Ultra high permittivity and significantly enhanced electric field induced strain in PEDOT:PSS–RGO@PU intelligent shape-changing electro-active polymers

Abstract

Large deformation of soft materials is harnessed to provide functions in the nascent field of soft machines. In this work, PEDOT:PSS noncovalent functionalized graphene–polyurethane dielectric elastomer composites (PEDOT:PSS–RGO@PU) have been synthesized as promising candidate materials for micro-actuator electromechanical applications. PEDOT:PSS conducting polymer chains not only reinforce the interaction between the polyurethane matrix and graphene sheets, but also prevent graphene sheets from aggregating and connecting, which are beneficial to forming microcapacitors in the matrix and suppressing the leakage current. The PEDOT:PSS–RGO@PU composite exhibits ultra high permittivity (350 at 1 kHz), low dielectric loss (∼0.2 at 1 kHz), low loss modulus (200 MPa), and low loss tangent (∼0.4), all being essential to create a high performance electric-induced strain material. The maximum thickness strain of 164% is significantly higher than reported values for polyurethane elastomers and nanocomposites.

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Introduction

An exciting field of engineering is emerging that uses soft active materials to create soft machines.^1,2 A soft machine uses the large deformation of soft materials to assist humans,^3,4 operate robots,^5,6 monitor living tissues,^7,8 sense environment,^9,10 shape light,¹¹ and harvest energy.¹² Dielectric elastomers comprise one class of soft active materials, macromolecules that respond to external electrical stimulation by displaying a significant change in shape or size and thus converting electrical energy into mechanical force and movement.^5,6,13 Typical dielectric elastomers include acrylic,^5,14 silicone,^5,15,16 and polyurethane.^17–20 Dielectric elastomers and their composites with improved electromechanical properties have been developed over the past decade. Nevertheless, some issues still persist. It remains a grand challenge to increase the dielectric constant of elastomer films to meet the needs in electromechanical devices. One strategy to increase the dielectric constant of elastomer films is to use electrically conductive fillers (e.g. carbon-based nanostructures).^21–29

Although previously reported composite films with one-dimensional (1-D) carbon nanotubes (CNTs) filler had a high dielectric constant, these films might not be suitable for electronic device applications because of their high dielectric loss.^30,31 For the two-dimensional (2-D) reduced graphene oxide (RGO) platelets, polymer composite films with partial chlorination of the RGO filler yielded an enhanced dielectric constant with a low dielectric loss due to the polar and polarizable C–Cl bonds.²³ The RGO platelets are perhaps a better ideal conductive filler material compared to CNTs because RGO platelets can be chemically functionalized by a simple method and can be easily dispersed in organic solvents.^23,32 Surface functionalization of graphene provides a promising way to improve dispersion and interfacial interaction between graphene and polymer matrix. Two methods to improve the dispersion of graphene have previously been reported, namely covalent and non-covalent functionalization.^33,34 Generally covalent functionalization of graphene tends to disrupt the sp²-hybridized network required for good electron/hole conduction, thus compromising the electrical conductivity. Noncovalent functionalization of graphene sheets through CH–π and/or π–π interactions are the preferred choice for tuning the interfacial properties without compromising conductivity.³⁵ Conducting polymers are promising materials in noncovalent functionalization of graphene.

In the study described herein, we report a simple approach to a poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS) conducting polymer noncovalent functionalized graphene nanocomposite (PEDOT:PSS–RGO) by using evaporation induced self-assembly (EISA) method. Moreover, the synthesized PEDOT:PSS–RGO nanocomposite was used as conductive filler to increase the dielectric performance of the polyurethane (PU) dielectric elastomer and the electromechanical actuation performance of the resulting PEDOT:PSS–RGO@PU diaphragm type actuator unit. After introducing PEDOT:PSS–RGO into the PU dielectric elastomer, PU composite with high dielectric constant and low dielectric loss is expected to be obtained because of the formation of a large number of micro-capacitors (high dielectric constant) and the coating of the functional groups (low loss). The flexible coating of PEDOT:PSS conducting polymer chains onto graphene sheets reinforces the interaction between the PU matrix and graphene sheets, thus it greatly reduces the loss modulus of the corresponding PU hybrids. According to the well-known Maxwell equation as previously reported, to obtain a DE with high actuated strain at a low electric field, a high electromechanical sensitivity (b) is required, which is defined as the ratio of the dielectric constant (k) to the elastic modulus (Y) (b = k/Y).⁵ Therefore, the superior electric-induced strain performance of PEDOT:PSS–RGO@PU is expected here.

