Programmable Polymer Actuators Perform Continuous Helical Motions

Subscriber access provided by Bibliothèque de l'Université Paris-Sud

Applications of Polymer, Composite, and Coating Materials

Programmable Polymer Actuators Perform Continuous Helical Motions Driven by Moisture Qing Chen, Xiunan Yan, Han Lu, Ning Zhang, and Mingming Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06398 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Programmable Polymer Actuators Perform Continuous Helical Motions Driven by Moisture Qing Chen, Xiunan Yan, Han Lu, Ning Zhang, Mingming Ma*

ABSTRACT: Powerful soft actuators that can perform programmable actuations are highly desired for the development of soft robotics. Herein we report a moisture-driven polymer actuator: PPA, which is a composite of poly(3,4-ethylenedioxythiophene)/polyvinyl alcohol/copolymer of acrylic acid and 2-acrylanmido-2-methylpropanesulfonic acid. PPA can not only generate powerful actuation with a contractile stress up to 13 MPa, but also perform programmable helical motions. PPA films with internal stress along the radial directions were prepared by a simple solution casting method. Driven by moisture, rectangular strips cut from the same PPA film but with different cutting angles (the oblique angle between the long axis of PPA strip and the radial axis of PPA film) can perform direct bending, left-handed or right-handed helical motions, demonstrating the generation of chirality from asymmetric internal stress. By modulating the distribution of internal stress in PPA strips, their moving direction and speed are readily prescribed. The powerful and programmable PPA strips can be used to make soft devices, such as moisture-responsive switches and transporters. Our strategy of generating and utilizing internal stress in responsive polymers represents a promising platform for fabricating smart soft actuators. Key words: Responsive polymers, Programmable soft actuators, Helical motion, Internal stress, Moisture * Corresponding author. E-mail address: [email protected] Postal address: CAS Key Laboratory of Soft Matter Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China 1. INTRODUCTION In nature, anisotropic movements and deformations of plants can be triggered by environmental humidity variations to achieve various biological functions1-2, such as coiling of Cucumber tendrils3, chiral opening of bean pods4, and helical winding of Erodium seed awns5. Due to the variation in the orientation of cellulose microfibrils across the thickness of the plant tissue, anisotropic movements and deformations occur when the tissue swells and expands2, 6. Designing stimuli-responsive polymeric materials to mimic these anisotropic movements and deformations are important for fundamental designing and the development of soft robotics. Inspired by biological systems, many materials based on photo/thermalresponsive polymers7-13 and moisture-responsive14-22 polymers have been designed. To achieve anisotropic movements or deformations, foreign molecules, multi-layers or paralleled structures that are inert to external stimuli are added into responsive polymers, which enable anisotropic shape or volume change of these responsive composites upon external stimuli23. For example, at molecular level, chiral dopants are incorporated into liquid crystalline network24-26; at microscopic level, carbon nanotubes or cellulose nanofibers are embedded into responsive polymer matrix27; at macroscopic level, various asymmetric multilayer 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

