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High-dielectric-permittivity silicone rubbers incorporated with polydopamine-modified ceramics and their potential application as dielectric elastomer generator
Materials Chemistry and Physics 241 (2020) 122373
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
High-dielectric-permittivity silicone rubbers incorporated with polydopamine-modified ceramics and their potential application as dielectric elastomer generator Liang Zhang *, Feilong Song, Xiang Lin, Dongrui Wang ** School of Chemistry and Biological Engineering, University of Science & Technology Beijing, Beijing, 100083, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A new kind of inorganic/polymer dielectric composite elastomer was prepared. � Surface modified high-dielectricpermittivity CCTO particles were used as fillers. � A bimodal silicone rubber system was used as matrix. � The composite had improved dielectric and mechanical properties fitting for DEG. � The energy conversion efficiency was 3.36%, increased by 68% compared to matrix. A R T I C L E I N F O
A B S T R A C T
Keywords: Composite materials Energy Dielectric elastomer Polymers
Exploiting green energy from environmental resources with high harvesting efficiency has always been an ambitious goal in materials science. Dielectric elastomer generator (DEG) is one of the most promising devices which could draw electrical energy from mechanical stress. However, the need for high deformability and dielectric properties poses great challenge. In this work, high permittivity inorganic fillers, copper calcium titanate (CCTO), were incorporated with a bimodal silicone rubber matrix aiming at improving dielectric, me chanical, and mechanic-electro conversion properties. The surfaces of the inorganic fillers were modified with polydopamine (PDA) layers first to enhance the interfacial interaction between the inorganic and polymer material, leading to better dispersion and greater dielectric property. Results showed at a bias voltage of 1500 V, the energy harvested by the 26 wt% CCTO@PDA incorporated composite was 0.69 mJ/cm3, twice as much as the polymer matrix. The energy conversion efficiency of 20 wt% CCTO@PDA incorporated composite elastomer was at the maximum of 3.36%, increased by 68% compared to the pure matrix.
1. Introduction Harvesting green energy has been a long-lasting pursuit since the rise
of modern electronics industry. Various technologies have been devel oped to convert mechanical energy directly to electricity. Dielectric elastomer generator (DEG) is one of the most promising method due to
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Zhang), [email protected] (D. Wang). https://doi.org/10.1016/j.matchemphys.2019.122373 Received 20 July 2019; Received in revised form 10 October 2019; Accepted 27 October 2019 Available online 30 October 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Materials Chemistry and Physics 241 (2020) 122373
its intrinsic merits including high energy density, fast response, low cost and feasibility for large- and small-scale fabrication [1–3]. Previous work has already shown prototypes of DEG drawing energy from envi ronmental sources, such as wind and ocean waves [4–6]. Generally, DEG is composed of dielectric elastomer membrane with large deformability, coated with compliant electrodes on both sides. It interactively works in a conditioning circuit. In a typical operation cycle, the DEG device is stretched to input mechanical energy first and charged by an external voltage source to input electric energy, and then released to convert the elastic strain energy to an increased electrical energy, which is harvested in the system. In other words, the DEG can be considered as a variable capacitor. The capacitance of a planar capacitor can be calculated from Equation (1): C¼
ε0 εr A d
with giant dielectric permittivity (>10000), was coated with PDA and then incorporated with silicone rubber to improve interfacial interaction and dispersion. A bimodal polymer network mixed by two kinds of polysiloxanes with different molecular weights was used as the matrix. The results showed the PDA coated CCTO increased overall ability of the inorganic/polymer composite. The as-prepared composites were tested in a conditioning circuit, evaluating the application of the CCTO@PDA particles incorporated composites as DEG. 2. Experimental section 2.1. Materials Tetrabutyl titanate, copper nitrate trihydrate, calcium acetate mon ohydrate, acetic acid, ethanol, tetrahydrofuran (THF), hydrochloric acid, tris(hydroxymethyl) aminomethane, tetraethyl orthosilicate and dibutyltin dilaurate were purchased from Beijing Chemical Plant (China). Dopamine hydrochlorides were purchased from Sigma Indus trial Corporation (China). Silicone rubber H107 with different molecular weights (90000 mPa s, 2000 mPa s) were obtained from Jinan Long cheng Silicone Co. Ltd. All chemicals were used as received.
(1)
where ε0 is the vacuum permittivity, εr is the relative permittivity, A and d represent the area and thickness of the dielectric. The electric energy stored in DEG, W for the polymer-based capacitors could be expressed by Equation (2), showing the stored energy is related to the dielectric permittivity εr and breakdown strength Eb. W ¼ 0:5CV 2 ¼ 0:5ε0 εr AdE2b
(2)
2.2. Preparation of CCTO particles
To maximize the output electric energy, the dielectric elastomer should have high dielectric permittivity which allow more stored charge and low dielectric loss for efficient energy transduction. High electric breakdown strength and mechanical properties of the elastomer mate rial are also required to ensure higher operating voltage tolerance and high strain at break for large elastic storage, respectively. Up till now, most works on DEG have been focused on circuit and device design using commercialized acrylic-based elastomers (VHB adhesives) [7–10]. However, the energy conversion efficiency is still not satisfactory, which poses great challenge for practical application. To this end, preparing elastomers with better dielectric and mechanical properties is in urgent need. Recently, several approaches have been devoted to synthesizing new hybrid elastomers by adding inorganic fillers to polymer matrix to improve the dielectric property while maintaining the deformability. Zhang’s group used barium titanate (BT) nanoparticles and natural rubber as fillers and polymer matrix. With the help of a plasticizer, the achieved-elastomer were tested as DEG, achieving an electric energy density of 0.71 mJ/cm3 and energy conversion efficiency of 3.8% [11]. Yang et al. also used BT as inorganic fillers but incorporated in poly urethane matrix. The dielectric permittivity raised to 8.6 at 1 kHz, but the conversion efficiency was only 1.56% [12]. Lee et al. prepared a composite with nanospring carbon nanotubes and silicone matrix. The best material developed had dielectric permittivity of 4.6 and strain-at-break of 270%. The output voltage of the DEG using the as-prepared composite increased from 8.8 V to 14.5 V [13]. Although some improvements were made, the reported materials were far from ideal. One of the reasons is the large disparity in dielectric permittivity and surface energy between the inorganic fillers and polymers, causing low dielectric permittivity boost, large dielectric loss, and poor me chanical property [14,15]. Therefore, it is necessary to modify the sur faces of the inorganic fillers to achieve better interfacial interaction. Polydopamine (PDA) modification is one of the most simple and ver satile method for altering the surface property and functionalization [16–18]. Dopamine could spontaneously deposit on almost any surface to creative a conformal PDA coating by oxidative polymerization. Due to the functional groups including o-quinone, carbonyl, amino, imine, and phenol, the PDA layer is compatible to polymer materials, and also could be used as a platform for secondary reactions. Our previous work showed the polydopamine modification was a useful strategy to enhance the dielectric property of polymer-matrix composites [19]. In this article, copper calcium titanate (CCTO), an inorganic ceramic
