Humidity- and Sunlight-Driven Motion of a Chemically Bonded

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Humidity- and Sunlight-Driven Motion of Chemically Bonded Polymer Bilayer with Programmable Surface Patterns Lidong Zhang, Xiaxin Qiu, Yihui Yuan, and Ting Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14112 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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ACS Applied Materials & Interfaces

Humidity- and Sunlight-Driven Motion of Chemically Bonded Polymer Bilayer with Programmable Surface Patterns Lidong Zhang,* Xiaxin Qiu, Yihui Yuan, Ting Zhang

Department of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200241, People’s Republic of China *Corresponding author: Lidong Zhang: E-mail:[email protected] ORCID: Lidong Zhang: 0000-0002-0501-6162 KEY WORDS: Smart materials, interfacial reactions, humidity and sunlight responses, programmed mechanical actuation, surface crosslinking patterns

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ABSTRACT We report a bilayer of sodium alginate/polyvinylidene fluoride (SA/PVDF) that is chemically bonded through a series of interfacial coupling reactions. SA layer is hydrophilic in structure, capable of strong interaction with water molecules, thus presenting high sensitivity to humidity, whereas PVDF layer is hydrophobic, inert to humidity. This structural feature results in the bilayer with asymmetric humidity-responsive performances that can thus make the shape change of the bilayer with directionality, which cannot be achieved in a SA single layer. The responsive process to humidity can be adjusted by exposure of the bilayer to sunlight by means of photothermal effect that accelerates dehydration of the bilayer to cause more rapid shape deformations. When the sunlight is removed, the bilayer adsorbs humidity again, and returns to its original shape, indicating good reversibility. To exactly regulate the shape deformations of the bilayer with external stimuli, we employ Ca2+-treated filter paper to customize crosslinking reactions in the SA layer as any desired patterns that are capable to cause different mechanical tensors and swellabilities in the bilayer, to regulate and control the actuations for self-folding, curling, twisting and coiling in response to sunlight and humidity. On the other hand, the chemically bonded bilayer has stronger interfacial toughness, capable of reaching 300 J m−2 that is around twelve times the interfacial toughness in physically combined bilayer, as a result the chemically bonded bilayer is capable to sustain continuous shape deformations without interfacial failure. The directionally mechanical actuations can be utilized in designing an indicator to roughly indicate the range of intensity of sunlight by coupling the chemically bonded bilayer into a typical electric circuit, in which the range of intensity of sunlight can be easily estimated by visual observation of the light-emitting diodes.


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INTRODUCTION Humidity is a type of eco-friendly, inexhaustible and ready energy source. Thus materials that are capable of response to humidity have attracted great attentions. In recent years, various types of humidity-responsive materials have been developed such as single layer,1 bilayer,2 multilayers3 and fibers,4 and corresponding actuators have also been created.5-8 Some of them even demonstrated promising performances in conversion of humidity energy to mechanical work such as humidity-driven “electricity generator”6 and “engines”.8 However, there are still apparent pitfalls in the humidity-responsive materials. For instances: 1) humidity-responsive materials are easily saturated with water molecules which often causes very short response period; 2) for humidity-induced shape change, a fast shape restoration is expected; however, after shape deformation with hydration, the materials may require quite long time for the reshaping process because the dehydration is much slower relative to the hydration at the same temperature. To address these issues, we designed a combined actuating system of humidity and sunlight, in which the sunlight could dramatically accelerate the dehydration process through photo-thermal effect to improve the reshaping speed of the humidity-deformed films. In the design of humidity-responsive bilayer, the materials, sodium alginate (SA) and polyvinylidene fluoride (PVDF) were selected and combined chemically, where the hydrophilic SA can generate strong interaction with water molecules to provide high sensitivity to humidity, and hydrophobic PVDF is innert to humidity that can induce the response with directionality. Shape deformation-controllable materials are capable of mimicking biological systems with shaping-matched mechanical responses of curling, bending, twisting, or self-folding.9-11 These efficient and elegant actuations enable applications for soft robotics,12,13 artificial


