Carbon Nanotube

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Article pubs.acs.org/IECR

Low-Cost Preparation of High‑k Expanded Graphite/Carbon Nanotube/Cyanate Ester Composites with Low Dielectric Loss and Low Percolation Threshold Lei Cao, Wei Zhang, Xinhua Zhang, Li Yuan, Guozheng Liang,* and Aijuan Gu* Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Materials Science and Engineering College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, China S Supporting Information *

ABSTRACT: Expanded graphites (EGs) or modified EGs (eEGs) were blended with multiwalled carbon nanotubes (MWCNTs) in a big weight ratio to prepare low-cost composites, coded as (EG-MWCNT)/cyanate ester (CE) or (eEGMWCNT)/CE. The structures and properties of the composites are closely related to the loading of total conductors ( f). When f < 0.75 wt %, EGs are beneficial for preparing high-k composites with low dielectric loss; however, when f ≥ 1.05 wt %, eEGs have superior advantages. When f = 1.5 wt %, the dielectric constant and loss of (eEG-MWCNT)1.5/CE composite are about 1.6 and 0.6 times that of the (EG-MWCNT)1.5/CE composite, respectively. The origin behind these interesting results was intensively discussed. Attractively, (EG-MWCNT)/CE and (eEG-MWCNT)/CE composites have similar percolation thresholds, so the surface modification of EGs does not increase the percolation threshold; moreover, ternary composites prepared herein have much better dielectric properties than both traditional EG/CE and MWCNT/CE composites. of filler such as clay,13 BaCO3,14 and graphite nanosheets,15 etc.; however, only a few studies in the literature focused on preparing high-k composites, and the resultant composites have very high dielectric loss. In 2012, our group added expanded graphites (EGs) into multiwall carbon nanotube (MWCNT)/cyanate ester (CE) composites with a fixed MWCNT concentration and found that the presence of a small loading (0.08 wt %) of EGs can significantly increase the dielectric constant and reduce the dielectric loss of the composites.16 These attractive results suggest a valuable possibility for preparing high-k materials with low dielectric loss. However, when more EGs were added into the MWCNT/CE composites, the synergetic effect of EGs and MWCNTs gradually decreased and even disappeared. This unexpected phenomenon demonstrates that it is necessary to find the condition for guaranteeing the appearance of the synergetic effect, and to reveal the nature behind it; otherwise it is not possible to prepare expected composites for actual applications. Unfortunately, to date, no literature reported the corresponding issues, no matter whether the aim of using hybrid fillers focused on the dielectric properties or not. On the other hand, “lower cost” and “higher performance” have been the eternal pursuit of people when they develop any new materials. EG is much cheaper than CNT, but EG does not have as good dielectric properties as those of CNT.16 Therefore, how to cheaply prepare composite with more EGs without degrading the dielectric property is of great importance, especially for actual applications.

1. INTRODUCTION High-k (high dielectric constant) polymeric composites have attracted increasing attentions of scientists and engineers worldwide because they become the base for fabricating highperformance products in many cutting-edge fields. For example, they can be used as high energy density capacitors,1 stress cone material for high-voltage cables of electric field,2 embedded microcapacitors,3 and artificial muscles and smart coat material for drug release,4 etc. The electric conductor/polymer composite is the main sort of high-k composites; its main advantage is that the composite with a small loading of dielectric conductors can get a high dielectric constant, so the composite has good processing characteristics.5−7 However, generally, high dielectric loss is the big disadvantage of this kind of composite. Previous researches have concluded that the sort of electric conductor and the good dispersion of the conductors in the polymer are the two main factors that determine the properties and cost of the composites. The carbon nanotube (CNT) is considered as a revolutionary conductor owing to its unique structure and outstanding integrated properties;8−10 however, original CNTs have poor dispersion in a polymer, and tend to form bundles because of very strong intertubular van der Waals attractions.11,12 Although chemical modification can effectively overcome the problem, this is not suitable for preparing highk materials because the chemical modification will destroy the structure, thus declining the outstanding conductivity of CNTs; moreover, this usually increases the percolation threshold, meaning that a larger loading of conductors is required to get a high dielectric constant, and, obviously, this is not good for actual applications. Recently, some scholars found that the dispersion of CNTs in the polymer matrix can be improved by adding another kind © 2014 American Chemical Society

Received: Revised: Accepted: Published: 2661

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Table 1. Formulations of (EG-MWCNT)/CE and (eEG-MWCNT)/CE Composites composite (EG-MWCNT)0.15/CE (EG-MWCNT)0.45/CE (EG-MWCNT)0.75/CE (EG-MWCNT)1.05/CE (EG-MWCNT)1.50/CE

or or or or or

EG or eEG (g)

MWCNT (g)

CE (g)