Copper nitrate trihydrate (4.87 g) and calcium acetate monohydrate (1.38 g) were dissolved in 30 ml of ethanol under magnetic stirring for 2 h to prepare Solution A. Meanwhile, tetrabutyl titanate (9.2 ml) and acetic acid (2.1 ml) were added to 15 ml of ethanol and stirred for 1 h to prepare Solution B. Then, Solution B was added into Solution A dropwise and stirred for another 1 h before the resultant solution was placed into an 80� C-water bath to remove most of the solvent. When the solution began to gel, stirring was stopped. After gelation completed, it was transferred into an oven to be calcined at 750 � C for 2.5 h. Finally, the product was grinded to achieve finer CCTO particles. 2.3. Modification of the CCTO particles CCTO particles (3 g) were dispersed in 50 ml of Tris-HCl solution (10 mM, pH ¼ 8.5) by sonification for 2 h. Then dopamine was added into the solution with a concentration of 2 mg/ml. The solution was heated at 60 � C in a water bath under vigorous stirring for 12 h. Then the solution was centrifuged and washed by deionized water for several times. The raw product was dried in an oven at 60 � C to obtain polydopamine-modified (CCTO@PDA) particles. 2.4. Preparation of CCTO@PDA/silicone rubber composites The silicone rubber matrix was fabricated from hydroxyl-terminated polysiloxane precursors by using tetraethyl orthosilicate and dibutyltin dilaurate as crosslinker and catalyst, respectively. To optimize the me chanical property of the elastic matrix, two polysiloxanes with different molecular weights (Long-chain: 90000 mPa s, Short-chain: 2000 mPa s) were mixed under various weight ratios (WLong-chain/WShortchain ¼ 100:0, 90:10, 80:20, 70:30, 60:40). After addition of the cross linker and catalyst, the viscous mixtures were poured into moulds and vulcanized for 8 h under a pressure of 20 MPa at room temperature. After mechanical characterization, the silicone rubber with precursor ratio WLong-chain/WShort-chain ¼ 80:20 was chosen as the matrix for the following composite fabrications. CCTO@PDA particles were dispersed in THF by sonification in advance. Certain amount of THF-diluted polysiloxane mixtures was mixed into the dispersion and stirred at 60 � C for another 2 h. Then the mixture was dried by using rotary evaporation at 60 � C for 2 h to remove the THF, and vulcanized. The vulcanization process was the same as described above. For comparison, silicone rubber membrane with pris tine CCTO particles were also prepared following the same procedure. 2
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Materials Chemistry and Physics 241 (2020) 122373
Fig. 1. SEM images of (a) pristine CCTO particles and (b) CCTO@PDA particles. Scale bar: 500 nm. (c) FTIR spectra and (d) TGA curves of CCTO and CCTO@PDA particles.
2.5. Characterization
s. The samples were immersed in silicone oil during the measurement. The energy harvesting performance of the composite films were evalu ated by using a set of instruments including oscilloscope, high voltage source, tensile machine and a pair of circular clamps, which was re ported previously [1].
The morphology of the particles and composites was observed by scanning electron microscopy (SEM, HitachiS 4700, Japan) with an accelerating voltage of 20 kV. Fourier transform infrared (FTIR) spectra were recorded by Nicolet Nexus 6700, (Nicolet, USA). Thermal property was investigated by thermal gravimetric analysis (TGA, Beijing Ever lasting Scientific Instrument Factory, HCT-1). Dielectric permittivity and loss tangent were measured by using an impedance analyzer (Agi lent 4294A) over the frequency range of 100–1 M Hz at room temper ature. Tensile properties were examined by a universal mechanical testing machine (Shimadzu AG-IC, Japan). A CS2674A high voltage amplifier with a sphere-to-sphere electrodes configuration was used to measure the electrical breakdown strength under the ramp rate of 200V/
3. Results and discussion High-dielectric-permittivity CCTO particles with diameters in the range of 100–400 nm were prepared, verified by SEM image (Fig. 1a) and XRD pattern (Fig. S1). The dielectric permittivity of pressed CCTOparticles membrane was ~103 at 100 Hz (Fig. S2). To make the CCTO particles more compatible to the polymeric matrix, a thin layer of PDA was coated onto the surfaces of CCTO particles through in-situ poly merization of dopamine (Fig. 1b). To confirm PDA was successfully deposited, the CCTO@PDA particles were characterized by FTIR and TGA. The FTIR spectrum shows that after PDA modification, two new peaks appear at 3460 cm 1 and 3180 cm 1, which are corresponding to the O–H stretching vibration (Fig. 1c). The peaks at 1640 cm 1 and 1450 cm 1, representing the stretching vibrations of aromatic rings and – C bonds, can also be found [20]. These changes suggest that PDA was C– coated on the surface of CCTO particles. The TGA curves show that the pristine CCTO particles had only a mild weight loss due to the trace of water and impurities (Fig. 1d). As for the CCTO@PDA particles, severe weight loss took place, started at 350 � C. This weight loss should mainly be ascribed to the decomposition of PDA layer. According to the weight loss numbers at 800 � C, it can be deducted that the ratio of PDA in CCTO@PDA is approximately 2% of the total weight. It is noteworthy that the PDA did not decompose before 150 � C, which is useful in practical applications. Silicone rubber was chosen to be the polymer matrix due to its chemical stability, good electrical and mechanical properties, and fast
Fig. 2. Tensile properties of silicone rubbers with different weight contents of short chain precursor. 3
