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Materials and Interfaces

Enhanced Thermal Conductivity of Segregated Poly(Vinylidene Fluoride) Composites via Forming Hybrid Conductive Network of Boron Nitride and Carbon Nanotubes Zhi-Guo Wang, Yan-Fei Huang, Guoqiang Zhang, Han-Qin Wang, Jia-Zhuang Xu, Jun Lei, Lei Zhu, Feng Gong, and Zhong-Ming Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01764 • Publication Date (Web): 04 Jul 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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Industrial & Engineering Chemistry Research

Enhanced Thermal Conductivity of Segregated Poly(Vinylidene Fluoride) Composites via Forming Hybrid Conductive Network of Boron Nitride and Carbon Nanotubes

Zhi-Guo Wang,† Yan-Fei Huang,† Guo-Qiang Zhang,‡ Han-Qin Wang,† Jia-Zhuang Xu,*,† Jun Lei,† Lei Zhu,‡ Feng Gong,§ and Zhong-Ming Li†

†

College of Polymer Science and Engineering, State Key Laboratory of Polymer

Materials Engineering, Sichuan University, Chengdu 610065, China ‡

Department of Macromolecular Science and Engineering, Case Western Reserve

University, Cleveland, Ohio 44106-7202, United States §

School of Energy Science and Engineering, University of Electronic Science and

Technology of China, Chengdu 611731, China

* Corresponding author. Tel.: +86-28-8540-6866; Fax: +86-28-8540-6866. E-mail: [email protected] (J.-Z. X.).

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ABSTRACT: This work placed an emphasis that constructing segregated boron nitride (BN)/carbon nanotube (CNT) hybrid network brought an immense benefit to enhance the thermal conductivity (TC) of poly(vinylidene fluoride) (PVDF) composites. The segregated composites ((CNT + BN)@PVDF) showed a high TC of 1.8 W/mK at the total filler fraction of 25 vol%, outperforming PVDF composites with random structure (CNT/BN/PVDF) and segregated BN structure (BN@PVDF) by 169% and 50%, respectively. Infrared thermal images further demonstrated that (CNT + BN)@PVDF exhibited superior capability to dissipate heat compared to BN/PVDF. The segregated architecture increased the effective utilization of fillers and interfacial thermal resistance between neighboring BN platelets was reduced by the bridging effect of CNTs. Molding pressure and temperature governed the integration of segregated networks and thus the enhancement efficiency of TC. The design of hybrid segregated structure holds promise in a broad range of the preparation of thermal management materials. Keywords: Thermal conductivity; Segregated structure; Hybrid filler network; Poly(vinylidene fluoride)


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1. INTRODUCTION The continuing surge of slimmer and microminiaturized electronic devices is setting

stringent

requirements

and

imperious

demands

for efficient

heat

management.1-7 Rapid transfer of the heat has become mandatory in modern electronic devices. By virtue of superior piezoelectric and pyroelectric properties, poly(vinylidene fluoride) (PVDF) attracts intriguing attentions as a material of choice for transducers, actuators, supercapacitors, batteries, etc.8-10 However, the widespread application of PVDF is plagued by the intrinsic low thermal conductivity (TC), typically in a value of 0.22 W/mK.11 The TC enhancement of PVDF is indispensable to tackle the challenges in advanced electronic equipment. Incorporation of thermal conductive fillers is the most popular approach to improve the TC of PVDF.4,12-18 However, the conventional way by maximizing filler content and randomly dispersing them in polymers as much as possible poses a limitation on the enhanced efficiency of TC. More seriously, it causes severe deterioration of both mechanical performance and economic benefit.19-20 To obtain high TC at relatively low filler content, the key is to reduce thermal contact resistance, which is regarded as the pitch point for phonon propagation at the filler/matrix and/or filler/filler interface21 Surface modification of the thermal conductive fillers could strengthen their affinity to the polymer matrix and reduce phonon scattering. It was reported that 10 wt% polyvinylpyrrolidone-modified carbon nanotubes (CNT) increased the TC of PVDF to 0.63 W/mK, 34% higher than that of the unmodified CNT/PVDF counterpart.22 The use of hybrid fillers with different size, shape,



