Hydrophilic Poly(vinylidene Fluoride) - ACS Publications - American

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Hydrophilic Poly(vinylidene Fluoride) Film with Enhanced Inner Channels for Both Water- and Ionic Liquid-Driven Ion-Exchange Polymer Metal Composite Actuators Dongjie Guo,*,† Yubing Han,† Jianjian Huang,† Erchao Meng,† Li Ma,† Hao Zhang,‡ and Yonghui Ding*,§ †

State Laboratory of Surface & Interface, Zhengzhou University of Light Industry, Zhengzhou 450002, China College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China § Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309, United States

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‡

S Supporting Information *

ABSTRACT: This study presents a novel and facile strategy to fabricate a hydrophilic poly(vinylidene fluoride) (PVDF) electrolyte film with enhanced inner channels for a highperformance and cost-effective ion-exchange polymer metal composite (IPMC) actuator. The resultant PVDF composite film is composed of hierarchical micro/nanoscale structures: well-defined polymer grains with a diameter of ∼20 μm and much finer particles with a diameter of ∼390 nm, producing three-dimensional interconnected, hierarchical inner channels to facilitate ion migration of IPMC. Interestingly, the electrolyte matrix film has a high porosity of 15.8% and yields a high water uptake of 44.2% and an ionic liquid (IL, [EMIm]·[BF4]) uptake of 38.1% to make both water-driven and IL-driven IPMC actuators because of the introduction of polar polyvinyl pyrrolidone. Compared to the conventional PVDF/IL-based IPMC, both water-driven and IL-driven PVDF-based IPMCs exhibit high ion migration rates, thus effectively improving the actuation frequency and producing remarkably higher levels of actuation force and displacement. Specifically, the force outputs are increased by 13.4 and 3.0 folds, and the displacement outputs are increased by 2.2 and 1.9 folds. Using an identical electrolyte matrix, water-driven IPMC exhibits stronger electromechanical performance, benefiting to make IPMC actuator with high levels of force and power outputs, whereas IL-driven IPMC exhibits a more stable electromechanical performance, benefiting to make long lifetime IPMC actuator in air. Thus, the resultant IPMCs are promising in the design of artificial muscles with tunable electromechanical performance for flexible actuators or displacement/vibration sensors at low cost. KEYWORDS: ionic exchange polymer metal composite (IPMC), electromechanical response, poly(vinylidene fluoride) (PVDF), polyvinyl pyrrolidone (PVP), ionic liquid (IL), inner channel

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toward the cathode direction.1,2 Therefore, fast movement of the hydronium ion is critical for the strong electromechanical response of an IPMC actuator. However, it is difficult for the pure Nafion IPMC to produce strong force and power outputs because the native inner channels with diameter less than 4 nm12 cannot support substantial ion migration with high speed. In addition, several other drawbacks of pure Nafion IPMCs also hamper its wide applications, such as high cost, short working lifetime, and evident back relaxation.13−16 The commonly used strategy for increasing the force output is increasing the size and stiffness of the electrolyte film.17−19

INTRODUCTION

Ion-exchange polymer metal composites (IPMCs) have been considered as promising candidates for robotic actuators, artificial muscle, and dynamic sensors1−4 because of their large bending displacements and fast electromechanical responses under a low voltage.5−9 To make conventional IPMC actuators, a polymer electrolyte film (i.e., Nafion) is sandwiched between two inert metal sheets used as working electrodes.10 The main chain of Nafion consisting of a linear hydrophobic structure crystallizes into a nonionic phase. The hydrophilic branch chains of sulfonic groups form an ionic phase with the attracted water and serve as the inner channel for migrating hydrated cations.11 When a voltage is applied, IPMC starts bending toward the anode direction because of the movement of the hydronium ion along the inner channel © XXXX American Chemical Society

Received: October 16, 2018 Accepted: December 24, 2018

A

DOI: 10.1021/acsami.8b18098 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the actuation mechanisms for water- and IL-driven IPMCs. (a) represents the composite film of PVDF/PVP/IL; (b) represents the PVDF/PVP composite film with enhanced inner channels by removing ILs and partial PVP from the PVDF/PVP/IL film; (c,e) are water- and IL-soaked PVDF/PVP films, respectively. Both (d) water-driven IPMC and (f) IL-driven IPMC bend toward the anode side, although the water-driven one shows larger deformation than the IL-driven one because of its higher ion conductivity.

