Photoplastic Transformation Based on Dynamic Covalent Chemistry

Jun 11, 2019 - ... chemistry based on thermodynamic equilibrium reactions,(5,6) such .... in 3–6 min, accompanied with photochromism (Figures 3a and...
2 downloads 0 Views 5MB Size
Download PDF
Research Article www.acsami.org

Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23623−23631

Photoplastic Transformation Based on Dynamic Covalent Chemistry Shi-Li Xiang,†,§ Qiong-Xin Hua,†,§ Wen-Liang Gong,† Nuo-Hua Xie,† Peng-Ju Zhao,† Gary J. Cheng,*,‡ Chong Li,*,† and Ming-Qiang Zhu*,† †

Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China ‡ School of Industrial Engineering, Purdue University, West Lafayette, Indiana 47907, United States

Downloaded via BUFFALO STATE on July 20, 2019 at 08:09:50 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The magical fantasy of decades-old transformer characters is becoming closer to scientific reality, as transformable materials can change their shapes in response to thermal, mechanical, electrical, and chemical stimuli. However, precise and prompt control of plastic shaping remains to be wanted. Photoresponsive materials provide a promising alternative for rapid optomechanical shaping with limited success. Here, we report a new class of photoplastic transformation based on dynamic covalently crosslinked polytriazole (PTA) networks, in which crosslinking points are comprised of photocleaveable hexaarylbiimidazole (HABI). Upon sub500 nm light irradiation, HABI is dissociated into two triphenylimidazole radicals (TPIRs) followed by spontaneous recombination back to the initial state. This photoswitching effect is demonstrated to generate nonthermal shape change in the PTA-HABI gel network at will upon light stimulus. A unique photoalignment phenomenon has also been discovered which can form oriented nanoscale patterning in the PTA-HABI gel network upon laser irradiation. The solventfree PTA-HABI elastomer exhibits photoenhanced automatic self-healing properties at temperatures ranging from 25 °C to freezing points, which is attributed to the dynamic equilibrium between TPIRs and HABI. A photoplastic spring is fabricated and exhibits photoswitchable plastic behavior, i.e., a reversible transformation between plastic strain and elastic strain upon light irradiation. HABI-based polymer networks, including solvated gel and solvent-free elastomer, are promising as smart materials for nonthermal photoactivated shape changing, transformation, and self-healing applications. KEYWORDS: dynamic covalent bond, hexaarylbiimidazole, photoinduced deformation, crosslinked polymer, self-healing

■

INTRODUCTION The animations of transformers have always triggered our imagination on remolding the appearance of plastic or metallic materials at will. Typically, reshaping and/or remolding polymer materials require thermal processing. Crosslinked polymer systems with multiple melting or glass transition temperatures can retain mechanical strain until reshaped by relatively slow thermal processing. However, permanently crosslinked polymers are difficult to be reprocessed by heat or with solvent once manufactured.1 Alternatively, supramolecular polymers, which provide similar mechanical properties to permanently crosslinked polymers, can have promising applications as reusable materials.2−4 Another strategy is dynamic covalent chemistry based on thermodynamic equilibrium reactions,5,6 such as dioxaborolane metathesis,7 malleable polyurethanes,8 etc. However, such thermoplastic behavior is not competent for machining in high-precision spatiotemporal precision. It is desirable to develop novel dynamic crosslinked polymer systems with high-accuracy plastic processing capability. Light has been used as a convenient approach to control the properties of materials, such as fluorescence9 and electroconductivity.10 Photochromic materials can have a reversible change in color or shade when exposed to light with the © 2019 American Chemical Society

selective frequency or intensity. Previous investigations demonstrate that molecular level photochemical reactions may be translated into macroscopic behaviors through elaborate molecular design.11−14 Photocontrolled reshaping of photochromic materials in single crystals,15 liquid-crystal polymers,16,17 and host−guest hydrogel18 based on transformations between cis−trans forms of azobenzenes, openclosed forms of dithienylethenes, and cinnamoyl crosslinking has been reported. However, such photoresponsive polymers would be more useful and effective in optomechanical and selfhealing applications if they produce more significant shape change and could be repeatedly remolded, even after being damaged or even broken into pieces.19−21 Distinct from molecular photoswitches based on azobenzenes, dithienylethenes, spiropyrans, and cinnamoyl moieties, the hexaarylbiimidazole (HABI) molecule exhibits photoinduced cleavage of the resonance-stabilized C−N bond between two triphenylimidazoles forming two isolated relatively stable radicals,22 which can recombine back into HABI spontaneously.23 Traditional preparation methods of Received: April 18, 2019 Accepted: June 11, 2019 Published: June 11, 2019 23623

DOI: 10.1021/acsami.9b06608 ACS Appl. Mater. Interfaces 2019, 11, 23623−23631

Research Article

ACS Applied Materials & Interfaces HABI-containing crosslinked polymers employing heat or radical initiators, often cause the failure of photochromism due to the thermodynamic and radical nature of the colored species.24 Therefore, HABI groups are usually introduced into polymers by two methods. One is that the vinyl monomers of triphenylimidazole as the precursors of hexaarylbiimidazole are radically co-polymerized with other functional monomers to form linear polymers and then oxidized to crosslinking network.14 The method can avoid the interference of photoinitiated radicals of HABIs, but the macromolecular oxidation reaction to produce HABIs is usually low efficient. Therefore, the use of nonradical polymerization methods to introduce HABI units into polymer systems is preferable. Previous reports on the direct polymerization of HABI have designed HABIs with only two functionalized groups as monomers, which are co-polymerized with nonphotosensitive, conventional multiple functionalized crosslinkers.25 Here, we report a new class of photoplastic transformation based on dynamic covalently crosslinked polytriazole (PTA) networks, in which crosslinking points are comprised of photocleaveable HABIs with multiple functionalized groups. When the HABIs with tri- or tetra-functionalized groups are directly used as the crosslinking agent of polymer networks, the crosslinking and decrosslinking of polymer architectures can be controlled by light, causing tunable optomechanical behaviors.