aspect-ratio, and dimension has been demonstrated to bring synergistic effects on the TC enhancement of the composites.23-25 Zhang et al. reported that addition of only 1.0 wt% graphene oxide in 10 wt% CNT/PVDF composites could increase the TC from 0.47 W/mK to 0.95 W/mK.26 The PVDF composites containing 2 wt% CNT and 20 wt% boron nitride (BN) showed the TC of 1.30 W m-1 K-1, exceeding the 20 wt% BN/PVDF composite by 34%.27 The combined use of graphene nanoplatelets (GNPs, 20 wt%) and CNT (2 wt%) in PVDF achieved a TC of 1.92 W/mK, 17% higher than the composites loaded with 20 wt% GNPs (1.64 W/mK).28 On the other hand, control over the distribution of the fillers could increase their connectivity to form more thermal conductive paths in the composites.29-34 The TC of immiscible PVDF/polystyrene blends was increased from 0.4 W/mK to 1.85 W/mK by selective localization of CNTs (2.9 vol%) and silicon carbide (11.4 vol%) in minor PVDF phase.35 But the surface tension and wetting coefficient between the filler and matrix are required to be matched closely. Recently, a great deal of efforts have been devoted to fabricating the composites with segregated structure, in which the functional fillers are primarily located at the interfaces between polymer granules instead of being randomly arranged throughout the entire matrix.36-41 The segregated network circumvents the interparticle thermal barrier and thermal conductive paths can readily form, leading to a very low percolation threshold for TC enhancement.42 On the basis of the above context, we presented a facile and viable method to fabricate PVDF composites with high TC by constructing segregated structure of BNs and CNTs. It was found that TC of the segregated hybrid PVDF composites was


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significantly increased compared to TC of the random hybrid composite or segregated composite with BN only. The enhancement efficiency of TC was bound up with the integration of hybrid segregated structure, which was verified by controlling the processing pressure and temperature.

2. EXPERIMENTAL SECTION 2.1 Materials PVDF (Solef 6020) powders with a melt flow rate of 2.0 g/10 min (230 oC/21.6 kg) and a density of 1.78 g/cm3 was purchased from Solvay, China. The diameter of PVDF powders is 100 ~ 200 µm (Supporting Information, Figure S1). Hexagonal BN with an average plate size of 30 µm and the thickness of ~ 40 nm (Figure S2) was kindly offered by Ya’an Bestry Performance Materials Co., Ltd., China. CNT (NC7000) with an average diameter of 9.5 nm and length of 1.5 µm were supplied by Nanocyl S.A., Belgium. 2.2 Preparation of hybrid segregated composites Fabrication process of the segregated hybrid PVDF composites was schematically shown in Figure 1. PVDF granules were mechanically wrapped by different volume content of CNTs and BN plates in sequence using a high-speed mixer at 22000 r/min for 3 min and 2 min, respectively. To obtain uniform distribution, the mixture was then subjected to ball-milling treatment at a rotation speed of 500 r/min for 30 min. Subsequently, the segregated hybrid composites were consolidated by compression molding at the preset temperatures (190, 200, 210, 220, and 230 oC) and under different pressures (10, 100, 200, 300, and 400 MPa). For the sake of



brevity, the segregated hybrid composites were labeled as (CNTx + BNy)@PVDF, where the x:y is the volume ratio of CNTs to BN in the composites. Note that the total content of fillers was fixed at 25 vol% unless otherwise specified. PVDF with randomly distributed CNTs and BN (denoted as CNTx/BNy/PVDF) were fabricated as control samples, which were melt mixed by an internal mixer (Haake RC-90, Germany) at 190 oC under 40 rpm/min for 10 min and then compression molded at 190 oC, 10 MPa for 5 min.

Figure 1. Schematic representation showing the preparation process of the (CNTx + BNy)@PVDF composites.