E-51 (g)

0.015 0.045 0.075 0.105 0.150

0.03 0.09 0.15 0.21 0.30

30 30 30 30 30

0.3 0.3 0.3 0.3 0.3

(eEG-MWCNT)0.15/CE (eEG-MWCNT)0.45/CE (eEG-MWCNT)0.75/CE (eEG-MWCNT)1.05/CE (eEG-MWCNT)1.50/CE

mixture, which was then maintained at 150 °C for 12 h with continuous stirring to get a prepolymer. After that, the prepolymer was degassed to remove entrapped air at 130 °C for 1 h in a vacuum oven, followed by curing and postcuring using a procedure of 180 °C/(2 h) + 200 °C/(2 h) + 220 °C/ (2 h) + 240 °C/(4 h). Finally the cured composite was demolded and coded as (EG-MWCNT)m/CE or (eEGMWCNT)m/CE, where m was the weight fraction of the total amount of fillers in the CE resin. 2.6. Characterization. A scanning electron microscope (SEM, Hitachi S-4700, Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS) was employed to observe the morphologies of EG, eEG, and the composites. The samples were sputter coated with a thin layer (about 10 nm) of gold. All samples should be dried at 100 °C for 6 h before tests. Thermogravimetric (TG) analyses were performed on TA Instruments SDTQ600 (New Castle, DE, USA) from 25 to 800 °C under a nitrogen atmosphere with a flow rate of 100 mL/ min and a heating rate of 10 °C/min. Fourier transform infrared (FTIR) spectra were recorded between 750 and 3750 cm−1 with a resolution of 2 cm−1 on a Pro-star LC240 infrared spectrometer (Varian Medical Systems, Inc., Palo Alto, CA, USA). X-ray diffraction (XRD) analyses were carried out on a MERCURY CCD X-ray diffractometer (Rigaku, Tokyo, Japan) with Cu Kα radiation. The 2θ angle ranged from 10 to 60°, and the scan rate was 2°/min. Raman spectra were obtained using an Almega dispersive Raman spectrometer (Thermo Nicolet, Madison, WI, USA) with an Ar+ laser (514.5 nm) at room temperature. Differential scanning calorimeter (DSC) measurements were performed with a DSC 2010 instrument (TA Instruments) ranging from room temperature to 320 °C at a heating rate of 10 °C/min under a nitrogen atmosphere. Dynamic mechanical analysis (DMA) scans were performed with a single-cantilever blending mode using a dynamic mechanical analyzer TA Q800 (TA Instruments) from 30 to 350 °C at a heating rate of 5 °C/min and a frequency of 1 Hz. The dimensions of each sample were (35 ± 0.02) × (13 ± 0.02) × (3 ± 0.02) mm3. Flexural properties were measured using a universal tester according to Chinese Standard GB/T2570-2008. For each property of a system, at least five samples were tested, and the average value was taken as the tested value. The electric resistances of EG-MWCNT and eEG-MWCNT powders were measured from 10−1 to 107 Hz using Zahner elektrik IM6ex (Kronach, Germany). The electric and dielectric properties, impedance magnitude, and the impedance phase of composites were measured using broadband dielectric spectroscopy (Novocontrol Concept 80, Hundsangen, Germany) at room temperature over a frequency range from 102 to 107 Hz.

Based on our previous research and the aim for developing composites with “low cost” and “high performance”, a series of (EG-MWCNT)/CE composites with a big ratio (0.5:1) of EG to MWCNT were prepared in this work. Meanwhile, the epoxy resin modified EG (eEG) was also prepared to fabricate another kind of ternary composites, (eEG-MWCNT)/CE, for investigating the surface nature of EGs on the structures (chemical and phase structures) and the dielectric properties of the composites. Some interesting results on the synergetic effect were found, and the nature behind them was discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. The CE used was 2,2′-bis(4-cyanatophenyl)isopropylidene, which was bought from Jiangdu Wuqiao Resin Plant, Yangzhou, China. Diglycidyl ether of bisphenol A type epoxy resin (E-51) with an epoxide equivalent weight of 196 g/ mol was purchased from Wuxi Resin Plant, Wuxi, China. MWCNTs (the average diameter is about 8−15 nm, and the length is about 50 μm) were obtained from the Chinese Academy of Sciences in Chengdu. Expandable graphite with a mesh of 80 was bought from Baixing Graphite Co., Ltd., Qingdao, China. 4,4′-Dimethylformamide, triphenylphosphine, and dichloromethane were commercial products with analytical grades and used as received. 2.2. Preparation of EGs. Appropriate quantities of expandable graphite were put into a muffle furnace (900 °C) and maintained for 30 s to generate expanded graphite particles, which were then separated into dispersed sheets under sonication in an acetone bath followed by drying. The resultant product was coded as EG. 2.3. Surface Modification of EG. EG (3 g) and 4,4′dimethylformamide (2500 mL) were blended in a three-necked flask; the flask was sonicated at room temperature for 0.5 h. And then a solution consisting of E-51 (60 g), 4,4′dimethylformamide (100 mL), and triphenylphosphine (0.3 g) was added into the flask. Subsequently, the mixture was heated to 80 °C and maintained at that temperature with stirring for 48 h. After that an excess amount of dichloromethane was added into the mixture for filtration. The filter cake was dried in a vacuum oven to get surface-treated EG, coded as eEG. 2.4. Preparation of Cured CE Resin. Appropriate amounts of CE and E-51 with a weight ratio of 100:1 were thoroughly blended at 150 °C for 120 min to get a prepolymer. The prepolymer was degassed to remove entrapped air at 130 °C for 1 h in a vacuum oven, followed by curing and postcuring using a procedure of 180 °C/(2 h) + 200 °C/(2 h) + 220 °C/ (2 h) + 240 °C/(4 h). The resultant cured resin is CE resin. 2.5. Preparation of Ternary Composites. According to Table 1, appropriate amounts of CE and E-51 with a weight ratio of 100:1 were thoroughly blended at 150 °C for 10 min, and then preweighed MWCNTs were added into the blend with stirring for 10 min to get a homogeneous mixture. An appropriate quantity of EGs (or eEGs) was added into the 2662

