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Synthesis and characterization of functionalized Fe3O4/ boron nitride as magnetically alignable 2D-nanofiller to improve the thermal conductivity of epoxy nanocomposites Mehdi Salehirad, Mir Mohammad Alavi Nikje, and Leila Ahmadian-Alam Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03540 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018

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Industrial & Engineering Chemistry Research

Synthesis and characterization of functionalized Fe3O4/ boron nitride as magnetically alignable 2D-nanofiller to improve the thermal conductivity of epoxy nanocomposites Mehdi Salehirada,b, Mir Mohammad Alavi Nikjea,*, Leila Ahmadian-Alamb a

Department of Chemistry, Faculty of Science, Imam Khomeini International University, P.O. Box 34148 - 96818, Qazvin, Iran. Tel-Fax: +982813780040

b

Niroo Research Institute, Chemistry and Materials Division, Chemistry and Processing Group, Tehran, Iran, P.O. Box 14665/517, Tel-Fax: +982188078296 Correspondence to: Mir Mohammad Alavi Nikje ([email protected])

ABSTRACT In the electronic industry, there is an enormous request for high thermal conductivity polymer. In this regard, we propose the novel synthetic route to produce magnetically alignable 2Dnanofiller. In our work, Surface modification of boron nitride nanosheets (BNNs) with polyacrylamide brushes are explored. For this purpose, the post-polymerization reaction of BNNs is followed by the redox polymerization of acrylamide onto the surfaces of already hydroxyl-functionalized BNNs. Afterwards, magnetic iron oxide nanoparticles are contributed to modified BNNs through polyacrylamide chelating effect. Finally, to provide high thermal conductive and magnetic responsive nanocomposites, the functionalized Fe3O4/ BNNs are dispersed in epoxy matrix and aligned under an external magnetic field. According to the result of thermal conductivity, thermal transport of the nanocomposites increases to 0.37 W/mK by adding only 10% of functionalized Fe3O4/ BNNs (at 25 °C). Mechanical and Thermal properties of these nanocomposites are compared with the pristine epoxy polymer as a control sample.

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1. INTRODUCTION Introducing a new electronic packaging has been extensively investigated by many researchers in order to develop a new generation of electronics packaging and replace the conventional materials used in many electronic devices.1,2 In recent years, polymer composites are known as suitable and alternative materials for packaging applications because of their low costs as well as their lightness.3,4 In this case, the performances of polymer composites as electronic packaging materials is affected by their coefficient of thermal expansion (CTE), electrical insulating and thermal conductivity properties. Recently, conventional filled polymeric materials have attracted attention in the current electronic devices packaging. However, obtaining a polymeric packaging with excellent characteristic that improves the thermal management, mechanical and adhesion properties needs considerable improvements. Nanodimensional materials as nanoscale additives have attracted a considerable attention to prepare new polymeric packaging with thermal-enhancing properties. This is achieved through providing interconnected thermal transport channel in a polymer matrix and also enhanced mechanical, thermal and adhesive properties of polymers.5-10 To prepare a new electronics packaging as an advanced thermal interface material (TIM), ceramic fillers,11-13 carbon based fillers14-19 and so on have been investigated due to their high intrinsic thermal conductivity, specific surface area, surface modification with various technique and also market accessibility of these fillers. Nowadays, hexagonal boron nitride (HBN) fillers have attracted a lot of consideration due to their higher thermal conductivity and lower CTE compared to conventional fillers.20-23 However, filler geometries,24,25 orientations,26 and interfacial properties27-29 affect the performances of composites, regardless of the intrinsic properties of fillers. Several approaches to align fillers in polymer matrix are applied which includes the use of prealigned fillers, carbon

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nanotube arrays, 30, 31 gravitational alignment, 32, 33 shear alignment34-38 and magnetic alignment3, 39-42

.