2.3 Characterization Thermal diffusivities (α) and specific heat capacity (Cp) were measured using a laser flash diffusivity instrument (LFA467, NETZSCH, Germany) from 25 to 105 oC. Thermal conductivity (λ) was calculated by the equation λ = α*Cp*ρ, where ρ is the density of the composite. At least three specimens were tested for each group and all tested specimens were punched from the center of the as-prepared sheets. A field-emission scanning electron microscopy (FE-SEM, Nova Nano450, FEI, USA) was used to observe the filler distribution at an accelerating voltage of 5 kV. The


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cryo-fractured surface of the composites was sputter-coated with a thin layer of gold before SEM observation. Energy dispersive X-ray spectrometry (EDS, X-Max Extreme, OXFORD, Britain) mapping was performed to analyze the composition of elements and further confirm the filler network. The variation of the surface temperature was recorded by an infrared thermograph (IR-160P, RNO, USA). The samples were held at 70 oC for 2 h on a hot stage to attain the thermal equilibrium and then transferred to a thermal insulating foam plate at environment temperature rapidly. Schematic diagram of an infrared thermograph experiment was shown in Figure S3. The surface temperature difference between composites and PVDF is defined as ∆T(t) = Tcomposite (t) – TPVDF (t).

3. RESULT AND DISCUSSION Figure 2a plots the TCs of (CNT + BN)@PVDF with respect to the volume ratio of CNTs to BN (total volume fraction of fillers is 25 vol%). The results of CNT/BN/PVDF are also present for comparison. The TC of both the (CNT + BN)@PVDF and CNT/BN/PVDF composites exhibits an ascending tendency with CNTs until the volume ratio of CNTs to BN platelets is 1:12. It indicates that there is a synergistic effect between BN platelets and CNTs on the enhancement of TC. More intriguingly, the (CNT + BN)@PVDF composites always show much higher TC than CNT/BN/PVDF composites at the same volume ratio. For example, the TC of (CNT1 + BN12)@PVDF is 1.32 W/mK, almost being twice as much as that of CNT1/BN12/PVDF (0.67 W/mK). The synergistic effect is amplified by constructing



the hybrid segregated network, due to the formation of more efficient conductive paths. To intuitively compare the synergistic efficiency, the enhancement percentage of TC (f) is defined as follows:

f =

T

_T BN / PVDF

composite

T

(1)

BN / PVDF

where Tcomposite and TBN/PVDF represent the TC of the composites loaded with hybrid fillers and single BN platelets, respectively. As shown in Figure 2b, the varied tendency of f is consistent with that of TC of the composites. The maximum f is ~ 165% for the composites with random structure, while it reaches as high as ~490% for the composites with hybrid segregated structure. Given that the microstructure of polymer composites is significantly influenced by molding condition, the effect of molding pressure on the TC of hybrid segregated composites is explored. As illustrated in Figure 2c, a monotonously upward tendency of TC is observed with increasing molding pressure for both the (CNT + BN)@PVDF and CNT/BN/PVDF composites. The TC of (CNT1 + BN12)@PVDF climbs from 1.32 to 1.80 W/mK as the molding pressure increases from 10 to 400 MPa. It is ascribed to the suppression of defects or porosity in the composites, being beneficial to reduce the thermal contact resistance. The high molding pressure allows CNTs to better bridge BN platelets in the segregated conductive paths. Thus, a more remarkable improvement in the f is obtained for (CNT + BN)@PVDF composites than BN@PVDF composites (Figure 2d). Figure 2e shows a temperature dependence of TC for (CNT + BN)@PVDF and BN@PVDF. There appears a decrease in TC for both composites from 25 to 105 °C.


