Preparation of Highly Thermally Conductive Polymer Composite at

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Preparation of Highly Thermally Conductive Polymer Composite at Low Filler Content via a Self-Assembly Process between Polystyrene Microspheres and Boron Nitride Nanosheets Xiongwei Wang† and Peiyi Wu*,†,‡ †

State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, P. R. China ‡ State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Chemistry, Chemical Engineering and Biotechnology, Center for Advanced Low-Dimension Materials, Donghua University, Shanghai 201620, China S Supporting Information *

ABSTRACT: Rational distribution and orientation of boron nitride nanosheets (BNNSs) are very significant for a polymer/BNNS composite to obtain a high thermal conductivity at low filler content. In this paper, a high-performance thermal interface material based on exfoliated BNNSs and polystyrene (PS) microspheres was fabricated by latex blending and subsequent compression molding. In this case, BNNSs and PS microspheres first self-assembled to form the complex microspheres via strong electrostatic interactions between them. The as-prepared complex microspheres were further hot-pressed around the glass transition temperature, which brought the selective distribution of BNNSs at the interface of the deformed PS microspheres. As a consequence, a polymer composite with homogeneous dispersion and high in-plane orientation of BNNSs in PS matrix was obtained. Benefitted from this unique structure, the resultant composite exhibits a significant thermal conductivity enhancement of 8.0 W m−1 K−1 at a low filler content of 13.4 vol %. This facile method provides a new strategy to design and fabricate highly thermally conductive composites. KEYWORDS: boron nitride, polystyrene microspheres, electrostatic interaction, orientation, thermal conductivity, polymer composite

1. INTRODUCTION Polymer materials with high thermal conductivity (TC) have attracted tremendous attention as the thermal interface materials (TIMs) because of their easy fabrication, light weight, and chemical resistance.1 Unfortunately, the TC value of a polymer material usually falls within a low range of 0.1−0.5 W m−1 K−1.2,3 To improve its thermal conductivity, various highTC inorganic fillers such as metal powders,4 carbon black,5 and ceramic particles6 have been introduced into the polymer matrix. However, the enhancement of TC is still limited due to the high thermal resistance between fillers and polymer matrix.7,8 To this end, a large amount of fillers are always required to construct a thermally conductive filler network throughout the whole sample, but this results in the high material cost and deteriorated mechanical properties.9−11 In recent years, a variety of new fillers, including carbon nanotubes (CNTs),12−14 graphene nanosheets,15−17 and boron nitride nanotubes (BNNTs)18,19 and nanosheets (BNNSs),20,21 have been extensively explored to composite with various polymer materials to achieve an improved heat conducting performance at relatively low filler content due to their superhigh thermal conductivity and large aspect ratio. Compared with graphene-based fillers, the BNNS has been © 2017 American Chemical Society

regarded as a promising candidate because it possesses a thermal conductivity comparable to that of graphene and large electrical resistance with a wide bandgap of ∼5.5 eV.22−25 As a typical two-dimensional nanosheet, BNNS has an anisotropic feature in thermal conductivity.18,26 The in-plane TC of BNNS can reach 2000 W m−1 K−1, much larger than that of the through-plane.11,24,25 Thus, the TC of BNNS-based composites should be largely associated with the BNNS alignment in polymer matrix.10,27,28 In previous studies, several strategies have been proposed to rationally control the alignment of BNNSs in polymer matrix such as hot-pressing,29 electrical field,26,28 strong shearing,27 vacuum-filtration,20,30 and doctor blading.8 Although these methods can effectively induce the BNNS orientation along a certain direction, the operation is relatively complex, and the filler aggregation is unavoidable. To sufficiently take advantage of the TC anisotropy of BNNSs, homogeneous dispersion as well as well-ordered orientation of BNNSs are both very important. However, there is still a great challenge to achieve these two factors at the same time. Received: April 4, 2017 Accepted: May 23, 2017 Published: May 23, 2017 19934

DOI: 10.1021/acsami.7b04768 ACS Appl. Mater. Interfaces 2017, 9, 19934−19944

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the formation mechanism of PS−BNNS complex microspheres and PS/oriented BNNS composites.

