Letter Cite This: ACS Macro Lett. 2018, 7, 1087−1091
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Freezing-Induced Mechanoluminescence of Polymer Gels Sota Kato, Kuniaki Ishizuki, Daisuke Aoki, Raita Goseki, and Hideyuki Otsuka* Department of Chemical Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
ACS Macro Lett. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/28/18. For personal use only.
S Supporting Information *
ABSTRACT: Mechanochromism can be triggered by different mechanical stimuli, such as tension, compression, shearing, and sonication. Freezing a polymer gel also induces mechanical stress on the polymer network. Herein, freezing-induced mechanoluminescence is demonstrated for the first time by introduction of a tetraarylsuccinonitrile moiety as a light-emitting mechanochromophore at the cross-linking points of a polymer network, in which the mechanical stress induces not only a color change but also light emission. The detailed mechanism and characteristics of this freezing-induced mechanoluminescence were quantitatively evaluated by electron paramagnetic resonance spectroscopy.
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olvent freezing is powerful enough to cleave covalent bonds, as evident from the previously discovered freezinginduced mechanochromism (FIM) in polymer gels, in which mechano-cleavable diarylbibenzofuranone (DABBF) moieties1−10 with weak covalent C−C bonds were incorporated at the cross-linking points. The DABBF moieties act as colorchanging mechanophores11−22 (mechanochromophores) via the homolytic cleavage and afford blue radicals in response to the force induced by solvent freezing.4 Although it has been reported that freezing a polymer solution is not able to induce molecular chain breakage,23 but merely changes in the polymer chain morphology24 and stretching,25,26 the introduction of relatively week covalent bonds at the cross-linking points of polymer gels may enable such freezing events to induce chain breaks. FIM has a huge potential not only from a materials science perspective but also with respect to fundamental studies in polymer science, particularly in the areas of polymer solutions and gels. In this paper, the phenomenon freezinginduced mechanoluminescence (FIML) is demonstrated for the first time, which enables the visualization of mechanical stress and chain cleavage induced by freezing via a color change and via light emission. For that purpose, we employed a tetraarylsuccinonitrile (TASN) derivative (Figure 1a)27 as the mechanochromophore that can be cleaved in response to mechanical stress. This cleavage generates pink radicals that emit yellow light under UV irradiation, which endows this FIM system with light-emission properties. Since only two reports on FIM have been published so far,4,28 the FIM behavior still needs to be investigated in detail in order to broaden its potential applications. In addition, the emission of yellow light derived from the TASN skeleton should enhance the sensitivity of detection, and there is no report on FIML. Therefore, TASN-containing cross-linked polymers (Figure 1b) constitute attractive probes for the detection of mechanical © XXXX American Chemical Society
Figure 1. (a) Chemical structure of a tetraarylsuccinonitrile derivative and its equilibrium after cleavage of the central C−C bond. (b) Structure of TASN-tetraol with four hydroxy groups and synthesis of TASN-containing cross-linked polyurethane.
stress and to gain insight into the mechanism underlying FIM. In this study, we synthesized a TASN-containing cross-linked polyurethane polymer and investigated its FIM and FIML behaviors. In order to introduce the TASN moiety at the cross-linking points of the polymer, TASN-tetraol, which bears four hydroxy groups, was synthesized as a cross-linker in four steps (Scheme S1). Prior to introducing the TASN skeleton into the polymer structure, the mechanochromic properties of TASN-tetraol were investigated. TASN-tetraol was ground using a ball mill (30 Hz, 5 min), which induced a color change from white to pink, and emission of yellow light under UV irradiation (λex = 365 nm) (Figure S11). Electron paraReceived: July 13, 2018 Accepted: August 23, 2018
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DOI: 10.1021/acsmacrolett.8b00521 ACS Macro Lett. 2018, 7, 1087−1091
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ACS Macro Letters
those spectra (2.003) is typical for carbon radicals, confirming the homolytic dissociation of the central C−C bond of the TASN skeleton during the cooling process. Figure 2b shows the temperature-dependent dissociation ratio of TASN in TASN gel, estimated by the peak intensity using 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) as a radical standard, and the photographs corresponding to each temperature. It was confirmed that the dissociation ratio of TASN during the cooling process increased remarkably below 0 °C, i.e., near the melting point of 1,4-dioxane (11.8 °C). On the other hand, solutions of a linear polymer with TASN linkages and control cross-linked polymer gels swollen with 1,4-dioxane did not exhibit such color changes during the cooling process (Figures S9−S10 and S16−S19). These results suggest that mechanical stress induced by freezing of the swelling solvent affects the polymer chains, resulting in the dissociation of TASN and the associated mechanochromism. The observed dissociation ratio of TASN (estimated by EPR measurements) is fully consistent with the visual changes, i.e., the intensity of the pink color and the appearance of a yellow emission, supporting the mechanical-stress-induced generation of radicals. Furthermore, light emission under UV irradiation was detected below −10 °C. The appearance of pink color, on the other hand, was observed at −30 °C, demonstrating that the introduction of the light-emitting TASN moiety aids the visualization of the changes occurring under mechanical stress. To gain further insight into the FIML of TASN gel, the reversibility of the mechanochromism was investigated. Figure 3 shows the dissociation ratio of TASN upon cycling the