Experimental

Reduced graphene oxide (RGO)

Graphene oxide (GO) was prepared from natural graphite via the modified Hummers method.³⁶ RGO was prepared by reduction of GO with hydrazine hydrate.

PEDOT:PSS–RGO

PEDOT:PSS–RGO was prepared by a simple evaporation induced self-assembly (EISA) method. In a typical experiment, reduced graphene oxide (50 mg) was dispersed in deionized water (10 ml) by ultrasonic treatment, and then the PEDOT:PSS aqueous solution (2.5 ml, Alfa Aesar) was added into the graphene mixture under magnetic stirring and then ultrasonically stirred for at least 4 h. The weight ratio of PEDOT:PSS/graphene was about 1 : 1. The mixed solution was then poured onto a Petri dish and dried at 60 °C in air for 24 h. The PEDOT:PSS–RGO was prepared by evaporation induced self-assembly of PEDOT:PSS conducting polymers onto the surface of graphene.

PEDOT:PSS–RGO@PU and RGO@PU films

PEDOT:PSS–RGO@PU and RGO@PU films were prepared using solution cast method as described elsewhere.³⁷ In a typical experiment, PU (a polyether-type thermoplastic polyurethane elastomer TPU58887, Estane) granules were completely dissolved in DMF at around 75 °C. Subsequently, Fillers were added to the DMF solution under magnetic stirring and then ultrasonically stirred for at least 4 h. The weight percentage of functionalized graphene was limited to below 3.0%. The mixed solution was put into refrigerator for degassing. Then the mixed solution was then poured onto a Petri dish and dried at 60 °C in air for 12 h, then in vacuum at 60 °C for 12 h to remove DMF. The films, were removed from the Petri dish using a little of alcohol, were dried again at 130 °C for 4 h under air. Samples without any additives, denoted pure PU, were also prepared for comparison. The film thicknesses varied between 80 and 120 μm.

Characterization

XRD patterns were recorded by a Bruker D8 Advance diffractometer. FT-IR spectra were recorded with the sample/KBr pressed pellets using a Bruker Vector-22 FT-IR spectrometer. Raman spectra were obtained with a French JY HR800 microscopic Raman system. SEM images were taken on a JSM-5610LV scanning electron microscope. TEM images were taken on a FEI Tecnai G2 transmission electron microscopy. AFM images were taken on a Shimadzu SPM-9500J3 atomic force microscope with the non-contact mode. Dielectric properties were measured by a HP4294A impedance analyzer. The samples were prepared by depositing gold films on both sides of the samples. TG curves were recorded by a Diamond TG/DTA thermogravimetric analysis in N₂ at a heating rate of 10 °C min⁻¹. DMA curves were measured by using a TA Model Q-800 dynamic mechanical analyzer. Samples are heated from −150 to 200 °C at the heating rate of 3 °C min⁻¹ and at a constant frequency of 1 Hz with a static force of 0.02 N.

For electric field induced strain measurement, the films were cut into discs with a diameter of 25 mm. 6 mm Pt electrodes (diameter) with 1 mm connection bands (width) were sputtered anti symmetrically on each side of the sample. We designed a diaphragm device to fix the film sample and connect the wire. The diaphragm device, presented in Fig. 1, was a sample holder. It was fabricated in 5 mm-thick PMMA sheets. A 5.5 mm diameter hole was drilled into the lower part of the device and an 18 mm hole drilled into the upper part. The 5.5 mm hole constituted the diaphragm. Two copper wire conductors were respectively fixed onto each part of the device between the hole and the edge of the device and acted as electrical connections. The sputtered electrodes were then aligned with the 5.5 mm hole and the copper wire on the lower part of sample holder. The sample adhered strongly onto the sample holder by electrostatic forces only. The upper part of the sample holder was then aligned in order to make the connection from the sample to the holder. The deflection at the diaphragm centre is measured at room temperature using the MTI Fotonic Sensor (MTI 2100). A micro mirror was placed on the upper electrode to reflect the light beam to determine the displacement. The chosen electrical voltage was applied to the device using a function generator (The Institute of Fluid Science in Tohoku University L-013-442) and amplified 1000 fold by a high-voltage amplifier (TREK 10/10B-HS) at 1 Hz. Fig. 1 also shows the experiments set-up for strain measurements. The “thickness strain” is defined as S = d/t, where d is the deflection thickness displacement measured at the centre of the diaphragm and t is the sample thickness.