Given that high filler loading always causes many limitations, great efforts have been devoted to attaining the high TC at the filler content as low as possible. One approach uses a freestanding filler framework as the heat transfer medium, including graphene aerogel,31−33 BNNS aerogel,34,35 and complex aerogel,36−38 and then filling with polymer matrix. However, this method normally needs multiple processes and is difficult for large-scale fabrication. Another approach is the selective distribution of nanofillers in an immiscible polymer blend with cocontinuous structure, which can reduce the thermal percolation threshold to some extent but is still insufficient because of the narrow cocontinuous window.7,39,40 Compared to the selective distribution of nanofillers in one phase, selective distribution of nanofillers at the interfaces is expected to be an ideal strategy to achieve the continuous filler network at the lowest filler loading.17,41 However, it is greatly difficult for the filler to accurately distribute at the interface of an immiscible polymer blend. Recently, latex blending exhibits significant advantages to fulfill the distribution of nanofillers at the interface.42−52 The nanofillers first self-assemble with the polymer latex microspheres, and then the obtained complex microspheres have two routes to achieve the target polymer composites. One route further disperses the complex microspheres in a suitable polymer matrix, and the adsorbed fillers would naturally locate at the interface of these two polymer phases.42,43 Another one is directly hot-pressing these complex microspheres into polymer composite at above the glass transition temperature.44−46 During compression molding, the polymer latex microspheres would direct the nanofillers around them to form an three-dimensional (3D) filler network at the interface.44−46,49 Additionally, the latex blending can also effectively eliminate the filler aggregation that usually presented in melt and solution blending. However, previous research has mainly focused on graphene oxide (GO) nanosheets with various polymer microspheres due to their excellent mechanical flexibility and amphiphilic feature. For example, Yan et al. prepared a high-performance electromagnetic interference shielding composites with continuous interface filler network based on polystyrene/reduced graphene oxide complex micro-

spheres.53 In view of the hydrophobicity and rigidity of BN nanosheets, self-assembly with polymer microspheres is relatively difficult compared with GO nanosheets. Therefore, to the best of our knowledge, the composites fabricated from the polymer/BNNS complex microspheres via latex blending are rarely reported. Herein, we report the preparation of high-TC polymer composites through the rational distribution and orientation of boron nitride nanosheets in PS matrix. First, a PS−BNNS complex microsphere was prepared via the electrostatic interaction induced self-assembly. Afterward, the obtained PS/BNNS microspheres were further hot-pressed around the glass transition temperature into PS/oriented BNNS composites. As a result, a homogeneous and well-oriented BNNS filler network was constructed at the interface of the deformed PS microspheres. This filler network can act as a thermally conductive pathway for heat transfer. Benefitted from this unique structure, a high thermal conductivity 8.0 W m−1 K−1 was delivered at the BNNS content of 13.4 vol %, much higher than that of the PS/BNNS composites prepared by solution methods. This work may provide a facile method to design and fabricate the highly thermally conductive TIM.

2. EXPERIMENTAL DETAILS 2.1. Materials. Details of the used reagents and materials are given in the Supporting Information. 2.2. Preparation of Positively Charged Polystyrene Microspheres. Monodisperse PS microspheres with a mean size of ∼1.4 μm were prepared through a dispersion polymerization approach.54 Details of the preparation procedure of PS microspheres are given in the Supporting Information. Then, the obtained PS microspheres were washed by ethanol several times and redispersed in 200 mL of water with 0.5 wt % PDDA. The mixture was stirred at room temperature for 12 h and then washed by water 5 times to remove the excess PDDA, obtaining the PDDA modified PS microspheres (marked as PS@ PDDA). 2.3. Liquid Exfoliation of BN. The exfoliation of BN was performed by the method of Coleman et al.55 Details of the liquid exfoliation of BN powder are given in the Supporting Information. 2.4. Preparation of PS/BNNS Complex Microspheres. The PS−BNNS complex microspheres were prepared by dropwise adding 19935


