Largely Enhanced Thermal Conductivity and High Dielectric Constant

Article pubs.acs.org/JPCC

Largely Enhanced Thermal Conductivity and High Dielectric Constant of Poly(vinylidene fluoride)/Boron Nitride Composites Achieved by Adding a Few Carbon Nanotubes Yan-jun Xiao, Wen-yan Wang, Ting Lin, Xi-jia Chen, Yu-tong Zhang, Jing-hui Yang, Yong Wang,* and Zuo-wan Zhou School of Materials Science & Engineering, Key Laboratory of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu 610031, China ABSTRACT: A small amount of carbon nanotubes (CNTs) was added into poly(vinylidene fluoride) (PVDF)/boron nitride (BN) composites through melt blending processing. The thermal conductivity, microstructure changes including the crystallization behavior of PVDF matrix and the dispersion states of fillers in the composites, and the electrical conductivity of the composites were comparatively investigated. The results demonstrated that compared with the PVDF/BN composites at the same BN content, the ternary PVDF/BN/CNT composites exhibited largely enhanced thermal conductivity. In the PVDF/BN/CNT composites, the crystallinity of the PVDF matrix was slightly increased while the crystal form remained invariant. BN particles exhibited homogeneous dispersion in the PVDF/BN composites, and they did not affect the rheological properties of the PVDF/BN composites when the BN content was lower than 10 wt %. The presence of CNTs did not affect the interfacial adhesion between BN and PVDF, but they facilitated the formation of denser BN/CNT network structure in the composites. The mechanisms were then proposed to explain the largely enhanced thermal conductivity of the PVDF/BN/CNT composites. Furthermore, the dielectric property measurements demonstrated that the PVDF/BN/CNT composites containing relatively low BN content exhibited a high dielectric constant with a low dielectric loss. This endowed the PVDF/BN/CNT composites with a greater potential application in the field of electronic devices.

1. INTRODUCTION Polymers have been well-known as excellent heat insulating material due to their extremely low thermal conductivity. Generally, the thermal conductivity of most polymers is lower than 0.5 Wm−1 K−1 at room temperature (25 °C),1 and therefore, they are widely used in the field of thermal protection. However, there are still many fields that need quick transfer of the heat so that the temperature of the product can be maintained at a relatively low level. The related application fields include generators, heat exchangers in power generation, automobiles, and microelectronic devices, among others. Now, the heat dissipation capability has become one of the key parameters that must be considered when designing the electronic devices. The most efficient strategy to enhance the thermal conductivity of polymers is introducing the thermal conductive fillers into a polymer matrix, such as metal powder,2,3 metal oxide,4 inorganic compound,5,6 and allotropes of carbon ranging from carbon black (with zero-dimensional structure), carbon nanotubes (with one-dimensional structure) to graphite and graphene (with two-dimensional structure).7−11 Among these fillers, carbon nanotubes (CNTs) and graphene have attracted much attention of researchers due to their extremely © XXXX American Chemical Society

high thermal conductivity. In theory, CNTs and graphene exhibit thermal conductivity of about 2000−6000 and 5000 Wm−1 K−1,12−16 respectively. However, adding thermal-conductive fillers into polymers does not ensure the great enhancement of the thermal conductivity of the composites. This is because that the thermal conductivity of polymer composites is greatly influenced by many factors, including the microstructure of polymer matrix, the interfacial interaction between thermal conductive fillers and polymer matrix, and the dispersion states of fillers.17−19 The microstructure of polymers is mainly related to the degree of crystallinity, and generally, if there is a higher degree of crystallinity, there will be a higher thermal conductivity of the polymers.20−23 The relatively high interfacial thermal resistance, which exists at the interfaces between fillers and polymer matrix and between fillers, has been believed to be the most important factor that results in the low thermal conductivity of polymer composites.8,24,25 Improving interfacial interaction through surface modification of fillers has Received: December 27, 2015 Revised: March 8, 2016

A

DOI: 10.1021/acs.jpcc.5b12651 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Schematic representations showing the preparation procedures of the PVDF composites. (a) PVDF, BN, and CNTs were dissolved in DMF to prepare the PVDF/DMF, BN/DMF and CNT/DMF solutions, respectively. (b,c) The three mixtures were compounded together and further stirred for 1 h, after that, the temperature was enhanced up to 100 °C to remove most of DMF. (d,e,f) The mixture was further dried in an oven to obtain the PVDF-based composite, and the composite was further melt-compounded in a microextruder at 200 °C for 6 min; (g−i) the pellets of the composite were compression-molded to obtain the final sample.

