Improved dielectric properties, mechanical properties

Applied Surface Science 439 (2018) 186–195

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Improved dielectric properties, mechanical properties, and thermal conductivity properties of polymer composites via controlling interfacial compatibility with bio-inspired method Mengnan Ruan a,c, Dan Yang b,c,⇑, Wenli Guo a,b,c,⇑, Liqun Zhang a, Shuxin Li b,c, Yuwei Shang b,c, Yibo Wu b,c, Min Zhang b,c, Hao Wang b,c a

College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China College of Materials Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China c Beijing Key Lab of Special Elastomeric Composite Materials, Beijing 102617, China b

a r t i c l e

i n f o

Article history: Received 23 October 2017 Revised 25 December 2017 Accepted 29 December 2017 Available online 6 January 2018 Keywords: Al2O3 particles Nitrile rubber Thermal conductivity Dielectric properties Mechanical properties

a b s t r a c t Surface functionalization of Al2O3 nano-particles by mussel-inspired poly(dopamine) (PDA) was developed to improve the dielectric properties, mechanical properties, and thermal conductivity properties of nitrile rubber (NBR) matrix. As strong adhesion of PDA to Al2O3 nano-particles and hydrogen bonds formed by the catechol groups of PDA and the polar acrylonitrile groups of NBR, the dispersion of Al2O3-PDA/NBR composites was improved and the interfacial force between Al2O3-PDA and NBR matrix was enhanced. Thus, the Al2O3-PDA/NBR composites exhibited higher dielectric constant, better mechanical properties, and larger thermal conductivity comparing with Al2O3/NBR composites at the same filler content. The largest thermal conductivity of Al2O3-PDA/NBR composite filled with 30 phr Al2O3-PDA was 0.21 W/m K, which was 122% times of pure NBR. In addition, the Al2O3-PDA/NBR composite filled with 30 phr Al2O3-PDA displayed a high tensile strength about 2.61 MPa, which was about 255% of pure NBR. This procedure is eco-friendly and easy handling, which provides a promising route to polymer composites in application of thermal conductivity field. Ó 2018 Elsevier B.V. All rights reserved.

1. Introduction In recent years, the demands in industrial and electronic equipment are increased with the rapid development of electronic information industry. However, heat accumulation produced by increasing electronics and industrial equipment may lead to efficiency reduction and even equipment damage [1]. In order to reduce the damage caused by overheating of the electronic equipment during operation, it is important to improve the thermal diffusion and reduce the heat accumulation of materials [2]. Due to low cost and good processability, polymeric matrix has been commonly used as electronic packaging materials, but the low thermal conductivity limits its application. In order to improve the thermal conductivity of polymeric matrix, many researchers added conductive fillers (carbon fiber, carbon nanotube, graphite [3–5], and metallic (Ag, Cu, Au) [6–8])

⇑ Corresponding authors at: College of Materials Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China. E-mail addresses: [email protected] (D. Yang), [email protected] (W. Guo). https://doi.org/10.1016/j.apsusc.2017.12.250 0169-4332/Ó 2018 Elsevier B.V. All rights reserved.

into polymeric matrix. As the interface compatibility between organic phase and inorganic phase is poor, the intrinsic defects and high thermal contact resistance lead to a limited improved thermal conductivity of composites. In addition, high dielectric loss and low dielectric strength are always accompanied, limiting the wide application in electronic packaging materials [9]. The second method to improve thermal conductivity of polymeric matrix is adding ceramic fillers, such as (aluminum oxide (Al2O3) [1,2,10], aluminum nitride (AlN) [11–13], boron nitride (BN) [14–16], silicon nitride (Si3N4) [17,18], silica (SiO2), and silicon carbride (SiC) [19]). Among those ceramic fillers, Al2O3 nano-particles are widely used in electronic circuit devices due to their high thermal conductivity (32 W/m K), low cost, and stable chemical performance [20]. However, the unmodified Al2O3 nano-particles have a weak interfacial bonding force between polymeric matrixes, leading to an unacceptable increased thermal resistance. Thus, surface modification of the Al2O3 nano-particles is an important route to enhance the thermal conductivity of polymer composites. Traditional method is chemical grafting silane coupling agent to polymer chains and filler particles. Permal et al. [9] functionalized Al2O3