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muscles,14 and optical devices.15 Various protocols for achievement of programmed actuations have been developed including direct 3D/4D printing,16,17 assembling polymer bilayers,18,19 patterning alignments,20,21 and electric or light assistances.22-25 Of them, introducing patterning alignments onto the surface of materials has stood among the top choices, since it is indeed a facile yet versatile way to trigger programmed actuations of soft materials. For example: printed ink on the surface of the pre-strained polymer sheets discriminately absorbs light, which causes relief of strain to induce programmable self-folding into various geometries;26-32 patterning infrared-active alignments on the surface of soft polymer bilayers demonstrated high efficiency to trigger programmed actuations.33,34 Herein we also introduced pattern structures into the bilayer to regulate and control the humidity- and sunlight-driven actuations. On basis of the conventional methods, we developed a new and more convenient patterning protocol that only required shape-customized filter paper strips that were pre-treated with saturated sorption of aqueous solution containing Ca2+, to cover on the surface of alginate layer. The crosslinking reaction thus took place between Ca2+ and carbonyl groups of alginate through electrostatic interactions.35 The pre-treated filter paper could be easily cut to any shapes to program the crosslinking reactions as any imaginable patterns in the SA layer. The Ca2+-crosslinking patterns are expected to cause different mechanical tensors and swellabilities to induce shape-controlled mechanical actuations of the bilayer. The controlled actuations of the bilayer could be utilized in an indicator that could visually tell the range of the intensity of sunlight by observing the light-emitting diodes.


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EXPERIMENTAL SECTION Materials: Polyvinylidene fluoride (PVDF powder, Mw = ~800000 g/mol, from Solvay, America),

3-aminopropyltriethoxysilane

(APTES),

water-soluble

dye,

2-(N-

morpholino)ethanesulphonic acid (MES), N-hydroxysulfosuccinimide sodium salt (HNS), N-(3dimethylaminopropyl)-N-ethylcarbodiimide (EDC), dimethyl formamide (DMF), CaCl2, and sodium alginate (SA, Mw = ~50000 g/mol) were purchased from Shanghai Excellent Chemical Co. Ltd. China, and all materials were used without further purification.

Preparation of chemically bonded bilayer of SA/PVDF: PVDF powder (5 g) was dissolved in DMF (100 mL) at 120 ºC under vigorously stirring until the powder fully dissolved and this process usually required 1 hour to obtain homogeneous PVDF/DMF solution. Then the solution (5 ml) was extracted with a 10-mL syringe and casted onto the surface of a glass template (10 cm x 10 cm x 1 mm). The glass template was then carefully moved to vacuum oven and dried at 75 ºC for 2 hours to give PVDF single layer. The glass template was pre-cleaned with ethanol and acetone, and dried by nitrogen flow. The SA layer was bonded to the surface of PVDF layer via EDC-Sulfo-NHS chemistry reactions following previously reported protocols.36,37 Briefly, prepared PVDF film on the glass template was treated with oxygen plasma for 5 min to introduce hydroxyl groups onto the surface, and was covered immediately with 3 ml of the amino-siloxane solution (100 ml deionized water, 2 wt% of APTES) for 2 hours at room temperature. After incubation, the unreacted reagent was washed out from the PVDF film surface with ethanol solvent. The APTES-functionalized PVDF film was further incubated in SA anchoring solution at pH 6.0 measured with pH meter (PHSJ-4A) (100 ml of aqueous MES buffer (0.1M MES), 1 wt% SA, Sulfo-NHS (molar ratio of 30:1 to SA) and EDC (molar ratio of 25:1 to SA)) for 12 h.


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Incubated PVDF film was then coated with 5-mL SA solution, and dried at 30 ºC for two days to give chemically bonded bilayer of SA/PVDF.

Preparation of physically combined bilayer of SA/PVDF: PVDF film was prepared on the glass substrate as above methods, and SA solution was then casted on its surface and dried at 30 ºC for two days to give physically combined bilayer of SA/PVDF. The thickness of the bilayer was dependent on amount of PVDF/DMF and SA solutions.

Interfacial toughness measurement. We conducted all tests in ambient air at room temperature. The interfacial toughness of chemically/physically combined bilayers was measured respectively with the standard 90-degree peeling test on a tensile testing machine (HY-0580, Shanghai Hengyi Test Instrument Co., Ltd,), and 50 N load cell was used, equipped with a 90-degree peeling fixture. To prevent bilayer’s elongation along the peeling direction, the bilayer was adhered to a glass slide (7.5 cm × 2.5 × 0.1 cm) from PVDF surface by dual adhesive tape, and the SA surface was fixed to a rigid tape backing. Thus, the measured interfacial toughness is equal to the steady-state peeling force per width of the bilayer. For better observation, the SA layer was stained with dye. All 90-degree peeling tests were performed with a constant peeling speed of 200 mm min−1. The measured force without apparent fluctuation as the peeling process reached the steady state, and this plateau force was calculated by averaging the measured force values in the steady-state region on excel software. We then divided the plateau force by the width of the bilayer strip to get the interfacial toughness value.


