b) stress of PU-A′-6 and PU-0 with M (Mn = 63 kDa) at a strain rate of 20 s−1. Insets showed the enlarged figures for PU-0 with M during stretching. The molar fraction of M in PU-0 is equal to A in PU-A′-6 (film size: 25 × 5.3 × (0.20 ± 0.01) mm3).
Figure 3. Total light intensity emitted upon straining films of PU-A′-2 (Mn = 60 kDa, film size: 25 × 5.3 × (0.24 ± 0.01) mm3). Points represent average and standard deviation from four parallel experiments. (b) Stress−strain curves of PU-A′-2 at different strain rates.
mixed films since dioxetane proximity to fluorescent dye could greatly improve energy transfer efficiency. We chose 9,10bis(phenylethynyl)anthracene (M) as the fluorescent dye to physically mix with PU-0 (Table S1) for comparison with PUA′-6. As seen in Figure 2, the cumulative light intensity from PU-A′-6 film (Video S1) increased markedly with stress and strain, while the curves for PU-0 with M (Video S2) rose slowly. Moreover, upon deformation, for instance, at a low stress of 30 MPa, the total light intensity from PU-A′-6 is determined to be 25.2, which is 18 times higher than that of PU-0 with M (1.4). Further increasing the amount of mixed M caused aggregation-induced fluorescence quenching (Figure S12).28 On the other hand, to capture the same amount of light, especially at the initial deformation stage, for instance, with light intensity (au) of 10, the required force for PU-A′-6 and PU-0 with M was found to be 22 and 86 MPa, respectively. All these results confirmed that more light could be captured under the same condition by PU-A′-6, and the force threshold to activate chemiluminescence was also decreased remarkably, offering more detailed and sensitive information about failure events (Figure S13). Besides, an optomechanical test performed on the blending film of PU-1 and PU-2 (Table S1) demonstrated that the intrachain and interchain energy transfer jointly contributed to the increased sensitivity of mechanoluminescence (Figure S14). Therefore, these PUs would be more applicable as a self-sensing polymer on account of the higher sensitivity of their mechanoluminescent response. Effect of Strain Rate on Light Intensity. Assuming that the cumulated light intensity is proportional to the number of dioxetane scission events, the summed intensities of all frames
up to a certain time is thus a measure of the total number of broken dioxetane bonds up to that moments. And as for linear polymers, chain slippage during elastic deformation would dominantly dissipate mechanical energy before bond breakage, without measurable optical signals. Accordingly, the quantitative analysis of mechanoluminescence will be able to discriminate the failure mechanisms between chain slip and chain scission. It is well-known that the strong strain rate dependence characterizes the large plastic deformation and fracture behavior of the semicrystalline polymers,29 and the crazing in the hard domain is favored at high deformation rates while the plastic deformation predominates at low strain rates.30 To uncover the effect of strain rate on mechanoluminescence, films of PU-A′-2 samples were stretched at different strain rates. As shown in Figure 3, with strain rate increasing gradually from 1 to 10 s−1, the stress applied in polymer films went up incrementally and the total emission intensity enhanced accordingly. Therefore, at lower strain rate, only partial activation of 1,2-dioxetanes in PU was achieved. This observation could be associated with the inter- and intrachain slip processes with the increased strain rate. When the strain rate came up to 20 s−1, the detected light intensity decreased slightly, which was most probably due to the saturation of camera detector and the time resolution of camera shutter. Effect of Molecular Weight on Light Intensity. It is well-known that transduction and accumulation of force along an extended polymer chain scale as its molecular weight.31 To study the effect of molecular weight on the mechanoluminescent response of these PUs, PU-A′-1 to PU-A′-4 whose Mn varied from 33 to 99 kDa were subjected to optomechanical testing. Figure 4 shows that the cumulated light intensity D
DOI: 10.1021/acs.macromol.8b01668 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. (a) Cumulative light intensity of PU-A′-1 to PU-A′-4. (b) Stress−strain curves of PU-A′-2 to PU-A′-4 at a strain rate of 1 s−1 (film size: 25 × 5.3 × (0.24 ± 0.01) mm3).