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Figure 1. Schematic routing for preparing eEG.

Figure 2. SEM images of EG and eEG.

3. RESULTS AND DISCUSSION 3.1. Structure of Surface Modified EG. CE can react with compounds containing active hydrogen.17 In order to improve the dispersion of EGs in CE resin, epoxy resin was used to modify EGs through the mechanism shown in Figure 1. Figure 2 gives the SEM photographs of EGs and eEGs. There are many spirals or interlocking grains on the surface of EG; while no grains can be observed on the surface of eEG; instead, eEG seems to be covered by a “coating”. Following the method reported in the literature,12 the grafting degree of eEGs was calculated to be 4.8 wt % through the TG curves of EG and eEG (Figure S1 in the Supporting Information). The element contents measured using EDS technique (Table 2) show that the ratio of C element to O element for eEG is significantly

Table 2. Elemental Compositions of EG and eEG from EDS Analyses elemental composition (wt %) sample

C

O

EG eEG

96.37 88.36

3.63 11.64

lower than the corresponding value of EG, indicating that E-51 resin has reacted with the carboxyl groups on the sheets of EG. The preceding statement is confirmed by the FTIR spectra shown in Figure 3. Specifically, multipeaks (2859−2920 cm−1), assigned to the stretching vibration of the C−H group,18 appears in the FTIR spectra of both eEGs and E-51 epoxy resin, 2663

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band at ca. 1326 cm−1, the G band at ca. 1579 cm−1, the D* band at ca. 2683 cm−1, and the E′ 2g band.20 eEG has a Raman spectrum similar to that of EG; however, the intensity ratio of D band to G band (ID/IG) in the spectrum of eEG is slightly greater than the corresponding value of EG, indicating a slight increase of the disorder degree of eEG. 3.2. Effect of the Surface Nature of Expanded Graphite on the Structure of Composites. 3.2.1. Morphological Structure. The dispersion of the inorganic phase in the organic resin matrix and the interfacial adhesion between two phases are known to be the key factors that affect the overall performances of the composite. Figure 6 shows the SEM pictures of (EG-MWCNT)/CE and (eEG-MWCNT)/CE composites. It can be seen that EGs aggregate in the CE resin, and the phenomenon becomes obvious as the content of EGs increases. Although hydroxyl and carboxyl groups exist on the surfaces of EGs, the concentration of these groups is low, so this cannot guarantee a good dispersion of EGs in the CE resin when the loading of EGs in the composites is high. The preceding disadvantage does not appear in the composites based on eEGs owing to the existence of active groups on the surfaces of eEGs. Specifically, these active groups not only provide the good dispersion of eEGs in the CE resin but also increase the curing reactivity at low temperature and thus can fix the good dispersion state of eEGs in the CE resin during the curing process. Besides SEM micrographs, more techniques (XRD and Raman spectra) were utilized to give additional strong evidence about the dispersion of fillers. Figure 7 shows XRD patterns of the basic components and the ternary composites. A clear diffraction peak at ca. 26° emerges in the XRD pattern of EGs, eEGs, or MWCNTs, which is the characteristic diffraction peak of the graphite crystal structure for EGs and eEGs19 and also represents the ordered arrangement of the concentric cylinders of graphitic carbon in the MWCNTs.21 In the case of the composites, the appearance of the characteristic peak is closely related to the total loading of the conductors in the composite ( f). Specifically, the peak is observed only when f ≥ 0.75 wt %, and which enhances as the f value increases. On the other hand, the intensity of the peak is dependent on the surface nature of the expanded graphite and the f value. When f < 1.5 wt %, the peak intensities of all composites are the same; however, when f = 1.5 wt %, the intensity of the characteristic peak for (EGMWCNT)1.50/CE composite is significantly higher than that of (eEG-MWCNT)1.50/CE composite. According to previous research,22,23 these results suggest that when the loading of conductors is high, the surface modification of expanded graphite is beneficial to get a good dispersion of fillers in the composites, while this superiority is not obvious when the f value is low. Figure 8 shows the Raman spectra of the basic components and their composites. All composites have similar spectra as CE resin. The peak referring to the G band for MWCNTs, EGs, or eEGs appears at 157724 or 1579 cm−1, respectively;20 while the G band peak is not obviously observed in the composites, maybe resulting from the facts that the peak nears the characteristic peak (at 1607 cm−1) of the CE resin, and the loading of conductors is not large enough. However, with enlarged images, the broad peak from 1550 to 1650 cm−1 can be divided into two peaks: the first one is the characteristic peak at 1607 cm−1 of CE resin, and the second one is the peak at 1588 cm−1 ((EG-MWCNT)1.05/CE) or 1592 cm−1 ((eEGMWCNT)1.05/CE) as shown in Figure 9. Compared with the

