Protective Coating with Crystalline Shells to Fabricate Dual-Stimuli Responsive Actuators

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jan 2022Protective Coating with Crystalline Shells to Fabricate Dual-Stimuli Responsive Actuators Peixin Xu, Qi Yu, Yao Chen, Peng Cheng and Zhenjie Zhang Peixin Xu State Key Laboratory of Medicinal Chemical Biology, College of Chemistry, Nankai University, Tianjin 300071 , Qi Yu State Key Laboratory of Medicinal Chemical Biology, College of Chemistry, Nankai University, Tianjin 300071 Shandong Provincial Key Laboratory of Fine Chemicals, School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology, Jinan 250353 , Yao Chen State Key Laboratory of Medicinal Chemical Biology, College of Chemistry, Nankai University, Tianjin 300071 , Peng Cheng State Key Laboratory of Medicinal Chemical Biology, College of Chemistry, Nankai University, Tianjin 300071 Key Laboratory of Advanced Energy Materials Chemistry, Ministry of Education, Nankai University, Tianjin 300071 and Zhenjie Zhang *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Medicinal Chemical Biology, College of Chemistry, Nankai University, Tianjin 300071 Key Laboratory of Advanced Energy Materials Chemistry, Ministry of Education, Nankai University, Tianjin 300071 https://doi.org/10.31635/ccschem.021.202000663 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The hybridization of mechanically responsive molecular crystals with polymers has proven to be an efficient approach to fabricate smart hybrid materials that can produce multiple motions upon external stimulus actuation. However, this fabrication approach occasionally displays limitations due to the solubility or poor dispersibility of molecular crystals in the polymer matrix solution. To address these challenges, we have created a facile and versatile strategy to use metal–organic frameworks (MOFs) as a protective coating against external perturbations while also improving the dispersibility of molecular crystals. As such, a series of hybrid smart materials with reversible photomechanical performance were successfully fabricated. Notably, the afforded smart materials can combine the photoresponsive properties of mechanically responsive molecular crystals with the vapor-responsive properties of certain polymers in one system, thereby obtaining dual-stimuli responsive actuators that can perform complicated motions (e.g., crawling). These results pave the way for the fabrication of multistimuli responsive smart materials and broaden the applicable scope of MOFs. Download figure Download PowerPoint Introduction Mechanically responsive smart materials that can convert chemical energy into mechanical movement have been attracting considerable interest in a wide range of advanced applications such as robotics, actuators, and sensors.1–8 This energy transformation can be achieved by the reversible deformation of materials (e.g., contraction, expansion, rotation, bending, twisting, and curling) under external stimuli such as electrical fields, temperature, light, pH, vapor, and pressure.9–12 At present, most mechanically responsive materials are based on polymeric materials, including organic polymers, hydrogels, and liquid crystal elastomers,13–17 while relatively less focus has been placed on molecular crystals that possess precisely determined structures with ordered molecular packing. Molecular crystals exhibit many advantages as smart materials that can surpass traditional polymeric materials such as rapid response, faster relaxation recovery, and a higher Young's modulus.9,18 Nevertheless, the small size, brittleness, and poor mechanical properties make molecular crystals unsuitable for sufficient energy conversion into useful motion (work) or macroscale device fabrication. To address these and other challenges, Lan and Chen19 and Yu et al.20 have developed facile strategies to fabricate macroscale hybrid materials by combining nano- or microscale molecular crystals with connective polymers [e.g., polyvinylidene fluoride (PVDF) and poly(vinyl alcohol) (PVA)]. Lang's group21 first reported a photomechanical metal–organic framework (MOF) based on photoresponsive olefinic species, and then mixed the MOF with PVA to generate a photomechanical film. Although such an approach can generate various actuators, due to the ease of size and shape control and the simplicity of the fabrication process, this method showed severe limitations in certain situations. For instance, some mechanically responsive crystals can dissolve in polymer solutions and thereafter lose their crystallinity and responsive behaviors during the fabrication process. In other cases, molecular crystals can exhibit poor dispersibility and compatibility in polymer solutions that seriously hinder the formation of uniform hybrid materials. Moreover, the incorporated molecular crystals may decompose upon external perturbation, such as exposure to acids/bases or oxidation. Therefore, it is necessary to develop new strategies with high versatility and increased