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response rate during electromechanical transduction. However, con ventional silicone rubbers are weak in tear strength, jeopardizing their usage as thin films. To solve this problem, here two types of poly siloxanes with significantly different molecular weights were selected as precursors to form a bimodal crosslinked network. Previous reports have confirmed that bimodal polydimethylsiloxane (PDMS) networks could show better mechanical properties than unimodal ones, because the long chains in the bimodal network provide extensibility and prevent possible rupture, while the short chains supplement the toughness of the elastomer [21–24]. The synergy of long chains and short chains deter mined the overall elasticity. In this work, two hydroxy-terminated pol ysiloxanes of different molecular weights were mixed under different weight ratios. As shown in Fig. 2, at first, by increasing the amount of short chain polysiloxane in the network, the crosslinked silicone rubber network gained higher tensile strength and elongation at break. How ever, when the short chain concentration reached 40 wt%, the me chanical property of the silicone rubber dropped intensively. Excessive short chains were not crosslinked and made the extensibility become worse. It is noteworthy that under 50% strain, the difference between the bimodal and unimodal silicone rubber did not have much difference. But when larger than 50%, the bimodal silicone rubber showed lower stress. It means at the same strain, the bimodal silicone rubber needed less mechanical energy to deform, which was beneficial for the energy conversion efficiency. In the following experiments, the silicone rubber with a bimodal network using 20 wt% of short chain precursor was selected as the polymeric matrix due to its high extensibility. The CCTO@PDA and CCTO particles were incorporated with the bimodal silicone rubber matrix, respectively. From the SEM images (Fig. 3), it can be seen that the CCTO particles with PDA modification had better dispersion than those without PDA. Even at high filler loading, there was no aggregation in the CCTO@PDA/silicone rubber
Fig. 3. SEM images of cross-sections of the composites incorporated with different mass contents of CCTO and CCTO@PDA particles. Scale bar: 30 μm.
Fig. 4. Tensile stress-strain responses of silicone rubber composites filled with various mass contents of (a) CCTO particles and (b) CCTO@PDA particles. (c) Comparison of the mechanical properties (elongation-at-break, tensile strength and Young’s modulus) of the CCTO and CCTO@PDA incorporated composites. 4
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Materials Chemistry and Physics 241 (2020) 122373
Fig. 5. Dielectric permittivity of the composites with different mass fraction of (a) CCTO and (b) CCTO@PDA as a function of frequency. (c) Dielectric permittivity and (d) dielectric loss of the composites filled with different mass fraction of CCTO and CCTO@PDA particles at 1 kHz frequency.
composite. In comparison, the CCTO particles started to aggregate at 13 wt% and the aggregation became more severe at higher content. This result shows the PDA coating is of great importance in enhancing the interaction between the particles and the polymer matrix. The mechanical properties of the elastomer have vital influence on the mechano-electrical energy conversion. It is important to obtain high strain-at-break and low Young’s modulus simultaneously. Fig. 4 shows the tensile stress-strain response of the CCTO and CCTO@PDA particles incorporated silicone rubbers at various contents. By increasing the filler content, the tensile strength also increased due to particle enhancement for both composites. The strain-at-break was different for the composites with and without PDA medium layer. For the pure CCTO particles, strain-at-break gradually decreased with higher filler contents, because the fillers aggregated and damaged the bimodal network. As for the CCTO@PDA fillers, the low filler loading (5 wt% and 13 wt%) helped the increase of the strain-at-break. It could be derived from the reactions between the PDA and the crosslinker, leading to the decrease of the crosslinking density of the silicone rubber network. Therefore, the mo lecular chains were more prone to be stretched. At high filler loading, crosslinking of the system went to a higher level of and restrained the movement of the molecular chains. As a result, the strain-at-break of the CCTO@PDA silicone rubber composite decreased. In addition, the Young’s modulus of the CCTO@PDA filled composites was lower than their counterparts blended with pristine CCTO at 5 wt%, 13 wt% and 20 wt%. One possible reason is that the decrease in the crosslinking density due to the presence of PDA. Also, the PDA layer can perform as a mechanical lubricator. All these results revealed that PDA modification of the fillers improved the mechanical properties of the composite elastomer. Fig. 5 shows the dielectric properties of the CCTO/silicone rubber composite with and without the PDA modification. Both kinds of the composites had dielectric permittivity increased along with the content
Fig. 6. Influences of CCTO and CCTO@PDA on the breakdown field strength of the composites at different contents.
of the fillers, indicating that CCTO had a positive effect on the dielectric permittivity as expected. To show more clearly the difference that PDA modification had made, the dielectric permittivity and dielectric loss of the two composites at 1 kHz are given in Fig. 5c and d. The CCTO@PDA/ silicone rubber had higher dielectric permittivity and lower dielectric loss at every filler loading. When the filler content was 20 wt%, the CCTO@PDA composite had dielectric permittivity of 4.8 and dielectric loss of 0.007. When the content reached to 26 wt%, the numbers shifted to 5.5 and 0.009, respectively. The dielectric losses were relatively small (<0.1) and could favor the energy conversion process. These properties derived from the compatibility that PDA brought, which led to better dispersion and the improved interfacial polarization. In addition, PDA has strong dipoles such as hydroxyl and amino groups in its chain 5
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Materials Chemistry and Physics 241 (2020) 122373
composites under the bias voltage of 500 V, 1000 V and 1500 V, respectively. Results show that the higher the dielectric permittivity the composite had, the larger the capacity was. In other words, at the same bias voltage and stretching strain, the composites with higher dielectric permittivity generated more electric energy. With the increase of the bias voltage, the energy gained had a large increase due to the increase of the electrode’s resistance and relatively small voltage loss of the system. At 1500 V, the electric energy achieved by the 26 wt% CCTO@PDA incorporated composite was 0.69 mJ/cm3, which was nearly twice to the pure polymer matrix (0.32 mJ/cm3). Fig. 8b shows the mechano-electric energy conversion efficiency calculated from the mechanical energy. The efficiency is dependent on the electric and mechanical input. The best efficiency was 3.36% achieved at 20 wt% CCTO@PDA filler content. Compared to the polymer matrix, the con version efficiency increased by 68%. The comparison of the energy harvesting abilities with reported literatures were summarized in Table 1. Because the electrical and mechanical input had an influence on the conversion properties of DEG, operating voltage and area expansion of the sample were also listed. The barium titanate/polyurethane (BT/ PU) systems had higher energy harvesting energy (1.71 and 2.88 mJ/ cm3), but the conversion efficiency was quite low [1,12]. The energy harvesting density and conversion efficiency reported in this work (0.69 mJ/cm3, 3.36%) were close to the BT/natural rubber system (0.71 mJ/cm3, 3.8%) [11]. In addition, this CCTO/silicon rubber sys tems were allowed to work at higher operating voltages and the me chanical input was rather small (~40% area expansion), showing considerable advantages.
Fig. 7. The DEG operating circuit employed to measure the energy conversion performance of the composite elastomers.
structure, which can be orientationally polarized under electric field, causing further increase in dielectric permittivity [14]. The breakdown field strength of the composites was also investigated (Fig. 6). The silicone rubber matrix had breakdown strength of about 73 V/μm. With the increase of the filler content, the breakdown strength of both the composites decreased. But the CCTO@PDA incorporated composites decreased more gently and was higher than the CCTO-filled composites at every filler content. The main reason should be the PDA layer improved the dispersion of fillers to avoid high level of defects and suppress electric field distortion. In addition, the PDA layer interacted with the silicone rubber which made the network denser, also leading to a higher breakdown strength. When the content of CCTO@PDA was 26 wt%, the breakdown strength was still higher than 50 V/μm, considered as a relatively good level. The above result shows the CCTO@PDA incorporated composites had better mechanical and dielectric property than those filled by pristine CCTO, including larger strain-at-break, relatively low tensile strength, higher dielectric permittivity and breakdown strength, and lower dielectric loss. To exploit the application as DEG, we tested the electricity harvesting performance of the composite elastomers using a circuit shown in Fig. 7. The stretching height and the stretching speed of the elastomer films were fixed to 20 mm and 20 mm/min, and the charging time for the stretched film was 20 s. When the stretched films were released, the generated high voltage outputs were measured. Fig. 8a shows the volumetric energy density generated by the
4. Conclusion In conclusion, CCTO particles were prepared and coated with PDA. The fillers were incorporated into a bimodal silicone rubber. The com posites showed increased dielectric permittivity and lower dielectric loss compared to the ones filled with unmodified CCTO particles. The me chanical properties and the breakdown strength were also improved by the CCTO@PDA incorporation. The PDA layer helped the dispersion of the ceramic fillers and interacted with the hydroxyl groups of silicone
Fig. 8. (a) The volumetric energy density and (b) the mechano-electrical conversion efficiency of the composite elastomers with various CCTO@PDA mass contents. Table 1 Comparison of the energy harvest abilities of reported DEGs. Materials a
BT/plasticizer/PU BT/PU BT/plasticizer/natural rubber CCTO/silicone rubber a
Conversion efficiency (%)
Energy harvesting density (mJ/cm3)
Operating voltage
Area expansion
Ref
~0.71 1.56 3.8 3.36
1.71 2.88 0.71 0.69
900 V 900 V 1400 V 1500 V
80% ~40% ~64% ~40%
[1] [12] [11] This work
BT: Barium titanate; PU: polyurethane. 6
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Materials Chemistry and Physics 241 (2020) 122373
rubber, which led to the above physical properties. The composite was also tested as DEG in a designed system. Results showed at 1500 V bias voltage, the energy generated by the 26 wt% CCTO@PDA incorporated composite was twice as much as the pure polymer matrix. As for the energy conversion efficiency, 20 wt% CCTO@PDA composite elastomer was at the maximum of 3.36%, increased by 68% compared to the pure matrix.
[3] A. O’Halloran, F. O’Malley, P. McHugh, J. Appl. Phys. 104 (2008), 071101. [4] R.D. Kornbluh, R. Pelrine, H. Prahlad, A. Wong-Foy, B. McCoy, S. Kim, J. Eckerle, T. Low, in: Y. BarCohen, F. Carpi (Eds.), Electroactive Polymer Actuators and Devices, 2011. [5] P. Jean, A. Wattez, G. Ardoise, C. Melis, R. Van Kessel, A. Fourmon, E. Barrabino, J. Heemskerk, J.P. Queau, in: Y. BarCohen (Ed.), Electroactive Polymer Actuators and Devices, 2012. [6] T. Andritsch, P.H.F. Morshuis, J.J. Smit, P. Jean, R. van Kessel, A. Wattez, A. Fourmon, 2012 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 2012, pp. 786–789. [7] T.G. McKay, B.M. O’Brien, E.P. Calius, I.A. Anderson, Appl. Phys. Lett. 98 (2011) 142903. [8] H. Wang, C. Wang, T. Yuan, Appl. Phys. Lett. 101 (2012), 033904. [9] J. Huang, S. Shian, Z. Suo, D.R. Clarke, Adv. Funct. Mater. 23 (2013) 5056–5061. [10] S. Shian, J. Huang, S. Zhu, D.R. Clarke, Adv. Mater. 26 (2014) 6617–6621. [11] D. Yang, Y. Xu, M. Ruan, Z. Xiao, W. Guo, H. Wang, L. Zhang, AIP Adv. 9 (2019), 025035. [12] Y. Yang, Z.-S. Gao, M. yang, M.-S. Zheng, D.-R. Wang, J.-W. Zha, Y.-Q. Wen, Z.M. Dang, Nano Energy 59 (2019) 363–371. [13] Y.J. Lee, P. Caspari, D.M. Opris, F.A. Nuesch, S. Ham, J.-H. Kim, S.-R. Kim, B.-K. Ju, W.K. Choi, J. Mater. Chem. C 7 (2019) 3535–3542. [14] L. Zhang, D. Wang, P. Hu, J.-W. Zha, F. You, S.-T. Li, Z.-M. Dang, J. Mater. Chem. C 3 (2015) 4883–4889. [15] J. Hu, L. Zhang, Z.-M. Dang, D. Wang, Compos. Sci. Technol. 148 (2017) 20–26. [16] L. Haeshin, S.M. Dellatore, W.M. Miller, P.B. Messersmith, 318 (2007) 426-430. [17] L. Zhang, J. Wu, Y. Wang, Y. Long, N. Zhao, J. Xu, J. Am. Chem. Soc. 134 (2012) 9879–9881. [18] L. Zhang, H. Yu, N. Zhao, Z.-M. Dang, J. Xu, J. Appl. Polym. Sci. 131 (2014) 41057. [19] L. Zhang, S. Yuan, S. Chen, D. Wang, B.-Z. Han, Z.-M. Dang, Compos. Sci. Technol. 110 (2015) 126–131. [20] D.R. Dreyer, D.J. Miller, B.D. Freeman, D.R. Paul, C.W. Bielawski, Langmuir 28 (2012) 6428–6435. [21] G.D. Genesky, B.M. Aguilera-Mercado, D.M. Bhawe, F.A. Escobedo, C. Cohen, Macromolecules 41 (2008) 8231–8241. [22] A.G. Bejenariu, L. Yu, L. Skov, Soft Matter 8 (2012) 3917–3923. [23] N. Seetapan, A. Fuongfuchat, D. Sirikittikul, N. Limparyoon, J. Polym. Res. 20 (2013), https://doi.org/10.1007/s10965-013-0183-8. [24] F.B. Madsen, A.E. Daugaard, C. Fleury, S. Hvilsted, A.L. Skov, RSC Adv. 4 (2014) 6939–6945.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors acknowledge the financial support from National Nat ural Science Foundation of China (No. 21574012, 51773019) and Fundamental Research Funds for the Central Universities (No. FRF-TP18-009B1). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.122373. References [1] G. Yin, Y. Yang, F. Song, C. Renard, Z.-M. Dang, C.-Y. Shi, D. Wang, ACS Appl. Mater. Interfaces 9 (2017) 5237–5243. [2] R. Pelrine, R. Kornbluh, J. Eckerle, P. Jeuck, S.J. Oh, Q.B. Pei, S. Stanford, Smart Structures and Materials 2001: Electroactive Polymer Actuators and Devices, 2001, pp. 148–156.
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
High-dielectric-permittivity silicone rubbers incorporated with polydopamine-modified ceramics and their potential application as dielectric elastomer generator Liang Zhang *, Feilong Song, Xiang Lin, Dongrui Wang ** School of Chemistry and Biological Engineering, University of Science & Technology Beijing, Beijing, 100083, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A new kind of inorganic/polymer dielectric composite elastomer was prepared. � Surface modified high-dielectricpermittivity CCTO particles were used as fillers. � A bimodal silicone rubber system was used as matrix. � The composite had improved dielectric and mechanical properties fitting for DEG. � The energy conversion efficiency was 3.36%, increased by 68% compared to matrix. A R T I C L E I N F O
A B S T R A C T
Keywords: Composite materials Energy Dielectric elastomer Polymers
Exploiting green energy from environmental resources with high harvesting efficiency has always been an ambitious goal in materials science. Dielectric elastomer generator (DEG) is one of the most promising devices which could draw electrical energy from mechanical stress. However, the need for high deformability and dielectric properties poses great challenge. In this work, high permittivity inorganic fillers, copper calcium titanate (CCTO), were incorporated with a bimodal silicone rubber matrix aiming at improving dielectric, me chanical, and mechanic-electro conversion properties. The surfaces of the inorganic fillers were modified with polydopamine (PDA) layers first to enhance the interfacial interaction between the inorganic and polymer material, leading to better dispersion and greater dielectric property. Results showed at a bias voltage of 1500 V, the energy harvested by the 26 wt% CCTO@PDA incorporated composite was 0.69 mJ/cm3, twice as much as the polymer matrix. The energy conversion efficiency of 20 wt% CCTO@PDA incorporated composite elastomer was at the maximum of 3.36%, increased by 68% compared to the pure matrix.
1. Introduction Harvesting green energy has been a long-lasting pursuit since the rise
of modern electronics industry. Various technologies have been devel oped to convert mechanical energy directly to electricity. Dielectric elastomer generator (DEG) is one of the most promising method due to
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Zhang), [email protected] (D. Wang). https://doi.org/10.1016/j.matchemphys.2019.122373 Received 20 July 2019; Received in revised form 10 October 2019; Accepted 27 October 2019 Available online 30 October 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Materials Chemistry and Physics 241 (2020) 122373
its intrinsic merits including high energy density, fast response, low cost and feasibility for large- and small-scale fabrication [1–3]. Previous work has already shown prototypes of DEG drawing energy from envi ronmental sources, such as wind and ocean waves [4–6]. Generally, DEG is composed of dielectric elastomer membrane with large deformability, coated with compliant electrodes on both sides. It interactively works in a conditioning circuit. In a typical operation cycle, the DEG device is stretched to input mechanical energy first and charged by an external voltage source to input electric energy, and then released to convert the elastic strain energy to an increased electrical energy, which is harvested in the system. In other words, the DEG can be considered as a variable capacitor. The capacitance of a planar capacitor can be calculated from Equation (1): C¼
ε0 εr A d
with giant dielectric permittivity (>10000), was coated with PDA and then incorporated with silicone rubber to improve interfacial interaction and dispersion. A bimodal polymer network mixed by two kinds of polysiloxanes with different molecular weights was used as the matrix. The results showed the PDA coated CCTO increased overall ability of the inorganic/polymer composite. The as-prepared composites were tested in a conditioning circuit, evaluating the application of the CCTO@PDA particles incorporated composites as DEG. 2. Experimental section 2.1. Materials Tetrabutyl titanate, copper nitrate trihydrate, calcium acetate mon ohydrate, acetic acid, ethanol, tetrahydrofuran (THF), hydrochloric acid, tris(hydroxymethyl) aminomethane, tetraethyl orthosilicate and dibutyltin dilaurate were purchased from Beijing Chemical Plant (China). Dopamine hydrochlorides were purchased from Sigma Indus trial Corporation (China). Silicone rubber H107 with different molecular weights (90000 mPa s, 2000 mPa s) were obtained from Jinan Long cheng Silicone Co. Ltd. All chemicals were used as received.