poly(ether ether ketone)-doped PVDF composite films.32−37 Jho’s group grafted sulfonic groups and carboxyl groups on the PVDF side chain38,39 because these groups provide the free charge carrier, that is, the hydrated H+ ion, and thus increase the ionic conductivity of the composite film. The composite film exhibits higher water uptake (WU) and better IEC capability, benefiting the formation of the ionic phase. Moreover, these groups take electrostatic adsorption with the precursor of the Pt(0) electrode, [Pt(NH3)4]2+, during the process of ion exchange and thus favor the stable deposition of Pt nanosheet electrodes. Therefore, it is expected that the PVDF composite film containing more polar groups such as sulfonic groups would increase the number of hydrated cations and the size and density of the inner channels and subsequently further improve IPMC performance. The PVDF-based, water-driven IPMC actuators showed much larger actuation with quicker response than the Nafion-based IPMC actuator.32,33 Additionally, this kind of IPMC was actuated toward the anode side without “back relaxation,” as done by the Nafion-based IPMC. However, very limited strategies are available to increase the amount and rate of ion migration, which hinder the further improvement of IPMC performance. Herein, we introduce a novel and facile approach to significantly improve the actuation force and displacement outputs of IPMC by inducing a large amount of ion migration at a high speed (Figure 1). Specifically, a basic hydrophilic polymer of polyvinyl pyrrolidone (PVP) was introduced into the PVDF matrix to increase the chemical compatibility between PVDF and the IL of [EMIm]+[BF4]−. All IL of [EMIm]+[BF4]− and partial PVP can be removed when this film was soaked into water because of their solubility, thus forming a polar PVDF/PVP film with highly porous structures, which would serve as enhanced inner channels and support significant ion migration. Notably, the resultant composite film can adsorb both polar water and IL to fabricate either water- or IL-driven IPMC actuator by introducing PVP.

For example, Jho and Park’s group reported that the blocking force of IPMC reached 250 mN maximum and could lift 124 g coins by stacking 5−7 layers of Nafion films to get thick electrolyte films with 1−1.4 mm thickness;17,18 the IPMC actuator produced by Chen’s group showed a Newton output blocking force by three layers of composite electrolyte films with ∼7 mm thickness;19 our group and other groups made SiO2 particles or montmorillonite flakes-doped Nafion film to improve the stiffness of the electrolyte film, producing an enhanced force output.16,20−22 However, these IPMC actuators always exhibited relatively low displacement outputs. Poly(vinylidene fluoride) (PVDF) has emerged as a promising candidate to replace Nafion, considering its outstanding thermal stability, chemical resistance, and mechanical strength.23 However, the intrinsic hydrophobicity of PVDF makes it difficult to attract water. Moreover, PVDF does not show the ion-exchanging capability (IEC) because there are no ion-exchanging groups such as sulfonic groups and carboxyl groups on its side chain. Therefore, it is a great challenge for PVDF to be used as the water-driven IPMC actuator such as Nafion. Meanwhile, PVDF can absorb some organic salts, that is, ionic liquid (IL), providing an opportunity to make IL-driven IPMC actuators.24−29 By introducing IL into the PVDF matrix, the electrostatic interactions between the IL ions and the dipolar moments of PVDF chains will induce crystallization of PVDF chains into a nonionic, polar β-phase, whereas the IL-surrounded PVDF crystallines construct the continuous ionic phase, thus forming the inner channels. The cations and anions of the IL are redistributed by the electric field, leading to volumetric change and therefore bending motions due to steric repulsion and electrostatic effects between the charged ions.28,29 ILs composed of various cations, for example, methylimidazolium (EMIm), and anions, for example, trifluoromethane sulfonate (TfO), had been used for making PVDF-based IPMC actuators.24−31 Size effects derived from the volume difference of cations and anions were discussed in detail as well as their concentration and viscosity effects.31 Those papers showed that the IL-driven IPMCs present improved stabilities but slower responses compared to water-based IPMCs, resulting from slow migration of the highly viscous, large ILs in narrow inner channels.25 On the other hand, some hydrophilic groups such as sulfonic groups and carboxyl groups have been introduced into the PVDF matrix to mitigate its hydrophobicity.32−37 Park’s group and Oh’s group made polystyrene sulfonic acid and sulfonated

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EXPERIMENTAL SECTIONS

Preparation of PVDF Family Films. PVDF (2.50 g, MW 900 000 g/mol, Sigma-Adrich) powder was dissolved in 20.0 mL of N,Ndimethylformamide (DMF) at room temperature under magnetic stirring. The precursor mixture was poured in a home-made glass model with dimensions 100 × 30 × 2.0 mm3 and incubated at 70 °C for 6 h, yielding a pure PVDF film (film a) with a light brown color (Figure 2). PVDF/IL ([EMIm]·[BF4], Sigma-Aldrich) film (film b) B


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nanoindentation (FT-S, Switzerland), and the Young’s modulus was calculated by eq S2. Fabrication and Characterization of PVDF-Based IPMC. The conductive graphite (Gr, ∼2000 meshes, Alfa Aesar) pretreated by dopamine (Sigma-Aldrich) was dispersed into PVDF solution to make a Gr electrode slurry.41,42 The dry, porous film d was sandwiched by the electrode slurry with a Gr/PVDF mass ratio (WGr/WPVDF) of 140% to prepare PVDF/PVP-based IPMC. After curing at 70 °C for 6 h, the prepared IPMC was cut into strips (30 × 5 mm) and soaked in the LiCl solution (0.1 N) and pure IL of [EMIm]·[BF4] to make the hydrated lithium ion-driven IPMC (Li− IPMC) actuator and the IL-driven IPMC (IL−IPMC) actuator, respectively. As a control, film b was also sandwiched by the electrode slurry to make the PVDF/IL-based IPMC actuator. The above IPMCs underwent low-temperature cracking under liquid N2 for field emission scanning electron microscopy (SEM, LEO 1530 VP) observations. Electrochemical properties of IPMCs were detected by an electrochemical workstation (CH660e) (Supporting Information 3), and the ion conductivities were obtained from the electrochemical impedance spectra (EIS). Setup for IPMC Performance Measurement. A signal generator (SP1651, Nanjing) with a multifunction data collection card (NI, 6024E) and a signal amplifier (TI, OPA548) were used to generate a sinusoidal wave with varying voltages of 5.0−15.0 V and a certain frequency of 0.1−5 Hz for IPMC actuation. To facilitate the detections of force and displacement outputs, all IPMC actuators were actuated in air. A camera (Apple VIII) was used to capture the IPMC actuation images. A laser displacement sensor (Keyence LK3001A, Japan) with the sensitivity of 0.1 μm was adopted to collect the deformation data. The laser beam was focused on the cantilevered IPMC strip (30 × 5 mm2), and the detected maximum displacement was counted by half of the difference between the wave peak and bottom. The blocking force was detected by aligning the end of IPMC to the tip of a force sensor (CETR-UMT) with the sensitivity of 0.01 mN under the balance position.

Figure 2. Optical images of film PVDF (a), PVDF/IL (b), PVDF/ PVP/IL (c), PVDF/PVP (d), film d containing LiCl solution (e), and the PVDF/PVP film containing IL (f). was casted according to the mass ratio of PVDF/IL = 3:1. Because of incompatibility, the prepared composite film was nontransparent and showed a white color. The film of PVDF/PVP/IL (film c) was casted according to the mass ratio of PVDF/PVP/IL = 2:1:1. Notice: IL will leak from the matrix; IL-doped samples need to be used soon after they are made. Film c was immersed into hot water at 70 °C and stirred for 2 h to remove the IL and partial PVP, thus forming a porous film of PVDF/PVP (film d). Films a−d were annealed for 5 min at 120 °C. Films e and f were prepared by putting film d into LiCl solution (0.1 N) and IL of [EMIm]·[BF4] separately. It is worth noting that the PVDF concentration of 12% (w/w) was selected to keep the balance between flexibility and mechanical strength for better performance of resultant actuators. At the room temperature, the solubility of PVDF in DMF is in the range of 7−21%.40 On the basis of our experience, the resultant film with a PVDF concentration of 14% (w/w) was very stiff, which went ill with the bending of IPMC, whereas the resultant film with a PVDF concentration of 10% (w/w) was too soft to handle and peel off from the container. Composition and Mechanical Property Detections of PVDF Films. The chemical composition and polymer crystallization of the prepared PVDF family films were characterized by attenuated total reflection Fourier transform infrared spectroscopy with a Bruker IFS66/S spectrometer. The beta phase content was calculated by eq S1. X-ray diffraction (XRD) patterns were obtained by a Bruker D8ADVANCE X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) at an accelerating voltage and current of 40 kV and 100 mA, respectively. Mechanical properties of films a−f were determined by

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RESULTS AND DISCUSSION Chemical Components and Thermal Properties of PVDF Family Films. Figure 3a shows the infrared (IR) spectra of PVDF-related films. For the PVDF powder or film a,

Figure 3. Chemical component characterization. Absorbance mode IR spectra of (a) PVDF-related polymers and (b) the enlarged regions of IR spectra for illuminating the phase transformation, (c) evolutions of β-phase concentration, and (d) XRD patterns of the PVDF-related polymers. C


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ACS Applied Materials & Interfaces Table 1. Comparisons of IPMC-Related Parameters for Films a−f films

thickness/μm

PVDF ratioa /wt %

a b c d e f

211 286 253 251 323 312

100 75 50 77.2 53.5 55.8

PVP ratioa /wt %

25 22.8 8.9 9.7

IL or WU/% 33.3 33.3 44.2 38.1

β-phase concentration/%

melt point/°C

compressive modulusb /MPa

tensile strengthc /MPa

± ± ± ± ± ±

162 157 142 159

1120 447 263 525 61.3 87.5

32.6 16.3 15.2 19.4 4.1 6.2

45 67 76 83 74 71

1 7 10 12 8 8

porosity/% 0.22 5.33 3.68 15.8

a

Derived from the IR analyses by using OPUS software; around 51.2% PVP remained. bDerived from the compression test by a nanoindentor. Derived from the tensile test by an automated material testing system.

c

peaks of 3024, 2989, 2916, and 2849 cm−1 were derived from the asymmetric and symmetric C−H stretching vibrations, whereas peaks of 1400, 1166, and 870 cm−1 belonged to C−H bending, C−F stretching, and C−C backbone stretching vibrations, respectively. The peak of 840 cm−1 resulted from the vibration absorption of the β crystalline phase, whereas peaks of 766 and 615 cm−1 were derived from the vibration absorptions of the α crystalline phase.43 After the introduction of ILs, the characteristic peaks of ILs appeared in the spectrum of film b: the B−F stretching band was from 1050 to 1037 cm−1; the stretching vibration of C−H bond related to the imidazole ring was at 3122 and 3166 cm−1; and the C−H bending vibration was at 1573 cm−1. The introduction of PVP induced a strong absorbance of CO at 1669 cm−1 resulting from the lactam group of PVP.44 As expected, the spectrum of film c showed all characteristic absorbance bands from PVDF, PVP, and IL. After soaking into hot water to remove ILs and partial PVP, the spectrum of film d showed the characteristic absorbance peaks of PVDF and PVP. Compared to its precursor of film c, two evident changes were seen on the spectrum of film d: (1) the absorbance peaks derived from the B−F band and the imidazole ring completely disappeared, indicating the removal of IL; (2) the CO vibration peak of 1669 cm−1 still existed, but its intensity was significantly attenuated, which might be due to the removal of ILs and partial PVP. Majority of the ILs were removed because of their high solubility in water and weak interactions with PVDF. PVP was only partially removed because of their strong electrostatic interactions with the active hydrogen group on PVDF, so that the remaining PVP could make the composite film remain hydrophilic. Another evidence of the formation of the polar PVDF composite film comes from the crystalline phase transformation of PVDF. In general, PVDF can crystallize in several polymorphs depending on the processing conditions. The polar β-phase is the more interesting one for sensor and actuator applications because of its well-known piezoelectric response.45 Figure 3b,c shows the evolution of β-phase based on calculations using eq 1 by assuming that the sample contains α and β phases only. Clearly, after dissolving in the polar solvent of DMF, the concentration of β-phase greatly increased from 1.2% in the PVDF powder to 45.3% in film a (Table 1); for films b−d, the concentrations of β-phase were 66.5, 76.0, and 83.1%, respectively. These results demonstrated that the introduction of polar substances such as DMF, PVP, IL, and water into the PVDF matrix would induce crystallization of the polymer into the piezoelectric polar βphase from the nonpolar α-phase. The increase in the polarity of the PVDF film facilitates further absorption of some polar substances such as water and IL.

XRD patterns provided further evidence to the β-phase transformation (Figure 3d). Diffraction peaks at 2θ = 18.36, 19.91, and 26.67° were assigned to (020) and (110) of the αphase, whereas peaks at 2θ = 20.60 and 36.54° resulted from (110) and (200) crystal planes of the β-phase. The intensities of peaks from the α-phase decreased, whereas the intensities of peaks from the β-phase increased by the introduction of DMF, IL, and PVP into PVDF. Both the melting point and the decomposition temperature of the fluoropolymer backbone shifted to lower temperature (Supporting Information 4): the former shifted to 157, 142, and 159 °C for films b−d from 162 °C of the pure PVDF (film a) (Table 1); the latter shifted to 395.6, 395.6, and 371.4 °C for films b−d from 410.9 °C of the pure PVDF (film a) (Figure S1), suggesting the existence of strong electrostatic interactions among PVP, IL, and PVDF. Morphology and Hydrophilicity Characterization of PVDF Family Films. SEM observation showed a typical plastic fracture surface of a pure PVDF macromolecule, and no evident inner channels were observed (Figure 4a). Thus, film a

Figure 4. SEM cross-sectional images of films a−d. Images (a−d) correspond to films a−d, respectively. The inserted image shows their water CAs (Figure S5).

could not take much water to support IPMC actuation. For the PVDF/IL composite film (film b), the mixture of DMF, PVDF, and IL took apparent phase separation after the evaporation of DMF, according to Flory−Huggins theory.40 PVDF macromolecules crystallized into many irregular polymer grains (Figure 4b), and inner channels with a width of around 600 nm were formed in their free spaces for IL migration, supporting IPMC actuation. The white amorphous substance (indicated by the arrow) might be the aggregated ILs, D


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Figure 5. Mechanical properties of PVDF family films. (a) Stress−strain curves (the dash-dotted lines reveal the slopes of linear regions) of films a−f. (b) Young’s moduli of films a−f.

porosity, thus water or IL may easily enter into the film. According to eq S4, film e had a much higher WU of 44.2% than the pure Nafion of 19.6%;52 film f also exhibited a high IL uptake of 38.1% (Table 1). Notably, the electromechanical responses of the IPMC actuator are determined by the “granular damming effect,” which resulted from the ion migration blocked by the electrodes.53 Increased WU (or IL uptake) would provide more ions to migrate, whereas enhanced inner channels benefit numerous ions migrating at the same time under an electric field and thus might produce strong force and power outputs. In addition, water-driven IPMC actuators always experience water shortage due to significant evaporation and electrolysis when the hydrated cations move fast between the two electrodes.13 Thus, high WU is important to ensure their actuation stability. Mechanical Property of PVDF Family Films. PVDF is thought as a semicrystalline polymer,23 where the crystalline phase provides mechanical strength and impact resistance to support the generation of the strong blocking force, whereas the amorphous phase offers flexibility and benefits the generation of large deformation of IPMC. Figure 5a shows the stress−strain curves for PVDF family films. All films exhibited a linear elastic region followed by a nonlinear deformation. According to eq S2, the Young’s moduli were calculated and are reported in Figure 5b and Table 1. The pure PVDF film (film a) showed the highest compressive Young’s modulus of 1.12 GPa and the highest tensile strength of 32.6 MPa, indicating the strongest mechanical strength among these films. Introduction of PVP or IL into the PVDF matrix reduced the crystalline regularity and then decreased the mechanical strength. The Young’s moduli of film b and film c substantially decreased by 60 and 77% in comparison to film a, which resulted from the plasticizing effects of IL and weak interfaces between PVDF and IL.54 The Young’s modulus of film d increased to a higher value of 525 MPa because of the removal of ILs and good chemical compatibility between PVP and PVDF; but its Young’s modulus was still lower than that of the pure PVDF film because of the formation of porous structures. Because the PVP component belongs to the hydrophilic polymer, the porous film d could absorb a large amount of polar substance such as water or IL. Both films e and f exhibited significantly decreased Young’s moduli of 61.3 and 87.5 MPa, respectively, which were similar to that of the commercial Nafion film.55 Compared to the precursor of film d, film e and film f have much higher flexibility and might produce larger deformation under an electric field. The tensile strengths for films a−f were 32.6, 16.3, 15.2, 19.4, 4.1, and 6.2 MPa, respectively (Figures S6, S7, and Table S1), which were consistent with the compressive moduli. In