Figure 5. Light intensity of bulk films of (a) PU-A′-2, PU-A′-5, and PU-A′-6 and (b) PU-B′-1, PU-B′-2, and PU-B′-3 at a strain rate of 1 s−1 (film size: 25 × 5.3 × (0.24 ± 0.01) mm3). Points represent average and standard deviation from four parallel experiments for each polymer.
to acceptor unit A or B was realized. It should be pointed out that the high dynamic range of the emission light led to a saturation of the camera sensor, so the actual light intensity of PU-A′-6 and PU-B′-3 was underestimated. In addition, the light intensity of PU-A′-X was higher than that of PU-B′-X at the same ratio of acceptor/dioxetane. The observation was reasonable due to the higher fluorescence quantum efficiency of A (ΦF = 86.3%) than that of B (ΦF = 57.7%) (Table S2). These results imply that by delicately choosing the sorts and ratio of covalently embedded fluorophores, optical properties, regarding the emission color and efficiency, controlled by mechanical force could be readily available, which would be important to fulfill the needs of diverse applications.
increased with the increase of molecular weight, which was corresponding to the evolution of stress from the relevant polymers. In detail, as for PU-A′-1 with lowest Mn (33 kDa), mechanically induced light was not probed due to the poor mechanical strength of the film. This indicated that there was a limiting molecular weight, below which mechanochemical chain scission could not happen. In contrast, as for PU-A′-4 with the highest Mn (99 kDa), the detected light intensity increased significantly. The molecular weight dependent bond breakage thus reflected the mechanical nature of the luminescence. Besides, as mentioned above, the mechanoluminescence helps us to discriminate between failure mechanisms; for PU from lower molecular weights to higher molecular weights, the transition in mechanism from chain slip to scission could be characterized very sensitively. Effect of Acceptor Contents on Light Intensity. To gain further insights into the relationship of luminescence intensity and the ratio of 1,2-dioxetane unit (cleavage site to form donor) and A or B (photoharvesting luminophore as acceptor), optomechanical tests were conducted on three polymer films with varied ratios of acceptor/dioxetane (i.e., 0.1, 0.2, and 0.5 for PU-A′-2, PU-A′-5, and PU-A′-6, respectively). As expected, their mechanoluminescence intensity is highly dependent on the content of the acceptor repeating unit. Because the three PU films have similar molecular weights (Mn = 60−64 kDa), their mechanical strength was not influenced by the ratio of dioxetane and acceptor unit, as is evidenced by their resemble stress−strain curves (Figure S15). This observation is very important for evaluating the energy transfer effect on mechanoluminescent intensity. As shown in Figure 5, the total light intensity increased almost linearly with the increase of acceptor fraction, meaning that enhanced energy transfer from broken dioxetane
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CONCLUSIONS In conclusion, by covalent incorporation of 9,10-bis(phenylethynyl)anthracene or 4,7-diphenylbenzo[c][1,2,5]thiadiazole moiety as the fluorescent repeating units, two kinds of mechanoluminescent PUs were developed. Upon straining to failure, all samples can emit bright light (i.e., green and greenyellow fluorescence) of the acceptor units, which is influenced by the molecular weight of the polymer, ratio of mechanophore/acceptor unit, and strain rate. Compared to the previously reported PU films with physically doped acceptors, energy transfer across the polymer chain from broken dioxetane to these covalently embedded dyes not only enhanced the visibility and emission intensity for the detection of mechanical stress but also simplified the procedure to prepare polymer films. Notably, by well control of the energy transfer process, the improved mechanical sensitivity efficiently lowered the force threshold to activate the mechanoluminescent events. Therefore, these elastic polyurethanes not only are a kind of promising functional material for mechanical sensors E