films27-30 and multi-component hydrogels31-33 have been developed (see Table S1 for detailed summary). However, it has been complicated to organize or pattern these foreign molecules or materials inside responsive polymers, which typically requires printing or lithographic techniques. The addition of these inert molecules or materials into responsive polymers could also reduce the sensitivity of responsive polymers to external stimuli. As an alternative strategy, modulation of internal stress in photo-polymerized composites by controlling the cross-linking density has been utilized to make shape-shifting materials34-36. Without adding foreign molecules, multilayered or paralleled structures, generation and modulation of internal stress in responsive polymers could become an effective strategy to achieve stimuli-driven anisotropic movements or deformations. Herein we report a moisture-responsive polymer: PEDOT/PVA/PAA-AMPS (PPA), which was prepared by polymerization of 3,4-ethylenedioxythiophene (EDOT) in the solution of polyvinyl alcohol (PVA) and the copolymer of acrylic acid and 2-acrylamido-2methylpropane sulfonic acid (PAA-AMPS). Strong and flexible PAA films were prepared by a simple solution casting method, which can perform fast and continuous actuation driven by moisture, and output a contractile stress up to 13 MPa. Internal stress along radial directions was generated in PPA films due to the inhomogeneous solvent evaporation. A large PPA film can be cut into many pieces of rectangular strips. Driven by moisture, these PPA strips can perform direct bending, left-handed or right-handed helical motions. The motion pattern is mainly controlled by the oblique angle between the long axis of the strip and the direction of internal stress. Therefore, by modulating the distribution of internal stress, the motion of PPA strips is readily programmable. These powerful and programmable PPA strips can be used to fabricate soft devices, such as moisture-responsive switches and cargo-transporting actuators. Most of previously reported moisture-responsive actuators either perform random motion14, or need the assistance of external field to perform directional motion37, while our PPA strips can perform programmable directional motion without external assistance. Our strategy of generating and utilizing internal stress in responsive polymers represents a promising method in the development of smart soft actuators. 2. EXPERIMENTAL 2.1 Preparation of PPA films and PPA strips. 20 μL EDOT, 200 μL PVA solution (10% wt/wt in H2O) and 667 μL PAA-AMPS solution (30% solid content) are mixed at room temperature. Then, 0.22 mmol APS dissolved in 50 μL water is added, and NH3·H2O was used to adjust the reaction pH to 3-4. Finally, 2.2 mmol FeCl3·6H2O dissolved in 200 μL water was added and the mixed solution was stirred for 10 min. The obtained PPA suspension was cast onto a glass slide and dried at room temperature for 8 hours, resulting in a black film sticking to the glass slide. The film was immersed in water for 1 hour to remove excessive reactants and ions. After washing and drying at room temperature, the as-prepared PPA film was easily peeled off. To make PPA strips, a large PPA film (9 cm × 9 cm × 12 μm) was prepared and cut into strips with different locations and sizes using scissors. The cutting angle was controlled by a protractor. 2.2 Locomotion analysis. Locomotion of PPA films or PPA strips on a moist substrate (nonwoven paper containing ~ 50% water by weight) in ambient conditions (24 °C, 30% relative humidity, no detectable air flow) was recorded by a video camera. The environment temperature and humidity was monitored by a commercial hydro-thermometer. For each condition, three one-minute videos were recorded and analyzed to calculate the average flipping frequency. The data of moving direction, flipping frequency, and radius of trajectory of PPA strips were collected based on the videos in supporting information. 3. RESULTS AND DISCUSSION 2 ACS Paragon Plus Environment

Page 2 of 16

Page 3 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.1 Design and synthesis of PPA film. Poly(3,4-ethylenedioxythiophene) (PEDOT) is a rigid and hydrophobic conducting polymer, which can be easily synthesized by polymerization of EDOT in aqueous solution38. Polyvinyl alcohol (PVA) is a hydrophilic polymer that can form strong fibers and films. PAA-AMPS is an ionic copolymer of acrylic acid (AA) and 2-acrylamido-2-methylpropane sulfonic acid (AMPS), which could serve as an anionic dopant for PEDOT and help solvating the hydrophobic PEDOT in aqueous solution. Therefore, we choose PEDOT, PVA and PAA-AMPS to construct PPA. APS (ammonium persulfate) and FeCl3 were used as oxidants, where Fe3+ ion could also coordinate with the carboxylate groups in PAA-AMPS and act as crosslinkers39. Therefore, hydrophobic PEDOT, hydrophilic PAA-AMPS and PVA are crosslinked by hydrogen bond, electrostatic interaction and coordination to form a supramolecular network (Figure 1a, b). The dynamic assembly of hydrophobic and hydrophilic polymers enables PPA a powerful moisture-responsive material. The detailed synthesis procedure of PAA film is decribed in the experimental section and in Scheme S1. Briefly, EDOT monomer was added to the aqueous solution of PVA and PAAAMPS. The solution was stirred at room temperature to get an uniform milky solution. Then, ammonium persulfate (APS) and FeCl3 were added and the mixture was stirred to get a darkblue suspension. This suspension was cast onto a glass slide and dried at room temperature for 8 hours, resulting in a black film. This film was immersed in pure water for 1 hour to wash away excessive reactants and ions. The remaining black film was dried at room temperature, and easily peeled off from the glass slide to give the PPA film. PPA films lacking PVA would crack into pieces upon drying (SI Figure S1), which indicates the critical role of PVA in the formation of a uniform PPA film. 3.2 Structural characterization of PPA film. Scanning electron microscope (SEM) images (Figure 1c, SI Figure S2) of the top, bottom and cross-sectional surfaces of PPA film indicate a uniform microstructure with evenly distributed micropores. The porous structure will benefit the fast absorption and desorption of water from PPA film, accompanied by fast and reversible deformations. TGA curve of PPA film showed < 5% weight loss up to 200°C, which demonstrates a good thermal stability (SI Figure S3). The chemical structure of PPA film was studied by X-ray diffraction (XRD), Raman and Infrared (IR) spectroscopy. The bands at 438, 991, and 1256 cm-1 in Raman spectrum (SI Figure S4) are attributed to the C-OC deformation, oxyethylene ring deformation, and Cα-Cα inter-ring stretching deformation of PEDOT, respectively. Remarkably, the strong band at 1425 cm-1 due to C=C stretching in the thiophene ring indicate a long conjugation length in the backbone of PEDOT. In XRD spectrum, the broad peak at 2θ = 24o indicates the amorphous nature of PPA (SI Figure S5). In the IR spectrum (SI Figure S6), 1048 cm-1, 1515 cm-1 and 1315 cm-1 are assigned to C-O-C bending vibration, C=C asymmetric stretching and Cα-Cα inter-ring stretching of PEDOT, respectively. The peak at 842 cm-1 is due to the C-S-C bond in thiophene ring,40-41 and peak at 1088 cm-1 is attributed to C-O-C stretching in PVA. These results indicate the successful assembly of PEDOT, PVA and PAA-AMPS polymers in PPA (SI Figure S7). Besides the three polymers, the remaining Fe3+ ion in PPA film is also important as crosslinkers: PAA-AMPS contains a large amount of carboxylate groups, which could coordinate with Fe3+ ion to form crosslinking points between polymers, contributing to the formation of PPA network. When a piece of PPA film was soaked in 0.1 M HCl solution, the carboxylate-coordinated Fe3+ ions were dissolved into solution, and detected by the formation of red-colored Fe(SCN)63- ions. After Fe3+ ions were removed by acid, PPA network was partially destroyed, therefore the PPA film became fragile and brittle, which suggests the importance of Fe3+ as crosslinkers to enhance the mechanical property of PPA. 3 ACS Paragon Plus Environment