indicating that IL of [EMIm]·[BF4] is not compatible with PVDF. To tune the hydrophilicity of PVDF, PVP was introduced into the polymer. PVDF and PVP cocrystallized and formed the composite grain backbone of film c because of strong electrostatic interactions between the lactam groups on PVP and the active hydrogen groups on PVDF.46 Figures 4c and S2a show irregular macroscale polymer grains and some flake structures appearing at the grain boundary, which might result from the introduction of PVP and IL. The hydrophilic IL of [EMIm]·[BF4], which is located among PVDF/PVP grains through a weak electrostatic interaction, would be removed after immersing into water. The regular well-defined polymer grains with a diameter around 20 μm were observed for film d (Figures 4d and S2b), which resulted from the recrystallization of PVDF/PVP composites. A number of three-dimensional (3D) interconnected inner channels with a size of ∼2.2 μm were formed among those polymer grains. Further, the formation of 3D porous structure was quantified by mercury intrusion porosimetry (Micromeritics Instrument Auto Pore IV 9500), and their pore size distribution curves are shown in Figure S4. Clearly, the detected pore radius and porosities were greatly increased with the additions and then removals of PVP and IL in film d compared to films a−c. The beneficial effects of porous structures on the electromechanical response of actuators were also observed in this study, as discussed in the later sections. The IL concentration plays important roles in the phase content, polymer microstructure, and electromechanical response.31,47−51 Kwon and Ng31 demonstrated that the actuator displacement increased with decreasing IL content in the polymer gel electrolyte, and maximum strain was observed from an actuator with an IL content of ∼23%. They explained that lowering the IL concentration led to a more compact ion distribution in the electrode layers and then the increased strain in the actuators. The initial IL content of 25% was adopted in our study to get the desired electromechanical response while avoiding ILs leaking from the matrix film. To further study the influence of IL content on the polymer microstructure, we increased the IL concentration from 25 to 33 and then 50%. After immersing into the hot water, the morphology of PVDF/PVP grains became more irregular, their porosities were also increased, and more defects appeared among the grains with the increase of the IL content (Figure S3). The evolution of water contact angle (CA) (Figure 4) showed that pure PVDF is highly hydrophobic with a CA of 112.6°, whereas films b−d become more hydrophilic because of the addition of PVP and IL. Additionally, film d has a high E


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diameter around 390 nm, creating numerous nanoscale inner channels among those nanoscale particles (Figure 6d). Notably, the width of the nanoscale inner channel (around 23 nm) was much larger than the intrinsic inner channels in the pure Nafion film.12 Therefore, such hierarchical structures provide interconnected 3D micro/nanoscale inner channels to promote ion migration during IPMC actuation. Electrochemical Performance Analyses of IPMCs. Electrolyte ion migration drives the electromechanical response of IPMC actuators,58−61 and thus electrochemical behaviors would affect their electromechanical performance. To analyze the electrochemical behaviors, the EIS spectra of PVDF family IPMCs in water and IL environments were measured (Figure 7). At IL environment (Figure 7a,c), PVDF/ IL-based IPMC (control) and IL−IPMC exhibited similar profiles: a rather flattened semicircular curve at a highfrequency range and a regular Warburg line at a low frequency appeared with increasing time. Both EIS curves exhibited nearly constant curves, and no evident change was observed within 180 min. Such high stability resulted from the nonvolatility and electrochemical stability of ILs,62 suggesting that IL-driven IPMC actuator has a long lifetime. Compared to the PVDF/IL-based IPMC, the IL−IPMC had higher capability for ion diffusion because of strong interactions between PVP and IL and highly porous structures; thus, the slope of the Warburg line was evidently higher than the control. It is worth mentioning that the IL of [EMIm]·[BF4] was adopted as the electrolyte in this study because its anion has a smaller volume with a van der Waals volume of 49 Å3, which is smaller than those of other common anions, such as TfO of 80 Å3 and ClO4− of 54 Å3,63 thus producing a high ion migration rate and larger electromechanical deformation.7,64 Li−IPMC exhibited a typical Nyquist diagram (Figure 7b), that is, a regular semicircular curve at a high frequency followed by a regular Warburg line at a low frequency, suggesting effective Li+ ion migration. By contrast, the EIS of film a showed a semicircular curve at a high frequency range with no evident Warburg line (Figure S11), indicating that there was no effective ion migration, and therefore, this film could not be used as the electrolyte film for IPMC.59−61 Such a huge difference would result from the fact that pure PVDF was extremely hydrophobic, whereas the PVDF/PVP film became hydrophilic because of the addition of polar PVP. The diameter of the semicircular curve of Li−IPMC increased with time and the slope of the Warburg line decreased. These results indicated that the migration rate of hydrated lithium ions was reduced and the electrochemical resistances were increased because of the loss of water. In general, two intercepts of the semicircular curve at x-axis are related to their Ohm and electrochemical resistances, whereas the slope of the Warburg line represents their capability of ion migration.59 As shown in Figure 7a−c and Table 2, Li−IPMC exhibited the lowest Ohm and electrochemical resistances and the highest sheet resistance. The sheet resistance reflects the resistance of electrode surfaces, whereas the Ohm resistance reflects the electronic and ionic transmissions inside the electrode. Therefore, the lower sheet resistances of the control and IL−IPMC resulted from the presence of ionic transmission at the surfaces. The low Ohm resistance of Li−IPMC resulted from the fast ion exchange and migration inside the electrode because of their hydrophilicity and highly porous structure.

