Photoplastic Self-Healing Polyurethane Springs and Actuators

Jun 28, 2019 - A novel class of photoplastic polyurethane elastomers based on ... was determined using differential scanning calorimetry (DSC) (-60-80...
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Photoplastic Self-Healing Polyurethane Springs and Actuators Shi-Li Xiang, Qiong-Xin Hua, Peng-Ju Zhao, Wen-Liang Gong, 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

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S Supporting Information *

ABSTRACT: Typical cross-linked polymer networks have rubbery elasticity and do not exhibit thermoplasticity such as linear polymers. However, thermodynamically cross-linked polymer networks exhibit certain thermoplasticity that allows for thermal processing. Here, we introduce a new concept of photoplasticity, that is, under light illumination, the covalently cross-linked polymer elastomers become flexible and plastic-like linear polymers. A novel class of photoplastic polyurethane elastomers based on dynamically covalent crosslinker hexaarylbiimidazole (HABI) and permanent cross-linker glycerol is designed and synthesized. Under the dual actions of light and stretching, the photoplastic elastomer exhibits reversible elongation and contraction-like springs. Because of the photoinduced reversible dissociation/recombination of HABI, two cut samples of solvent-free elastomers can be healed under irradiation at room temperature. Under asymmetric illumination, the photoplastic elastomer exhibits the appreciative phototropism, which demonstrates that artificial crucifer actuators are selectively driven by light. Photoplastic elastomers integrate photodriven healing, reversible stretching, and bending deformation, which offers the potential for new applications as optical actuators through optimized design of molecular structure, composition, and geometry.

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INTRODUCTION Smart actuators or artificial muscles that respond to external stimuli with changes in shape or size have recently attracted considerable attention.1−3 Polymers play an important role in these smart materials as they provide several key advantages such as low manufacturing cost, the ease of processing, flexibility, low density, and so forth.4 Reversible stimuliresponsive smart polymeric elastomers containing photochromic molecular motifs, such as azobenzenes, spiropyrans, naphthopyran, and diarylbibenzofuranones, were investigated as sensors and actuators.5−17 The distinguished molecular architectural transformation based on the mechanisms such as cis−trans isomerization and cyclization/cycloreversion upon light irradiation has been reported to control the fluorescence, conductivity, and fluidic properties of materials.18−20 Compared with popular photochromic molecules, hexaarylbiimidazole (HABI) undergoes photoinduced cleavage of the C−N bond of two triphenylimidazoles upon light irradiation to give a pair of isolated triphenylimidozole radicals (TPIRs), which can undergo spontaneous annihilation upon encountering adjacent radicals.21,22 Honda and Toyota synthesized the solvent-free HABI cross-linking polymer networks showing isothermal reversible liquid−nonliquid conversion, while the synthesis procedures involving star-shaped precursors are relatively complicated, and subsequent oxidation cross-linking efficiency of triphenylimidozoles is limited.23 Ahn et al. prepared the HABI-incorporating, covalently cross-linking gels that have the ability of rapidly self-healing with the aid of solvents.24 Despite that the investigations of HABI in selfhealing and liquid−solid conversion were reported, the photodriven deformation and multifunctional integration© XXXX American Chemical Society

based HABI motif remains to be developed for novel photoactuators and optomechanical systems. Herein, we designed a novel kind of dual-cross-linking photoplastic polyurethane (PPU) elastomers with multifunctional integration involving photochromism, photodriven deformation, and self-healing functions. The dual-cross-linking PPU elastomers were fabricated by stepwise addition polymerization, in which four hydroxyl-decorated HABI and glycerol were used as dynamic and fixed cross-linking agents, respectively (Figure 1a). The cross-linking density in the PPU network depends on the reversible cleavage and recombination of the imidazole−imidazole covalent bond in HABI (Figure 1b). Upon UV irradiation, the HABI moieties as dynamic cross-linking points are dissociated into two TPIRs, causing the cross-linking density of whole polyurethane network to decrease remarkably with apparent elongation. This work provides a new idea to create novel soft smart materials and systems integrating multiple functions.

