Polymer Supramolecular Assemblies

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Tetra-Carboxylated Azobenzene/Polymer Supramolecular Assemblies as High-Performance Multiresponsive Actuators Chengqun Qin, Yiyu Feng, Haoran An, Junkai Han, Chen Cao, and Wei Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15075 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 14, 2017

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ACS Applied Materials & Interfaces

Tetra-Carboxylated Azobenzene/Polymer Supramolecular Assemblies as High-Performance Multiresponsive Actuators Chengqun Qina, Yiyu Fenga,c,d, Haoran Ana, Junkai Hana, Chen Caoa, Wei Fenga,b,c,d* a

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, P.

R China. b

Collaborative Innovation Centerof Chemical Science and Engineering (Tianjin),

Tianjin 300072, P. R China. c

Key Laboratory ofAdvanced Ceramics and Machining Technology, Ministry of

Education, Tianjin 300072, P. R China. d

Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, P.

R China.

KEYWORDS: multistimuli-responsive polymers, azobenzene, polyelectrolytes, supramolecular chemistry, actuator

ABSTRACT: Multistimuli-responsive polymers are materials of emerging interest, but synthetically challenging. In this work, supramolecular assembly was employed as a facile and effective approach for constructing 3,3′,5,5′-azobenzenetetracarboxylic acid (H4abtc)/poly(diallyldimethylammonium chloride) (PDAC) supramolecules. Structural transformations of H4abtc can be induced by light, mechanical force, and 1 ACS Paragon Plus Environment


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heat, and influenced by free volume. Thus, the fabricated freestanding H4abtc/PDAC film underwent bending/unbending movements upon treatment with light, humidity, or temperature, as asymmetric structural transformations on either side of the film generated asymmetric contraction/stretching forces. Fast rates of shape recovery were achieved for the film on exposure to gently flowing humid nitrogen. The bending/unbending motions are controllable, reversible, and repeatable. Hence, this light-, humido-, and thermoresponsive film has great potential in device applications for advanced functions.

INTRODUCTION

Stimuli-responsive polymers (SRPs) are notable because of their ability to respond to a particular stimulus (temperature, humidity, electric field, pH, light, etc.) by altering their physical and/or chemical properties.1-11 Several classes of SRPs have been developed with enhanced performance and new functions, including multiresponsive polymers (MRPs), which are capable of responding to multiple stimuli.12-19 Multifaceted responsiveness could greatly enhance the versatility of MRPs for novel applications in extreme, changeable and complicated conditions where multiple tuning ways provide a more reliable guarantee.1,

20-24

Hence, the

design and development of novel MRP systems and mechanisms is essential for further expanding the applications, such as artificial muscles, robot skin, actuators, and switching devices.2, 3, 25-27 2 ACS Paragon Plus Environment

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Over the past two decades, a large variety of SRPs has been successfully synthesized owing to the development of new methods for polymerizing functional monomers.28, 29 Subsequently, the synthesized SRPs can be combined with functional organic/inorganic molecules or other SRPs to yield MRPs via covalent/non-covalent interactions. In particular, supramolecular assembly is emerging as a smart and facile approach for responsive materials, as they are easy to make from readily available and inexpensive commercial components. Compared with covalently synthesized polymers, the compositions, structures and properties of supramolecular assembly can be easily tailored by tuning parameters such as mixing ratios, pH values, ionic strength and temperature during and even after the preparation of polymeric complexes.2, 30-33 Recently, various supramolecular MRPs have been reported based on supramolecular interactions including electrostatic interactions,34, bonding,36-38

halogen

bonding,39-42

coordination

bonding,43

35

hydrogen

charge-transfer

interactions,44 and guest-host interactions.45 Therein, electrostatic assembly of small organic molecules and polyelectrolytes (PEs) is of particular interest, as it combines the excellent stimuli-responsive properties of organic molecules and the high mechanical flexibility of PEs.13,

