Synergetic Improvement in Thermal Conductivity and Flame

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Applications of Polymer, Composite, and Coating Materials

Synergetic Improvement in Thermal Conductivity and Flame Retardancy of Epoxy/Silver Nanowires Composites by Incorporating “Branch-like” Flame Retardant-functionalized Graphene Yuezhan Feng, Xiongwei Li, Xiaoyu Zhao, Yunsheng Ye, Xingping Zhou, Hu Liu, Chuntai Liu, and Xiaolin Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05221 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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ACS Applied Materials & Interfaces

Synergetic Improvement in Thermal Conductivity and Flame Retardancy of Epoxy/Silver Nanowires Composites by Incorporating “Branch-like” Flame Retardant-functionalized Graphene Yuezhan Feng,†,§ Xiongwei Li,† Xiaoyu Zhao,† Yunsheng Ye,*,† Xingping Zhou,*,† Hu Liu,§ Chuntai Liu§ and Xiaolin Xie,†,‡ †

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of

Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. ‡

State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong

University of Science and Technology, Wuhan 430074, China. §

National Engineering Research Center for Advanced Polymer Processing Technology,

Zhengzhou University, Zhengzhou 450002, China KEYWORDS: thermal conductivity, flame retardancy, synergistic effect, flame retardantfunctionalized graphene, silver nanowires

ABSTRACT: The significant fire hazards on the polymer-based thermal interface materials (TIM) used in electronic devices are but often neglected. Also, high filler loading with the incident deterioration of mechanical, thermal and processing properties limits the further application of the traditional polymer-based TIMs. In this work, a ternary TIMs with epoxy resin

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(EP) matrix, silver nanowires (AgNWs) and a small amount of flame retardant-functionalized graphene (GP-DOPO) were proposed to address the above questions. Briefly, a facile “branchlike” strategy with a polymer as the backbone and flame retardant molecule as the branch was firstly used to functionalize reduced graphene oxide (RGO) toward increasing the flame retardant grafting ratio and RGO’s compatibility in matrix, and the resulted GP-DOPO was then in situ introduced into EP/AgNW composites. As expected, the incorporation of GP-DOPO (2 wt%) can increase the thermal conductivity to 1.413 W/mK at very low AgNW loading (4 vol%), which is 545% and 56% increments comparing to pure EP and EP/AgNW, respectively. The prominent improvement in thermal conductivity was put down to the synergetic effect of AgNW and GPDOPO, i.e., the improving dispersion and bridging effect for AgNWs by adding GP-DOPO. Moreover, the high flame retardant grafting amount and the excellent compatibility of GP-DOPO resulted in a strong catalytic charring effect on EP matrix, which further formed a robust protective char layer by combining the AgNW and graphene network. Therefore, the flame retardancy of EP/AgNW was significantly improved by introducing GP-DOPO, i.e., the peak heat release rate (PHRR), total heat release (THR) and total smoke production (TSP) reduced by 27.0%, 32.4% and 30.9% reduction compared to EP/AgNW, respectively.

1. INTRODUCTION With the high-degree miniaturization, integration and multi-functionalization in modern electronics, thermal interface materials (TIMs), used to fill up the gaps and transfer heat between heat sources and heat sink, are widely served as the essential ingredients of thermal management to avoid the problems induced by heat deposition, such as heat shock, thermal aging, fire hazard etc.1-2 Polymer-based thermally conductive composites (PTCs) have been regarded as the ideal


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TIMs based on their characters of light weights, flow processing and high sealing.3-5 Nevertheless, the essential fire hazards of polymers, reinforcing by the high energy losses and heat transference in PTCs, threatens the safety of electronic equipment seriously, but has not received much attention. Besides, commercial PTCs, fabricated by filling metal6 or ceramics particles (i.e., α-Al2O3,7 hBN,8 AlN,9 SiC10), require high filler loading (>50 %) to achieve thermal conductivity (Kc) >1 W/mK at room temperature, which inevitably results in the missing of the above-mentioned PTCs’ advantages. Therefore, exploring a suitable strategy to prepare PTCs with high Kc and high flame retardancy at low filler loading is still an important and challenging topic. Many efforts have been reported to improve Kc of graphene-based PTC by various strategies, such as aligning fillers vertically/horizontally11-12 or distributing fillers selectively in matrix13-14, constructing segregated structure15-16 or filler network in matrix17 and introducing hybrid fillers with synergetic effect in matrix.18-21 In view of the preparation process complexity, generating a synergetic effect by the hybridization of nano-sized and/or micro-fillers in a polymer matrix to create a thermally conductive network, seems to be a simple but effective approach to massproduce PTCs in industry.22-23 But even so, the relative high micro-filler loading (such as Al2O3,24 hBN,23 AlN,18) is still required in the final PTCs thereby limiting its practical application. Graphene and its derivatives, because of its intrinsic extreme in-plane Kc and high aspect ratio, have been introduced into various polymer matrices to fabricate low-loading PTCs for electrically conducting TIMs. For instance, the well-dispersed graphene-based PTC showed a Kc of 1.53 W/mK at only 10 wt% filler loading.25 Therefore, introducing the concept of synergetic effect, the hybridization of 2D graphene with different dimensional nano-fillers, such as 1D CNTs, show a significant synergetic enhancement in the Kc of PTCs under low filler