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Figure 2. (a) TEM image of exfoliated BN nanosheets, and the inset is the high-resolution TEM image of the BNNS edge. (b) AFM image and (c) the corresponding height curve of exfoliated BN nanosheets. Thickness (d) and length (e) statistical distribution of exfoliated BNNS according to AFM and TEM images. the BNNS/IPA dispersion into the PS@PDDA/isopropanol dispersion under vigorous stirring for 30 min to ensure sufficient adsorption of BNNSs on the PS@PDDA microspheres. After that, the precipitate was collected by centrifugation at 1000 rpm and dried in vacuum oven at 50 °C overnight to remove the solvent. 2.5. Fabrication of PS/Oriented BNNS Composite and PS/ BNNS Composite. The as-prepared PS−BNNS complex microspheres were compressed into a composite sheet at 120 °C for 15 min with a pressure of 10 MPa, and then the composite sheet was further cold-pressed for 5 min with a pressure of 10 MPa. The obtained composites were denoted as PS/oriented BNNS-X, where X is the volume fraction of BNNS in the composite. As a reference, the PS/ BNNS composites with relatively irregular distribution of fillers were fabricated by a solution blending method. In a typical preparation, the BNNSs and PS microspheres were added into 20 mL of DMF and stirred at 70 °C for 24 h. Then, the mixture was directly poured into methanol. The obtained precipitates were dried under a vacuum at 80 °C overnight. After that, the composite products were further ground into powders and then compressed into a composite sheet at 120 °C for 15 min under a pressure of 10 MPa. The obtained PS/BNNS composites were denoted as PS/BNNS-Y and Y is the volume fraction of filler. 2.6. Characterization. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), X-ray diffractometery (XRD), atomic force microscopy (AFM), ζ-potential, differential scanning calorimetry (DSC), energy dispersive X-ray spectrometry (EDX), and a thermal diffusivity

instrument and universal electronic tensile machine were used to characterize the structure and features of the complex microspheres and the final composite sheets. The details are given in the Supporting Information.

3. RESULTS AND DISCUSSION The formation procedures of the PS−BNNS complex microsphere and PS/oriented BNNS composites are schematically described in Figure 1. Bulky BN power was first exfoliated to a few layers by vigorous sonication in isopropanol. After centrifugation, TEM was used to study the morphology of the exfoliated BNNSs. As shown in Figure 2a, BN nanosheets with lateral size above 500 nm are slightly transparent to the electron beam, suggesting their ultrathin feature. The highresolution TEM (HRTEM) image reveals the edge of a BN nanosheet with eight layers. Atomic force microscopy (AFM) was further used to probe the thickness of the exfoliated BNNSs (Figure 2b). The corresponding height marks show the thickness of about 3.8 nm for BNNS, which is consistent with the TEM results. According to the AFM and TEM images, the statistical distribution of thickness and length through counting over 120 pieces of BN nanosheets indicates that the exfoliated BNNSs are mainly 2−5 nm in thickness (Figure 2d) and 0.5− 1.5 μm in length (Figure 2e). Moreover, the ζ-potential of the 19936


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Figure 3. (a) ζ-potential of pure PS microspheres, PS@PDDA microspheres, exfoliated BNNSs, and PS−BNNS complex microspheres in isopropanol. (b) Photographs of BNNSs in isopropanol before and after addition of positively charged PS microspheres.