Fig. 1 Schematic view of the device comprising the sample holder and electroded sample; schematic view of the measurement structure.

Results and discussion

Fig. 2a shows a schematic diagram of the fabrication process of the PEDOT:PSS–RGO hybrid. Fig. 2b shows the chemical structure of PEDOT:PSS. As shown in Fig. 2, scanning electron microscope (SEM) and transmission electric microscope (TEM) were performed to interrogate the microstructure of PEDOT:PSS–RGO. After the PEDOT:PSS was diffused into the stacked graphene, the PEDOT:PSS and the surface of the graphene sheets have π–π interactions with each other. TEM view confirms the formation of a thin layer of PEDOT:PSS in Fig. 1c. The π–π interaction of graphene and PEDOT:PSS forms a tightly coated layer on the graphene surface. Wrinkles appear on the thin film surface due to the processes of solvent evaporation and polymer matrix distortion, which, as shown in Fig. 1d, are attributed to the reaggregation of polymers. Under the polymer film are the graphene bundles and lamellas which consisted of aggregates of graphene.

Fig. 2 (a) Schematic diagram of the fabrication process of the PEDOT:PSS–RGO hybrid, (b) chemical structure of PEDOT:PSS, (c) TEM image of PEDOT:PSS–RGO, and (d) SEM image of PEDOT:PSS–RGO.

The chemical structures of PEDOT:PSS, graphene oxide (GO), reduced graphene oxide (RGO), and PEDOT:PSS–RGO were characterized and confirmed by Fourier transform infrared spectra (FT-IR) and Raman spectra as shown in Fig. 3 and 4. It can be seen from the FTIR spectrum of PEDOT–PSS that the band at 892 cm⁻¹ ascribed to the bending mode of C–H in EDOT monomer disappears after the polymerization, indicating the formation of PEDOT molecular chains with α–α′ coupling (Fig. 3). And the vibrations at 1513 and 1326 cm⁻¹ are corresponding to the C=C and C–C stretching in the thiophene ring. Bands at 837 and 930 cm⁻¹ could be ascribed to vibration modes of C–S bond in the thiophene ring. Peaks at 1145 and 1056 cm⁻¹ are assigned to the stretching modes of the ethylenedioxy group. Peaks at 1090 and 1205 cm⁻¹ are corresponding to the sulfone groups –SO₂ and –SO₃⁻ in molecule of PSS. The absorptions of PEDOT:PSS–RGO in the composite are not observed because either they are too weak or they overlap with the PEDOT:PSS hydrogel characteristic peaks. Fig. 4a and b shows the main types of ring stretching vibration of PEDOT and PSS: namely C–C inter-ring stretching (1260 cm⁻¹); single C–C stretching (1368 cm⁻¹); C=C symmetrical stretching (1436 cm⁻¹); C=C asymmetrical stretching (1509 cm⁻¹); and C=C antisymmetric stretching (1567 cm⁻¹). When the PEDOT:PSS–RGO is formed, the peak intensities of PEDOT:PSS decrease or even vanish, whereas the peaks at 1590 cm⁻¹ (G-band) and 1357 cm⁻¹ (D-band) both dominate. These phenomena occurred due to the change in the structure of PEDOT:PSS. When graphene and PEDOT:PSS were mixed, PEDOT polymer chains were absorbed to the surface of the graphene. The π–π interaction of graphene and PEDOT:PSS forms a tightly coated layer on the graphene surface.

Fig. 3 FT-IR spectra of PEDOT:PSS, graphene oxide (GO), reduced graphene oxide (RGO), and PEDOT:PSS–RGO.

Fig. 4 Raman spectra of GO and RGO (a); Raman spectra of PEDOT:PSS and PEDOT:PSS–RGO (b).