21

The responsive properties of such polymer

composite usually largely depend on the selection of materials and the mixing ratios for supramolecular assembly. Azobenzene (AZO) and its derivatives are widely used as light-responsive units in self-assembled structures due to their reversible photoisomerization. Moreover, transitions between the E- and Z-isomers can also be induced by mechanical 3 ACS Paragon Plus Environment


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force.46-48 Therefore, when AZO molecules are attached to polymer networks, the isomerization of AZO is closely related to the shrinking/swelling of the polymer matrix due to the generated mechanical force and free volume change around AZO. Sun and co-workers13, 21 showed that PE films have a great ability to adsorb/desorb water, resulting in shrinking/swelling of freestanding films. Hence, supramolecular assembly of ionic AZO and oppositely charged PEs can realize high-performance MRPs. In this paper, 3,3′,5,5′-azobenzenetetracarboxylic acid (H4abtc) (Figure 1a) was combined with poly(diallyldimethylammonium chloride) (PDAC) via electrostatic interactions. Compared with previously reported supramolecular assemblies coupled by one or two functional groups,31, 32, 49 the four carboxylate groups of H4abtc largely increase the supramolecular cross-linking of PDAC chains to form stable networks, thus integrating the isomerization of H4abtc with the chain movement of PDAC. A multiresponsive free-standing H4abtc/PDAC film was prepared by an optimized drop-coating technique (Figure 1b). The size of the film, in principle, is only limited by the size of the Teflon dish. The film was cut into a suitable size for the fabrication of actuators. This obtained actuators show an excellent light-, humido- and heat-responsive properties by a large shape deformation. Moreover, the large shape deformation and recovery of H4abtc/PDAC actuators can be easily tuned and controlled by changing different stimulus. This H4abtc/PDAC actuator shows a novel route for the construction of multiresponsive system by supramolecular interaction.

MATERIALS AND METHODS 4 ACS Paragon Plus Environment

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Materials. All commercial starting materials and solvents were used as supplied. Poly(diallyldimethylammonium chloride) (PDAC, 20% (w/w) in water, Mw = 200 000–350 000) was purchased from Sigma-Aldrich. Synthesis of H4abtc. H4abtc was synthesized by a typical azobenzene coupling reaction (Figure S1). 5-Aminoisophthalic acid (1.81 g, 10 mmol), NaOH (0.8 g, 20 mmol), and NaNO2 (0.76 g, 11 mmol) were dissolved in 50 mL of distilled water, and the mixture was slowly added into an HCl solution (50 mL, 1 mol L-1) in an ice bath (0–5 ºC) for diazotization. The diazonium salt solution was slowly added to a suspension of isophthalic acid (1.66 g, 10 mmol) and NaOH (0.8 g, 20 mmol) at 0–5 ºC under the protection of argon. The pH of the reaction was adjusted to 7–9 by adding NaHCO3 dropwise. HCl solution was used to precipitate H4abtc from the reaction solution. The resultant raw materials were recrystallized using EtOH/H2O (1:1), and H4abtc (2.43 g, 6.8 mmol, yield: 68%) was collected after washing with di water, filtration, and drying under vacuum overnight at 60 °C. 1H NMR (500 MHz, DMSO-d6): δ = 13.38 (br s, 4H; COOH), 8.58 (d, 4H; Ar–H), 8.61 ppm (t, 2H; Ar–H). HRMS-ESI: m/z: 359.0509 (calcd. for [M + H]+, 359.0515). Preparation of H4abtc/PDAC films. H4abtc (358 mg, 1 mmol), NaOH (160 mg, 4 mmol), and PDAC (20 mL) were mixed and heated to 80 °C under stirring to obtain a homodisperse solution. Therefore, H4abtc/PDAC supramolecules were acquired by electrostatic assembly in solution. Subsequently, NaCl was removed by repeated dialysis. Typically, the H4abtc/PDAC films were prepared by drop-coating the H4abtc/PDAC solution (5 mL, 3 mg mL-1) mixed with 0.25 mL ethanol into a Teflon 5 ACS Paragon Plus Environment