Figure 4. SEM images of PS−BNNS complex microspheres with different BNNS loading: (a) 0, (b) 3.0, (c) 5.9, (d) 8.5, (e) 11.1, and (f and g) 13.4 vol %. (h and i) TEM images of PS−BNNS complex microspheres with 13.4 vol % BNNSs.

exfoliated BNNSs in isopropanol is estimated to −34 mV (Figure 3a). Monodisperse PS microspheres with a particle size of ∼1.4 μm were synthesized via a dispersion polymerization (Figure S1). The ζ-potential value of −8 mV for the synthesized PS microspheres indicates the inaccessible self-assembly with the negatively charged BNNSs. Thus, the surface modification was conducted by poly(disllyldimethylammonium chloride) (PDDA) to endow PS microspheres with positive charges. After modification, the resultant PS@PDDA microsphere exhibits a ζ-potential value of 14.5 mV. Next, these PS@ PDDA microspheres were utilized to assemble with BNNSs to render these BN nanosheets adsorbed on the surface of PS microspheres. Figure 3b shows the photographs of BNNSs in isopropanol before and after adding PS@PDDA microspheres.

BNNSs show a homogeneous dispersion in isopropanol. While the PS@PDDA and BNNSs are mixed, the BNNSs fillers quickly settle to the bottom owing to the electrostatic selfassembly between them. Field emission scanning electron microscopy (FESEM) was performed to investigate the morphology of PS−BNNS complex microspheres. Figures 4a−g show the SEM images of neat PS microsphere and PS−BNNS complex microspheres with different BNNSs loading. A smooth surface is observed for the neat PS microspheres (Figure 4a). In contrast, the PS−BNNS complex microspheres show a rough structure due to the decoration of BNNSs on their surface, and the adsorbed BNNS content greatly increases with the BNNSs feeding (Figures 4b−e). When the BNNSs loading increases to 13.4 vol %, majority of the PS microspheres are coated by 19937


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Figure 5. Cross-section SEM images of the PS/oriented BNNS composite sheets with different BNNS loading: (a) 0 vol % (inset illustrates the observation angle of the cross-section), (b) 3.0, (c) 5.9, (d) 8.5, (e) 11.1, and (f) 13.4 vol %.

Figure 6. Cross-section SEM images of the PS/BNNS-13.4 composite sheet (a) and PS/oriented BNNS-13.4 composite sheet (b).

interface of deformed PS microspheres when the BNNS loading increases to a certain value. As shown in Figure 5a, the fractured surface of the neat PS sheet indeed shows many pielike PS microspheres with good interfacial adhesion along the in-plane direction (Figure S5). As for the composites filled with BNNSs, they all exhibit a homogeneous dispersion and high inplane orientation of BNNSs in the PS matrix, which are mainly profited from the uniform hybridization of PS microspheres with BNNSs in advance (Figures S6b−f). Additionally, the magnified cross-section images demonstrate that BNNSs mainly disperse at the interfaces of the deformed PS microspheres and gradually contact with each other to form a filler pathway as the BNNS content increases (Figures 5b−f). When the filler content is increased to 13.4 vol %, the majority of the BNNNs are directly connected together to construct a 3D filler network throughout the PS matrix. This filler network can be regarded as a thermally conductive pathway when a heat source is supplied on the composite. To further understand the distribution state of BNNS in the PS matrix, the cross-section morphology of the sample containing 13.4 vol % was further characterized by TEM, and the results are displayed in Figure S7. Obviously, the homogeneous dispersion and in-plane orientation of BNNSs in the PS matrix are also observed (Figure S7a). It should be noted that the voids in the cross section are mainly resulted from the pull-out of BNNS. Furthermore, one can see that the BNNSs contact directly with each other to form the continuous filler network (Figure S7b). For comparison, cross-section morphology of the composite

