Fabrication of Thermal Conductivity Enhanced Polymer Composites

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Applications of Polymer, Composite, and Coating Materials

Fabrication of Thermal Conductivity Enhanced Polymer Composites by Constructing an Oriented Three-dimensional Staggered Interconnected Network of Boron Nitride Platelets and Carbon Nanotubes Zheng Su, Hua Wang, Jing He, Yulan Guo, Qiqi Qu, and Xingyou Tian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09703 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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ACS Applied Materials & Interfaces

Fabrication of Thermal Conductivity Enhanced Polymer Composites by Constructing an Oriented Three-dimensional Staggered Interconnected Network of Boron Nitride Platelets and Carbon Nanotubes Zheng Sua,b, Hua Wanga,c*, Jing Hea,b, Yulan Guoa,b, Qiqi Qua,b, Xingyou Tiana,c** a

Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese

Academy of Sciences, Hefei 230031, P. R. China. b

University of Science and Technology of China, Hefei 230036, China.

c

Key Laboratory of Photovoltaic and Energy Conservation Materials, Chinese

Academy of Sciences, P. R. China. *

Corresponding author 1, Tel.: +86 551 65393698; E-mail: [email protected].

**

Corresponding author 2, Tel.: +86 551 65393666; E-mail: [email protected].

Abstract: The orientation of ultrahigh aspect ratio thermally conductive fillers could construct heat transfer path to enhance the thermal conductivity of composite materials effectively with low filler loading. Nevertheless, single orientation (vertical or horizontal) limited the application of these materials when need isotropic heat transferring. Here we reported a novel strategy to prepare thermally conductive flexible cycloaliphatic epoxy resin (CER) nanocomposites with an oriented three-dimensional staggered interconnected network of vertical aligned h-BN (hexagonal boron nitride) platelets and randomly dispersed CNT-NH2 (aminated carbon nanotubes). In this structure, h-BN platelets coated with magnetic particles could response to the external magnetic field, however, the CNT-NH2 couldn’t. The obtained composites exhibited both through-plane (0.98±0.037 W/m•K) and in-plane 1

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(0.99±0.001 W/m•K) thermal conductivity enhancement at low h-BN loading of 30 wt%, and also presented excellent electrical insulating properties (95%, -NH2>0.45%) from Beijing DK nano technology Co., Ltd. Silane coupling agent N-[3-(Trimethoxysilyl)propyl]ethylenediamine (TPEDA, 95%) was obtained from Aladdin Industrial Corporation. Sodium hydroxide (NaOH, AR) was purchased from Sinopharm Chemical Reagent Co., Ltd. Wuhan Sen Mao Fine Chemical Co., Ltd supported the cycloaliphatic epoxy resin (CER-170). Polyetheramine (PEA, BASF EC 301) was purchased from BASF SE as the curing agent. Deionized water was used for all experiments. All the reagents were used as received. Surface Treating of Nano Iron Oxide (nano-Fe3O4). Fig. 1 (a) showed the process of surface treating of iron oxide nanoparticles (nano-Fe3O4). The iron oxide nanoparticles (2 g) were dispersed in 100 mL of ethanol and 2 g of deionized water, and then 0.2 g of N-[3-(Trimethoxysilyl)propyl]ethylenediamine (TPEDA) was added. The mixture was heated at 80 °C for 5 h. The obtained product was washed for several time by deionized water and ethanol, and then dried in a freeze drying oven for 12 h and coded as TPEDA@Fe3O4. Surface Treating of Hexagonal Boron Nitride Platelet (h-BN). This step was presented in Fig. 1 (b), 7 g sodium hydroxide was added to the 6


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water/ethanol mixed solution to prepare the 5M sodium hydroxide solution and then the 2.5 g h-BN platelets were put into. The mixture system was further transferred to a poly(tetrafluoroethylene) (PTFE)-lined stainless steel autoclave and heated at 120 °C for 48 h. After cooling down to room temperature, the product was washed with deionized water repeatedly until the pH of the filtrate was close to neutral. The as-obtained sample was dried in a freeze drying oven for 12 h and coded as m-hBN. In this part, we changed the weight ratio of h-BN platelets and sodium hydroxide to choose the best suitable one (Fig. S1, S2, S3, S4 and Table 1). Magnetic Treating of m-hBN (Fe@hBN, TPEDA@Fe3O4-decorated h-BN) The mixture of 3.5 g TPEDA@Fe3O4 and 5.0 g m-hBN was dispersed in 250 mL ethanol solution with ultrasonic treatment for about 1 h. And then, the constant temperature incubator shaker was used to achieve the electrostatic self-assembly of TPEDA@Fe3O4 and m-hBN by shaking for 24 h with 185 rpm at 24.1 °C. The obtain product was dried in a freeze drying oven for 12 h and coded as Fe@hBN. Preparation of Composite Film CER/Fe@hBN and CER/CNT-NH2/Fe@hBN. In this work, cycloaliphatic epoxy-based composites (CER/h-BN@hBN and CER/CNT-NH2/Fe@hBN) were fabricated shown in Fig. 1 (c). Firstly, epoxy resin (CER, cycloaliphatic epoxy resin) and curing agent (PEA, polyetheramine, BASF EC 301) were added to the Teflon beaker with continuous mechanical stirring for 10 min. Secondly, various amount of filler Fe@hBN (10 wt%, 20 wt% and 30wt% according to the content of h-BN) was put in the epoxy resin and continue stirring for 1 h. The mixture of uncured CER with the fillers was Poured into the Teflon mold and carried 7



out curing process for 72 h with magnetic field at room temperature. Finally, the composite CER/Fe@hBN was prepared. For the composites CER/CNT-NH2/Fe@hBN, the filler Fe@hBN and CNT-NH2 (2 wt%) were added one by one. All the prepared composites would be tested in the future. Characterizations Techniques. The morphology of filler and cross-section of composites coated with Au were examined using scanning electron microscopy (SEM, Sirion-200 FEI, America). The Au uncoated samples were used to obtain the energy dispersive spectroscopy via SEM equipped with an energy dispersive spectroscopy (EDS) detector. The zeta-potential of the m-hBN and TPEDA@Fe3O4 were measured using Malvern Zetasizer 3000HSA. Nicolet 8700 FT-IR spectrometer (Thermo Scientific Instrument Co. U.S.A) was used to characterize the functional groups using KBr pellet method. The XRD (X-ray diffraction) analysis of h-BN and m-hBN were carried out with a Philips X’ Pert Pro MPD X-ray diffractometer with Cu-Ka radiation (λ=0.154 nm, 40 kV, 40 mA). The magnetic properties were carried out on the equipment called physical property measurement system (PPMS- 9/PPMS ECⅡ-9T). The water contact angles of material surface were measured using contact angle goniometer (CD-100D, Shanghai Innuo Precision Instruments Co., Ltd.) via average of 5 drops for each one sample at different locations. The TGA tests (thermal gravimetric analysis, Q5000 IR) were used to obtain the thermal properties of the composites with a heating rate of 10 °C/min under nitrogen conditions. The thermal diffusivity of the composite materials were obtained by using a LFA 467 (Netzsch) instrument through the laser 8


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flash method according to ASTM E_1461-1. The thermal conductivity of the composite materials were calculated by κ = α • ‫ܥ‬௣ • ߩ, where ‫ܥ‬௣ and ρ were the heat capacity and density of the composite materials, respectively. The heat capacity of the composite materials were measured by thermal differential scanning calorimeter (DSC, PerkinElmer Pyris Diamond). The density of the composites was tested by ASTM D972. All related thermal measurements were carried out at room temperature. The high resistivity meter (LK2679A) was carried out to test the insulating property of the composite materials.26 The mechanical properties (such as tensile strength, Young’s modulus, and elongation at break) were evaluated by tensile machine (CMT, SANS) according to ASTM D3039 at room temperature. 3. Results and Discussion. Preparation of the CER/Fe@hBN and CER/CNT-NH2/Fe@hBN composites.