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Ca2+-induced crosslinking patterns in SA layer: A filter paper was cut to various equal-size strips (5 mm in width and 8 cm in length), and immersed in saturated CaCl2 aqueous solution. The treated paper strips were then paved on the SA surface of the bilayer as wanted patterns (Figure S1 in Supporting Information). At paper-covered areas, CaCl2 solution permeated into the SA layer, resulting in the crosslinking reactions between Ca2+ and carboxylates. In comparison to UV light-assistant crosslinking patterning techniques, this process is faster that could be completed in less than 5 minutes to provide the SA layer with good crosslinking performances, and is more convenient to make materials with periodic variation of mechanical tensors in the structure to induce shape-controllable actuations by external stimuli.

Response to sunlight irradiation: A sunlight simulator (OSRAM, 300 W) was used in all sunlight-responsive tests of the bilayer of SA/PVDF, and the light intensity was determined by sunlight irradiatometer (SM206). As the bilayer of SA/PVDF was placed under sunlight irradiation, the photo-thermal effect would cause the bilayer to dehydrate and contract towards SA layer. The temperature on the film surface caused by sunlight irradiation was determined by thermal infrared camera (FLIR C2).

Response to humidity: The humidity was supplied through a moist filter paper containing 30−40wt% of water content. As the bilayer of SA/PVDF was in contact with the moist paper, the bilayer would adsorb humidity and expand from SA layer, resulting in the bilayer with shape changing towards PVDF layer.


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Indicator of sunlight intensity: The indicator was composed of five independent electrical circuits and the bilayer of SA/PVDF was used in the electrical circuit as a switch that was capable of processing circuit through sunlight-induced bending. As the sunlight was removed, the bilayer adsorbed humidity from surrounding environment, and bended back to break the circuit. We assembled the electrical circuits to make the bilayer require different intensities of sunlight to establish connection (that is to say the different bending degrees of the bilayer is required to establish connection). It should be noted that the bilayer of SA/PVDF is nonconductive, thereby a soft gold wire was combined with the bilayer by adhesive tape. The movements of soft wire could be driven by bending motion of the bilayer to establish or break the connection.

Plasma Treatment: Plasma treatment of PVDF surface was performed on a plasma cleaner (PDC-32G-2). A rotary pump was connected to achieve base pressure of 1 × 10−3 mbar. Airflow was used to supply oxygen for the treatment process of the samples, and the treatment time was 5 min for each sample.

X-ray photoelectron Spectroscopy (XPS): The spectra were obtained using an Al K-α X-ray photoelectron spectrometer (Thermo Fischer Scientific, ESCALAB 250Xi, USA) at a work voltage of 12.5 kV, an anode current of 16 mA, and a pressure of 8 × 10‒10 Pa. The pass energy was 40 eV. The measurements were carried out after one week of plasma treatment.

Contact Angle Measurement: We measured the water contact angle (WCA) with distilled water on a tester (JC2000C1). The plasma-treated PVDF films were mounted on glass slides


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prior to measurement using double side adhesive tape. Six measurements of each PVDF sample condition were taken to obtain an averaged WCA data. The WCA was then calculated based on the captured images after 10 s of water droplet contact time, by using Theta v4.1.9.8 (Biolin scientific, Stockholm, Sweden) software. All the measurements were carried out within 1 hour of plasma treatment.

Infrared spectroscopy: An ATR-IR spectrometer (Thermo Scientific Nicolet IS10) was used to detect the bare and modified PVDF surfaces at 25 °C. All the samples were measured within two days of preparation.

Scanning electron microscopy (SEM): SEM images were recorded on an electron microscope (S-4800, SYST TA PRO 1156) with primary electron energy of 2 kV. The film samples were attached to silicon wafer with adhesive carbon tape and coated with 5 nm thick gold layer prior to SEM observation. The micrographs were recorded at room temperature and pressure of 8.8 × 10−7 Pa. For the measurements of energy dispersive X-Ray spectroscopy (EDX), the film samples were coated with carbon for 10 min.

