Inorganic–Organic Hybrid Janus Fillers for Improving the Thermal

Letter Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Inorganic−Organic Hybrid Janus Fillers for Improving the Thermal Conductivity of Polymer Composites Xiao Han, Leijie Wu, Hongbo Zhang, Aihua He,* and Huarong Nie* Shandong Provincial Key Laboratory of Olefin Catalysis and Polymerization, Key Laboratory of Rubber-Plastics (Ministry of Education), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao, Shandong 266042, China

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S Supporting Information *

ABSTRACT: Janus fillers represent a combination of inorganic thermally conductive silver nanoparticles and organic polystyrene brushes on one entity but different sides. They are of practical importance for polymer composites with high thermal conductivity because of the improved dispersion and reduced interfacial heat resistance. Moreover, benefiting from the sheetlike structure and singleside deposition of inorganic particles, Janus fillers tend to align such that the heat pathway is constructed in the composite films, when fabricated by layer-by-layer doctor blading. As a result, the in-plane thermal conductivity of the polymer composite is as high as 4.57 W m−1 K−1, with only 10 vol % Janus filler loading. KEYWORDS: inorganic−organic hybrid Janus fillers, silver nanoparticles, polystyrene brushes, thermal conductivity, aligned structure

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Janus particles (JPs), a special material with two distinct compositions and chemical-physical properties on two opposite sides,24 can be attached easily with polymer brushes and even metal particles onto a surface by selective modification.25−27 Hence, Janus fillers concentrated with polymer brushes and metal particles on two opposite sides are of special relevance for good dispersion in a polymer matrix and excellent thermal conduction. Further, if the architecture of fillers is provided in the polymer matrix by special methods, high thermal conductivity can be achieved. Therefore, fillers with anisotropic characteristics and a facile orientation to form a network, such as graphene, h-BN, and Al2O3 sheets, are primarily used in a polymer matrix to construct a “brickwork structure” by doctor blading,9 magnetically self-aligning,28 and ice templating.29 In this study, a special inorganic−organic hybrid Janus filler was fabricated, wherein high-thermal-conductivity silver nanoparticles (AgNPs) and polystyrene (PS) brushes were anchored on opposite sides of a silica nanosheet by selective surface modification. To enable orientation of the Janus fillers in polymer matrix, the Janus fillers were dispersed in polymer matrix by layer-by-layer doctor blading. Consequently, there were several distinct advantages of using polymer composites with JPs, such as (1) the dense deposition of AgNPs on one side of the Janus filler enabled the construction of a heat pathway within the Janus sheet, which contributed to low loading of thermally conducting particles; (2) the attachment

o prolong the lifetime and maintain the high performance of some heat-accumulating polymer materials, such as electrical devices, computer, mobile phone, and LED packaging, the development of polymer composites with high thermal conductivity is necessary and has attracted extensive attention.1−4 In such entities, ceramic materials including Al2O3,5−7 h-BN8−11 and SiC,12,13 carbon materials,14−16 and metal particles2,17−19 are used as fillers to transfer heat via dispersion in the polymer matrix. The ease of the process and low cost of these composites make them popular and widely fabricated.4,20,21 However, these fillers are inhomogeneous with the polymer matrix, and their consequent poor dispersion in the matrix leads to low thermal conductivity. Although surface modification is believed to be an efficient way to improve the dispersion of fillers in a polymer matrix,22,23 it is hard to surface-modify some fillers, especially the highly thermally conductive metals, thereby hindering the improvement of thermal conductivity. In addition, direct surface modification on the filler surface could also lead to a high interfacial thermal resistance because the organic coating impedes the direct connection of thermally conducting particles. For instance, much effort is devoted to developing h-BN filler to improve the thermal conductivity of polymer composites.8−11 However, the chemical inertia of h-BN makes it difficult to react with other substance for surface modification to improve its dispersion in matrix. Moreover, the destroyed crystal structure with surface modification causes the reduced intrinsic thermal conductivity of h-BN, which is not conducive to the heat transfer in polymer composites. Indeed, the desirable filler should have high thermal conductivity and easily disperse in polymer matrix. © XXXX American Chemical Society