Obviously, reducing BN content is significant, and in this condition, the materials can maintain good processing ability. Considering the extremely high thermal conductivity of CNTs, it is easy to ask what happens if CNTs and BN are simultaneously introduced into the composites. It has been demonstrated in the literature that once the one-dimensional fillers and two-dimensional fillers are simultaneously present in the composites, they can form the hybrid three-dimensional filler network structure, which is favorable for the improvement of physical properties of composites.11,40−42 For example, in our previous work,40 a small amount of graphene oxide (GO) was introduced into the poly(vinylidene fluoride) (PVDF)/ CNT composites, and the results demonstrated the formation of the denser hybrid network structure of CNT/GO in the composites, which was suggested to be one of the main reasons for the largely enhanced thermal conductivity. Park et al.8 introduced surface-modified CNTs (1 wt %) into the BN-filled polyphenylene sulfide (PPS) composites with 50 wt % BN, and they found that CNTs and BN exhibited a synergistic effect in improving the thermal conductivity of the composites. They also demonstrated that the thermal conductivity of the composites was greatly dependent upon the surface modification of CNTs. Furthermore, they proposed that the formation of the three-dimensional thermal transfer pathways between the BN and CNTs was possibly the main reason for the enhancement of the thermal conductivity, although they did not demonstrate the formation of such three-dimensional network structure. In this work, we attempt to introduce a small amount of CNTs (2 wt %) into the BN-filled PVDF composites to seek an efficient but simple strategy to enhance the thermal conductivity of such composites on one hand. On the other hand, it is expected that this strategy will enable a better understanding of the microstructure−performance relationship of such thermal conductive composites. It is interesting to observe that the presence of CNTs promotes the formation of the three-dimensional network structure of fillers in the PVDF

been demonstrated to be an efficient strategy to enhance the thermal conductivity of polymer composites.7,26−28 Furthermore, if fillers form an effective thermal conductive path in the composites, the thermal conductivity of composites can be also enhanced to a great degree.29,30 However, the formation of the thermal conductive path is greatly dependent upon the filler content, dispersion state of fillers in the composites, and the geometric parameters of fillers.11,30,31 To obtain the composites with relatively high thermal conductivity, high content of fillers are required. This also results in other problems, including the deterioration of processing flow ability, the rising of product cost, the deterioration of product appearance, and so on. Therefore, it is urgent and also significant to seek another strategy to enhance the thermal conductivity of polymer composites at relatively lower filler content and simultaneously with low product cost. As one of the ceramic materials with relatively high thermal conductivity, recently, boron nitride (BN) has attracted much attention of many researchers.32−34 BN exhibits the typical twodimensional structure, which is very similar to the platelet-like structure of graphene. The thermal conductivity of BN is about 250−300 Wm−1 K−1.35 Although the value is much lower than that of the graphene, BN exhibits very low electrical conductivity. The high thermal conductivity and low electrical conductivity features endow BN-filled polymer composites with special applications. For example, they can be used as the encapsulating material of electronic devices.36 To date, many researches have been carried out to prepare BN-filled polymer composites with high thermal conductivity. Similarly, the attentions are mainly focused on improving the interfacial interaction between BN and polymer matrix,37 constructing BN thermal conductive path in the composites through increasing BN content,38 etc. For example, Sato et al. introduced 60 vol % BN into polyimide (PI) and the thermal conductivity of the composites was as high as 7 Wm−1 K−1.39 However, it seems that very high content of BN must be required so that the thermal conductivity could be enhanced to a high level. B


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were prepared through compression molding processing had a diameter and a height of 30 mm and 7 mm, respectively. Before measurement, the sample surface that contacted the measuring probe was carefully polished and then cleaned using ethanol to eliminate the negative effects of surface roughness and dust on the thermal conductivity of sample. 2.4. Differential Scanning Calorimetry. The melting and crystallization behaviors of samples were detected using a differential scanning calorimetry (DSC) STA449C Jupiter (Netzsch, Germany). During the measurements, the weight of each sample was maintained at about 8 mg. The DSC scanning program was set as follows: the sample was first heated from 0 to 200 °C at a heating rate of 10 °C/min, then the temperature was maintained at 200 °C for 3 min to erase the thermal history, and then the sample was cooled down to 0 °C at a cooling rate of 5 °C/min. All the measurements were carried out in nitrogen atmosphere to avoid the possible oxidative degradation of sample. The degree of crystallinity (Xc) of PVDF matrix was calculated according to the following relation:

composites, which not only induces the great enhancement of thermal conductivity but also induces the apparent change of electrical conductivity and dielectric properties of the composites.

2. EXPERIMENTAL SECTION 2.1. Materials. The materials used in this work are commercially available. PVDF (with a trade name of Kynar 720) was purchased from Arkema (France). It exhibited a melt flow rate (MFR) of 22.8 g/10 min (230 °C/3.8 kg) and a density of 1.78 g/cm3. Hexagonal BN (with a purity of 99.99% and a density of 2.3 g/cm3) was purchased from Chengdu UPAM applied materials Co., LTD (China). CNTs with carboxyl content of about 0.51 wt % were obtained from Chengdu Institute of Organic Chemistry, Chinese Academy of Science (Chengdu, China). The corresponding parameters of CNTs could be seen in our previous work.7 2.2. Sample Preparation. To achieve the good dispersion of fillers in the composites, the PVDF-based composites were prepared through a two-step processing strategy (i.e., solution compounding with melt compounding). The detailed procedures are schematically represented in Figure 1. First, PVDF, BN and CNTs were dissolved in the N,N-dimethylformamide (DMF) to prepare the PVDF/DMF, BN/DMF and CNT/ DMF solution (Figure 1a), respectively. The dissolution of BN and CNT were carried out at 55 °C with the aid of sonication for 2 h and the dissolution of PVDF was stirred for 20 min. Second, the three mixtures were mixed together with sonication and continuously stirring at 55 °C for 2 h, and then the solution temperature was increased up to 100 °C and maintained at this temperature for about 2 h to remove most of the solvent (Figure 1b,c). After that, the mixture was transferred into a watch glass and placed in an oven with a setting temperature of 80 °C for more than 24 h until the weight of the sample was not invariant, and in this condition, the residual solvent could be completely removed (Figure 1d). Subsequently, the composites were continuously melt-compounded using a microextruder for about 6 min (Figure 1e,f). The melt compounding was carried out in a nitrogen atmosphere to avoid the thermal degradation of PVDF. Then, the final composites were prepared. To obtain the sample for physical property measurement and microstructure characterization, the pellets of the composite were compression-molded at a melt temperature of 200 °C and a pressure of 10 MPa for about 8 min (Figure 1g−i). In the present work, different contents of fillers were introduced into PVDF. The sample notation and the corresponding compositions are listed in Table 1. 2.3. Thermal Conductivity Measurement. A Transient Hot Disk TPS 2500S instrument (Hot Disk AB, Gothenburg, Sweden) was used to measure the thermal conductivity of sample according to ISO/CD 22007-2. The diameter of the measuring probe was 6.403 mm. The cylindrical samples that