magnetic resonance (EPR) spectroscopy measurements clearly supported the generation of radicals derived from the dissociation of TASN, whose content was drastically increased upon grinding (0.0005% → 0.15%). Cross-linked polyurethane that contains TASN moieties at the cross-linking points was synthesized from hexamethylene diisocyanate (HDI), polyethylene glycol (PEG, Mn = 1000), and TASN-tetraol (HDI/PEG/TASN-tetraol = 4.0/2.0/1.0). The polyaddition reaction proceeded quantitatively under concomitant disappearance of the fluidity of the original solution. It should be noted that the resulting gel (TASN gel) can be molded into different shapes (e.g., cubes) by pouring the polymer solution into suitable molds at the initial stage of the reaction. TASN gel cubes swollen with 1,4-dioxane were frozen using liquid nitrogen, resulting in a color change to passion pink and the emission of brilliant yellow light under UV irradiation (λex = 365 nm) during the cooling process (Figure 2a). EPR measurements were carried out to characterize the FIM and FIML properties of TASN gel, in which the peak intensity rapidly increased below 0 °C (Figure S21). The g value of
Figure 3. Dissociated TASN (%) in TASN gel swollen with 1,4dioxane (Q = 0.79) in response to reversible temperature change cycles (−80, −30, and 25 °C).
temperature repeatedly between 25, −30, and −80 °C. As shown in Figure 3, even after ten cycles, the amount of dissociated TASN radicals remains almost constant, while the characteristic color change and the yellow emission under UV irradiation appear as expected. This excellent reversibility, which is not observed for other mechanochromic luminescent materials such as organic crystalline compounds,29,30 liquid crystalline compounds,31 and other mechanophores,13,20,21 demonstrates the high stability and easy handling of TASN gel for the future development of functional materials. As shown, TASN gel exhibits FIML with high stability and reusability, and its shape can be easily modified. Subsequently, we carried out a fundamental study on the FIML behaviors. The effect of the cross-linking density, which is the most important factor influencing the properties of swollen gels, on the FIML of TASN gel was evaluated. The intriguing
Figure 2. (a) FIM and FIML of TASN gel swollen with 1,4-dioxane under ambient conditions and under UV irradiation (λex = 365 nm) and (b) dissociated TASN (%) in TASN gel swollen with 1,4-dioxane (Q = 2.3) at different temperatures (−50 to 25 °C) during the cooling process together with the corresponding photographs at each temperature (−50 to 20 °C). 1088
DOI: 10.1021/acsmacrolett.8b00521 ACS Macro Lett. 2018, 7, 1087−1091
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ACS Macro Letters
2.3. Other FIM tests also support the existence of the optimized network structure (Figures S22−S26). These results confirm that FIM is clearly affected by the voids in the network structure, through which the solvent molecules penetrate the polymer network. Then, we investigated the effect of the solvent affinity for the polymer chains on the FIM behavior. For that purpose, we recorded the occurrence of FIM in a variety of solvents. The results revealed that some solvents induce FIM, while others do not (Table S2). Figure S20 summarizes the relationship between the melting point of the swelling solvents and the Q values at 20 °C, where the filled circles indicate that FIM was observed, while the open circles indicate the opposite. From this plot, the following trend can be inferred: FIM was only observed in solvents that combine high melting points and high degrees of swelling, which is consistent with the results on previously reported DABBF gels.4 To further describe the behavior of FIM from the viewpoint of the affinity between the polymer chains and swelling solvents, the Hansen solubility parameter (HSP) distance (Figure S30, Table S3), which is a solubility parameter measuring the effects of the interactions of the swelling solvent on the polyurethane skeleton, was evaluated (Figure 6a).34 To
properties of TASN gel comprise not only its ability to exhibit FIML but also the dynamic nature of the swollen gel with network reorganization32,33 in response to increasing temperatures derived from the presence of dynamic covalent bonds at the cross-linking points.6 In other words, the cross-linking density of the polymer can be tuned after its synthesis via postmodification of the resulting gels by simply increasing the temperature. Figure 4 shows the dynamic nature of TASN gel, i.e., the ability to reorganize its network. The swelling degree (Q) of a
Figure 4. Swelling behavior of a THF-swollen TASN-containing cross-linked polymer exposed to temperatures alternating between 0 and 40 °C, together with the proposed mechanism for the network reorganization in TASN gel.