BNNSs (Figure 4f). Energy dispersive X-ray (EDX) elemental mapping plotted in Figure S2 also shows the uniform distribution of B and N on the PS microspheres. Moreover, it can be more clearly seen from the magnified image in Figure 4g that the small BN nanosheets are tightly coated on the PS surface, while the large BN nanosheets are partially anchored on the PS surface. Further increased BNNS feeding is not expected to increase the adsorption of BNNSs on the PS microspheres because of the adsorption saturation. As shown in Figure S3, the supernatant of the PS/BNNS dispersion with 15.6 vol % BNNS feeding after centrifugation is white and turbid (left), while the supernatant of the PS/BNNS dispersion with 13.4 vol % BNNS feeding exhibits transparency (right). The compact adsorption of BNNSs on the PS microsphere surface can be further observed by the TEM images (Figures 4h and i and S4a and b). The as-obtained PS−BNNS complex microspheres were further compressed into a composite sheet at 120 °C, which is slightly higher than the glass transition temperature of PS. Thus, the PS microspheres are easy to deform and form pie-like structures along the in-plane direction (Figure 1). Though the PS molecular chain segments are unfrozen at this temperature, the mobility of the whole molecular chains are still confined, which would prevent the BNNSs from penetrating into the PS phase. As a result, the adsorbed BNNSs are directed to distribute along the interface of the pie-like PS microspheres, resulting in the orientation of BNNS in the in-plane direction. Therefore, a continuous filler network is constructed at the 19938


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Figure 7. (a) Photographs of the neat PS sample and PS/oriented BNNS-8.5 sample for in-plane thermal conductivity measurement. (b) XRD patterns of the PS/oriented BNNS composite with different BNNS loading. Stress−strain curves (c) and volume resistivity (d) of PS, PS/oriented BNNS, and PS/BNNS composites.

that the peak intensity ratio of (002) and (100) can reveal the orientation degree of BNNSs in polymer matrix.8,26 The I002/ I100 ratio of PS/BNNS-8.5 complex microsphere powders is only 8.5, but this ratio sharply increases to 987 after compression, suggesting a high in-plane orientation of BNNSs in the PS matrix. The BNNS loading has a little influence on the I002/I100 ratio of the composites. Moreover, the orientation degree of samples prepared by solution blend method was also investigated. Figure S9 shows that the I002/I100 ratio of PS/oriented-BNNS composite is about two times that of PS/BNNS composite, indicating the higher orientation degree of BNNS in PS/oriented BNNS composite in comparison with the PS/BNNS composite. Thermal stability is one of the important properties for a composite in processing and application. Therefore, TGA and DSC were used to study the influence of filler on the thermal decomposition and glass transition of PS. Figure S10 depicts the thermal decomposition curves of the composite sheets with different filler content. The result shows that addition of BNNS has only a small influence on the thermal decomposition temperature of PS. The glass transition temperatures of the composite sheets were determined by DSC. One can see that Tg of PS exhibits a slight decline with the increase in BNNS content (Figure S11). Similar phenomena were also observed in some other polymer/BNNS composites.23,57 The reason might be attributed to that the increased BNNS loading facilitates the thermal transfer in the composite and then decreases the temperature gradient inside the sample. That is to say, the sample with high thermal conductivity has an earlier glass transition than that of the sample with low thermal conductivity. Moreover, stress−strain curves of the PS/oriented BNNS, PS/NNS-13.4, and PS composite sheets were also