M. Ruan et al. / Applied Surface Science 439 (2018) 186–195

nano-particles and boron nitride (BN) nanoplatelets with (3-aminopropyl)triethoxysilane (KH550) to reduce the thermal interfacial resistance between fillers and epoxy matrix. It was observed that the resultant epoxy composites provided a higher thermal conductivity of 0.57 W/m K than that of neat epoxy (0.17 W/m K). Wang et al. [20] pretreated Al2O3 particles by silane-coupling agent bis-(3-triethoxy silylpropyl)-tetrasulfide (Si69) and stearic acid (SA) to improve particles dispersion and filler-rubber interaction of ethylene propylene diene monomer (EPDM). The results showed that the maximum thermal conductivity of Al2O3-SA/EPDM was up to 0.45 W/m K, about three times higher than that of pure EPDM. However, these studies and techniques limited in the applications of industrialization due to complicated procedures, high cost instruments, strict reaction conditions [21–24], and toxic reagents [2]. Therefore, it is important to develop an easy-handled and eco-friendly method to modify the fillers to improve the thermal conductivity of polymer composites. Recently, poly(dopamine) (PDA) was used in surface functionalization of thermal conductivity fillers due to strong adhesion to all kinds of substance. This bio-inspired method is convenient and effective for its simple reagents, mild reaction conditions, and no-harmful properties [25–27]. Shen et al. [2] modified h-BN micro-platelets by PDA to prepare h-BN@PDA/PVA thermal conductivities composites. The results showed that the thermal conductivity of h-BN@PDA/PVA composite containing 10 vol% filler is 5.4 W m1 K1, which is about 1.5 times higher than that of hBN/PVA composite at the same filler loading. Chen et al. [28] coated PDA on the surfaces of BN to reduce the interfacial thermal barrier and enhance the thermal conductivity of PP based composites. Comparing with PP composite filled with unmodified BN, the PP composite filled with modified BN showed an increase of 214% at 25 wt% filler loading. However, dopamine was applied to modify the commercial Al2O3 particles to improve thermal conductivity of NBR has rarely been reported. In this work, Al2O3 particles was selected as thermal conductivity filler. Although the thermal conductivity of Al2O3 particle (31.7 W/m K) is lower than that of BN particle (56.9 W/m K), the used thermal conductive filler is Al2O3 particle, which is a commonly known ceramic material with low cost and non-toxic. As it is cheap to procure the versatile range of particles sizes, Al2O3 has been widely used in industry [9]. The NBR was chose as polymer matrix due to their high dielectric constant (>10), good processability, abrasion resistance, and heat and chemical resistance. The PDA with abundant catechol groups was introduced to functionalize the surface of Al2O3 particles. The dispersion can also be improved because polar acrylonitrile (ACN) groups in NBR chains can interact strongly with catechol groups in the PDA through hydrogen bonds. The dielectric properties, mechanical properties, and thermal conductivity properties of Al2O3/NBR and Al2O3-PDA/NBR composites were investigated comprehensively.

2. Experimental 2.1. Materials As the polymer matrix, NBR (DN2850, acrylonitrile content of 28%) was purchased from Shanghai NESSEN international Trading Co., Ltd. a-Al2O3 particles with an average diameter of about 300 nm and density (qAl2O3) of 3.9 g/cm3 were bought from Jiangsu Lianlian Chemical Co.,Ltd. (China). The crosslinking agent dicumyl peroxide (DCP) was obtained from Beijing Chemical Reagents Co., Ltd. (China). The epoxidized soybean oil (ESO) of epoxy value 6.0 was purchased from Tianjing Guangfu Fine Chemical Institute (China). Tris(hydroxymethyl)-amino-methane (Tris) and dopamine were purchased from J&K Scientific Ltd. (China).

