Electrically Insulated Epoxy Nanocomposites Reinforced with …composites.utk.edu/papers in pdf/Wu_et_al-2017-Macromolecular_Chemistry... · 2017/7/13 · 2@MWCNTs nanoparticles

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Electrically Insulated Epoxy Nanocomposites Reinforced with Synergistic Core–Shell SiO2@MWCNTs and Montmorillonite Bifillers

Zijian Wu, Sheng Gao, Lei Chen, Dawei Jiang, Qian Shao, Bing Zhang, Zhaohui Zhai, Chen Wang, Min Zhao, Yingyi Ma, Xiaohong Zhang, Ling Weng, Mingyan Zhang,* and Zhanhu Guo*

DOI: 10.1002/macp.201700357

1. Introduction

Carbon nanotubes (CNTs) have remark-able electrical, thermal, and mechanical properties such as extremely high con-ductivity and Young’s modulus, which can efficiently improve the physical properties of polymers even at low CNT loadings.[1–4] Especially for electrical properties, the addition of a small amount of conductive CNTs to an insulating polymer matrix results in nanocomposites with highly enhanced electrical conductivity.[5] The critical content of CNTs that character-izing a drastic increase in conductivity is commonly termed as the electrical per-colation threshold. When the CNTs con-tent exceeds this electrical percolation threshold, the CNTs conductive pathway inside the poly mer matrix is formed. The electrical percolation threshold of most CNTs/polymer nanocomposites is much below 1 vol%.[6–9] The polymer nanocomposites with high electrical con-ductivity are of high demand for electro-magnetic interference shielding, antistatic device, energy storage/conversion/saving,

Electrical Insulation

With unique physicochemical properties, multiwalled carbon nanotubes (MWCNTs) have enabled major achievement in polymer composites as rein-forcing fillers. Nevertheless, high conductivity of raw MWCNTs (R-MWCNTs) limits their wider applications in certain fields, which require outstanding thermal conductivity, mechanical, and insulation properties simultane-ously. In this article, silica (SiO2) coated MWCNTs core–shell hybrids (SiO2@MWCNTs) and organically modified montmorillonite (O-MMT) are employed to modify epoxy (EP) simultaneously. The epoxy-clay system is cured by using anhydride curing agent. The impact strength and flexural strength of final nanocomposites are greatly improved. Meanwhile, the final composites remain in high electrical insulation. Compared to mixed acid treated MWCNTs (C-MWCNTs) (0.5 wt%)/EP nanocomposites, the volume resistivity of the O-MMT(4 wt%)/SiO2@MWCNTs(0.5 wt%)/EP nanocom-posites increases more than six orders of magnitude. Synergistic toughening effect occurs when using core–shell SiO2@MWCNTs and MMT bifillers. The electrical insulation is attributed to the suppressed electron transport effect by SiO2 layer on the CNTs surface, and the blocked conductive CNTs network by the buried 2D structural O-MMT. The SiO2@MWCNTs core–shell hybrids also benefit to decrease the dielectric constant and dielectric loss of CNTs/EP composites. This work provides guidance to using CNTs as reinforcement fillers to toughen the polymers for electric insulating applications.

Dr. Z. Wu, Prof. X. Zhang, Dr. L. Weng, Prof. M. ZhangKey Laboratory of Engineering Dielectrics and Its ApplicationMinistry of EducationHarbin University of Science and TechnologyHarbin 150040, ChinaE-mail: [email protected]. Z. Wu, S. Gao, Z. Zhai, C. Wang, Dr. Y. Ma, Dr. L. Weng, Prof. M. ZhangCollege of Material Science and EngineeringHarbin University of Science and TechnologyHarbin 150040, ChinaDr. L. Chen, Dr. M. ZhaoSchool of Chemical Engineering and TechnologyHarbin Institute of TechnologyHarbin 150001, China

Dr. D. JiangHeilongjiang Key Laboratory of Molecular Design and Preparation of Flame Retarded MaterialsNortheast Forestry UniversityHarbin 150040, ChinaDr. Q. Shao, Dr. B. ZhangCollege of Chemical and Environmental EngineeringShandong University of Science and TechnologyQingdao 266590, ChinaDr. M. Zhao, Dr. Z. GuoIntegrated Composites Laboratory (ICL)Department of Chemical & Biomolecular EngineeringUniversity of TennesseeKnoxville, TN 77966, USAE-mail: [email protected] ORCID identification number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/macp.201700357.