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loading, which mainly originates from the constructing of thermal conductive filler network by bridging these nano-fillers together.22, 26 Unfortunately, owing to the presence of lattice defects in common reduced graphene oxide (RGO),27 relatively high fillers loading is still required to achieve desired Kc, which would severely limit their processibility due to very high viscosity in resultant nano-hybrid mix slurry caused by their enormous aspect ratio and specific surface.22 Thus, it is essential to explore a new type of nano-hybrid materials with a synergistic effect in a polymer matrix to obtain a thermal percolation threshold value superior to currently used nanohybrid materials under low filler loading. For the issue on flame retardancy of PTCs, RGO functionalized with flame retardant have been regarded as promising flame retardant additives to enhance their charring capacity for polymers, thereby enhancing the flame retardancy for resultant nanocomposites without deteriorating mechanical and thermal properties, while minimizing the requirement of flame retardant loading to achieve high flame retardancy.28-31 Phosphorus-containing compounds have been believed to be one of the most promising and eco-friendly modifiers for improving the flame retardant efficiency of graphene, which has been widely studied by Hu and his coworkers.32-35 Nevertheless, by limited to the less reactive sites owned by RGO and its high steric hindrance, the grafting ratios of organic flame retardants are usually far less than what we expected in reported literatures.32, 36 To address this problem, grafting polymeric flame retardant onto RGO is usually used to increase the grafting ratio.24 However, the grafted polymeric flame retardant composed of the rigid benzene skeleton backbone,32 often led to a limited improvement in RGO’s compatibility in liquid solvent and polymer matrices, is adverse to its application in polymer nanocomposites.


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1D Silver nanowire (AgNW) with a moderate aspect ratio (~100), intrinsic high Kc (~400 W/mK) and excellent mechanical property can be regarded as a promising nano-filler for PTCs. Although the excellent processibility of the composites reinforced by a relatively small amount of AgNW can be achieved, however, the enhanced compatibility between polymer and metal phases are required in the premise of reasonable level AgNW dispersion within polymer matrix, which limits the practical application of this material.37 Very recently, we found a synergetic effect in 0D silica-1D AgNWs interconnection inhibit the agglomeration nano-fillers, which endows the resulting epoxy-based PTCs with excellent processability as well as enhanced Kc.37-38 In comparison to nano-SiO2, 2D graphene is expected to more potential to synergistically improve the Kc for incorporated AgNW composites under a small loading since its much higher intrinsic Kc and aspect ratio. Combining the flame retardant effect of graphene, here, we demonstrate the rational design and fabrication of PTCs with high Kc and flame retardancy simultaneous by the introduction of a new type nano-hybrid material composing of 1D AgNW and 2D graphene under low filler loading. Inspired by the high grafting amount of in situ polymer-grafted graphene,39 a facile and novel “branch-like” strategy with two steps as shown in Scheme S1 in Supplementary Information, i.e., poly(glycidyl methacrylate) (PGMA) chains were firstly grafted onto RGO surface via grafting-through approach (step 1), then flame retardant molecules (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, DOPO) were decorated on the PGMA chains (step 2), was employed to prepare “branch-like” flame retardantfunctionalized graphene (GP-DOPO). The presence of decorated PGMA-DOPO chains on the surface of RGO results in a promising nanofiller, in which (1) the flexible backbones of PGMA chains provide good dispersion in polymer matrix, facilitating the resultant GP-DOPO used as one component of the nano-hybrid have a surfactant-like characteristic at matrix-AgNW


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interfacial interface, (2) the residual unreacted epoxy groups from PGMA present the effective gluing of nanofillers to polymer matrix via covalent bonding, which reduce the thermal resistance between nano-hybrid and polymer matrix, (3) a large amount of decorated DOPO molecules around RGO provide a maximum enhancement in the flame retardancy for the composites and (4) a unique “branch-like” structure in graphene provide excellent compatibility and interfacial interaction with matrix induce the formation of an excellent char layer. Therefore, as a synergistic nanofiller with AgNW, the resulted GP-DOPO is expected to improve the Kc and fire resistance of PTCs simultaneously. 2. EXPERIMENTAL 2.1 Materials. Diglycidyl ether of bisphenol-F epoxy (DGEBF, YDF-170) was supplied by KUKDO Chemical Co., Ltd. 2-ethyl-4-methylimidazole (EMI-2,4, AR) used as cured agent, DOPO (97%), 2, 2-azobisisobutyronitrile (AIBN, 98%) and glycidyl methacrylate (GMA, 97%) were purchased from Aladdin Industrial Corporation. Polyvinylpyrrolidone (PVP, Mw=360, 000 g mol-1) was provided by TCI Development Co., Ltd. N, N-Dimethylformamide (DMF), ethylenediamine (EDA), ethylene glycol (EG), 1-Methyl-2-pyrrolidinone (NMP), sodium chloride (NaCl), silver nitrate (AgNO3), acetone and ethanol with analytically reagent were supported by Sinopharm Chemical Reagent Co., Ltd.. GMA monomers were further purified by basic alumina column chromatography to remove the inhibitor. AIBN was recrystallized in ethanol. 2.2 Synthesis of “branch-like” flame retardant-functionalized graphene (GP-DOPO). Graphene oxide (GO) was prepared by chemically exfoliating natural graphite based on our previous work.24 GP-DOPO was synthesized by a two-step functionalization process as shown in Scheme 1. In brief, GO sheets were chemically reduced by EDA to prepare amino-functionalized


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graphene (G-NH2) based on the previous report.40 The resultant G-NH2 sheets and the purified GMA monomers were ultrasonically dispersed into NMP (500 mL). The mixture was heated to 70 oC with argon shield and continuous stirring for 4 h, to graft some of GMA monomers onto the surface of G-NH2, followed by adding AIBN into the system for other reaction time of 20 h, to obtain PGMA-grafted graphene (G-PGMA). After that, a DMF solution containing equimolar DOPO was injected into the above system with a further reaction time of 24 h at 80 oC, to graft the DOPO molecules into the side chain of PGMA. The products (GP-DOPO) were centrifuged and filtered several times to remove the free chains before they were freeze-dried at lyophilizer. The designed GP-DOPO with “branch-like” structure, i.e., the wrapped PGMA chains and its side chain’s DOPO molecules, was used as a flame retardant additive for composites. As a reference, DOPO molecules were directly grafted into GO according to the previous literature.36