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2.2. Modification of Al2O3 particles by PDA First, 0.8 g dopamine was dissolved in deionized water to obtain a solution with a concentration of 2.0 g/L. Second, 0.6 g tris was added into the above solution until the pH of the solution reached to 8.5. Then, 60 g Al2O3 particles were putted into the above dopamine solution and mechanical agitation about 24 h. Last, the Al2O3 particles which were modified with PDA solution were washed with distilled water and then dried at 45 °C in vacuum oven. The procedure for fabrication of Al2O3-PDA particles is shown in Fig. 1. 2.3. Preparation of Al2O3/NBR and Al2O3-PDA/NBR composites The uncured NBR with different contents of Al2O3, Al2O3-PDA were prepared through physical mixing on a 6 in two-roll mill. Recipe (3.2) in our experiments contained different contents Al2O3 or Al2O3-PDA (0 phr, 10 phr, 20 phr, 30 phr), 2 phr DCP, 15 phr ESO, and 100 g of NBR, denoted as Al2O3/NBR or Al2O3-PDA/ NBR composites. Each composite was cured at the pressure of 15 MPa at 160 °C for its optimum cure time as determined by a disk oscillating rheometer (GT-M2000-FA, Goteah Testing Machines Inc., Taiwan) to obtain the corresponding cured composite. Usually the thermal conductivity composites are applied in flexible electronic systems, such as communication, automotive, biomedical, and aerospace. In this study, the content of thermal conductive filler was below 30 phr in order to keep flexibility and stretchability of polymer composites. Too much rigid Al2O3 particles will decrease the flexibility of the thermally conductive composite. In addition, more particles in the system will increase the probability of interfacial failure, though the PDA will postpone the interfacial failure, a disadvantage for long-time stability and lifetime. 2.4. Characterization A high-resolution transmission electron microscope (HR-TEM) (Hitachi H9000, Japan) was used to examine the morphologies of the Al2O3 and Al2O3-PDA particles at a voltage of 300 kV. XPS spectra of Al2O3 and Al2O3-PDA particles were recorded to study the element composition using an ESCALAB 250 XPS system which was made in American Thermo Electron Corporation. In the XPS analysis, a monochromatic Al Ka X-ray source was operation with pass energy of 1486.6 eV. Scanning electron microscopy using the FEI NanoSEM 430 scanning electron microscope was used to observe distribution of fillers in composites. Prior to observation, fractured surfaces of samples were sputtered with a layer of gold to avoid charge accumulation. The tensile tests of Al2O3 particles and Al2O3-PDA particles filled NBR composite were tested by using a tensile apparatus (Instron 3366, America) at 25 °C according to Chinese Standards GB/T5281998. The testing speed was 50 mm/min. The elastic modulus of pure NBR and NBR composites were determined by the slope of the stress-strain curves at 5% strain. The experimental data of elastic modulus is the average of the results obtained from at least five samples under the same conditions. On an atomic scale, macroscopic elastic is manifested as small changes in the interatomic spacing and the stretching of interatomic bonds. As a consequence, the magnitude of elastic modulus is a measure of resistance to separation of adjacent atoms/ions/molecules. Moreover, the stressstrain characteristics at low stress levels are virtually the same for both tensile and compressive situations, to include the magnitude of the modulus of elasticity [29]. So we choose the tensile apparatus to characterize the mechanical property in our study. The dielectric properties of the samples were measured by an impedance analyzer (Alpha-A, Novochtrol, Germany) over the frequency range of 101 to 107 Hz at room temperature and at the elec-

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Fig. 1. Schematic diagram illustrating of preparation process of Al2O3-PDA/NBR composite.