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dish (Figure 1b). After drying in air, a freestanding H4abtc/PDAC film was easily released from the Teflon substrate. Characterization. Fourier-transform infrared (FT-IR) spectroscopy was carried out on a Bruker Tensor 27 spectrometer. 1H NMR spectra were obtained on a Varian INOVA 500 MHz spectrometer with trimethylsilyl as an internal standard. High-resolution mass spectrometry (HRMS) was carried out on a Bruker Daltonics APEXIV 4.7 T Fourier Transform Ion Cyclotron Resonance Mass Spectrometer. The morphologies of the H4abtc/PDAC films were studied using field-emission scanning electron microscopy (FESEM, Hitachi S-4800). X-ray photoelectron spectroscopy (XPS) analyses were performed on a PHI 1600 spectrometer with a 450 W Mg K radiation. The differential scanning calorimetry (DSC) measurement was carried out using a TA Instruments DSC Q20 with a heating rate of 10 °C/min in a nitrogen atmosphere. Time-evolved UV-Vis absorption spectra of H4abtc/PDAC solutions and films were recorded using a Hitachi 330 UV-Vis spectrophotometer. The light-induced bending of the actuator was performed by a UV gun (LED-200), and the intensity of light to the film was 30 mW cm-2 measured by the light density meter (Beijing Zhongjiaojinyuan Co., Ltd.). The UV light-indued shape deformation of the H4abtc/PDAC film was tested in closed environment to keep a constant temperature (25 °C) and humidity (20% RH). The humido-response properties of the H4abtc/PDAC film were observed at 25 °C in air without shielding from light. The heat-induced shape deformation of the H4abtc/PDAC film was recorded at a humidity of 20% RH in air without shielding from light. 6 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION Fourier-transform infrared (FT-IR) spectra were carried out to confirm the chemical structures of H4abtc, PDAC, and the H4abtc/PDAC film (see Table S1 for detailed assignments, Supporting Information). Multiple peaks in the spectrum of H4abtc/PDAC (Figure 2a), including ν(ring C–C) at 1608 cm-1, δ(CH2) at 1476 cm-1, and ν(C–N) at 1141 cm-1, indicate the presence of both H4abtc and PDAC. Furthermore, the formation of supramolecular interactions between H4abtc and PDAC is indicated by the disappearance of the strong C=O stretching band of the carboxylic acid (observed at 1700 cm-1 in the spectrum of neutral H4abtc), the appearance of new bands at 1560 cm-1 (νas(COO–)) and 1366 cm-1 (νs(COO–)), and the disappearance of δ(OH) at 1420 cm-1 and γ(OH) at 924 cm-1, which indicates the formation of carboxylate anions. Moreover, the shoulder band at 2927 cm-1 assigned to the asymmetric –CH3 stretching vibration of PDAC shifts to 2940 cm-1 in H4abtc/PDAC. This stretching vibration is sensitive to the polarity of the nitrogen atom in the dimethyl diallylammonium group of PDAC. Thus, the observed shift is indicative of an electrostatic interaction between H4abtc and PDAC.49, 50 Hence, the FT-IR results demonstrate that the H4abtc/PDAC assembly forms by electrostatic interactions. Moreover, the supramolecular assembly of H4abtc/PDAC was also confirmed by examining the solubility of the film in organic solvents (Figure S2, Supporting Information). Unlike H4abtc, which is soluble in all the selected solvents, including acetone, ethyl acetate, N-methyl-2-pyrrolidinone, N,N-dimethylformamide (DMF), and NaOH aqueous solution, the H4abtc/PDAC film is only soluble in NaOH aqueous 7 ACS Paragon Plus Environment


