Research Article www.acsami.org
Building a Novel Chemically Modified Polyaniline/Thermally Reduced Graphene Oxide Hybrid through π−π Interaction for Fabricating Acrylic Resin Elastomer-Based Composites with Enhanced Dielectric Property Sen-Qiang Wu,‡ Jing-Wen Wang,*,† Jing Shao,‡ Lei Wei,‡ Kai Yang,§ and Hua Ren∥ †
College of Materials Science and Technology, Nanjing University of Aeronautics & Astronautics, 29 Yudao Street, Nanjing 210016, P. R. China ‡ Department of Materials Science and Engineering, College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, P. R. China § College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, P. R. China ∥ Department of Materials Science and Engineering, Nanjing University, 22 Hankou Road, Nanjing 210093, P. R. China S Supporting Information *
ABSTRACT: Sustainability urgently demands low dielectric loss and low elastic modulus as fostering high permittivity (HiK) conductor/polymer composites. This work introduces a ternary composite system, consisting of acrylic resin elastomer (AR), chemically modified polyaniline (HBSiPA), and the thermally reduced graphene oxides (TrGOs), for applying to actuators, of which AR was fabricated by free radical polymerization. The unique hybridized graphene (HBSiPA− TrGO) was prepared by a two-step procedure, including the doped polyaniline modified by the hyperbranched polysiloxane via a ring opening reaction, followed by the decoration of HBSiPA on the surface of TrGO, the conductivity of which is desired to be the same as that of graphene. Afterward, diverse filler contents of HBSiPA−TrGO were put into the AR matrix to fabricate composites with the solution casting method and TrGO/ AR composites were fabricated as well for comparison. Unlike TrGO, HBSiPA has plenty of polyaniline chain segments that ensure better dispersion of graphene hybrids in the AR, and thus the composites inherit the excellent electrical property of graphene. The permittivity and dielectric loss of the HBSiPA−TrGO/AR composite at 100 Hz are 3.5 and 0.27 times that of the TrGO/AR composite, respectively, when the loading of fillers approaches the percolation threshold (fc), which originates from the HBSiPA anchored onto the graphene serving as spacer and thus decreases the leakage currents induced by the contact of graphene sheets. Besides, the elastic modulus of 2.83 vol % HBSiPA−TrGO/AR composite was lower than 5 MPa. KEYWORDS: acrylic resin, polyaniline, graphene, π−π interaction, dielectric property electrical-induced strain, and higher breakdown field strength compared with the other polymer matrices.9 Actuated relative area strains were showed to be up to 215% with VHB 4910 acrylic elastomer.10 Howbeit AR suffers from a low dielectric constant (≈3), which restricts its practical applicability in actuators and energy harvesters. Therefore, an efficacious approach to improve its actuation strain s is to increase the permittivity (εr) of AR according to a formula Pelrine proposed for s
1. INTRODUCTION There have been eximious achievements in dielectric materials because electroactive polymers (EAPs) hold a wide foreground of applications in numerous cutting-edge fields, including MEMS devices, biomedicine, artificial muscles, and tactile displays.1−4 In particular, dielectric elastomers (DEs) that are capable of changing shape or volume once applied with electric field, such as acrylic resin elastomer (AR),5 silicone rubber,6 polyurethane elastomer,7 and nitrile rubber,8 have aroused wide-spread interest due to their immanent advantages, such as fast response, excellent load matching, high efficiency of electromechanical conversion, strong environmental adaptability, lightness, and low cost. Among the dielectric elastomers, AR is one of the appropriate selections in the present stage for dielectric elastomer actuators and supercapacitors because the AR furnishes a higher energy density (≈3.4 J cm−3), larger © XXXX American Chemical Society
s=
εrε0 2 E Y
(1)
Received: June 1, 2017 Accepted: August 4, 2017 Published: August 4, 2017 A
DOI: 10.1021/acsami.7b07785 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces where ε0 is the dielectric constant of vacuum (8.85 × 10−12 F m−1), Y is the modulus of elasticity, and E is the electric field intensity.11 Not only should the desired dielectric materials applicable for DEs have a high εr and extremely low conductivity, the modulus of elasticity Y is also of great importance for it directly affects the machinability and achievable deformation of the devices. Accordingly, AR, with a lower elastic modulus obtained by adjusting the proportion of soft monomer and hard monomer, shows more strain under the identical electric field strength and more output stress is available while the composites have a high dielectric constant. Recently, DEs filled with varieties of fillers, such as ceramics,12,13 metal particles,14 carbon black,15−17 and conductive polymers,18,19 have received tremendous attention owing to the synergistic effects between the fillers and the high breakdown voltage of the polymer matrices.12−15 Particularly, ceramic fillers, including PbTiO3, BaTiO3, CaCu3Ti4O12, Li 0 . 