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It is because that phonon collision along the conductive paths is aggravated with the increase of temperature. Despite this, the TC of (CNT + BN)@PVDF is superior to the BN@PVDF in the range of test window and the corresponding f still maintains at a high level (Figure 2f). Table 1 compares the TC of our segregated hybrid PVDF composites and the other PVDF composites reported in the literature. It could be found that the segregated hybrid PVDF composites exhibit relatively high TC at low volume fraction of the fillers, emphasizing the superiority of our proposed strategy in terms of cost saving, light weight, and enhancement efficiency. The TC of (CNT1 + BN12)@PVDF composite is 1.8 W/mK at 25% filler loading, which is the highest value among all the reported PVDF composites at the same filler loading.20 Though the 60 vol% barium titanate (BT)/SiC/PVDF and 60 vol% Al/β-SiCw/PVDF composites show the TC of 1.67 and 2.52 W/mK, respectively,12,15 overloading the fillers could results in a sacrifice of the processability and mechanical performance.



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Figure 2. TC (a) and the corresponding enhancement percentage (b) of CNT/BN/PVDF and (CNT + BN)@PVDF composites as a function of the volume ratio of the hybrid fillers; TC (c) and the corresponding enhancement percentage (d) of BN@PVDF and (CNT1 + BN12)@PVDF composites under different molding pressure; TC (e) and the corresponding enhancement percentage of BN@PVDF and (CNT1 + BN12)@PVDF composites under 400 MPa with respect to molding temperature.

Table 1. Comparison on the TC of our hybrid segregated PVDF composites with the previously reported PVDF composites. Type of filler BN/CNT

Filler content

TC (W/mK)

Ref

25 vol%

1.80

This work


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CNT

10 wt%

0.63

22

GO

10 wt%

0.58

11

Zn

28 vol%

1.20

20

AlN

60 vol%

5.02

4

GO/CNT

11 wt%

0.95

26

BN/CNT

22 wt%

1.3

27

GNP/CNT

22 wt%

1.92

28

BT/SiC

60 vol%

1.67

12

Al/β-SiCw

60 vol%

2.52

15

In order to visually identify the heat dissipation capacity of the segregated hybrid composites,43 the evolution of the surface temperature with cooling time were recorded by an infrared thermograph. As shown in Figure 3a, initial thermal equilibrium is established by holding the specimens at 70 oC for 2 h and then quickly transferred to a thermal insulating foam stage at room temperature. It could be observed that the decline of surface temperature follows the sequence: 400 MPa-(CNT1 + BN12)@PVDF (the composite is consolidated under 400 MPa) > (CNT1 + BN12)@PVDF > BN/PVDF. To be more specific, the surface temperature of BN/PVDF and (CNT1 + BN12)@PVDF is 37.9 and 37.6 oC, respectively, at the time of 90 s, while it is 37.2 oC for 400 MPa-(CNT1 + BN12)@PVDF. Figure 3b illustrates the surface temperature difference between the PVDF composites and the neat PVDF. It is noted that 400 MPa-(CNT1 + BN12)@PVDF shows the fastest heat dissipation



among all three samples. The maximum temperature difference exceeds 1.3 oC for 400 MPa-(CNT1 + BN12)@PVDF at the cooling time of 160 s, significantly higher than (CNT1 + BN12)@PVDF and BN/PVDF. It manifests that the heat dissipation is much faster for the segregated composites than the random composites, especially for the segregated hybrid composites. Combining the synergistic effect of employed hybrid fillers and segregated structure provides a highly feasible means to prepare thermal interface materials with high TC.

Figure 3. Infrared thermal images (a) and surface temperature variation (b) as a function of time for BN/PVDF (black), (CNT1 + BN12)@PVDF composites (blue), and (CNT1 + BN12)@PVDF composite under 400 MPa (red).

It has been well established that the TC of polymer composites is donimated by thermally conductive network composed by the fillers. As shown in Figure 4a, hybrid fillers are interspersed in the CNT1/BN12/PVDF composites randomly. More information about the distribution of the fillers and matrix can be gained from the EDS mapping, where the fluorine (F), boron (B), nitrogen (N) elements exhibit a