sheet prepared by solution blend method was also investigated. Different from the latex blending, the solution blending method commonly leads to the irregular distribution and inevitable aggregation of BNNSs, indicating the effect of latex blending on achieving the homogeneous and oriented dispersion of BNNSs in the PS matrix (Figures 6a and b). In addition, influence of the molding temperature of PS−BNNS complex microspheres on the microstructure of the final composites was also investigated. At the low molding temperature of 80 °C, the PS chain segments are frozen. Thus, the PS microspheres deformed into multifacets with many visible gaps at the interfaces, resulting in the largely deteriorated mechanical properties for the composites (Figures S8a and b). As the molding temperature of PS−BNNS complex microspheres increases to 150 and 180 °C, which are much larger than the glass transition temperature, the mobility of the PS chain segments is further enhanced. It can be seen that the in-plane orientation of BNNSs is still maintained in matrix, but the PS microspheres are completely fused together without visible phase interfaces (Figures S8c−f). In this case, the BNNSs directly penetrate into the PS microspheres, and the continuous filler network is broken. Figure 7a shows the photograph of the neat PS sheet and PS/oriented BNNS composite sheet. The neat PS sheet exhibits a high transparency because of its amorphous feature, while the resultant PS/oriented BNNS composite shows the white color. XRD characterization was recorded to further detect the alignment structure of BNNSs in the composite.3,8 As depicted in Figure 7b, the broad diffraction peak at 19.8° is associated with the amorphous PS matrix, and the peaks at around 27.1 and 42.0° represent the (002) and (100) planes of BN, respectively.27,56 Many previous studies have manifested 19939


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Figure 8. (a) In-plane TC and (b) through-plane TC of PS/oriented BNNS composites and PS/BNNS composites with different BNNS loading. Schematic illustration of heat transfer in PS/oriented BNNS composite (c) and PS/BNNS composite (d).

of the PS specimen without PDDA modification (0.76 W m−1 K−1). The increment of in-plane TC in PDDA modified PS might be due to the higher TC of PDDA compared with PS.58 For the PS/oriented BNNS composites, an analogous percolation behavior, which is commonly presented in the electrically conductive polymer composite, can also be observed in this work. At the BNNSs content below 8.5 vol %, the TC increases slowly with the BNNSs content and reaches 3.6 W m−1 K−1 at 8.5 vol % of BNNS. Because the polymer mediates between adjacent BNNSs, the BNNSs cannot sufficiently contact each other. In this case, the interfacial mismatch between PS matrix and BNNS will cause a drastic phonon scattering and eventually result in low thermal conductivity. However, with a further increase of BNNS content, the in-plane TC value exhibits a sharp enhancement due to the formation of continuous filler network with the increased number of direct BNNS/BNNS contacts in the in-plane direction, which can also be confirmed by the corresponding cross-section morphology in Figure 5. When the BNNS content increases to 13.4 vol %, a high in-plane TC of 8.0 W m−1 K−1 is achieved. Similar “thermal percolation behavior” has also been observed in some previously reported articles.27,52,59 Additionally, further increase in BNNS is not supposed to enhance the TC value due to the adsorption saturation of PS microspheres at the 13.4 vol % of BNNS. Thus, excess BNNSs will be removed by centrifugation during the preparation process. These results reveal that an oriented and continuous filler network was constructed throughout the whole sample in the in-plane direction at 13.4 vol % BNNSs. Compared with the PS/oriented BNNS sample, the thermal conductivities of PS/BNNS samples are relatively

measured, and the results are displayed in Figure 7c. All of the four samples show a fragile feature with a low elongation at break. The tensile strength of the PS/oriented BNNS-13.4, PS/ oriented BNNS-8.5, PS/BNNS-13.4, and neat PS are 43.3, 40.1, 46.1, and 37.8 MPa, respectively. The larger tensile strength of PS/BNNS-13.4 in comparison with PS/oriented BNNS-13.4 is mainly ascribed to the more effective loading transfer from the PS matrix to the BNNSs. When applied as a TIM in electronic devices, the electrical insulation property is another important factor to be considered. Figure 7d exhibits the volume electrical resistivity of neat PS, PS/oriented BNNS-8.5, PS/oriented BNNS-13.4, and PS/BNNS-13.4 specimens. The difference between these volume resistivities are considered to be caused by the accidental error in measurement. Therefore, we think that incorporation of BNNS has a negligible influence (in the range of corresponding error bars) on the volume electrical resistivity of the composites in comparison to the neat PS specimen. All of the samples possess a high volume electrical resistivity of more than 1016 Ω cm, suggesting the excellent electrical insulation property. The in-plane and through-plane TC of the composite sheets were measured by laser-flash method at room temperature, and the corresponding schematic diagram was provided in Figure S12. Figure 8a demonstrates the in-plane TC of PS/oriented BNNS composites versus BNNS content. PS/BNNS composites prepared by solution blend, as the reference, were also investigated under the same conditions. One can observe that the in-plane TC of these two composites both increases with the addition of BNNSs. The neat PS with PDDA modification has an in-plane TC of 1.07 W m−1 K−1, which is larger than that 19940