Magnetic alignment is an attractive method for this purpose due to its ability to align filler in all directions.3, 40 For example, attachment of superparamagnetic nanoparticles such as iron oxide onto the surface of high aspect ratio fillers such as hexagonal boron nitride as a support for magnetic nanoparticles is an effective strategy to prepare magnetically responsive HBN. This approach, which is known as the hierarchical nanostructures, has received considerable attention regarding to the suitable dispersion of nanosheets into the polymer matrix, thereby enhancing thermal and mechanical properties of polymeric composites. Furthermore, post-polymerization of fillers via different grafting techniques such as grafting from, grafting to and grafting through is an effective route to prepare highly dispersible nanofillers into organic-based matrix and also provides unique supports.43-46 In related literatures, Ejaz et al. synthesized glycidyl methacrylate/boron nitride and styrene/boron nitride hybrid nanoparticles via surface-initiated atom transfer radical polymerization (SI-ATRP).43 Anisotropically alignable magnetic boron nitride platelets were synthesized by Lim and co-workers.44 In that work, after grafting of poly (sodium 4styrenesulfonate) to BN, magnetic boron nitride platelets were prepared by attachment of Fe3O4 nanoparticles onto the polymer-grafted BN. It is expected that grafting polymerization of a monomer carrying desired functionality can create an active polymeric shell that can be used as a support for the attachment of different nanoparticles. In this paper, we utilize the redox polymerization to prepare modified HBN nanosheet as an effective support to attain magnetically responsive HBN. Furthermore, we aim to prepare epoxy nanocomposites filled with aligned HBN nanosheets for electronic packaging applications. We

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investigated the effect of the aligned-HBN nanosheets content on thermal conductivity behavior, thermal and mechanical properties of the prepared epoxy nanocomposite in vertical and horizontal magnetic field.

2. EXPERIMENTAL SECTION 2.1. Materials Hexagonal boron nitride (70 nm, Lower Friction), acrylamide (AM, 99% from Sigma), cerium (IV) ammonium nitrate (CAN, 99.99% from Sigma-Aldrich), sulfuric acid (98% from Aldrich), di-tert-butyl peroxide (DTBP, 95% from Sigma-Aldrich), N-methyl-2-pyrrolidinone (NMP, 97% from Sigma-Aldrich), Magnetic iron oxide nanoparticles (98%, APS: 20-30 nm from Nanosany corporation), hydrogen peroxide solution (30 % (w/w) in H2O from Sigma) and Triethylenetetramine (TETA, ≥97% from Aldrich) were used without more purifications.

2.2. Synthesis of hydroxyl-functionalized boron nitride nanosheets (FHBN) Hydroxyl functionalized boron nitride nanosheets were prepared by using previously reported method.47 0.2 gr of Boron nitride nanosheets were spread out in NMP (20 ml) through sonication for 3 h. The supernatant was collected by centrifuge and after that dried at 80 °C in a vacuum oven. The exfoliated HBN nanosheets (0.1 g) were dispersed in 20 ml of NMP and the content was mixed with 30 ml of DTBP in a flask. This suspension was stirred for 20 h at 120 ° C under N2 atmosphere. Following centrifuge, the tert-butoxy-functionalized HBN was washed with ethanol, chloroform and dried under vacuum. Afterwards, the tert-butoxy-functionalized HBN was dispersed in H2SO4 (35 ml) through sonication for 45 min. The reaction was carried out by slow addition of hydrogen peroxide solution (10 ml). This mixture was stirred at room

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temperature for 2 h. After centrifuge, the hydroxyl-functionalized HBN nanosheets were washed with chloroform and water and then dried in a vacuum oven at 50 ºC.

2.3. Synthesis of polyacrylamide-grafted boron nitride nanosheets (mHBN) FHBN (0.4 g, dispersed in 12 ml of 1 N HNO3 solution) was dispersed in AM monomer solution (1.2 g, 0.017 mol, dissolved in 3 ml distilled water) and then the reactor was degassed with N2. The reaction was carried out by adding CAN (40 mg, 0.07 mmol), the temperature was elevated to 70 ºC and stirred for 2 h. Acquired mixture was then diluted by adding an excess of water. After centrifuging, the modified HBN nanosheets were washed with water three times and freeze-dried for further use.

2.4. Preparation of magnetic iron oxide nanoparticles-modified boron nitride hierarchical nanostructures (FeNPs-mHBN) Magnetic iron oxide nanoparticles-modified boron nitride composite was prepared by the addition of a known amount of iron oxide nanoparticles into mHBN nanosheets. The prepared mHBN nanosheets (0.5 g dispersed in 10 ml distilled water) were stirred for 1 h. Then, iron oxide nanoparticles (0.01 g) were added to the suspension, and the mixture was sonicated for 5 h and the resulting mixture was incubated overnight and then dried by freeze-drying.