Xc − DSC =

a

sample

PVDF (wt %)

BN (wt %)

CNTs (wt %)

100 80−99 98 78−97

0 1−20 0 1−20

0 0 2 2

ϕ × ΔHm0

× 100% (1)

where ΔHm was the value of fusion enthalpy of sample obtained during the DSC heating scan, ΔH0m was the fusion enthalpy of the completely crystalline PVDF, and ϕ was the relative weight fraction of PVDF in the composites. Here, ΔH0m of PVDF was selected as 104.7 J/g.43 2.5. Scanning Electron Microscopy (SEM). A scanning electron microscope (SEM) Fei Inspect (FEI, The Netherlands) was used to characterize the dispersion states of fillers in the composites. During the characterization, an accelerating voltage of 20 kV was selected. Before SEM characterization, the sample was first immersed into nitrogen for 30 min, and then it was cryogenically fractured. The fractured surface was then characterized. To investigate the dispersion of BN particles, the sample surface was coated with a thin layer of gold before SEM characterization. However, to clearly understand the dispersion of CNTs, the sample surface was directly characterized using SEM. 2.6. Rheological Measurement. The rheological responses of samples were detected using a stress-controlled rheometer DHR-1 (TA Instrument, USA). The sample disk was prepared through a compression molding method, and it had a diameter and a thickness of 20 and 1 mm, respectively. During the rheological measurement, the melt temperature of the sample was set at 190 °C and the measuring frequency range of 0.01−100 Hz was applied. Furthermore, it was stressed that the measurement was conducted within the linear viscoelastic strain range. All the measurements were carried out in nitrogen atmosphere to avoid the possible oxidative degradation. 2.7. Electrical Conductivity Measurement. The electrical conductivity was evaluated through measuring the electrical resistivity of samples that was conducted on a universal meter (DT9208, China). A rectangular cross-sectional bar was cut from the previously compression-molded plate, and the bar had a length of 30 mm and a width of 6 mm. To reduce the effect of contact resistance, the two ends of the bar were coated with a thin layer of conductive silver paint. The measurements were carried out through a two-point technique.

Table 1. Sample Notation and the Compositions of the Samplesa PVDF PVDF/BN-x PVDF/CNT PVDF/BN-x/CNT

ΔHm

x represents the relative weight fraction of BN in the composites. C


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Figure 2. (a) variations of thermal conductivity versus the content of BN in the composites, (b) comparison of the thermal enhancement factor between PVDF/BN and PVDF/BN/CNT composites, and (c) the synergistic efficiency of BN and CNTs versus the content of BN in the composites.

thermal conductivity of 0.30 Wm−1K−1 while the value of the PVDF/BN-20/CNT sample is enhanced up to 1.30 Wm−1K−1, which is nearly 34% higher than that of the PVDF/BN-20 sample (0.97 Wm−1K−1), and even about 465% higher than that of the neat PVDF. In the report of Park et al.,8 when 50 wt % BN and 1 wt % CNTs were simultaneously introduced into PPS, the thermal conductivity was enhanced from 0.31 Wm−1K−1 of pure PPS to 1.74 Wm−1K−1 of the composites. Although the value of thermal conductivity was relatively higher, the too high BN content most likely weakened the processing ability of the composite. Obviously, from a viewpoint of enhancement efficiency per unit weight of fillers, this work provides relatively higher enhancement efficiency. Teng et al.37 also introduced functionalized BN and CNTs into epoxy and found that simultaneous addition of 30 vol % BN and 1 vol % CNTs resulted in a 743% increase in thermal conductivity (1.913 Wm−1K−1, compared to 0.227 Wm−1K−1 of neat epoxy). However, it should be stressed that the matrix was a thermoset polymer rather than the thermoplastic polymer. To better understand the variation of thermal conductivity of the composites, two parameters, i.e. thermal enhancement factor (ϕ), which represents the degree of enhancement of thermal conductivity of composites, and synergistic efficiency ( f), which represents the synergistic effect of CNTs and BN on thermal conductivity of the PVDF/BN/CNT composites, are defined as follows:36,45

2.8. Dielectric Property Measurement. The dielectric property measurements were conducted on a broad frequency dielectric spectrometer Concept 50 (NOVOCONTROL, Germany). The sample disk (with a diameter of 20 mm and a thickness of 3 mm) was also cut from the previously compression-molded plate. During the measuring process, the frequency range of 10−107 Hz and an operating voltage of 220 V were selected.