The coating of PEDOT:PSS onto the graphene surface produces interfacial layers with specific structure and dynamics, leading to more uniform distributions within the polyurethane dielectric elastomer composite system, as depicted in Fig. 5a. SEM images of the fracture surface in Fig. 5b and c highlight the difference between RGO and PEDOT:PSS–RGO dispersed in polyurethane dielectric elastomer. The obvious layered structure of elastomer was shown in RGO@PU composites. There is apparent phase separation that due to the poor dispersion and large graphene aggregates. In the sample using functionalized graphene the result is very different. The PEDOT:PSS functionalized graphene laminates appear to be perfectly dispersed in the polymer matrix throughout the sampled area. It is believed that the PEDOT:PSS coated RGO is fully encapsulated into the polyurethane matrix. Considering the results in Fig. 5, significant improvements in the dielectric properties as well as the electromechanical performance of the PEDOT:PSS–RGO@PU sample were expected.

Fig. 5 (a) Schematic illustration on the reinforced interface of graphene@PU by noncovalent functionalized graphene with PEDOT:PSS conducting polymer, (b) SEM pattern of RGO@PU from the cross-section view, and (c) SEM pattern of PEDOT:PSS–RGO@PU from the cross-section view.

X-ray powder diffraction pattern (XRD) and thermogravimetric analysis (TG) patterns of PU, RGO@PU, and PEDOT:PSS–RGO@PU were shown in Fig. 6 and 7. The XRD results reveal that both the hard segment and soft segment of PU are in the amorphous phase and there is no detectable crystalline phase in the material. Moreover, we can see that wide-angle XRD patterns of RGO@PU and P–RGO@PU do not show any obvious characteristic signals for graphene, indicating the very high dispersion of graphene sheets in PU matrix which is in agreement with the results of SEM. The pure PU possesses a single stage in the thermal degradation around 300–400 °C in the TG curves. Both the RGO@PU and P–RGO@PU samples exhibit similarly significant improvements in the thermal stability compare with the PU. The results confirm that the nanofillers can improve the thermal stability of the PU. The TG curves of 3%-RGO@PU and P–RGO@PU hybrid show the amount of weight residue increased by 5% in comparison with the pure PU at 600 °C. The increase in thermal stability was caused by the presence of thermally stable graphene. As a result, adding the graphene nanofiller could slow down the weight loss during thermal degradation.

Fig. 6 XRD patterns of PU, RGO@PU, and PEDOT:PSS–RGO@PU.

Fig. 7 TG curves of PU, RGO@PU, and PEDOT:PSS–RGO@PU.

We investigated that the PEDOT:PSS noncovalent functionalized graphene sheets served as fillers toward enhancing the dielectric performances of polyurethane dielectric elastomer composites. Fig. 8 shows dielectric properties of RGO@PU, and PEDOT:PSS–RGO@PU with the same graphene backbone content (3 wt%) in the frequency range of 40 Hz to 110 kHz at room temperature. The dielectric constant of all samples decreased as the frequency increased and the dielectric loss increased on the contrary. Clearly, a significant enhancement of permittivity for PEDOT:PSS–RGO@PU is observed at the low frequencies. For instance, the permittivity at 1 kHz reaches more than 350, 88 times higher than that of the pure PU. The permittivity value is also much higher than those of RGO@PU. This result indicates that the interfacial modification between graphene plates and polymer matrix plays an important role in enhancing the dielectric performances of graphene-based composites. For the RGO@PU, the aggregation of RGO still serves as the electrode material to form a microcapacitor network in the matrix for increasing the permittivity. However, the number of microcapacitors is restricted because of the aggregation of RGO. As to PEDOT:PSS–RGO@PU, noncovalent functionalization improves the dispersibility of RGO in polymeric matrix. Furthermore, the PEDOT:PSS conducting polymer on RGO can serve as dielectric layers to prevent graphene backbones from direct contact in the composites. These factors allow for the construction of more microcapacitors in PEDOT:PSS–RGO@PU composites, leading to a higher permittivity. More importantly, the dielectric loss of PEDOT:PSS–RGO@PU displays a relatively lower dielectric loss in compassion with the RGO@PU composite. As a sequence, the PEDOT:PSS–RGO@PU shows superior dielectric performances.