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solution. This feature suggests that the free diffusion of AZO moieties in the film is limited, except in NaOH aqueous solution, owing to supramolecular interactions between H4abtc and PDAC. X-ray photoelectron spectroscopy (XPS) was carried out to estimate the elemental composition of H4abtc/PDAC. The principal peak at ~402 eV in the N1s XPS spectra (Figure 2b) is attributed to the charged nitrogen (N+) of PDAC,51, 52 and a peak at 400 eV corresponding to –N=N– (or N–C) in H4abtc is also observed.53 Quantitative calculations based on the N1s XPS data suggests that every H4abtc molecule is supported by approximately 8–9 PDAC structural units through electrostatic interactions. Moreover, Figure 2c shows the morphology of the freestanding H4abtc/PDAC film, as observed by field-emission scanning electron microscopy (FESEM). The film exhibits a smooth surface and no aggregates were observed. The image of the cross section indicates that the thickness of the film is 15 µm. The distribution of H4abtc in the PDAC polymer matrix was examined by energy-dispersive X-ray (EDX) spectroscopy elemental mapping. Figure 2d and 2e show uniform distributions of N and O throughout the area of the film shown in Figure 2c, indicating that supramolecular self-assembly is a simple but efficient technique for obtaining a homodisperse structure at the molecular level for small organic molecules/polymer composites. The unique photophysical properties and photoinduced isomerization processes of H4abtc and the H4abtc/PDAC film were studied by UV-Vis absorption spectroscopy. Figure 3a shows the E-enriched/Z-enriched photostationary states (PSS) of H4abtc in 8 ACS Paragon Plus Environment

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DMF and the H4abtc/PDAC film. The strong absorption at approximately 332 nm is due to the π–π* transition of the E-H4abtc isomer (Figure 1a). In the film, the maximum absorption of H4abtc/PDAC is located at 354 nm, which is red-shifted by 22 nm compared with that of H4abtc in solution. This shift has been observed previously and is attributed to the formation of H4abtc salt in the films (Figure S3, Supporting Information). Moreover, irradiation with UV light at 365 nm causes E→Z isomerization, resulting in a decrease of the π–π* band and a change from the E-enriched PSS to the Z-enriched PSS (Figure 3a). The detailed time-evolved absorption spectra of the H4abtc/PDAC film during irradiation are shown in Figure 3b. The absorbance of the H4abtc/PDAC film at 354 nm decreases continuously from 1.04 to 0.85 (18.3% reduction of the initial absorbance) after irradiation for 5 min owing to E→Z photoisomerization. This feature has been widely observed in previous AZO/polymer photoresponse systems.15, 17, 54 The reversion of the H4abtc/PDAC film (Figure 3c) in darkness was also studied. Owing to the instability of the Z-isomer, the H4abtc/PDAC film shows excellent recovery with a half-life (τ1/2) of 30 s, and the intensity of the band at 354 nm completely recovers its initial intensity in 1 h. Interestingly, the photoinduced isomerization and reversion of the H4abtc/PDAC film are remarkably slower than those of H4abtc/PDAC in solution (Figure S4, Supporting Information). This result is further confirmed by the first-order rate constants for photoinduced isomerization (kp) (equation S1, Supporting Information) and reversion in darkness (krev) (equation S2). As shown in Figure 3d, the kp and krev values of the H4abtc/PDAC film are 1.87 × 10-2 s-1 and 3.70 × 10-3 s-1, respectively, which are 9 ACS Paragon Plus Environment


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lower than those in solution (3.80 × 10-2 s-1 and 7.77 × 10-3 s-1, respectively, Figure S5). Owing to the electrostatic assembly of H4abtc and PDAC, the transformation of AZO moieties is affected by the free volume in the composite and the degree of freedom for the movement of polymer chains.46,