3. RESULTS AND DISCUSSION 3.1. Thermal Conductivity. The thermal conductivities of the samples with and without different fillers were first measured. As shown in Figure 2, pure PVDF sample exhibits a relatively low thermal conductivity, i.e. 0.23 Wm−1K−1, which is very close to the value reported in the literature.44 The presence of BN particles induces the enhancement of thermal conductivity of the PVDF/BN composites, but the enhancement is greatly dependent upon the content of BN particles. For example, the PVDF/BN-1 sample exhibits the thermal conductivity of only 0.26 Wm−1K−1. However, when the BN content is increased up to 20 wt %, the thermal conductivity is enhanced up to 0.97 Wm−1K−1, which is 321.7% higher than that of the pure PVDF sample. In addition, the thermal conductivity of the PVDF/CNT sample containing only 2 wt % CNTs was also measured and the value is 0.28 Wm−1K−1. It can be seen that the 2 wt % CNTs exhibit the similar role to that of the 1 wt % BN particles in improving the thermal conductivity of the PVDF composites. Interestingly, it is observed that adding the same content of CNTs (2 wt %) into the PVDF/BN composites promotes the apparent enhancement of the thermal conductivity. The PVDF/BN-1/CNT sample exhibits the

ϕ=

λcom − λPVDF × 100% λPVDF

(2)

where λcom and λPVDF represent the thermal conductivity of PVDF-based composites and pure PVDF sample, respectively. D


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The Journal of Physical Chemistry C f=

λBN/CNT − λPVDF (λCNT − λPVDF) + (λBN − λPVDF)

(3)

where λ BN/CNT , λ BN , and λCNT represent the thermal conductivities of the ternary PVDF/BN/CNT composites, the binary PVDF/BN and PVDF/CNT composites, respectively. If the value of f is bigger than 1, BN particles and CNTs exhibit a synergistic effect in improving the thermal conductivity of PVDF. Obviously, a high f value means more probability to achieve high thermal conductivity. The variations of ϕ and f of samples are illustrated in Figure 2b,c. It can be seen that ϕ of the PVDF/BN samples increases with increasing BN content. This indicates that BN particles with high content induce more apparent enhancement in thermal conductivity. With the presence of 2 wt % CNTs, ϕ of the ternary PVDF/BN/CNT samples exhibits more apparent enhancement with increasing BN content. Furthermore, for all the ternary composites, f is higher than 1, which clearly indicates the existence of a synergistic effect between BN and CNTs. However, it can be seen that for the investigated samples, the PVDF/BN-5/CNT sample exhibits the maximum f, and further increasing BN content leads to the slight decrease of f. Obviously, it is very significant to understand why the thermal conductivity of the PVDF/BN/CNT composites is dramatically enhanced by adding a small amount of CNTs. This will be carried out in the following sections. 3.2. Crystallization Behavior of PVDF Matrix. For most polymers, the electron is constrained, and in this condition, heat transfer is realized through vibration of the crystal lattice. Generally, the quantum that describes the vibration of crystal lattice is named a phonon. That is the reason why the thermal conductive mechanism of polymer is a phonon conductive mechanism.46 So far, it has been demonstrated that the crystalline structure of the polymer matrix is one of the key factors that influences the thermal conductivity of composites. Generally, relatively high Xc and more integrated crystalline structure facilitate the enhancement of thermal conductivity.20−23 PVDF is a typical semicrystalline polymer with polymorphic structures, including α, β, γ, δ, and ε, among others.47 Both the crystal form and Xc can be influenced by adding fillers that exhibit nucleation effect for PVDF crystallization. For example, several researches have already demonstrated that the presence of functionalized CNTs induces the increase of Xc and/or promotes the formation of β-form crystallites.48,49 Here, BN and CNTs exhibit relatively high aspect ratio, and therefore, it is easy to ask whether they can induce the change of PVDF crystalline structure in the composites or not. The crystalline structures of the representative samples were first characterized using WAXD. As shown in Figure 3, pure PVDF sample exhibits three characteristic diffraction peaks at 2θ = 17.8°, 18.4°, and 20.0°, which can be attributed to the diffractions of (100), (020), and (110) crystal planes of α-form PVDF,50 respectively. The similar characteristic diffraction peaks are also observed for the other three samples. This indicates that all the composites exhibit the α-form crystalline structure. Although the crystal form of PVDF matrix is not influenced by adding CNTs and/or BN particles, from the variations of relative intensities of diffraction peaks, one can still differentiate the slight differences in crystalline structures among samples. For the pure PVDF sample, the (110) crystal plane exhibits the highest intensity, whereas the (100) crystal plane exhibits the smallest one, indicating that the preferential

Figure 3. WAXD profiles showing the crystalline structures of some representative samples as indicated in the graph.