Figure 1. Representative structure, photoresponsibility, and photoprocessing of PTA-HABI polymer networks. (a) The schematic structure of PTA-HABI polymer network. Photoresponsibility: (b) molecular structures and colors of HABI before and after UV irradiation. (c) Photoinduced EPR. (d) Photochromism of HABI. (e) Photocycling of PTA-HABI organogel. Photoprocessing: (f) photomelting. (g) Photoremolding. (h) Photocutting. (i) Photodrilling. (j) Photopatterning. (k) Photohealing. The samples used in (b) and (d) are HABI-doped poly(methyl methacrylate) films with a concentration about 1 wt %, which were made by spin coating. The samples in (c) and (e) are PTA-10%HABI-based dimethylformamide (DMF) organogel with a thickness of 1 mm. The samples used in (f) and (h) are PTA-5%HABI-based DMF organogels, whereas the samples in (g) are PTA-5% HABI hydrogels. The samples in (i) and (j) are PTA10%HABI-based DMF and ethanol organogels, respectively, with a thickness of 1 mm. The sample in (k) is a dried gel of PTA-10%HABI with an original length of 1.5 cm. All of the gel samples had reached their swelling equilibrium in DMF or ethanol or water before using (the maximum swelling ratio is recorded in Figures S3 and S4).

■

RESULTS AND DISCUSSION Synthesis, Photochromism, and Characterization of the PTA-HABI Gel. In this paper, we design the HABI crosslinker with tetra-functionalized groups and prepare the polymer network by introducing HABIs as dynamic covalent crosslinking points. We select the Click reaction between azides and alkynes26 as the polymerization method, which has been extensively used to synthesize various polymer networks in a highly efficient way to construct crosslinked polymers named as polytriazoles (PTAs).27,28 We present the concept of photoplastic transformation based on PTA-HABI polymer organogel network, which exhibits solid−liquid transformation by controlling the vulcanization and disintegration of polymer architectures due to the reversible transformation between HABIs and TPIRs. Upon light irradiation, the HABI as a crosslinking point is dissociated into two TPIRs, causing the polymer architecture from crosslinking state to disintegration state. It is visible to naked eyes that the PTA-HABI polymer organogel simultaneously shows color changes and evident deformation from solid to liquid state upon light irradiation. The light stimulus can be used to reversibly shape the PTAHABI organogel network to various shapes. The photoalignment of the PTA-HABI organogel network demonstrates that the oriented surface patterning is directed by light. In addition, due to the dynamic equilibrium between HABI and TPIR, the solvent-free PTA-HABI photoplastic elastomers exhibit automatic self-healing properties at room temperature, which is significantly accelerated upon light irradiation. Finally, the photoplastic spring based on PTA is fabricated with dual crosslinking agents and exhibits the reversible transformation between plastic strain and elastic strain upon light irradiation. We have designed a photochromic crosslinking agent, 2bromo-terapropargyl-HABI, by introducing four propargyl groups into the molecular structure of HABI (Figure 1a). One HABI molecule decomposes into two colored triphenylimidazole radicals (TPIRs), which recombine back into HABI spontaneously at room temperature (Figure 1b). The HABIs

possess great photoresponsibility including photoinduced electron paramagnetic resonance (EPR), photochromism with great photocycling capability (Figure 1c−e). The most interesting discovery on crosslinked PTA-HABI polymer gels is their great photoprocessing capability. It is demonstrated that PTA-HABI gels or solvent-free elastomers can be plastically processed such as photomelting, photoremolding, photocutting, photodrilling, photopatterning, and photohealing (Figure 1f−k). We have synthesized PTA-HABI gel with dipropargylPEG1000 as a hydrophilic unit, 1,6-diazidohexane as a linker, and 2-bromo-tetrapropargyl-HABI as a crosslinker. The halogen atom at the ortho position in HABI would speed up the fading kinetics of colored species and improve the fatigue resistance.29,30 The photochromism of 2-bromo-terapropargylHABI is investigated in solution by UV−vis spectra. Before irradiation, no absorption peak is detected in the visible region, whereas two new peaks at 355, 580 nm and a broad absorption band around 750−1000 nm are observed after 5 s of irradiation (Figure S1a). The solution of 2-bromo-terapropargyl-HABI becomes blue after irradiation (the inset in Figure S1a). The optical densities at maximum absorption peaks keep increasing until are saturated upon 60 s of irradiation, which could be attributed to the competition between the photoinduced dissociation of HABI and the spontaneous backward recombination of TPIRs. The fading kinetic investigation of 23624

DOI: 10.1021/acsami.9b06608 ACS Appl. Mater. Interfaces 2019, 11, 23623−23631

Research Article

ACS Applied Materials & Interfaces

Figure 2. Synthesis, photochromism, and photoinduced reversible EPR changes of PTA-HABI gel. (a) The schematic synthetic procedure of PTAHABI gel. (b) UV−vis spectra of PTA-HABI organogel before and after irradiation. (c) Absorbance change measured at 580 nm of PTA-HABI organogel subjected to UV irradiation and spontaneous thermal discoloration cycles. Purple areas: irradiation at 405 nm for 10 s. Gray areas: after ceasing 405 nm laser. (d) Photoinduced EPR changes of PTA-HABI organogel upon irradiation at 405 nm (100 mW/cm2) for various time. (e) Spontaneous fading of EPR spectra of PTA-HABI organogel after removing UV irradiation. All of the samples in (b)−(e) are PTA-10% HABIbased DMF organogels with a maximum swelling ratio of 8.25 and a thickness of 1 mm.