Figure 3. The FTIR spectra of EG, eEG, and E-51 epoxy resin.

but no similar peak is observed in the spectrum of EG, clearly demonstrating that epoxy resin molecules have been chemically linked on the surfaces of EGs. In order to evaluate whether the surface modification has an impact on the structure and crystallization of EG, XRD and Raman spectra were recorded. Figure 4 shows the XRD

Figure 4. XRD patterns of EG and eEG.

patterns of EG and eEG. Each spectrum clearly shows the characteristic diffraction peak (2θ = 26.43°) of the crystal structure of graphite,19 indicating that the surface modification of EG does not change the crystalline form of EG. Figure 5 shows the Raman spectra of EG and eEG. The spectrum of EG shows four main Raman signals; they are the D

Figure 5. Raman spectra of EG and eEG. 2664

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Figure 6. SEM images of (EG-MWCNT)/CE and (eEG-MWCNT)/CE composites.

Figure 9. Enlarged Raman spectra for (EG-MWCNT)1.05/CE and (eEG-MWCNT)1.05/CE composites.

Figure 7. XRD patterns of basic components and their composites.

disentanglement of MWCNTs in the resin matrix,23,25−27 especially for the MWCNTs in the (eEG-MWCNT)1.05/CE composite. In other words, MWCNTs have better dispersion in the (eEG-MWCNT)1.05/CE composite than those in the (EG-MWCNT)1.05/CE composite. 3.2.2. Chemical Structure and Cross-Linking Density. It is well-known that the performance of thermosetting resin is primarily dependent on its chemical structure and the crosslinking density, while the latter is determined by the curing behavior and mechanism of the resin. Therefore it is necessary to study the curing behaviors and mechanisms of (EGMWCNT)/CE and (eEG-MWCNT)/CE composites. Figure 10 gives the DSC curves of CE, (EG-MWCNT)1.05/ CE, and (eEG-MWCNT)1.05/CE prepolymers. The whole exothermal peak of (EG-MWCNT)1.05/CE or (eEGMWCNT)1.05/CE prepolymer appears at about 31 or 44 °C lower temperature than that of the CE prepolymer, indicating that the curing mechanism of CE has changed due to the

Figure 8. Raman spectra of basic components and their composites.

G band peak of MWCNTs at 1577 cm−1, the second peak respectively up-shifts 11 or 15 cm−1, demonstrating the 2665

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catalyze the homopolymerization of CE but also coreact with the -OCN groups of CE. The former tends to increase the ρ value, while the latter will decrease the ρ value. From the data shown in Figure 11, it is known that the former aspect plays the dominant role. Figure 11 also shows that the ρ value of the composite is closely related to the surface nature of the expanded graphite; (eEG-MWCNT)/CE composite has a lower ρ value than (EGMWCNT)/CE with the same loading of conductors. The good dispersion of eEGs is responsible for this phenomenon. Specifically, graphite sheets of eEG are easier to be peeled off and dispersed in the resin, meaning that more active groups of eEG contact with CE resin, so more copolymers will be produced in the (eEG-MWCNT)/CE composite, and the (eEG-MWCNT)/CE composite has lower ρ value than the (EG-MWCNT)/CE composite. 3.3. Effect of the Surface Nature of Expanded Graphite on the Flexural Properties of the Ternary Composites. Flexural property is a typical sort of mechanical property and usually used for evaluating the integrated mechanical properties of a material because the flexural loading is very complicated and may contain multitype loadings such as tensile, shearing, and/or compressing loading.33,34 Figure 12 gives flexural strengths and moduli of (EGMWCNT)/CE and (eEG-MWCNT)/CE composites. As the f