2.5. Preparation of aligned epoxy/FeNPs-mHBN nanocomposite A desired amount of FeNPs-mHBN nanosheets was added into the epoxy resin, and the mixture was stirred for 5 h in order to disperse the nanomaterials. After complete homogenization, TETA as the hardener was added and nanocomposite samples were prepared by casting into a silicon mold located between two parallel magnets overnight. Finally all samples were post-cured at 50 5



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and 80 ºC for 1 h, respectively. The nanofiller content in the prepared nanocomposite samples was determined as a weight percentage of FeNPs-mHBN relative to the epoxy resin. Filler loading (FeNPs-mHBN) in the prepared nanocomposite samples was chosen to be 5% (I), 10% (II), 15% (III) and 20% (IV). Nanocomposites samples were placed horizontally and vertically between two parallel magnets, designated as N and M, respectively.

2.6. Characterization Fourier-transform infrared spectroscopy (FTIR) was performed on a Bomem FTIR instrument. X-ray diffraction (XRD) patterns were recorded on an X-ray diffraction instrument (Siemens D5000) at room temperature from 2θ= 5 to 100°. The thermal behavior of all samples was evaluated by a PL thermogravimetric analyzer (TGA 1000, UK). All samples were analyzed under nitrogen atmosphere from ambient temperature to 600 °C at the heating rate of 10 °C/min. The morphology and element detection of sample was characterized by a field emission scanning electron microscopy (FESEM) (Zeiss SUPRA 35VP) equipped with an energy dispersive X-ray (EDX) system. The mechanical properties of all samples were measured by Z030 Zwick/Roell testing machine using three Dog-bone shaped specimens with a tensile rate of 1 mm/min at ambient temperature and an average value reported. Transmission electron microscopy (TEM) (Philips EM 120) with an accelerating voltage of 150 kV was utilized to monitor nanofiller modification. The thermal diffusivity (α) of all nanocomposites were measured by the laser flash method using a LFA 471 (Netzsch). Thermal conductivity was calculated by the following Equation 1

k = α CP ρ

(1)

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Where ρ and CP are the density and heat capacity of nanocomposite samples. The CP was determined by differential scanning calorimetry (DSC, NETZSCH DSC 200 F3, Netzsch Co, Germany) by heating of the samples from 0 °C to 250 °C at a heating rate of 10 °C/min.

3. RESULTS AND DISCUSSION The synthetic routes of hydroxyl-functionalized HBN nanosheets (FHBN), polymer-grafted HBN nanosheets (mHBN) and iron oxide nanoparticles-modified boron nitride hierarchical nanostructure (FeNPs-mHBN) are shown in scheme 1. Scheme 1.

As shown in the scheme 1, the hydroxyl functional groups onto the HBN nanosheets are activated with CAN via a redox polymerization method. Subsequently, these active sites are utilized for post-polymerization modification. The successful preparation of FHBN and mHBN were confirmed by FTIR and TGA analysis. The FTIR spectra of HBN, FHBN, mHBN and FeNPs-mHBN are demonstrated in Fig. 1. The characteristic peaks at 1397 and 816 cm-1 are attributed to in-plane B−N stretching and out-of-plane bending vibration, respectively.47 For FHBN sample, the absorption band of around 1,080 cm-1 can be allocated to the C-O bond. The characteristic vibration bands of the hydroxyl stretching band appeared at 3400 cm−1 (Fig. 1), indicating that grafted-tert-butoxy groups onto the surface of HBN nanosheets have been successfully converted to hydroxyl groups.47 The absorption peaks of the polyacrylamide are illustrated in FTIR spectrum of the polymer-grafted HBN nanosheets (Fig. 1). For HBN/PAM hybrid nanosheets (mHBN in Fig. 1), the carbonyl peak at 1,657 cm-1 has overlapped with the BN bands of HBN nanosheets. The aliphatic C–H bands were observed at about 2,850–2,930 cm1 48

.

The absorption infrared spectrum of mHBN bands located at 3343 and 3185 cm-1 are

attributed to tensional stretching of –NH amid functional group. For FeNPs-mHBN sample, after 7



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adding and treatment of Fe3O4 nanoparticle to mHBN, slightly change was observed in –NH amid functional group absorption band. The band related to tensional stretching of –NH amid appeared at 3414 cm-1 which shows not only a shift from its former absorption frequency at IR spectrum of mHBN but also a change in absorption band shape. We propose that this observation is an indication of -NH bonding to Fe3O4 nanoparticle which has been illustrated in scheme 1. Consequently, the FTIR spectra demonstrated that the surface of the HBN nanosheets has been effectively modified by the PAM polymer shell. Figure 1.