THF-swollen TASN gel changes upon exposing the polymer to repeated temperature cycles between 0 and 40 °C. As shown in Figure 4, significant changes in Q were not observed at 0 °C after reaching the equilibrium swelling, but the Q value abruptly increased at 40 °C. This network reorganization originates from changes in the ratio between the number of cross-linking points and the loop structures, i.e., the segments that do not serve as cross-linking points. A reduction of the number of cross-linking points and an extension of the loop structure were observed by changing the temperature from 0 to 40 °C repeatedly (Figure S27). By exploiting this dynamic nature of TASN gel (Figure S28), we prepared seven samples with the same composition but different cross-linking densities, as denoted by their Q values at 20 °C. The dissociation ratio of TASN in these samples, indicative of the degree of force transmission induced by freezing, was plotted as a function of the cross-linking density (Q) (Figure 5, Figure S29). The mechanical force induced by FIM was most effectively transmitted in the network structure of the sample with Q =
Figure 6. (a) HSP distance for polyurethane in 1,4-dioxane/benzene (ϕ1,4‑dioxane: 0−100%) and swelling degree (Q) of TASN gel in those solvent mixtures and (b) dissociated TASN (%) at −100 °C in TASN gel swollen with 1,4-dioxane/benzene (Q = 0.79).
precisely measure such affinity effects, i.e., to eliminate any effects from the melting point of the swelling solvent, we chose solvent mixtures of 1,4-dioxane and benzene. These two solvents have almost the same melting point in a relatively high temperature range (1,4-dioxane: 11.8 °C; benzene: 5.5 °C), but different solvent polarity (Figure S20). The HSP distance between the polymer chains and the swelling solvents was controlled by changing the mixing ratio of these solvents. Figure 6a shows the HSP distance and Q values as a function of the volume ratio of 1,4-dioxane in the mixtures (ϕ). Smaller HSP distances correspond to higher solvent affinity of the solvent toward the polymer chain, which supports that Q reflects the affinity between the polymer chains and the solvent molecules. Figure 6b shows the TASN dissociation percentage during the freezing process as a function of the ϕ value. As shown in Figure 6b, a relatively good correlation between the HSP distance (or Q value) and the dissociation ratio of TASN was observed, revealing that the affinity between the polymer chains in TASN gel and the solvent is a critical factor for the efficient transmission of the mechanical force generated by freezing onto the mechanochromophore at the cross-linking points. In conclusion, freezing-induced mechanoluminescence (FIML) was achieved for the first time by introducing a
Figure 5. Dissociated TASN (%) in TASN gel swollen with 1,4dioxane at different temperatures (−60 to 25 °C) and swelling degrees (Q). 1089
DOI: 10.1021/acsmacrolett.8b00521 ACS Macro Lett. 2018, 7, 1087−1091
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ACS Macro Letters TASN moiety (a fluorescence-generating mechanochromophore) at the cross-linking points of a polymer gel, which enhanced the visibility and light emission for the detection of mechanical stress induced by freezing. The FIML is caused by the generation of colored radicals from the dissociation of TASN, which is induced by freezing of the swelling solvent. The FIM and FIML mechanisms were studied by EPR spectroscopy, which revealed that the affinity and freezing point of the swelling solvent are the critical factors affecting FIM and FIML. As the TASN gel exhibits highly stable FIML, is reusable, and is easily molded into different shapes, this gel not only is a promising functional material for mechanical sensors but also should be an effective probe for fundamental studies on polymer solutions, especially at low temperatures. We believe that this report on FIML 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/acsmacrolett.8b00521. Instrumental methods; synthesis, NMR, and IR spectra; DSC and GPC profiles; mechano- and thermochromism of TASN-tetraol; swelling behaviors of TASN gels; EPR measurements of TASN-containing linear polymer; characteristics of control cross-linked polymer gels; solvent effects on FIM and EPR spectra; swelling degree effects on FIM; solvent polarity effects on FIM; demonstration of FIM’s great reversibility; and equilibirum states in cooling and warming run (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hideyuki Otsuka: 0000-0002-1512-671X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by KAKENHI grants 17H01205 (H.O.) and 15K17907 (R.G.) from the Japan Society for the Promotion of Science (JSPS), as well as by the ImPACT Program of the Council for Science, Technology, and Innovation (Cabinet Office, Government of Japan).
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