(1)
where ε0 is the vacuum permittivity, εr is the relative permittivity, A and d represent the area and thickness of the dielectric. The electric energy stored in DEG, W for the polymer-based capacitors could be expressed by Equation (2), showing the stored energy is related to the dielectric permittivity εr and breakdown strength Eb. W ¼ 0:5CV 2 ¼ 0:5ε0 εr AdE2b
(2)
2.2. Preparation of CCTO particles
To maximize the output electric energy, the dielectric elastomer should have high dielectric permittivity which allow more stored charge and low dielectric loss for efficient energy transduction. High electric breakdown strength and mechanical properties of the elastomer mate rial are also required to ensure higher operating voltage tolerance and high strain at break for large elastic storage, respectively. Up till now, most works on DEG have been focused on circuit and device design using commercialized acrylic-based elastomers (VHB adhesives) [7–10]. However, the energy conversion efficiency is still not satisfactory, which poses great challenge for practical application. To this end, preparing elastomers with better dielectric and mechanical properties is in urgent need. Recently, several approaches have been devoted to synthesizing new hybrid elastomers by adding inorganic fillers to polymer matrix to improve the dielectric property while maintaining the deformability. Zhang’s group used barium titanate (BT) nanoparticles and natural rubber as fillers and polymer matrix. With the help of a plasticizer, the achieved-elastomer were tested as DEG, achieving an electric energy density of 0.71 mJ/cm3 and energy conversion efficiency of 3.8% [11]. Yang et al. also used BT as inorganic fillers but incorporated in poly urethane matrix. The dielectric permittivity raised to 8.6 at 1 kHz, but the conversion efficiency was only 1.56% [12]. Lee et al. prepared a composite with nanospring carbon nanotubes and silicone matrix. The best material developed had dielectric permittivity of 4.6 and strain-at-break of 270%. The output voltage of the DEG using the as-prepared composite increased from 8.8 V to 14.5 V [13]. Although some improvements were made, the reported materials were far from ideal. One of the reasons is the large disparity in dielectric permittivity and surface energy between the inorganic fillers and polymers, causing low dielectric permittivity boost, large dielectric loss, and poor me chanical property [14,15]. Therefore, it is necessary to modify the sur faces of the inorganic fillers to achieve better interfacial interaction. Polydopamine (PDA) modification is one of the most simple and ver satile method for altering the surface property and functionalization [16–18]. Dopamine could spontaneously deposit on almost any surface to creative a conformal PDA coating by oxidative polymerization. Due to the functional groups including o-quinone, carbonyl, amino, imine, and phenol, the PDA layer is compatible to polymer materials, and also could be used as a platform for secondary reactions. Our previous work showed the polydopamine modification was a useful strategy to enhance the dielectric property of polymer-matrix composites [19]. In this article, copper calcium titanate (CCTO), an inorganic ceramic
Copper nitrate trihydrate (4.87 g) and calcium acetate monohydrate (1.38 g) were dissolved in 30 ml of ethanol under magnetic stirring for 2 h to prepare Solution A. Meanwhile, tetrabutyl titanate (9.2 ml) and acetic acid (2.1 ml) were added to 15 ml of ethanol and stirred for 1 h to prepare Solution B. Then, Solution B was added into Solution A dropwise and stirred for another 1 h before the resultant solution was placed into an 80� C-water bath to remove most of the solvent. When the solution began to gel, stirring was stopped. After gelation completed, it was transferred into an oven to be calcined at 750 � C for 2.5 h. Finally, the product was grinded to achieve finer CCTO particles. 2.3. Modification of the CCTO particles CCTO particles (3 g) were dispersed in 50 ml of Tris-HCl solution (10 mM, pH ¼ 8.5) by sonification for 2 h. Then dopamine was added into the solution with a concentration of 2 mg/ml. The solution was heated at 60 � C in a water bath under vigorous stirring for 12 h. Then the solution was centrifuged and washed by deionized water for several times. The raw product was dried in an oven at 60 � C to obtain polydopamine-modified (CCTO@PDA) particles. 2.4. Preparation of CCTO@PDA/silicone rubber composites The silicone rubber matrix was fabricated from hydroxyl-terminated polysiloxane precursors by using tetraethyl orthosilicate and dibutyltin dilaurate as crosslinker and catalyst, respectively. To optimize the me chanical property of the elastic matrix, two polysiloxanes with different molecular weights (Long-chain: 90000 mPa s, Short-chain: 2000 mPa s) were mixed under various weight ratios (WLong-chain/WShortchain ¼ 100:0, 90:10, 80:20, 70:30, 60:40). After addition of the cross linker and catalyst, the viscous mixtures were poured into moulds and vulcanized for 8 h under a pressure of 20 MPa at room temperature. After mechanical characterization, the silicone rubber with precursor ratio WLong-chain/WShort-chain ¼ 80:20 was chosen as the matrix for the following composite fabrications. CCTO@PDA particles were dispersed in THF by sonification in advance. Certain amount of THF-diluted polysiloxane mixtures was mixed into the dispersion and stirred at 60 � C for another 2 h. Then the mixture was dried by using rotary evaporation at 60 � C for 2 h to remove the THF, and vulcanized. The vulcanization process was the same as described above. For comparison, silicone rubber membrane with pris tine CCTO particles were also prepared following the same procedure. 2
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Fig. 1. SEM images of (a) pristine CCTO particles and (b) CCTO@PDA particles. Scale bar: 500 nm. (c) FTIR spectra and (d) TGA curves of CCTO and CCTO@PDA particles.
2.5. Characterization
s. The samples were immersed in silicone oil during the measurement. The energy harvesting performance of the composite films were evalu ated by using a set of instruments including oscilloscope, high voltage source, tensile machine and a pair of circular clamps, which was re ported previously [1].
The morphology of the particles and composites was observed by scanning electron microscopy (SEM, HitachiS 4700, Japan) with an accelerating voltage of 20 kV. Fourier transform infrared (FTIR) spectra were recorded by Nicolet Nexus 6700, (Nicolet, USA). Thermal property was investigated by thermal gravimetric analysis (TGA, Beijing Ever lasting Scientific Instrument Factory, HCT-1). Dielectric permittivity and loss tangent were measured by using an impedance analyzer (Agi lent 4294A) over the frequency range of 100–1 M Hz at room temper ature. Tensile properties were examined by a universal mechanical testing machine (Shimadzu AG-IC, Japan). A CS2674A high voltage amplifier with a sphere-to-sphere electrodes configuration was used to measure the electrical breakdown strength under the ramp rate of 200V/
3. Results and discussion High-dielectric-permittivity CCTO particles with diameters in the range of 100–400 nm were prepared, verified by SEM image (Fig. 1a) and XRD pattern (Fig. S1). The dielectric permittivity of pressed CCTOparticles membrane was ~103 at 100 Hz (Fig. S2). To make the CCTO particles more compatible to the polymeric matrix, a thin layer of PDA was coated onto the surfaces of CCTO particles through in-situ poly merization of dopamine (Fig. 1b). To confirm PDA was successfully deposited, the CCTO@PDA particles were characterized by FTIR and TGA. The FTIR spectrum shows that after PDA modification, two new peaks appear at 3460 cm 1 and 3180 cm 1, which are corresponding to the O–H stretching vibration (Fig. 1c). The peaks at 1640 cm 1 and 1450 cm 1, representing the stretching vibrations of aromatic rings and – C bonds, can also be found [20]. These changes suggest that PDA was C– coated on the surface of CCTO particles. The TGA curves show that the pristine CCTO particles had only a mild weight loss due to the trace of water and impurities (Fig. 1d). As for the CCTO@PDA particles, severe weight loss took place, started at 350 � C. This weight loss should mainly be ascribed to the decomposition of PDA layer. According to the weight loss numbers at 800 � C, it can be deducted that the ratio of PDA in CCTO@PDA is approximately 2% of the total weight. It is noteworthy that the PDA did not decompose before 150 � C, which is useful in practical applications. Silicone rubber was chosen to be the polymer matrix due to its chemical stability, good electrical and mechanical properties, and fast
Fig. 2. Tensile properties of silicone rubbers with different weight contents of short chain precursor. 3
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response rate during electromechanical transduction. However, con ventional silicone rubbers are weak in tear strength, jeopardizing their usage as thin films. To solve this problem, here two types of poly siloxanes with significantly different molecular weights were selected as precursors to form a bimodal crosslinked network. Previous reports have confirmed that bimodal polydimethylsiloxane (PDMS) networks could show better mechanical properties than unimodal ones, because the long chains in the bimodal network provide extensibility and prevent possible rupture, while the short chains supplement the toughness of the elastomer [21–24]. The synergy of long chains and short chains deter mined the overall elasticity. In this work, two hydroxy-terminated pol ysiloxanes of different molecular weights were mixed under different weight ratios. As shown in Fig. 2, at first, by increasing the amount of short chain polysiloxane in the network, the crosslinked silicone rubber network gained higher tensile strength and elongation at break. How ever, when the short chain concentration reached 40 wt%, the me chanical property of the silicone rubber dropped intensively. Excessive short chains were not crosslinked and made the extensibility become worse. It is noteworthy that under 50% strain, the difference between the bimodal and unimodal silicone rubber did not have much difference. But when larger than 50%, the bimodal silicone rubber showed lower stress. It means at the same strain, the bimodal silicone rubber needed less mechanical energy to deform, which was beneficial for the energy conversion efficiency. In the following experiments, the silicone rubber with a bimodal network using 20 wt% of short chain precursor was selected as the polymeric matrix due to its high extensibility. The CCTO@PDA and CCTO particles were incorporated with the bimodal silicone rubber matrix, respectively. From the SEM images (Fig. 3), it can be seen that the CCTO particles with PDA modification had better dispersion than those without PDA. Even at high filler loading, there was no aggregation in the CCTO@PDA/silicone rubber
Fig. 3. SEM images of cross-sections of the composites incorporated with different mass contents of CCTO and CCTO@PDA particles. Scale bar: 30 μm.
Fig. 4. Tensile stress-strain responses of silicone rubber composites filled with various mass contents of (a) CCTO particles and (b) CCTO@PDA particles. (c) Comparison of the mechanical properties (elongation-at-break, tensile strength and Young’s modulus) of the CCTO and CCTO@PDA incorporated composites. 4
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Fig. 5. Dielectric permittivity of the composites with different mass fraction of (a) CCTO and (b) CCTO@PDA as a function of frequency. (c) Dielectric permittivity and (d) dielectric loss of the composites filled with different mass fraction of CCTO and CCTO@PDA particles at 1 kHz frequency.
composite. In comparison, the CCTO particles started to aggregate at 13 wt% and the aggregation became more severe at higher content. This result shows the PDA coating is of great importance in enhancing the interaction between the particles and the polymer matrix. The mechanical properties of the elastomer have vital influence on the mechano-electrical energy conversion. It is important to obtain high strain-at-break and low Young’s modulus simultaneously. Fig. 4 shows the tensile stress-strain response of the CCTO and CCTO@PDA particles incorporated silicone rubbers at various contents. By increasing the filler content, the tensile strength also increased due to particle enhancement for both composites. The strain-at-break was different for the composites with and without PDA medium layer. For the pure CCTO particles, strain-at-break gradually decreased with higher filler contents, because the fillers aggregated and damaged the bimodal network. As for the CCTO@PDA fillers, the low filler loading (5 wt% and 13 wt%) helped the increase of the strain-at-break. It could be derived from the reactions between the PDA and the crosslinker, leading to the decrease of the crosslinking density of the silicone rubber network. Therefore, the mo lecular chains were more prone to be stretched. At high filler loading, crosslinking of the system went to a higher level of and restrained the movement of the molecular chains. As a result, the strain-at-break of the CCTO@PDA silicone rubber composite decreased. In addition, the Young’s modulus of the CCTO@PDA filled composites was lower than their counterparts blended with pristine CCTO at 5 wt%, 13 wt% and 20 wt%. One possible reason is that the decrease in the crosslinking density due to the presence of PDA. Also, the PDA layer can perform as a mechanical lubricator. All these results revealed that PDA modification of the fillers improved the mechanical properties of the composite elastomer. Fig. 5 shows the dielectric properties of the CCTO/silicone rubber composite with and without the PDA modification. Both kinds of the composites had dielectric permittivity increased along with the content
Fig. 6. Influences of CCTO and CCTO@PDA on the breakdown field strength of the composites at different contents.