49

The free volume in the

H4abtc/PDAC film is relative small compared with that in solution, and the movement of polymer chains in the film is more difficult. Therefore, E-Z isomerization and Z-E reversion is slower in the film than that in solution, indicating that the microstructure of a supermolecule affects the transformation of AZO. As shown in Figure 3e, the absorbance of the E- and Z-enriched H4abtc/PDAC film can be reversibly controlled by using UV irradiation (365 nm) for 5 min or darkness for 60 min. The H4abtc/PDAC film exhibits excellent reversibility and cycling stability, without obvious decay over 50 cycles. Such behavior is an important indicator for the long-time performance of high-performance light-driven actuators. A photoresponsive actuator was fabricated by cutting the freestanding H4abtc/PDAC film into a 3.5 cm × 1.5 cm strip. The deformation of the unfixed strips was investigated under UV light, as shown in Figure 4a, Figure S6, and Movie S1. The film exhibited continuous bending toward the UV light in the first 16 s. When the light was turned off, the film recovered its original flat state in the next 38 s. Similar to previous studies,16, 17, 46, 49 the light-driven deformation and shape recovery of the H4abtc/PDAC film results from the isomerization of AZO moieties and the resultant segmental motion of the polymer. As H4abtc was anchored to PDAC through electrostatic interaction between the four carboxyl groups of AZO and the quaternary 10 ACS Paragon Plus Environment

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ammonium cationic group of PDAC (Figure 1b), microscopic motion of the AZO moieties is converted into macroscopic changes of the film. More specifically, as E-Z isomerization of AZO largely reduces the molecular length from 9 to 5.5 Å, a contraction force is generated in the film. Owing to the large absorption coefficient of the AZO units, the incident light is mainly absorbed by the front surface of the film (facing the UV light), resulting in a faster and higher degree of E-Z isomerization than that at the back surface. Therefore, the contraction force is much larger at the front surface than at the back surface, and this asymmetric force bends the film towards the light. Similarly, the shape recovery is caused by an asymmetric tensile force, resulting from the higher degree of Z-E reversion at the front surface.

To observe the humido-response properties of the H4abtc/PDAC film (Figure 4b, Figure S7 and Movie S2), the film was held parallel to water in a glass dish with one side clamped by tweezers. This film was almost flat at room temperature (25 °C) and 20% RH. Moving the film near the water surface caused obvious bending away from the water surface within 20 s. PDAC in the film adsorbs water and expands when exposed to highly humid environments.13, 21, 45 H4abtc/PDAC films adsorb water to ~15% of its original mass when the RH increases from 0% to 80% (calculated according to ref. 46). The absorbed water swells the H4abtc/PDAC film, producing a mechanical force that stretches the Z-isomer, resulting in the Z-E isomerization of the H4abtc in film (Figure S8, Supporting Information). Simultaneously, the bottom of the film (near the water surface) has a larger free volume than the top of the film. Hence, the synergetic effect of the mechanical force and free volume results in asymmetric 11 ACS Paragon Plus Environment


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Z-E isomerization in the film. The film shows obvious shape recovery when moved away from water (Figure 4b), relaxing to its original configuration in 46 s. When the environmental humidity decreases from 60% to 20% RH, water molecules are lost from the film, decreasing the free volume and generating an extrusion force on the E-isomers. Therefore, mechanical-force-induced E-Z isomerization on the bottom of the film leads to shape recovery. These results indicate the structural transformation of AZO in H4abtc/PDAC supramolecules can be controlled by changing the microstructure (free volume) and resultant mechanical force. The shape deformation response using heat as a stimulus was characterized by placing the film on a heating element at 50 °C. As shown in Figure 5a and Movie S3, both edges of the H4abtc/PDAC film bent upwards, forming a U-shaped film in 5 s. This shape deformation is much faster and larger than that induced by UV light. Different from provious reported temperature-responsive polymer systems based on phase transition, no phase transition was observed from the differential scanning calorimetry (DSC) measurement of pure PDAC film and H4abtc/PDAC film (Figure S9, Supporting Information). Moreover, control experiments revealed that freestanding PDAC films do not undergo bending movements with temperature changing, which indicates that H4abtc plays an important role in the thermally driven deformation. In previous studies,46-48, 54-56 isomerization of AZO was induced by light, heat, ultrasound, mechanical force, etc. Herein, the Z-E isomerization the H4abtc molecules can be induced by heat (Figure S10, Supporting Information). While a temperature increasing from 20 to 50 °C induces Z-E isomerization of H4abtc in 12 ACS Paragon Plus Environment