Figure 1. Synthesis, structure and moisture-responsive property of PPA film. (a) Synthesis of PPA composites. (b) Supramolecular interactions between PEDOT, PVA and PAA-AMPS. (c) SEM image of the cross-section of a PPA film showing the porous structure. (d) The moisture-responsive behavior of a PPA film at molecular (right), microscopic (middle) and macroscopic (left) levels. 3.3 Mechanism of moisture-responsive properties of PPA film. The dynamic network structure in PPA that is formed by the hydrophobic PEDOT and the hydrophilic PVA and PAA-AMPS polymers is the key to enable PPA the moisture-responsive properties. Water can be quickly absorbed by hydrophilic PVA and PAA-AMPS, which cause PPA film to swell (Figure 1d, right), and the hydrophobic PEDOT helps water to be desorbed from PPA film. Therefore, when the moisture concentration in environment varies, the PPA film could quick absorb and desorb water, leading to fast swelling and shrinking deformations (Figure 1d, left). The water-induced swelling of PPA was examined under optical microscope. As shown in Figure 1d (middle), in a dry PPA film, PEDOT fibers (dark blue) are randomly distributed in the matrix of PVA and PAA-AMPS. When the PPA film is swelled by water, ridges and wrinkles appears, and the film became more transparent (SI Figures S8, S9, Movie S1). We also used ATR-IR spectrum to probe the chemical structure changes of PPA film upon water absorption and desorption. A piece of PPA film was dipped into liquid H2O for 10 s, then taken out and examined by ATR-IR. The broad peaks at 3250 cm-1 and 1647 cm-1 are corresponding to the O-H stretching and binding vibration, which quickly decreased within 5 min (SI Figure S10). This result indicates that the H2O content in PPA quickly decreased due to fast diffusion and evaporation of H2O from PPA film. In addition, the water exchange in PPA film was studied by D2O-H2O exchange experiment. A piece of PPA film was soaked in D2O for 10 s, then taken out and examined by ATR-IR (SI Figure S11). The fast decrease of peaks at 2400 cm-1 and 1162 cm-1, and the evolution of peaks at 3250 cm-1 and 1647 cm-1 indicated continuous exchange of D2O and HDO with H2O in air. The results from optical 4 ACS Paragon Plus Environment