Fig. 2 shows TGA thermograms of the pristine and modified HBN nanosheets. For mHBN nanosheets, a distinct weight loss at around 320 ºC is attributed to the degradation of grafted PAM onto the surface of HBN nanosheets. Grafting efficiency of post-polymerization modification can be estimated from the weight loss observed for modified HBN nanosheets in comparison with the pristine HBN. From TGA curves, grafting percentage (Equation 2) and conversion of polymerization (Equation 3) were found to be 19 and 6.3% for mHBN.49

G ( %) = ( m200 − m600 m600 ) ×100

(2)

Where m200 and m600 are the weights of sample remained at 200 ºC and 600 ºC, respectively. The conversion of graft polymerization is calculated according to the following Equation 3.49 G (%)  ×W S 100  G × WS   Conversion(%)= × 100 = WM WM

(3)

Where WM and WS are the initial amount of monomer and FHBN nanosheets applied in the graft copolymerization, respectively. TGA and FTIR results confirms successful grafting of PAM onto the surface of HBN nanosheets. Figure 2. 8


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The morphology and particle size of HBN nanosheets before and after graft polymerization were monitored by SEM images (Fig. 3). The flake-shaped nanoparticles approximately 70 nm in length are observed for the HBN in the quite uniform distributions (Fig. 3a). The average nanosheets length changed from 70 to 300 nm while the thickness decreased (see also AFM results). The SEM micrographs demonstrated an increase in aspect ratio of modified HBN nanosheets (flake length divided by thickness). Increasing aspect ratio parameter of nanoparticles is one of the researcher priorities due to its very important impact on various properties of materials. To prepare nanocomposites with enhanced characteristics, synthesizing nanofillers with high aspect ratio is a key parameter in controlling the interfacial adhesion between polymer matrix and the nanofillers. The large surface area of HBN nanosheets and their flake shape could be responsible for HBN agglomerate, thereby the aggregation of HBN remains unchanged during functionalization step. Moreover, SEM images illustrated that HBN nanosheet nearly connected with other particle as an outcome of post-polymerization. Low aggregation of HBN nanosheets could be due to relatively large particle size of pristine HBN nanosheets as is evident by SEM images (Fig. 3a1 and 3a2). Considering that the PAM grafting efficiency onto the HBN’s surface is relatively low (19%), therefor the modified HBN nanosheet with larger particle size than the unmodified one can be attributed to HBN aggregation during graft polymerization of acrylamide. It is assumed that several HBN nanosheets possibly form a nucleus with PAM shell (see TEM images). 50 Figure 3.

Additionally, the probability of interparticle termination between the polymer-grafted HBN nanosheets during the polymerization can also lead to the formation of HBN agglomerate and result the larger HBN nanosheets. It is supposed that polymer grafting strategy could make proper compatibility and reliable dispersion of nanosheets in epoxy resin and enhances the

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superior performance of the resulted nanocomposite samples. In addition, it is expected that modified HBN nanosheets as a thermal transport material can induce the interconnected thermal transport channel in the polymer bulk and improve the thermal conductivity of epoxy nanocomposites. The thickness change of HBN nanosheets after modification was monitored by AFM images. Images of HBN and mHBN reveal appropriate distribution of nanosheets with uniform thickness. As shown in Fig. 4a, most of the HBN is approximately 60-80 nm thick. In case of the mHBN sample (Fig. 4b), AFM images showed a high degree of exfoliation and sheet thickness of about 4 nm. Additionally, the AFM results indicate that the interlayer space of HBN nanosheets is controlled by the grafted-polymer chain length and the efficiency of HBN modification. Figure 4.

As mentioned before, the amide groups of polyacrylamide grafted onto the HBN nanosheets were used as active sites for attachment of the iron oxide nanoparticles onto the nanosheets (see scheme 1). Formation of hierarchical nanostructures of iron oxide nanoparticles-functionalized HBN (FeNPs-mHBN) was detected by XRD patterns. A high crystalline structure is evident for HBN nanosheets in the range of 5–100º (Fig. 5). The characteristic peaks of polyacrylamide at about 14-21º further confirms polymer grafting onto the surface of HBN nanosheets.48 Additionally, the appearance of new peaks at around 30.1, 35.4, 57 and 62.6º indicated that the iron oxide nanoparticles were successfully attached onto the modified HBN nanosheets. Figure 5.