Scheme 1. Schematic diagram of the two-step synthetic route of “branch-like” flame retardantfunctionalized graphene (GP-DOPO). 2.3 Synthesis of silver nanowires (AgNWs). AgNW was synthesized based on the previous works with some modifications.41 Typically, PVP (4.01 g) was dissolved into EG (70 mL) with moderate stirring and heated to 168 oC. After forming a homogeneous solution, 120 µL of NaCl


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solution (0.2 M, in EG) was injected into the PVP solution with continuous stirring. After 2 min, 12 mL AgNO3 solution (1 M) was added onto the system at a rate of 0.4 mL/min for 2 min, followed by the quick addition of the residual AgNO3 solution with a rate of 9.4 mL/min. After that, standing the solution for 15 min with a color change from brownish red to greyish-green (glistening) and gas release. The products (AgNWs) were purified by filtration with ethanol several times and saved in ethanol. 2.4 Preparation of epoxy/AgNW/GP-DOPO composites. Epoxy-based composites containing AgNW and GP-DOPO sheets were prepared by solution and mechanical blending and program-controlled curing. Typically, GP-DOPO sheets were ultrasonic-dispersed in ethanol for 1 h, followed by adding stoichiometric epoxy monomers dissolved in acetone and quantified AgNWs dispersed in ethanol for other ultrasonic treatment for 1 h. Then the solvent was quickly evaporated using a rotary evaporator and further dried overnight at 60 oC. Then the obtained mixture with 6 wt% EMI-2, 4 (curing agent) was mixed and outgassed by a planetary stirrer. Finally, the obtained EP/AgNW/GP-DOPO mixture was placed in a mold and cured based on our previous work.24 For the sake of comparison, EP/AgNW, EP/AgNW/GO-DOPO, EP/GODOPO and EP/GP-DOPO composites were fabricated using the same process. The characterizations for the structures and properties of GP-DOPO and EP-based composites were shown in Supplementary Information. 3. RESULTS AND DISCUSSION 3.1 Characterizations of GP-DOPO. In this work, PGMA chains are grafted onto RGO surface before the functionalization of flame retardant (DOPO). The grafted PGMA chains provided a large number of active sites (epoxy groups) for RGO, thereby maximum increasing its grafting amount of flame retardant. The surface morphology of the obtained GP-DOPO with


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“branch-like” structure was characterized by using AFM, TEM and SEM technologies. GO and G-NH2 sheets show a transparent morphology with some flexible wrinkles (Figure S1 in Supplementary Information) and present only one-atoms thickness (0.69 nm, Figure 1a).42 After grafting PGMA chains, an increasing layer thickness of 2.38 nm and a darkening surface feature show in the resultant G-PGMA (Figure 1b, d), respectively. Further wrapping DOPO molecules onto the side chain of grafted PGMA, Further decorating the side chain of grafted PGMA by DOPO molecules via ring-opening reaction puts the flat PGMA chains up to form a “branchlike” structure, the obtained GP-DOPO sheets are found to be thicker (4.51 nm) in the thickness and darker on the surface as shown in Figure 1c and e, respectively. Its amplifying image (Figure 1f) reveals the formation of “branch-like” structure, which is in contrast to the surface morphology of GO functionalized with DOPO directly (Figure 1g), only heterogeneous dark spots can be observed. Such the “branch-like” structure with rough and wrinkled morphology can be further confirmed in its SEM image more intuitively (Figure 1i), which is expected to improve its dispersion and compatibility in polymer matrix. In contrast, GO-DOPO shows a similar smooth surface morphology (Figure 1h). All of the morphology results mean that such a “two-step” method is capable of realizing the “branch-like” functionalization for graphene with PGMA backbone and flame retardant (DOPO) lateral branch. Moreover, the EDX spectrum of GP-DOPO (Figure 1j, k) also confirm the presence of phosphorus element corresponding to decorated DOPO molecules on its surface. All of the morphology results mean that such a “twostep” method is capable of realizing the “branch-like” functionalization for RGO with PGMA backbone and flame retardant (DOPO) lateral branch.


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Figure 1. AFM images and cross-section height profiles of (a) GO, (b) G-PGMA and (c) GPDOPO; TEM images of (d) G-PGMA, (e, f) GP-DOPO and (g) GO-DOPO; SEM images of (h) GO-DOPO, (i, j) GP-DOPO and (k) EDX spectrum of GP-DOPO. The chemical structures of GP-DOPO were confirmed by using FTIR and XPS spectroscopic analyses. As exhibited in their FTIR spectra (Figure 2a), the characteristic peaks of the oxygencontaining groups of GO were drastically weaken after EDA reduction, with the presence of amino at 3230 cm-1 (G-NH2).43 For G-PGMA, the characteristic absorption peaks of ester (1726 cm-1) and epoxy (1152 cm-1) groups arising in its FTIR spectrum, combining with the disappearing amino group, mean the successful grafting PGMA to G-NH2 surface by covalent bond. Further decorating DOPO to the side chain of the grafted PGMA chains would result in the decrease of epoxy groups, the increase of hydroxyl groups and the appearance of DOPO


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characteristic peaks (i.e., P-O-Ph at 754 cm-1, P=O at 1205 cm-1 etc.),36,

44

which can be

confirmed in GP-DOPO spectrum. Moreover, the elimination of the reactive P-H characteristic peak (DOPO, 2432 cm-1) in GP-DOPO spectrum directly confirms the covalent grafting of DOPO onto RGO. Besides, steric hindrance effect arises from the surface of G-PGMA replaced by DOPO gradually, resulting in that lots of residual epoxy groups still exist in the resultant GPDOPO sample.

(b)

GO 3412

Transmittance (%)

G-NH2

1734

1049

1620 1546

3230

G-PGMA 2921 2858

1726

GP-DOPO

1152 1569

536

1635

DOPO

O 1s

GP-DOPO

C/O atomic ratio

Intensity (a.u.)