Page 4 of 16

Page 5 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


microscope and IR spectrum suggested that PPA film continuously “breathing” water from environment (Figure 1d), which should enable its fast and reversible response to environmental humidity variations. Upon the increase of environmental humidity, the hydrophilic PVA and PAA-AMPS polymers can quickly absorb H2O, resulting in the quick expansion of PPA film. Upon the decrease of environmental humidty, water could quickly diffuse, leading to a quick contraction of the film. And the hydrophobicity of PEDOT polymer and its interaction with the hydrophilic PVA and PAA-AMPS could be the key for the fast water adsorption and desorption of the PPA film.14 3.4 Moisture-responsive motion of PPA film. When a piece of PPA film was put on a moist substrate, it rapidly and continuously flipped across the substrate. This motility required a water vapor gradient, rather than just water. When placed in a closed chamber saturated by water vapor, the PPA film would be swelled and remain static. Volatile polar organic solvents (such as alcohol and acetone) also caused the PPA film to bend, but no flipping motion could be achieved, since the desorption of these solvent by PPA was slow. Similar to the autonomous motion of other water-responsive polymers previously reported by Langer 14 and Vaia,42 a rectangular PPA film performs a locomotive cycle composed of five stages (Figure 2a, Movie S2). When it is placed on a moist substrate, the film undergoes asymmetric swelling and curls upward (I). Then the film’s gravity center rises (II), and the film topple over (III) and move horizontally (IV). Finally, the upper surface of the film come into contact with the substrate (V) to start a new flipping cycle (I). During the flipping process (stages I to V), the water gradient between substrate and air was reflected in the asymmetric film deformation. Thus, the chemical potential from the water gradient, combined with the film’ gravity and the friction between the film and substrate, lead to asymmetric film deformation that drive the film to flip continuously. We further studied the effect of temprature of the substrate, thickness and aspect ratio of the film on the motion of PPA film. Firstly, temperature would affect the water exchange kinetics inside the PPA film. As the temperature of the substrate was raised from 20 °C to 60°C, the flipping frequency of PPA increased at first and then decreased. The optimal temperature for the highest flipping frequency was found at 40-50°C (Figure 2b). Secondly, the flipping frequency of PPA films decreased from 30 to 6.8 min-1, as the film thickness increased from 10 to 40 μm (Figure 2b). The optimal film thickness was 10-20 μm. Flipping of thinner films (30 μm) was limited by their stiffness. Furthermore, we found that rectangular PPA films with aspect ratio between 1.5:1 to 2:1 flip most effectively. In order to probe the critical role of the supramolecular structure of PPA on the moistureresponsibility, we prepared a PEDOT:poly(styrenesulfonate)(PSS)/PVA/PAA-AMPS (PSPA) film by substituting EDOT solution to a commercial PEDOT:PSS (PH1000) solution (Figure S12, see SI for the synthesis procedure). In PEDOT:PSS, PEDOT chains are doped and surrounded by anionic PSS, and form amorphous particles43. While in the PPA film, PEDOT chains are doped and surrounded by PAA-AMPS and PVA, and form microfibers (Figure 1d). In their XRD spectra (SI Figure S13), the narrower peak around 24° also suggests a more ordered polymer packing in PPA film than that in PSPA film. In the Raman spectrum (SI Figure S14), PPA shows strong and sharp peaks at 1425, 991, 576, 438 cm-1, indicating that PEDOT chains in PPA have a quite long and uniform conjugation structure44. In contrast, the PSPA shows much weaker and broader peaks in its Raman spectum, suggesting a shorter and non-uniform conjugation structure of PEDOT in PSPA film. Due to the different chemical structure and micro-structure, the moisture-responsive property of PSPA film is much weaker 5 ACS Paragon Plus Environment


and slower than that of PPA film (Movie S2). This comparison clearly demonstrates that the unique moisture-responsive property of PPA film is due to the in-situ formed network structure of PEDOT, PVA and PAA-AMPS polymers. In PSPA film, PEDOT chains have been wrapped by PSS and could not form efficient interactions with PVA and PAA-AMPS, resulting in the low moisture-responsibility.