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from a single reactive moiety incorporated into polymer chains. Angew. Chem., Int. Ed. 2016, 55, 3040−3044. (9) Black, A. L.; Lenhardt, J. M.; Craig, S. L. From molecular mechanochemistry to stress-responsive materials. J. Mater. Chem. 2011, 21, 1655−1663. (10) Imato, K.; Irie, A.; Kosuge, T.; Ohishi, T.; Nishihara, M.; Takahara, A.; Otsuka, H. Mechanophores with a reversible radical system and freezing-induced mechanochemistry in polymer solutions and gels. Angew. Chem., Int. Ed. 2015, 54, 6168−6172. (11) Balkenende, D. W.; Coulibaly, S.; Balog, S.; Simon, Y. C.; Fiore, G. L.; Weder, C. Mechanochemistry with metallosupramolecular polymers. J. Am. Chem. Soc. 2014, 136, 10493−10498. (12) Diesendruck, C. E.; Steinberg, B. D.; Sugai, N.; Silberstein, M. N.; Sottos, N. R.; White, S. R.; Braun, P. V.; Moore, J. S. Protoncoupled mechanochemical transduction: a mechanogenerated acid. J. Am. Chem. Soc. 2012, 134, 12446−12449. (13) Hickenboth, C. R.; Moore, J. S.; White, S. R.; Sottos, N. R.; Baudry, J.; Wilson, S. R. Biasing reaction pathways with mechanical force. Nature 2007, 446, 423−427. (14) May, P. A.; Moore, J. S. Polymer mechanochemistry: techniques to generate molecular force via elongational flows. Chem. Soc. Rev. 2013, 42, 7497−7506. (15) Ducrot, E.; Chen, Y.; Bulters, M.; Sijbesma, R. P.; Creton, C. Toughening elastomers with sacrificial bonds and watching them break. Science 2014, 344, 186−189. (16) Davis, D. A.; Hamilton, A.; Yang, J.; Cremar, L. D.; Van Gough, D.; Potisek, S. L.; Ong, M. T.; Braun, P. V.; Martínez, T. J.; White, S. R.; Moore, J. S.; Sottos, N. R. Force-induced activation of covalent bonds in mechanoresponsive polymeric materials. Nature 2009, 459, 68−72. (17) Chen, Y.; Spiering, A. J. H.; Karthikeyan, S.; Peters, G. W. M.; Meijer, E. W.; Sijbesma, R. P. Mechanically induced chemiluminescence from polymers incorporating a 1, 2-dioxetane unit in the main chain. Nat. Chem. 2012, 4, 559−562. (18) Kean, Z. S.; Hawk, J. L.; Lin, S.; Zhao, X.; Sijbesma, R. P.; Craig, S. L. Increasing the maximum achievable strain of a covalent polymer gel through the addition of mechanically invisible cross-links. Adv. Mater. 2014, 26, 6013−6018. (19) Clough, J. M.; Creton, C.; Craig, S. L.; Sijbesma, R. P. Covalent bond scission in the mullins effect of a filled elastomer: real-time visualization with mechanoluminescence. Adv. Funct. Mater. 2016, 26, 9063−9074. (20) Chen, Y.; Sijbesma, R. P. Dioxetanes as mechanoluminescent probes in thermoplastic elastomers. Macromolecules 2014, 47, 3797− 3805. (21) Clough, J. M.; Van Der Gucht, J.; Sijbesma, R. P. Mechanoluminescent imaging of osmotic stress-induced damage in a glassy polymer network. Macromolecules 2017, 50, 2043−2053. (22) Schuster, G. B.; Turro, N. J.; Steinmetzer, H. C.; Schaap, A. P.; Faler, G.; Adam, W.; Liu, J. Adamantylideneadamantane-1, 2dioxetane. Chemiluminescence and decomposition kinetics of an unusually stable 1, 2-dioxetane. J. Am. Chem. Soc. 1975, 97, 7110− 7118. (23) Hirose, T.; Matsuda, K. Self-assembly of amphiphilic fluorescent dyes showing aggregate-induced enhanced emission: temperature dependence of molecular alignment and intermolecular interaction in aqueous environment. Chem. Commun. 2009, 5832− 5834. (24) Iacono, S. T.; Budy, S. M.; Moody, J. D.; Smith, R. C.; Smith, D. W., Jr. Modular approach to chromophore encapsulation in fluorinated arylene vinylene ether polymers possessing tunable photoluminescence. Macromolecules 2008, 41, 7490−7496. (25) Fudickar, W.; Linker, T. Why triple bonds protect acenes from oxidation and decomposition. J. Am. Chem. Soc. 2012, 134, 15071− 15082. (26) Stryer, L.; Haugland, R. P. Energy transfer: a spectroscopic ruler. Proc. Natl. Acad. Sci. U. S. A. 1967, 58, 719−726.
but also should be an effective probe for fundamental studies on fracture of PUs. By virtue of structural tunability and fruitful optical properties of fluorescent dyes, acceptor monomers suitable for energy transfer of 1,2-dioxetane-containing polymers are far beyond the scope of this study. We believe that the strategy described herein will be broadly applicable to other mechanoluminescent polymers, and this report should open a new research avenue in polymer and materials sciences.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01668. Experimental details, characterization data, strain−stress data, and additional figures (PDF) Video S1: chemiluminescence upon stretching a bulk film of PU-A′-6 (AVI) Video S2: chemiluminescence upon stretching a bulk film of PU-0 with M (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yulan Chen: 0000-0001-6017-8888 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support by National Natural Science Foundation of China (Grants 21522405, 51503142, and 21734006), National Key Research and Development Program of China (Grants 2017YFA0204503 and 2017YFA0207800), and Thousand Youth Talents Plan and Natural Science Foundation of Tianjin (Grants 15JCYBJC52900 and 17JCJQJC44200) is gratefully acknowledged.
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