(a)

1205

1049 754

GO

C 1s

2.26

140

GO-DOPO

P 2p

GO-DOPO

135

130

2.51 N 1s

GP-DOPO

5.03

G-PGMA

3.74

G-NH2

9.43

P 2s

P 2p

2432

4000

3500

3000

2500

2000

1500

1000

500

1200

1000

-1

400

200

0

(d)

G band

D band

600

GO-DOPO

ID/IG=0.94

GP-DOPO

ID/IG= 1.23

G-PGMA

ID/IG= 1.50

G-NH2

ID/IG= 1.16

GO

ID/IG= 0.94

Intensity (a.u.)

(c)

800

Binding Energy (eV)

Wavenumber (cm )

Intensity (a.u.)

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


GP-DOPO G-PGMA G-NH2

500

1000

1500

2000

2500

GO

3000

10

20

-1

Raman shift (cm )

30

40

50

60

70

80

2 theta (degree)

Figure 2. (a) FTIR, (b) XPS, (c) Raman and (d) XRD spectra of GO, G-NH2, G-PGMA, GPDOPO and GO-DOPO. In the XPS spectra (Figure 2b), the peaks at 285 and 529 eV corresponding to C 1s and O 1s as well as low C/O atomic ratio of 2.26 can be found in the original GO. After reduction by EDA, a significant increase in C/O atomic ratio (9.43) and a new peak corresponding to N 1s at


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400.1 eV can be observed in the G-NH2 sample. When G-NH2 grafting with PGMA chain, which composited by high ratio of oxygen atoms, leading to a noticeable decrease in C/O ratio (3.74), while further decorating with DOPO molecules resulting in the increase of C/O ratio (5.03) due to the introducing high ratio of carbon atoms by biphenyl structure from DOPO molecules. In contrast to GP-DOPO, GO-DOPO prepared by directly grafting DOPO onto GO,36 exhibits very weaker reduction degree (Figure S2a, b in Supplementary Information) and much lower phosphorus element content compared to GP-DOPO, meaning that the grafting ratio of DOPO by our method is much higher than that by previous reports.36, 45 The C1s core-level spectra of functionalized graphene (Figure S2c-e in Supplementary Information) can be curve-fitted to six peaks: C-C/C=C (284.5 eV), C-OH (285.6 eV), C-O-C (286.9 eV), C=O (287.5 eV) and C(O)OH (289.0 eV).46 The C-O-C fitting peak intensity in GP-DOPO decreases obviously compared to that of G-PGMA, due to the ring-opening reaction of epoxy groups and DOPO, but it is still more intense as compared to G-NH2. This result suggests that there still exist residual unreacted epoxy groups in grafted PGMA chains, which is in consistent with previous FTIR result. The structural changes of various graphene derivatives were investigated by Raman spectroscopy and XRD measurements. It is well-known that defective graphene shows two distinct peaks of D band and G band at ~1334 and 1573 cm-1, respectively. The structural defects of graphene can be thus determined by the ratio of ID/IG peak intensity.47 As expected, severe oxidation causes defects in the synthesis of GO, increasing the ID/IG ratio compared with graphite (Figure 2c). After chemical reduction, the value of ID/IG ratio increase from 0.94 to 1.16 due to the destruction of crystal structure and size for GO sheets during chemical reduction.46 The obtained RGO grafting with PGMA chains raised its ID/IG ratio to 1.50 by the introduction of sp3


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carbon atoms, however, its ID/IG ratio decreased to 1.23 by further decorating DOPO molecules due to the DOPO molecule mainly composed by biphenyl structure of sp2 carbon. By contrast, the GO-DOPO exhibits a similar ID/IG ratio with GO due to the weak reduction of DOPO molecules. In XRD pattern (Figure 2d), the sharp peak at 2θ=11.8o corresponding to the (002) diffraction peak of graphitic lattice is shown in GO with a d-spacing of 7.5 Å.48 EDA reduction can shift the (002) diffraction peak to ~23.2o (broad peak), resulting from the aggregation induced by reduction. Another broad peak is also formed at ~10.2o due to the intercalation of EDA molecules in graphene layers. Covalently grafted organic layer on RGO surface effectively suppresses the aggregation of RGO and increases the d-spacing based on the extended and lowangle broad peak in G-PGMA and GP-DOPO pattern. These results combined with previous FTIR and XPS results indicate the successful the functionalization of “branch-like” flame retardant. 100 G-NH2

80

Weight (%)

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


GP-DOPO G-PGMA (29.8%)

60 40 20

PGMA

0 100

200

300

400

500

600

700

o

Temperature ( C)

Figure 3. TGA curves of GO, G-NH2, G-PGMA and GP-DOPO under nitrogen atmosphere with a heating rate of 10 oC/min. TGA and EA results as presented in Figure 3 and Table 1, respectively, are employed to reveal the composition and grafting ratio of our “branch-like” flame retardant-functionalized


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RGO. As expected, a slight decrease of mass loss (~7 wt%) due to the very weak reduction of DOPO, which is similar to thermally unstable GO as previous reports.49 In contrast, G-NH2 exhibits a lower weight loss compared to GO due to the high reducibility of EDA for GO.40 When grafting with PGMA, the TGA curve displays an obvious mass loss above ~250 oC, due to the thermal decomposition of grafted PGMA. Through quantitatively analyzing the residual weight between G-NH2, PGMA and G-PGMA, the grafting ratio of PGMA chains can be calculated to be ca. 29.8 wt%. It should be noted that GP-DOPO exhibits higher thermal stability than G-PGMA, including thermal degradation temperature and char yield, suggesting that decorating DOPO onto PGMA’s side chain would induce the improvement in its thermal stability. However, the final char yield of G-PGMA becomes unpredictable when it decorated with DOPO molecules because an increase in DOPO grating ratio increased both char yield and thermal stability of resultant materials. Therefore, the total graft amount is hard to calculate just based on TGA results. For this purpose, we employ EA to accurate elements’ contents, especially phosphorus element. As shown in Table 1, it can be clear to see that the phosphorus content of GP-DOPO is much higher than that of GO-DOPO, meaning the higher flame retardant (DOPO) grafting ratio on RGO used by “branch-like” grafting strategy. In the quantitative analysis, combining with the TGA result of G-PGMA, the total grafting ratio is about 41.6 wt% for GP-DOPO. Based on the hypothesis of all carbon structure of G-NH2 or GO, the grafting density of flame retardant is easily calculated to be 160.5 molecules per 104 carbon atoms for GP-DOPO, which is ~5 times higher than that of GO-DOPO (only 32.6 molecules per 104 carbon atoms for GP-DOPO). Therefore, these results confirm the reasonability and effectiveness of our “branch-like” structure grafting strategy to improve the flame retardant grafting ratio.


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Table 1. Elemental analysis results of GO-DOPO and GP-DOPO. Samples

C wt%

H wt%

O wt%

N wt%

P wt%

GO-DOPO

57.690

2.529

38.986

--

0.795

GP-DOPO

68.340

3.647

20.648

7.365

2.420

(c)

(b)

(a)

20 µm

20 µm

(f)

(e)

(d)

Homogeneous dispersion 20 µm

2 µm

(h)

(g)

500 nm

20 µm

GO-DOPO agglomeration

Homogeneous dispersion 20 µm

20 µm

Monodisperse AgNWs

2 µm

Figure 4. SEM images of the fracture surfaces for (a) pure EP, (b) EP/GO-DOPO, (c, d) EP/GPDOPO, (e) EP/AgNW, (f) EP/AgNW/GO-DOPO, (g, h) EP/AgNW/GP-DOPO with 2 vol% AgNW and/or 2 wt% graphene. 3.2 Morphology and structure of EP/AgNW/GP-DOPO composites. As above-mentioned, the residual epoxy groups in the grafted PGMA chains and the “branch-like” structure are expected to increase the compatibility of graphene in matrix. The XRD results (Figure S5 in Supplementary Information) confirm that the aggregated structure of epoxy or EP/AgNW has a slight change after introducing GO-DOPO or GP-DOPO. The morphology analyses reveal that pure EP shows a typical brittle fracture (Figure 4a). The fracture roughness of EP is signally improved by incorporating 2 wt% graphene. However, a severe aggregating behavior is observed


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in the EP/GO-DOPO with aggregation size of 10-20 µm (Figure 4b), while the GP-DOPO sheets homogeneously disperse in matrix due to the improved compatibility by the grafted PGMA chains grafting (Figure 4c), which is further confirmed by the uniform distribution of phosphorus element based on the EDX result (Figure S6a, b in Supplementary Information). Moreover, the high magnification SEM image (Figure 4d) reveals the strong interfacial interaction between GPDOPO and EP matrix. For EP/AgNW composite, AgNWs exhibit a severe aggregating behavior in matrix (Figure 4e), which would deteriorate the final properties of composites seriously. The introduction of the undispersed GO-DOPO shows a limited improvement in the aggregating behavior of AgNWs (Figure 4f). In contrast, the AgNWs almost monodisperse into matrix when incorporating GP-DOPO into EP/AgNW composite (Figure 4g). The phosphorus EDX mapping image (Figure S6c, d in Supplementary Information) and the high magnification SEM image (Figure 4h) of EP/AgNW/GP-DOPO confirm the uniform distribution and strong interfacial interaction of GP-DOPO in matrix. Such the improvement in AgNWs’ dispersion is main ascribed to the inhibiting settlement action of high-density AgNWs in ethanol by GP-DOPO, where a strong hydrogen bonding can be formed between grafted PGMA chains in GP-DOPO and PVP layer on the surface of AgNW. Moreover, the increased matrix’s viscosity by graphene (Figure S7 in Supplementary Information) further prevented the combination of AgNWs during the curing process.50


16

(a)

1.0

(b) 1.5

60 EP/AgNW EP/AgNW/GP-DOPO

EP/AgNW/GP-DOPO AgNW content: 2 vol%

1.2

0.8

Kc (W/mK)

Kc (W/mK)

0.9

0.7 0.6 0.5

40

GP-DOPO content: 2 wt%

0.9

20

0.6 0 0.3 -20

0.1 0.0

0.0 0.0

0.5

1.0

1.5

2.0

0

Graphene content (wt%)

3

4

(d) 10 8

Kc/Km

/A

2

2

Rb = 0 m K/W

-8

2

-7

2

Rb = 4.19×10 m K/W

6 Rb = 2.20×10 m K/W

4

2

AgNW hBN CNT or Graphene

Si O

EP

/A

gN W

@

EP

1.0

gN W

/S i

O

2

[3 8] EP /G

Th i

1.5

s W or k EP 2 EP [5 [3 /G PV 0 ] /G N EP 7] P/ EP N D P N /S SW P F/ V P@ /C W [ D C 26 N N N N F/ A T T ] T T C l 2O [2 [2 -P N [2 2] 3 6] VP T/ 6] [5 G 1] PS [5 O 2] [ 2 /G PA 2 E E EP NP ] P 6/ P/ /B G /E /B hB N N G N NS P/ N@ /M NS B @ W N G [5 A N NS O 3] gN T [5 PV [58 [5 P 6] 3] D ] [ EP 5 F/ 5] /h h B B EP N N@ /C /fG NT G N O P [2 [5 [5 3] 8 ] PE 4] G EP /h /h B N B /G N/ N M P W [5 C N 9] T [6 0] EP /h B N @ G O [5 EP 7] /h B N/ M W CN T [6 0]

2.5 2.0

1

AgNW content (vol%)

Si O

(c) Kc (W/mK)

gN

W @

0.5

PI /A

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


(Kc-K0)/K0 (%)

Page 17 of 40

2

0.0 0

5

10

15

20

25

30

35

0.00

0.02

Volume fraction (%)

Figure 5. (a) Kc of EP/AgNW/GP-DOPO as a function of

0.04

0.06

0.08

0.10

f

GP-DOPO content, (b) Kc of

EP/AgNW/GP-DOPO and synergistic effect strength of GP-DOPO and AgNWs as a function of AgNWs content, (c) statistical results for the Kc of heterogeneous composites containing AgNW, carbon-based nanofiller and hBN in previous literatures,22-23, 26, 37-38, 51-61 (d) the model fitting curves and corresponding thermal conductivity data of EP/AgNW and EP/AgNW/GP-DOPO (dots and lines are the experimental data and fitting curves of modified EMA model, respectively). 3.3 Thermal conductivity of EP/AgNW/GP-DOPO composites. In this work, 1D AgNWs with a mean length of 10 µm and mean diameter of 90 nm (Figure S4 in Supplementary Information), synthesized by a two-step-injection polyol method,41 was used as thermal conductive nanofiller due to its high intrinsic Kc (~400 W/mK) and high aspect ratio. Combination of 2D graphene (GP-DOPO) with 1D AgNW inducing a synergetic effect to endow the composites with high Kc is a critical composite concept for the first time. Figure 5a shows the


17


Page 18 of 40

incorporated 2 vol% AgNW EP-based composites as a function of GP-DOPO content from 0.5 to 2.0 wt%. The results demonstrate that the Kc of the composites is a positive correlation with the GP-DOPO content, and reaches to 0.88 W/mK at 2 wt% GP-DOPO content, which increases by ~34% comparing to the composites without GP-DOPO. The GP-DOPO content is fixed as 2 wt% in next section to investigate the synergistic effects in Kc of composites. As presented in Figure 5b, both of EP/AgNW and EP/AgNW/GP-DOPO show a continuous increasing Kc values as a function of AgNW content from 1 to 4 vol%. The thermal conductivity enhancements (TCE, comparing to pure EP) reach to 313% and 545% at 4 vol% AgNW, respectively. An synergistic enhancement in Kc is easily found by comparing the results of EP/AgNW and EP/AgNW/GPDOPO composites, e.g., the Kc of the composites with 4 vol% AgNW increases from 0.90 to 1.41 W/mK, representing an increment of 56.3% by incorporating 2 wt% GP-DOPO. Defining (Kc1-Kc0)/Kc0 as the strength of the synergistic effect, where Kc1 is the Kc of EP/AgNW/GPDOPO, Kc0 is the Kc of EP/AgNW, the results are shown in Figure 5b. It is found that the synergistic effect strength increase with the AgNWs loading due to the increasing inter-particle interactions.21, 60 All of these results reveal that the synergistic effect of AgNWs and GP-DOPO in Kc exists in the ternary composites, especially with relatively high filler loading, and the mechanism would be discussed in the next section. To highlight the advantage of the synergistic enhancement, a statistical comparison about the Kc of homogeneous composites in previous reports was shown in Figure 5c. It can be easily found that high filler loading was required to achieve the desired Kc for the composites containing micro-fillers (hBN), while the alternative nanofillers (CNTs, graphene or AgNWs) were capable of reducing the loading level. In contrast to carbon-based nanofillers, AgNWs show some advantages in terms of its loading requirement for obtaining desired Kc and the viscosity of resultant mix slurry. The comprehensive comparison


18

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reveals that the incorporated AgNW/GP-DOPO nano-hybrid composite in this work exhibits a best superiority by balancing filler loading and Kc. Interfacial thermal resistance (Rb) caused by phonon acoustic mismatch between fillers and matrix, is widely considered to be the main reason for the lower Kc of composite than its theoretical value.62-63 We used effective medium approximation (EMA) theory, proposed by Garnett64 and developed by Nan et al.,65 to predict the Rb. For the randomly one-dimensional fillers, the normalized Kc of Kc/Km for composites can be simplified as follow by Huang et al.:66

Kc fp = 1+ Km 3

K p /K m p +

2Rb K m K p d Km

(2)

where Kc, Km and Kp are the Kc of composites, matrix and fillers, respectively. f and p are the volume fraction and aspect ratio of fillers. In this work, AgNWs with a high p of ~110 and Kp of ~400 W/mK were used filler, and other parts of composites (EP in EP/AgNW, EP and GPDOPO in EP/AgNW/GP-DOPO) were regarded as matrix with Km of 0.219 and 0.240 W/mK, respectively. The model fitting results are exhibited in Figure 4d. The values of Rb for EP/AgNW and EP/AgNW/GP-DOPO are 2.20×10-7 and 4.19×10-8 m2K/W, respectively. Although the experimental data are still much lower than the ideal values when Rb= 0 m2K/W (red line in Figure 5d), however, it worth noting that the introduction of GP-DOPO is capable of weakening the Rb greatly, which may be ascribed to the improvement of modulus mismatch between matrix and AgNWs by GP-DOPO.67


19


(c)

(b)

(a) AgNW bundles

Page 20 of 40

AgNWs

Graphene bridges

Graphene bridges

Gaps

3 µm

5 µm

500 nm

(e)

(d)

Heat paths

Heat paths Gaps

Figure 6. The high-magnification SEM and TEM images of the fracture surfaces for (a) EP/AgNW and (b, c) EP/AgNW/GP-DOPO; Proposed thermally conductive models for (d) EP/AgNW and (e) EP/AgNW/GP-DOPO. The morphology analyses (Figure 4e) reveal that the AgNWs show a poor dispersion in EP/AgNW composite. Its high-magnification SEM image of fracture surface (Figure 6a) further reveals that most of AgNWs distribute in EP/AgNW in bundle form because of their high surface energy, and many gaps are observed between adjacent AgNW bundles, which would go against the formation of thermally conductive paths. Interestingly, incorporating GP-DOPO into EP/AgNW composites can improve the dispersion of AgNWs (Figure 4g) dramatically, and almost all AgNWs show a mono-dispersion distribution in the EP/AgNW/GP-DOPO (Figure 6b). Moreover, its TEM image (Figure 6e) reveals that the segregated AgNWs (yellow arrows)


20

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are bridged connection by GP-DOPO (red arrows), where a connectional thermally conductive path is presented as a dashed line. Such the bridging effect acted by GP-DOPO can effectively weaken the effect of gaps on Kc. Based on the morphological analyses, the heat transfer mechanisms for incorporated AgNW and AgNW/GP-DOPO nano-hybrid composites are proposed in Figure 6d and e, respectively. The synergistic enhancement in Kc for our EP/AgNW/GP-DOPO composite mainly by (1) the presence of surfactant-like GP-DOPO sheets in the mixture preventing the agglomeration of AgNWs during curing process; (2) the incorporated GP-DOPO sheets improving matrix-AgNW interfacial interface as well as bridging AgNWs together to form a highly effective heat-conductive networks in the final composites. Combination of improved dispersion of AgNWs in the matrix and the decreasing of Rb analyzed by EMA model play a dominant role in enhancing Kc for our composite. 3.4 Thermal stability of EP/AgNW/GP-DOPO composites. Figure 7 exhibits the thermal stability of pure EP and its composites. One-stage and two-stage degradation behaviors to be observed in their TGA curves under nitrogen and air shielding, corresponding to the pyrolysis of epoxy chains and its chars, respectively. With introducing graphene (GO-DOPO and GP-DOPO) into EP matrix or EP/AgNW composite, four information can be found in their TGA and DTG results, i.e., reducing the onset thermal decomposition temperature (Tonset), weakening the weight loss rate, increasing the char yield and improving the char’s oxidation stability, which are in accordance with previous reports.24, 68 It is generally recognized that the decomposed products of decorated organic flame retardant (DOPO in this work) catalyze the decomposition of organic matrix,33 resulting in the increase of char’s yield, these char combining with graphene forms an effective barrier layer in composites, which restrains the release of flammable volatile products and prevents the transfer of oxygen and heat during thermal decomposition.69-70 Such the


21


decomposition process, similar to the combustion process, is beneficial to analyze the flame retardation mechanism of composites. In contrast, GP-DOPO has a higher flame retardant grafting amount and reactive surfaces as well as abundant interfacial bonding with better compatibility in matrix, which induce the production of more and better char layers, further increasing its thermal stability.

(b) 2.0

100

100

Weight (%)

60

Neat EP EP/GO-DOPO EP/GP-DOPO EP/AgNW EP/AgNW/GO-DOPO EP/AgNW/GP-DOPO

o

95

80

Deriv. weight (%/ C)

(a)

90 320

360

400

440

40

100

1.5 1.0

2.0


420

200

300

400

500

600

700

100

800

200

o

o


300

400

(d) 2.5

100

90 360

400

440

20 0 100

480

500

600

700

800

o

95

40

450

Temperature ( C)

Deriv. weight (%/ C)

(c)100

60

1.2

0.5

Temperature ( C)

80

1.6

0.0

20

Weight (%)

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

Page 22 of 40

2.0 1.5

2.4


2.0 1.6 1.2 420

440

460

1.0 0.5 0.0

200

300

400

500

600

700

800

100

200

o

Temperature ( C)

300

400

500

600

700

800

o

Temperature ( C)

Figure 7. TGA and DTG curves of pure EP and EP-based composites with 2 wt% functionalized graphene and (or) 2 vol% AgNW under (a, b) nitrogen and (c, d) air atmosphere. 3.4 Flame retardancy of EP/AgNW/GP-DOPO composites. The synergetic effect of GPDOPO and AgNWs on the flammability of EP-based composites can be confirmed by using cone calorimeter (CC), limit oxygen index (LOI) and UL-94 vertical burning test. The resultant curves and corresponding flame retardant parameters are exhibited in Figure 8 and Table 2,


22

Page 23 of 40

respectively. Flammable EP exhibits a high peak heat release rate (PHRR) of 1137.6 kW/m2, total heat release (THR) of 81.6 MJ/m2 and total smoke production (TSP) of 62.4 m2. Incorporating GO-DOPO sheets (2 wt%) into EP matrix results in a slight reduction in PHRR, THR and TSP, due to its low DOPO grafting amount and poor compatibility with the matrix. By contrast, the “branch-like” functionalization with flexible PGMA chains and high DOPO grafting amount not only improves the interfacial compatibility between RGO and matrix, but also reinforces its catalytic carbonization for polymer chians.24, 68 The introduction of GP-DOPO is capable of reducing the values of PHRR, THR and TSP to 853.5 kW/m2, 63.2 MJ/m2 and 40.1 m2, respectively, corresponding to 25.0%, 22.5% and 34.8% reductions compared to pure EP. Besides, it should be noted that the char combustion exothermic peak of EP (150-200 s, Figure 8a) disappears after adding GO-DOPO or GP-DOPO in the heat release rate curves. This result indicates the increase of their chars’ oxidation resistance, which is consistent with their TGA results (Figure 7c, d). Based on the effective improvement in the flame retardancy of EP/GPDOPO, GP-DOPO was thus expected to improve the fire resistance of TIMs (EP/AgNW in this work), which is more urgent due to their work environment.