tric voltage of 3 V. The samples had a diameter of 25 mm and a thickness of 1 mm. A resistivity meter (EST 121, Beijing Huajinghui, China) was used to measure the volume resistivity of samples. The volume resistivity (qm) was calculated by using the following equation:

qv ¼ 4L=ðRv p d2 Þ

ð1Þ

where L is the thickness of the specimen, Rm is the resistance of the specimen, and d is the diameter of the electrode. The thermal conductivity of the thermally conductive elastomer composite was measured using a HC-110 thermal conductivity meter (Laser Comp. Inc., USA). The temperature of the hot and cold plate of the thermal conductivity meter was 20 °C and 40 °C, respectively with pressure of 414 kPa. 3. Results and discussion 3.1. Surface modification of Al2O3 particles by PDA The overall procedures of surface modification of Al2O3 particles by the polymerized poly(dopamine) derivatives are presented in Fig. 1. A possible reaction mechanism for PDA is explained as follows. The dihydroxyl group protons in dopamine become dopamine-quinone under the condition of oxidation. Then the dopamine-quinone undergoes oxidation to form dopamine chrome. At last, through intra-molecular cyclization, the main polymerized products are formed, including 5,6-dihydroxyindolerich compounds (DHI) and polymerized dopamine derivatives. The polymerization of DHI to polydopamine (PDA) are not invariable due to the various oxidation states of DHI depending on medium pH, and thus the resulting PDA have high structure diversity [30]. The schematic diagram illustrating the self-polymerization for PDA is shown in Fig. S1.

Fig. 2 displays the HR-TEM images of Al2O3 particle and Al2O3PDA particle. It is clearly found that the surface of Al2O3 particle is smooth and neat [31]. However, after modification with PDA, it can be seen from Fig. 2(b) that the surface of Al2O3 particle is rough, because a layer of amorphous substance (PDA layer) with a thickness of about 3 nm was absorbed on the surface of the Al2O3 particle [32]. Surface elemental composition of Al2O3 particles is detected by comparing the XPS spectra of particles surface. Fig. 3(a) and Fig. 3 (b) shows the XPS wide scan spectra of the Al2O3 particles and Al2O3-PDA particles, respectively. Compared with Fig. 3 (a) and (b), a new characteristic peak at 398 eV is observed, which is due to the presence of N 1s electron. Fig. 3(c) and (d) shows the N 1s peak spectrum of the Al2O3 particles and Al2O3-PDA particles. From Fig. 3(c), we cannot easily find an obvious N 1s characteristic peak, scribing to there is no N element in the pristine Al2O3 particles. As shown in Fig. 3(d), the N 1s core-level spectrum of the Al2O3-PDA particles can be curve-fitted with two peak components at 398.5 eV for the amine (ANAH) groups, and 399.5 eV for the imine (@NA) groups. The (ANAH) groups is due to dopamine, while the imine (@NA) groups are formed by the indole groups in the process of the dopamine self-polymerization. The XPS results indicate that dopamine is indeed adhesive on the Al2O3 surface. 3.2. Microstructure and mechanical properties The dispersion of fillers in composites can be observed directly by using the SEM. The SEM micrographs of the fractured surfaces of the Al2O3/NBR and Al2O3-PDA/NBR composites are shown in Fig. 4. From Fig. 4(a) and (c), we can observe large agglomerations of Al2O3 particles in the NBR composite. The phenomenon could be interpreted by the large difference in surface energy between the Al2O3 particles and the NBR matrix [33]. As the super strong adhe-


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Fig. 2. HR-TEM images of (a) Al2O3 particles and (b) Al2O3-PDA particles.

Fig. 3. X-ray photoelectron spectroscopy wide-scan spectra of (a) Al2O3 particles and (b) Al2O3-PDA particles. N 1s core-level spectrum of (c) Al2O3 particles and (d) Al2O3PDA particles.