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RESULTS AND DISCUSSION The dual-cross-linking PPU elastomer was prepared from the copolymerization of polytetramethyleneglycol (PTMG1000), hexamethylene diisocyanate (HDI), di-n-butyltin dilaurate catalyst (DBTDL), glycerol, and four-armed photochromic cross-linker HABI (2-Cl-4-diol-HABI) (Figure 1a). As fixed cross-linking points, glycerol-based networks maintain the permanent elasticity of PPU materials, while dynamic covalent Received: March 11, 2019 Revised: June 27, 2019 Published: June 28, 2019 A

DOI: 10.1021/acs.chemmater.9b00983 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 1. Design and photochromism of dual-cross-linking PPU elastomers. (a) Schematic representation of chemical composition and the network structure of PPU elastomers. (b) Cleavage and reformation of the dynamic covalent bond in HABI. (c) UV−vis absorption spectra of 2Cl-4-diol-HABI and PPU elastomer after 405 nm irradiation. (d) Absorbance change measured at 600 nm of PPU subjected to UV irradiation and thermos-recover cycles. Pink areas: irradiation at 405 nm for 10 s. Cyan areas: after ceasing 405 nm laser. (e) Fading kinetics of the colored PPU film. t1/2 is the half-life time of the TPIR absorbance in the PPU film.

bonds of HABI enable the photoplastic and self-healing performance of PPU materials due to the dynamic equilibrium between HABIs and TPIRs under mild conditions. The molar ratio of 2-Cl-4-diol-HABI, glycerol, PTMG, and HDI is optimized to be 1:3:12.5:13.7 to ensure the resilience of the PPU network before and after photoinduced dissociation of cross-linker HABI (Table S1 and Note S1). The halogen atom at the o-position would speed up the fading kinetics and improve the fatigue resistance.25 The photochromism of 2-Cl4-diol-HABI is investigated in tetrahydrofuran (THF) solution by UV−vis spectra. Before irradiation, no absorption peak is detected in the visible region, while two new peaks at 385 and 600 nm emerge after 2 s of irradiation (Figure S1a). The color change of 2-Cl-4-diol-HABI in solution before and after irradiation has been observed (Figure S1a inset). The fading kinetics of the colored species indicates that the optical density decreases rapidly in the first 100 s and then becomes flat after 300 s (Figure S1b). The half-life time (t1/2) of the colored species is calculated to be 60 s. The curve plot of 1/[At]−t indicates that the recombination of TPIRs is first-order reaction (Figure S1b inset). The absorbance of 2-Cl-4-diolHABI can be switched with periodic UV irradiation, demonstrating that the photochromism of 2-Cl-4-diol-HABI possesses high reversibility (Figure S1c). The transparent thin film (film thickness: 0.1 cm) of the dual-cross-linking PPU elastomer is prepared by reactive casting. Upon irradiation with 405 nm laser for 10 s, the asprepared colorless PPU film becomes blue, and an absorption peak around 600 nm has been detected, which is in accordance with the photochromism of 2-Cl-4-diol-HABIs (Figure 1c). The fatigue resistance of PPU indicates that the photoinduced process could be reversibly performed for more than eight times (Figure 1d). Upon 405 nm laser irradiation for 10 s, the optical density increases rapidly due to the photoinduced

dissociation of HABI to TPIRs (pink area in Figure 1d). After ceasing the light, the optical density decreases to the initial state (cyan area in Figure 1d). It is worth noting that the optical density in the PPU film decreases to half in 5 s (t1/2,PPU = 5 s) (Figure 1e), which is much faster than 2-Cl-4-diol-HABI in the solution state (t1/2,solution = 60 s) (Figure S1b), indicating that TPIRs in the PPU film possess faster recombination capability. The PPU elastomers show the excellent macroscopic photohealing behavior at room temperature. Two dumbbellshaped PPU specimens containing HABI were cut with a razor blade (Figure 2a). The cut pieces from different specimens were pressed together. One of the PPU specimen was stained with Rhodamine B during preparation so that two PPU blocks could be distinguished with different colors in the picture. After healing for 12 h under illumination from a table lamp at room temperature, the combinatorial PPU specimen with different colors did not fracture at the cut/resealed position even under manual stretching stress. In comparison, the cut pieces of PU control with the glycerol cross-linker only could not be coalesced and are vulnerable to breaking into two pieces by manual stretching after placing it under visible light for 12 h. These results indicate that the self-healing ability originates from the photoinduced dynamic HABI linkages (Figure 2b). To quantitatively evaluate this healing behavior, tensile tests were performed using the dumbbell-shaped specimens. The effects of the healing time and temperature on the healing process were systematically investigated. Figure 2c shows typical stress−strain curves for PPU before and after healing for 1, 6, and 12 h under visible light at room temperature. The stress−strain curves of the self-healing PPU specimens gradually approached that of the original intact specimen by prolonging healing time. The strain at break of the PPU sample after healing for 6 h becomes twice of that after healing for 1 h B