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ACS Applied Materials & Interfaces lower in the whole filler range. When the filler content increases to 13.4 vol %, it shows only a low TC of 3.0 W m−1 K−1. These observations bring us to a conclusion that rational distribution and alignment of BNNS in polymer matrix contributes to the large improvement of thermal transfer. Moreover, the in-plane TCs of the PS/oriented BNNS-13.4 composites prepared under different temperatures are also investigated. As shown in Figure S13, the in-plane TC of the PS/oriented BNNS-13.4 shows a great decline from 8.0 to 3.9 and 3.2 W m−1 K−1 when the molding temperature increases from 120 to 150 and 180 °C. At such a high temperature, the melt viscosity of PS greatly drops, and the BNNSs easily penetrate into the PS regions. As a result, the effective BNNS concentration to form thermally conductive pathways in the composite significantly decreases, resulting in the decline of thermal conductivity. In view of the different orientation of BNNSs in the in-plane and throughplane direction, the through-plane TCs of PS/oriented BNNS and PS/BNNS composites were also investigated. Different from the in-plane TC results, the through-plane TC of PS/ oriented BNNS composite shows only a slight increment with the addition of BNNSs (Figure 8b). At the filler content of 13.4 vol %, only 70% TC enhancement is achieved. While for the PS/BNNS composite, its through-plane TC is much larger than that of PS/oriented BNNS composite under all filler content, which is related with the different dispersion of BNNSs in these two composites. Moreover, compared with the in-plane TC, the low thermal transfer in the through-plane direction is attributed to the large interfacial thermal resistance between BNNSs and PS matrix and the intrinsic TC anisotropy of BNNS. Although thermal conductivity, as an intrinsic property of a certain material, is independent of the shape of the material, the practically measured thermal conductivity is inevitably affected by the surface roughness, thickness, and geometry of the sample due to the heat loss caused by them. Therefore, the effect of sample thickness on the thermal conductivity was also studied in this work. Figure S14 depicts the in-plane TCs of PS/oriented BNNS-13.4 composites with different thickness. One can see that the in-plane TC shows a large decline when the sample thickness increases to 480 μm. This observation is mainly resulted from the larger heat loss in the through-plane direction for the sample with large thickness. Detailed information is also provided in the Supporting Information. The excellent heat conducting performance of the PS/ oriented BNNS composites at such low BNNS content is mainly attributed to the latex blending between BNNSs and PS microspheres as well as subsequent compression molding around the glass transition temperature. These two factors lead to the homogeneous distribution of BNNSs at the interface of the deformed PS phases and meanwhile render them highly oriented along the in-plane direction, which contributes to the formation of a thermally conductive network at low filler content. Therefore, the heat can readily transfer along the shortest heat transfer channels from one side to another side (Figure 8c). As for the PS/BNNS composite, the fillers are directly dispersed in the PS matrix accompanied by serious aggregation (Figure 8d). Therefore, a larger filler addition is required to form the continuous heat transfer pathway. Namely, the heat transfer is discontinuous and easily hindered by the large thermal resistance between the PS and BNNSs at low filler content. Compared with the previous investigation focused on the polymer/BN composites, higher filler content is required to achieve the similar value of in-plane TC as in our work (Table 1). Then, we further define the TC increment per

Table 1. Comprehensive Comparison on In-Plane TC and TC Increment per Vol % Filler of Polymer/BN Composites typical sample

filler loading (vol %)

in-plane TC (W m−1 K−1)