0 5 Ti 0 . 0 2 Ni 0 . 9 3 O, PbZr x Ti 1 − x O 3 , PbMg 1 / 3 Nb 2 / 3 O 3 , K1−xNaxNbO3, and their mixtures, have been widely used in dielectric composites due to their extremely high permittivity of several thousands.12,13,20,21 Tangboriboon et al. reported that PbTi1−xO3 and PbZrxO3 particles could consumedly improve the electrical activity and the dielectric constant of the ARbased nanocomposite.22 Despite such enhancements via adding inorganic ceramics, these ceramic/polymer composites demand higher filler concentration for improving their dielectric properties; thus, the increased concentration of these inorganic fillers inevitably deteriorates the flexibility and breakdown voltage of the resultant composites. Moreover, the leaded ceramics may destroy the environment to some degree. On the basis of these facts thoroughly discussed above, more suitable fillers with environmental friendliness, high flexibility, and low filler loading are required for the construction of the comprehensive high-performance AR-based actuators. High dielectric constant (marked as Hi-K) carbon conductor/polymer nanocomposites possess superiorities of cheaper cost, quite lower filler loadings, better flexibility, as well as better mechanical properties compared with the other forms of composite materials.16,17 Graphene, a single-atom-thick honeycomb planar architecture of sp2 carbon atoms, has become one of the most attractive and preferential fillers depending on its prominent conductivity, thermal stability, electron mobility, and large specific surface area.23,24 Nevertheless, a remarkable improvement of the dielectric constant simultaneously brings an increasing dielectric loss, which conflicts with the application indexes of the actuators and energy harvesters. To facilitate the compatibility between graphene and the acrylic resin elastomer, therefore, researchers attempt to decorate conductors with an insulator layer to reduce the dielectric loss. So far, covalent (chemical)25−27 and noncovalent (physical)28−30 approaches have been utilized to modify graphene. Especially, noncovalent modification forms covering anchored organics onto graphene sheets through physical action, for example, π−π conjugation, van der Waals forces, and electrostatic force, are used to cut off the leakage current generated by the conductive path from the interconnection between conductors and to reduce the tunneling current. This powerful interaction isolates the direct connection of graphene sheets; thereby, the dielectric loss decreases effectively. Moreover, in contrast to covalent modification, noncovalent modification guarantees the excellent integrated properties which preserve the intrinsic unique π-conjugated structure of graphene. For instance, Wang et al. reported that graphene
modified by the surface active agent poly(vinylpyrrolidone) resulted in better dispersion in the AR matrix and significantly intensified the dielectric and thermal properties of the ultimate nanocomposites.31 Hence, it is of great necessity to design novel conductive fillers for developing Hi-K composites based on graphene. Doped polyaniline (d-PANI) is one of the most promising candidates for noncovalently modifying graphene owing to its outstanding virtues, such as handy preparation, reversible redox behavior, and good conductivity.32 Moreover, the d-PANI of nanometer sizes ( fc
(3)
σDC ∝ (fc − f )−s for f < fc
(4)
where σDC is the DC conductivities of composites, fc is the percolation threshold, f is the volume fraction of fillers, and t, s are the critical parameters.46 As displayed in the inset plots of Figure 5a,b, the resultant calculated fc values of TrGO/AR and HBSiPA−TrGO/AR nanocomposites are 1.87 and 2.42 vol %, respectively. The percolation threshold of HBSiPA−TrGO/AR nanocomposites increasing slightly is attributable to the following reasons: On one hand, the dispersion of TrGO in the matrix improves significantly owing to HBSiPA, which restricts the contact of TrGO and thereby postpones the formation of conductive networks, as shown in the Scheme 3, whereas on the other hand, the conductivity of hybrids is fractionally lower than that of graphene; thus, it is of essence to append higher content of the graphene hybrids. As displayed in Figure 6a,b permittivity depends on the frequencies from 100 to 106 Hz of TrGO/AR and HBSiPA− TrGO/AR composites with different filler contents. In detail,
Figure 4. SEM images of fracture surfaces of TrGO/AR (a, b) and HBSiPA−TrGO/AR (c, d). XRD spectra (e) of AR, TrGO/AR, and HBSiPA−TrGO/AR.
TrGO/AR composite (Figure 4a,b). By contrast, HBSiPA− TrGO presents a homogeneous and smooth layered structure Scheme 2. Interaction of HBSiPA−TrGO and AR
I
DOI: 10.1021/acsami.7b07785 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
is originated from the directional alignment of some dipoles, which is capable of reversing the applied field with the electrical field due to the change of frequency. (2) When the frequency increases, the reversion of dipoles is unable to keep up with the change of electrical fields on account of resistance in the materials, which consequently forms relaxation.47 (3) While the frequency reaches very high, some dipoles cease reversing that results in making no contribution to the permittivity. Observing Figure 6a,b, it is easily found that the permittivity of 2.83 vol % HBSiPA−TrGO/AR at 102 or 103 Hz is as high as 355 and 325, approximately 3.5 and 2.8 times that of 1.69 vol % TrGO/AR, respectively. These phenomena can be illustrated by the following causes: (1) the Maxwell−Wagner−Sillars (MWS) polarization effect.48 It is universally known that provided there is a disparity of the permittivity between the insulating phase and conducting phase, a great deal of charge carriers accumulate on the interface of both phases and then interface polarization occurs. Especially, compared with TrGO/ AR composites, HBSiPA−TrGO/AR composites consist of interfaces of HBSiPA-AR and HBSiPA−TrGO besides the interface of TrGO/AR, which possesses more interfaces than that of TrGO/AR composites resulting in stronger MWS polarization, which is the specific essence that endows HBSiPA−TrGO with a particular virtue in enhancing the permittivity. (2) HBSiPA−TrGO has better dispersion than that of TrGO in the AR, forming larger number of microcapacitors that are very significant for improving the charge-storage capacity and achieving high dielectric constant. Moreover, the unsatisfactory dispersity of graphene in TrGO/ AR composites easily forms conductive paths, resulting in worse permittivity performance. In contrast, the much better dispersity of HBSiPA−TrGO hybrid in the composite, which is beneficial to the better dielectric property, leads to the delay in the formation of conductive networks.49 Figure 6c presents the dependence of the permittivity (100 Hz) on the filler contents of TrGO/AR and HBSiPA−TrGO/ AR nanocomposites. The difference of the trend between the DC conductivities is that the permittivity decreases drastically while the filler loading goes beyond the percolation threshold, which indicates that the resultant composites turn from insulators into conductors. The foregoing results are attributed to the following reasons: (1) The quantity of the microcapacitors formed when the graphene sheets act as the electrode plates and AR is served as the dielectric is relatively small as the filler content is far low, which leads to a slow ascend of the permittivity. (2) Thus, the more the filler content
Figure 5. DC conductivity as a function of the filler loading of TrGO/ AR (a) and HBSiPA−TrGO/AR (b) composites. The inset shows the log(σ) − log( fc − f) plot for fc > f and log(σ) − log( f − fc) plot for fc < f.
the permittivity practically is independent of the frequency with no filler in the AR, which illustrates that there is no sufficient accumulation of charges on the interface within the neat AR. As a comparison, the dielectric constant decreases in pace with frequency increasing with the addition of fillers, which is ascribed to the following explanations: (1) The dielectric constant of materials is caused by polarization; the polarization
Scheme 3. Composites of Graphene Sheets (Black) in AR (Left)b and Composites of Graphene Sheets Coated by HBSiPA (Green) in AR (Right)a
Overlapped conductive sheets forming a conductive path. bGraphene sheets are polarized when applied with an electric field; therefore, positive and negative charges gather at the surface and the sheets begin to feel each other leading to mutual attraction. a
J
DOI: 10.1021/acsami.7b07785 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 6. Dependence of the dielectric constant on the frequency of TrGO/AR (a) and HBSiPA−TrGO/AR (b) composites at room temperature with different mass fractions of fillers. The dielectric constant of TrGO/AR and HBSiPA−TrGO/AR composites as a function of the filler content measured at 100 Hz and room temperature (c). Dependence of dielectric loss on the frequency of TrGO/AR (d) and HBSiPA−TrGO/AR (e) composites at room temperature with different mass fractions of fillers.