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Figure 2. Photohealing of PPU elastomers. (a) Photographs of self-healing dual-cross-linking PPU compared with HABI-free PU control under visible light at room temperature. Original, cut, healed (12 h), and stretched state. Scale bars, 0.5 cm. (b) Schematic of the self-healing PPU elastomer, indicating that the self-healing ability originates from the photoinduced cleavage and reformation of C−N in HABI. (c) Stress−strain curves healing for different healing times (1, 6, and 12 h) of the PPU elastomers under visible light at room temperature. (d) Dependence of fracture strain and maximum stress on healing time under visible light from (c). (e) Stress−strain curves for the PPU after healing for 12 h at different temperatures (40, 50, and 60 °C) without irradiation. (f) Dependence of fracture strain and maximum stress on healing temperatures from (e). (g) Stress−strain curves for the PPU healing for different times (3, 6, 12, and 24 h) at 60 °C without irradiation. (h) Dependence of fracture strain and maximum stress on healing time at 60 °C. Note that the fracture strain and the maximum stress are the mean value of three repeated experiments. Error bars show standard deviation.

tests and tensile tests of them were performed to demonstrate the effect of chain mobility on self-healing efficiency of these materials under illumination (Figure S4). The results reveal that the longer chain of PTMG and higher content of photoresponsive HABI will bring greater healing performance. (Note S3 and S4). The temperature dependence of self-healing in the dark is examined using typical stress−strain curves for the PPU specimens before and after healing for 12 h at 40, 50, and 60 °C (Figure 2e). After self-healing for 12 h at 40 °C, the strain at break recovers to ∼10%. Increasing to 60 °C, the recovery of strain is still below 40% (Figure 2f). The recovery of stress is also poor. The self-healing performance at higher temperature is more efficient than that at room temperature but much poorer than that with light illumination. Furthermore, Figure 2g shows typical stress−strain curves for PPU before and after healing for 3, 6, 12, and 24 h at 60 °C without light irradiation. It is demonstrated that the recovery performance becomes better by increasing healing time. The approximately 80% recovery of the original fracture strain and maximum stress could be achieved after healing at 60 °C for 24 h in the dark (Figure 2h). In comparison, light illumination for 12 h at room temperature attains the recovery of 90% (Figure 2d). Therefore, it is concluded that light illumination causes much better healing performance than elevating temperature.

while half of that after healing for 12 h (Figure 2d). More than 80% of original fracture maximum stress is recovered while the strain at break becomes slightly over 100% after healing for 12 h, indicating an excellent self-healing performance. Considering that permanently cross-linked glycerol networks have no capacity of self-healing after cutting under visible light, the total cross-linking density and elastic modulus are somewhat decreased in the PPU specimens after healing. As a result, the maximum strain after healing for 12 h under visible light is slightly larger than that of the original one, while the fracture stress cannot return to that of the original one. This is possibly because the partially decreased toughness of the PPU under relatively small stress after 12 h healing upon illumination causes the photo-healed PPU to be stretched slightly longer than the original PPU without any treatment. The self-healing efficiency of the PPU materials could be further improved when less glycerol is used (Figure S3). For the sample of PPU1 containing a higher content of HABI dynamic cross-linked points, more than 99% of original fracture maximum stress would be recovered after healing for 6 h. Self-healing performance involves the mobility of the polymer chains. A good mobility of the polymer chains is conducive to the selfhealing process. Three polyurethane elastomers (PPU, PPU2, and PPU3) with different PTMG chain lengths (PTMG1000, PTMG850, and PTMG2000) were compared. Stress relaxation C