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H4abtc/PDAC film (Figure S11, Supporting Information), owing to the temperature gradient in the thickness direction of the film, the asymmetric Z-E isomerization rate between the lower and upper surfaces continuously generates a large gradient stretching force that decreases from the lower to the upper surface. Further, similar to the humido-response process, a larger free volume and a microscopic stretching mechanical force on AZO were generated in the surface of the film near the heating element. Therefore, compared with that in the upper surface, the H4abtc molecules in the lower surface show a larger degree of Z-E isomerization, resulting in a larger stretching force, which bends the film back from the heating element. Hence, the temperature response of H4abtc and the heat-expansion of PDAC synergistically contribute to the bending deformation of the film. More interestingly, over time, the U-shaped film moves like a tumbler and wags from side to side (Figure 5b and Movie S3). This is due to the imbalance between gravity and the bracing force from the heating platform, resulting in the formation of a moment of force.57 In detail, the contact point between the film and heating element changes continuously with the heat-induced shape deformation. Thus, the force balance of the film is inevitably broken by its asymmetry, allowing the film to rock to left. Simultaneously, a right-rotation resisting moment is produced, enabling the film to recover its initial position, and then rock to the right owing to the force of inertia. Similarly, a left -rotation resisting moment makes the film rock to the left. Hence, the heat-induced bending deformation of the H4abtc/PDAC film makes the film move like a tumbler. 13 ACS Paragon Plus Environment


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We quantified the deformation by bending angle during the light-, humido-, and thermoresponsive processes. In the light-responsive process, the bending angle increases with irradiation time, reaching a maximum angle of 60° at 16 s (Figure 6a). A decrease of the bending angle is then observed during the recovery process (38 s). The bending/unbending movement is completed in 54 s, and no decay time is observed. Similarly, the humido-responsive process shows a bending angle of 50° at 20 s and recovery of 46 s. Interestingly, the thermoresponsive process exhibits the shortest responsive time of 5 s and the largest bending angle of 70°, which shows an obvious advantage for the application in fast-responsive actuator. However, the shape recovery of the bent film is rather slow, with a time of 600 s. This feature may seriously hinder the use of the film in applications requiring quick reversion and repeatability. Considering the multiresponsiveness of the film, an easy method was found to obtain fast shape recovery. As shown in Figure 6b, the upper surface of the bent films was exposed to a gentle flow of humid nitrogen (0.15 m3 h-1), which increased the environmental humidity from 20% to 60% RH. Owing to the adsorption of water molecules, Z-E isomerization occurs in the upper surface owing to the synergetic effect of the mechanical force and free volume. As a result, the shape recovery time of the light-, humido-, and thermo-induced bent films decrease from 38, 46, and 600 s to 5, 12, and 9 s, respectively. This improvement is less significant for the humido-induced bent film. As UV irradiation enriched the Z-isomer content of the upper surface, enhanced Z-E isomerization on exposure to humid nitrogen flow 14 ACS Paragon Plus Environment

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contributes to fast unbending. In the thermoresponsive process, most water in the film was lost during heating, leading to a decrease of the free volume around AZO. Therefore, the thermo-induced bent film is more sensitive to the humid nitrogen flow, which can swell the upper side of the film, enlarging the free volume. Moreover, the bending/unbending motions are controllable, reversible, and repeatable, with the film maintaining its multiresponsiveness after 10 repetitions of this bending/unbending cycle (Figure S12, Supporting Information). The multiresponsive properties give this film great potential for application in various environments.