sion of PDA to the surface of inorganic fillers and NBR macromolecules, there is strong interfacial force between filler and matrix, leading to a good dispersion of Al2O3-PDA particles in NBR matrix (as shown in Fig. 4(b) and (d)). With the increasing content of Al2O3-PDA particles, the filler network structure could be formed in system, benefit for increasing thermal conductivity of the Al2O3-PDA/NBR composites. Fig. 5(a) and (b) illustrate the stress-strain curves response of Al2O3/NBR and Al2O3-PDA/NBR composites, respectively. Generally, the composites show an enhancement of tensile strength after they are added with inorganic fillers. Also, in our work, the tensile strength increases with the increasing content of Al2O3 and Al2O3PDA particles. Furthermore, the tensile strength of Al2O3-PDA/NBR composites is higher than that of Al2O3/NBR composites at the

same filler content. This may be attributed to the strong hydrogen bonding from the abundant catechol groups of PDA and the polar acrylonitrile (ACN) groups of NBR mobility suppressed the mobility of polymer chains of polymer chains, thus increasing the tensile strength [34]. The hydrogen bonding was demonstrated to exist by the molecular dynamics (MD) simulation in Fig. S2. Moreover, we can observe that all NBR composites display larger elongation at break than that of pure NBR. This maybe is attributed to crosslink density of the NBR composites decreased by Al2O3 and Al2O3-PDA particles, consisting with previous studies [35]. In addition, the elongation at break of Al2O3-PDA/NBR composites is also larger than Al2O3/NBR composites at the same filler content, and the largest elongation at break of Al2O3-PDA/NBR composite reaches to 160%. That may be related to the poor dispersion and

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Fig. 4. SEM micrographs of fractured surfaces of NBR composites filled with different contents of particles: (a) 10 phr of Al2O3, (b) 10 phr of Al2O3-PDA, (c) 30 phr of Al2O3, and (d) 30 phr of Al2O3-PDA.

defect in Al2O3/NBR composites. Usually, the aggregation of Al2O3 particles which act as additional stress centers would cause the low elongation at break [36]. Fig. 5(c) shows the elastic modulus of NBR composites filled with different content of Al2O3 and Al2O3-PDA. From Fig. 5(c), we can see that the elastic modulus of Al2O3/NBR and Al2O3-PDA/ NBR composites increases with the increasing content of fillers. As ceramic fillers are tough and have a higher modulus than that of pure NBR, the elastic modulus of composites increases with increasing content of filler, whether Al2O3 or Al2O3-PDA particles. In addition, the elastic modulus of Al2O3-PDA/NBR composites is always lower than that of Al2O3/NBR composites at the same content of filler. The above phenomenon might be explained by two competing effects. One is the hydrogen bonding in Al2O3/NBR composite is weaker than of the Al2O3-PDA/NBR composite. The strong hydrogen bonding between Al2O3-PDA particles and NBR matrix will improve filler dispersion and suppressed the mobility of polymer chains of polymer chains, thus increasing the elastic modulus. In addition, the weak bond between Al2O3 fillers and the matrix may cause an interfacial failure under tension. With the increase of load, interfacial debonding first occurs at the pole of the particle and propagates down to the equator, and then matrix plastic damage happens at the vicinity of the interfacial debonding; finally, interfacial debonding at different locations is linked by the matrix cracks [37,38]. As the weak interfacial bond between the Al2O3 particles and polymeric matrix, the Al2O3 fillers play a negligible role in load-bearing capacity of the composite. However, the strong hydrogen bond between Al2O3-PDA fillers and the matrix will

postpone the interfacial failure, thus improving the role of the fillers in load-bearing capacity of the composite. Second is the PDA layer is relatively soft. When the rigid Al2O3 particles are coated on the PDA, the elastic modulus of Al2O3-PDA particles is lower than that of Al2O3 particles, leading to a lower elastic modulus of Al2O3-PDA/NBR composites than that of Al2O3-PDA/NBR composites [39]. As the second reason is dominant during the competing effects of the above two reasons, leading to the elastic modulus of Al2O3-PDA/NBR composite is lower than that of Al2O3/NBR composite. The basic equation of volume of the relatively soft PDA layer (VPDA) coated on the Al2O3 particles is roughly calculate by the number of Al2O3 particle in the matrix (n Al2O3) and the thickness of PDA (h) (3 nm):

V Al2 O3 4pr 2 h V SAl2 O3 mAl2 O3 4pr 2 h ¼ 4pr 2 h qAl2 O3 43 pr3

V PDA ¼ nAl2 O3 4pr2 h ¼ ¼

mAl2 O3

qAl2 O3 V SAl2 O3

ð2Þ

where r and VSAl2O3 are the radius and volume of a single Al2O3 particle, m Al2O3 is the weight of all the Al2O3 particles in the matrix (30 g). The volume of PDA layer was about 0.23 cm3, while the volume of Al2O3 particles (VAl2O3) was 7.6 cm3 in about 100 cm3 NBR. Moreover, the elastic modulus of the Al2O3-PDA/NBR composites (E) was calculated from moduli of matrix (Em) by the Guth-Gold equation (shown in Fig. 5(c)) [40]: 2

E ¼ Em ð1 þ 2:5f P þ 14:1f P Þ

ð3Þ


Fig. 5. Stress-strain curves of the NBR composites filled with different contents of (a) Al2O3 particles and (b) Al2O3-PDA particles. (c) Elastic modulus of NBR composites filled with different contents of Al2O3 particles and Al2O3-PDA particles.

where fp is the volume fraction of filler. From Fig. 5(c), the elastic modulus of Al2O3-PDA/NBR composites are close to the elastic modulus calculated by the Guth-Gold equation, showing that the reinforcing effect of the Al2O3-PDA is lower than that of the Al2O3 particles [40]. 3.3. Dielectric properties, thermal properties, and volume resistivity of NBR composites The dielectric constant (1 kHz) of Al2O3/NBR and Al2O3-PDA / NBR composites at room temperature is shown in Fig. 6(a). From

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Fig. 6. (a) Dielectric constant of NBR composites filled with different contents of Al2O3 particles and Al2O3-PDA particles at a frequency of 1 kHz. Frequency dependence of (b) dielectric constant and (c) dielectric loss tangent of NBR composites at different contents of Al2O3-PDA particles.

Fig. 6(a), the dielectric constant of Al2O3-PDA/NBR composites is always higher than that of Al2O3/NBR composites. This may be explained by two reasons. The first one is good dispersion of Al2O3-PDA particles in the NBR matrix results in more interfacial polarization. The second reason is the hydrogen bonds formed by the catechol groups of PDA and the polar acrylonitrile groups of NBR are of high polarizability, which is benefit for improving dielectric constant [41]. Moreover, the insulating dopamine coating can prevent leakage of electricity under the electric field, which may lead to the increasing dielectric constant of Al2O3/NBR composites as shown in Fig. 10. The Fig. 6(b) and (c) show the dielectric constant and dielectric loss tangent of Al2O3-PDA /NBR composites as a function of frequency in range from 102 to 106 Hz at room

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temperature are presented. From Fig. 6(b), it can be see that the dielectric constant of NBR composites increases with the increasing content of Al2O3-PDA particles at the same frequency. The main reason is the more interfacial polarization brought by large amount of filler particles. However, the dielectric constant of NBR composites decreases with the increasing frequency. It is possible that two main factors are responsible for this phenomenon. One is the dipole reversal cannot keep up the speed of field at high frequency. Another is the dielectric loss increases at high frequency (shown in Fig. 6(c)), leads to decrease of the dielectric constant [21]. However, the dielectric loss of all samples is less than 0.35 over all range of the frequency, a distinct advantage for materials using in electronic field. The temperature dependence of the dielectric constant of pure NBR and NBR composites filled with 30 phr Al2O3 and Al2O3-PDA particles in the temperature range of -80 to 100 °C are descripted in Fig. 7(a). From Fig. 7(a), we can find the maximum dielectric constant of the composites appeared at 0 °C. The phenomenon might be attributed to the competing effect including the mobility and thermal expansion of the composites [42]. The interfacial interaction bounded the thermal expansion as the temperature is increased from 80 to 0 °C. When temperatures are higher than 0 °C, the effect of thermal expansion is dominant. However, the difference of coefficient of thermal expansion between NBR and Al2O3 is great that it might overwhelm the effect of polymer mobility on the dielectric constant. As a result, the dielectric constant decreases with increasing temperature at high temperature.