TC increment (per vol %)

PS/oriented BNNS PVA/h-BN PVA/BNNS SiR/BNNS PCL/PCL-gBNNS EP/ cellulose@ BN EP/BN network PMMA/ BNNS PVA/BN LCER/ BNNS

13.4

8.0

0.52

30 38.3 30.8 12.0

8.8 8.5 5.47 1.96

0.28 0.22 0.18 0.15

this work 20158 201611 201627 201621

20

5.22

0.25

201643

2.85

0.3

201535

9.2

yearref

68

14.7

0.21

201610

50 25

13 7.8

0.26 0.31

201260 201661

vol % BNNS loading (η) as η= (TC − TC0)/Vf, where TC and TC0 are the thermal conductivities of the composites with given BNNS content and pure PS, respectively; Vf represents the volume fraction of BNNS in composite. As shown in Table 1, we can find that the η value of our work is the highest among these studies to the best of our knowledge. To further demonstrate the potential application as a TIM in electronic devices, PS/oriented BNNS-13.4 and PS/BNNS13.4 samples were further utilized as the substrates to support a light-emitting-diode (LED) chip using silver paste (Figure 9a). Surface temperature variations of the substrate in the edge and center with LED working time are detected by a portable thermocouple (Figure 9b). Figure 9c shows the surface temperature variations of PS/BNNS-13.4 and PS/oriented BNNS-13.4 specimens in the edge within 300 s. One can observe that the PS/oriented BNNS-13.4 sample shows a heating behavior faster than that of PS/BNNS-13.4 sample. After 300 s, the edge temperature of PS/oriented BNNS-13.4 increases to 32.8 °C, while the edge temperature of PS/BNNS13.4 sample shows only a slight enhancement of 28.6 °C. In contrast, the PS/BNNS-13.4 sample shows a surface temperature of around 47.8 °C, larger than that of the PS/oriented BNNS-13.4 sample (41.3 °C) in the center position after 300 s (Figure 9d). These results further confirm the better heat dissipation of PS/oriented BNNS-13.4 composite than that of PS/BNNS-13.4.

4. CONCLUSION In summary, we reported a facile latex blending method to fabricate the highly thermally conductive PS/oriented BNNS composites by electrostatic adsorption between positively charged PS microspheres and negatively charged BN nanosheets, followed by compression molding around the glass transition temperature. Homogeneous dispersion, high in-plane orientation, and selective distribution at the interface of BNNSs in PS matrix were successfully achieved. The obtained PS/ oriented BNNS composites show a sharp increase in thermal conductivity upon BNNSs loading of 8.5 vol % due to the formation of thermally conductive network and reach up to 8.0 W m−1 K−1 when the BNNS loading increases to 13.4 vol %. We believe that this simple and facile method may provide a 19941


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Figure 9. (a) Optical image of PS/oriented BNNS-13.4 and PS/BNNS-13.4 specimens as the LED chip substrates. (b) Optical image of the surface temperature measurement of the composite sheet by a thermocouple. The edge temperature variation (c) and center temperature variation (d) of these two composites with LED working time (inset shows the setting position of thermocouple in the substrate).

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new strategy to design and fabricate highly thermally conductive composites.

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ASSOCIATED CONTENT

S Supporting Information *

This materials are available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04768. Additional cross-section SEM images of the PS/oriented BNNS composite, EDX mapping of PS−BNNS complex microspheres, cross-section TEM image of the PS/ oriented BNNS-13.4 composite, photographs of PS/ BNNS microsphere dispersion in IPA after centrifugation, XRD patterns of composites, TGA and DSC curves of PS/oriented BNNS composites, and in-plane thermal conductivity of PS/oriented BNNS composites prepared at different temperatures (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Peiyi Wu: 0000-0001-7235-210X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Science Foundation of China (NSFC) (Grant 51473038) 19942


Research Article

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Preparation of Highly Thermally Conductive Polymer Composite at

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