Figure 2. Moisture-responsive property of PPA films. (a) Flipping motion of a PPA film on a moist substrate. (b) Effect of the film thickness (10-40 m) and the temperature (20-50 °C) of moist substrate on the flipping frequency of PPA film. (c) The contractile force produced by a 10-mg PPA film during 20 swelling-shrinking cycles. (d) Load-dependent displacement during the swelling and shrinking of a 10-mg PPA film. 3.5 Mechanical performance of PPA film. The mechanical performance of PPA films were studied by Instron mechanical analyzer at a relative humidity (RH) of 30-34%. The tensile strength and elongation-at-break of PPA film was 45 MPa and 3.9%, respectively (SI Figure S15). Then we tested the contractile force generated during the swelling and shrinking of PPA film. A 10-mg PPA film was clamped between two holders. Then, the film was swelled by a piece of wet paper and stretched to keep the film straight and tight. The loaded force on this swelled PPA film was 0.05 N. When the moist paper was removed, the PPA film shrinked and stiffened, creating a contractile force up to 1.5 N. The highest contractile stress could reach 13 MPa (Figure 2c), which is ~37 times higher than the highest stress production of mammalian skeletal muscle (~0.35 MPa).45 By alternately changing the environmental humidity, PPA film can expand and contract periodically. This expand-contract cycle could be repeated hundreds of times, indicating the outstanding mechanical stability of PPA film. Moreover, we also measured the load-dependent displacement of PPA films. When we preload a 10-mg PPA film with a constant stress and decrease the environmental humidity, PPA film contracted, and the displacement was proportional to the load exerted (Figure 2d), signifying that PPA actuator was working in its elastic range (SI Figures S16, S17). We also investigated the ability of PPA film to do mechanical work. Upon water adsorption, a 10-mg PPA film could deform and lift a 0.29-g glass cover slide to a height of 6 mm within 3 s (SI Figure S18 a). The same PPA film could also lift a 2.4-g glass slide to 2 mm, which is 266 times heavier than itself (SI Figure S18 b), demonstrating the potential of PPA actuators to perform robust mechanical work. Work performance of PPA in the weight-lifting process was calculated as 47 μJ, and the work performance normalized to the mass of PPA was 4.7 mJ g-1. 6 ACS Paragon Plus Environment

Page 6 of 16

Page 7 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.6 Generation of internal stress in PPA film. During the casting process, radial internal stress was generated in the as-prepared PPA film due to the inhomogeneous water evaporation. To visualize the formation of internal stress, we cast PPA suspension onto Parafilm® M as a soft substrate (4×4 cm2), and the curvature of the substrate was monitored during the solvent evaporation process. As seen in Figure 3a, the Parafilm substrate curled inward from the edges, and the curling angle increased as evaporation proceeds (SI Figure S19). In the PPA suspension, the hydrophilic PVA and PAA-AMPS are fully hydrated. As water evaporates, PPA film shrinks and causes the Parafilm substrate to curl. Since the evaporation rate in the center of PPA film is slower than at the edges, anisotropic internal stress is generated in PPA films. As shown in Figure 3b, the internal stress gradient exists in radial directions, while there is no significant difference in the film thickness and chemical composition (SI Figure S20). Based on the thickness, Young’s modulus, and the curvature of the Parafilm substrate (SI Figure S21), we calculate the elastic potential energy to be 2.94 J m-2 at the corner of the 4×4 cm2 PPA film (Figure 3c, 3d, see SI for the details).

Figure 3. Generation of anisotropic internal stress in PPA films. (a) Photographs of PPA (4×4 cm2) films cast on Parafilm®M (left) or on a glass slide (right), respectively. The different shadow regions (right) indicate the remaining wet area at certain time points. (b) Schematic diagram showing the directional drying and generated internal stress in PPA films cast on a soft substrate (left) or a hard substrate (right), respectively. (c) The elastic potential energy at the edge of a PPA film upon drying. (d) The distribution of elastic potential energy in a dried PPA film. (e) Programmed deformation of PPA strips. Top: schematic diagram showing two groups of PPA strips cut from two PPA films (1.5×6 cm2), and the cutting angles were 60° (left group) and 90° (right group), respectively. Bottom: photographs showing the curling or bending patterns of the two groups of PPA strips. One as-prepared PPA film can be cut into many pieces of rectangular PPA strips. The cutting angle (θ) is defined as the oblique angle between the long axis of PPA strip and the 7 ACS Paragon Plus Environment


