Controlled Dielectric Properties of Polymer Composites from Coating

Article pubs.acs.org/IECR

Controlled Dielectric Properties of Polymer Composites from Coating Multiwalled Carbon Nanotubes with Octa-acrylate Silsesquioxane through Diels−Alder Cycloaddition and Atom Transfer Radical Polymerization Wenjing Zhang,† Zheng Zhou,‡ Qifang Li,*,‡ and Guang-Xin Chen*,† †

Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, P. R. China ‡ College of Material Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China ABSTRACT: A synthetic strategy of coating multiwalled carbon nanotubes (MWCNTs) with cross-linkable octa-acrylate polyhedral oligomeric silsesquioxane (POSS) by combining Diels−Alder cycloadditions with atom transfer radical polymerization in a controlled manner is reported. First, the furfuryl-2-bromoisobutyrate is synthesized by an esterification reaction from 2-bromoisobutyryl bromide and furfuryl alcohol. Then, the MWCNT-based initiators of atom transfer radical polymerization (ATRP) are synthesized through a Diels−Alder reaction between MWCNTs and furfuryl-2-bromoisobutyrate with controlled grafted content. The MWCNT initiators are used to initiate the ATRP reaction of the octa-acrylate POSS on the MWCNT surface. Finally, a core−shell structure with a MWCNT at the center is obtained, with the thickness of the POSS shell adjusted through different initiators from 5, 10, to 15 nm. The POSS-coated MWCNT is compounded with polyvinylidene fluoride to obtain a kind of composite which has both high dielectric permittivity and low dielectric loss.

1. INTRODUCTION With the rapid development of electronic information and electric and power industries, the preparation of low-cost polymer composites with a high dielectric constant (high-k) and low dielectric loss has been a hot and important topic.1−4 High-k composites can be divided into two types: one is ceramic particle/polymer composite, and the other is electric conductor/polymer composite.5 Compared with the former, the latter can not only achieve higher dielectric constant at a smaller content of conductors but also shows better mechanical properties. Within the past decade, carbon nanotubes (CNTs) have gained great attention worldwide owing to their special structure and properties. More interestingly, with the addition of an extremely small content of CNTs to a polymer, the dielectric constant will be greatly increased, suggesting that CNT/polymer composites generally have an extremely low percolation threshold.6−10 Therefore, CNT/polymer composites have been regarded as potential materials for preparing high-k composites. However, the combination of a large surface area and a high aspect ratio with attractive van der Waal interaction forces in CNTs make them tend toward aggregated bundles; thus, they are often difficult to mix with polymers.11−13 The composites become conductive not dielectric if CNTs contact each other. As a result, the dielectric loss of these percolative composites is usually quite high because of the insulator−conductor transition near the percolation threshold. Normally, CNT/polymer composites are good conductive composites but not good for dielectrics because of their high dielectric loss. One way to suppress the overwhelming van der Waals interaction is to modify the surface of the CNTs through chemical reactions with small molecules or polymers.14,15 The © 2014 American Chemical Society

covalent grafting of polymers on CNTs can be performed through “grafting to”16,17 and “grafting from”18 strategies. Grafting high molecular weight polymers through a grafting to method is inefficient because of the entropy penalty associated with the conformation of macromolecules. The grafting from strategy, also called surface initiated polymerization, is the most promising method because it provides control over the functionality, density, thickness, composition, and architecture of the grafted polymers.19 An appropriate functional group is introduced on the surface of CNTs and then modified with a potential initiator moiety to induce a controlled vinyl polymerization. In general, the acid groups generated on the CNTs are used for the attachment of an initiator through esterification or amidation.20,21 Direct reactions like 1,3-dipolar, Diels−Alder (DA) [4 + 2], and nitrene [2 + 1] cycloadditions on the sp2 carbons of CNTs are other common methods used for the introduction of functional groups that allow the attachment of potential initiator moieties.22−28 Yameen et al.27 have reported on the functionalization of pristine single-walled carbon nanotubes with a functional polymer via a one-pot strategy by employing a novel cyclopentadienyl functionalized RAFT agent. All known types of living and controlled polymerization techniques have been utilized for grafting polymers covalently on CNTs.29,30 The polymerization methods employed to initiate vinyl and cyclic monomers from the surface of CNTs include atom transfer radical polymerization (ATRP),31−34 Received: Revised: Accepted: Published: 6699

December March 25, March 27, March 27,

26, 2013 2014 2014 2014

dx.doi.org/10.1021/ie404204g | Ind. Eng. Chem. Res. 2014, 53, 6699−6707

Industrial & Engineering Chemistry Research

Article

reversible addition−fragmentation chain transfer polymerization,35 nitroxide mediated radical polymerization,36 anionic and cationic polymerization,37 and ring-opening polymerization.38 Homopolymers and diblock copolymers, V-shaped, and dendrimers have been grafted on all types of CNTs. However, to the best of our knowledge, no paper describing the surface-initiated multifunctional monomers on the surface of CNTs via ATRP has been published. To avoid the cross-linking of POSS itself, ATRP, one of the most powerful controlled radical polymerization techniques, was applied. Mild conditions are the other reason that we choice the ATRP reaction, in which the radical concentration can be controlled by controlling the ratio of CuI/CuII catalyst and the amount of alkyl halide initiator in the system. We report a synthetic strategy of coating multiwalled CNTs (MWCNTs) with multifunctional polyhedral oligomeric silsesquioxane (POSS) through a method combining DA cycloadditions with ATRP. The MWCNT-based initiator (CNT-initiator) of ATRP is synthesized through a DA reaction between MWCNTs and furfuryl-2-bromoisobutyrate (FBB) obtained by an esterification reaction. The CNT-initiator is used to induce the initiated polymerization of octa-acrylate POSS on the MWCNT surface. The thickness of the POSS layer grafted on MWCNTs is controlled by changing the grafting degree of the CNT-initiator. The cross-linkable octaacrylate POSS-coated MWCNTs (POSS@CNTs) are compounded with polyvinylidene fluoride (PVDF) because PVDF and its copolymers exhibit the largest piezoelectric and pyroelectric coefficients. Cross-linked POSS was used to coat MWCNTs for the following reasons. First, the acrylate group in POSS has good compatibility with host polymer PVDF, which ensures the good dispersion of MWCNTs in the matrix. Second, there is strong chemical interaction between the surface of MWCNTs and the POSS shell, resulting in firm coating of the POSS to MWCNTs. Third, the POSS has quite low conductivity relative to all of the polymers.39 Thereby, the leakage current of the PVDF/POSS@CNTs composites can be effectively depressed. A percolating nanocapacitor network with interparticle barriers originated by the POSS shells, which syncretizes with the polymer matrix, can lead to high dielectric constant and low dielectric loss.

Scheme 1. Covalent Functionalization of MWCNTs through DA Cycloaddition with FBB and Subsequent Coating of Octa-acrylate POSS by ATRP

2. EXPERIMENTAL METHODS 2.1. Materials. MWCNTs were purchased from Chinese Academy of Sciences Chengdu Organic Chemicals Co., Ltd. Octa-acrylate POSS ((C 6 H 9 O 2 ) n (SiO 1.5 ) n n = 8), the structureof which is shown in Scheme 1, was purchased from Hybrid Plastics America. Reagent grade 2-bromoisobutyryl bromide, furfuryl alcohol (FA), triethylamine (TEA), cuprous bromide (CuBr), and N,N,N′,N′,N″-pentamethyl diethylenetriamine (PMDETA) were all purchased from Alfa Aesar (Tianjin, China) and were used without further purification. Alkaline alumina (Al2O3) was purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. Tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), ethyl acetate (EA), N,Ndiethylacetamide, anisole, chloroform, and petroleum ether (Tianjin chemical works) were used after being refined by the method of the solvent handbook and kept with 4A molecular sieves. The PVDF used was the commercially available KF850 (Kureha Chemical, Japan), which was dried in a vacuum oven at 80 °C for 24 h before processing. 2.2. Synthesis of FBB. The synthesis of FBB performed between FA and 2-bromoisobutyryl bromide was similar to a

under nitrogen and kept in an oil bath at 80 °C for 48 h. The products were then washed thoroughly using THF to remove the excess FBB. Three kinds of CNT-initiators with varying grafting amounts of FBB were obtained from such a reaction induced in a mild environment. The CNT-initiators were named as shown in Table 1. 2.4. Synthesis of POSS@CNTs. CNT-initiator (30 mg) and 10 mg (0.07 mmol) of CuBr were placed in a 50 mL eggplant-type flask, which was then sealed with a rubber plug. The flask was evacuated and filled with nitrogen. A 12 mg (0.07 mmol) sample of N,N,N′,N′,N″-pentamethyldiethylenetriamine, 3 mL of 1,2-dichlorobenzene, and 300 mg of octaacrylate POSS were injected into the flask through a syringe. The reaction was induced in an oil bath for 24 h at 90 °C. Finally, the products were washed thoroughly by THF to remove the residual polymer that is not covalently attached. Three kinds of POSS@CNTs were obtained: POSS@CNT1 from CNT-initiator1, POSS@CNT2 from CNT-initiator2, and POSS@CNT3 from CNT-initiator3. A control experiment was carried out where the nonfunctional CNT was used as the ATRP initiator to synthesize the modified CNT (control

typical esterification reaction (Scheme 1). A crucial step taken was the cooling of the reactant in an ice bath when the 2bromoisobutyryl bromide was mixed. The reaction was conducted at room temperature for 24 h. The FBB was obtained after thoroughly washing FA with water, passing it through a basic aluminum column to remove 2-bromoisobutyryl bromide, and drying it with magnesium sulfate. 2.3. Synthesis of CNT-Initiators. As in a typical experiment (Scheme 1), 50 mg of MWCNTs and 0.3 mL of FBB were placed in a three-neck flask and varying amounts of anisole were charged (Table 1). The reaction was performed Table 1. Synthesis and Grafting Amount of CNT-Initiators sample CNTinitiator1 CNTinitiator2 CNTinitiator3

6700

MWCNT (mg)

FBB (mL)

anisole (mL)

weight loss (%)

weight loss (mg)

grafting density (mmol/g)

50

0.3

10

4.7

2.47

0.15

50

0.3

5

13.1

7.54

0.46

50

0.3

3

16.5

9.88

0.60



Article

sample) at the same reaction conditions as the ATRP process of POSS@CNTs. 2.5. Preparation of Composites. PVDF (5 g) was dissolved in 50 mL of N,N-diethylacetamide. Different amounts of POSS@CNT2 (0.5, 1, 3, 5, and 7 wt %) were added into the solution. After 30 min of ultrasonic vibration, a black suspension of POSS@CNT2 with homogeneous dispersion was obtained. The suspension was stirred for a few days to obtain the PVDF/POSS@CNT2 composites. They were dried in a vacuum at 100 °C for 3 days to remove the residual solvent. The thin-film composites of PVDF/POSS@CNT2 were prepared through thermocompression at 220 °C. Meanwhile, PVDF/MWCNT composites were prepared in the same way as that used to prepare PVDF/POSS@CNT2. 2.6. Characterization. 1H NMR spectroscopy measurements were carried out on a Bruker arx-500 spectrometer at room temperature with CDCl3 as a solvent and TMS as an internal reference. Fourier transform infrared (FT-IR) spectra (collected from a Bruker Tensor 27 FT-IR system) were used to characterize the molecular structure. The samples were imbedded in KBr disks. X-ray photoelectron spectroscopy (XPS) data were recorded on ESCALAB 250 spectrometer (Thermo Electron Corp.) in the fixed analyzer transmission mode with the Mg Kα X-ray source and a magnetic lens system that yields high spatial resolution and high sensitivity. The pressure in the analysis chamber was maintained at 2 × 10−10 mbar during measurement. Raman spectroscopy (RenishawinVia) was used to confirm the structure of the POSS@CNTs operating at 514 nm with a resolution of 1.5 cm−1. Thermo gravimetric analysis (TGA, Netzsch TG209F3) was conducted in nitrogen atmosphere. Samples were heated at a rate of 10 °C/min from 50 to 600 °C to determine the weight loss. The morphologies of the coated MWCNTs were carried out by transmission electron microscopy (TEM, Tecnai G220). The samples were dissolved in DMSO then dropped in the micro grid. UV−vis absorption measurements were carried out on a Shimadzu 1800 UV−vis spectrophotometer. Dielectric properties were measured by an Agilent 4294A instrument with a 16451B fixture (40 Hz to 110 MHz) at a constant temperature of 25 °C. Samples were prepared by flat rheometer to obtain a thickness of about 1 mm and then measured by three different areas of samples from 40 Hz to 30 MHz.

Figure 1. 1H NMR of the furfuryl-2-bromoisobutyrate.

Figure 2. FT-IR spectra of (a) pristine MWCNT, (b) CNT-initiator1, (c) CNT-initiator2, and (d) CNT-initiator3.

3. RESULTS AND DISCUSSION 3.1. Synthesis of FBB by Esterification Reaction. The whole synthesis scheme is outlined in Scheme 1. The FBB was synthesized through an esterification reaction from the 2bromoisobutyryl bromide and FA. As shown in Figure 1, the resulting FBB was characterized by 1H NMR spectroscopy; the relevant signals are labeled, and the peaks are assigned to their chemical structure. The 1H NMR spectroscopy result confirmed that the FBB, the diene of the DA reaction, was successfully obtained. 3.2. Synthesis of CNT-Initiators by DA Cycloaddition. The FT-IR spectra of pristine MWCNT and CNT-initiators are shown in Figure 2, and the detailed data of CNT-initiators are shown in Table 1. The adsorption bands at about 562.43 cm−1 can be ascribed to the C−Br, and those at 2993.4 cm−1 to the −CH3 of FBB. The absorption peak at 1721.63 cm−1 is a characteristic of the CO of the ester group, whereas the two absorption peaks at 1195.5 and 1120 cm−1 are related to the C−O−C stretching vibration of the ester group. Their intensities were greatly enhanced after functionalization,

Figure 3. TGA curves of (a) pristine MWCNT, (b) CNT-initiator1, (c) CNT-initiator2, and (d) CNT-initiator3.

compared with that of pristine MWCNT. This enhancement is a result of the attachment of alkyl groups on the surface of MWCNTs. The amount of the FBB grafted on the MWCNT, defined as the ratio of the mass of the immobilized FBB to that of the CNT-initiator, was estimated from the TGA as shown in Figure 3. The grafted amount of CNT-initiators are summarized in Table 1. All of the CNT-initiators exhibited major weight loss in the temperature range of 160−260 °C, a result of the degradation of the FBB grafted on the MWCNT through the 6701



Article

Figure 6. Raman spectra of (a) pristine MWCNT, (b) control sample, (c)POSS@CNT1, (d) POSS@CNT2, and (e) POSS@CNT3.

Figure 4. FT-IR spectra of (a) pristine MWCNT, (b) control sample, (c) POSS@CNT1, (d) POSS@CNT2, (e) POSS@CNT3, and (f) Octa-acrylated POSS.

Figure 7. TGA curves of (a) pristine MWCNT, (b) control sample, (c) POSS@CNT1, (d) POSS@CNT2, (e) POSS@CNT3, and (f) octa-acrylate POSS.

DA addition reaction, as shown in Figure 3. The residual fraction at temperatures above 600 °C corresponds mainly to the MWCNT. The amount of the FBB bonded to the MWCNT ranged from 4.7 to 16.5 wt % when the usage of anisole decreased from 10 to 3 mL at 80 °C for 48 h. The grafted amount of CNT-initiators is bound up with the

Figure 5. XPS curves of (a) POSS@CNT1, (b) POSS@CNT2, and (c) POSS@CNT3.

Table 2. Grafting Amount of POSS@CNTs From XPS atom (%)

sample POSS@ CNT1 POSS@ CNT2 POSS@ CNT3

sample

Si

O

C

Si/C (×10−2)

POSS@CNT1 POSS@CNT2 POSS@CNT3

1.76 3.42 9.74

6.75 12.12 21.21 From TGA

91.49 84.46 69.05

1.92 4.05 14.11

CNT- initiator (mg)

all weight loss (%)

weight loss of POSS (%)

30

9.5

4.5

30

20.3

30

37.9

grafting percent of POSS (%)

weight of POSS@ CNT (mg)

grafting weight of POSS (mg)

Grafting density of POSS (mmol/g)

10

33.3

3.33

0.08

11.2

24.9

39.9

9.93

0.25

20.5

45.6

55.15

25.15

0.64

6702



Article

Figure 9. UV−vis absorption spectra of POSS@CNT2 in chloroform in different concentrations.

O−Si stretching vibration appeared at 1120 cm−1, CO vibration peak at 1730 cm−1, and C−H vibration peaks at 2835−3010 cm−1, which indicates that the POSS was attached on MWCNTs. Their intensities were greatly enhanced after ATRP, compared with that of pristine MWCNT. This enhancement is explained by the attachment of POSS on the surface of MWCNTs. Almost all the characteristic peaks became more obvious with the increased amount of FBB on CNT-initiators, which can be attributed to the triggering ability of initiators. The control sample does not show any significant absorbance peaks of POSS. As shown in Figure 5, signals at about 285.0 (C1s) and 533.0 eV (O1s) are clearly detected for POSS@CNTs. The POSS@CNTs also show the peak at 103 eV, which is the characteristic of Si2p from the attached POSS. This phenomenon is in accordance with the observation of FTIR. Quantitative analyses are also carried out to compare the different POSS@CNTs in Table 2, where the Si/C ratio was achieved. As for POSS polymerized on CNT-initiator, the Si/C ratio increases from 1.92 (POSS@CNT1) to 14.11 (POSS@ CNT3). The Raman spectra of pristine MWCNT and POSS@CNTs are shown in Figure 6. For all the samples, the D and G bands at ca. 1325 and 1580 cm−1 are clearly detected and associated with the defects/disorder-induced modes and the vibration of sp 2 -bonded carbon atoms in a 2D hexagonal lattice, respectively. The grafted-layer may form a discontinuous phase because of the random distribution of defects on the surfaces of MWCNTs, which makes it difficult or even impossible to totally absorb and reflect all the excited energy for the polymer phase. Hence, the ratios of D- to G-band intensity (ID/IG) exhibit the degrees of disorder in MWCNTs. The values of ID/IG for POSS@CNTs (POSS@CNT1, 0.62; POSS@CNT2, 0.77; and POSS@CNT3, 0.90) are greater than that of pristine MWCNTs (0.58), which indicates the increase of defects in MWCNTs because of the covalent functionalization. The value of ID/IG for the control sample (0.59) is almost the same as that of pristine MWCNTs, which indicates that POSS does not react with the nonfunctional MWCNTs in the same reaction conditions as the ATRP process of POSS@ CNT2. TGA is used to determine the relative amount of coated polymer of POSS@CNTs (Figure 7) as the defunctionalization

Figure 8. TEM images of (a) POSS@CNT1, (b) POSS@CNT2, and (c) POSS@CNT3; general views of (d) POSS@CNT2 and (e) control sample.

concentration of reactants in the DA reactions, and for this reason the grafted reaction becomes easier to control. 3.3. Synthesis of POSS@CNTs by ATRP. The FT-IR spectra and XPS curves of POSS@CNTs are shown in Figures 4 and 5, respectively. As can be seen clearly in Figure 4, the Si− 6703



Article

Figure 10. Dependence of the (A) dielectric permittivity, (B) loss of PVDF/MWCNT composites, (C) dielectric permittivity, and (D) loss of PVDF/POSS@CNT2 composites on frequency at room temperature.

of modifed CNTs can be realized through thermal decomposition. Compared with the weight loss of CNT-initiators (Figure 3 and Table 1), POSS@CNTs demonstrate a considerable weight loss in the 400−600 °C range, corresponding to the decomposition of the organic molecules of the attached POSS, whereas the pristine MWCNT shows only a 2.0% weight loss up to 600 °C under nitrogen conditions. Significant weight loss does not appear at a specific temperature range in the TGA curves of the control sample and pristine MWCNTs. The entire weight loss (the 50−600 °C range) of the coated MWCNT (9.5% of POSS@CNT1, 20.3% of POSS@CNT2, and 37.9% of POSS@CNT3) is in accordance with the coating thickness observed through TEM. The percentage of the coated POSS on the POSS@CNTs is relative to the percentage of the FBB (Table 1) on CNTinitiators. The former increases as the latter increases. Furthermore, in contrast to the TGA curve of CNT-initiators, two stages of weight loss are observed from the POSS@CNTs. The stage from 50 to 400 °C should come from the thermal degradation of the FBB molecules used for the initiation of ATRP. The weight loss in the 400−600 °C range corresponds to the decomposition of the attached POSS (weight loss of POSS). On the one hand, the wide temperature range of weight loss can be attributed to the protective effect of the coated POSS layer on the degradation of FBB. On the other hand, the functionalization of MWCNTs with FBB is performed through

the DA reaction, which is thermally reversible. The DA adduct, CNT-initiator, can carry out a retro-DA reaction at high temperatures to regenerate the precursors utilized in the DA reaction. The performance of the retro-DA reaction and the regeneration of precursors have also been demonstrated by Liu et al.40 The weight loss data of POSS@CNTs from TGA are summarized in Table 2. Because pristine POSS decomposes 45 wt % in the 50−600 °C range, the grafting percent of POSS is calculated by the weight loss of POSS/45%. Therefore, the weight loss and grafting density of POSS can be easily obtained. Normally, the “grafting from” strategy via ATRP can offer higher grafting densities, reaching up to 0.2 mmol/g.28 In our study, the grafting densities range from 0.08 to 0.64, which depend on the concentration of initiators. The attachment of POSS on MWCNTs is directly observed through TEM (Figure 8). Unlike the clean and uniform surface of the pristine MWCNT, a core−shell structure with the MWCNT at the center can clearly be seen in POSS@CNTs, indicating that the MWCNTs have been coated by a POSS layer. The thickness of the POSS shell coating the MWCNTs shows obvious correlation with the grafted amount of CNTinitiators. When CNT-initiator1 is used to initiate the ATRP, the layer thickness is about 5 nm (Figure 8a). When CNTinitiator2 is used, the coating thickness reaches about 10 nm (Figure 8b). With a further increase of the amout of FBB bonded to MWCNT (CNT-initiator3), the coating thickness 6704



Article

Figure 11. Dielectric permittivity (a) and loss (c) of PVDF/MWCNT composites and the dielectric permittivity (b) and loss (d) of PVDF/POSS@ CNT2 composites at 100 kHz.

increases to about 15 nm, as shown in Figure 8c. Except for the grafting amount of CNT-initiators, the coating thickness of POSS does not show any obvious increase with a change in other reactive conditions of ATRP, such as temperature, time, concentration, and so on. This phenomenon is a result of the steric hindrance effect of the POSS cages. Therefore, by adjusting the initiators of ATRP, the controllable coating on the surface of MWCNT, namely 5, 10, and 15 nm in thickness, is achieved successfully. As shown in Figure 8d, POSS@CNT2 is well -distributed in the form of an extended network over a large area, and many entangled clusters of POSS@CNT2 are observed. However, in Figure 8e, we cannot observe any crosslinked POSS and the nonfunctional MWCNTs are still naked. The POSS@CNT dispersion efficiency in organic solvent increased significantly on the modified POSS layer. Figure 9 displays the UV−vis absorption spectroscopy of the POSS@ CNT2 with the middle thickness of POSS layer in chloroform. As can clearly be seen, the addition of POSS@CNT2 induces a significant increase in light absorption intensity in the visible light region. The continuous absorption band in the 250−500 nm range is caused by the addition of POSS@CNT2. The light absorption intensity in the visible light region increases, accompanied by the augmented additional amount of POSS@CNT2. The relationship of absorbance versus solution concentration is linear, which corroborates the uniformity of the POSS@CNT2 dispersion in organic solvent. 3.4. Dielectric Properties of PVDF/CNT Composites. The well-controlled POSS@CNT hybrid is a kind of new material with advantages. First, chemical interaction is induced

between the surfaces of MWCNTs and POSS, resulting in the firm POSS coating of MWCNTs. Second, the cross-linkable POSS has good compatibility and reactivity with the host matrix, which ensures the effective dispersion of MWCNTs in the matrix. Third, the POSS has quite a low dielectric constant compared with almost all polymers. Thereby, the leakage current of its composites can be effectively depressed when it is used to produce devices or composites.5,41−43 In this study, POSS@CNT2 was selected to compound with PVDF. As shown in Figure 10, the dielectric permittivity and loss of PVDF/POSS@CNT2 and PVDF/MWCNT composites was measured as a function of different frequencies. Both the dielectric permittivity and loss of PVDF/MWCNT composites tremendously improved with an increase in MWCNT content (Figure 10A,10B). In general, high permittivity is coupled with high dielectric loss for percolative composite materials. The permittivity reached 382, and at the same time the dielectric loss reached 95 at 100 kHz when 7 wt % of pristine MWCNT was added. The permittivity of PVDF/POSS@CNT2 is relatively lower than the results of PVDF/MWCNT, a normal phenomenon due to the insulation of POSS (Figure 10C). However, the dielectric loss of PVDF/POSS@CNT2 was evidently lower than that of PVDF/MWCNTs (Figure 10D). The dielectric loss reached only 1.1, whereas the permittivity reached 120 at 100 kHz when 7 wt % of POSS@CNT2 was added in the PVDF matrix. Figure 11 exhibits the drift of dielectric properties related to the percentage of conductive fillers kept at 100 kHz. The POSS layer effectively reduced the dielectric loss of the percolative composite by adding POSS@ 6705



Article

CNT in the polymers,44 in which the dielectric loss of these percolative composites is usually quite high because of the insulator−conductor transition near the percolation threshold.5 This result implies that the PVDF filled with POSS-coated MWCNTs could be a prospective actuator and supercapacitor material because it can enhance conductivity and permittivity.45 In this way, a kind of material with both high permittivity and low dielectric loss is obtained.

polymer composites based on SiC-carbon nanotubes micro/nano hybrid. Appl. Phys. Lett. 2011, 98, 032901. (8) Zhao, X.; Koos, A. A.; Chu, B. T. T.; Johnston, C.; Grobert, N.; Grant, P. S. Spray deposited fluoropolymer/multi-walled carbon nanotube composite films with high dielectric permittivity at low percolation threshold. Carbon 2009, 47, 561. (9) Li, Q.; Xue, Q. Z.; Hao, L. Z.; Gao, X. L.; Zheng, Q. B. Large dielectric constant of the chemically functionalized carbon nanotube/ polymer composites. Compos. Sci. Technol. 2008, 68, 2290. (10) Dang, Z.-M.; Yao, S.-H.; Yuan, J.-K.; Bai, J. Tailored Dielectric Properties based on Microstructure Change in BaTiO3-Carbon Nanotube/Polyvinylidene Fluoride Three-Phase Nanocomposites. J. Phys. Chem. C 2010, 114, 13204. (11) Iijima, S. Helical microtubules of graphitic carbon. Nature 1991, 354, 56. (12) Moniruzzaman, M.; Winey, K. I. Polymer nanocomposites containing carbon nanotubes. Macromolecules 2006, 39, 5194. (13) Jin, H.-J.; Choi, H. J.; Yoon, S. H.; Myung, S. J.; Shim, S. E. Carbon Nanotube-Adsorbed Polystyrene and Poly(methyl methacrylate) Microspheres. Chem. Mater. 2005, 17, 4034. (14) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Solution Properties of Single-Walled Carbon Nanotubes. Science 1998, 282, 95. (15) Chen, G. X.; Li, Y. J.; Shimizu, H. Ultrahigh-shear processing for the preparation of polymer/carbon nanotube composites. Carbon 2007, 45, 2334. (16) Riggs, J. E.; Guo, Z.; Carroll, D. L.; Sun, Y.-P. Strong Luminescence of Solubilized Carbon Nanotubes. J. Am. Chem. Soc. 2000, 122, 5879. (17) Chen, G.-X.; Kim, H. S.; Park, B. H.; Yoon, J. S. Controlled functionalization of multiwalled carbon nanotubes with various molecular-weight poly(L-lactic acid). J. Phys. Chem. B 2005, 109, 22237. (18) Sakellariou, G.; Priftis, D.; Baskaran, D. Surface-initiated polymerization from carbon nanotubes: Strategies and perspectives. Chem. Soc. Rev. 2013, 42, 677. (19) Chen, G. X.; Kim, H. S.; Park, B. H.; Yoon, J. S. Synthesis of poly(L-lactide)-functionalized multiwalled carbon nanotubes by ringopening polymerization. Macromol. Chem. Phys. 2007, 208, 389. (20) Karousis, N.; Tagmatarchis, N.; Tasis, D. Current Progress on the Chemical Modification of Carbon Nanotubes. Chem. Rev. (Washington, DC, U.S.) 2010, 110, 5366. (21) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chemistry of carbon nanotubes. Chem. Rev. (Washington, DC, U.S.) 2006, 106, 1105. (22) Menard-Moyon, C.; Dumas, F.; Doris, E.; Mioskowski, C. Functionalization of single-wall carbon nanotubes by tandem highpressure/Cr(CO)6 activation of Diels−Alder cycloaddition. J. Am. Chem. Soc. 2006, 128, 14764. (23) Sarkar, S.; Bekyarova, E.; Niyogi, S.; Haddon, R. C. Diels−Alder Chemistry of Graphite and Graphene: Graphene as Diene and Dienophile. J. Am. Chem. Soc. 2011, 133, 3324. (24) Munirasu, S.; Albuerne, J.; Boschetti-de-Fierro, A.; Abetz, V. Functionalization of Carbon Materials using the Diels−Alder Reaction. Macromol. Rapid Commun. 2010, 31, 574. (25) Nebhani, L.; Barner-Kowollik, C. Functionalization of Fullerenes with Cyclopentadienyl and Anthracenyl Capped Polymeric Building Blocks via Diels−Alder Chemistry. Macromol. Rapid Commun. 2010, 31, 1298. (26) Zydziak, N.; Huebner, C.; Bruns, M.; Barner-Kowollik, C. OneStep Functionalization of Single-Walled Carbon Nanotubes (SWCNTs) with Cyclopentadienyl-Capped Macromolecules via Diels−Alder Chemistry. Macromolecules 2011, 44, 3374. (27) Yameen, B.; Rodriguez-Emmenegger, C.; Ahmed, I.; Preuss, C. M.; Durr, C. J.; Zydziak, N.; Trouillet, V.; Fruk, L.; Barner-Kowollik, C. A facile one-pot route to poly(carboxybetaine acrylamide) functionalized SWCNTs. Chem. Commun. (Cambridge, U. K.) 2013, 49, 6734.

4. CONCLUSIONS The monomer containing multiple CC bond, octa-acrylate POSS, was successfully coated on the surface of MWCNT in a controlled manner by combining DA cycloadditions with ATRP. Varying thicknesses of POSS on MWCNT, ranging from 5 to 15 nm, were achieved by adjusting the grafting degree of MWCNT-initiators. The POSS-coated MWCNT was welldispersed in the organic solvent. Its PVDF composite showed controlled dielectric performance. The dielectric loss was effectively reduced compared with that of pristine MWCNTbased composite because of the insulative POSS layer. The multifunctional molecules coating MWCNTs provided a potential application for the preparation of composites with controlled properties. As such, the coating technique is helpful in designing novel hybrids based on CNT chemistry.

■

AUTHOR INFORMATION

Corresponding Authors

*Tel.: 86 10 64445680. Fax: 86 10 64421693. E-mail: gxchen@ mail.buct.edu.cn. (G.X.C.) *Tel.: 86 10 64445680. Fax: 86 10 64421693. E-mail: qflee@ mail.buct.edu.cn. (Q.L.) Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support of this work from the Natural Science Foundation of China (NSFC) (51173009).

■

REFERENCES

(1) Zhang, Q. M.; Bharti, V.; Zhao, X. Giant Electrostriction and Relaxor Ferroelectric Behavior in Electron-Irradiated Poly(vinylidene fluoride-trifluoroethylene) Copolymer. Science 1998, 280, 2101. (2) Yuan, J.-K.; Yao, S.-H.; Dang, Z.-M.; Sylvestre, A.; Genestoux, M.; Bai, J. Giant Dielectric Permittivity Nanocomposites: Realizing True Potential of Pristine Carbon Nanotubes in Polyvinylidene Fluoride Matrix through an Enhanced Interfacial Interaction. J. Phys. Chem. C 2011, 115, 5515. (3) Xie, L.; Huang, X.; Wu, C.; Jiang, P. Core-shell structured poly(methyl methacrylate)/BaTiO3 nanocomposites prepared by in situ atom transfer radical polymerization: A route to high dielectric constant materials with the inherent low loss of the base polymer. J. Mater. Chem. 2011, 21, 5897. (4) Dang, Z.-M.; Yuan, J.-K.; Zha, J.-W.; Zhou, T.; Li, S.-T.; Hu, G.H. Fundamentals, processes and applications of high-permittivity polymer matrix composites. Prog. Mater. Sci. 2012, 57, 660. (5) Nan, C. W.; Shen, Y.; Ma, J. Physical Properties of Composites Near Percolation. Annu. Rev. Mater. Res. 2010, 40, 131. (6) Hayashida, K.; Tanaka, H. Ultrahigh Electrical Resistance of Poly(cyclohexyl methacrylate)/Carbon Nanotube Composites Prepared Using Surface-Initiated Polymerization. Adv. Funct. Mater. 2012, 22, 2338. (7) Yuan, J. K.; Li, W. L.; Yao, S. H.; Lin, Y. Q.; Sylvestre, A.; Bai, J. B. High dielectric permittivity and low percolation threshold in 6706



Article

(28) Zydziak, N.; Yameen, B.; Barner-Kowollik, C. Diels-Alder reactions for carbon material synthesis and surface functionalization. Polym. Chem. 2013, 4, 4072. (29) Matyjaszewski, K.; Xia, J. Atom Transfer Radical Polymerization. Chem. Rev. (Washington, DC, U.S.) 2001, 101, 2921. (30) Braunecker, W. A.; Matyjaszewski, K. Controlled/living radical polymerization: Features, developments, and perspectives. Prog. Polym. Sci. 2007, 32, 93. (31) Kong, H.; Gao, C.; Yan, D. Controlled Functionalization of Multiwalled Carbon Nanotubes by in Situ Atom Transfer Radical Polymerization. J. Am. Chem. Soc. 2004, 126, 412. (32) Qin, S.; Qin, D.; Ford, W. T.; Resasco, D. E.; Herrera, J. E. Polymer Brushes on Single-Walled Carbon Nanotubes by Atom Transfer Radical Polymerization of n-Butyl Methacrylate. J. Am. Chem. Soc. 2003, 126, 170. (33) Yao, Z.; Braidy, N.; Botton, G. A.; Adronov, A. Polymerization from the Surface of Single-Walled Carbon Nanotubes − Preparation and Characterization of Nanocomposites. J. Am. Chem. Soc. 2003, 125, 16015. (34) Baskaran, D.; Mays, J. W.; Bratcher, M. S. Polymer-Grafted Multiwalled Carbon Nanotubes through Surface-Initiated Polymerization. Angew. Chem., Int. Ed. 2004, 43, 2138. (35) Cui, J.; Wang, W.; You, Y.; Liu, C.; Wang, P. Functionalization of multiwalled carbon nanotubes by reversible addition fragmentation chain-transfer polymerization. Polymer 2004, 45, 8717. (36) Dehonor, M.; Masenelli-Varlot, K.; González-Montiel, A.; Gauthier, C.; Cavaillé, J.-Y.; Terrones, H.; Terrones, M. Nanotube brushes: Polystyrene grafted covalently on CNx nanotubes by nitroxide-mediated radical polymerization. Chem. Commun. (Cambridge, U. K.) 2005, 5349. (37) Viswanathan, G.; Chakrapani, N.; Yang, H.; Wei, B.; Chung, H.; Cho, K.; Ryu, C. Y.; Ajayan, P. M. Single-step in situ synthesis of polymer-grafted single-wall nanotube composites. J. Am. Chem. Soc. 2003, 125, 9258. (38) Xu, Y.; Gao, C.; Kong, H.; Yan, D.; Jin, Y. Z.; Watts, P. C. P. Growing Multihydroxyl Hyperbranched Polymers on the Surfaces of Carbon Nanotubes by in Situ Ring-Opening Polymerization. Macromolecules 2004, 37, 8846. (39) Liu, Y.-L.; Fangchiang, M.-H. Polyhedral oligomeric silsesquioxane nanocomposites exhibiting ultra-low dielectric constants through POSS orientation into lamellar structures. J. Mater. Chem. 2009, 19, 3643. (40) Chang, C.-M.; Liu, Y.-L. Electrical Conductivity Enhancement of Polymer/Multiwalled Carbon Nanotube (MWCNT) Composites by Thermally-Induced Defunctionalization of MWCNTs. ACS Appl. Mater. Interfaces 2011, 3, 2204. (41) Bergman, D.; Imry, Y. Critical Behavior of the Complex Dielectric Constant near the Percolation Threshold of a Heterogeneous Material. Phys. Rev. Lett. 1977, 39, 1222. (42) Stauffer, D. Introduction to Percolation Theory; Taylor & Francis: London, 1992. (43) Nan, C. W. Physics of Inhomogeneous Inorganic Materials. Prog. Mater. Sci. 1993, 37, 1. (44) Yang, C.; Lin, Y.; Nan, C. W. Modified carbon nanotube composites with high dielectric constant, low dielectric loss and large energy density. Carbon 2009, 47, 1096. (45) Zhao, X.; Zhao, J.; Cao, J.-P.; Wang, X.; Chen, M.; Dang, Z.-M. Tuning the Dielectric Properties of Polystyrene/Poly(vinylidene fluoride) Blends by Selectively Localizing Carbon Black Nanoparticles. J. Phys. Chem. B 2013, 117, 2505.

6707


Controlled Dielectric Properties of Polymer Composites from Coating

Recommend Documents