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Fig. 1. Schematic of the procedure to prepare the (a) TPEDA@Fe3O4, (b) m-hBN, and (c) CER/Fe@hBN and CER/CNT-NH2/Fe@hBN composite materials. To prepare filler Fe@hBN platelets, m-hBN platelets were dispersed in ethanol and then nano-Fe3O4 particles were added. Nano-Fe3O4 particles were coated by cationic surfactants and had a zeta-potential of 24.0 mV, indicating the positive surface charge (Fig. 1 (a)). The h-BN platelets were treated in the sodium hydroxide solution and the obtained m-hBN had negative surface charge of –4.2 mV (Fig. 1 (b)). 10


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Therefore, the there was a strong electrostatic interaction between m-hBN and nano-Fe3O4 particles. When we mixed m-hBN and nano-Fe3O4 particles, these magnetic nano-Fe3O4 particles would attached onto the m-hBN platelets’ surface quickly. And now, the h-BN platelets would have the ability to respond to the external magnetic field shown in Fig. 1 (c).

Fig. 2. (a) FT-IR spectra of pristine h-BN (p-hBN) and surface treated h-BN (m-hBN); (b) The XRD pattern of the pristine h-BN (p-hBN) and surface treated h-BN (m-hBN); SEM images of (c) pristine h-BN (p-hBN) and (d) surface treated h-BN (m-hBN) with EDS (the red line circled the deformation on the surface of h-BN). Pristine h-BN platelets were treated with sodium hydroxide at 120 C° in an 11



autoclave. The successful modification of h-BN platelets was confirmed by FT-IR spectra of p-hBN and m-hBN shown in Fig. 2 (a). Two strong characteristic absorption peaks of p-hBN could be identified clearly at 1369 cm-1 and 815 cm-1, which were attributed to the B-N stretching vibration and B-N-B out-of-plane bending.27 The peak at around 1448 cm-1 was according to the characteristic absorption band of B-O bond. After XRD characterization, the peaks at about 26.9º, 41.7º and 55.2º were assigned to the (002), (100), and (004) crystallographic planes of h-BN platelets, respectively.18, 28 We found that the surface treating reduced the intensity of (002) peak, which might destroy the crystal structure of h-BN platelets and reduce the inherent thermal conductivity of h-BN platelets (Fig. 2 (b)). Therefore, we needed a suitable procedure to reduce this negative impact. The SEM images showed in Fig. 2 (c) and (d) revealed that the surface change and confirmed the successful modification of h-BN platelets. Further, the energy dispersive spectroscopy (EDS) and elemental mapping analysis of pristine h-BN and m-hBN platelets showed the presence and a uniform distribution of hydroxyl group.

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Fig. 3. (a) FT-IR spectra of nano-Fe3O4 and surface treated Fe3O4 (TPEDA@Fe3O4); (b) Zeta-potential of the m-hBN, nano-Fe3O4, and TPEDA@Fe3O4; (c) SEM images TPEDA@Fe3O4-decorated h-BN (Fe@hBN) with EDS and elemental mapping; (d) Magnetic hysteresis loops for nano-Fe3O4 particles coated h-BN platelet with different weight ratio. The inset showed a close view of the hysteresis loops; (e) Fe@hBN particles dispersed in solution with magnetic separation (left) and h-BN platelets without magnetic response (right). The N-[3-(Trimethoxysilyl)propyl]ethylenediamine (TPEDA) as silane coupling agent was used to treat the surface of nano-Fe3O4 particles. The analysis technology of FT-IR and zeta-potential measurement were used to verify the effective modification. As shown in the FT-IR spectra (Fig. 3 (a)), the characteristic peaks of nano-Fe3O4 particles presented at 3400 cm-1 and 570 cm-1.29 The N-H shearing vibration and C-N stretching vibration of the amide group could be found at 1505 cm-1 and 1182 cm-1. 13