protection for the fabrication of new types of smart hybrid materials. Coating with a protective shell can efficiently protect materials from external perturbations (e.g., solvents or chemicals), thereby enhancing material stability and reducing solubility. For example, Morrissey and co-workers reported the preservation of fragile antibodies on a biochip surface via coating with ZIF-822,23 (as called MAF-4, first reported by Chen), a widely studied MOF. It was also reported in the literature that ZIF-8 ( MAF-4) can be uniformly mixed with water-soluble polymers, such PVA, due to its excellent compatibility with polymers.24 Moreover, some MOFs, such as ZIF-8 ( MAF-4), are transparent in the visible (vis) and near-infrared (NIR) window and would be an ideal class of material for use as a protective shell for photoresponsive actuators. Thus, to overcome the existing challenges in the fabrication process of hybrid actuators based on molecular crystals, we propose an original strategy (Scheme 1) to coat molecular crystals with MOFs to protect the molecular crystals while also improving their dispersibility into polymer matrices. Scheme 1 | Illustration of the MOF-coating strategy to facilitate the formation of mechanically responsive hybrid materials (bottom), which cannot be directly obtained without MOF-coating (upper). Download figure Download PowerPoint In this contribution, to demonstrate "proof-of-concept," we selected a series of photomechanical molecular crystals and employed ZIF-8 ( MAF-4)25,26 as the protective shell material to assist the fabrication of hybrid photoresponsive actuators (Scheme 1). Finally, we combined the photoresponsive property of the molecular crystals with the vapor-responsive property of PVDF polymer27 in one system to achieve novel dual-stimuli responsive actuators, which can perform complicated motions. Experimental Methods Synthesis of p-crystal 9-((Ethylimino)methyl)anthracene ( 9EA) was synthesized according to literature with slight modifications.28 Commercial 9-methylanthracene ( 9MA), 9-bromoanthracene ( 9BA), 9-anthraldehyde ( 9AA), and 3-(9-anthryl)-acrylaldehyde ( 3AA) were directly used without further purification to grow crystals. CH2Cl2 was added to a powder of 9EA (1.0 g) until completely dissolved to obtain a saturated solution, which was evaporated at room temperature for 1 day to yield yellow needle-like crystals. Commercial 9MA, 9BA, 9AA, and 3AA were directly used without further purification to grow crystals via a similar procedure as 9EA. Among these crystals, 9BA, 9AA, and 3AA formed yellow needle-like crystals, and 9MA formed pale yellow sheet-like crystals. 9MA (CCDC#227661), 9BA (CCDC#1244955), and 9AA (CCDC#1510568) possessed the reported crystal structures, while 9EA (CCDC#1982847) and 3AA (CCDC#1982846) generated two new structures. Commercial 4-aminoazobenzene ( AAB)29 was directly used without further purification to grow crystals via a similar procedure as 9EA. AAB (CCDC#761536) formed orange needle-like crystals. Acylhydrazone crystals ( Ac-1a) were synthesized according to literature30: 4-methoxybenzaldehyde (1.0 equiv) was added to a solution of 4-chlorobenzhydrazide (1.0 equiv) in ethanol and refluxed for 3-4 h with the addition of a catalytic amount of glacial acetic acid. Upon completion of the reaction, the obtained precipitates were filtered and recrystallized from ethanol. Finally, upon slow evaporation from the acetonitrile solution, sheet-like Ac-1a (CCDC#1861511) crystals were generated. Synthesis of [email protected] About 0.100 g of preground p-crystal and 0.420 g of 2-methylimidazole were dissolved in 6.0 mL of deionized water. About 0.022 g of Zn(NO3)2·6H2O was dissolved in 6.0 mL deionized water. Subsequently, these two solutions were mixed together at room temperature and stirred for 1 h. The precipitate was harvested via centrifugation (9500 rpm, 10 min) to remove the unreacted 2-methylimidazole, and then washed three times with deionized water. The product was obtained via drying at 45 °C under vacuum. Synthesis of [email protected]@PVDF About 0.100 g of [email protected] was added to 1.0 mL of PVDF solution [7.5 wt % in dimethylformamide (DMF)]. The combined [email protected]@PVDF suspension was sonicated in an ultrasonic bath until completely dispersed. The suspension was then cast on glass substrates. Mixed matrix membranes (MMMs) were formed by drawdown coating with a glass rod using a spacer thickness of 750.0 μm. After drying at 45 °C under vacuum, a high-quality membrane was obtained. All membranes used in this study were 750.0 μm thick. Synthesis of [email protected]@PVA The [email protected] and 7.5 wt % PVA polymer aqueous solution were mixed together at a weight ratio of 1:2. A cross-linker solution [4.0 wt % of 1 M sulfuric acid, 6.0 wt % of glutaraldehyde (GA), and 90.0 wt % of water] at a weight ratio of 4:1 was then added and sat at room temperature for 30 min.24 [email protected]@PVA solution was carefully poured on the glass substrates, and the hybrid membrane was formed by