of the fillers, indicating that CCTO had a positive effect on the dielectric permittivity as expected. To show more clearly the difference that PDA modification had made, the dielectric permittivity and dielectric loss of the two composites at 1 kHz are given in Fig. 5c and d. The CCTO@PDA/ silicone rubber had higher dielectric permittivity and lower dielectric loss at every filler loading. When the filler content was 20 wt%, the CCTO@PDA composite had dielectric permittivity of 4.8 and dielectric loss of 0.007. When the content reached to 26 wt%, the numbers shifted to 5.5 and 0.009, respectively. The dielectric losses were relatively small (<0.1) and could favor the energy conversion process. These properties derived from the compatibility that PDA brought, which led to better dispersion and the improved interfacial polarization. In addition, PDA has strong dipoles such as hydroxyl and amino groups in its chain 5
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composites under the bias voltage of 500 V, 1000 V and 1500 V, respectively. Results show that the higher the dielectric permittivity the composite had, the larger the capacity was. In other words, at the same bias voltage and stretching strain, the composites with higher dielectric permittivity generated more electric energy. With the increase of the bias voltage, the energy gained had a large increase due to the increase of the electrode’s resistance and relatively small voltage loss of the system. At 1500 V, the electric energy achieved by the 26 wt% CCTO@PDA incorporated composite was 0.69 mJ/cm3, which was nearly twice to the pure polymer matrix (0.32 mJ/cm3). Fig. 8b shows the mechano-electric energy conversion efficiency calculated from the mechanical energy. The efficiency is dependent on the electric and mechanical input. The best efficiency was 3.36% achieved at 20 wt% CCTO@PDA filler content. Compared to the polymer matrix, the con version efficiency increased by 68%. The comparison of the energy harvesting abilities with reported literatures were summarized in Table 1. Because the electrical and mechanical input had an influence on the conversion properties of DEG, operating voltage and area expansion of the sample were also listed. The barium titanate/polyurethane (BT/ PU) systems had higher energy harvesting energy (1.71 and 2.88 mJ/ cm3), but the conversion efficiency was quite low [1,12]. The energy harvesting density and conversion efficiency reported in this work (0.69 mJ/cm3, 3.36%) were close to the BT/natural rubber system (0.71 mJ/cm3, 3.8%) [11]. In addition, this CCTO/silicon rubber sys tems were allowed to work at higher operating voltages and the me chanical input was rather small (~40% area expansion), showing considerable advantages.
Fig. 7. The DEG operating circuit employed to measure the energy conversion performance of the composite elastomers.
structure, which can be orientationally polarized under electric field, causing further increase in dielectric permittivity [14]. The breakdown field strength of the composites was also investigated (Fig. 6). The silicone rubber matrix had breakdown strength of about 73 V/μm. With the increase of the filler content, the breakdown strength of both the composites decreased. But the CCTO@PDA incorporated composites decreased more gently and was higher than the CCTO-filled composites at every filler content. The main reason should be the PDA layer improved the dispersion of fillers to avoid high level of defects and suppress electric field distortion. In addition, the PDA layer interacted with the silicone rubber which made the network denser, also leading to a higher breakdown strength. When the content of CCTO@PDA was 26 wt%, the breakdown strength was still higher than 50 V/μm, considered as a relatively good level. The above result shows the CCTO@PDA incorporated composites had better mechanical and dielectric property than those filled by pristine CCTO, including larger strain-at-break, relatively low tensile strength, higher dielectric permittivity and breakdown strength, and lower dielectric loss. To exploit the application as DEG, we tested the electricity harvesting performance of the composite elastomers using a circuit shown in Fig. 7. The stretching height and the stretching speed of the elastomer films were fixed to 20 mm and 20 mm/min, and the charging time for the stretched film was 20 s. When the stretched films were released, the generated high voltage outputs were measured. Fig. 8a shows the volumetric energy density generated by the
4. Conclusion In conclusion, CCTO particles were prepared and coated with PDA. The fillers were incorporated into a bimodal silicone rubber. The com posites showed increased dielectric permittivity and lower dielectric loss compared to the ones filled with unmodified CCTO particles. The me chanical properties and the breakdown strength were also improved by the CCTO@PDA incorporation. The PDA layer helped the dispersion of the ceramic fillers and interacted with the hydroxyl groups of silicone
Fig. 8. (a) The volumetric energy density and (b) the mechano-electrical conversion efficiency of the composite elastomers with various CCTO@PDA mass contents. Table 1 Comparison of the energy harvest abilities of reported DEGs. Materials a
BT/plasticizer/PU BT/PU BT/plasticizer/natural rubber CCTO/silicone rubber a
Conversion efficiency (%)
Energy harvesting density (mJ/cm3)
Operating voltage
Area expansion
Ref
~0.71 1.56 3.8 3.36
1.71 2.88 0.71 0.69
900 V 900 V 1400 V 1500 V
80% ~40% ~64% ~40%
[1] [12] [11] This work
BT: Barium titanate; PU: polyurethane. 6
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rubber, which led to the above physical properties. The composite was also tested as DEG in a designed system. Results showed at 1500 V bias voltage, the energy generated by the 26 wt% CCTO@PDA incorporated composite was twice as much as the pure polymer matrix. As for the energy conversion efficiency, 20 wt% CCTO@PDA composite elastomer was at the maximum of 3.36%, increased by 68% compared to the pure matrix.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors acknowledge the financial support from National Nat ural Science Foundation of China (No. 21574012, 51773019) and Fundamental Research Funds for the Central Universities (No. FRF-TP18-009B1). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.122373. References [1] G. Yin, Y. Yang, F. Song, C. Renard, Z.-M. Dang, C.-Y. Shi, D. Wang, ACS Appl. Mater. Interfaces 9 (2017) 5237–5243. [2] R. Pelrine, R. Kornbluh, J. Eckerle, P. Jeuck, S.J. Oh, Q.B. Pei, S. Stanford, Smart Structures and Materials 2001: Electroactive Polymer Actuators and Devices, 2001, pp. 148–156.
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