Received: December 21, 2018 Accepted: March 20, 2019 Published: March 20, 2019 A

DOI: 10.1021/acsami.8b22278 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces Scheme 1. Fabrication Processes of Inorganic−Organic Hybrid Janus Fillers and Polymer Composites

of PS brushes on the other side of the Janus filler enabled good dispersion of the filler and helped reduce the interfacial thermal resistance between the thermally conducting particles; (3) the sheetlike structure and the deposition of AgNPs on one side made the Janus filler tend to align in polymer matrix so as to construct the heat pathway. As a result, the achieved polymer composites with inorganic−organic hybrid Janus fillers showed excellent thermal conductivity as high as 4.57 W m−1 K−1 with only 10 vol % loading. Notably, the diversity in chemical reaction of the hydroxyl groups on JPs affords a benefit to enhance the thermal conductivity for a large number of polymers. Scheme 1 shows the detailed procedure for the fabrication of the inorganic−organic hybrid Janus fillers and polymer composites. In the first step, the hollow silica spheres30 with amine groups on the outer surface but hydroxyl groups on the inner surface were used as the template, on the outer surface of which AgNPs were deposited by a redox reaction. Herein, the amine groups could coordinate with silver ions, so that the AgNPs were well-adsorbed on the surface of hollow spheres, in the event of disengagement between the AgNPs and silica template during ultrasonication which would impede the conduction of heat. Subsequently, the hollow spheres were smashed by ultrasonication to obtain Janus sheets with the exposed hydroxyl groups on the inner surface. Moreover, to improve the dispersion and compatibility of Janus fillers with polymer matrix, we further functionalized the hydroxyl groups to introduce the PS brushes, as shown in the pane of Scheme 1. Figure 1a shows the TEM image of the original hollow silica sphere fabricated by self-assembly sol−gel method30 and used as the template for the deposition of AgNPs. The hollow spheres are collapsed with a flat structure, indicating the ultrasonication is simple and suitable for obtaining two-

Figure 1. TEM images of (a) silica Janus hollow sphere, (b) silica Janus hollow sphere with the deposition of Ag nanoparticles, (c) Agsilica nanosheet obtained from the ultrasonication of abovementioned hollow spheres. (d) SEM image of the ultimate Janus fillers.

dimensional sheet structure. The wall thickness of the silica template is 15−20 nm (Figure S1). With the deposition of AgNPs, the color of the silica solution turns brown from white (Figure S2), and a dense particle layer is observed on the outer surface of the hollow spheres (Figure 1b). By TGA analysis, the mass ratio of AgNPs is about 74.8 wt % (Figure S3). After smashed by ultrasonication, the obtained Janus nanosheet with discrete and aggregated AgNPs is shown in Figure 1c. Each AgNP is 50−100 nm in size. Notably, the AgNPs are strictly deposited only on one side of the Janus sheets (Figure 1d). The clean wall of the inner side with the preserved hydroxyl B

DOI: 10.1021/acsami.8b22278 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

the Janus fillers in the samples (Figure 3a). As a control, Janus fillers were dispersed in polymer matrix with a disordered manner by processing the samples with a simple casting method (Figure 3b). With the layer-by-layer method, less than 2 μm thickness of monolayer is lower than the diameter of silica nanosheets, and the deposition of AgNPs on one side leads to a higher density of Janus fillers in the plane. Both factors cause the sheet-shaped Janus fillers to lie flat in the polymer matrix and form a somewhat oriented structure. Unfortunately, rapid solidification and the high viscosity of the polymer composites during fabrication disallow a strict parallel orientation of the Janus fillers. Thus, the heat pathway resulting from the orientation of the Janus fillers is constructed (Figure 3a and Figure S5). Apart from the good dispersion of the Janus fillers in polymer matrix, the excellent compatibility between the PS brushes and PS matrix leads to good interfacial stability between the fillers and matrix. For the obtained polymer composites with Janus fillers, the thermal conductivity and diffusivity were examined with a laser flash technique (NETZSCH, LFA 467). The in-plane thermal conductivity was calculated as follows:

groups contributes to the subsequent selective modification to attach PS brushes. Further, the deposition of AgNPs was also confirmed by XRD analysis (Figure 2a). Compared to the original hollow

K = αρCp Figure 2. (a) XRD spectra of silica hollow sphere template before and after deposition of Ag nanoparticles. (b, c) FTIR and 1HNMR spectra of silica nanosheets with PS graft. (d) Dispersion phenomena of Agsilica (left) and Ag-silica-PS (right) Janus nanosheets in toluene.

(1)

Where K and α are the thermal conductivity and thermal diffusivity, respectively. ρ is the density, and Cp is the specific heat capacity. The thermal conductivity enhancement (TCE) was also calculated to evaluate the difference between the thermal conductivity of the composite and pure matrix. TCE is expressed as K − Km TCE = c Km (2)