growth direction of PVDF crystal is possibly along the direction of the (110) crystal plane. For the PVDF/CNT sample, although the (110) crystal plane still exhibits the highest intensity, the intensity ratio between (020) and (110) crystal planes is increased. For the composites containing 10 wt % BN, the (020) crystal plane exhibits the highest intensity, whereas the intensity of the (100) crystal plane is greatly reduced. This indicates that the preferential growth direction of PVDF crystal is changed to the direction of (020) crystal plane possibly due to the heterogeneous nucleation effect of BN particles that promotes the preferential growth of PVDF crystal along its (020) crystal plane. Figure 4 exhibits the melting and crystallization behaviors of all the samples obtained through DSC measurements. The corresponding parameters are listed in Table 2. It can be seen that the PVDF/BN composites exhibit a melting point (Tm) of about 170.1−171.7 °C, which is slightly lower than that of the pure PVDF sample (172.5 °C). However, the presence of BN particles induces the slight change of Xc. For example, pure PVDF sample exhibits Xc of 34.3%, whereas the PVDF/BN-20 sample exhibits Xc of 30.3%. This indicates that BN restricts the integration of PVDF crystalline structure. From Figure 4b, one can see that the nonisothermal crystallization of PVDF matrix is apparently influenced by BN particles, and the crystallization temperature (Tc) is gradually increased with increasing BN content. Pure PVDF sample exhibits Tc of 139.2 °C, and the PVDF/BN-1 sample exhibits Tc of 145.8 °C. When the BN content is increased up to 20 wt %, Tc is enhanced up to 152.0 °C, which is apparently higher than that of the pure PVDF sample. However, it can be also shown that the crystallization peak of the PVDF composites becomes weaker with increasing BN content. As shown in Table 2, the crystallization enthalpy (ΔHc) decreases with increasing BN content, which means that fewer PVDF crystallize during the cooling process. In other words, the presence of BN particles restricts the integration of PVDF crystalline structures. The effects of CNTs on melting and crystallization behaviors of the PVDF matrix are shown in Figure 4c,d. Although CNTs also exhibit an excellent nucleation effect for PVDF crystallization, and Tc is increased from 139.2 °C of pure PVDF to 148.1 °C of the PVDF/CNT composite, the presence of CNTs does not induce the apparent changes of melting and crystallization behaviors of PVDF matrix in the ternary composites. Namely, the ternary composites exhibit similar E


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Figure 4. DSC heating (a,c) and cooling (b,d) curves showing the melting and crystallization behaviors of all the samples investigated in this work.

conductive mechanism, the dispersion states of fillers in the composites were characterized using SEM. Figure 5 shows the cryogenically fractured surface morphologies of the representative samples. Here, the cryo-fractured surfaces were coated with a thin layer of gold before SEM characterization. From Figure 5a, one can see that even though the content of BN achieves 10 wt %, BN particles exhibit relatively homogeneous dispersion in

Table 2. Melting and Crystallization Parameters of All Samples Obtained through DSC Measurements samples

ΔHm (J/g)

Xc (%)

Tm (°C)

Tc (°C)

ΔHc (J/g)

PVDF PVDF/BN-1 PVDF/BN-2 PVDF/BN-5 PVDF/BN-10 PVDF/BN-20 PVDF/CNT PVDF/BN-1/CNT PVDF/BN-2/CNT PVDF/BN-5/CNT PVDF/BN-10/CNT PVDF/BN-20/CNT

35.9 35.0 30.8 31.9 28.3 25.4 33.6 29.9 29.5 31.3 31.5 28.8

34.3 33.8 30.0 32.1 30.0 30.3 32.7 29.4 29.3 32.1 34.2 35.3

172.5 171.7 170.1 171.7 170.7 171.5 172.0 171.3 171.5 170.2 169.9 171.3

139.2 145.8 148.3 151.1 151.7 152.0 148.1 149.9 150.5 150.8 150.9 151.8

−49.1 −43.0 −42.8 −35.2 −30.2 −26.2 −43.0 −41.5 −34.3 −35.7 −34.2 −34.0

Tm and Tc to those of the binary composites. In other words, there is no synergistic effect between BN and CNTs in influencing the crystallization of PVDF matrix. However, one can see that the presence of CNTs really induces the slight increase of Xc. This is possibly one of the reasons that contributes to the improved thermal conductive properties of the PVDF/BN/CNT composites compared with the binary PVDF/BN composites. However, it should be stressed that among all these samples, the variations of Xc of PVDF matrix are relatively small, and therefore, it is easily to ask whether there is another factor contributing to the large enhancement of thermal conductivity or not. 3.3. Dispersion and Microstructure of Filler. It is wellknown to all that if the conductive fillers form the network structure in the matrix, the thermal conductivity of the material can be greatly enhanced.51 To better understand the thermal

Figure 5. SEM images showing the dispersion states of BN particles in the PVDF/BN and PVDF/BN/CNT composites. The cryo-fractured surfaces were coated with a thin layer of gold before SEM characterization. (a) PVDF/BN-10, (b) PVDF/BN-10/CNT, (c) PVDF/BN-20, and (d) PVDF/BN-20/CNT. F