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Figure 3. Photoplastic mechanical testing of PPU springs. (a) Photoinduced elongation and contraction of PPU springs. I: original state; II: elastic extension under loading weight (100 g); III: plastic deformation with loading weight upon 405 nm light irradiation; IV: unloading and removing the 405 nm light; V: recover state upon 405 nm irradiation. “F” represents “loading weight”. “−F” means “unloading weight”. The arrows represent the various level of strains. Gray as loading weight and blank as unloading. (b) Photoinduced repeatable elongation and contraction of PPU springs after self-healing for 12 h under visible ligh. I: original state; II: elastic extension under loading weight (100 g); III: plastic deformation with loading weight upon 405 nm light irradiation; IV: unloading and removing the 405 nm light; V: recover state upon 405 nm irradiation. “F” represents “loading weight”. “−F” means “unloading weight”. The arrows represent the various level of strains (gray means loading weight, while blank means unloading). (c) Reversible stress−strain curves of the original dumbbell-shaped PPU elastomer (3.6 cm × 0.38 cm × 0.22 cm) upon cycled light irradiation. (d) Reversible stress−strain curves of the photohealed PPU elastomer (3.6 cm × 0.38 cm × 0.22 cm) upon cycled light irradiation. (e) Loading−unloading curves from 50% strain to 150% strain of the PPU elastomer after healing for 12 h under visible light, which is obtained from 500 continuous cycles at 1 Hz. (f) Minimal (50% strain) and maximal stresses (150% strain) of the PPU elastomer after healing for 12 h under visible light (hollow red dots) and original one (black squares) in 500 continuous loading−unloading curves from 50% strain to 150% strain. Note that in the figure the end of the loading−unloading curve does not indicate specimen failure. (g) Stress relaxation modulus [G(t)] of PU control samples with or without 405 nm irradiation. (h) Stress relaxation modulus [G(t)] of original PPU samples with or without 405 nm irradiation.

removing the light and counterweights, the PPU elastomer began self-contracting. The PPU strip retains good resilience, but it could not recover to its original length (state IV). When we irradiated the PPU sample using the 405 nm laser again and placed it in the dark for a period of time, the sample was found to continue to shrink and finally closer to the original length (state V) (Figure 3a). The dual-cross-linking PPU elastomers exhibit great resilience both before and after light irradiation. At the beginning, the dual-cross-linking points of the PPU strip contribute to the resilience of the PPU film. Upon UV irradiation, most of HABI cross-linking points are dissociated; the resulted TPIRs are dislocated due to the vertical gravity (F) when photoinduced dissociation of the C−N bond in polyurethane occurs with loading weight. After ceasing UV irradiation, the PPU specimens enters a pseudo-steady state because of the dislocation rearrangement of HABI cross-

The photoinduced repeatable elongation and contraction of dual-cross-linking PPU elastomers act like artificial photoplastic springs, which are demonstrated by loading weight upon 405 nm light irradiation (Figure 3a). The PPU specimens were thermally molded into a dumbbell-shape (Figure S2a). At the beginning, the original sample (state I) was stretched for 30 min with loading weight (a counterweight, ca. 100 g), ensuring that the sample attained the fixed elastic extension (state II). The sample was then illuminated with 405 nm laser (100 mW). It is visible to naked eyes that the elastomer will continue to elongate gradually, exhibiting photoinduced plastic deformation accompanied with photochromism in 1−2 min (state III). The irradiated area becomes deep blue. As we moved the light resource to other area, the deep blue disappeared quickly (Video S1). The blue spot moved along with the movement of the light spot. After D