Figure 10. DSC curves of CE, (EG-MWCNT)1.05/CE, and (eEGMWCNT)1.05/CE prepolymers.

addition of fillers. It is known that CE can be cured through thermal cyclotrimerization to form a triazine ring network,28 and the cyclotrimerization of -OCN groups can be catalyzed with active hydrogens.29,30 Since there is a small amount of -OH groups on the surfaces of EGs, so the (EG-MWCNT)1.05/CE prepolymer has a lower curing temperature than the CE prepolymer. In the case of (eEG-MWCNT)1.05/CE prepolymer, the epoxy groups on the surfaces of eEGs can coreact with -OCN groups of CE at the temperature lower than the cyclotrimerization of -OCN groups,31 and thus significantly reduce the curing temperature. Cross-linking density (ρ) is also an important parameter of the network structure of a thermosetting resin, which can be calculated according to the classic semiempirical equation32 ρ=

G′ 3ϕRT

(1)

where G′ is the storage modulus of the material at the temperature (T) that is 20 °C higher than the glass transition temperature from DMA tests; Φ is the preexponential factor, which is assumed to be 1; R is the gas constant. As shown in Figure 11, each composite has a higher ρ value than CE resin, and the ρ value of the composite gradually increases as the content of conductors increases. These results are mainly attributed to both positive and negative impacts of the conductors on the curing of CE. As previously discussed, the active groups on the surfaces of EGs and eEGs will not only

Figure 12. Flexural strengths and moduli of (EG-MWCNT)/CE and (eEG-MWCNT)/CE composites.

value increases, the flexural strength or modulus of either kind of composite increases until it reaches the maximum value and then decreases. This trend is reasonable because EG and MWCNT have been proved to be effective reinforcements,35,36 so more loadings of fillers are beneficial to get higher flexural properties. However, on the other hand, the reinforcing effect is

Figure 11. Cross-linking densities of (EG-MWCNT)/CE and (eEGMWCNT)/CE composites. 2666

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closely related to the dispersion of inorganic fillers in the organic matrix;37 that is, only when these fillers have good dispersion, a high reinforcement appears, so the flexural property reaches the highest value at some loading of fillers. For the composites prepared herein, when f is 0.75 or 1.05 wt %, the corresponding (EG-MWCNT)/CE or (eEGMWCNT)/CE composite exhibits the highest flexural properties, reflecting that the two composites have different dispersions of fillers induced by the different surface nature between EG and eEGs. Based on the data in Figure 12, it is known that when f is not large (for example, 0.75 wt %), EG-MWCNT fillers have good dispersion in CE resin owing to the physical interaction between EG and MWCNT;16 however, when the content of EG-MWCNT fillers is large (larger than 0.75 wt %), the physical interaction could not guarantee the good dispersion of these fillers, so the flexural property of (EG-MWCNT)/CE composites obviously decreases. While, in this condition, the surface modification of EG is beneficial for obtaining good dispersion of fillers, the flexural property of (eEG-MWCNT)/ CE composites continuously increases until the loading of fillers reaches 1.05 wt %. 3.4. Effect of the Surface Nature of Expanded Graphite on the Conductivities of the Ternary Composites. Figure 13 provides the AC conductivity−frequency

Figure 14. Plots of AC conductivity as a function of frequency for EGMWCNT and eEG-MWCNT powders.

However, when the loading of EGs increases to some value, the limited amount of -OH groups on EGs cannot endow EGs with good dispersion in CE resin any longer, meaning that EGs lose their effect of assisting the dispersion of MWCNTs, and consequently, MWCNTs have poor dispersion in the (EGMWCNT)/CE composites. Comparatively, eEGs still have good dispersion in the CE resin, so the assisting effect on dispersing MWCNTs is still maintained. Therefore, although EGs have better conductivity than eEGs, the good dispersion of conductors and more conductive paths endow the (eEGMWCNT)/CE composites with similar or even higher conductivities than (EG-MWCNT)/CE composites. Figures 15 and 16 show the electrical conductivities (at 100 Hz, room temperature) of (EG-MWCNT)/CE and (eEG-

Figure 13. Plots of AC conductivity as a function of frequency for (EG-MWCNT)/CE and (eEG-MWCNT)/CE composites.

curves of CE resin and two kinds of ternary composites. The conductivity of each kind of composite increases as the f value increases. When f ≤ 0.75 wt %, the conductivity of the (eEGMWCNT)/CE composite is lower than that of the (EGMWCNT)/CE composite with the same loading of conductors at the same frequency; when f ≥ 0.75 wt %, the former is close to or higher than the latter. This phenomenon is attributed to the number of conductive paths. Note that the number of the conductive paths is closely related to the dispersion of conductors in the resin. When the content of expanded graphite is low, EGs and eEGs have similar dispersion in the CE resin, so the conductivities of the corresponding ternary composites mainly depend on the conductivity of the conductors. Because the presence of epoxy resin on the surfaces of eEGs will separate the progress of current, the eEG-MWCNT hybrid fillers have lower conductivity than the EG-MWCNT hybrid fillers (Figure 14), and consequently, the conductivity of (eEG-MWCNT)/CE composites is lower than that of (EG-MWCNT)/CE composites when the f value is low.

Figure 15. Dependence of AC conductivity on the mass concentration of conductors for (EG-MWCNT)/CE composites at 100 Hz.

MWCNT)/CE composites, respectively. According to eq 2 based on the percolation theory,38 the corresponding curves of log σ versus log (fc − f) are obtained as shown in the insert plots of Figures 15 and 16. σ ∝ (fc − f )−s

for fc > f

(2)

where σ is the electrical conductivity, fc is the percolation threshold, and s is the critical parameters of conductivity. From these insert plots, the fc values of (EG-MWCNT)/CE and (eEG-MWCNT)/CE composites are respectively found to be 1.45 and 1.50 wt %. This result is attractive because the surface modification (grafting or coating) of conductors generally increases the percolation threshold of the composites 2667

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the insulating epoxy resin, so the effective area of the two conductive plates is reduced, and consequently, the corresponding capacitance between two graphite sheets of eEG decreases according to eq 3, and the (eEG-MWCNT)/CE composites have lower dielectric constants than the (EGMWCNT)/CE composites. C=

εS 4πkd

(3)

where ε is the dielectric constant between the two polar plates, S is the plate area, k is the electrostatic constant and d is the distance between the two plates. When f ≥ 0.75 wt %, compared with (EG-MWCNT)/CE composites, the good dispersion of eEGs and MWCNTs endows the (eEG-MWCNT)/CE composites with more microcapacitors, and thus higher dielectric constants. For example, the dielectric constants of the (eEG-MWCNT)1.5/ CE and (EG-MWCNT) 1.5/CE composites are 525 and 340, respectively. Figure 18 is the curves of dielectric loss versus frequency of (EG-MWCNT)/CE and (eEG-MWCNT)/CE composites at

Figure 16. Dependence of AC conductivity on the mass concentration of conductors for (eEG-MWCNT)/CE composites at 100 Hz.

because the surface modification improves the dispersion of conductors, and thus more conductors are needed to achieve the percolation.39,1 The ternary composites prepared herein do not follow the trend, so there is another factor (or synergistic effect) that is responsible for the formation of the conductive paths. 3.5. Effect of the Surface Nature of Expanded Graphite on the Dielectric Properties of the Ternary Composites. Figure 17 gives the plots of dielectric permittivity

Figure 18. Dielectric loss as a function of frequency for (EGMWCNT)/CE and (eEG-MWCNT)/CE composites.

room temperature. With the increasing of the content of conductors, the dielectric loss of the composites increases, and its dependence of the frequency is enhanced. Similar to that of dielectric constant, the order of the dielectric loss between the two kinds of composites is also closely related to the f value. When the f value is low, the (eEG-MWCNT)/CE composite has lower dielectric loss than the (EG-MWCNT)/CE composite with the same loading of conductors. This is because the insulating polymer on the surfaces of eEGs reduces the conductivity and partially hinders the generation of leakage current in (eEG-MWCNT)/CE composites. When f > fc, as the f value increases, the dielectric loss of (eEG-MWCNT)/CE composites is lower than that of (EGMWCNT)/CE composites. Interestingly, the dielectric losses of the (eEG-MWCNT)1.5/CE and (EG-MWCNT)1.5/CE composites are 12, and 18, respectively. This phenomenon results from following two opposite aspects. First, owing to the good dispersion of conductors, (eEG-MWCNT)/CE composites have more conductive paths than (EG-MWCNT)/CE composites, so the former tends to have larger dielectric loss induced by the leakage current. Second, the presence of the insulating resin on the graphite sheets of eEGs also tends to hinder the generation of leakage current, and thus reduce the dielectric loss.

Figure 17. Dielectric constant as a function of frequency for (EGMWCNT)/CE and (eEG-MWCNT)/CE composites.

versus the frequency of (EG-MWCNT)/CE and (eEGMWCNT)/CE composites at room temperature. When f < fc, as the f value increases, the number of formed microcapacitors increases, resulting in increased dielectric constant. On the other hand, the order of the dielectric constants of the two composites is closely related to the f value. When f ≤ 0.75 wt %, the dielectric constant of (eEG-MWCNT)/CE composite is slightly lower than that of (EG-MWCNT)/CE composite with the same loading of conductors, while the former is higher than the latter when f ≥ 0.75 wt %. This phenomenon is originated from the difference in the dispersion of conductors. Briefly, when f is small, the surface modification of EGs does not bring an obviously different morphological structure of the composite, meaning that either EGs or eEGs will form a similar amount of microcapacitors in the corresponding composites. However, the capacitance (C) of the microcapacitors based on eEGs is lower than that based on EGs. This is because graphite sheets of eEG are wrapped with 2668

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Figure 19. Equivalent circuit of (EG-MWCNT)/CE and (eEG-MWCNT)/CE composites.

Figure 20. Plots of complex impedance plane (hollow circle) and the simulation results (solid line) for (EG-MWCNT)/CE and (eEG-MWCNT)/ CE composites.

Table 3. Parameters from the Simulating Results for (EG-MWCNT)/CE Composites (EG-MWCNT)0.15/CE L (H) CPE 1(F) n1 R1 (Ω) CPE2 (F) n2 R2 (Ω) CPE3 (F) n3 R3 (Ω) Rt (Ω) CPEt (F)

1.01 × 3.06 × 0.9426 1.84 × 1.40 × 0.9272 1.00 × 4.96 × 0.9341 5.63 × 7.56 × 9.42 ×

−5

10 10−11 109 10−11 108 10−11 109 109 10−11

(EG-MWCNT)0.45/CE 7.03 × 1.80 × 0.9566 5.03 × 3.12 × 0.9453 7.86 × 1.10 × 0.9301 9.47 × 2.24 × 1.59 ×

(EG-MWCNT)0.75/CE

−5

8.59 × 1.10 × 0.9374 3.22 × 1.84 × 0.8590 1.95 × 8.02 × 0.8941 7.30 × 1.25 × 3.75 ×

10 10−11 108 10−11 108 10−10 108 109 10−10

Comparing the ternary composite prepared herein with binary composites (EG/CE and MWCNT/CE), it can be

−5

10 10−10 108 10−10 108 10−11 108 109 10−10

(EG-MWCNT)1.05/CE 6.57 × 1.30 × 0.9101 2.29 × 1.27 × 0.8767 2.38 × 5.08 × 0.9915 1.78 × 4.85 × 7.65 ×

−5

10 10−10 108 10−10 108 10−10 107 108 10−10

(EG-MWCNT)1.5/CE 1.42 × 1.79 × 0.8290 1.43 × 1.93 × 0.7744 2182 9.58 × 0.8426 2.49 × 3.94 × 2.08 ×

10−4 10−8 105 10−9

10−10 105 105 10−8

found that when the loading of EGs (or MWCNTs) is 4.98 wt % (or 0.7 wt %), the EG/CE (or MWCNT/CE) composite 2669

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Table 4. Parameters from the Simulating Results for (eEG-MWCNT)/CE Composites (eEG-MWCNT)0.15/CE L (H) CPE1 (F) n1 R1 (Ω) CPE2 (F) n2 R2 (Ω) CPE3 (F) n3 R3 (Ω) Rt (Ω) CPEt (F)

2.05 × 1.01 × 0.5233 9.99 × 1.63 × 0.9727 3.37 × 5.04 × 0.9907 1.27 × 1.16 × 2.24 ×

10−6 10−12 109 10−11 108 10−12 109 1010 10−11

(eEG-MWCNT) 0.45/CE 4.61 × 3.10 × 0.9642 4.53 × 1.19 × 0.9714 5.76 × 7.62 × 0.8946 1.00 × 6.11 × 5.05 ×

(eEG-MWCNT) 0.75/CE

10−6 10−11 109 10−11 108 10−12 109 109 10−11

9.87 × 1.45 × 0.8273 1.00 × 3.29 × 0.9183 1.11 × 1.66 × 0.8740 5.18 × 5.39 × 3.44 ×

10−6 10−10 108 10−11 108 10−10 109 109 10−10

(eEG-MWCNT) 1.05/CE 9.79 × 1.02 × 0.8860 2.09 × 1.80 × 0.8196 1.00 × 2.98 × 0.8694 1.50 × 2.52 × 1.50 ×

10−6 10−9 105 10−10 107 10−10 107 107 10−9

(eEG-MWCNT) 1.5/CE 1.00 × 1.94 × 0.9682 1.29 × 1.97 × 0.7855 2605 5.70 × 0.9581 1.22 × 2.54 × 1.95 ×

10−5 10−7 105 10−10

10−10 105 105 10−7

either dispersion of expanded graphites or the synergetic effect between expanded graphites and MWCNTs. When f ≥ 0.75 wt %, the magnitude orders of CPEt and Rt between the (eEG-MWCNT)/CE and (EG-MWCNT)/CE composites with the same f value show different trends. This is because, for a large loading of conductors, EGs and eEGs have obviously different dispersion as previously discussed. Briefly, when the content of EGs increases, the agglomeration of EGs becomes more obvious and its effect on assisting the dispersion of MWCNTs weakens, so the agglomeration of MWCNTs in the (EG-MWCNT)/CE composites is serious. For (eEGMWCNT)/CE composites, the dispersion of eEGs in CE resin remains good, and eEGs can effectively assist the dispersion of MWCNTs, leading to the formation of more microcapacitors and thus higher dielectric constant. These trends are consistent with the regular pattern of the dielectric constant and conductivity of the two composites.

gets its highest dielectric constant (120 or 240); meanwhile the dielectric loss is 110 or 18.16 Obviously, the ternary composites prepared herein have much better dielectric properties than both EG/CE and MWCNT/CE composites, especially (eEGMWCNT)1.5/CE shows superior advantages in much higher dielectric constant and lower dielectric loss as well as low cost. 3.6. Equivalent Circuits. To reveal the relationship between the microstructure and properties of (EGMWCNT)/CE and (eEG-MWCNT)/CE composites, AC impedance spectroscopies were recorded and simulated. Results show that the equivalent circuit shown in Figure 19 can simulate the actual impedance spectra of composites very well (Figure 20). Due to similar compositions, the two kinds of composites can be simulated using a similar equivalent circuit that consists of an inductance (L) and three parallel circuits in series. The CE resin is an insulating material, so resistors R1, R2, and R3, exist in the equivalent circuit. The larger aspect ratio of MWCNTs makes them easy to curl in composites, and thus form coils, which can be represented by an inductance. In addition, the conductors that are very close but not connected in composites will produce microcapacitors, so capacitors should be a part of the equivalent circuit; however, to obtain a better fitting result, the capacitive element is replaced by the constant phase element (CPE1, CPE2, or CPE3). Although the equivalent circuits of the two composites have the same form, their analog values of the respective elements are different (Tables 3 and 4), confirming that the different surface nature between EG and eEG produces different effects on the dielectric performance of the composites. Note that the sum of the capacitance (coded as CPEt), which is directly proportional to the dielectric constant of composites, and the sum of resistances (designed as Rt) reflecting the total resistance of the circuit are also summarized. With the same loading of conductors, the inductances of (eEG-MWCNT)/CE composites are smaller than those of (EG-MWCNT)/CE composites, indicating that the curl degree of MWCNTs in (eEG-MWCNT)/CE composites is smaller than that in the (EG-MWCNT)/CE composites; in other words, eEGs and MWCNTs have a bigger synergetic effect that makes MWCNTs have better dispersion and bigger difficulty of curling. When f ≤ 0.75 wt %, compared with (EG-MWCNT)/CE composite, the (eEG-MWCNT)/CE composite with the same f value has smaller CPEt and bigger Rt values. These results are expected because, under this condition, the presence of epoxy resin on the surface of eEGs decreases the conductivities of expanded graphites, but does not bring an obvious difference in

4. CONCLUSION When f ≤ 0.75 wt %, eEGs show no advantage in improving the dispersion of MWCNTs compared with EGs, and the surface modification of EGs is not beneficial to prepare a ternary composite with high dielectric constant and low dielectric loss. When f ≥ 1.05 wt %, eEGs have obvious merits than EGs because EGs do not have good dispersion any longer, and the agglomerated EGs make EGs difficult to assist the dispersion of MWCNTs. Compared with the (EG-MWCNT)/CE composites, the (eEG-MWCNT)/CE composites have superior dielectric properties, reflected by much higher dielectric constant and lower dielectric loss.

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ASSOCIATED CONTENT

S Supporting Information *

TG curves of EG and eEG under a nitrogen atmosphere (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(A.G.) Tel.: +86 512 65880967. Fax: +86 512 65880089. Email: [email protected]. *(G.L.) Tel.: +86 512 65880967. Fax: +86 512 65880089. Email: [email protected]. Notes

The authors declare no competing financial interest. 2670

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ACKNOWLEDGMENTS We thank the Natural Science Foundation of China (Grant 51173123), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Major Program of Natural Science Fundamental Research Project of Jiangsu Colleges and Universities (Grant 11KJA430001), Suzhou Applied Basic Research Program (Grant SYG201141), and the Scientific Innovation Research of College Graduate in Jiangsu Province (Grant CXZZ12_0814) for financially supporting this project.

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