Fig. 7(b) shows the variation of dielectric loss tangent of NBR composite filled with 30 phr Al2O3-PDA particles at different temperature (60 to 100 °C). From Fig. 7(b), we can see that the dielectric loss of the Al2O3-PDA/NBR composite decreases with increasing temperature, indicating that the dielectric loss are dominated by the dielectric loss originates from the relaxations [43]. In addition, we can also find that the dielectric loss is more dependent on the frequency at high temperature than that of at low temperature. This can be explained as follows. In the high temperature range, the relaxation of polar groups in polymer chains is dominant, whereas in the low temperature range the dielectric response of the NBR composite is determined by the glass transition of the polymer. Fig. 8 displays the thermal conductivity on the Al2O3/NBR composites and Al2O3-PDA/NBR composites. From Fig. 8, we can find that the thermal conductivity of composites increased with the increasing content of Al2O3 particles, whether the Al2O3 particles were coated with dopamine or not. That can be explained that the thermal conductivity of Al2O3 particles (32 W/m K) is much greater than that of NBR (0.172 W/m K). Compared with Al2O3/ NBR composites, it is found that the Al2O3-PDA/NBR composites show higher thermal conductivity at the same filler content. The phenomenon can be ascribed to Al2O3-PDA/NBR composite had a more uniform dispersion, leading to a better interface compatibility than that of Al2O3/NBR composite, and Al2O3-PDA particles thermal conductive path were formed in the Al2O3-PDA/NBR composites (as shown in Fig. 8(b)) [2]. Furthermore, the strong interface bonding force between Al2O3 particles and NBR matrix through hydrogen bond between AOH on PDA and ACN on NBR chains will reduce the interfacial thermal resistance and improve thermal conductivity of composites [32]. The hydrogen bonding was demonstrated to exist by the MD simulation in Fig. S2. Moreover, many researchers demonstrated that there is hydrogen bonding between abundant catechol groups in PDA and polar groups in the polymeric matrix [44–46]. In addition, the thermal conductivity of 30 phr Al2O3-PDA/NBR composite and some advanced thermal composites with addition of 6.9 vol% thermal conductive filler were compared in the Table S1 in the supplementary information. From Table S1, we can find that 30 phr Al2O3-PDA/NBR composite has an obvious advantage than other thermal composites. Several equations have been used to study the thermal conductivity of two-phase mixtures [47–50]. The Maxwell-Eucken model was calculated to according to the following equation:

kc ¼ km

½2km þ kp þ 2f p ðkp km Þ ½2km þ kp f p ðkp km Þ

ð4Þ

where kc, km, kp are the thermal conductivity of the Al2O3/NBR composite, NBR matrix, and Al2O3, respectively. fp is the volume fraction of Al2O3 particles. Bruggeman model: 1=3

1 fp ¼

ðkp kc Þðkm =kc Þ ðkp km Þ

ð5Þ

Nielsen-Lewsi model:

1 þ ABfp 1 wBfp

ð6Þ

kp =km 1 kp =km þ A

ð7Þ

kc ¼

B¼

Fig. 7. (a) Temperature dependence of the dielectric constant of the pure NBR, 30 phr Al2O3/NBR composite, and 30 phr Al2O3-PDA/NBR composite in the temperature range of 80 to 100 °C. (b) Frequency dependence of dielectric loss tangent of NBR composite filled with 30 phr Al2O3-PDA particles in the temperature range of 60 to 100 °C.

w¼1þ

1 wr w2r

fp

ð8Þ

where wr is the maximum accumulation volume fraction of the filler. The values of A and wr are 1.5 and 0.637, respectively.


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Fig. 8. (a) Thermal conductivity of the NBR composites filled with different contents of Al2O3 particles and Al2O3-PDA particles. (b) Schematic diagrams of Al2O3 particles and Al2O3-PDA particles dispersed in NBR matrix.