spheres, the Ag-deposited Janus nanosheets show peaks at 38.1, 44.3, 64.4, and 77.5°, attributed to the face-centered cubic structure of AgNPs. Regarding the PS graft, the peaks at 3050, 2923, and 2894 cm−1 are assigned to the stretching vibration of the CH2CH2 groups, and asymmetric and symmetric stretching vibration of the −CH2 groups (Figure 2b). Weaker characteristic peaks for the stretching vibration of −OH at 3400 cm−1 and the stronger cross-plane vibrational peaks for the monosubstitution of benzene at 697 cm−1 are observed as well. Prior to PS attachment, only the stretching vibration and bending vibration of hydroxyl groups appear at 3400 and 1500 cm−1 on the silica nanosheets. Notably, the characteristic peaks at 6.6 and 7.1 ppm resulting from the benzene of the PS brushes are also revealed in the NMR spectra of the Janus fillers (Figure 2c). Thus, it is concluded that the PS brushes are successfully grafted onto the silica surface. As expected, due to the attachment of PS, the dispersion stability of the Janus fillers is greatly improved in toluene, indicating the promising affinity of Janus fillers with a nonpolar polymer matrix (Figure 2d). Figure 3 shows the cross-section morphology of the obtained polymer composites with 10 vol% of Janus fillers. There is relatively good orientation and connectivity between

Where Kc and Km are the thermal conductivity of the composites and pure matrix, respectively. As shown in Figure 4, the thermal conductivity of the pure PS matrix is 0.15 W

Figure 4. Thermal conductivities and thermal diffusivities of polymer composites with different filler contents.

m−1 K−1 because of poor heat transfer. When 10 vol % AgNPs (50−100 nm in size) are loaded, PS composites show almost no changes in the thermal conductivity (0.154 W m−1 K−1). With only 2 vol % loading of Janus fillers, the thermal conductivity increases to 2.56 W m−1 K−1, representing a nearly 17-fold increase. The TCE value of the PS composite with 2 vol % filler loading reaches 1618%. This indicates that

Figure 3. Cross-sectional SEM images of polystyrene composites with 10 vol % Janus fillers that were prepared by (a) layer-by-layer doctor blading and (b) simple casting. C

DOI: 10.1021/acsami.8b22278 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

ACS Applied Materials & Interfaces

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the dense deposition of AgNPs on the Janus filler contributes to reducing the loading of high-thermal-conductivity particles because of their easy connection on JPs. When Janus filler loading increases to 10 vol%, the thermal conductivity reaches 4.57 W m−1 K−1 (Table S1), which is at least 3 times higher than the conductivity of composites with a similar volume fraction of fillers (Table S2). Actually, the concentration of thermally conducting AgNPs is just around 7.13 vol %. With 15 vol % loading of Janus filler, the thermal conductivity increases to 5.11 W m−1 K−1. By comparison, the thermal conductivity of epoxy/BN-AgNPs composite films with 11 vol % filler is around 1.1 W m−1 K−1. Even here, for samples are fabricated by simple casting with 5 vol % Janus fillers, the thermal conductivity is just 0.81 W m−1 K−1 (Figure S4). Although Kargar et al.16 reported epoxy/graphene composites with the thermal conductivity of about 8 W m−1K−1, the loading was above 55 wt %. Figure S5 schematically depicts the high capacity of polymer composites with Janus fillers. First, the dense silver layer on the Janus filler enables the heat pathway within the Janus sheets. Second, the orientation of sheet-shaped fillers in the polymer matrix contributes to the construction of a filler bridge for heat conduction. Third, the good dispersion of Janus fillers favors the homogeneity of polymer composites, leading to the larger TCE even at low filler loading. Because of the alignment of Janus filler in polymer matrix, the in-plane thermal conductivities are obviously higher than the cross-plane thermal conductivities (Figure S6). In summary, we fabricated Janus fillers to develop polymer composites with high thermal conductivity. The Janus fillers were characterized as the dense layer of high-thermalconductivity AgNPs and PS brushes on two opposite sides of a silica nanosheet template. By layer-by-layer doctor blading, the Janus fillers tended to lie flat in the polymer matrix, thereby constructing an oriented structure. With 10 vol % Janus filler loading, the thermal conductivity reached 4.57 W m−1 K−1, which is approximately 30 times that of pure PS matrix.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51773105), the National Basic Research Program of China (2015CB654700, (2015CB654706)), the Significant Basic Research Program of Shandong province (ZR2017ZA0304), the Natural Science Foundation of Shandong Province (ZR2016EMM05) and Taishan Scholar Program.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b22278.

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Letter

Experimental section, thickness of Janus sheets, dispersion of Janus sheets, TGA analysis of Ag-silica sheets, the thermal conductivity of polymer composites fabricated by simple casting, the thermal conducting route in PS/Janus filler composites, the in-plane and cross-plane thermal conductivities of polymer composites with different filler contents, and the comparison of thermal composites beteween polymer composites with different fillers (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.N.). *E-mail: [email protected] (A.H.). ORCID

Aihua He: 0000-0002-7535-8379 Huarong Nie: 0000-0003-0007-5865 Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acsami.8b22278 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.8b22278 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX