Hydrogen Bond-Regulated Boron Nitride Network Structures for

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Hydrogen Bond Regulated Boron Nitride Network Structures for Improved Thermal Conductive Property of Polyamide-imide Composites Fang Jiang, Siqi Cui, Na Song, Liyi Shi, and Peng Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03522 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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ACS Applied Materials & Interfaces

Hydrogen Bond Regulated Boron Nitride Network Structures for Improved Thermal Conductive Property of Polyamide-imide Composites †

‡

†

†

Fang Jiang , Siqi Cui , Na Song , Liyi Shi , and Peng Ding*,

†

†

Research Center of Nanoscience and Nanotechnology, Shanghai University, 99 Shangda Road,

Shanghai 200444, PR China ‡

School of Materials Science and Engineering, Shanghai University, 99 Shangda Road, Shanghai

200444, PR China

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Highly thermal conductive polymer composites with minimized the content of fillers are desirable for handling the issue in thermal management in modern electronics. However, the difficulty of filler dispersion restricts the heat dissipation performance of thermoplastic composites and the intermolecular interaction is another crucial factor in this problem. In the present study, the hydrogen bond was used to regulate the formation of the three dimensional boron nitride (3D BN) interconnected network to act as a high thermal conductive network in thermoplastic polyamide-imide (PAI) materials. The prepared electrical insulated PAI/3D-BN composites have a thermal conductivity of 1.17 W·m-1·K-1 at a low BN loading of 4 wt%/2 vol%, and exhibit thermal conductivity enhancement of 409%. We attribute the increased thermal conductivity to the construction of 3D BN interconnected network and the hydrogen bond regulated between hydroxylated BN and polyvinyl alcohol, in which an effective thermal conductive network is constructed. This study provides a guided hydrogen bond strategy for thermally conductive polymer composites with good mechanical and electrical insulation properties in thermal management and other applications. Keywords: Boron nitride, interconnected network, hydrogen bond, thermal conductivity, thermoplastic composites

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Introduction Polymer composites with enhanced thermal conductive property are highly desirable for realizing the effective thermal management materials.1-2 Such materials are enabling to solve the heat removal problem and maintain the lifetime, performance and reliability of electrical devices.3-4 In recent years, some thermally conductive polymer composites have been developed. However, these composites usually require high filler loadings5-6, and the difficulty of filler dispersion restrict their heat dissipation performance1, 7-8. Therefore, decreasing the content of fillers while achieving simultaneous high thermal conductivity (TC) remain a challenge. Recent

researches

demonstrated

that

constructing

three-dimensional

(3D)

interconnected network in the polymer matrix could be expected to overcome this challenge.9-10, 11-13 For example, Chen et al.14 utilized boron nitride (BN) fabricating 3D BN network by self-assembly of BN nanosheets on a 3D cellulose skeleton via sol-gel, and highly thermal conductive composites were then prepared by impregnating epoxy resin into the 3D BN network. Hu et al.15 reported a fabrication of epoxy/3D BN network composites with high TC of 4.42 W·m-1·K-1 at 34 vol% BN loading through ice-templating followed by infiltration technology. Both of the above studies were carried out in thermosetting resin systems. Recently, there is increasing interest in filler-reinforced thermoplastic polymers for thermal management because of their ability to reshape and reform the part after consolidation, and high recycling potential of the composites. However, it is proved to be difficult to get thermoplastic composites with high TC comparing with thermoset resin due to the well wettability

3



of fillers with thermoset monomers.16 Therefore, it is highly desirable to construct a 3D BN interconnected network for realizing high thermal conductive thermoplastic composites. In this work, we developed a facile crosslinking-freeze-drying-infiltration approach to prepare thermoplastic composites with favorable thermal conductive properties by using 3D BN interconnected structures as high thermal conductive networks. Herein, besides the construction of 3D BN networks, it is an important factor that we regulate the hydrogen bonds to control the formation of 3D BN interconnected network and thus to modify the intermolecular interaction between the BN and the polymer matrix. It is well known that the intermolecular interaction is a crucial factor in affecting the thermal conductive properties of polymer composites. One of the most important interactions in polymer composites is the hydrogen bond.17 There have been reports of hydrogen bonding interactions between the hydroxylated BN and polyvinyl alcohol (PVA) chains form the interpenetrating network structure lead to an outstanding TC (for example, 5% increase in TC with 0.12 wt%).18 Additionally, the assembly process usually required the use of templates, vacuum, hazardous chemicals and sophisticated fabrication techniques. Meanwhile, the hydrogen bond cross linking strategy can obtain ultra light 3D BN interconnected network. In particular, we demonstrate that the prepared polyamide-imde (PAI) composites has an enhanced TC up to 1.17 W·m-1·K-1 at a low BN loading of 4 wt% (corresponding to 2 vol%) than that of weak hydrogen bond formed PAI/3D-BN composites (0.67, 0.61, and 0.63 W·m-1·K-1 for the PAI/3D-BN (B2P1, B1P1 and

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B1P3) composites). Moreover, through regulating the hydrogen bond of 3D BN, the PAI composites show electrical insulation properties and obtain an enhancement in ductility and toughness. We attribute the increased thermal conductivity to the construction of 3D BN interconnected network and the hydrogen bond regulated between hydroxylated BN and PVA, in favor of the construction of an effective thermal conductive network. This study provides a guided hydrogen bond strategy for thermally conductive polymer composites in thermal management and other relative applications.

Experimental Section Materials. h-BN powder (1 μm) and sodium cholate (SC) were provided by Sigma-Aldrich Co., Ltd. Dimethylformamide (DMF, 99%) and PVA were purchased from Sinopharm Chemical Reagent CO., Ltd. PAI powder was provided by Shanghai Huayi Group Technology Research Institute (Shanghai, China), and the general chemical structure was shown in Figure S1. Preparation of BNNSs-OH. Hydroxylated BN nanosheets (BNNSs-OH) were prepared by solution exfoliation of the h-BN powder with a tip-type sonication process.19 In brief, 1 g BN powder and 100 mL 0.05 wt% SC water solutions were sonicated for 24 h using a tip-type sonication machine (Bilon 1000Y, 600 W, Shanghai Bilon Instrument Manufacturing Co., Ltd), and the BNNSs-OH solution (10 mg·mL-1) was obtained. Preparation of 3D BN Interconnected Network. 12 mL of BNNSs-OH solution (10

5



mg·mL-1) was mixed with 3 mL 8 wt% PVA solution. The mixture of BNNSs-OH and PVA was magnetic stirred for 30 min, and followed by further ultrasonic treatment for 30 min to form a homogeneous BNNSs-OH/PVA solution. Next, the prepared mixture was placed in ambient temperature for 3 days to obtained hydrogel. The hydrogel was then immersed in an ice bath to freeze for 24 h. After freeze-drying for 52 h, the 3D BN interconnected network with weight ratio of BNNSs-OH and PVA is 1:2 was obtained. Similar process was conducted to prepare 3D BN interconnected network with weight ratio of BNNSs-OH and PVA is 2:1, 1:1 and 1:3. The different weight ratio of BNNSs-OH and PVA is noted as B2P1, B1P1, B1P2 and B1P3, respectively. (It should be noted that 2 wt% PVA was supplemented to ensure the form 3D BN interconnected network.) Preparation of the PAI/3D-BN Composites. The PAI/3D-BN composites were fabricated using vacuum-assisted impregnation method. First, PAI powder was diluted with DMF, and 25 wt% PAI solution was obtained. The 3D BN interconnected network was then immersed completely into the 25 wt% PAI solution. Then the samples were transferred into a vacuum oven at ambient temperature for 24 h for the 3D BN interconnected network to be infused with PAI and to remove air bubbles. Finally, the PAI/3D-BN sample was taken out from PAI solution and evaporates solvent at 60 oC for 24 h. Excessive PAI solution adhered on the 3D BN interconnected network surface was removed before evaporation. The BNNS loading in the composite was determined by measuring the weight of the 3D BN interconnected network before infusion and the composite after PAI infusion.

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Characterization. The morphology and microstructure of BNNSs-OH, 3D BN interconnected network

and PAI/3D-BN composites

were investigated by

high-resolution transmission electron microscopy (HRTEM, JEM-2010F, JEOL, Japan),

scanning

electron

microscopy

(SEM,

JSM-6700F,

JEOL,

Japan),

Fourier-transform infrared (FTIR, AVATAR370, Nicolet, USA), X-ray diffraction (XRD, D/MAX-2200/PC,Rigaku, Japan) and X-ray photoelectron spectroscopy (XPS, RBD upgraded PHI5000CESCA, Perkin Elmer, USA). The size of BNNSs was determined by laser particle size analyzer (Mastersizer 3000, Malvern Instrument Ltd. U.K.). The glass transition temperature was measured using differential scanning calorimetry (DSC, TA, Q20). Thermogravimetric analysis (TGA) was performed using TA Q500 HiRes Thermogravimetric analyzer at a heating rate of 10 oC·min-1. The thermal diffusivities (α) of the composites were performed on a Netzsch LFA447 NanoFlash at 25 oC. The TC (λ, W·m-1·K-1) was calculated as a multiplication of density (ρ, g·cm-1), specific heat (Cp, J·g-1·K-1), and thermal diffusivity (α, mm2·s-1). Namely, λ =ρ×Cp×α. Where Cp obtained by differential scanning calorimetry (DSC, TA, Q20). The heat transport properties of the sample were characterized by IR thermal imaging spectrometer (OPTRIS PI400, Germany) (for detail, see Supporting Information). Volume resistivity of composites was tested using an Agilent 4339B high-resistance meter, and the measured voltage and current were 500 V and 2 mA, respectively. A universal tensile tester (Instron 5569A) was introduced to measure the mechanical properties of the sample.

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Results and Discussion Preparation and Structure of 3D-BN and PAI/3D-BN Composites

Figure 1. a) Scheme illustrate of the preparation progress of PAI/3D-BN composites

and

schematic diagram of the structure of the PAI/3D-BN composites. b) Schematic of the hydrogen bond between PVA and BNNSs-OH.

Figure 1a illustrates the synthesis procedure of the 3D BN interconnected network and its PAI composites. In general, BN is considered to be insoluble in water.20 However, BN can be effectively exfoliated by water through SC-assisted solution exfoliation in the present study, because SC has a strong affinity with the basal plane of BNNSs via van der Waals. (see Experimental Section for more details).11,

21-22

The solution containing exfoliated BNNSs-OH display obvious

Tyndall effect and can be stable for more than one month. The exfoliated BNNSs-OH 8


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comprises of isolated flat nanosheets with a particle size about 400 nm (Figure S2a,b). The TEM image (Figure 1a) reveals that the BNNSs-OH are flexible and the surface of BNNSs-OH is smooth. The thickness of BNNSs-OH is 5 nm, which indicated that BN powder was exfoliated into small ones by vibration-induced exfoliation (Figure 1b). The fast Fourier transform (FFT, Figure 1a inset) of the BNNSs-OH displays typical sixfold symmetry of h-BN, indicating that the exfoliated BNNSs retain the structural integrity of h-BN. The content of hydroxyl group grafted onto the BN is estimated by XPS spectra and TGA. As shown in Figure 2c, the XPS spectrum of BN shows two strong N1s (398.1 eV) and B1s (191.1 eV) peaks and two weak C1s (285.1 eV) and O1s (533.1 eV) peaks.23-24 These peaks also occur in the XPS spectrum of BNNSs-OH. However, the atom percentage of oxygen for BNNSs-OH is much higher than that of BN, and the atomic ratio of boron and oxygen (B/O) of BNNSs-OH is about 3.0, much lower than that of BN (21.0), indicating that the pristine BN is hydroxylated through the treatment. The B1s spectrum could be deconvoluted into two peaks located at 191.0 eV and 190.3 eV, which are attributed to the B-O and B-N bondings, respectively (Figure S3a). While only one peak centered at 397.8 eV assigning to the N-B bonding is observed for the N1s spectrum (Figure S3b), so it is reasonable to conclude that the –OH groups are bonded to B atoms. As for TGA analysis (Figure 2d), the pristine BN is thermal stable whereas the BNNSs-OH has the decomposition due to the removal of oxygen functional groups at the surface. These results support that the pristine BN is hydroxylated through the treatment.

9



Figure 2. a) TEM images of BNNSs-OH (inset) corresponding FFT of the image. b) HRTEM image of BNNSs-OH. c) XPS spectra and d) TGA curves of BN and BNNSs-OH.

In the following procedure, the BNNSs-OH/PVA interpenetrating hydrogels were successfully fabricated because of the hydrogen bond between the hydroxyl groups from BNNSs-OH and PVA chains, where the PVA chains served as cross-linking agent to link the BNNSs. The 3D BN interconnected network was obtained by freeze-drying method. Figure 3a and 3b present the SEM images of the 3D BN (B1P2) interconnected network. It can be seen that the 3D BN shows interconnected network morphology and has a typical homogeneously distributed porous structure with pore diameter of 1-10 μm. From the SEM images, no obvious filamentous structure PVA chain and BNNSs-OH can be found in the 3D BN interconnected network, suggesting

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that BNNSs-OH and PVA chain combine tightly through strong hydrogen bond cross linking. While the other 3D-BN (B2P1, B1P1, and B1P3) show 3D interconnected network with obvious PVA filamentous structure (Figure S3), suggesting of the weak hydrogen bond crosslinking in these samples. The above results indicate that individual BNNSs can be connected to each other because of the abundant hydroxyl groups in PVA can form hydrogen bond with hydroxyl groups on the edges of BNNSs, thus forming strong and stable 3D interconnected structures (Figure 1b).18

Figure 3. The SEM image of a) and b) 3D BN (B1P2) interconnected network, c) and d) PAI/3D-BN (B1P2) composite.

To characterize the hydrogen bond in 3D BN interconnected network, FTIR spectroscopy was employed, which is standard analytical tool commonly used for the purpose.5,

17, 25-26

FTIR spectrum of BNNSs-OH (Figure S4) confirming the

functionalization of BN27-28, and the introduced hydroxyl groups on the BN layers 11



form hydrogen bond with PVA. From Figure 4b of the FTIR region 3000-3900 cm-1, the peak has a shift of hydroxyl (-OH) stretching band in 3D BN interconnected network, in which the largest is in B1P2 (40 cm-1 and 52 cm-1), modest is in B1P1 (16cm-1 and 28 cm-1) and B2P1 (14 cm-1 and 26 cm-1), and smallest is in B1P3 (9 cm-1). The downshift of the peak confirms the existence of hydrogen bonds in the 3D BN interconnected network and the value difference indicates the strongest hydrogen bonding interaction between BNNSs-OH and PVA in B1P2 sample at 1:2 weight ratio. In addition, the XRD patterns in Figure S5a also suggest that some interactions between the PVA and BNNSs-OH may take place because the intensity of the PVA peak decreased and became broader, and showing a decrease in crystallinity.29 The existence of hydrogen bond affects the alignment of BN in the 3D BN interconnected network. The XRD patterns of the 3D BN interconnected network were obtained as shown in Figure 4b (see Figure S5b for more details). The intensity ratio I002/I100 value elucidates the orientation degree of h-BN platelets.8, 15, 30 For pure BN, the ratio is 15.08, and reducing with the reduction of the weight ratio of BNNSs-OH and PVA. It is worth noting that the ratio of B1P2 sample becomes 5.93, because hydrogen bonding interactions with PVA matrix make BNNSs more easily oriented, which indicates that the hydrogen bond is strongest at this ratio. The intermolecular hydrogen bond interaction also has influences in glass transition behavior and thermal stability of the 3D BN interconnected network. Figure 4c depicts DSC thermogram of PVA and 3D BN interconnected network. Pure PVA exhibits a low glass transition temperature (Tg) of 66.8 oC.25 After PVA was

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introduced and crosslinking with BNNSs-OH, the obtained 3D BN interconnected network presents a slightly higher Tg. The Tg of B1P2 is the highest (81.1 oC), which is increased by 14.3 oC compared to that of PVA, indicating strong hydrogen bond between BNNSs-OH and PVA at 1:2 ratio.6 The significant enhancement in Tg is attributed to the existence of hydrogen bond hinder the free movement and arrangement of molecular chains and exhibits strong confinement effect on the mobility of the interpenetrating networks.5, 31-32 As for B1P3, another Tg peak at 49.0 o

C can also be seen, which is belong to the free PVA molecules. But the Tg is much

lower than pure PVA, because the introduction of BNNSs cause the dissociation of the hydrogen bond in PVA molecules and there are not extra BNNSs-OH can form hydrogen bond with hydroxyl groups in PVA.25 The initial degradation temperature (Ti) is an important parameter for evaluating the thermal stability of a material.33 Almost all 3D-BN display poorer thermal stability than PVA, except for B1P2. The Ti of B1P2 is about 228 oC, slight higher than that of pure PVA (~222 oC) (Figure S6a). These results also confirming the strong hydrogen bond in 3D BN at 1:2 weight ratio.

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Figure 4. a) The part of FTIR spectra of BNNSs-OH, PVA and 3D-BN with different weight ratio of BNNSs-OH and PVA at wavenumber 3000-3900 cm-1. b) XRD pattern of PVA, BN and 3D-BN with different weight ratio of BNNSs-OH and PVA. c) DSC thermogram of PVA and 3D BN interconnected network with different ratio of BNNSs-OH and PVA.

Finally, the PAI/3D-BN composites were prepared by vacuum-assisted impregnation of PAI (Figure 1a). After impregnation by PAI, the pores in 3D BN interconnected network are full-filled with PAI and the 3D BN interconnected network exhibits good adhesion with the PAI (Figure 3c,d and Figure S6). Moreover, the 3D BN interconnected network is well maintained in the PAI matrix. The same downward trend was found in the pure PAI and PAI/3D-BN (B1P2) composite (Figure S6b). Whereas, the TGA curve of the composite displayed a slowly downward sloping

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line, indicating of the enhanced thermal stabilities due to the existence of the steady 3D BN interconnected network. Moreover, the residue up to 600 oC was detected to be about 4 wt% BN for PAI/3D-BN composite consistent with the results of weighing depict in experiment section.

Thermal Conductive Properties of the PAI/3D-BN Composites Figure 5 shows the TC of the PAI/3D-BN composites using the transient laser flash technique (details see experimental methods section). As shown in Figure 5a, the TC of pure PAI is 0.23 W·m-1·K-1. With the introduction of 3D BN interconnected network, the TC of the PAI/3D-BN composites was obviously improved. For example, the TC of the PAI/3D-BN (B2P1, B1P1 and B1P3) composites reached 0.67, 0.61, and 0.63 W·m-1·K-1, respectively. It is noteworthy that the TC of the PAI/3D-BN (B1P2) composites reached the maximum up to 1.17 W·m-1·K-1. This PAI/3D-BN (B1P2) composite exhibits the thermal conductivity enhancement (TCE) of 409% at a BN loading of 4 wt% (corresponding to 2 vol% BN). In order to demonstrate the advantage of the hydrogen bond regulated 3D BN interconnected network in enhancing the TC of composites, we also summarizes previous work on TC and TCE of BN and graphene based polymer composites (Table 1). It can be seen that the present PAI/3D-BN composites exhibit a high TC and TCE at low filler content among the reported BN and graphene thermoset and thermoplastic polymer composites.

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Figure 5. a) The TC and the thermal diffusivity of the PAI and PAI/3D-BN composites with different weight ratio of BNNSs-OH and PVA. b) Infrared thermal images and surface temperature variation with cooling and heating time of PAI and PAI/3DBN composites.

In order to demonstrate the thermal management capability of the prepared PAI/3D-BN (B1P2) composites, the heat transfer rate of PAI and the composites can be recorded.14,

34-38

As shown in Figure 5b, the temperature decrease of the

PAI/3D-BN composite is much faster than pure PAI. It is worth noting that, after 100 s cooling, the surface temperature is very close to that of the hot plate, but the temperature of PAI is much higher than that of the hot plate. As for heat absorption capability, the surface temperature of the composite is obviously higher than that of 16


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PAI, and the composite increases with time at a higher rate. After 35 s heating, the surface temperature is about 70

o

C for the PAI/3D-BN composite, which is

significantly higher than that of PAI (47 oC). The high TC of PAI/3D-BN allows heat to transfer rapidly and have better thermal response. The above results illustrate that the PAI/3D-BN composites have high potential for thermal management application. All these results suggest that the hydrogen bond regulated 3D BN interconnected network plays a dominant role in enhancing the TC of the thermoplastic polymer composites. Table 1. TC and TCE of our 3D-BN/thermoplastic polymer composites in this study, and BN/graphene polymer composites which were previously reported. Filler

Matrix

3D-BN

Polymer Type

Filler

TC -1

-1

TCE

References

Content

(W·m ·K )

(%)

and Year

Epoxy

14.3 vol%

1.34

269

201715

3D-C-BNNS

Epoxy

~2.6 vol%

~0.75

308

201714

PCB-BN

Epoxy

20 wt%

0.71

269

201739

h-BN@rGO

Epoxy

Thermo-

30 wt%

0.94

390

201640

h-BN

Epoxy

set

40 wt%

0.80

220

201641

mBN

PI

30 wt%

0.70

300

201742

f-BN

PI

50 wt%

0.74

469

201443

BN-c-MWCNTs

PI

3 wt%

0.39

106

201444

BNNSs

SBR

27.5 vol%

0.55

189

201745

BN

PEG

30 wt%

0.79

158

201746

GO/BN scaffolds

PEG

20 wt%

1.24

289

201647

BN

PVDF

20 wt%

0.97

322

201648

21.5 wt%

0.66

326

201549

Thermo-

s-GH/s-BN

PS

3DBGF

PA6

8.4 wt%

0.89

355

201650

graphene framework

PP

5 wt%

0.82

272

201751

graphene aerogel

PMMA

2.5 vol%

0.70

250

201552

3D-BN

PAI

4 wt%/2 vol%

1.17

409

this work

plastic

Discussion on the Thermal Conductive Mechanism

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3D BN interconnected network plays an important role in the enhancement of thermal conductive properties of the composites. Due to the construction of 3D BN interconnected network, the BNNSs become continuous phase in the composites. This interconnected structure allows heat to be conducted seamlessly across the entire composites without experiencing any thermal boundary resistance caused by the polymer matrix. Moreover, the effect of interface thermal resistance between BNNSs and PAI on the TC can be effectively reduced by pre-constructing a 3D BN interconnected network and then complex with PAI.53-55 Several control experiments through changing the dispersion of the fillers (4 wt%) in the composites were carried out, and the TC were shown in Figure 6. For example, 1) PAI/BNNSs composite was synthesized using BNNSs as filler, and the BNNSs in the PAI matrix tend to orderly arranged.56 The ordered arrangement of BNNSs is beneficial to the formation of continuous thermal conductive pathways. The TC of the PAI/BNNSs composites is 0.52 W·m-1·K-1, which is lower than PAI/3D-BN (B1P2) composites. It was revealed that 3D BN interconnected network were more efficiency in constructing thermal conductive pathways than 2D BNNSs. 2) 3D BN interconnected network were comminuted into powder and then complex with PAI. The 3D-BN powder was broken into isolated units, and some of 3D-BN powder would aggregate together and formed the PAI/cracked-3DBN composites. Besides these, the introduction of PVA increases the thermal resistance of composites. The TC of the PAI/crack-3DBN composites is 0.41 W·m-1·K-1, which is obviously lower than PAI/3D-BN composites. It demonstrated that an integrated interconnected 3D-BN network was critical to achieve

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high TC. 3) The PAI/3D-BN composites were destroyed to form crack-PAI/3D BN composites by hot-pressing. After hot-pressing, the 3D composites is flattened and the TC of the crack-PAI/3DBN is 0.34 W·m-1·K-1, which is obviously lower than the other composites. The 3D BN interconnected network is compressed to a plane and changed into multi-layer structure so the original 3D thermal conductive pathway change into a flat structure. The PAI matrix between layers lead to the increase of thermal resistance, meanwhile, the PVA in the composites also cause the increase of thermal resistance. These results demonstrate that the 3D BN interconnected network plays a dominant role in enhancing the TC of the PAI composites.

Figure 6. The TC of the PAI/BNNSs composite, PAI/crack-3DBN composite, crack-PAI/3DBN and PAI/3D-BN (B1P2) composites.

Hydrogen bond regulation is another fundamental reason for the enhancement of the TC of the composites. Adding PVA molecules which interact with BNNSs-OH via hydrogen bond would then remove the gaps and bridge the BNNSs, and thus reduces the number of effective phonon scattering centers, leading to the improvement of 19



TC.21 The hydrogen bonding interactions between BNNSs-OH and PVA chain make BNNSs form contact to each other (Figure 1a), so that an interconnected and penetrated thermal conductive internet formed in the composites. The BNNSs act as a thermal carrier to continuously and effectively diffuse the heat throughout the composites. The number of the hydrogen bonds was calculated in Table S1. From the calculation results, when the number of hydrogen bonds in the BN is equal to that of PVA, the hydroxyl groups on the PVA and BNNSs-OH can almost completely form hydrogen bonds.57 Whereas, the number of hydrogen bond in the BN is over than PVA, suggesting that the existence of free BNNSs-OH. While the number of hydrogen bond in the BN is less than that of PVA, which means that the existence of excess PVA chain without interacting with BNNSs-OH. The detail hydrogen bond crosslinking network is shown in the Figure 7. From the analysis of theoretical calculation, when the weight ratio of BNNSs-OH and PVA is about 1:2 (in B1P2), the hydroxyl groups on the PVA and BNNSs-OH can form strong hydrogen bond, and the PVA molecules link the BNNSs-OH firmly and then BNNSs can act as thermal carrier. When the weight ratio of BNNSs-OH and PVA is below 1:2 (in B2P1 and B1P1), there exist free BNNSs-OH and some of them will aggregate together. They cannot form an effective three dimensional network and then lead to the decrease of TC of the composites. When the weight ratio of BNNSs-OH and PVA is over 1:2 (in B1P3), the decrease of TC is assigned to the existence of excess PVA without interacting with BNNSs-OH, thereby leading to part isolation of BNNSs-OH, which will increase the thermal resistance of the composites. Therefore, comparing with PAI/3D-BN(B1P2),

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PAI/3D-BN(B2P1), PAI/3D-BN(B1P1) and PAI/3D-BN(B1P3) shows lower thermal conductivity. Additionally, the calculation give us a guided hydrogen bond strategies between fillers and polymers such as poly(acrylic acid) (PAA) with carboxyl58 and poly (4-vinyl phenol) (PVPh) with phenolic hydroxyl, and these results also make us have a new understanding of hydrogen bond.

Figure 7. The hydrogen bond network and possible interactions of 3D BN interconnected network with different weight ratio of BNNSs-OH and PVA.

Mechanical and Electrical Properties of the PAI/3D-BN Composites The mechanical properties could be another important parameter for the application of these composites in thermal management. Figure 8, Figure S7 and Table S2 display the mechanical properties of the PAI matrix and the PAI/3D-BN composites. An obvious enhancement of ductility and toughness are observed for 21



PAI/3D-BN (B1P2) composites when comparing with other PAI/3D-BN composites. The elongation at break of PAI/3D-BN (B1P2) is 4.73 % and the toughness of the composite is 33.81 (J/m3), whereas the composites with other weight ratio are much lower than those of PAI/3D-BN (B1P2) (Figure 8a,d). This is mainly because the hydrogen bond between BN and PVA make PVA chain acts as an effective cross-linker connecting the joints between individual BNNSs in the 3D-BN, thereby preventing slip behavior under tensile loading and stress concentration between the BNNSs in the PAI/3D-BN composites. The PAI/3D-BN (B1P2) composites have better 3D BN interconnected network because of stronger hydrogen bond 59-60, so at low load, the composites appears to maintain its structural integrity. As the load increases, the composites with other weight ratio undergoes irreversible damage and breaks immediately, whereas, the pores in the PAI/3D-BN (B1P2) composite can be stretched deform greater than the other without break immediately, therefore the PAI/3D-BN (B1P2) composite has better ductility and toughness. For electrical insulation applications, electrical insulation property of the composites is important. The volume resistivity of the PAI/3DBN composites (B1P2) is 5.06×1010 Ω·cm, which is high than the critical resistance for electrical insulation.57, 61

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Figure 8. The mechanical properties of the PAI and PAI/3D-BN composites with different weight ratio of BNNSs-OH and PVA: a) tensile strain-stress curves b) tensile strength c)Young’s modulus and d) toughness.

Conclusion In summary, we have successfully fabricated PAI/3D-BN composites by crosslinking-freeze-drying-infiltration technology. The obtained electrical-insulated composites show enhanced TC (1.17 W·m-1·K-1) at a BN loading of 4 wt%/2 vol%, where the TCE of 409% is documented. We attribute the improved thermal conductivity to the following two factors: (1) the establishment of 3D BN interconnected network to act as a high thermal conductive network in polymer composites;(2) the intermolecular hydrogen bond to regulate the formation of the effective 3D interconnected network. Such composites can open up future opportunities to design a well 3D structure for thermal conductive polymer 23



composites which suggesting strong potential in heat dissipation materials for electronics.

Associated Content Supporting Information General chemical structure of PAI, SEM image of BNNSs-OH and 3D-BN (B2P1, B1P1 and B1P3), BNNSs-OH size distribution measurement. XPS spectra, FTIR spectra, XRD pattern and TGA curves of BN, PVA and 3D-BN (B2P1, B1P1 and B1P3). TGA curves of PAI and PAI/3D-BN (B1P2), IR thermal imaging, calculation of the number of the hydrogen bond, mechanical properties of the PAI (PDF) Author Information Corresponding Author *

E-mail: [email protected] (P. Ding)

Notes The authors declare no competing financial interest. Acknowledgements This work was financially supported by the Program of Shanghai Academic/Technology Research Leader (No. 17XD1424400), the National Natural Science Foundation of China (No. 51703122), the Natural Science Foundation of Shanghai (No. 17ZR1440700, 17HKC09), the Development Fund for Shanghai Talents (No.2017014), and the PetroChina Innovation Foundation (No. 2016D-5007-0508).

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Hydrogen Bond-Regulated Boron Nitride Network Structures for

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