Macromol. Chem. Phys. 2017, 218, 1700357

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actuators, anticorrosive coating, and sensors.[10–35] Nevertheless, high conductivity of CNTs was an obstacle to wider applications of the CNTs/polymer nanocomposites in some special fields, which require not only outstanding thermal and mechanical properties but also insulation simultaneously (e.g., electronic packaging). However, few researchers pay attention to the remarkable thermal and mechanical properties of CNTs in the insulating fields.[36,37]

A great synergistic reinforcing effect of nanofillers with two different dimensions such as 2D nanomaterials (e.g., gra-phene and clay platelets) and 1D nanomaterials (e.g., CNTs and nanofibers) in polymer matrix has been observed.[38,39] The mechanical properties of the polymer nanocomposites are sig-nificantly improved when compared with the neat polymer, which is due to the formation of much jammed fillers net-work with 1D nanomaterials and 2D nanomaterials combined together.[40,41] Layered silicate as a good nanoscale reinforce-ment for polymer matrix has a similar 2D planar nanostructure to graphene.[42] Polymer/layered silicate nanocomposites have attracted great industrial and academic interests, because at low filler content (i.e., lower than 5 wt%) they show a remarkable improvement of mechanical, thermal, and physical properties compared with the pristine polymer, conventional micron and macrocomposites. Montmorillonite (MMT) clay, consisting of nanometric silicate layers, has been used as inorganic fillers to modify epoxy (EP)-based materials. Compared to conductive graphene, the insulating properties of layered silicate make it better suited to strengthen insulating polymer. Some function-alized CNTs have been fabricated and used as fillers to improve the properties of polymers, especially aimed to enhance the thermal conductivity while remaining high electrical insula-tion.[43] However, no attempts have been made to obtain the EP composites with both improved mechanical properties and high electrical insulation using silica (SiO2) coated multiwalled carbon nanotubes (SiO2@MWCNTs) and MMT together.

In this work, an insulating silica layer coated on the surface of MWCNTs was used to create SiO2@MWCNTs core–shell hybrids, which were used as reinforcement fillers to modify the epoxy composites with anhydride as curing agent. Bisphenol-A epoxy/anhydride system was synergistically reinforced using core–shell SiO2@MWCNTs and montmorillonite as fillers, while remaining electrical insulation of epoxy composites. A comprehensive analysis on synergistic toughening mechanism and dielectric properties of EP nanocomposites is presented.

2. Experimental Section

2.1. Materials

E-44 epoxy resins with epoxy value between 0.41 and 0.47 were purchased from Wuxi Resin Phoenix Co. Ltd., China. Methyl-hexahydrophthalic anhydride and pyridine supplied by Zhe-jiang Alpharm Chemical Technology Co., Ltd , China were used as curing agent and accelerator, respectively. Raw sodium montmorillonite (R-MMT) with a layer spacing of 1.26 nm was purchased from Xi′an Comprehensive Test Center Rock Min-eral Chemical Institute, China. Octadecyl quaternary ammo-nium salt was purchased from Sinopharm Chemical Reagent

Co. Ltd, China. R-MWCNTs (length: 5–10 µm, diameter: 40–60 nm) prepared by chemical vapor deposition method were purchased from Shenzhen Nanotech Port Company. Tetraethy-lorthosilicate (TEOS) and other reagents were purchased from Aladdin Chemical Reagent Co. Ltd. All the chemicals were used as received without any further treatment.

2.2. Preparation of O-MMT

In a typical preparation, a certain amount of the original mont-morillonite was placed into a three-necked flask, and stirred at 80 °C in deionized water for 3 h. Next, a certain amount of octa-decyl quaternary ammonium salts were dissolved in deionized water to prepare organic modifier and stirred at 80 °C for 3 h, then slowly added to the MMT/water mixture and stirred vig-orously for 10 h. The mixture was washed several times with deionized water until no white precipitate was detected by drop-ping AgNO3 solutions in the filtrate. At last, organically modi-fied MMT (O-MMT) were collected after washing and drying. Finally, the MMT clay was modified with octadecyl quaternary ammonium salts and was intercalated into the interlayer region of MMT-Clay.

2.3. Preparation of SiO2@MWCNTs

2 g R-MWCNTs were first placed into a flask, then concentrated H2SO4 and HNO3 in a volume ratio of 3:1 were poured into the flask, the mixture was treated at 65 °C for 10 h under ultrasoni-cation. The resulting mixture was washed with acetone several times by high-speed centrifuge until the pH of the filtrate was neutral and then collected by freeze-drying treatment, marked as C-MWCNTs. Next, 1 g C-MWCNTs, 160 mL ethanol, and 40 mL deionized water were mixed together in a three-necked flask, and ultrasonically dispersed for 1 h to form a stable dispersion. The solution containing 2 g TEOS and 20 mL ethanol was added drop-wise to the C-MWCNTs solution. The solution was then stirred at the speed of 400 rpm for 12 h at 50 °C. Finally, the dispersion was vacuum filtered through a 0.45 µm teflon membrane, and thoroughly rinsed with dimethyl formamide (DMF) and deion-ized water at least 5 times. The as produced SiO2@MWCNTs nanoparticles were lyophilized to avoid the aggregation.

2.4. Preparation of EP Nanocomposites

Taking O-MMT (4 wt%)/SiO2@MWCNTs (0.5 wt%)/EP nano-composites as the example, 100 g EP and 30 mL acetone were mixed, then the 0.5 g SiO2@MWCNTs and 4 g O-MMT were introduced to the mixture, stirring at 80 °C for 10 h under high frequency ultrasonic vibration. Then 74 g curing agent of methyl hexahydrophthalic anhydride was added to the mix-ture and stirred 20 min to get a uniform mixture. Finally, 0.5 g accelerator of pyridine was added to the mixture. The mixture was poured into a preheated steel mold and cured in vacuum oven at certain heating procedures. After cooling, the SiO2@MWCNTs/EP and O-MMT/SiO2@MWCNTs/EP nanocompos-ites were obtained, respectively.




2.5. Characterization

The SiO2@MWCNTs particles were observed by a transmission electron microscope (TEM, Hitachi H-7650, Japan) operated with an accelerating voltage of 200 kV. The fracture surfaces of the specimens were observed by scanning electron microscope (SEM, Hitachi S-4700, Japan). The surface functional groups of the nanofillers were analyzed using a Fourier Transform Infrared (FTIR) spectrophotometer (Nicolet, Nexus 670, USA). The nanofillers were milled with KBr and then compressed into a thin pellet for characterization. Before surface analysis, the samples were dried for 2 h under vacuum at 150 °C. The impact strength and flexure strength were tested by K-12 Charpy impact tester and SHIMADZU AGS-J electronic uni-versal testing machine, respectively. Each test was repeated at least 5 times and the average results were reported.

The volume resistivity of the nanocomposites was meas-ured using a high-resistance meter (Agilent 4339B, Agilent Technologies). Novocontrol broadband dielectric spectroscopy were employed to measure the dielectric constant and dielec-tric loss of the nanocomposites, the samples were cut into 30 × 30 × 0.8 mm3 and deposited aluminum as a circle elec-trode with diameter of 25 mm on the both sides of samples.

3. Results and Discussion

3.1. Characterization of SiO2@MWCNTs

Figure 1a shows the digital photographs of R-MWCNTs and SiO2@MWCNTs dispersed in ethanol after standing 24 h. Remarkable differences of the dispersion can be observed between R-MWCNTs and SiO2@MWCNTs. It is worth noting that the SiO2@MWCNTs can be dispersed in ethanol to form a stable solution and keep stable with negligible sediment for a long time at ambient condition. It indicates that the polarity change of CNTs from the hydrophobic to hydrophilic nature, due to existence of OH. Figure 1b displays the FTIR spectra of the R-MWCNTs and SiO2@MWCNTs. The specimen of SiO2@MWCNTs presents the characteristic bands of the typical SiO2 at ≈1120 cm−1 (υas (SiOSi)). The peak at ≈710 cm−1 is attributed to the SiC stretching. A characteristic band at

3430 cm−1 is assigned to the stretching vibration of the hydroxyl groups (OH), indicating that massive polar groups have been introduced to the surface of SiO2@MWCNTs. Thus, a layer of SiO2 was attached to the surface of MWCNTs via covalent bond (SiOC).[44] Massive silicon hydroxyl groups on MWCNTs surface will be conducive to improving the interfacial adhesion between the SiO2@MWCNT and resin.

Figure 2a,b shows the TEM images of R-MWCNTs and SiO2@MWCNTs, respectively. As shown in Figure 2a, long R-MWCNTs seem easy to entangle with each other. The bound-aries between the side-wall and cavity of CNTs can be dis-tinguished clearly. Figure 2a shows relatively good structure integrity of R-MWCNTs. The outline of R-MWCNTs is relatively smooth, without any coating or impurities observed. The diam-eter of R-MWCNTs is around 60 nm and the length ranges from a few to dozens of micrometers. The existence of SiO2 layer coated on MWCNTs surface and its thickness are evidenced by TEM, Figure 2b. The diameter of SiO2@MWCNTs is around 80 nm and the length ranges from hundreds of nanometers to micrometers. After the SiO2 coating treatment, the agglomera-tion of MWCNTs was greatly reduced, which could be explained by the increased silicon hydroxyl groups and the reduced length of SiO2@MWCNTs. Simultaneously, the surface of SiO2@MWCNTs evidently became rougher than that of R-MWCNTs. The TEM images of SiO2@MWCNTs (Figure 2b) indicate that MWCNTs is completely encapsulated with a uniform SiO2 shell, and the thicknesses of SiO2 coating is about 10 nm.

3.2. Characterization of O-MMT

Figure 3 shows the (a) FTIR spectra and (b) X-ray diffrac-tion (XRD) patterns of R-MMT and O-MMT, respectively. As shown in Figure 3a, the specimen of O-MMT presents the characteristic bands at ≈1470(v(CN)), 2855(vs(CH2)), and 2925(vas(CH2)) cm−1, respectively, indicating that massive octa-decyl quaternary ammonium salts were intercalated to the space between MMT layers by ion exchange. The diffraction peak (001) of R-MMT is about 7.1°, indicating that its interlayer space is about 1.26 nm based on the Bragg equation (2dsinθ = nλ). After the interposition of octadecyl quaternary ammonium salts, the interlayer space of O-MMT increased to 2.38 nm.


Figure 1. a) Optical image of suspension of R-MWCNTs and SiO2@MWCNTs (30 mg mL−1) in ethanol after standing 24 h. b) FTIR spectra of R-MWCNTs and SiO2@MWCNTs.



3.3. Impact Properties

The optimum content of O-MMT is deter-mined to be under the range between 2.0 and 4.0 wt%. Under this doping content range, the O-MMT plays the biggest role in enhancing the overall performance of EP system. Therefore, the doping content of O-MMT for EP/anhydride system is fixed to 4.0 wt%.[45,46] Figure 4 shows the impact strength and flexural strength of SiO2@MWCNTs/EP and O-MMT/SiO2@MWCNTs/EP nanocomposites. The max-imum impact strength of SiO2@MWCNTs/EP and O-MMT/SiO2@MWCNTs/EP nano-composites was significantly improved by about 112.8 and 135.1%, respectively, as compared to that of pristine EP. In theory, the elastic hollow tubular structure and larger surface area of the MWCNTs make them more conducive to absorb impact energy.[47] A transition layer of SiO2 formed in the interface of MWCNTs and EP, the modulus of which lied between high modulus of MWCNTs and low mod-ulus of EP matrix. The gradient modulus makes the interface region act as a stress transfer medium and the load can be transferred from weak matrix to stronger MWCNTs gradually. The mechanical inter-locking structures of SiO2@MWCNTs and O-MMT prevent the impact crack propa-gation and make the crack path deviate away from the original transfer direction to the hybrid structure orientation in the matrix.[48] However, with increasing the particle loading, agglomeration happened and led to a decreased impact strength of SiO2@MWCNTs/EP and O-MMT/SiO2@MWCNTs/EP nanocomposites.

As shown in Figure 2b, a uniform layer of SiO2 can be found on the surface of MWCNTs. Compared to the chemical


Figure 2. TEM images of a) R-MWCNTs and b) SiO2@MWCNTs.

Figure 3. a) FTIR spectra and b) XRD patterns of R-MMT and O-MMT.



oxidation in a mixture of concentrated acids, SiO2 coating treat-ment can offer much more polar groups, such as OH. The existence of hydroxyl groups would improve the dispersion of SiO2@MWCNTsin the EP matrix. Simultaneously, unique hollow tubular structures of the SiO2@MWCNTs could effec-tively absorb impact energy under external forces by elastic deformation. Moreover, numerous CNTs could induce more cracks when the major crack passed them, which could effi-ciently absorb fracture energy.[48–51]

Compared to SiO2@MWCNTs/EP, the impact strength and flexural strength of the O-MMT/SiO2@MWCNTs/EP are improved by about 10.5 and 9.7%, respectively, with the same SiO2@MWCNTs content (0.5 wt%). Synergistic effect on the toughening of EP/anhydride system took place when 1D and 2D nanostructured materials were introduced to the polymer matrix simultaneously, which could be attributed to the formation of more mechanical interlocking network with 1D CNTs and 2D O-MMT combined together. Furthermore, 1D–2D hybrid nanostructures are beneficial to disperse stress uniformly, as shown in Figure 5.

Figure 6 shows the SEM images of the fractured mor-phology of nanocomposites. Figure 6a shows the relatively smooth fractured morphology with a river-like structure which

is characteristic morphology of brittle fracture.[48,52,53] After introducing C-MWCNTs into EP matrix, the cross-section mor-phology (Figure 6b) of nanocomposites becomes rough, part of EP resins near the CNTs was torn apart. C-MWCNTs nanopar-ticles are embedded and tightly held to the matrix, and exhibit a better dispersion. Furthermore, the cross-section morphology (Figure 6c) of the SiO2@MWCNTs/EP composites seems much rougher than that of the C-MWCNTs/EP composites and much more resins were torn apart. It should be noted that the fracture surface of SiO2@MWCNTs/EP composites still exhibited river-like structure, which means it is still a kind of brittle fracture. However, the mechanical properties of SiO2@MWCNTs/EP composites are better than that of C-MWCNTs/EP composites. It could be preliminarily deduced that SiO2@MWCNTs has better interfacial adhesion with EP than that of C-MWCNTs.

No obvious CNTs could be found in Figure 6. But to a certain extent, it was an indication of a better CNTs dispersion in epoxy. In fact, most of SiO2@MWCNTs were buried in the resin, and no CNTs pull-out was observed, just some tube tops of SiO2@MWCNTs were exposed at the cross-section. For one hand, the surface treatment is beneficial to strengthen the interfacial bonding. For another hand, a large number of CNTs were cut off to less than several micrometers by the surface treatment. The shortened length and the stronger interfacial bonding make it very difficult for CNTs to be pulled out from the matrix. In our previous work, untreated commercial CNTs with the length of 40–60 µm were introduced to prepare CNTs/epoxy composites. It is interesting to see that CNTs were much easier to be observed in the untreated CNTs/epoxy composites. How-ever, it could be just observed in some particular area, most of other areas of the fracture sections are relatively smooth. And the mechanical properties of untreated CNTs/epoxy composites were far less than that of SiO2@MWCNTs/EP composites. The areas where CNTs could be observed easier were just the areas where agglomeration occurred.

The good dispersion of SiO2@MWCNTs in EP matrix should stem from the existence of OH groups in SiO2@MWCNTs, which could react with the EP resins, resulting in larger specific surface area of SiO2@MWCNTs and strong interfacial adhesion with polymer. These unique characteristics promote efficient stress transfer between the SiO2@MWCNTs and the EP matrix, and thus improve the impact toughness of nanocomposites.

Nevertheless, two kinds of failure modes were noticed in the fracture morphology of the EP nanocomposites (Figure 6c,d),


Figure 4. a) Impact strength and b) flexural strength of SiO2@MWCNTs/EP and O-MMT/SiO2@MWCNTs/EP nanocomposites.

Figure 5. Schematic diagram of the stress condition of CNTs in polymer matrix a) with and b) without O-MMT.



representing two different models of crack evolution in duc-tile failure process. Compared to the fracture morphology in Figure 6c, massive nest-like patterns were detected in Figure 6d. More complex fracture morphology and the increased surface roughness manifest that the path of the crack is distorted by the hybrid structure of O-MMT and SiO2@MWCNTs in the EP matrix. The 1D–2D mechanical interlocking structure prevents the impact crack propagation and makes the crack path deviate away from the original transfer direction to the hybrid structure orientation in the EP matrix.

3.4. Insulating and Dielectric Properties

Owing to high electrical conductivity of CNTs, the electrical percolation threshold of most CNTs/polymer nanocompos-ites is much below 1 vol%, which means almost all the CNTs/poly mer nanocomposites are electrically conductive. As shown in Figure 7, the volume resistance declines from 3.3 × 1015 Ω m for EP to 2.1 × 107 Ω m for the C-MWCNTs/EP, suggesting that the addition of C-MWCNTs (0.5 wt%) leads to the transforma-tion of the EP from insulator to semiconductor.

By contrast, the volume resistance increases to 1.2 × 1013 Ω m for SiO2@MWCNTs (0.5 wt)/EP nanocomposites. A layer of insulated SiO2 coating on CNTs surface effectively restrains the formation of CNTs conductive network in the EP nanocom-posites. As anticipated, the volume resistance further increases to 8.6 × 1013 Ω m for O-MMT/SiO2@MWCNTs (0.5 wt)/EP nanocomposites, which is a signature of the insulating mate-rial (>1.0 × 109 Ω m). The satisfactory insulating properties were attributed to the suppression of electron transport by SiO2

coating on the surface of MWCNTs and O-MMT layer inserted in the MWCNTs conductive network, which could be expressed as a schematic diagram (Figure 8).

As the average feature size in ultralarge-scale integrated cir-cuits continues to decrease according to Moore’s law, reducing the dielectric constant (K) and dielectric loss of interlevel dielectrics becomes more and more stringent to reduce the resistance–capacitance delay, power consumption, and cross-talk noise. Low-K insulating materials are urgently desired in the field of electronic devices. The effect of SiO2 coating on


Figure 6. The fracture morphology of a) raw EP, b) C-MWCNTs (0.5 wt)/EP, c) SiO2@MWCNTs (0.5 wt)/EP, and d) O-MMT/SiO2@MWCNTs (0.5 wt)/EP nanocomposites.

Figure 7. Volume resistivity of (a) raw EP, (b) C-MWCNTs (0.5 wt)/EP, (c) SiO2@MWCNTs (0.5 wt)/EP, and (d) O-MMT/SiO2@MWCNTs (0.5 wt)/EP nanocomposites.



CNTs surface and 1D–2D hybrid nanostructure on dielectric properties for the EP nanocomposites was thus investigated.

Figures 9 and 10 present the dielectric constant and dielectric loss of the composites on the frequency. The dielectric constant of all samples decreased with increasing the measuring fre-quency, which was consistent with the general rule of dielectric physics.[54] In the low-frequency region, polarization is easier to build.[55] In the high-frequency areas, reducing relaxation time means that the dipole polarization cannot be established in time. Previous researches[56] indicated that introducing a certain amount of CNTs (>1 wt%) to the polymer could lead to a rise in the dielectric constant. On the contrary, adding a small amount (0.5 wt%) of C-MWCNTs and SiO2@MWCNTs can lead to a decrease in the dielectric constant. Especially for the composites filled with SiO2@MWCNTs, a sharp decrease in both dielectric constant and loss is observed.

A small amount of CNTs is much easier to be dispersed in epoxy matrix and formed abundant interface areas. Compared to the pristine EP, the interfacial polarization has become a new factor influencing the dielectric constant of the modified EP composites.[57,58]

The dielectric constant for carbon nanomaterials/polymer composites could be enhanced dramatically when the particle

content was near the threshold.[59–61] But for the nanocompos-ites with low content of carbon nanomaterials, including CNTs and graphene, the decrease in the dielectric constant could occur in some research.[62] Based on the dielectric theory, the interfacial polarization occurs in the low frequency region (1–103 Hz) and contributes to the increased dielectric constant of the nanocomposites. However, the nanocomposites with low CNTs contents show an obvious reduction of dielectric constant at low frequency region. The lowest dielectric constant of the SiO2@MWCNTs/EP composites at 103 Hz is 3.3, far less than that of the pristine EP. One reasonable explanation for the die-lectric constant reduction is that the nanoparticles with larger surface area and increased interfacial strength can effectively limit the dipole orientation and movement in the interface zone,[63] resulting in a low energy storage in an electromag-netic field, which could lead to a decrease of dielectric constant. Second, the greater electrical resistance of SiO2 shell is also beneficial to suppress the polarization behavior in the interface zone. Third, the interface polarization of composites does make a certain contribution to the improvement of dielectric constant when an external electric field is acted on it.[64] Thus, for the polymer nanocomposites, the interface plays a leading role in


Figure 8. Schematic diagram of the inhibitory effect of SiO2 coating and O-MMT on the formation of MWCNTs conductive network.

Figure 9. Dielectric constant of raw EP, C-MWCNTs (0.5 wt)/EP, SiO2@MWCNTs (0.5 wt)/EP, and O-MMT/SiO2@MWCNTs (0.5 wt)/EP nanocomposites.

Figure 10. Dielectric loss of raw EP, C-MWCNTs (0.5 wt)/EP, SiO2@MWCNTs (0.5 wt)/EP, and O-MMT/SiO2@MWCNTs (0.5 wt)/EP nanocomposites.




the dielectric constant. At low filler contents, the CNTs are easy to be dispersed in the EP matrix and generate large numbers of interface. Massive interface area sharply suppresses the dipole polarization and makes a decrease of the dielectric constant. The interface polarization contributes to the increased dielec-tric constant to some extent during this process. Nevertheless, the suppressing effect of interface dose makes a predominant effect on the dielectric constant of nanocomposites. Moreover, the dielectric constant of CNTs itself did not make much contri-bution to the dielectric constant of final nanocomposites.

The dielectric constant of the nanocomposites was also observed to increase when the nanofillers loading is high.[65] The nanofillers are beneficial for improving the dielectric con-stant of the nanocomposites owing to their higher dielectric constant than polymer matrix. CNTs have good conductivity and high dielectric constant than EP. The dielectric constant of CNTs itself will play a leading role in the dielectric con-stant of final nanocomposites when CNTs content was further increased, which makes contribution to the improvement of dielectric constant. Furthermore, CNTs aggregation caused by high filler contents reduces the area of the interface region, so the inhibitory effect[66] of the interface on the orientation of molecule dipoles has been diminished. The interface zone decreased as the CNTs content increased, however, the contri-bution of interface polarization on the dielectric constant of nanocomposites was increased.

The composites filled with SiO2@MWCNTs have the lowest dielectric constant and loss. SiO2@MWCNTs core–shell struc-ture has abundant polar groups (OH), which can react with EP matrix to create stronger interfacial bonding and form a thicker interface layer between CNTs and matrix. Stronger interfacial bonding is beneficial to suppress the dipole polariza-tion and thus decreases the dielectric constant. Moreover, the SiO2 layer can effectively suppress the electron transfer and accumulation in the composites. When 4 wt% O-MMT was introduced to the composites system, the dielectric constant increased to 3.5 at 103 Hz. On one hand, the O-MMT itself has a higher dielectric constant than EP. On the other hand, the O-MMT with lamellar structure cannot create massive interface zone like CNTs with 1D nanostructure. The reduced dielec-tric loss in the SiO2@MWCNTs/EP nanocomposites could be attributed to the reduction of their charge transportation.[44] The strong interfaces between the nanofillers and the polymer and the entanglements of the EP molecular chains are benefi-cial to restrain the motion of charge carriers.[67]

4. Conclusion

Synergistic reinforcement while remaining electrical insula-tion of epoxy composites using core–shell SiO2@MWCNTs and montmorillonite as fillers were reported. Compared to pris-tine EP, the maximum impact strength of SiO2@MWCNTs/EP and O-MMT/SiO2@MWCNTs/EP nanocomposites was significantly improved by about 112.8 and 135.1%, respec-tively. Meanwhile, the volume resistivity of the O-MMT/SiO2@MWCNTs (0.5 wt%)/EP nanocomposites was increased more than six orders of magnitude than that of the C-MWCNTs (0.5 wt%)/EP nanocomposites. The nanocomposites containing

SiO2@MWCNTs and O-MMT showed excellent comprehensive properties, partly because of the synergistic effect of 1D nano-structure and 2D nanostructure, as well as the inhibitory effect of charge transport by SiO2 insulating layer and the O-MMT with 2D layer nanostructure embed in CNTs conductive net-work. For the nanocomposites filled with SiO2@MWCNTs, a steep decrease in dielectric constant and loss was observed. The decreased dielectric constant of the EP composites at low filler loadings was attributed to the restriction effect of inter-face region on the dipoles orientation. The reduction of the dielectric loss in nanocomposites systems at low nanoparticles loadings could be attributed to the reduction of their charge transportation.

AcknowledgementsThis work was supported by the National Natural Science Foundation of China (Grant Nos. 51677045, 51577045, and 21604019), China Postdoctoral Science Foundation Funded Project (Grant Nos. 2017M610212 and 2016M601402), and the Heilongjiang Postdoctoral Fund (Grant Nos. LBH-Z16089 and LBH-Z16004).

Conflict of InterestThe authors declare no conflict of interest.

Keywordscarbon nanotubes, electrical insulation, epoxy nanocomposites, mechanical property, montmorillonite

Received: July 13, 2017Revised: August 25, 2017

Published online: October 11, 2017

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