CONCLUSION

In summary, we constructed light-, humido-, and thermoresponsive supramolecular polymeric films based on H4abtc and PDAC using electrostatic interactions. Interestingly, the H4abtc/PDAC film undergoes bending/unbending movements when exposed to UV light, humidity, or heat. Unlike the deformation toward the light source caused by light-induced E-Z isomerization, the humido- and thermoresponsive bending

away

from

the

humidity

and

heat

sources

is

attributed

to

mechanical-force-induced Z-E isomerization, which largely depends on the free volume around H4abtc. Although a bent film reverted to its original flat shape in air, the recovery rate improved significantly on exposure to a gentle flow of humid nitrogen. The bending/unbending motions are controllable, reversible, and repeatable. The present study provides a facile and efficient approach to prepare novel MRP film materials for future application in smart actuators. 15 ACS Paragon Plus Environment


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ASSOCIATED CONTENT

Supporting Information

Figure S1: synthetic route of H4abtc. Table S1: the detailed FT-IR peaks and assignments. Figure S2: photographs of H4abtc and H4abtc /PDAC films in different solvents. Figure S3: UV-Vis absorption spectra of the H4abtc salt in DMF. Figure S4 and S5: time-evolved absorption spectra and the first-order rate constants of the H4abtc/PDAC solution. Figure S6 and S7: time-evolved photographs of the bending/unbending movement under light and humidity. Figure S8: UV-Vis absorption spectra of the H4abtc/PDAC film on quartz substrate under different humidity.

Figure S9: DSC thermograms of the PDAC film and H4abtc/PDAC film.

Figure S10 and S11: temperature-evolved absorption spectra of the H4abtc in DMF and H4abtc /PDAC film. Figure S12: light-, humido- and thermo-response cycling performance.

The

real-time

recording

of

the

light-,

humido-

and

thermo-responsiveness are shown in Supporting Movie S1-S3. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Tel: +86-22-87402059 Fax: +86-22-27404724

Notes The authors declare no competing financial interest. 16 ACS Paragon Plus Environment

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ACKNOWLEDGMENTS

This work was financially supported by the National Key Research and Development Program of China (NO. 2016YFA0202302), the State Key Program of National Natural Science Foundation of China (NO. 51633007), the National Natural Science Funds for Distinguished Young Scholars (51425306), and National Natural Science Foundation of China(NO. 51373116, 51573125 and 513111129), and Natural Science Foundation of Tianjin City (No. 14JCZDJC37900).

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Figure 1. (a) Reversible isomerization of H4abtc. (b) Schematic illustration of the preparation and structure of AZO/PDAC films.

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Figure 2. (a) FT-IR spectra of H4abtc, PDAC, and H4abtc/PDAC. (b) N1s XPS spectra of the H4abtc/PDAC film. (c) SEM image of a freestanding H4abtc/PDAC film on a Si substrate (inset: SEM image of a cross section of the film). (d) N- and (e) O-elemental mapping of the H4abtc/PDAC film.


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Figure 3. (a) UV-Vis absorption spectra of the PSSE and PSSZ of H4abtc in DMF and the H4abtc/PDAC film at room temperature (25 °C). (b, c) Time-evolved absorption spectra of the H4abtc/PDAC film (b) under irradiation at 365 nm and (c) in darkness after irradiation. (d) Determination of the first-order rate constants for E-Z (kp) and Z-E (krev) transitions of the H4abtc/PDAC film. (e) Fifty E-to-Z-to-E isomerization cycles of the H4abtc/PDAC film, as monitored at 354 nm after UV irradiation for 5 min and keeping in the dark for 60 min.



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Figure 4. Photographs of the bending and unbending movements of a freestanding H4abtc/PDAC film upon (a) UV irradiation (30 mW cm-2) and (b) humidity changes and shape recovery in air.


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Figure 5. (a) Photographs of thermo-induced bending and unbending movements of the H4abtc/PDAC film. (b) Top row: the tumbler motion of the H4abtc/PDAC film induced by heat; bottom row: schematic illustration of the heat-induced tumbler motion. G and N represent gravity and the bracing force.



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Figure 6. Time-dependent bending angles of the H4abtc/PDAC film with UV irradiation, humidity (∼40% RH), and heat (50 °C) treatment, and the shape recovery process (a) in air and (b) under a gentle flow of humid nitrogen (0.15 m3 h-1).


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Table of Contents Graphic


Polymer Supramolecular Assemblies

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