As shown in Fig. 8(a), the Maxwell-Eucken, Bruggeman models, and Nielsen-Lewsi models are used to fit the thermal conductivity of Al2O3/NBR composites. From Fig. 8(a), we can find that the predicted values of thermal conductivity are approximate to the experimental data of 10 phr Al2O3/NBR composite. However, the predicted value of NBR composite filled with 30 phr particles is higher than that of experimental data and is more approximate to the experimental data of Al2O3-PDA/NBR composite. That may be explained that the Maxwell-Euckenl and Bruggeman mode assumed that the shape of the filler was spherical and the dispersion state of filler in composite was confined to the uniform dispersion state [50,51]. With addition of 10 phr particles, the thermal conductivity of predicted value was lower than that of Al2O3PDA/NBR composite and approximate to the experimental data of Al2O3/NBR composite, which can be explained that the interfacial thermal resistance was the dominant factor and the dispersion of NBR composite had a small impact on thermal conductivity. However, when the filler content increases to at 30 phr, the thermal conductivity of predicted value was higher than that of Al2O3/ NBR composite and approximate to the experimental data of Al2O3-PDA/NBR composite, which can be explained that the good dispersion of NBR composite was the dominant factor. In conclusion, we could infer that PDA coated on Al2O3-PDA particles reduced the interfacial thermal resistance and improved the performance of thermal conductivity of composites.

Fig. 9 and Table 1 show the volume resistivity of Al2O3/NBR and Al2O3-PDA/NBR composite with different contents of fillers. From Fig. 9 and Table 1, we can see that the volume resistivity decreased

Fig. 9. Volume resistivity of the NBR composites filled with different contents of Al2O3 particles and Al2O3-PDA particles.

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Table 1 Dielectric properties, thermal conductivity, and mechanical properties of different contents of Al2O3 and Al2O3-PDA particles. Sample

Dielectric constant, 103 Hz

Thermal conductivity, W/m K

Volume resistivity, 1010 X mm

Elastic modulus, MPa

Tensile strength, MPa

0 phr Al2O3 10 phr Al2O3 20 phr Al2O3 30 phr Al2O3 10 phr Al2O3-PDA 20 phr Al2O3-PDA 30 phr Al2O3-PDA

9.97 10.36 10.78 10.96 10.59 11.15 11.43

0.172 0.183 0.196 0.204 0.186 0.201 0.211

8.08 5.86 4.72 4.14 6.13 5.27 4.96

2.01 2.37 2.52 2.67 2.10 2.37 2.59

1.02 1.19 1.66 1.87 1.39 1.84 2.61

Fig. 10. Schematic diagrams of dispersions of (a) Al2O3 particles and (b) Al2O3-PDA particles in NBR composites.

with the increasing content of Al2O3 or Al2O3-PDA particles. In addition, the volume resistivity of Al2O3-PDA/NBR is higher than that of Al2O3/NBR composite, which can be explained as following. As PDA is a kind of insulating polymer, it will hinder the electron transition of the composites, thus improving the electrical insulation of the NBR composites. However, volume resistivity of all of samples is higher than 4⁄10E9, indicating the composites are insulate, which can be used in electrical insulation field.

4. Conclusion An improved dielectric properties, mechanical properties, and thermal conductivity properties of NBR composites were obtained via controlling interfacial force. In order to improve interfacial compatibility between filler and matrix, mussel-inspired poly(dopamine) was used to functionalize Al2O3 nano-particles. As super strong adhesion of PDA to the surface of inorganic fillers

and hydrogen bonds formed by the catechol groups of PDA and the polar acrylonitrile groups of NBR, resulting in the well dispersion and strong interfacial force, thus leading to Al2O3-PDA/NBR composites exhibit better mechanical properties, higher dielectric constant, and larger thermal conductivity than that of Al2O3/NBR. At last, the largest thermal conductivity of 0.21 W/m K was obtained by of 30 phr Al2O3-PDA/NBR composite, which was higher than that of pure NBR (0.17 W/m K). This work provides some new insight in improving thermal conductivity, mechanical properties, and dielectric properties of electronic packaging materials. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51503019), Beijing Natural Science Foundation (No. 2162014), Beijing Science and Technology Project of Beijing Municipal Education Commission (KM201710017005).


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