DOI: 10.1021/acs.chemmater.9b00983 Chem. Mater. XXXX, XXX, XXX−XXX

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second cycle which is also the recovered state after the first cycle, the stress at 100% strain is 0.62 MPa, while the stress drops back to 0.34 MPa after 405 nm irradiation. At the initial state of the third cycle, the stress at 100% strain falls to 0.60 MPa and drops to 0.35 MPa again after 405 nm irradiation (Figure 3d). To investigate the fatigue resistance of PPU after healing for 12 h under visible light, the original PPU and photohealing PPU endured 500 continuous fatigue cycles from 50% strain to 150% strain with oscillation frequency at 1 Hz (Figures 3e, S5a, and Video S4), which shows that they both have the same decay trend of the stress (Figure 3f). Considering that permanently-cross-linked glycerol networks have no ability to self-healing after cutting under visible light, the total cross-linking density and elastic modulus are somewhat decreased in the PPU specimens after healing. It is reasonable that the stress of PPU after healing for 12 h under visible light is smaller than that of the original one. The photoplastic properties and fatigue resistance of self-healed PPU specimens are similar to the original photoplastic PPU elastomers, indicating the good recyclability, excellent repeatability, and fatigue resistance of photoplastic PPU materials. In addition, the loading−unloading curves in Figure 3e show clear hysteresis. To prove that the HABI dynamic cross-linked points are not mechanically labile and will not cause energy dissipation to PPU elastomers, fatigue resistance tests have been conducted to the PPU (with HABI cross-linkers) and PU control samples (without HABI cross-linkers) (Figure S5a,b). The dissipated energy curves of PPU and PU control are very close (Figure S5c). The energy dissipation of original PPU at each of the same cycle is only slightly lower than that of the PU control sample. It can be regarded that the PU control sample and the original PPU have almost the same hysteresis. It should be noted that the main difference in the structure between PU control and the PPU sample is that the HABI cross-linkers in PPU are totally replaced by glycerol crosslinkers, although their hysteresis curves are almost the same. Therefore, the above mentioned results reveal that the HABI dynamic cross-linkers are relatively mechanically stable. Both PU control and PPU samples have nearly the same decay trend of the stress during the test cycles (Figure S5d). It is further confirmed that the dynamic cross-linked points are mechanically stable and does not contribute to the energy dissipation. To further elucidate the importance of dynamic covalent bonds in HABI from the view of rheology, stress relaxation tests were performed to demonstrate the distinction on chain mobility of the PU control samples and PPU with/without 405 nm irradiation.26,27 Within 500 seconds, the attenuations of stress relaxation modulus [G(t)] of the PU control sample and PPU before 405 nm irradiation are measured to be 0.14 and 0.15 MPa, respectively (Figure 3). Both of the PPU or control samples have almost the same attenuation of G(t) without 405 nm irradiation. It indicates that the polymer chains of the original PPU and PU control sample themselves have a certain mobility, which is beneficial to self-healing in some degree. As expected, upon 405 nm irradiation, the G(t) attenuation of the PU control sample is about 0.13 MPa which is almost unchanged (Figure 3g), while that of PPU under irradiation is 0.24 MPa upon 405 nm (Figure 3h). It suggests that the chain mobility of PPU materials containing dynamic HABI crosslinked points has been greatly improved upon 405 nm irradiation. Thus, the self-healing ability of PPU is prominently superior to PU control upon light irradiation. PPU materials

linking points. Removing loading weight, the PPU specimens would partially contract due to the existence of fixed crosslinking points based on glycerol but could not recover to the original state because of the dislocation of HABI cross-linking points. At this time, UV irradiation would help the sample to recover to the original state. Upon UV irradiation again, HABIs decompose into TPIRs, which will be moved because of the elastic or memory effect of the glycerol cross-linking system. To minimize the internal stress of the glycerol cross-linking system, the dislocated TPIRs rearrange to find the TPIRs at the original sites upon the second light irradiation. Thus, the sample will shrink further until it reaches its initial length. This unique behavior of the HABI-containing cross-linking polymer is attributed to photoresponsive bond exchange of HABI linkages and the resulting network rearrangement. We repeated the above experiment three times with the same sample (Videos S2 and S3). After three cycles, the PPU sample can still recover almost to the initial length, which shows that the sample has good repeatability and fatigue resistance. The optomechanical response of PPU elastomers is demonstrated by the static tensile experiment. The PPU samples possess excellent elasticity without UV irradiation (Figure S2b). The PPU shows repeatable photoinduced elastic deformation within 100% strain before and after 405 nm light irradiation for three cycles (Figure 3c). From the typical stress−strain profiles, it is observed that the mechanical properties of PPU elastomers are almost recovered after three cycles, further proving that the materials have excellent repeatability and fatigue resistance. The photohealing PPU elastomers are also examined as photoplastic springs using the same way as original PPU elastomers. We have conducted the photoplastic experiment of photohealing PPU springs, that is, photoinduced repeatable elongation and contraction of PPU specimens after self-healing for 12 h under visible light. Similar to original PPU photoplastic springs, the self-healing PPU sample goes across five states: (I) original state, (II) elastic elongation with 100 g of loading weight, (III) photoplastic elongation with 100 g of loading weight under UV irradiation, (IV) elastic contraction without UV irradiation and loading weight, and (V) final photoinduced plastic contraction. First, the original samples (state I) are stretched for 30 min with loading weight (state II), ensuring that the samples attain fixed elastic deformation. The samples are then irradiated with a 405 nm laser. It is visible to naked eyes that the elastomers will continue to elongate gradually, exhibiting photoinduced plastic deformation accompanied with photochromism (state III). The irradiated area exhibits a deep blue color change, which moves as we move the light resource to another area. After removing the counterweights, the PPU springs begin to contract, but their lengths cannot recover to their original ones (state IV). Upon 405 nm light irradiation for 10 s followed by keeping in the dark, the PPU samples continue to shrink and finally approach to their original lengths (state V) (Figure 3b). The experiments have been repeated three times. The stress− strain behaviors of photoplastic self-healing PPU springs are recorded. The measured results also indicate that the mechanical properties of the cut specimens are recovered after healing under visible light for 12 h. It is observed that photoplastic self-healing PPU springs exhibit reversible stress− strain behavior. The initial stress at 100% strain is about 0.65 MPa. The stress at 100% of strain becomes 0.36 MPa after 405 nm irradiation during the first cycle. At the initial state of the E

DOI: 10.1021/acs.chemmater.9b00983 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 4. Phototaxis behaviors of PPU elastomers. (a) Photographs of the elongation with loading weight (a counterweight, ca. 20 g), the negative and the positive phototaxis of the PPU strip (4 cm × 0.5 cm × 0.1 cm), UV irradiation from the right side. (b) Photographs of the elongation with loading weight (a counterweight, ca. 20 g), the negative and the positive phototaxis of the PPU strip (4 cm × 0.5 cm × 0.1 cm), UV irradiation from the left side. Scale bars, 1 cm. (c) Schematic of photoinduced actuators. It was proposed according to the observed behaviors in (a). Each state in the schematic is consistent with the state in (a). SF and PF represent the shady face and phototropic face of the PPU strip, respectively. F represents the vertical gravity of the counterweight. F1 represents the contraction stress of the shady face in state IV. F2 represents the contraction stress of the phototropic face in state IV. The red arrow represents the direction of the force. L1, L2, and L3 is the length of the shady face and phototropic face of the PPU strip in states I, II, and III, respectively. L4 and L5 is the length of the shady face and phototropic face of the PPU strip in state IV, respectively. The red line represents the polymer chains. (d) Localized photodriven actuators of a blooming crucifer flower.

obtain better chain mobility under illumination, which is the key mechanism to self-healing and photoplastic deformation. The photoinduced repeatable elongation and contraction of dual-cross-linking PPU elastomers act like photoplastic springs, which is promising in photoinduced deformation or actuators in optomechanical systems. An interesting phototaxis behavior of dual-cross-linking PPU elastomers is observed upon asymmetrical illumination (Figure 4a). The PPU specimens were cut into strips (4 cm × 0.5 cm × 0.1 cm). At the beginning, the original sample was stretched with loading weight (a counterweight, ca. 20 g). The sample was then illuminated from one side (right) of the PPU strip with 405 nm laser (100 mW). At the beginning, it is visible to naked eyes that the elastomer will continue to elongate gradually, exhibiting photoinduced deformation accompanied with photochromism. The irradiated area becomes deep blue. Afterward, removing the light and counterweights, the PPU elastomer begins bending to the left along with previously described self-contraction, indicating the negative phototaxis. It is more surprising that the bent PPU strip was straightened again upon further UV irradiation from the right side, which is the positive phototaxis instead (Video S5). Similarly, the stretched PPU strip was illuminated from the left side bent to the right after removing the UV irradiation and counterweight (Video S6). Further UV irradiation from the left side straightened the PPU strip again (Figure 4b). The whole phototactic phenomena of the PPU strip can be repeated for many times. This is of much significance because repeating light irradiation from the same side led to the bending and

straightening behavior of the PPU strip at different situations. We propose the molecular schematic of photoinduced actuators according to the observed behavior (Figure 4c). It is well known that light is directional, which causes asymmetric illumination from one side, or focused in a fixed spot with precise localization. Therefore, photoinduced actuators can be more easily controlled than thermoinduced behavior. The PPU strip elastomer is stretched when loading weight due to the great elasticity, which results from the dual-cross-linking of glycerol and HABI. However, the PPU strip is further stretched upon UV irradiation. It is worth noting that the extinction coefficient of HABI moieties at approximately 405 nm is large (about 0.6 × 104 L mol−1 cm−1), and more than 99% of the incident photons are absorbed by the surface with a thickness of less than 120 μm (Supporting Information Note S2). Therefore, more HABI groups are decomposed into TPIRs only in the phototropic face upon exposure to UV light, causing much lower cross-linking density than that at the shady face. After removing light but still with counterweight, the resulting TPIRs recombine into HABIs to form new crosslinking points with a lot of distinct dislocations. By removing the counterweight, the resulting contraction stresses between phototropic and shady face are unbalanced despite that crosslinking density in total has returned to the initial level. The dual cross-linking points including glycerol and HABI both contribute to the formation of contraction stress (F1) in the shady face. However, in the phototropic face, only glycerol is responsible for the contraction stress (F2), while dislocated HABIs stabilize the stretched PPU strip instead. Obviously, F2 F

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poral controllability than thermoplasticity. In addition, the PPU has been verified to be of moderate long term stability even it is exposed to air and sunlight (Figure S8 and Note S5). It is imaginable that HABIs are integrated into more types of functional materials, such as adhesives, coating, photoplastics, and photocurable resins, for developing more microoptomechanical applications.

is smaller than F1. Therefore, after removing counterweight, the PPU strip begins to shrink and the shady face of the PPU strip contracts more than the phototropic face. The final length of the shady face and phototropic face after contracting is L4 and L5 (L4 < L5), respectively, resulting in the negative phototaxis. It is the unbalance of contraction stress between phototropic and shady faces that causes the strip bend to the shady face. The internal stress in the shady face of the bent PPU strip has been released, while there is still a balance between contraction stress and stretching stress in the phototropic face of the PPU strip, which results from glycerol and HABI cross-linking points, respectively. Upon second time of UV irradiation, the dislocated HABI cross-linking points in the phototropic face of the PPU strip split again and stretching stress is released. The remaining contraction stress from glycerol cross-linking points in the phototropic face are responsible for the contraction of the PPU strip, causing the recovery of the PPU strip to the original state, showing a positive phototaxis effect. The phototaxis effect of PPU elastomers enables the photodriven movement of soft robots. We fabricate a crucifer to demonstrate the localized photodriven actuators (Figures 4d and S7). In the beginning, the AB branch is stretched followed by UV irradiation from the lower side of the AB branch. The stretched AB branch of the PPU crucifer bends to upper direction after removing UV irradiation and stretching. Similarly, the CD branch also can bend to the upper direction to take advantage of the negative phototaxis effect of PPU elastomers. It acts as a photodriven gripper when the PPU crucifer is exposed to light from the lower side. After light irradiation from the lower side again, the gripper opens four hands just like a blooming flower (Video S7). It is probably the first or at least one of the rare reports about the negative and positive phototaxis effects of PPU elastomers. The dual-cross-linking PPU networks are the promising photocontrol plastic elastomers, which integrates photohealing, photodriven reversible elastic deformation, and bendability. This unique behavior of HABI-containing crosslinking polymers is attributed to photoresponsive bond dissociation and exchange of the HABI linkages and the resulting rearrangement of cross-linking points in the polymer network. As we know, the thermoplasticity of linear polymers provides great convenience for the processing of synthetic resins. However, cross-linking resins possessing better thermomechanical properties are difficult to thermally process. To explore the processability of novel cross-linking resins, many dynamic cross-linking polymer networks have been designed, such as supramolecular cross-linking systems, ionomers, and dynamically covalently cross-linking polymers. However, these polymers have limitations such as mechanical strength, fine controllability, and sensitivity. The exploration of the new dynamic covalent cross-linking system is in the ascendant. We propose to use HABI as a light-driven dynamic cross-linking system, which not only has the solvent-free photohealing function at room temperature but also has photodriven stretching and bending deformation capabilities. The optomechanical performance of HABI-based photoswitchable polymer networks, and their optical and electromagnetic properties which were previously reported, are eventually determined by the photoswitchable molecular structure change. Compared to thermoplasticity, light enables precise positioning of optical actuators in time and space. Therefore, photoplasticity possesses much better spatiotem-