The elemental mapping by EDX showed homogeneous grafting of polymer and iron oxide nanoparticles onto the surface of HBN nanosheets. The presence of carbon, oxygen, iron elements in mHBN and FeNPs-mHBN samples were indicated by EDX and EDX mapping. For mHBN sample (Fig. 6), EDX mapping images show a uniform distribution and high element 10


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concentration of C and O elements. Observation of a uniform distribution of iron element immobilized onto the surface of mHBN nanosheets confirms that the post-polymerization modification is a novel and effective route for achievement of iron oxide nanoparticles-modified boron nitride hierarchical nanostructures. Uniform dispersion of iron oxide nanoparticles as supermagnetic nanoparticles and managing the alignment of HBN nanaosheets in magnetic field is a key issue to attain the interlinked and continuous thermal transport pathway in polymer matrix, which provides an efficient way for heat dissipation from the nanocomposites. The amount of the immobilized iron oxide nanoparticles onto the mHBN nanosheets was determined by EDX analysis to be 1.9 Wt%. Figure 6.

TEM micrographs of HBN and FeNPs-mHBN nanosheets are shown in Fig. 7. The successful immobilization of iron oxide nanoparticles onto the mHBN surface was also confirmed by TEM images (Fig. 7a1 and 7a2). TEM was used to detect the level of the Fe3O4 nanoparticles dispersion on the mHBN surface and the size of immobilized Fe3O4 nanoparticles as well. TEM characterization showed the spherical Fe3O4 nanoparticles with a diameter range of 1–10 nm and a quite uniform size distribution. In addition, relatively homogeneous distribution of Fe3O4 nanoparticles on the mHBN surface was observed. The branched network morphology or agglomeration observed for FeNPs-mHBN nanosheets resulted from interparticle van der Waals interactions between the HBN nanosheets which is confirmed by TEM images. Formation of PAM shell around the HBN agglomerate as a core (Fig. 7b), which diminishes distinct boundary between HBN nanosheets makes it possible to observe HBN nanosheets with larger length size. 50

Figure 7.

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The FeNPs-mHBN composite as a high thermal conductive material was used to improve the thermal conductivity of epoxy nanocomposites. It is expected that magnetically responsive HBN (FeNPs-mHBN) can react to an external magnetic field. The aligned epoxy nanocomposite samples with different loading of FeNPs-mHBN were provided by applying a vertical and horizontal magnetic field. In the following discussion, we will explore various properties of prepared aligned nanocomposite. The alignment of the HBN nanosheets has an essential role in the thermal transport and conductivity behavior of polymer matrix. Thus, the filler alignment in prepared epoxy nanocomposites was studied by XRD patterns (Fig. 8). The appearance of HBN peaks verifies the existence of HBN nanosheets in the nanocomposite matrix. The I002/I100 ratio of each nanocomposite samples was estimated from XRD patterns. The prepared epoxy nanocomposites in vertical magnetic field showed low I002/I100 ratio compared with the other nanocomposite samples. This means that most platelets are parallel to the magnetic field direction. However, XRD patterns show the existence of both 002 and 100 HBN nanosheet orientation in the prepared nanocomposites. Figure 8.

Additionally, we used SEM as a complimentary technique to detect the magnetic alignment of HBN nanosheets in the nanocomposite samples. SEM Images of epoxy nanocomposites with different loading of FeNPs-mHBN nanosheets reveal distinct HBN nanosheets in cross section of all nanocomposites. In addition, a uniform distribution of HBN nanosheets was also observed in the nanocomposite cross-section. Moreover, the SEM micrographs of all nanocomposites demonstrate both vertically and horizontally oriented nanosheets which are characterized as 2D plates (blue arrows) and 1D rods (red arrows), respectively (Fig. 9). According to the results obtained from XRD and SEM analysis, alignment of FeNPs-mHBN nanosheets under magnetic field has been schematically illustrated in scheme 2. 12


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Scheme 2.

We observed that effective alignment of HBN nanosheets in polymer matrix was decreased by increasing HBN nanosheets loading. This was due to high viscosity and decrease free volume of HBN plates in epoxy resin, which inhibits the free orientation of HBN nanosheets in epoxy resin. Overall, when nanofiller loading is decreased the role of magnetic alignment for orientating of HBN nanosheets in polymer matrix is more prominent. Figure 9.