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uniform distribution (Figure 4b, 4c, and 4d). Such a fact illustrates that a high filler loading is reqiured to built the connection between the fillers, resulting in a high percolation threshold for TC enhancement. In contrast, the hybrid fillers in (CNT1 + BN12)@PVDF composites are primarily located at the interfacial regions between the PVDF granules. Close inspection of EDS mapping images reveals that the thermal conductive network is readily formed by the merit of segerated structure (Figure 4f, 4g, and 4h). The crystalline structure of the polymer matrix is also a key factor to influence the TC of the composites.2,44-47 The melting behavior of as-prepared composites is compiled in Table S1. All the samples present a similar melting point as well as crystallinity. For example, the melting point and crystallinity of the (CNT1 + BN12)@PVDF sample is 173.1 oC and 46.1%, in comparison to CNT1/BN12/ PVDF (173.6 oC and 47.0%). Thus, the improvement of the TC for (CNT + BN)@PVDF composites results exclusively from the segregated thermal conductive networks created by the hybrid fillers.



Figure 4. SEM fracture micrographs of CNT1/BN12/PVDF (a) and (CNT1 + BN12)@PVDF composites (e); The electron mapping images of CNT1/BN12/PVDF and (CNT1 + BN12)@PVDF composites: F (b, f), N (c, g), and B (d, h).


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Figure 5. The TC of the (CNT1 + BN12)@PVDF composite molded at different compression temperature. The insets show the morphology of the hybrid filler network.

To demonstrate the contribution of segregated network to the TC enhancement, we measured the TC of (CNT + BN)@PVDF composites molded at the different temperature. As shown in Figure 5, one can easily observe that the TC gradually declines with the compression temperature. Accordingly, the fillers permeate into the PVDF matrix and the integrity of the hybrid segregated structure was damaged. Based on the above results, the enhancement mechanism is explained and illustrated in Figure 6. Benefiting from the fact that CNTs act as bridges between BN platelets for phonon transport, the use of BN platelets and CNTs in the PVDF composites leads to the synergy enhancement of TC. For CNT/BN/PVDF composites,



the thermal network is isotropic with very few percolated conducting pathways, owing to the random dispersion of hybrid fillers (Figure 6a). Thermal dissipation via cooperation of BN and CNTs occurs across the whole sample volume, resulting in a limited enhancement. While constructing the segregated structure favors the creation of a percolated network, in which the phonons could be propagated along the conductive channels formed between PVDF granules (Figure 6b). In this situation, the CNTs have more chance to stack with the BN platelets, effectively reducing the thermal contact resistance. The synergistic effect is thus strengthened and a notable enhancement efficiency is achieved.

Figure 6. Schematic diagrams for the filler network of CNT/BN/PVDF (a) and (CNT + BN)@PVDF (b) composites. The thickness of the yellow dash line means the thermal conductive capability along the fillers and the matrix.


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4. CONCLUSION The construction of segregated conductive network with hybrid fillers was verified to bring an intensive synergistic enhancement in TC of the (CNT + BN)@PVDF composites. As demonstrated by infrared thermal images, the segregated hybrid composites showed higher heat dissipation capacity than the rival with randomly distributed fillers. The morphological observation manifested that the concentrated fillers were located at the interfacial regions between the PVDF granules, enabling CNTs to intimately bridge between neighboring BN platelets, so that TC could be further enhanced. Moreover, lower molding temperature and higher molding pressure during composite preparation are desirable to form more well-defined thermal conducting networks. Our work introduces a new and effective approach to prepare high-performance thermal conductive polymer composites for thermal management applications and other applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at. The SEM image of PVDF powders and BN platelets. Melting enthalpy, crystallinity and melting point of CNT/BN/PVDF and (CNT + BN)@PVDF composites. The schematic diagram of infrared thermograph experiment.



AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]. Tel.: +86-28-8540-0211. Fax: +86-28-8540-5402

ORCID Jia-Zhuang Xu: 0000-0001-9888-7014 Zhong-Ming Li: 0000-0001-7203-1453 Lei Zhu: 0000-0001-6570-9123 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors gratefully thank the financial support from the National Natural Science of China (51533004, 51773135, and 51528302), Supported by the Program of Introducing Talents of Discipline to Universities (B13040), and Outstanding Young Scholars Research Fund of Sichuan University (2016SCU04A17).

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