Fig. 8 Relative permittivity (a) and dielectric loss (b) curves of PU, RGO@PU, and PEDOT:PSS–RGO@PU.

We further measured the storage modulus and loss modulus and tan δ values of the graphene@PU composites with a dynamic mechanical analyzer (Fig. 9a–c). The storage modulus relates the ability of the material to store energy when oscillatory force is applied and the loss modulus relates the ability to lose the energy. All the storage modulus and loss modulus and tan δ of the RGO@PU composite are much higher than that of the pure PU and PEDOT:PSS–RGO@PU at the temperature range studied here. This phenomenon may be explained by the different interface interaction of graphene/functionalized graphene and PU matrix. The flexible coating of PEDOT:PSS conducting polymer chains onto graphene reinforces the interaction between the PU matrix and graphene sheets, thus it greatly reduces the loss modulus and tan δ of the corresponding PU hybrids. This is of great benefit to the electromechanical properties.

Fig. 9 (a) Storage modulus–temperature plots of PU, RGO@PU, and PEDOT:PSS–RGO@PU, (b) loss modulus–temperature plots, and (c) tan δ–temperature plots.

We measured the electric field induced strain of the obtained high permittivity PEDOT:PSS–RGO@PU film with the Fotonic displacement sensor by designing a diaphragm device to facilitate the measure system. Fig. 10a shows the typical result of deflection displacement of PEDOT:PSS@PU actuator unit versus time with 2 cycles of a sinusoidal-shaped electric field. The displacement signal has sinusoidal shape the same as the electric field. As the electric field increases there is a corresponding decrease in film thickness. It was also observed that the strains were fully recovered as the electric field decreased to zero. We found an extra high electric-induced strain of PEDOT:PSS–RGO@PU diaphragm type actuator unit here. Fig. 10b shows the field-induced strain (out-of-plane strain) along the thickness direction of the neat PU, RGO@PU and PEDOT:PSS–RGO@PU composite films as a function of the applied electric field. As shown in Fig. 10b, it is clear that the PEDOT:PSS–RGO@PU sample presented much higher strain levels than the pure PU and RGO@PU counterparts. Strain curve of PEDOT:PSS–RGO@PU shows a steep slope whereas curves of RGO@PU and PU are relatively smooth. Excitingly more than 160% electric induced strain of PEDOT:PSS–RGO@PU was achieved at 32.2 MV m⁻¹. This is much higher than the deflection obtained on the other polyurethane elastomer hybrids.^16–19 To the best of our knowledge, it is the biggest electric induced thickness strain of the polyurethane dielectric elastomer composite.

Fig. 10 Thickness deflection displacement of PEDOT:PSS–RGO@PU at super high sinusoidal electric field of 3200 V (a); maximum thickness strain as a function of the applied electric field for pure PU, RGO@PU and PEDOT:PSS–RGO@PU (b).

Conclusions

In summary, electroactive PEDOT:PSS–RGO@PU intelligent shape-changed polymer composites were fabricated. To improve their interfacial interaction with the elastomer matrix and ensure their homogeneous dispersion, graphene sheets were noncovalently functionalized with PEDOT:PSS conducting polymer. The dielectric constant and the electromechanical actuation strain were improved dramatically in comparison with the pure PU and RGO@PU. PEDOT:PSS–RGO@PU also has low dielectric loss, low loss modulus, and low loss tangent. The improved dielectric, mechanical, and electromechanical actuation performance of the PEDOT:PSS–RGO@PU composites offers a broad range of potential applications such as artificial muscles, micro-air vehicles, microrobotics, pumps and valves in microfluidic systems, ventricular assist devices for failing hearts, and switchable optics, sensors, actuators, and micro-electronic devices for these unique intelligent shape-changed composites.

Acknowledgements

This work was supported by Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the National High Technology Research and Development Program of China (863 Program: 2013AA041105), the National Natural Science Foundation of China (11372133), the Aeronautical Science Foundation of China (no. 2014ZF52070), the Funding for Outstanding Doctoral Dissertation in NUAA (BCXJ13-11), the Funding of Jiangsu Innovation Program for Graduate Education (KYLX_0264) and the Fundamental Research Funds for the Central Universities (5645004, NZ2013307).

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