From LFA data (Fig. 10), the thermal conductivity of neat epoxy and its nanocomposites was calculated by Eq. 1. The thermal conductivity of nanocomposite samples containing mHBN (F) was measured and shown in Fig. 10. As shown in Fig. 10, thermal conductivity of epoxy nanocomposite at room temperature increases by the increase of mHBN nanosheets loading. In addition, the thermal conductivity of the aligned nanocomposite increases by the addition of FeNPs-mHBN nanosheets. This improvement has been mainly attributed to high innate thermal conductivity of the HBN nanosheets. It is expected that alignment of HBN nanosheets as thermal transport materials is able to provide the interconnected thermal transport channel in polymer matrix and improve the heat transport at low HBN loading. From thermal conductivity of the aligned nanocomposites in two magnetic field direction, it could be obserdved that nanocomposites (designated as M) display higher thermal conductivity in comparison with the nanocomposites (designated as N) at low FeNPs-mHBN loading. More thermal enhancement of nanocomposites containing mHBN (F) in comparison with the aligned nanocomposites can be attributed to the absence of Fe3O4 nanoparticles in these nanocomposites. Addition of Fe3o4 nanoparticles can result in larger thermal boundary resistance between mHBN (F) and matrix interface, thereby decreasing thermal conductivity of these nanocomposites. Figure 10.

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These results possibly attributed to the filler alignment and its significant role in dissipating heat from polymer matrix. Nanocomposite containing 10 % FeNPs-mHBN nanosheets (M) showed higher thermal conductivity and 1.7 times enhancement of thermal conductivity in comparison with the other nanocomposites. We propose that in low filler concentration which alignment can be easily occurred, this function is a key factor to increase the thermal conductivity of composites. In other words, in higher filler loadings not only proper alignment is more difficult but also more Fe3O4 nanoparticles concentration with larger thermal boundary resistance makes it possible to consider that the interfacial interaction now is the promising factor for thermal conductivity improvement. The comparative thermal conductivity enhancement of the prepared nanocomposites in respect to the other polymer/HBN nanocomposites is summarized in Table 1. Table 1. Thermal Conductivity Enhancement of various polymer/HBN nanocomposites.

Material Epoxy/HBN Polyimide/HBN Polyimide/HBN Polyimide/HBN Epoxy/HBN FeNPs-mHBN-epoxy

Filler size (µm)

Loading (wt %)

12 60-100 1 and 0.07 0.07 0.7 5 0.07

20 20 20 20 40 20 10

Thermal Conductivity Enhancement (Kcomposite/Kresin) 3 5.6 1.5 1.5 6.8 5.7 1.7

References 51 12 13 29 3 This work

Thermal stability behavior of the neat epoxy and its nanocomposites was studied by TGA analysis (Fig. 11). As demonstrated in Fig. 11, the neat epoxy and its nanocomposites decomposed in a one-step process (around 300 ºC). Thermal stability of all nanocomposites did not show a considerable increase in comparison to the neat epoxy. Ash content of the nanocomposite samples after complete thermal degradation is approximately in agreement with the amount of HBN used for preparing of each nanocomposites. Figure 11.

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Stress–strain measurement was used to investigate the mechanical properties of all samples. These measurements are given in Table 2. A reduction in elongation at break is observed by the addition of FeNPs-mHBN nanosheets. Moreover, the tensile strength of all nanocomposite samples rises by increasing FeNPs-mHBN nanosheets loading. As expected, improvement of the tensile strength can be probably due to the uniform dispersion and appropriate interfacial interaction of FeNPs-mHBN nanosheets within the resin matrix. Therefore, the higher tensile strength value of these nanocomposites in comparison with neat epoxy might be related to the homogeneous dispersion of FeNPs-mHBN nanosheets and the effective crosslinker role of polymer-graft containing reactive functionality such as amine or amid groups which covalently cross-linked into the polymer network. 52 Table 2. Tensile results of neat epoxy and its nanocomposites with different HBN nanosheets content.