radial axis of PPA film (Figure 3e). We prepared a series of PPA strips with cutting angles at the same absolute value (θ = 60° or -60°), but at different distances from the center of the same PPA film, and placed them on a substrate with low humidity. These strips formed lefthanded or right-handed helicoids with the same helicity but different curling pitches (Figure 3e left). The curling pitch of strips cut from the inner part of PPA film were larger than those from the outer part, demonstrating that the stress gradient was shaper at the outer part than the inner part of PPA film. This result is in accord with the observation of curled PPA film on the soft Parafilm substrate. We also cut a series of PPA strips with θ = 90° and placed them on the substrate with low humidity. These strips folded into rolls with no helical preference (Figure 3e right). When these two groups of PPA strips were put on a moist substrate, they performed either direct-bending or continuous helical motions driven by moisture. 3.7 The motion patterns of PPA strips. The motion patterns of PPA strips could be prescribed by the cutting angle θ. In the circumstance of θ=0° or θ=90°, the internal stress gradients in PPA strips is symmetric, and the strips display non-helical random motions. For example, when a PPA strip with θ = 0° (Figure 4a) is placed at 1 cm above a moist substrate (Figure 4b), the average periodity for this strip (5 mm × 10 mm) to bend and unbend was 5 s (Figure 4c). When contacting water vapor, the strip bends and folds away within 0.1 s (Movie S3). When the strip unbends and contacts moisture again, it starts a new cycle, and this process can repeat dozens of times (Figure 4d). In the circumstance of θ ≠ 0° or 90° (Figure 4e, 4i), the asymmetric internal stress gradients in PPA strips would cause helical motions (Figure 4f-h, j-l). The internal stress in PPA strips can be eliminated by annealing: when a strip that perform continuous left-handed helical motion was heated at 80°C for 2 hours, this annealled PPA strip can only perform random non-helical motion (Movie S4). Based on these results, we know that the helical motions of PPA strips are due to two asymmetric factors: (1) the formation of internal stress gradient along the radial directions, and (2) the cutting angle, which determines the asymmetric internal stress distribution within a specific PPA strip. By properly adjusting these two factors, continuous helical motions of PPA strips driven by moisture can be successfully achieved. By tracking the gravity centers of PPA strips during their helical motions, we found that the trajectories of helical-locomotive PPA strips are also helical (Figure 4h, l, Movie S5). Smilarly to the light-driven materials in the work of White et. al46, the helicities (left-handed and right-handed) of their trajectories (Figure 4h, l) are the same with their single flip (Figure 4f, j), and helical flipping motion (Figure 4g, k). While in this work, PPA performs moisturedriven helical motion contnuously instead of simply helical deformations. Therefore, the helicity of PPA strips’ helical motions are achieved at three different levels: (1) the helicity of a single flip, (2) the helicity of one helical motion cycle, and (3) the helicity of the motion trajectory.

8 ACS Paragon Plus Environment

Page 8 of 16

Page 9 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Different motion patterns of PPA strips. (a-d) Direct bending of PPA strips. (a) Cutting direction of PPA strip is parallel to the radial directions (θ=0°). (b) Schematic diagram showing the bending movement of a PPA strip (θ=0°) above a moist substrate (distance: 1 cm). The changes of bending angle of the PPA strip within one (c) and (d) multiple bend-unbend cycles were measured by analyzing recorded movies. (e-l) Righthanded and (i-l) left-handed helical motions of PPA strips. The schematic diagrams show one flip (f, j), one helical motion cycle (g, k), and two representative motion trajectories of PPA strips (clockwise and counter-clockwise) that are based on the analysis of Movie S5 (h, l). 3.8 Programmable helical motion of PPA strips. By analyzing the helical motion video of multiple PPA strips, we found that a PPA strip is composed of an active end and a passive end (Figure 5a). The active end has a sharper internal stress gradient than the passive end, and works like a flexible oar that initiates and drives the helical motion. The curling direction of PPA strip is perpendicular to the direction of internal stress gradient (Figure 5a). Thus, when the long axis of PPA strip is not parallel or perpendicular to the direction of internal stress gradient, the PPA strip would curl and perform helical motions. Firstly, we studied the effect of cutting angle on the helical motion pattern of PPA strips. PPA strips with the same size (0.8×3.2 cm2) and radial distance to the center of PPA film, but different cutting angles were prepared from the same PPA film (9×9 cm2) (Figure 5b). Strips with 90°

Programmable Polymer Actuators Perform Continuous Helical Motions

Recommend Documents