The bending vibration of -CH2- presented at 1460 cm-1. Because of the hydrolysis condensation of alkoxy, the characteristic peak showed at 1080 cm-1 ascribing to the stretching vibration of Si-O-Si bond the hydrolysis condensation. The peak at 618 cm-1 belonged to the Fe-O-Si stretching vibration. Therefore, the TPEDA@Fe3O4 had the opposite surface charge with m-hBN platelets (Fig. 3 (b)). After the mixture between TPEDA@Fe3O4 and m-hBN platelets to obtain the Fe@hBN, the SEM characterization revealed that the size of nano-Fe3O4 particles ranged from 20-25 nm and confirmed the successful surface attachment of the nano-Fe3O4 particles to the m-hBN platelets (Fig. 3 (c)). In addition, the EDS analysis and elemental mapping showed the uniform distribution of nano-Fe3O4 particles on the surface of m-hBN platelets. The magnetic properties of the h-BN platelets coated with nano-Fe3O4 were investigated by using a physical property measurement system at room temperature, as shown in Fig. 3 (d). The nano-Fe3O4 particles coated h-BN platelets exhibited a typical ferromagnetic behavior due to a large coercive force (Hc) with just a small applied magnetic field. The coercive forces ranged from 108.67 to 122.03 Oe. The saturation magnetization (Ms) values of these nano-Fe3O4 particles coated h-BN platelets could be found the dependent on the content of absorbed magnetic nano-Fe3O4 particles. The Ms values for nano-Fe3O4 particles coated h-BN platelets were 13.44 and 25.85 emu/g for weight ratio of h-BN and nano-Fe3O4 as 1:0.2 and 1:0.7, respectively. Fig. 3 (e) presented the nano-Fe3O4 coated h-BN platelets could be magnetically separated from solution, however, the h-BN platelets would not.

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Fig. 4. The cross-sectional SEM images of (a), (b), (c): the CER/Fe@hBN composites and (d), (e), (f): the CER/CNT-NH2/Fe@hBN composites (the yellow and blue arrow represent the Fe@hBN in CER/Fe@hBN and CER/CNT-NH2/Fe@hBN, respectively, the red circle represent the dispersion of CNT-NH2). The composites CER/Fe@hBN and CER/CNT-NH2/Fe@hBN were fabricated using solvent-free method, as described in the experimental section (Fig. 1 (c)). During the curing process, the mold filled with uncured epoxy resin was placed between two magnets (Fig, 1 (c)). The external magnetic field could achieve the vertical orientation of Fe@hBN platelets in the composites. The cross-sectional SEM images of samples filled with Fe@hBN platelets prepared without external magnetic field were presented in Fig. S7. The filler Fe@hBN would disperse randomly or stack along the horizontal direction without the external magnetic field. The amount of nano-Fe3O4 particles on m-hBN platelets surface would affect the magnetic alignment. We changed the weight ratio of m-hBN platelets and nano-Fe3O4 particles from 1.0:0.05, 1.0:0.2, 1.0:0.5, 1.0:0.7, to 1.0:1.0 (Fig. S5). The nano-Fe3O4 particles coating on the h-BN platelets’ surface would reduce the inherent thermal conductivity 15



of h-BN platelets due to the phonon spectral mismatched. In terms of the magnetic alignment and thermal conductivity, we chosen the ratio of 1.0:0.7 to achieve the highest performance enhancement and reduce the side effect from iron coating. Because of the no magnetic of CNT-NH2, CNT-NH2 would disperse randomly in the composites. The similar structure and van der Waals interaction provided a strong connection between h-BN platelets and CNT-NH2.30 The SEM images of the cross-section of the CER/Fe@hBN and CER/CNT-NH2/Fe@hBN composites were shown in Fig. 4. The roughness of the cross-section of the composites compared to that of pure CER showed in Fig. S6 (a), indicating strong interfacial interaction between Fe@hBN and m-hBN platelets. The orientation of the Fe@hBN platelets could be visualized from the SEM images as shown in Fig. 4. It could be seen that in CER/Fe@hBN and CER/CNT-NH2/Fe@hBN composites, the Fe@hBN platelets oriented along the direction of the magnetic field, which were marked out by the yellow and blue arrows. In CER/CNT-NH2/Fe@hBN composites, the CNT-NH2 without magnetic dispersed randomly in the polymer matrix and connected the Fe@hBN platelets along the horizontal direction. It was clear that, in Fig. 4 (d), (e), and (f), the red circles represented the distribution and agglomerate of CNT-NH2 in composites. When we increased the loading of Fe@hBN platelets, the degree of agglomerate of CNT-NH2 decreased obviously (Fig. S6 (b) and (c)). The observed SEM morphology was very similar to the top-view illustration drawing in Fig. 5 and presented the distribution of Fe@hBN and affective of CNT-NH2 on the formation of networks in composites. The FT-IR spectrums of composites in Fig. S8 and S9 16