process could be reversibly performed for more than 10 times (Figure 2c). Upon 10 s of 405 nm laser irradiation, the optical density at 580 nm increases due to the photoinduced dissociation of HABI to TPIRs (the purple area in Figure 2c). After ceasing the light, the optical density decreases rapidly to the initial state (the gray area in Figure 2c). Noteworthily, the optical density of PTA-HABI organogel at 580 nm decreases to half in 16 s (t1/2,gel = 16 s), which is much shorter compared with 2-bromo-terapropargyl-HABI in the solution state (t1/2,solution = 110 s). This might be attributed to the fact that HABIs as crosslinkers are immobilized in the polymer framework, thus restricting the diffusion and escape of TPIRs. Electron paramagnetic resonance (EPR) is usually used to detect the radical nature of the chemical process. We have performed real-time dynamic EPR measurements of PTAHABI organogel upon light irradiation. Without light irradiation, no EPR signal is detected (Figure 2d, black line), whereas a strong peak around 3360 G is observed after irradiation (2 s). The signal continues increasing with the irradiation time. After ceasing UV irradiation, the EPR signal decreases gradually with time and completely disappears after 60 s (Figure 2e). The in situ and real-time monitoring of EPR intensity at 3360 G has provided direct information on the photoinduced radicals of HABIs and the recombination of TPIRs (Figure S2). Photoplastic Processing. We found that laser can be transplanted to the photoplastic processing of PTA-HABI gel (Figure 3). DMF is used as the solvent due to the high swelling

the colored species after UV irradiation indicates that the optical density at 580 nm decreases rapidly in the first 200 s, then becomes flatten after 500 s with a half-life time of the colored species of 110 s (Figure S1b). The linear plot of 1/ [At]∼t indicates that the recombination of TPIRs is a secondorder reaction (the inset in Figure S1b). As presented in Figure 2a and the Supporting Information, 2-Br-tetrapropargyl-HABI, dipropargyl-PEG1000, and 1, 6-diazidohexane were mixed with optimized molar ratio of 0.1:0.8:1 in DMF using CuBr as the catalyst under a N2 atmosphere, which afforded the best swelling factor and tensile strength. It is noteworthy that the reaction happens spontaneously in a few minutes at room temperature with noticeable exothermic characters, and the reaction could be controlled by decreasing the reaction temperature. The resulting DMF organogel is dialyzed against ethylenediaminetetraacetate (EDTA) and water for a few days to remove ion impurities.The as-prepared gels were dried in vacuum to obtain solvent-free gels which could be subsequently immersed in different solvents to get corresponding gels. The absorption spectra of PTA-HABI-based DMF organogel under light irradiation are monitored (Figure 2b). Before irradiation, no absorption peaks in long-wavelength visible region have been observed. Upon irradiation with 405 nm laser for 10 s, the colorless organogel becomes blue, and an absorption around 600 nm has been detected. These characters of PTA-HABI organogel are in accord with the photochromism of 2-bromo-terapropargyl-HABI in the solution. The fatigue resistance of PTA-HABI organogel indicates that the 23625

DOI: 10.1021/acsami.9b06608 ACS Appl. Mater. Interfaces 2019, 11, 23623−23631

Research Article

ACS Applied Materials & Interfaces

Figure 3. Photoplastic processing based on PTA-HABI gels. (a) Laser melting: photoinduced sol−gel transformation of a PTA-HABI organogel doll. (b) Laser cutting: a piece of organogel (3 mm × 3 mm × 2.5 cm) hang on a needle is cut by light in the middle. (c) Laser drilling: a piece of organogel (a thickness is about 1 mm) is “drilled” by 405 nm laser in the center. Left: pore size 0.3 cm; right: grid, pore size 0.3 mm. (d) Laser remolding: photoplastic sol−gel cycling. According to the arrow direction: gel pieces, first light irradiation (melted), first gelation, second irradiation, second gelation, third irradiation, and third gelation. The cuvette size in (d) is 10 mm × 2 mm. (e) Photoplastic Surface Patterning: multiscale magnified scanning electron microscopy (SEM) images of the organogel surface after UV irradiation for 3 min. Left: large-scale surface patterning. Middle: selected-area SEM images of the gel surface from the red box. Right: selected-area SEM images of the gel surface from the blue box. The distinct difference between exposed channels (oriented wrinkles) and unexposed masks (random wrinkles) is observed. (f) Proposed mechanism of photoplastic transformation of PTA-HABI gel. The light source is 405 nm laser (100 mW/cm2) for all of the experiments. The samples used in (a) and (b) are PTA-5%HABI-based DMF organogels and the organogel strips in (b) have a length about 2.5 cm which is hang by a small syringe needle. The samples in (c) are PTA-10%HABI-based DMF organogels. The samples in (d) are PTA-5%HABI-based hydrogels. The sample in (e) is PTA-10%HABI-based ethanol organogel with a thickness of 1 mm. All of the above gel samples had reached their swelling equilibrium in DMF or ethanol or water before using (the maximum swelling ratio is recorded in Figures S3 and S4).

degree of PTA-HABI gel in nonvolatile DMF. All of the gel samples used in Figure 3 had reached a swelling equilibrium (Figure S3) in the solvent before using. For example, the most frequently used specimens, i.e., PTA-10%HABI DMF gels have a maximum swelling ratio of 8.27. The investigation on the crosslinker dependence of swelling degree indicates that the higher the content of crosslinker, the lower the swelling value (Figure S4). It is visible to naked eyes that a PTA-HABI-based DMF organogel doll with predesigned shape exhibits photoplastic transformation from “solid” to “liquid” (laser melting) in 3−6 min, accompanied with photochromism (Figures 3a and S5). After 10 s of 405 nm irradiation (100 mW), the organogel turns to blue because of the photochromic characteristic of HABI. Afterward, the irradiated spot exhibits a deep blue color change, whereas an emerging deterioration of organogel doll to liquid is observed. As we moved the light spot to other area, the same phenomenon is observed, and, finally, the organogel became flowable liquid onto the substrate (Movie S1). Another vertical gel transformation demonstrates that the organogel is “cut” upon 405 nm light irradiation across the gel center (laser cutting). The vertical organogel is cut by laser in 10 s (Figure 3b and Movie S2). The photoplastic effect of PTA-HABI organogels was demonstrated through the photocutting test of PTA-HABI organogels upon 405 and 640

nm irradiations (Figure S6). The sample temperatures are increased to 58 and 67 °C upon 30 s of irradiations of 405 and 640 nm lasers, respectively. As expected, the PTA-HABI organogel was not cut upon 640 nm irradiation, whereas it was cut into pieces upon 405 nm irradiation. Due to the photoinduced dissociation of HABI upon 405 nm irradiation, which is demonstrated in Figure 2, we could conclude that the transformation of PTA-HABI gels results from the photoplastic rather than thermoplastic process of PTA-HABI gels. Laser drilling is demonstrated that upon irradiation at 405 nm laser, the organogel center is penetrated in 5−10 s depending on the organogel thickness. The macroscale pores in the organogel are well defined with a minimal broadening (Figure 3c, left). The laser microdrilling is conducted by the aid of a photomask to afford a pore microarray with a pore size about 0.3 mm (Figure 3c, right). Bearing in mind the photoplastic transformation between the polymer sol and gel, we have cut the PTA-HABI hydrogel into small pieces for laser remolding (Figure 3d). Upon irradiation of 405 nm laser, the hydrogel pieces loaded in the cuvette turn blue, gradually “melted”, and finally become a sol in 1−2 min. After the sol in the cuvette stands in darkness for 10 min, the sol is recovered to the gel state. The photoplastic sol−gel cycling process can be repeated for many times, which exhibits the excellent solid−liquid transition 23626

DOI: 10.1021/acsami.9b06608 ACS Appl. Mater. Interfaces 2019, 11, 23623−23631

Research Article

ACS Applied Materials & Interfaces

because of light irradiation (18.2% at 100% strain) (Figure S13, red). The results could be repeated when we performed the stress−strain measurement at the third time with 405 nm irradiation, which is attributed to the photoplastic effect of PTA-HABI elastomers (Figure S13, blue). We presume that the photoplastic rather than thermoplastic effect of PTA-HABI elastomers arises from the decrease of crosslinking density upon light irradiation. The similar trend is observed in other samples with different thicknesses (Figure S14). Self-Healing. It is discovered that the PTA-HABI photoplastic elastomer exhibits an excellent self-healing behavior. We cut a piece of elastomer into two and simply leave the fracture interfaces of two pieces contacted together without loading any additional pressure. It is observed that the strain at break is only 89% when healing for 24 h in the dark (Figure 4a, green),

reversibility of the PTA-HABI gel. In comparision, upon irradiation at 405 nm laser, PTA-HABI hydrogels containing 10 and 5 mol % HABI groups become flowable in about 3 and 1 min, respectively. It indicates that the photoinduced sol−gel transformation of low-crosslinking-density samples is faster than high-crosslinking-density ones. Notably, the gels are also sensitive to white light, which is attributed to HABI photosensitivity to sub-500 nm blue light (Figure S7). As a control, tetraphenylethylene (TPE) without photochromic feature is used as the crosslinker to synthesize the PTA-TPE gel, which does not exhibit any photochromism and solid− liquid transition upon UV or white light irradiation (Figures S8 and S9). Thus, the photochromic HABI crosslinking points should be responsible for the photoinduced solid to liquid transition of the gel. The photosensitive molecules are often used for the direct patterning of gel grating. Therefore, photoinduced surface molecular reconfiguration and subsequent photopatterning are observed in the surface of PTA-HABI organogel (Figure 3e, left). It is interesting that the random ripples in the unexposed mask are observed (Figure 3e, middle), whereas the wrinkle structures become oriental cross the exposed channel between the shadowing mask (Figure 3e, right). We deduce that the polymer chains in the unexposed part serve as the templates or seeds to direct the orientation growth of the wrinkle structure in the exposed part. A possible explanation is, upon UV irradiation, the HABI crosslinking points are disintegrated, which causes the decay of compressive stress and the surface molecular reconfiguration of the gel.31 Thus, the wrinkle structures could be reconfigured due to the recombination of polymer gel crosslinking points, causing the gel grating from nonorient ripples to oriental wrinkle structure upon light irradiation. We deduce that the crosslinking could be destroyed by adding HABIs small molecules since the crosstalk recombinations of different TPIRs are a nonselective and diffusionlimited process.32 Thus, we immerse the PTA-HABI gel in a solution of 2-Br-HABI in DMF, which exhibits very similar properties with those of 2-bromo-terapropargyl-HABI (Figure S10). As presented in Figure S11, the colorless organogel turns to blue-purple solution upon irradiation. However, no obvious gelation has been observed when the sol stands for a long time, even 1 day, which is due to the crosstalk recombination between two kinds of TPIRs from small molecules and PTAHABI gel. Thus, we proposed a scheme to illustrate the mechanism of the photoinduced sol−gel transformation of PTA-HABI organogel in Figure 3f. Generally, the mechanical strength of PTA-HABI gel would dramatically decrease after the solvent adsorption. Therefore, we prepare a series of dried PTA-HABI gels, named as PTAHABI photoplastic elastomers, and investigate their optomechanical and self-healing properties. The optomechanical response of PTA-HABI photoplastic elastomer is demonstrated by the stress−strain experiment. The PTA-HABI elastomer shows repeatable elastic deformation within 100% strain during the first stretching without any light irradiation (Figure S12). The photoplastic stress−strain experiment is conducted using 405 nm laser. The first stress−strain measurement is conducted without any irradiation (Figure S13, black). Upon 405 nm laser irradiation, the PTA-HABI elastomer sample is stretched for the second time. Compared with the first stress−strain measurement without 405 nm irradiation, we observed a clear decrease in the stress value

Figure 4. Self-healing of solvent-free PTA-HABI photoplastic elastomer at room temperature. (a) Stress−strain curves of the PTA-HABI elastomer. The strain at break: two cut PTA-TPE elastomers with light irradiation for 24 h (20%), original PTA-HABI elastomer (363%), two cut PTA-HABI elastomers healing in the dark for 24 h (89%), and two cut PTA-HABI elastomers healing with white light irradiation (5 mW/cm2) at room temperature for 1 h (85%), 6 h (123%), 12 h (185%), and 24 h (338%). All tensile tests were performed by using a strain rate of 2 mm/min at 25 °C. (b) Photographs of the self-healing behavior of the PTA-HABI elastomer with white irradiation at room temperature (bottom) compared with PTA-TPE control (top) and PTA-HABI elastomer in the dark (middle). From left to right, cut, self-healed, and stretched state, respectively. (c) Proposed mechanism of the self-healing of PTAHABI photoplastic elastomer. The samples in (a) and (b) are PTA10%HABI-dried gels (15 mm × 3.8 mm × 3.8 mm), which has been dried in vacuum before using.

which is close to the value of that (85%) after healing for 1 h with white light irradiation (5 mW/cm2) (Figure 4a, blue). The strains at break are measured to be 123, 185, and 338% after healing for 6, 12, and 24 h with white light irradiation, respectively (Figure 4a). It is noteworthy that the strain at break of self-healing elastomer after only 1 h of healing with white light irradiation become close to the value in the dark for 24 h, which indicates that light significantly enhances the self23627

DOI: 10.1021/acsami.9b06608 ACS Appl. Mater. Interfaces 2019, 11, 23623−23631

Research Article

ACS Applied Materials & Interfaces healing of elastomer. Upon white light irradiation for 24 h, the strain at break of PTA-HABI elastomer increases to 338% (Figure 4a, deep red), which is closed to that of original PTAHABI elastomer (363%) (Figure 4a, dark). It is visible to naked eyes that PTA-HABI elastomer pieces could self-heal together after 24 h in dark at room temperature, which can be stretched to 1.8 times of the original length (Figure 4b, middle). Notably, the elastomer pieces have better self-healing property under white light (5 mW/cm2) at room temperature, which can be stretched to 4.6 times of the original length (Figure 4b, bottom). As a control, the strain at break of PTATPE elastomer with the same crosslinking content remains only 20% after cut followed by standing 24 h (Figure 4a, gray). Thus, no self-healing is achieved (Figure 4b, top). Thus, it is definite that PTA-HABI photoplastic elastomers exhibit excellent self-healing behavior, which is significantly enhanced by the involvement of light. We suppose that the self-healing behavior of PTA-HABI solvent-free elastomer generates from the thermodynamic nature of the HABI. Upon ambient light irradiation, the resulting TPIRs between the fracture interfaces from two pieces of elastomer could recombine to form HABIs. As a result, they could self-heal without any stimuli, and the corresponding mechanism has been shown in Figure 4c. The previous report indicates that the self-healing of elastomer takes place in need of the solvent and does not work well at low temperature.33 Here, we perform the self-healing experiment of the solvent-free elastomer, which holds built-in advantage in maintaining the mechanical strength of healed materials. The self-healing could work even in freezing temperature (0 °C) while a little longer healing time is required, which excludes the thermoinduced self-healing (Figure S15). Thus, this self-healing technique based on photoswitching of HABI is water resistant, unnecessary to heat or pressure, which is promising in soft tissue or wound healing, self-healing, or sealing of dry-state solid materials such as elastomer, rubber, plastics, etc. Photoplastic PTA-HABI-TPE Spring. In addition, we fabricate a photoplastic spring based on dual crosslinkers of HABI and TPE (Figure 5 and the Supporting Information). We have synthesized the PTA-HABI-TPE spring with dipropargyl-PEG1000 as the hydrophilic unit, 1,6-diazidohexane as the linker, 2-Br-tetrapropargyl-HABI and tetrapropargylTPE as crosslinkers (Figure 5a). The sample demonstration is set at five states, i.e., original state (state I), loading at 100 g weight (state II), loading 100 g weight plus 405 nm light irradiation (state III), light-free unloading (state IV), and final photohealing (state V). The elastic strain is observed between state I and state II followed by the further plastic strain from state II to state III upon 405 nm light irradiation (Movie S3). Upon loading 100 g weight, PTA-HABI-TPE spring shows an elastic elongation. However, the irreversible plastic deformation occurs in addition to reversible elastic elongation when the loaded spring is exposed to 405 nm light. Removing light and loading (state III to state IV), the elastic strain disappears, but plastic strain still preserves. However, the sample can be recovered to the original length (state V), and plastic strain can be eliminated upon additional light irradiation (Movie S4). The self-recovery process of PTA-HABI-TPE photoplastic spring can be repeated many times (Figure 5b). It is discovered that the PTA-HABI-TPE photoplastic spring exhibits reversible stress−strain behavior (Figure 5c). The initial stress at 60% strain is about 0.11 MPa. The stress at 60% of strain

Figure 5. Photoplastic PTA-HABI-TPE spring based on dual crosslinkers of HABI and TPE. (a) The schematic synthetic procedure of solvent-free PTA-HABI-TPE elastomers. (b) The photographs of cycling for three times of the photoplastic PTAHABI-TPE spring. From left to right, original state (state I), loading at 100 g weight (state II), loading 100 g plus 405 nm light irradiation (state III), light-free unloading (state IV), and final photohealing (state V). (c) Reversible stress−strain curves of PTA-HABI elastomer upon cycled light irradiation. First cycle: initial state and light irradiation, second cycle: initial state and light irradiation, third cycle: initial state and light irradiation. The insert is the plot of stress changes at 60% strain in three cycles. The stress−strain behavior is reversible. (d) Proposed mechanism of the transformation between elastic and plastic strains of PTA-HABI-TPE photoplastic springs. 405 nm laser: 100 mW/cm2. The samples in (b) and (c) are dried gel strip of PTA-7.5%HABI-2.5%TPE with a size of 3.8 mm (thickness) × 3.8 mm (width) × 20 mm (length).

becomes 0.09 MPa after 405 nm irradiation during first cycle. At the initial state of second cycle, i.e., the recovered state after the first cycle, the stress at 60% strain is 0.10 MPa, whereas the stress drops back to 0.09 MPa after 405 nm irradiation. The initial stress falls in 0.10−0.11 MPa, which drops to 0.08−0.09 MPa after 405 nm irradiation. It is surprising that the elastic strain and plastic strain can be converted upon light irradiation, which is demonstrated in Figure 5d. The spring shows elastic strain (Se) upon loading weight, whereas further plastic strain (Sp) occurs with light irradiation because of the reversible crosslinking and domain slippage in the elastomer. At this very moment, the elastic strain will disappear, and plastic strain will conserve if light and loading are ceased. Finally, the plastic strain of the spring will disappear upon additional light irradiation, which is the corporative result of permanent crosslinking of TPE and photoswitching crosslinking of HABI. 23628

DOI: 10.1021/acsami.9b06608 ACS Appl. Mater. Interfaces 2019, 11, 23623−23631

ACS Applied Materials & Interfaces The total effect is the photoinduced transformation between the plastic and elastic strain of the photoplastic spring. It is customarily supposed that compared with the reversible elastic strain, the plastic strain is irreversible. However, upon light irradiation, the plastic strain of PTA-HABI-TPE spring can be eliminated and become reversible just like the elastic strain. Therefore, we demonstrate a prototype of the photoplastic spring, which exhibits a photoswitchable plastic strain behavior, that is, the plastic strain is transformed into the elastic strain upon light irradiation. Hexaarylbiimidazoles (HABIs) become the representatives of “T” type photochromic materials because of their fast fading kinetics of colorful radical recombination and strong fatigue resistance. HABI-based gel and elastomer provides multiple photoresponsive effects, i.e., photochromism, photoswitchable EPR, and sol−gel transformation as well as surface photopatterning. Photoinduced dissociation of HABI produces two radicals with visible absorption and EPR activity, which is called as photochromic and photoswitchable EPR effect. HABI integration with fluorophore produces photoswitchable fluorescent HABIs with high fluorescence quantum yield and fluorescence switching ratio, which has been applied in superresolution imaging. HABI gel can be used for laser melting, laser cutting, laser drilling, and laser remolding. HABI gel exhibits surface wrinkles because of the modulus mismatch between the surface and the bulk of the gel, which is caused by the photoinduced dissociation and recombination nature of PTA-HABI gel. The optical, electromagnetic, and surface properties of HABI-based photoswitchable gels are eventually determined by the photoswitchable molecular structure change. It is imaginable that HABIs can be integrated into more types of functional materials, such as adhesives, membranes, photoplastics, and photocurable resins, for developing more micro-optomechanical applications in photocrosslinking, photoablation, photoreconfiguration, photohealing, and four-dimensional-printing.

Research Article

■

EXPERIMENTAL SECTION

■

ASSOCIATED CONTENT

The detail experiment information (Schemes S1−S8, Method and Synthesis sections in the Supporting Information) and characterizations (Figures S19−S37) of PTA-HABI and PTA-HABI-TPE gels are placed in the Supporting Information. General Synthesis Procedures of PTA-HABI Organogels. Into a 10 mL sample tube, PEG1000-propynyl, 2-Br-tetrapropargylHABI (5/10% equiv, 12.1/24.1 mg, 0.0125/0.025 mmol), 1,6diazidohexane (DAH, 1.0 equiv, 43 mg, 0.25 mmol), and PMDETA (52 μL, 0.25 mmol) were added and dissolved in dry DMF (500 μL). The solution was subjected to 3× freeze−pump−thaw cycles using an oil pump and high pure N2. At this moment, the catalyst CuBr (28 mg, 0.2 mmol) was added into the sample tube and quickly stirred to mix them thoroughly. At the same time, the sample tube was set in a N2 atmosphere by vacuumed with an oil pump and then filled with N2. Generally, the reaction was an exothermal reaction at room temperature and finished within 5 min. The sample tube was further heated to 40 °C for 1 h to ensure the complete conversion of the starting materials. The hydrogel was dialyzed in an aqueous solution of EDTA−2Na for 3 days to remove Cu2+ and, consequently, the color of the hydrogel changes from deep blue-green to light yellow. The as-prepared gels were placed in the vacuum and dried at 70 °C for 24 h to obtain solvent-free dried gels, i.e., elastomers, which are used for self-healing experiment. The sample size for mechanical properties tests is 3.8 mm (thickness) × 3.8 mm (width) × 20 mm (length). According to the feed molar ratio of HABI relative to 1,6diazidohexane (DAH), the gels are named PTA-5%HABI (5% HABI relative to DAH) and PTA-10%HABI (10% HABI relative to DAH). The dried gels were immersed in different solvents to reach the swelling equilibrium before using to get corresponding organogels. For example, the most frequently used specimens, i.e., PTA-10% HABI-based DMF organogels, have the maximum swelling ratio of 8.27. Synthesis Procedures of PTA-HABI-TPE Organogels. Into a 25 mL sample tube, PEG1000-propynyl (1170 mg, 1.2 mmol), tetrapropargyl-TPE (7.5% equiv, 20.6 mg, 0.0375 mmol), 2-Brtetrapropargyl-HABI (2.5% equiv, 72/110 mg, 0.075/0.1125 mmol), 1, 6-diazidohexane (1 equiv, 252 mg, 1.5 mmol), and PMDETA (64 μL, 0.3 mmol) were added and dissolved in dry DMF (5 mL). The solution was subjected to 3× freeze−pump−thaw cycles using an oil pump and high pure N2. At this moment, the catalyst CuBr (44 mg, 0.3 mmol) was added into the sample tube, then a quick stir to mix them thoroughly. At the same time, the sample tube was set in a N2 atmosphere, vacuumed with an oil pump, and then filled with N2. Generally, this is an exothermal reaction at room temperature. The sample tube was further heated to 40 °C for 1 h to ensure the complete conversion of the polymerization. At last, the hydrogel was dialyzed in an aqueous solution of EDTA−2Na for 3 days to remove Cu2+ and, consequently, the color of the hydrogel changes from deep blue-green to light yellow. The as-prepared gels were placed in vacuum and dried at 70 °C for 24 h to obtain solvent-free dried gels, i.e., elastomers, which are used for photoplastic springs. According to the feed molar ratio of HABI and TPE relative to DAH, the asprepared organogel is named PTA-2.5%HABI-7.5%TPE organogel (2.5% HABI and 7.5% TPE relative to DAH). The sample size for mechanical properties tests is 3.8 mm (thickness) × 3.8 mm (width) × 20 mm (length).

■

CONCLUSIONS In summary, we have designed a new class of photoplastic transformation based on dynamic covalently crosslinked polytriazole (PTA) networks with the crosslinking points comprised of photocleaveable HABI, which exhibits abundant optomechanical properties and applications. With the aid of Click reaction between alkynes and azides, the synthetic procedure of PTA-HABI polymer networks has overcome the drawback of traditional radical polymerization methods. Upon light irradiation, the gel exhibits a transition from solid gel to a flowable sol. The photocontrolled disintegration and vulcanization of HABI crosslinking point have enabled the gel to be melt, cut, drilled, remolded, and surface-patterned by light. Based on the photoenhanced thermodynamic behavior of HABI, we have demonstrated that the PTA-HABI elastomer exhibits automatic self-healing properties at room temperature, which is significantly enhanced by white light. The photoplastic spring is fabricated and exhibits photoswitchable plastic strain behavior, i.e., the plastic strain is transformed into elastic strain upon light irradiation. HABI-based polymer networks, including solvated gel and solvent-free elastomer, are promising as smart materials for nonthermal photoactivated shape changing, transformation, and self-healing applications.

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06608. Materials and methods: the synthesis procedures of PTA-HABI geIs; optical properties of 2-bromo-terapropargyl-HABI in toluene solution; photoexcitation and fading of EPR signals of PTA-HABI gels; swelling properties of PTA-HABI gels; transformation of PTAHABI gel under continuous excitation of UV light with 23629

DOI: 10.1021/acsami.9b06608 ACS Appl. Mater. Interfaces 2019, 11, 23623−23631

Research Article

ACS Applied Materials & Interfaces

■

(6) Maeda, T.; Otsuka, H.; Takahara, A. Dynamic covalent polymers: Reorganizable Polymers with Dynamic Covalent Bonds. Prog. Polym. Sci. 2009, 34, 581−604. (7) Röttger, M.; Domenech, T.; van der Weegen, R.; Breuillac, A.; Nicolay, R.; Leibler, L. High-Performance Vitrimers from Commodity Thermoplastics through Dioxaborolane Metathesis. Science 2017, 356, 62−65. (8) Zhang, Y.; Ying, H.; Hart, K. R.; Wu, Y.; Hsu, A. J.; Coppola, A. M.; Kim, T. A.; Yang, K.; Sottos, N. R.; White, S. R.; Cheng, J. Malleable and Recyclable Poly(urea-urethane) Thermosets bearing Hindered Urea Bonds. Adv. Mater. 2016, 28, 7646−7651. (9) Li, C.; Yan, H.; Zhao, L. X.; Zhang, G. F.; Hu, Z.; Huang, Z. L.; Zhu, M. Q. A Trident Dithienylethene-Perylenemonoimide Dyad with Super Fluorescence Switching Speed and Ratio. Nat. Commun. 2014, 5, No. 5709. (10) Jia, C.; Migliore, A.; Xin, N.; Huang, S.; Wang, J.; Yang, Q.; Wang, S.; Chen, H.; Wang, D.; Feng, B.; Liu, Z.; Zhang, G.; Qu, D.H.; Tian, H.; Ratner, M. A.; Xu, H. Q.; Nitzan, A.; Guo, X. Covalently Bonded Single-Molecule Junctions with Stable and Reversible Photoswitched Conductivity. Science 2016, 352, 1443−1445. (11) Lv, J.-A.; Liu, Y.; Wei, J.; Chen, E.; Qin, L.; Yu, Y. Photocontrol of Fluid Slugs in Liquid Crystal Polymer Microactuators. Nature 2016, 537, 179−184. (12) Lu, X.; Guo, S.; Tong, X.; Xia, H.; Zhao, Y. Tunable Photocontrolled Motions Using Stored Strain Energy in Malleable Azobenzene Liquid Crystalline Polymer Actuators. Adv. Mater. 2017, 29, No. 1606467. (13) Iamsaard, S.; Aßhoff, S. J.; Matt, B.; Kudernac, T.; Cornelissen, J. J. L. M.; Fletcher, S. P.; Katsonis, N. Conversion of Light into Macroscopic Helical Motion. Nat. Chem. 2014, 6, 229−235. (14) Honda, S.; Toyota, T. Photo-Triggered Solvent-Free Metamorphosis of Polymeric Materials. Nat. Commun. 2017, 8, No. 502. (15) Kobatake, S.; Takami, S.; Muto, H.; Ishikawa, T.; Irie, M. Rapid and Reversible Shape Changes of Molecular Crystals on Photoirradiation. Nature 2007, 446, 778−781. (16) Mamiya, J. I.; Kuriyama, A.; Yokota, N.; Yamada, M.; Ikeda, T. Photomobile Polymer Materials: Photoresponsive Behavior of CrossLinked Liquid-Crystalline Polymers with Mesomorphic Diarylethenes. Chem. - Eur. J. 2015, 21, 3174−3177. (17) Zhou, H.; Xue, C.; Weis, P.; Suzuki, Y.; Huang, S.; Koynov, K.; Auernhammer, G. K.; Berger, R.; Butt, H.-J.; Wu, S. Photoswitching of Glass Transition Temperatures of Azobenzene-Containing Polymers Induces Reversible Solid-to-Liquid Transitions. Nat. Chem. 2017, 9, 145−151. (18) Takashima, Y.; Hatanaka, S.; Otsubo, M.; Nakahata, M.; Kakuta, T.; Hashidzume, A.; Ymotoguchi, H.; Harada, A. ExpansionContraction of Photoresponsive Artificial Muscle Regulated by HostGuest Interactions. Nat. Commun. 2012, 3, No. 1270. (19) Chen, S.; Tang, F.; Tang, L. Z.; Li, L. D. Synthesis of CuNanoparticle Hydrogel with Self-Healing and Photothermal Properties. ACS Appl. Mater. Interfaces 2017, 9, 20895−20903. (20) Tang, F.; Ma, N.; Tong, L. Y.; He, F.; Li, L. D. Control of Metal-Enhanced Fluorescence with pH- and Thermoresponsive Hybrid Microgels. Langmuir 2012, 28, 883−888. (21) Peng, X.; Liu, T. Q.; Jiao, C.; Wu, Y. Q.; Chen, N.; Wang, H. L. Complex shape deformations of homogeneous poly(N-isopropylacrylamide)/graphene oxide hydrogels programmed by local NIR irradiation. J. Mater. Chem. B. 2017, 5, 7997−8003. (22) Hayashi, T.; Maeda, K. Preparation of a new phototropic substance. Bull. Chem. Soc. Jpn. 1960, 33, 565−566. (23) Kobayashi, Y.; Mishima, Y.; Mutoh, K.; Abe, J. Highly Durable Photochromic Radical Complexes Having No Steric Protections of Radicals. Chem. Commun. 2017, 53, 4315−4318. (24) Verstraeten, F.; Göstl, R.; Sijbesma, R. P. Stress-Induced Colouration and Crosslinking of Polymeric Materials by Mechanochemical Formation of Triphenylimidazolyl Radicals. Chem. Commun. 2016, 52, 8608−8611.