Designation

neat epoxy MI MII MIII MIV NI NII NIII NIV

Strength (MPa) 31.23 37.78 41.37 33.33 43.24 47.73 35.30 32.96 35.07

Elongation at break (%) 1.75 0.3 1 1.40 1.50 0.8 0.10 0.1 1.70

4. Conclusion We modified BNNs by grafting through redox polymerization of acrylamide from already modified BNNs by hydroxyl groups. FT-IR, TGA, SEM, TEM, XRD and EDX results confirmed the successful grafting of polyacrylamide and also the attachment of Fe3O4 nanoparticles onto the surface of HBN nanosheets. The prepared nanocomposites showed high thermal conductivity and significant mechanical properties in respect to the pristine epoxy polymer.

The

nanocomposite consisting of 10 % FeNPs-mHBN showed high thermal conductivity (0.37 W/mK at 25 °C) and suitable properties as compared with others. We conclude that epoxy/boron 15



nitride nanocomposites can be potentially used as thermal transport materials for a broad area in microelectronic applications. SUPPORTING INFORMATION Digital images (Fig. S1 and Fig. S2) of HBN, modified HBN nanosheets and aligned epoxy/FeNPs-mHBN nanocomposites samples. Digital images (fig. S3) of a) FeNPs-mHBN powder in the presence of an external magnet, b) FeNPs-mHBN powder and c) mHBN powder in the presence of an external magnet. Scheme S1: Schematic illustration of HBN agglomeration Scheme S2: Structure of interparticle termination between polymer-grafted HBN nanosheets during polymerization. Scheme S3: Structure of covalently cross-liked amine functional group, presented on polymer grafted chains, into the polymer network. ACKNOLEDGEMENT Financial supports granted by Niroo Research Institute (NRI) are gratefully acknowledged.

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(29) Sato, K.; Horibe, H.; Shirai, T.; Hotta, Y.; Nakano, H.; Nagai, H.; Mitsuishi, K.; Watari, K. Thermally conductive composite films of hexagonal boron nitride and polyimide with affinity-enhanced interfaces. J. Mater. Chem. 2010, 20, 2749. (30) Marconnett, A.M.; Yamamoto, N.; Panzer, M.A.; Wardle, B.L.; Goodson, K.E. Thermal Conduction in Aligned Carbon Nanotube-Polymer Nanocomposites with High Packing Density. ACS Nano 2011, 5, 4818. (31) Li, L.; Yang, Z.B.; Gao, H.J.; Zhang, H.; Ren, J.; Sun, X.M.; Chen, T.; Kia, H.C.; Peng, H.S. Vertically Aligned and Penetrated Carbon Nanotube/Polymer Composite Film and Promising Electronic Applications. Adv. Mater. 2011, 23, 3730. (32) Yousefi, N.; Gudarzi, M.M.; Zheng, Q.B.; Aboutalebi, S.H.; Sharif, F.; Kim, J.K. Selfalignment and high electrical conductivity of ultralarge graphene oxide–polyurethane nanocomposites. J. Mater. Chem. 2012, 22, 12709. (33) Liang, Q.Z.; Yao, X.X.; Wang, W.; Liu, Y.; Wong, C.P. A Three-Dimensional Vertically Aligned Functionalized Multilayer Graphene Architecture: An Approach for Graphene-Based Thermal Interfacial Materials. ACS Nano 2011, 5, 2392. (34) Song, W.L.; Wang, P.; Cao, L.; Anderson, A.; Meziani, M.J.; Farr, A.J.; Sun, Y.P. Polymer/Boron Nitride Nanocomposite Materials for Superior Thermal Transport Performance. Angew. Chem. Int. Ed. 2012, 51, 6498. (35) Terao, T.; Zhi, C.Y.; Bando, Y.; Mitome, M.; Tang, C.C.; Golberg, D. Alignment of Boron Nitride Nanotubes in Polymeric Composite Films for Thermal Conductivity Improvement. J. Phys. Chem. C 2010, 114, 4340. (36) Cooper, C.A.; Ravich, D.; Lips, D.; Mayer, J.; Wagner, H.D. Distribution and alignment of carbon nanotubes and nanofibrils in a polymer matrix. Compos. Sci. Technol. 2002, 62, 1105. (37) Xie, B.H.; Huang, X.; Zhang, G.J. High thermal conductive polyvinyl alcohol composites with hexagonal boron nitride microplatelets as fillers. Compos. Sci. Technol. 2013, 85, 98. (38) Kuang, Z.; Chen, Y.; Lu, Y.; Liu, L.; Hu, S.; Wen, S.; Mao, Y.; Zhang, L. Fabrication of Highly Oriented Hexagonal Boron Nitride Nanosheet/Elastomer Nanocomposites with High Thermal Conductivity. Small 2015, 11, 1655. (39) Cho, H.B.; Tokoi, Y.; Tanaka, S.; Suematsu, H.; Suzuki, T.; Jiang, W.; Niihara, K.; Nakayama, T. Modification of BN nanosheets and their thermal conducting properties in nanocomposite film with polysiloxane according to the orientation of BN. Compos. Sci. Technol. 2011, 71, 1046. (40) Erb, R.M.; Libanori, R.; Rothfuchs, N.; Studart, A.R. Composites Reinforced in Three Dimensions by Using Low Magnetic Fields. Science 2012, 335, 199. (41) Yan, M.; Fresnais, J.; Sekar, S.; Chapel, J.P.; Berret, J.F. Magnetic Nanowires Generated via the Waterborne Desalting Transition Pathway. ACS Appl. Mater. Interfaces 2011, 3, 1049. 19