drawdown coating with a glass rod using a spacer thickness of 750.0 μm. After drying at 45 °C under vacuum, a high-quality membrane was obtained. All membranes used in this study were 750.0 μm thick. Results and Discussion Design and preparation protocol Anthracene derivatives have long been employed to prepare photoresponsive smart materials because they can undergo reversible [4 + 4] dimerization under light irradiation and heating, which is accompanied by volume expansion or contraction.31,32 Therefore, we chose a series of anthracene derivatives as representative photoactuators, including 9EA, 9AA, 3AA, 9MA, and 9BA (Figure 1a). After crystallization in corresponding solvents, 9EA, 9AA, 3AA, and 9BA crystals were prepared as yellow needle-like crystals, and 9MA formed pale yellow sheet-like crystals ( Supporting Information Figure S1). Single-crystal X-ray diffraction (SCXRD) measurements revealed that 9AA, 9MA, and 9BA possessed the reported crystal structures,33–35 while 9EA and 3AA formed two new structures ( Supporting Information Table S1). Powder X-ray diffraction (PXRD) patterns further unveiled the bulky samples possessed characteristic patterns consistent with calculated patterns, indicative of their high purity ( Supporting Information Figure S2). SCXRD analysis revealed that anthracene moieties in all crystal structures are arranged in a similar parallel face-to-face manner in a π–π stacking fashion (Figure 1b and Supporting Information Figure S3). The plane–plane distance between adjacent anthracene groups is favorable to [4 + 4] photodimerization according to Schmidt's rule.36–38 Figure 1 | (a) Molecular structures of 9EA, 9MA, 9BA, 9AA, and 3AA. (b) (top) Illustration of the reversible structure transformation ([4 + 4] photodimerization) of 9EA triggered by visible light irradiation or heating (carbon in gray and nitrogen in blue), (bottom) the reversible bending behavior of 9EA crystal. (c) FT-IR spectra of 9EA before and after light irradiation and thermal backreaction. Download figure Download PowerPoint Next, we studied the photoresponsive characteristics of these five anthracene crystals. As shown in Figure 1b and Supporting Information Figure S4, crystals fixed onto glass fiber can gradually bend away from a unilateral blue light source. When irradiating from the opposite direction, or by heating, crystals can bend back to the initial position. To unveil the mechanism behind the observed photomechanical motions, Fourier transform IR (FT-IR), UV–vis, and 1H NMR spectroscopies were used to study the chemical reactions during light irradiation. FT-IR spectra clearly showed the appearance of new peaks (608–740 cm−1) (Figure 1c and Supporting Information Figure S5), which can be ascribed to the deformation vibrations of the cycloaddition of anthracene. After a thermal backreaction, we confirmed that these were completely reversible.39–41 These results indicate that the photomechanical properties were related to the photocycloaddition reaction of anthracene groups.42 Through the UV–vis spectra, the change in the absorption peak can be clearly observed, and the change of the absorption bands was associated with the [4 + 4] cycloaddition ( Supporting Information Figure S6). In the 1H NMR spectra, the appearance of new peaks at ∼6.00 ppm was observed as a result of the cross-linked C–H from anthracene dimerization ( Supporting Information Figure S7). To fabricate macroscale hybrid actuators, we subsequently attempted to directly combine anthracene crystals with PVDF polymer. However, all tested anthracene crystals were found to be highly soluble in DMF, which is the commonly used solvent to prepare PVDF membranes. Unsurprisingly, directly blending the anthracene crystals with PVDF in DMF failed to generate high-performance photoresponsive hybrid membranes. Instead, a negligible photoresponse was observed ( Supporting Information Figures S8 and S9). This can be ascribed to the high solubility of the anthracene crystals that hindered the reformation of crystalline domains in the final membranes, as revealed by PXRD data ( Supporting Information Figure S10). Thus, new fabrication approaches to solve these formidable challenges were required. Considering the advantages of ZIF-8 ( MAF-4), such as excellent coating effects, mild synthesis conditions (aqueous solution at room temperature), and low-cost, ZIF-8 ( MAF-4) was chosen as a representative MOF to demonstrate the MOF-coating strategy. ZIF-8 ( MAF-4) was expected to form a protective layer that could protect the photoresponsive crystals against external perturbations and thus, maintain the photomechanical performance after blending with PVDF. The anthracene crystals were preground to form smaller particles and then added during the synthetic process of ZIF-8 ( MAF-4). Transmission electron microscopy (TEM), elemental mapping analysis, and PXRD measurements were performed to ensure that ZIF-8 ( MAF-4) was successfully coated on the surface of the photoresponsive crystals (Figure 2 and Supporting Information Figures