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showed the completed curing of cycloaliphatic epoxy resin with the epoxy group disappeared at 910 cm-1. The inserted pictures in Fig.5 presented the water contact angle of the composites’ surface. Because of the hydrophilic group such hydroxyl and ether groups, pure CER obtained a contact angle of 76.08±4.20°. The loading of Fe@hBN increased the water contact angle of the composite materials due to the super high hydrophobicity of the h-BN. And then, the existence of amino-group on the surface of CNT-NH2 reduced the water contact angle of the composites (inserted pictures in Fig. 5).

Fig. 5. The top-view illustration of the composite films with increased content of Fe@hBN and CNT-NH2 (2 wt%); the inserted pictures showed the water contact angle of the surface of the composites. Thermal conductivity properties of the composite materials.

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Fig. 6. (a) and (b): Thermal conductivity of CER/Fe@hBN and CER/CNT-NH2/Fe@hBN composites at in-plane and through-plane direction; the cross-sectional illustration of (c) in-plane and (d) through-plane thermal conductivity of the composite materials. The thermal conductivity of the composites were enhanced by filler alignment. And the dispersion of CNT-NH2 also affected the thermal transport across the in-plane and through-plane direction. Fig. 6 (a) showed the anisotropic heat transferring of the CER/Fe@hBN composites. Because of the vertical orientation of Fe@hBN platelets, the through-plane thermal conductivity was higher than the in-plane direction thermal conductivity. Therefore, the low in-plane thermal conductivity limited its application as electronic packaging materials which needed isotropic heat transferring. 18


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Interestingly, when the loading of Fe@hBN arrived at 30 wt%, the thermal conductivity along the in-plane direction arrived at 0.99±0.001 W/m•K was a little higher than it along the through-plane direction (0.98±0.037 W/m•K). We also could believe that there were no difference between the two directions. The increasing of the loading of the vertical oriented Fe@hBN increased the contacting between the adjacent Fe@hBN platelets along the horizontal direction (Fig. 5). Some formation of the efficient thermal pathways made by the horizontal adjacent Fe@hBN platelets, which increased the in-plane thermal conductivity of CER/Fe@hBN composites. However, when we incorporated the CNT-NH2 into the composite CER/Fe@hBN, the CNT-NH2 connected the Fe@hBN platelets and enhanced the thermal conductivity, especially along the in-plane direction. When the loading of Fe@hBN was about 26.43 wt%, the in-plane and through-plane thermal conductivity would have the same value. The in-plane thermal conductivity of CER/CNT-NH2/Fe@hBN composites arrived at 0.85±0.002 W/m•K was higher than the through-plane thermal conductivity ~0.76±0.04 W/m•K with 30 wt% loading of Fe@hBN. But the incorporated CNT-NH2 between the adjacent Fe@hBN platelets increased the interface thermal resistance. Especially, when the loading of Fe@hBN was above 20 wt%, the through-plane thermal conductivity began to reduce slightly. Electrical properties of the composites.

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Fig. 7. Electrical conductivity of CER/Fe@hBN and CER/CNT-NH2/Fe@hBN composites at in-plane and through-plane direction. Fig. 7 presented the mass fraction of filler Fe@hBN and CNT-NH2 affecting on the electrical conductivity of CER/Fe@hBN and CER/CNT-NH2/Fe@hBN composites. Excellent electrical insulating properties (

Fabrication of Thermal Conductivity Enhanced Polymer Composites

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