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composites. Compared with the PVDF/CNT sample, the PVDF/BN-5/CNT sample (Figure 6b) exhibits more CNT aggregates in the SEM image and simultaneously, one can see that these CNT aggregates contact each other. Denser CNT aggregates between BN particles are observed for the PVDF/ BN-10/CNT sample (Figure 6c). In this condition, BN particles exhibit the role of volume exclusion, increasing the probability for the contact of CNTs.52 This implies that besides the thermal conductive path of BN particles, the other thermal conductive path arising from CNTs can also form between BN particles. However, it should be stressed that the effective thermal conductive path of CNTs is not formed in the whole sample but only formed in the local range because of the relatively low content of CNTs. Further increasing BN content up to 20 wt % (Figure 6d), due to the more serious accumulation of BN particles and the more apparent volume exclusion effect, most of the CNTs are covered or limited in a smaller zone and in this condition, and it is challenging to observe CNTs from the SEM image. Rheological measurement has been widely used to investigate the microstructure−performance relationship of materials, and especially, it provides not only the information about processing flow ability but also the microstructure information on fillers in the composite melt.53 It is widely accepted that if a plateau is observed in the storage modulus curve at low frequencies, one can judge that a network-like structure of fillers forms in the melt, because the melt exhibits more apparent elastic characteristic, namely, the storage modulus exhibits relatively lower dependence on shear frequency.54 Figure 7 exhibits the storage modulus (G′), loss modulus (G″), Cole−Cole plots of G′ versus G″, and the complex viscosity (η*) of all samples. According to the variations of G′, G″, and η*, one can see that adding a few BN particles (≤10 wt %), the PVDF/BN composites exhibit very similar rheological results to that of the pure PVDF sample. This means that a few BN particles do not influence the processing flow ability of the composites. Furthermore, from Figure 7d, one can see that at relatively lower frequency ranges, the PVDF/BN composites exhibit the feature of a Newtonian fluid with nearly invariant η*. The PVDF/BN-20 sample exhibits higher G′, G″, and η* compared with the pure PVDF sample. Specifically, one can observe an apparent plateau of the flow regime at low frequency ranges where G′ changes slightly with increasing frequency. This indicates the transition from liquid-like to solid-like viscoelastic behavior induced by the greatly restrained longrange molecular chain motion. Such a transition is generally induced by the presence of percolated filler network structure.53 Namely, the network structure of BN particles is present in the composite containing 20 wt % BN. The PVDF/CNT composite also exhibits the plateau of G′ at low frequency ranges, indicating the formation of the CNT network structure. Specifically, one can see that G′ of the PVDF/CNT sample is very similar to that of the PVDF/BN-20 sample. This implies that 2 wt % CNTs exhibit the similar role of 20 wt % BN particles in restricting the long-range molecular chain motion of the PVDF matrix. The different degrees of restriction for molecular chain motion between the onedimensional CNTs and two-dimensional fillers, such as montmorillonite, have been reported in the literature,55 and the mechanisms are mainly related to the different geometric shapes and aspect ratios of fillers. Interestingly, the PVDF/BN/ CNT composites exhibit higher G′, G″, and η* compared with the PVDF/CNT and PVDF/BN composites. Specifically, a

the PVDF matrix, and few BN particles contact each other. Furthermore, interfacial debonding that induces the gaps between BN particles and PVDF matrix is clearly observed. This implies the poor interfacial interaction in the composites. At 20 wt % BN content (Figure 5c), many BN particles can be seen in the whole range of view, and especially, these BN particles contact each other. In this condition, an effective thermal conductive path can be formed in the material. Compared with the dispersion states of BN particles in the PVDF/BN composites, it seems that the PVDF/BN/CNT composites (Figure 5b,d) exhibit more homogeneous dispersion of BN particles. The presence of CNTs (2 wt %) results in the great increase of melt viscosity of material, and in this condition, relatively larger shear stress is possibly brought to the BN particles, which facilitates the good dispersion of BN particles on one hand. On the other hand, CNTs that locate between BN particles also exhibit the steric hindrance effect, which prevents and/or delays the contact of adjacent BN particles. Furthermore, one can see that the surfaces of BN particles are rather smooth, which further demonstrates the poor interfacial interaction between BN and PVDF matrix. From Figure 5, one cannot clearly observe the dispersion states of CNTs in the composites, and therefore, the cryofractured surfaces were further characterized without coating gold. In this condition, only the conductive CNTs can be clearly observed. Similarly, the morphology of the PVDF/CNT composite was also characterized. As shown in Figure 6a, some

Figure 6. SEM images showing the dispersion states of CNTs in the composites. The cryo-fractured surfaces were directly characterized without coating gold. (a) PVDF/CNT, (b) PVDF/BN-5/CNT, (c) PVDF/BN-10/CNT, and (d) PVDF/BN-20/CNT.

aggregates of CNTs are observed in the PVDF/CNT sample. However, it is worth noting that these CNT aggregates are nearly sporadically dispersed in the PVDF matrix. Obviously, such dispersion state of CNTs does not facilitate the enhancement of thermal conductivity because there is no efficient thermal conductive path in the sample. This is also the reason why the PVDF/CNT sample exhibits only thermal conductivity of 0.28 Wm−1 K−1, as shown in Figure 2. Interestingly, apparent changes of CNT morphology with increasing BN content are observed for the PVDF/BN/CNT G


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Figure 7. Rheological properties of all the samples as indicated in the graphs. (a) Storage modulus, (b) loss modulus, (c) Cole−Cole plots of storage modulus versus loss modulus, and (d) complex viscosity.

indicate the formation of the percolated network structure.60 In this work, the electrical conductivity of the PVDF-based composites containing CNTs was also measured. As shown in Figure 8, the PVDF/CNT composite exhibits the electrical

more pronounced plateau of G′ is observed. This indicates that a denser network structures of fillers are formed in these samples. This can be further demonstrated from the variations of Cole−Cole plots of G′ versus G″ that is shown in Figure 7c. For the PVDF/BN/CNT composites, the increase of G′ is higher than the increase of G″ with increasing filler content, and a deviation from a linear relationship between G′ and G″ is obtained, which represents the presence of the filler network structures in the melts.56 The rheological results demonstrate that denser percolated network structure of fillers is formed in the PVDF/BN/CNT composites. Here, BN particles are two-dimensional (2d) structure, whereas CNTs are one-dimensional (1d) structure. Many researchers have demonstrated that with the simultaneous presence of 1d and 2d fillers, they can form the hybrid three-dimensional (3d) network structure in the polymer matrix.11,40−42 Such hybrid 3d network structure usually exhibits an excellent reinforcement effect, leading to the great enhancement of mechanical properties.57 Once the 1d and 2d fillers are electrically conductive fillers (e.g., CNTs and graphene), they can greatly enhance the electrical conductivity of the composites.58,59 Here, both BN and CNTs are thermally conductive fillers, and therefore, the formation of the hybrid 3d network structure of BN and CNTs mainly contributes to the enhancement of the thermal conductivity of the PVDF/BN/ CNT composites. 3.4. Electrical Conductivity. Another method that is widely adopted to demonstrate the variation of microstructure and/or the formation of the percolated network structure of conductive fillers is electrical conductivity measurement. For most polymer composites containing conductive fillers, the enhancement of electrical conductivity depends on the content of fillers. A term of percolation threshold, at which a step change of electrical conductivity occurs, is usually used to

Figure 8. Variations of electrical conductivity of samples containing CNTs.

conductivity of 8.4 × 10−1 S/m, which is much higher than that of the pure PVDF sample as reported in the literature.61 For the PVDF/BN/CNT composites, it can be thought as the scenario that adding BN particles into the PVDF/CNT composites and then investigating the effect of BN content on electrical properties of the composites. It can be seen that adding a small amount of BN particles facilitates the enhancement of electrical conductivity. Specifically, for the PVDF/BN-5/CNT sample, the electrical conductivity is enhanced up to 4.5 S/m, which is H


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probability for the contact of the adjacent BN particles, and the conductive network structure can be formed at relatively high BN content (Figure 9c). For the PVDF/CNT composite (Figure 9d), although it exhibits the electrically conductive path, which leads to the great improvement of electrical conductivity, it does not exhibit the effective thermally conductive path. Therefore, a relatively low thermal conductivity is achieved. However, adding CNTs into the PVDF/ BN composites, especially at relatively low BN content (≤10 wt %) (Figure 9e), results in CNTs forming the effectively thermal conductive path in a local range because of the volume exclusion of BN particles. Furthermore, the 1d CNTs and 2d BN also form the 3d network structure. Consequently, the largely enhanced thermal conductivity and improved electrical conductivity are simultaneously achieved. Further increasing BN content results in denser network structure of CNTs and BN (Figure 9f), which facilitates the further enhancement of the thermal conductivity of the composites. However, due to the increased steric hindrance effect of BN particles, the electrically conductive path of CNTs is destroyed, and accordingly, decreased electrical conductivity is achieved for the composites. It is well-known to all that the thermal conductivity of polymer-based composites is greatly influenced by the interfacial thermal resistance between polymer matrix and thermally conductive fillers. Generally, the strong interfacial interaction facilitates the transmission of the phonon across the interface and the reduction of phonon scattering occurred at the interface and finally, facilitating the enhancement of thermal conductivity of the composites. Furthermore, the contact thermal resistance among filler particles also influences the thermal conductivity of composites. Surface modification of thermal conductive fillers is proven as an efficient way to reduce the interfacial thermal resistance.7,26−28 In the present work, both BN and CNTs particles were directly used without any surface modification. Therefore, it is easy to ask whether the surface modification of BN and CNTs is favorable for the further enhancement of thermal conductivity of the ternary composites or not. Most likely, further enhanced thermal conductivity will be achieved at relatively lower filler content. This will be demonstrated in our future work. 3.6. Dielectric Properties. It is well-known that CNTs can be used to improve the dielectric properties of common polymers due to their excellent conductive properties.62 When the content of CNTs achieves the percolation threshold, the percolated network structure of CNTs can be formed in the whole matrix, and consequently, high dielectric constant (ε′) can be achieved for the composites. However, the high leakage current originated from direct contact between CNTs also results in the large dielectric loss (tan δ). Therefore, to achieve the excellent dielectric properties, the CNT network structure must be controlled in a small zone rather than in the whole sample.63 Here, the variation of electrical conductivity of the PVDF/BN/CNT composites with increasing of BN content implies that the composites possibly exhibit excellent dielectric properties. Therefore, the dielectric properties of the PVDF/ BN/CNT composites were also measured, and the results are shown in Figure 10. For better understanding, the comparison of ε′ and tan δ obtained at frequency of 100 Hz is shown in Figure 10c. For making a comparison, the results of pure PVDF and PVDF/CNT composites are also shown. It is worth noting that the dielectric properties of the PVDF/BN composites were not measured because there are no electrically conductive fillers

435.7% higher than that of the PVDF/CNT sample. This is because BN particles are insulating, and they do not contribute to the enhancement of electrical conductivity by themselves. Therefore, it is suggested that the change of CNTs microstructure mainly contributes to the enhancement of electrical conductivity. In other words, with the presence of BN particles, CNTs form a more effective conductive path. This can be demonstrated by the observations obtained from previous SEM characterizations. However, too many BN particles, especially when they form the percolated network structure, exhibit the steric hindrance effect and isolate the conductive path of CNTs. Consequently, a great decrease of the electrical conductivity is achieved for the PVDF/BN-10/CNT and PVDF/BN-20/CNT samples. 3.5. Thermal Conductive Mechanism. According to the above results obtained through conductive measurements and morphological characterization, the thermal conductive mechanisms can be drawn as follows. As described above, the thermal conductivity of polymer composites are mainly determined by several factors, including the crystalline structure of polymer matrix, the dispersion of conductive fillers, and the interfacial interaction between conductive fillers and polymer matrix.17−23 In this work, both BN and CNTs were directly used without any surface modification, and therefore, adding CNTs does not improve the interfacial interaction between BN and PVDF. In other words, the enhancement of thermal conductivity is not attributed to the reduction of interfacial thermal resistance of the composites. Although the presence of CNTs induces the increase of Xc of the PVDF matrix, the increase is rather small, and therefore, it is believed that the enhanced Xc is favorable for the enhancement of the thermal conductivity but not the determinable factor. Obviously, the formation of the 3d hybrid network structure of BN and CNTs mainly contributes to the great enhancement of the thermal conductivity. To better understand the mechanism for the enhancement of the thermal conductivity, more visualized illustrations are proposed and shown in Figure 9. For the PVDF/BN composites, at relatively low BN content, it is difficult for BN particles to form the thermally conductive path (Figure 9b). However, increasing BN content increases the

Figure 9. Schematic representations showing the morphological changes of fillers in the different PVDF composites. (a) Pure PVDF, (b) PVDF/BN composites with low BN content, (c) PVDF/BN composites with high BN content, (d) PVDF/CNT composite, (e) PVDF/BN/CNT composites with low BN content, and (f) PVDF/ BN/CNT composites with high BN content. I


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Figure 10. (a) Dielectric constant and (b) dielectric loss of samples as indicated in the graphs. (c) Comparison of the dielectric constant and dielectric loss of samples obtained at 100 Hz.

composites. However, a denser network structure of fillers is achieved for the PVDF/BN/CNT composites. Consequently, it is suggested that the mechanism for the largely enhanced thermal conductivity is mainly related to the formation of the denser 3d hybrid BN/CNT network structure. Furthermore, due to the volume exclusion and space steric hindrance effects of BN particles, CNTs form a more effective electrical conductive path in a local range, and consequently, the PVDF/BN/CNT composites exhibit high dielectric constant with low dielectric loss. Therefore, the PVDF/BN/CNT composites have great potential application as materials for electronic devices and they meet the requirement of further miniaturization of electronics.

in the composites, and in this condition, ε′ is very small and can be ignored. From Figure 10, one can see that although the PVDF/CNT composite exhibits high ε′ at low frequency ranges, it also exhibits high tan δ, which can be attributed to the formation of the CNT network structure in the whole sample, as demonstrated by the above rheological measurements. Interestingly, the PVDF/BN/CNT composites containing BN content lower than 10 wt % exhibit much higher ε′ compared with the PVDF/CNT sample, whereas tan δ of the respective samples is comparable. However, further increasing BN content results in the great decrease of ε′. Specifically, the PVDF/BN20/CNT sample exhibits very low ε′, which is very similar to that of the pure PVDF sample. This indicates that the dielectric properties of the composites are greatly weakened. Regardless, the previous results demonstrate that through the appropriate formulating of recipe, the PVDF/BN/CNT composites with simultaneously enhanced thermal, electrical, and dielectric properties can be obtained.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-28-87603042. Notes

The authors declare no competing financial interest.

4. CONCLUSIONS In conclusion, the PVDF/BN/CNT composites are fabricated by adding a few CNT (2 wt %) into the PVDF/BN composites. Compared with the PVDF/BN composites, largely enhanced thermal conductivity is achieved for the PVDF/BN/CNT composites. Studies on the crystalline structure of PVDF matrix demonstrate that the crystallinity of PVDF matrix is slightly increased by the presence of CNTs, whereas the crystal form remains invariant. Studies on the dispersion and microstructures of fillers in the composites demonstrate that BN particles exhibit relatively homogeneous dispersion in the PVDF matrix at relatively low BN content, which does not apparently influence the rheological properties of the

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51203129, 50973090) and the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (Grant No. sklpme2015-421).

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Largely Enhanced Thermal Conductivity and High Dielectric Constant

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