S11 and S12). Herein, 9EA was chosen as a representative example to interpret the detailed characterization. PXRD data revealed a combination of characteristic patterns of 9EA and ZIF-8 ( MAF-4), indicative of the successful coating of ZIF-8 ( MAF-4) on 9EA crystals (Figure 2a).43 TEM images clearly showed the coating of ZIF-8 ( MAF-4) onto the surface of 9EA particles (Figure 2b), and elemental mapping confirmed that the ZIF-8 ( MAF-4) coating was uniform across the surface of the molecular crystals ( Supporting Information Figure S13). Dynamic light scattering (DLS) was used to measure the hydrodynamic size distribution, which was consistent with that of the TEM analysis ( Supporting Information Figure S14). As predicted, due to the excellent protection effect from ZIF-8 ( MAF-4), the formed [email protected] (p-crystal = photoresponsive crystal; @ = encapsulating) composites became insoluble in common organic solvents such as DMF (Figure 2c), methanol, ethanol, acetone, and so on. In addition, we found that coating with ZIF-8 ( MAF-4) can significantly improve the chemical stability of the crystals. For instance, [email protected] can survive in 8 M NaOH (H2O∶Ethanol, 1∶1) for 5 h without dissolution, while 9EA crystals wholly dissolved upon the same treatment. PXRD patterns also confirmed the improved chemical stability of [email protected] ( Supporting Information Figure S15). The excellent protective effect of ZIF-8 ( MAF-4) guarantees the embedding of photoresponsive crystals into the polymer matrices without losing the crystallinity of the molecular crystals (Figure 2d). Figure 2 | (a) PXRD patterns of 9EA crystals, ZIF-8 (MAF-4), [email protected], and [email protected]@PVDF. (b) A TEM image showing 9EA cores coated by a ZIF-8 (MAF-4) shell. (c) 9EA completely dissolved in DMF (left) vs [email protected] suspended in DMF (right); (d) Image of the [email protected]@PVDF hybrid membrane (left) and SEM image of [email protected]@PVDF (right). Download figure Download PowerPoint Properties and performance of [email protected]@PVDF hybrid membranes We then tested whether [email protected] can be directly used to fabricate hybrid materials with DMF/PVDF. As predicted, high-quality freestanding membranes were obtained (Figure 2d and Supporting Information Figure S16). Scanning electron microscopy (SEM) images revealed that all [email protected] were uniformly dispersed into the PVDF matrix (Figure 2d and Supporting Information Figure S16). PXRD patterns further revealed that all [email protected]@PVDF hybrid membranes exhibited a combination of molecular crystals and ZIF-8 ( MAF-4) (Figure 2a and Supporting Information Figure S11). Notably, [email protected]@PVDF membranes inherited the photoresponsive behavior of the photomechanical crystals. That is, these membranes can bend away from light and recover their original shape upon light irradiation from the opposite side or thermal treatment (Figure 3a and Supporting Information Figure S17). We observed that coating of ZIF-8 ( MAF-4) did not affect the photoresponsive performance of hybrid membranes, which showed a faster response than pure p-crystals ( Supporting Information Figures S4 and S17). We used the displacement (D) and bending angle (θ) to evaluate the degree of the mechanical response process (Figure 3b). As shown in Figure 3a, an angle of θ = 35° was observed for [email protected]@PVDF upon irradiation for 5 s. The reversible bending process can be repeatedly cycled via alternating light from different directions ( Supporting Information Figure S18). As a comparison, the pure PVDF membrane and [email protected] membrane did not exhibit any light response ( Supporting Information Figures S19a and S19b). These results demonstrate that the photoresponsive behavior of the hybrid materials originated from the doped molecular crystals. Figure 3 | (a) Illustration of the photoresponsive bending behavior of [email protected]@PVDF. (b) Schematic illustration of the defined displacement (D) and bending angle (θ) in the mechanically responsive process. (c) Vapor-triggered response of [email protected]@PVDF. (d) Plot of the repeating cycles of [email protected]@PVDF via adding and removing vapor. Download figure Download PowerPoint Exploration of the generality of the MOF-coating approach To further explore the generality of the MOF-coating strategy, we also studied the hybridization of PVA (a widely used polymer matrix) with anthracene crystals. It was found that although all tested anthracene crystals do not dissolve in the aqueous solution of PVA, they are also unable to be homogenously dispersed ( Supporting Information Figure S20). This result critically hindered the formation of uniform hybrid membranes ( Supporting Information Figure S21). Notably, coating with ZIF-8 ( MAF-4) can significantly improve the dispersibility and compatibility of [email protected] with the PVA matrix.24 Ultimately, the formed [email protected]@PVA hybrid materials showed excellent photoresponsive performance, comparable with [email protected]@PVDF systems ( Supporting Information Figure S22). To further prove the universal applicability of this method to fabricate hybrid materials, we tried other types of widely studied photoresponsive crystals. Azobenzene crystals and acylhydrazone derivatives are representative organic photochromic molecules because they can undergo E → Z isomerization after light irradiation. According to literature synthesis procedures reported by the Nakano29 and Naumov group,30 we obtained AAB and Ac-1a (Figures 4a and 4b). PXRD data revealed the successful synthesis of desired crystals ( Supporting Information Figure S23). We then studied their photoresponsive performance ( Supporting Information Figure S24). The changes of the cis–trans isomerism of crystals before and after light irradiation were confirmed by UV–vis spectroscopy ( Supporting Information Figure S25). Following the aforementioned experimental procedures, we successfully coated ZIF-8 ( MAF-4) on AAB and Ac-1a, as confirmed by PXRD ( Supporting Information Figure S23), elemental mapping analysis ( Supporting Information Figure S26), and solubility comparison ( Supporting Information Figure S27). The formed hybrid membranes ( Supporting Information Figure S28) showed excellent photoresponsive performance (Figures 4c and 4d), while the pure PVDF membrane did not exhibit light-responsive performance ( Supporting Information Figure S29). These results further proved the universality and versatility of the MOF-coating strategy. Figure 4 | Molecular structures of (a) AAB and (b) Ac-1a. Illustration of the photoresponsive bending behavior of (c) [email protected]@PVDF and (d) [email protected]@PVDF membranes. Download figure Download PowerPoint Hybrid materials not only inherit the advantages of molecular crystals (e.g., crystallinity and photoresponse), but also the advantages of polymers (e.g., good processability, high mechanical stability, and flexibility). As reported in the literature, PVDF membranes show a fast bending response under the stimuli of acetone vapor and can rapidly recover their original shape upon drying in air ( Supporting Information Figure S19c).27 Our investigation revealed that [email protected]@PVDF membranes did show reversible vapor-responsive behavior similar to PVDF (Figure 3c), and the reversible vapor-response process can undergo >10 cycles without obvious exhaustion (Figure 3d). Taking advantage of the photoresponse of the anthracene crystals and vapor-responsive behaviors of PVDF, we proposed to combine the two types of stimuli-responsive properties to achieve an advanced dual-stimuli responsive actuator. Thereupon, we selected [email protected]@PVDF for demonstration. First, the [email protected]@PVDF membrane was cut into a strip (length, 1.7 cm; width, 1.0 mm) and then folded. It was found that the hybrid membrane exhibited interesting crawling behavior through alternating vapor and light stimuli. The strip can arch in the acetone vapor atmosphere and then bend to an opposite direction under light irradiation (Figures 5a and 5b). In this way, this "robot" can crawl forward >2 mm in as short as six cycles ( Supporting Information Video S1). This crawling performance can result from the inhomogeneous strain forces accumulated in the strip during light and vapor stimulation and is made viable by the reusability of [email protected]@PVDF.44 Figure 5 | (a) Schematic diagram and (b) photographs showing that the strip of [email protected]@PVDF can crawl through alternating vapor and light stimuli. Download figure Download PowerPoint Conclusion For the first time, we developed a facile and versatile coating strategy to fabricate hybrid actuators. To demonstrate this strategy, we chose the well-known ZIF-8 ( MAF-4) as the representative crystalline shell to coat on a series of photoresponsive crystals. It was found that this strategy not only can efficiently protect molecular crystals against external perturbations, but can also significantly improve their dispersibility and compatibility in the polymer solutions. These benefits can facilitate the formation of uniform hybrid membranes in both organic and aqueous phases. The formed hybrid materials exhibited excellent photoresponsive behaviors, while hybrid materials fabricated without the ZIF-8 ( MAF-4) coating showed no response under light irradiation. Moreover, we found that hybrid materials can inherit the intrinsic properties of polymers such as good processability (e.g., membrane formation), high flexibility, and vapor-responsive performance. Thereupon, we combined the photoresponse of anthracene crystals with the solvent response of PVDF polymer in one system, and finally achieved dual-stimuli responsive smart actuators that exhibited crawling behavior through alternating vapor and light stimulus. This study broadens the application scope of the hybridization strategy to fabricate smart materials and provides important guidance for future research to prepare multiresponsive actuators. 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