is put in, the larger the number of the microcapacitors and the dielectric constant gain. Importantly, the microcapacitors in the composites mutually overlap and progressively build a conductive network while TrGO or HBSiPA−TrGO approaches the percolation threshold, which realizes the transition from the insulator to the conductor of the composites, contributing to the steep increase of the permittivity. (3)
Nevertheless, the dielectric constant decreases remarkably when even more TrGO or HBSiPA−TrGO is added because the conductive network has been accomplished.50 In addition, it is easily to found that the permittivity of HBSiPA−TrGO/AR is much higher than that of TrGO/AR at the same filler content. This phenomenon is attributed to the much better dispersity of the HBSiPA−TrGO hybrid than that of graphene in the matrix. K
DOI: 10.1021/acsami.7b07785 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Furthermore, the better dispersity generates more microcapacitors that are capable of enhancing the ability of charge storage. As shown in the Figure 6d,e the dielectric loss depends on the frequency from 102 to 106 Hz of TrGO/AR and HBSiPA− TrGO/AR composites with different filler contents. In the wake of the increase of the filler loading, the dielectric loss appears in the analogous percolation phenomenon as the dielectric constant, which is the inevitable outcome of the improved conductivity of the composites. Moreover, the dielectric loss falls first and then rises slightly along with the increase of the frequency, which results from the following two parameters: (1) The relaxation phenomenon discussed above begins to emerge at the low frequency; thus, the relaxation polarization fails to fully build, which contributes to the sharp decline of the dielectric loss for the reactive current, which is inversely proportional to frequency. (2) The increase of polarization loss enforces the active current into increasing faster than the reactive current in the anomalous dispersion region; therefore, the dielectric loss ascends with the increase of frequency ranging from 105 to 106 Hz. Observing Figure 6a,b, it is easily summarized that dielectric loss of 2.83 vol % HBSiPA−TrGO/AR at 102 or 103 Hz is as low as 0.37 and 0.18, approximately 0.27 and 0.24 times that of 1.69 vol % TrGO/AR, respectively. Note that there exist two main currents for the electrical conductivity, including leakage current and tunneling current, of which the leakage current plays a more important role in conductivity than the latter because it is relevant to the formation of the conductive paths. Accordingly, the dispersion and direct touch of fillers result in the dielectric loss of nanocomposites. (1) The π−π interactions between the HBSiPA and TrGO make for a better dispersion in the AR matrix than that in TrGO. (2) Furthermore, coating the low-conductivity HBSiPA on graphene cuts the leakage current arising from the immediate contact of the graphene sheets. The two aforementioned reasons lead to the greatly reduced dielectric loss.51,52 To further detect the origin behind the distinction in electrical and dielectric performances between the TrGO/AR and HBSiPA−TrGO/AR composites, the results confirmed that two distinctive equivalent circuits should be utilized to simulate the electrochemical impedance spectra of these composites (Figure 7).53 The equivalent circuit of TrGO/AR (Figure 7a) is composed of matrix resistance (Rm), contact resistance (Rc ), charge-transfer resistance (Rct), matrix capacitance (Cm), contact capacitance (Cc), and charge-transfer capacitance (Cct). The constant phase angle element (CPE) is the calibration of the capacitance deviating from a pure capacitive behavior. Different from TrGO/AR, the equivalent circuit of the HBSiPA−TrGO/AR composite introduces an extra parallel circuit, including a CPEp and a polarization resistance (RP) (Figure 7b), which represents the outcome that the dispersity of graphene improves on account of introducing HBSiPA and thus enhances the interfacial polarization of HBSiPA-AR resin and HBSiPA−TrGO. It is universally known that the semicircle diameter of the Nyquist curve represents charge-transfer resistance (Rct) when it comes to dielectric materials. In the light of the formulas Z = ZR + ZP
Z P = RP +
1 jwC P
Figure 7. Experimental data (hollow circle) and fitting curves (solid line); the inset shows an equivalent circuit of the corresponding composite ((a) 1.69% TrGO/AR; (b) 2.83% HBSiPA−TrGO/AR). Impedance spectra of HBSiPA−TrGO/AR composites (c). The inset is an enlarged version of the circled area.