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CONCLUSIONS In summary, photoplastic dual-cross-linking PU elastomers are designed and prepared by stepwise addition polymerization of diisocyanates, PTMG, and the dual-cross-linking system constituted with glycerol and HABI cross-linkers. The PPU elastomers can work as photoplastic springs, exhibiting photoinduced repeatable elongation and contraction upon UV irradiation, which is attributed to the photocleavage of the C−N bond in HABI and the partial disintegration of elastomer networks. PPU elastomers also exhibit excellent self-healing performance at room temperature under visible light. The selfhealing PPU elastomers act as photoplastic springs in the same way as original PPU elastomers. The negative and positive phototaxis effects of PPU elastomers are observed in the same specimen, which are responsive for photodriven reversible bending behaviors. The dual-cross-linking PPU networks are promising as smart materials in biological systems and recycled materials. Our method provides a new idea to create new soft smart materials and systems integrating multiple functions.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Synthesis of PPU Elastomers. PTMG1000 (1.9 g, 1.9 mmol, 12.5 equiv) and DBTDL (8 mg, 0.013 mmol, 0.1 equiv) were dissolved in 5 mL anhydrous THF to form solution A. Tetrahydroxy-functionalized HABI (128 mg, 0.152 mmol, 1 equiv) and glycerol (42 mg, 0.46 mmol, 3 equiv) were dissolved in 5 mL anhydrous THF to form solution B. Solution A and B were mixed and stirred at room temperature for 30 min. HDI (350 mg, 2.08 mmol, 13.7 equiv) was then added dropwise into the mixed above solution. Then, the mixture was heated and stirred at 30 °C under vacuum to remove THF, degassed, and poured into a Teflon mold. The mold was kept in an N2-purged oven at 45 °C for 2 days. The final samples were removed from the mold and kept under ambient conditions for 12 h before testing. Photoplastic Mechanical Testing. Static tensile tests were carried out at room temperature at a speed of 2 mm/min. First, the stretch test was performed on the initial dumbbell-shaped PPU sample (3.6 cm × 0.38 cm × 0.22 cm) to 100% strain. The width of the dumbbell-shaped PPU is the middle width of the sample. The sample was then released to rebound to the initial length; the tensile test was carried out again to the same sample, which was irradiated continuously with a 405 nm laser. At the end of the first round of testing, the sample was placed in the dark environment for 12 h, followed by a second round of testing, which was repeated three times. S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.9b00983. Synthesis procedures; optical properties of 2-Cl-4-diolHABI; photo of the setup for deformation tests; stress− strain tests; stress relaxation tests; loading−unloading cycles; energy dissipation curves; infrared spectra and differential scanning calorimetry curve; demonstration of the photodriven PPU crucifer; characterization of long G

DOI: 10.1021/acs.chemmater.9b00983 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

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term utility; and NMR (1H and 13C) spectra and mass spectra (PDF) First cycle of photoinduced elongation (AVI) Second cycle of photoinduced elongation (AVI) Third cycle of photoinduced elongation (AVI) Loading−unloading cycles of the PPU after healing for 12 h (AVI) Phototaxis behavior (AVI) Phototaxis behavior (AVI) Localized photodriven actuators (AVI)

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AUTHOR INFORMATION

Corresponding Authors

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

Chong Li: 0000-0003-0453-2496 Ming-Qiang Zhu: 0000-0002-8886-4166 Notes

The authors declare no competing financial interest.

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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: 2019kfyXKJC035 and HUST: 2018KFYXKJC033), and the Nature Science Foundation of Hubei Province (2018CFB574). We also thank the Analytical and Testing Center of Huazhong University of Science and Technology and the Center of Micro-Fabrication and Characterization (CMFC) and the Center for Nanoscale Characterization & Devices (CNCD) of WNLO for use of their facilities.

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DOI: 10.1021/acs.chemmater.9b00983 Chem. Mater. XXXX, XXX, XXX−XXX