different times; the temperature change of organogels in DMF when irradiated with 405 nm and 640 nm lasers; Images of PTA-HABI gel under irradiation of white light with different time and 405 nm; fluorescence spectrum of cross-link PTA-TPE gel; images of cross-link PTATPE upon various time of 405 nm laser irradiation; photochromic properties of 2-Br-HABI in the solution; disintegration of PTA-HABI gel upon irradiation; mechanical properties of PTA-HABI elastomers before and after light irradiation; stress−strain curves of two cut elastomers after healing; IR spectra, thermogravimetric analysis curve, 1H and 13C NMR spectra and mass spectra (PDF) Laser melting (MP4) Laser cutting (MP4) State II to state III (MP4) State IV to state V (MP4)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.-Q.Z.). *E-mail: [email protected] (G.J.C.). *E-mail: [email protected] (C.L.). ORCID

Gary J. Cheng: 0000-0002-1184-2946 Chong Li: 0000-0003-0453-2496 Ming-Qiang Zhu: 0000-0002-8886-4166 Author Contributions §

S.-L.X. and Q.-X.H. contribute equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Basic Research Program (973) of China (Grant Nos. 2015CB755602 and 2013CB922104), the National Science Foundation of China (NSFC 51673077, 21474034, and 51603078) and the Fundamental Research Funds for the Central Universities (HUST: 2016YXMS029 and HUST: 2018KFYXKJC033), and the Nature Science Foundation of Hubei Province (2018CFB574). We thank J.D. on helping the EPR measurements. We also thank Analytical and Testing Center of Huazhong University of Science and Technology, the Center for Nanoscale Characterization & Devices (CNCD), WNLOHUST, and the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for use of their facilities.

■

REFERENCES

(1) Montarnal, D.; Capelot, M.; Tournilhac, F.; Leibler, L. SilicaLike Malleable Materials from Permanent Organic Networks. Science 2011, 334, 965−968. (2) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Supramolecular Polymers. Chem. Rev. 2001, 101, 4071−4098. (3) Burnworth, M.; Tang, L.; Kumpfer, J. R.; Duncan, A. J.; Beyer, F. L.; Fiore, G. L.; Rowan, S. J.; Weder, C. Optically Healable Supramolecular Polymers. Nature 2011, 472, 334−337. (4) Zhang, F.; Xiong, L. G.; Ai, Y. J.; Liang, Z.; Liang, Q. L. Stretchable Multiresponsive Hydrogel with Actuatable, Shape Memory, and Self-Healing Properties. Adv. Sci. 2018, 5, No. 1800450. (5) Rowan, S. J.; Cantrill, S. J.; Cousins, G. R. L.; Sanders, J. K. M.; Stoddart, J. F. Dynamic Covalent Chemistry. Angew. Chem., Int. Ed. 2002, 41, 898−952. 23630

DOI: 10.1021/acsami.9b06608 ACS Appl. Mater. Interfaces 2019, 11, 23623−23631

Research Article

ACS Applied Materials & Interfaces (25) Ahn, D.; Zavada, S. R.; Scott, T. F. Rapid, Photomediated Healing of Hexaarylbiimidazole-Based Covalently Cross-Linked Gels. Chem. Mater. 2017, 29, 7023−7031. (26) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Click Chemistry: Diverse Chemical Function from A Few Good Reactions. Angew. Chem., Int. Ed. 2001, 40, 2004−2021. (27) Qin, A.; Lam, J. W.; Tang, B. Z. Click Polymerization. Chem. Soc. Rev. 2010, 39, 2522−2544. (28) Qin, A.; Lam, J. W. Y.; Tang, B. Z. Click Polymerization: Progresses, Challenges, and Opportunities. Macromolecules 2010, 43, 8693−8702. (29) Shima, K.; Mutoh, K.; Kobayashi, Y.; Abe, J. Enhancing the Versatility and Functionality of Fast Photochromic Bridged Imidazole Dimers by Flipping Imidazole Rings. J. Am. Chem. Soc. 2014, 136, 3796−3799. (30) Iwasaki, T.; Kato, T.; Kobayashi, Y.; Abe, J. A Chiral BINOLBridged Imidazole Dimer Possessing Sub-Millisecond Fast Photochromism. Chem. Commun. 2014, 50, 7481−7484. (31) Hou, H.; Yin, J.; Jiang, X. Reversible Diels-Alder Reaction to Control Wrinkle Patterns: from Dynamic Chemistry to Dynamic Patterns. Adv. Mater. 2016, 28, 9126−9132. (32) Kimoto, A.; Niitsu, S.; Iwahori, F.; Abe, J. Formation of Hexaarylbiimidazole Heterodimers via the Cross Recombination of Two Lophyl Radicals. New J. Chem. 2009, 33, 1339−1342. (33) Imato, K.; Nishihara, M.; Kanehara, T.; Amamoto, Y.; Takahara, A.; Otsuka, H. Self-Healing of Chemical Gels CrossLinked by Diarylbibenzofuranone-Based Trigger-Free Dynamic Covalent Bonds at Room Temperature. Angew. Chem., Int. Ed. 2012, 51, 1138−1142.

23631

DOI: 10.1021/acsami.9b06608 ACS Appl. Mater. Interfaces 2019, 11, 23623−23631