the impedance can be regarded as a synergistic effect between conductivity impedance and polarization impedance, where ZR represents conductivity impedance, ZP, the polarization impedance, RP, the polarization resistance, CP, the polarization capacitance, w, the angular frequency, and j is the imaginary unit. The Nyquist plot of the 1.69% TrGO/AR composite (Figure 7a) presents a classical semicircle shape, which demonstrates
(5)
(6) L
DOI: 10.1021/acsami.7b07785 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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elastic modulus, it is still under the acceptance level for applying in actuators. Therefore, the composites are of practical significance.
that dielectric property results from the formation of conductive paths through directly contacting graphene. Consequently, the charge transfer inside the composite improves greatly. As comparison, the Nyquist plot of the 2.83% HBSiPA−TrGO/AR composite contains two overlapped semicircles (Figure 7b), one of which stands for RP in the lowfrequency region and another reflects the ZR of the composite in the high-frequency region as ZP can be ignored on account of polarization relaxation. Along with the increase of HBSiPA− TrGO content, the Nyquist curves change from a straight line to a semicircle, indicating conductive network forms gradually (Figure 7c). In detail, the double-semicircle Nyquist curve suggests that the formation of conductive channels of graphene as well as the enhanced MWS polarization are two significant elements for the increased dielectric constant of the HBSiPA− TrGO/AR composite. The enhanced MWS polarization is ascribed to more polarization interfaces arising from the interaction between dipoles in HBSiPA and TrGO, together with more microcapacitors induced by the desiring dispersity of hybrids in the AR matrix. To monitor the electrical strength of elastomer films, dielectric withstanding voltage tests were carried out. Eventually, the breakdown field strength or dielectric strength of 1.0 wt % HBSiPA−TrGO/AR composite (62 V μm−1) is higher than that of 1.0 wt % TrGO/AR composite (48 V μm−1), which is in virtue of better dispersion, delaying the formation of conductive paths. The elastic modulus is another important indicator to evaluate the suitability of composite material elastomers for actuators. Figure 8 shows the elastic modulus of composite
4. CONCLUSIONS By means of π−π noncovalent interaction, HBSiPA is coated on the graphene surface, building a unique type of graphene hybrid (HBSiPA−TrGO) with comparable conductivity as that of the neat graphene. The hybridization endows the graphene sheets with better dispersion in the AR matrix and exfoliates graphene sheets to a single or few-layered structure, which leads to improving the dielectric and thermal properties of the resultant HBSiPA−TrGO/AR composite. The permittivity and dielectric loss of the HBSiPA−TrGO/AR composite at 100 Hz are 3.5 and 0.27 times that of the TrGO/AR composite, respectively, when the loading of fillers approaches the percolation threshold ( fc), which results from the discrepant structures. In detail, the presence of HBSiPA in AR not only forms increasing microcapacitors but also decreases the leakage currents induced by the contact of graphene sheets with HBSiPA acting as a spacer. Moreover, the elastic modulus of the composite below 5 MPa is still suitable for applying in actuators.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07785. Molecular weight (Mw), polydispersity and quantity of epoxide groups of HBSi, 1H NMR, 13C NMR, and 29Si NMR spectra of HBSiPA, the solubility (the UV spectra at 600 nm with different solvents) and thermal stability (TG curves) differences between d-PANI and HBSiPA, dielectric property of d-PANI−TrGO/AR composites and HBSiPA−TrGO/AR composites with different filler loadings, and thermal property (DSC and TG curves) of HBSiPA−TrGO/AR composites (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jing-Wen Wang: 0000-0002-2268-500X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21174063), the Natural Science Foundation of Jiangsu Province (No. BK20131358), the Aeronautical Science Foundation of China (Nos. 2011ZF52063 and 2014ZF52069), and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
Figure 8. Elastic modulus of TrGO/AR and HBSiPA−TrGO/AR composites.
materials with different loadings. First, the elastic modulus increases along with the larger filler content both in TrGO/AR composites and HBSiPA−TrGO/AR composites. Moreover, in the case of the same loadings, the HBSiPA−TrGO/AR composites have a little higher elastic modulus than that of TrGO/AR composites. The phenomena can be accounted for by the following factors: (1) The movement of the chain segments of the polymer AR is hindered by the presence of graphene sheets on its large surface. (2) HBSiPA ameliorates the dispersion of TrGO in AR, which enhances the interfacial interaction between TrGO and AR. Although this improves the
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DOI: 10.1021/acsami.7b07785 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