addition, a nonlinear region was clearly observed following the linear region by showing a self-reinforcing behavior, that is, the modulus of the elastomer rises dramatically with increasing strain.56 Such nonlinearity might be due to a combination of the polymer chain uncoiling and bond stretching.57 It is worth noting that the deformation of IPMCs is usually in the nonlinear region; thus, larger deformation always leads to stronger stress and power outputs. Morphological Characterization of PVDF-Based IPMC. To obtain a flexible, conductive Gr electrode, we made a series of Gr-doped PVDF electrode slurries with varying Gr concentrations and then brushed them on the electrolyte film surfaces. Detections of four-point probe measuring system (FT-300D) revealed that the Gr/PVDF film with a WGr/WPVDF ratio of 140% exhibited a suitable conductivity of 109 Ω (Figures S8−S10). It is worth mentioning that the electrodes are usually stiffer than the electrolyte matrix film and would hamper the swing of the electrolyte matrix film and reduce the electromechanical response. Improving the flexibility and decreasing the thickness of the electrodes are two commonly used methods to reduce the resistance from the electrodes. Therefore, the electrode slurry of 140% was used to make the relative flexible electrodes with a thickness of only ∼16 μm because higher contents of Gr flakes result in stiffer electrodes.57 The typical sandwiched structure of the PVDF/PVP-based IPMC actuator was observed by SEM (Figure 6a−d). The

Figure 6. SEM observation of cross-sectional images of PVDF/PVP (film d)-based IPMC. (a) Profile of the sandwiched structure of IPMC; (b) interface between the electrode and matrix; PVDF/PVP electrolyte film with 3000 (c) and 20 000 (d) times magnifications.

total thickness of the matrix layer and electrode sheets was around 313 μm. The matrix layer with a thickness of ∼280 μm was composed of densely packed microscale polymer grains with an average diameter of 20 μm, and the electrode sheets with a thickness of ∼16 μm was composed of many overlapped Gr flakes wrapped by the PVDF adhesive. A close observation of the matrix−electrode interface revealed strong integrations between the Gr-doped PVDF electrode and the PVDF/PVP matrix (Figure 6b), which is critical to prevent the potential separation in actuation and ensure the stable electromechanical response.10 Interestingly, the observation of matrix fine structures revealed that each individual polymer grain consisted of closely packed nanoscale particles with the F


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Figure 7. Electrochemical analyses of PVDF family IPMCs. Evolutions of EIS curves with time for (a) the PVDF/IL-based IPMC as the control, (b) Li−IPMC, and (c) IL−IPMC; (d) evolutions of their ion conductivities within 180 min.

Table 2. Comparison of Conductive Properties for Water- or IL-Driven IPMCs IPMCs

thickness/μm

electrolyte

sheet resistance/Ω

Ohmic resistance/kΩ

electrochemical resistance/kΩ

area/cm2

conductivity/mS/cm

control Li−IPMC IL−IPMC

338 343 334

IL Li+/H2O IL

23.03 88.62 38.86

1.28 0.47 0.66

2.89 1.68 2.35

0.16 0.16 0.16

0.80 1.54 1.10

Ion conductivities for PVDF family IPMCs were counted according to eq S3, and their evolutions with time are shown in Figure 7d and Table 2. The detected ion conductivities of Li− IPMC and IL−IPMC were higher than those of the control. The detected ion conductivities of the PVDF family IPMCs were much lower than that of the Nafion film of 70−180 mS/ cm or of the sulfonated fluorine blocked polysulfone film of 20.59 mS/cm, as reported previously34,65,66 because the PVDF electrolyte film did not contain any ion-exchanging groups, that is, the sulfonic group, which can support the hydrated cations (i.e., H+) migrating from one sulfonic end to another end driven by the electric field. In addition, the ion conductivity of Li−IPMC continually decreased from 1.54 to 0.06 mS/cm within 180 min because of water loss, which is also the reason why most water-driven IPMCs can only maintain stable actuation for several minutes in air.67 However, ion conductivities of the control and IL− IPMC did not change and kept stable at 0.80 and 1.10 mS/cm, respectively. Collectively, compared to conventional PVDF/ILbased IPMC, the PVDF/PVP-based IPMC exhibited higher levels of ion migration because of enhanced inner channels in the water environment or IL environment. Water-driven IPMC exhibited higher levels of ion migration, whereas IL-driven IPMC exhibited more stable performance. Investigations of Possible Electromechanical Response of IPMCs. Actuations of IPMC actuators were captured and are shown in Figure 8. Under ac field, the control and IL−IPMC generated continuous electromechanical bending with maximum swing angles of ±36.5 and ±75.4°, respectively. For the control and IL−IPMC, [EMIm]+ cations and [BF4]− anions were redistributed under the electric field, that is, anions of [BF4]− moved to the anode because of its

Figure 8. Actuation images of PVDF/IL-based IPMC (control, top), Li−IPMC (middle), and IL−IPMC (bottom) actuated by a sinusoidal wave with a frequency of 0.1 Hz under an ac voltage of 15.0 V.

electrostatic interactions with the positive side of the PVDF dipolar moments,68,69 whereas the cations of [EMIm]+ moved to the cathode. Given the much larger volume of [EMIm]+ than [BF4]−, the cathode swelled while the anode shrunk, resulting in bending toward the anode. The highly porous structures of IL−IPMC provide enhanced inner channels and then induces greater amount and faster ion migration than the control, resulting in larger bending angle of IL−IPMC. For water-driven IPMC actuators, we adopted LiCl water solution as the electrolyte instead of pure water. Because the volume of hydrated Li+ ion is larger than the hydrated H+ ion, G


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Figure 9. Electromechanical properties of water- and IL-driven IPMCs. Tip displacement curves as a function of time under (a) a dc voltage of 9.0 V and (c) an ac voltage of 10.0 V; (b) typical displacement curves and (e) typical force curves under an ac voltage of 10.0 V; (d) displacement evolution in the frequency range of 0.1−5 Hz under an ac voltage of 10.0 V; (f) force curves as a function of time under an ac voltage of 10.0 V. For comparison, ac signals mostly adopted a sinusoid wave of 0.1 Hz. The mean actuation displacement was calculated from half of the difference of peak displacement and bottom displacement in one circle. The blocking force was collected at the balanced site and equaled the difference between the peak force and the bottom force in one circle.

Table 3. Comparisons of the Electromechanical Outputs for Water- or IL-Driven IPMCs dc of 9.0 V

ac of 10.0 V, 0.1 Hz

ac of 15.0 V, 0.1 Hz

actuators

steady swing angle/deg

steady displacement/mm

swing angle/deg

displacement/mm

force/mN

swing angle/deg

force/mN

control Li−IPMC IL−IPMC

17.4 31.3 22.2

6.2 9.6 7.3

16.4 46.3 32.1

5.7 12.6 11.1

2.48 33.33 7.62

36.5 81.1 75.4

4.21 48.56 11.54

Li+ ion-driven IPMC would generate a larger output force than H+ ion-driven IPMC.70,71 Actuated by ac field, hydrated lithium ions migrated from the anode to the cathode under the electric field, resulting in a water concentration gradient and subsequently a strong bending of IPMCs toward the anode.1,2 As shown in Figure 8, Li−IPMC exhibited much larger electromechanical deformation with a maximum swing angle of ±81.1° than the IL-driven IPMCs, which might result from two facts: (1) softer matrix film of PVDF/PVP and (2) much faster migration of hydrated Li+ ions with lower viscosity and smaller size than [EMIm]+ or [BF4]−. Overall, the data suggested that improving the hydrophilicity and increasing the porosity of the electrolyte matrix are effective strategies to enhance IPMC electromechanical deformation.

Electromechanical Performance Analyses of IPMCs. Displacement output is an important parameter for assessing IPMC electromechanical response.13,14 The “back-relaxation,” that is, moving back in the opposite direction (cathode) under a dc voltage, is always a problem associated with Nafion IPMCs,32,33 which is caused by the diffusion of water molecules to the anode side. Figure 9a and Table 3 recorded the tip displacement curves driven by a dc voltage of 9.0 V. Li− IPMC actuator was immediately actuated toward the anode side and reached a steady displacement of 9.6 mm within 10 s after applying the electric field. IL-driven IPMCs needed a longer time to reach the steady displacements of 6.2 and 7.3 mm for the control actuator and IL−IPMC actuator, respectively. Clearly, our PVDF family IPMCs here exhibited H


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the water loss. By contrast, the IL-driven IPMCs maintained stable electromechanical responses for a long time, produced the desired displacement, and showed smaller force output. Compared to the conventional PVDF/IL-based IPMC, the PVDF/PVP-based IPMCs, including Li−IPMC and IL−IPMC reported here, presented significantly enhanced force and displacement outputs because of the presence of the polar electrolyte film and the enhanced inner channels. Specifically, the displacement outputs increased by 2.2 and 1.9 folds, and the force outputs increased by 13.4 and 3.0 folds under 10.0 V. Notably, the detected electromechanical properties were wellaligned with the electrochemical detections, suggesting that the ion conductivities of water- or IL-driven IPMCs could be used as a bench mark for evaluating their electromechanical performance.

a very stable electromechanical response without backrelaxation, which was superior to Nafion IPMC. Typical displacement curves and quantification data are recorded in Figure 9b,c and Table 3. At the beginning, the displacement gradually swings to a maximum, which might be explained by the fact that all inner channels are not completely interconnected and the hydrated Li ions (or [EMIm]+ and [BF4]− of IL) migrate and gradually open the ion channels driven by the electric field. The maximum displacement of Li− IPMC was maintained at a maximum level for 6 min and then dropped quickly because of the loss of water. By contrast, the displacement of IL-driven IPMCs remains constant in the studied time course, that is, more than 10 min after reaching a plateau. The stability and reliability were generated from the high porous electrolyte matrix and the strong chemical affinity between the electrode and the elastomer matrix. Another cause came from the nonvolatility and electrochemical stability of ILs. Under actuation of 10.0 V, 0.1 Hz electric signal, the maximum swing angles were 16.4, 46.3, and 32.1° for the control, Li−IPMC, and IL−IPMC, respectively. The results showed that the electromechanical response of water-driven IPMC was stronger that of IL-driven IPMC because of the faster migration of hydrated Li+ ions with lower viscosity.67 The electromechanical response of IPMCs from film d was stronger that from film b (control) because of the enhanced inner channels. Additional results show a clear correlation between the swing angle and the driving voltage (Table 3). When the driving voltage increased to 15.0 V, the swing angles increased to 36.5, 81.1, and 75.4°, respectively, for the control, Li−IPMC, and IL−IPMC, demonstrating that PVDF/PVPbased IPMCs can be easily controlled by the applied voltage. The electromechanical performance under various driving frequencies showed that the actuation displacement of ILdriven IPMCs continually decreased with the increase of frequency (Figure 9d). The attenuation of displacement resulted from the hysteresis of ion migration responding to the change of electrical field, so that the deformation amplitude was always reduced at high frequency;14 but the electromechanical response of water-driven IPMC was slightly enhanced by increasing the driving frequency from 0.1 to 0.5 Hz and then attenuated when the driving frequency was higher than 0.5 Hz. These evolution trends should relate to the WU, the migration rate of hydrate Li+ ions, and the density/size of the inner channels. The enhanced displacement outputs of PVDF/PVP-based IPMCs in the frequency of 0.1−5 Hz demonstrated that increasing the density/size of inner channels is a useful strategy to improve the actuation frequency of the IPMC. Blocking force is another important parameter to evaluate the output power of IPMC. Typical force curves and quantification data are reported in Figure 9e,f and Table 3. Li−IPMC exhibited the largest force output under 10.0 V; its value was 33.3 mN, which was 4.37 folds that of IL−IPMC and 13.4 folds that of the control. The huge difference might come from the fast migration of the hydrated Li+ ions in the enhanced channels, which provided stronger power to generate larger “granular damming effect.” Similar to displacement data, the detected force output increased with the increase of the driving voltage. Collectively, it was found that the water-driven IPMCs showed a strong displacement and blocking force because of the high levels of WU, enhanced inner channels, and fast migrations of hydrated Li+ ions, but they will go recession with

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CONCLUSIONS We reported a polar electrolyte film of PVDF/PVP with enhanced inner channels by using an IL of [EMIm][BF4] as the template. The phase transformation of PVDF from the nonpolar α-phase to the polar β-phase and 3D hierarchical, porous structures contributed to enhanced inner channels and ion migration rate. A PVDF/PVP-based IPMC was fabricated by coating Gr/PVDF flexible electrodes on PVDF/PVP films. Interestingly, this IPMC could be used to adsorb LiCl solution or IL of [EMIm]·[BF4] to make a water-driven or IL-driven IPMC actuator. The investigation of the electrochemical and electromechanical properties generated from the water- and IL-driven IPMC actuators using an identical IPMC led to the following findings: (1) both water- and IL-driven IPMCs exhibited a steady electromechanical response without “backrelaxation,” which is superior to conventional Nafion IPMC; (2) the PVDF/PVP-based IPMCs including Li−IPMC and IL−IPMC produced much higher levels of force and displacement outputs compared to the conventional PVDF/ IL-based IPMC because of the presence of the polar electrolyte film and the enhanced inner channels; (3) the increase of the inner channel’s size and density can effectively improve the actuation frequency of IPMC; (4) by using the same electrolyte PVDF/PVP matrix, water-driven IPMC exhibited stronger electromechanical behavior, whereas IL-driven IPMC exhibited more stable electromechanical behavior; (5) electrochemical properties were well-aligned with their electromechanical performance in both water and IL environments, suggesting that ion conductivity was a useful bench mark to evaluate the electromechanical properties of IPMC. Thus, the resultant water- and IL-driven IPMCs can be used to design artificial muscles with tunable electromechanical performance for flexible actuators or displacement/vibration sensors.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b18098. Equations of counting beta phase content, compression Young’s modulus, ion conductivity, and WU or IL uptake; thermal properties and water contact results of PVDF family films; morphological characterization of film c and film d; structure comparison of film d with diverse IL contents; tensile test and porosity test of PVDF family films; preparation, morphological characterization, and electric conductive properties of Gr/ I


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PVDF electrodes; and electrochemical properties of PVDF IPMC (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.G.). *E-mail: [email protected] (Y.D.). ORCID

Dongjie Guo: 0000-0002-9426-7544 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Yangtze River Scholar Innovation Team Development Plan (IRT1187), the National Natural Science Foundation in China (21471046 and U1704149), the Henan Province University Science and Technology Innovation Talent (16HASTIT048), and the Natural Science Foundation of Jiangsu Province (grants BK20151473).

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