RESULTS AND DISSCUSION

The bilayer was prepared through interface-chemical reactions to enhance adhesion between SA and PVDF.36,37 As shown in Figure 1a and b, the PVDF single layer, obtained at 75 ºC according to reported method,19 was treated with oxygen plasma to introduce hydroxyl groups onto the surface,38,39 followed immediately by incubation in amino-siloxane solution (APTES).


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After two hours, unreacted reagent was washed out with ethanol solvent and alginate buffer solution was coated. The prepared bilayer of SA/PVDF was kept on static surface and dried at 30 ºC (see the details in experimental section).

Figure 1. Structures, preparation and performances of the bilayer of SA/PVDF. (a) Materials utilized in the bilayer. (b) Schematics of preparation of the bilayer.

To demonstrate the successful introduction of hydroxyl groups, the oxygen plasma-treated side of the PVDF film was examined with water contact angle measurements. As shown in Figure S2 in Supporting Information, the treated PVDF surface indeed changed from hydrophobicity to hydrophilicity with the water contact angle being decreased from 97 to 18° after treatment for 5 min on a plasma cleaner. The treatment time was determined based on a series of comparative trials, in which 5 min was enough to treat the surface with good hydrophobicity. To obtain more evidences on the formation of hydroxyl groups, the treated PVDF side was further characterized with ATR-IR spectroscopy and SEM/EDX measurements. As shown in Figure S3 in Supporting


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Information, the peak 1 represents hydroxyl groups formed on the treated PVDF surface (named as OH-PVDF), and in comparison to the bare PVDF the oxygen atom content in the OH-PVDF side got up to 0.44 wt% detected by EDX measurement (Figure S4a in Supporting Information). These results further proved that the hydroxyl groups have been introduced onto the PVDF film surface. After incubation of OH-PVDF in APTES solution, the oxygen atom content increased to 0.84 wt% and the silicon atom was detected to be 0.12 wt% (Figure S4b in Supporting Information). In ATR-IR spectra, the peak 2 represented primary amine groups that came from the APTES structure (Figure S3 in Supporting Information). The both results implied that the APTES was successfully reacted onto the OH-PVDF surface. On the other hand, the peak 3 and 4 represented the amide bond coupling of alginate and multiple hydroxyl groups of alginate, respectively, which indicated that the SA was chemically bonded with PVDF layer (Figure S3 in Supporting Information). To get more insights into the formation of amide bond, the APTESand SA-modified PVDF surfaces were analyzed with XPS measurements respectively. As shown in Figure S5 in Supporting Information, the main bonding energy of N1s shifted from 399.24 eV (Figure S5c in Supporting Information) to 401.88 eV (Figure S5d in Supporting Information) which indicated that the most of ‒NH2 groups have been converted to –NHCO by the reaction between –NH2 groups of APTES and HOOC groups of SA. In the meantime, the silicon content relatively decreased from 10.44 to 1.08 at% as a result of the introduction of SA content (Figure S5e,f, and peak table S1,2 in Supporting Information).

The chemically combined bilayer was expected to be able to sustain continuously mechanical shape deformations without interfacial failure. To demonstrate adhesive strength of the bilayer, we carried out a standard 90-degree peeling test at a peeling rate of 200 mm min−1 to measure the


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interfacial toughness between SA and PVDF layers (See experimental section). We measured four types of SA/PVDF bilayers that were combined with different treatment processes (Figure 2a). The bilayer that was physically combined, was easily detached (Figure 2d), and the measured interfacial toughness was only 26 J m−2, in which SA layer was non-crosslinked, while it was 27 J m−2 with SA layer crosslinked by Ca2+ (Figure 2b, and Movie S1 in Supporting Information). Such physically combined bilayers easily underwent interfacial failure with mechanical deformations by exposure to external stimuli (Figure 2e). As non-crosslinked SA layer was chemically bonded to surface of the PVDF layer, formed bilayer processed of robust adhesion strength with the measured interfacial toughness of 250 J m−2, calculated from the testing results in Figure 2b. After crosslinking of SA layer, the measured interfacial toughness was higher, capable of reaching 300 J m−2 (Figure 2b and c, and Movie S2 in Supporting Information), and we did not observe any interfacial failure when such a bilayer was subjected to continuous deformations driven by humidity and sunlight. These results demonstrate that the chemically combined bilayers are optimized utilized for various mechanical actuators.


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Figure 2. Tough bonding of the bilayers of SA/PVDF. (a) Schematics of various types of bilayer interfaces: the bilayers of SA/PVDF were physically or chemically combined respectively. (b) Curves of the peeling force per width of the SA layer versus displacement for various types of SA/PVDF bonding: physically combined bilayer with (Bilayer-1) and without (Bilayer-2) crosslinking reaction in the SA layer, chemically combined bilayer with (Bilayer-3) and without (Bilayer-4) crosslinking reaction in the SA layer. (c) Photo of the peeling process of a chemically combined bilayer that was hard to detach. (d) Photo of the peeling process of a physically combined bilayer that was easily detached. (e) Physically combined bilayer was easily separated during the humidity- and sunlight-driven deformations.

For Ca2+-induced crosslinking reactions in SA layer, a filter paper was cut to strips to customize the crosslinking patterns (see the details in experimental section). After removal of the paper strips, the film’s surface became rougher with excess CaCl2 solids attached in comparison to the non-crosslinked surface (see the SEM images in Figure S6a,b in Supporting Information). The EDX spectroscopy for crosslinked regions further confirmed that Ca2+ solution indeed permeated into SA layer because there were two obvious calcium element peaks in the EDX spectrum (Figure S6c in Supporting Information). When the crosslinking-patterned bilayer was immersed in water, the non-crosslinked regions swelled more dramatically than crosslinked regions reflected by the images in Figure S1b and c in Supporting Information, indicating higher hydroscopicity at non-crosslinking regions. But crosslinked region processes of higher mechanical strength that was confirmed with the mechanical tensile measurement of the bilayers (Figure S7 in Supporting Information), where the mechanical stress for crosslinked film was much higher than that for non-crosslinked film at any the same elongating positions. These


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differential physical properties caused by Ca2+-induced crosslinking reactions were expected to regulate shape change of the bilayer, because shape deformation might primarily take place along non-crosslinked patterns in response to humidity and sunlight. This hypothesis were verified by a series of demonstrations on humidity- and sunlight-induced shape deformations of the bilayers (see below).

When the crosslinking-treated 70-µm thick bilayer film was cut to a strip with crosslinked pattern perpendicular to its long axis, the strip absorbed water molecules from SA layer, and curved towards PVDF side upon exposure to humidity (Figure 3a); while irradiated with sunlight, the strip dehydrated and curved towards SA side (Figure 3c). However, note that the curling of the bilayer was unsmooth, but resembled a swimming snake, which was caused by differential hygroscopicities at crosslinked and non-crosslinked patterns in the bilayer stip. To get insight into this point, we carried out a series of quantitative measurements to compare the hygroscopicity for crosslinked and non-crosslinked SA layers, respectively. As shown in Figure S8 in Supporting Information, the humidity sorption ratio for non-crosslinked SA layer is 4.8 ± 0.3% that is higher than crosslinked one (1.6 ± 0.5%). While exposed to sunlight at 115 mW cm−2, the humidity desorption ratio in non-crosslinked SA layer is 11.2 ± 2.9% that is also higher relative to crosslinked SA layer (3.5 ± 0.2%) (Figure S9 and S10 in Supporting Information). Both the results implied that the faster shape deformation should occur always along noncrosslinked patterns because of the higher water-sorption/desorption capability upon exposure to humidity and sunlight.


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Figure 3. Directionality in the humidity- and sunlight-responsive bilayer. (a) Curling of the bilayer towards PVDF side with exposure to humidity. (b) Original state of the bilayer was not flat owing to the differential mechanical tensors at non-crosslinked and crosslinked areas. (c) Curling of the bilayer towards SA side by exposure to sunlight at 115 mW cm−2. (d) A crosslinking-patterned bilayer responded to sunlight irradiation with rapid self-folding towards SA side along the non-crosslinked patterns. (e) The self-folding of the bilayer was directional, which folded or bended always towards SA side when being exposed to sunlight. (f) A crosslinking-patterned single layer of SA demonstrated weak folding response by sunlight irradiation at 115 mW cm−2. (g–j) Self-folding processes of the bilayer from irregular configuration to regular self-folding shape in response to sunlight at 115 mW cm−2. The thickness of the bilayers is 70 µm.


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As shown in Figure 3d, the strip rapidly folded up along non-crosslinked regions upon irradiation of sunlight from SA side. This folding process was directional that was always towards SA side, no matter which way to shine the light (Figure 3e, Movie S3 in Supporting Information). This directional self-folding is highly subject to PVDF layer. When the PVDF layer was removed, the remained SA single layer was unable to fold up by sunlight, but exhibited a non-directional oscillating behavior (Figure 3f, Movie S3 in Supporting Information), similar to an azobenzenebased liquid crystalline polymer film that was capable of continuous chaotic oscillatory motion when exposed to ambient sunlight in air.40 The bilayer of SA/PVDF with sunlight-driven selffolding performance is expected to have promising potentials in design of artificial self-folding robots. As a rather rudimentary proof, we designed a miniature soft self-folding robot that was capable of reshaping itself from irregular state to regular self-folding shape in response to sunlight (Figure 3g to j, and Movie S4 in Supporting Information).

Under the irradiation of sunlight, the photothermal effect caused rapid dehydration from SA side that resulted in rapid shape deformation of the bilayer. To get more insights into the sunlight effect on the shape deformation, dehydration-induced shape changes were carried out in twodifferent environmental conditions, respectively. In the first condition, as a flat hydrous bilayer strip with the thickness of 70 µm was placed on a dry substrate at 26 °C, it dehydrated (Figure S9 and S10 in Supporting Information), and folded up slowly towards SA layer, in which the folding angle changed from 155 ° to 78 ° requiring 95 s, averagely 0.81° per second (Figure 4a and S11 in Supporting Information). This process was reversible, and the folded bilayer returned to its original shape in 33 s, upon again adsorption of humidity on a moist filter paper containing 30−40wt% of water content (Figure 4b and S12 in Supporting Information). In the second


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condition, the self-folding motion was more rapid when the bilayer was exposed to irradiation of sunlight at 115 mW cm−2, in which the bilayer folded up from 179 to 78° only taking 14 s, averagely 7.21° per second (Figure 4c and S13 in Supporting Information). Apparently, photothermal effect accelerated the shape deformations. The sunlight-triggered self-folding of the bilayer was also reversible, which could unfold to its original flat shape by humidity sorption on a moist substrate (Figure 4d and S14 in Supporting Information).

Figure 4. Self-folding and unfolding performances of the bilayers in response to sunlight and humidity. (a) The bilayer slowly bending to SA side along the non-crosslinked pattern by dehydration-induced contraction on a dry substrate at 26 °C, and (b) bending back to original flat shape by hydration on a moist substrate at 23 °C. (c) The bilayer quickly bending up towards SA side by rapid dehydration with sunlight at 115 mW cm−2, and (d) then bending back to original flat shape by rehydration on a moist substrate at 23 °C.


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The orientation of crosslinked and non-crosslinked patterns is expected to affect the elastic tensors that may direct shape change of the bilayer. To test this hypothesis, we cut out a bilayer strip with a non-crosslinking pattern aligned on SA side at an oblique angle with respect to the long axis. As a result, the deformation occurred along the non-crosslinked pattern, resulting in the bilayer strip into a twisting structure upon sunlight irradiation (Movie S5 in Supporting Information). The bilayer strip twisted in left-handedness when the oblique angle was +30° to the long axis, while it twisted in right-handedness at the oblique angle of −30° to the long axis (Figure 5a). As the twisted bilayer was removed away from sunlight irradiation, it quickly untwisted and returned to its original shape by adsorption of humidity from the surrounding environment. If the bilayer strip was cut out with periodic crosslinked/non-crosslinked patterns in SA side at the angle of +30° to the long axis, it would be capable of slowly coiling into a lefthanded helix by dehydration on a dry substrate at 26 °C (Figure 5b, Figure S15 in Supporting Information). This process was reversible upon exposure to humidity (Figure S16 in Supporting Information). The coiling motion could be dramatically speeded up with sunlight irradiation at 115 mW cm−2 that was able to heat the strip to ≈37 °C confirmed with thermal infrared camera (Figure 5c, Figure S17 and Movie S6 in Supporting Information). The higher temperature caused more rapid dehydration from non-crosslinked patterns, thus accelerating coiling of the bilayer strip. The sunlight-coiled strip could also be uncoiled by hydration on a moist substrate (Figure S18 in Supporting Information). This reversible process visually resembles the response to humidity of pinecone scales that deform towards peduncle upon hydration and recover their original shapes upon dehydration.41 As we made the periodic crosslinked/non-crosslinked patterns be aligned at −30° to the long axis, the bilayer strip demonstrated right-handed coiling motion with faster coiling also triggered by sunlight at 115 mW cm−2 (Figure 5d,e, and Movie S7


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in Supporting Information). Note that the coiling of the bilayer strips proceeded always along the non-crosslinked patterns, however, as the irradiation power of sunlight was over 200 mW cm−2, the deformation occurred also along the crosslinked patterns that would disable the controllability over the shape change.

Figure 5. Shape-controlled mechanical actuations of the bilayers in response to sunlight and humidity. (a) The bilayer twists along non-crosslinked pattern upon exposure to sunlight. (b) The bilayer with non-crosslinked patterns aligned at an angle of +30° with respect to the long axis, slowly coils into a left-handed helix on a dried substrate at 26 °C, and (c) quickly coils in left handiness by irradiation of sunlight at 115 mW cm−2. (d,e) The bilayer with non-crosslinked patterns aligned at an angle of −30° with respect to the long axis, slowly coils into right-handed


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helix by hydration on a dried substrate at 26 °C (d), and by irradiation of sunlight at 115 mW cm−2 (e).

To further demonstrate the crosslinking patterns-controlled motions of the bilayer composite, more shapes of the bilayer were fabricated. For instance, a cross-shaped bilayer that was designed with non-crosslinking patterns on SA side as shown in Figure 6a, exhibited controlled shape deformations with folding motion along the non-crosslinked patterns only under sunlight irradiation at 115 mW cm−2 (Movie S8 in Supporting Information). The bilayer actuator that has periodic equal crosslinked and non-crosslinked patterns on SA side, could close up into an irregular cylinder by exposure to sunlight irradiation, and then open itself back to the flat shape on a moist surface (Figure 6b). When unequal patterns were designed on SA side, the bilayer responded to sunlight with generating shape deformations highly resembling motion of caterpillars, indicating vividly mimetic performances (Figure 6c, and Movie S9 in Supporting Information). In another proof-of-concept functionality, the response to sunlight could also be used to make a rudimentary multi-armed soft robot that could twist and close itself up upon exposure to sunlight, simulating the response of the stems of S. lepidophylla to dehydration (Figure 6d).42


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Figure 6. Kinematics of various rudimentary soft robots in response to sunlight. (a) Crossshaped bilayer folded up along non-crosslinked patterns only. (b) Periodic crosslinked and noncrosslinked patterns on SA side guided the bilayer with curling into an irregular cylinder that then uncurled back to the flat shape on a moist surface. (c) Unequal patterns on SA side programmed the bilayer with shape deformations resembling the motion of a caterpillar. (d) Multi-armed soft robot closed by sunlight resembling the response of the stems of S. lepidophylla to dehydration.

The mechanism for the directional shape deformations of SA/PVDF bilayer actuators can be located within the different physical and chemical properties of the both layers. The SA layer contains numbers of hydrophilic groups in the structure, capable of adsorption of humidity that can lead to visible layer expansion, while the PVDF layer with hydrophobic groups in the structure, is inert to humidity. As a result, upon exposure to humidity, the expansion always


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occurs in SA layer, resulting in the shape deformation towards PVDF layer. While exposed to sunlight, the photothermal effect-induced dehydration occurs from SA layer that enables the contraction reaction against PVDF layer, and the bilayer thus curves to SA layer. When the crosslinked and non-crosslinked patterns are introduced, the elastic tensors at SA layer would be caused to be different between the patterns that direct the bilayer with programmed shape deformations.

Based on the potentials demonstrated above, the sunlight-driven shape deformation of the bilayer of SA/PVDF was utilized to roughly examine the intensity range of sunlight irradiation. Five electrical circuits were designed as shown in Figure 7a with the SA/PVDF film acting as a switch that bended requiring different power intensities of sunlight to establish connection respectively. In the electrical circuit (I), the power intensity of 38 mW cm−2 was required for the bilayer bending to turn the diode on, while in the electrical circuit (V), 200 mW cm−2 was required to turn the diode on. As a result, when assembling these electrical circuits together as shown in Figure 7b to make a simple device, it could indicate the range of intensity of sunlight by visual observation the light-emitting diodes. For example, under the sunlight irradiation, if the diodes in electrical circuit (I, II and III) were on, it implied that the intensity of sunlight was in the range of 98−130 mW cm−2, and if all diodes were on, the sunlight intensity would be over 200 mW cm−2. This demonstration is rather primary to disclose application potential of the SA/PVDF bilayer that is expected to be further improved in the future.


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Figure 7. The bilayer film utilized to roughly yet visually detect intensity range of sunlight. (a) The bilayer film-based five electrical circuits were designed respectively, in which the SA/PVDF film acted as switches that bended by sunlight to turn the diodes on. (b) Assembly of such five electrical circuits together to roughly estimate intensity of sunlight by visual observation of the diodes.

CONCLUSIONS We prepared self-actuating bilayers in response to humidity and sunlight based on two highly elastic polymers of SA and PVDF. The hydrophilic SA element combined with hydrophobic PVDF layer gave rise to the asymmetric hygroscopic response in the bilayer, which thus made the actuations of the bilayer with good directionality. The bilayer bended to the PVDF layer in


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response to humidity owing to the hydration-induced expansion of SA layer, whereas the bending towards the SA layer under sunlight was attributed to the photothermal effect-induced contraction of the SA layer. To achieve more complex actuations from the bilayer, a facile technique was utilized to pattern the bilayer with crosslinking-/non-crosslinking alignments on SA side. The crosslinking patterns were able to cause different mechanical tensors and swellabilities to induce deformation-controlled actuations of the bilayer, such as self-folding, curling, twisting and coiling in response to humidity and sunlight. In most of conventional bilayer actuators, the bilayer was physically combined that easily underwent interfacial failure upon suffering from external mechanical forces. In this work, the bilayer was chemically bonded through a series of interfacial coupling reactions that enabled the bilayer to process of robust adhesion strength with the measured interfacial toughness of reaching 300 J m−2, and thus it was capable of continuous deformations without any interfacial failure. The sunlight-responsive motion of the bilayer can be utilized for the design of a simple device to roughly examine the intensity of sunlight irradiation. These results highlight their potentials as smart materials for applications in soft robotics, sensors and energy conversions.

Supporting Information. Figures S1‒S18, and Movies S1‒S9. This material is available free of charge via the Internet at http://pubs.acs.org. Author Contributions L. Z contributed to all aspects of this study. X. Q. contributed to the interfacial reactions of the polymer bilayers. Y.Y. and T.Z developed part bilayer actuators. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes


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The authors declare no competing financial interest. ACKNOLEDGEMENTS The authors are grateful to East China Normal University, National Natural Science Foundation of China (Grant No. 51603068) and Natural Science Foundation of Shanghai (Grant No. 17ZR1440600) for financial support. We thank Dr. Haiying Sun (ECNU) for the help in water contact angle measurements and Prof. Xuesong Jiang (Shanghai Jiao Tong University) for his warm-hearted assistance with ATR-IR spectroscopy. REFERENCES (1) Liu, Y.; Xu, B.; Sun, S.; Wei, J.; Wu, L.; Yu. Y., Humidity- and Photo-Induced Mechanical Actuation of Cross-Linked Liquid Crystal Polymers. Adv. Mater. 2017, 1604792, 1 – 7. (2) Dai, M.; Picot, O. T.; Verjans, J. M. N.; de Haan, L. T.; Schenning, A. P. H. J.; Peijs, Ton.; Bastiaansen, C. W. M., Humidity-Responsive Bilayer Actuators Based on a Liquid-Crystalline Polymer Network. ACS Appl. Mater. Interfaces 2013, 5, 4945 − 4950. (3) Islam, M. R.; Li, X.; Smyth, K.; Serpe, M. J., Polymer-Based Muscle Expansion and Contraction. Angew. Chem. Int. Ed. 2013, 52, 10330 – 10333. (4) Cheng, H.; Hu, Y.; Zhao, F.; Dong, Z.; Wang, Y.; Chen, N.; Zhang, Z.; Qu, L., MoistureActivated Torsional Graphene-Fiber Motor. Adv. Mater. 2014, 26, 2909 – 2913. (5) Chen, X.; Mahadevan, L.; Driks, A.; Sahin, O., Bacillus Spores as Building Blocks for Stimuli-Responsive Materials and Nanogenerators. Nat. Nanotech. 2014, 9, 137 –141. (6) Ma, M.; Guo, L.; Anderson, D. G.; Langer, R., Bio-Inspired Polymer Composite Actuator and Generator Driven by Water Gradients. Science 2013, 339, 186 – 189. (7) Okuzaki, H.; Kuwabara, T.; Funasaka, K.; Saido, T., Humidity-Sensitive Polypyrrole Films for Electro-Active Polymer Actuators. Adv. Funct. Mater. 2013, 23, 4400 – 4407.


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ToC graphic:


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