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Table of contents

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Scheme 1: Structure of HBN nanosheets before and after graft polymerization of AM a) HBN, b) FHBN, c) mHBN, and d) FeNPs-mHBN. 236x129mm (300 x 300 DPI)


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Scheme 2: Schematic diagram of Fe3O4-mHBN alignment under different magnetic field directions. 85x56mm (300 x 300 DPI)



Figure 1: FTIR spectra of HBN and modified HBN nanosheets 72x81mm (300 x 300 DPI)


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Figure 2. TGA thermograms of HBN and modified HBN nanosheets. 50x48mm (300 x 300 DPI)



Figure 3. SEM Micrographs of: a1 and a2) HBN, b1, b2 and b3) mHBN nanosheets modified by surfaceinitiated redox polymerization. 84x30mm (300 x 300 DPI)


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Figure 3. SEM Micrographs of: a1 and a2) HBN, b1, b2 and b3) mHBN nanosheets modified by surfaceinitiated redox polymerization. 103x24mm (300 x 300 DPI)



Figure 4. AFM images of: a) HBN and b) mHBN. 124x44mm (300 x 300 DPI)


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Figure 4. AFM images of: a) HBN and b) mHBN. 71x37mm (300 x 300 DPI)



Figure 5. XRD patterns of HBN, mHBN and FeNPs-mHBN. 46x56mm (300 x 300 DPI)


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Figure 6. X-ray mapping of a) mHBN and b) FeNPs-mHBN. 131x145mm (300 x 300 DPI)



Figure 6. X-ray mapping of a) mHBN and b) FeNPs-mHBN. 130x140mm (300 x 300 DPI)


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Figure 7. TEM micrographs and EDX spectra of mHBN and FeNPs-mHBN nanosheets. 73x37mm (300 x 300 DPI)



Figure 7. TEM micrographs and EDX spectra of mHBN and FeNPs-mHBN nanosheets. 92x30mm (300 x 300 DPI)


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Figure 8: XRD patterns of the epoxy/ FeNPs-mHBN nanocomposite placed between two parallel magnets in a) vertical and b) horizontal position. 53x50mm (300 x 300 DPI)



Figure 8: XRD patterns of the epoxy/ FeNPs-mHBN nanocomposite placed between two parallel magnets in a) vertical and b) horizontal position. 53x50mm (300 x 300 DPI)


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Figure 9: SEM cross-section of the epoxy/ FeNPs-mHBN nanocomposite placed between two parallel magnets in a) vertical and b) horizontal position. 131x157mm (300 x 300 DPI)



Figure 9: SEM cross-section of the epoxy/ FeNPs-mHBN nanocomposite placed between two parallel magnets in a) vertical and b) horizontal position. 127x146mm (300 x 300 DPI)


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Figure 10: A) Thermal conductivity and B) Thermal enhancement of the epoxy nanocomposites. 556x547mm (96 x 96 DPI)



Figure 10: A) Thermal conductivity and B) Thermal enhancement of the epoxy nanocomposites. 175x169mm (300 x 300 DPI)


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Figure 11: TGA curves of the epoxy/ FeNPs-mHBN nanocomposite placed between two parallel magnets in a) vertical and b) horizontal position. 63x60mm (300 x 300 DPI)



Figure 11: TGA curves of the epoxy/ FeNPs-mHBN nanocomposite placed between two parallel magnets in a) vertical and b) horizontal position. 63x60mm (300 x 300 DPI)


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