600 400 200

50 60 Neat EP EP/GO-DOPO EP/GP-DOPO EP/AgNW EP/AgNW/GO-DOPO EP/AgNW/GP-DOPO

40 20

0

50

100

150

Time (s)

200

250

300

40 30


20 10

0

0

70 60

80

2

2

800

(c)

TSP (m )

1000

(b) 100

2


THR (MJ/m )

(a)1200 HRR (kW/m )

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


0 0

100

200

300

400

Time (s)

500

600

700

0

100

200

300

400

Time (s)

Figure 8. (a) Heat release rate, (b) total heat release and (c) total smoke production versus time curves of pure EP and its composites with 2 wt% functionalized graphene and (or) 2 vol% AgNW.


23

500

600

700


Page 24 of 40

Table 2. Combustion data of pure EP and its composites. TSP

FIGRA

LOI

UL-94

TTI

PHRR

THR

(s)

(kW/m2)

(MJ/m2)

Pure EP

67

1137.6

81.6

62.4

8.8

25.0

NR

EP/GO-DOPO

62

997.3

72.0

44.2

8.7

25.6

NR

EP/GP-DOPO

57

853.5

63.2

40.7

8.1

26.8

V-2

EP/AgNW

74

1053.7

92.0

55.4

8.1

26.2

NR

EP/AgNW/GO-DOPO

86

950.4

84.1

47.1

7.3

27.3

V-2

EP/AgNW/GP-DOPO

86

769.1

62.2

38.3

5.9

28.7

V-1

Samples

(m2) (kW/m2s) (vol%)

rating

As shown in Figure 8, EP/AgNW displays a similar flammability including PHRR (-7.4%), THR (+12.7%) and TSP (-11.2%) in comparison to pure EP. The introduction of 2 wt% GODOPO in EP/AgNW results in the slight decrease in PHRR, THR and TSP. However, incorporating GP-DOPO sheets can effectively reduce the combustion parameters to 769.1 kW/m2 (PHRR), 62.2 MJ/m2 (THR), and 38.3 m2 (TSP), respectively, corresponding to a 32.4%, 23.8% and 38.6% diminution in comparison to pure EP, and a 27.0%, 32.4% and 30.9% reduction when comparing to EP/AgNW. Besides, the fire growth rate index (FIGRA) of EP/AgNW/GP-DOPO also demonstrates a drastically decrease in contrast with pure EP and EP/AgNW (Table 2), meaning that flashover time of fire is delayed.33 Such the considerable reductions in the above-mentioned combustion parameters reveal the significant inhibition in the heat and smoke hazards of the flammable EP/AgNW by GP-DOPO. Additionally, pure EP and EP/AgNW with low LOI of 25.0% and 26.2% does not pass the UL-94 vertical burning test (Table 2), and a melt dripping phenomenon was observed simultaneously. Incorporating GPDOPO in EP or EP/AgNW matrix can observably increase the LOI value, meanwhile make them


24

Page 25 of 40 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


to pass the UL-94 burning test. The melt dripping phenomenon is effectively suppressed in EP/AgNW/GP-DOPO due the increasing melt viscosity. Therefore, this work demonstrates a promising way to prepare high Kc and fire resistant PTCs with low filler loading by the combination of AgNWs and a small amount “branch-like” flame retardant-functionalized graphene (GP-DOPO).

Figure 9. SEM images of the residues’ surface and section after CC test with different magnificent: (a, b, a’, b’) EP/AgNW, (c, d, c’, d’) EP/AgNW/GO-DOPO and (e, f, e’, f’) EP/AgNW/GP-DOPO. Condensed phase strategy is generally believed to be the primary flame retardation mechanism of the composites containing graphene or its derivatives, which can be confirmed by analyzing their residue chars. The morphologies and components of the chars after CC test were


25


Page 26 of 40

investigated. It is found the char yields (Figure S8 in Supplementary Information) and quality (integrality and compact degree, Figure S9 in Supplementary Information) after CC test are improved signally after incorporating GO-DOPO or GP-DOPO into both EP and EP/AgNW systems. Moreover, the residue char of EP/GP-DOPO with few holes and cracks seems to be more continuous and compact than that of EP/GO-DOPO (Figure S10 in Supplementary Information). This is main ascribed to the better catalytic carbonization of GP-DOPO due to its higher DOPO grafting ratio and stronger interfacial interaction than that of GO-DOPO in matrix. It is well known that the very low specific heat capacity (0.24 J/g) of silver leads to a high temperature increment when absorbing the same amount of heat, which results in a poor fire resistance of EP/AgNW in this work. Moreover, the wicking action of fiber or nanowire further deteriorates the flame retardancy of EP/AgNW.71 Therefore, a hoary thin char layer with lots of holes (Figure S9a in Supplementary Information and Figure 9a), which are regarded to be the transmission channels for volatile products,68 were left after the burning of EP/AgNW. As well as the melting AgNW structure (Figure 9b) with less organic components (Table 3) on the char’s surface was also found in the high magnification SEM image and EDX spectrum (Figure S11 in Supplementary Information). Such the poor char layer in only incorporated AgNW composite indicates that AgNW only have an insufficient ability to protect the matrix under the char layer, where the melting AgNW morphology (Figure 9a’, 9b’) and the less carbon and oxygen elements content of the residue’s section (Figure S11 in Supplementary Information and Table 3) were found. When incorporating 2 wt% GO-DOPO or GP-DOPO into EP/AgNW composite, the chars’ surfaces show remarkable changes including the increasing char’s thickness, the reducing number density for holes and cracks and the weakening melting behavior of the surficial AgNWs (Figure 9c-f). Moreover, the unbroken AgNWs structure (Figure 9c’-f’) and the more carbon and


26

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oxygen content (Table 3 and Figure S11 in Supplementary Information) are observed in the chars’ sections, which also confirm the effective barrier effect by the formed strong char layers. All of these results confirm the strong char layer on the surface of EP matrix, formed by combining the catalytic carbonization of functionalized-graphene and the incompletely melting AgNWs network. It worth noting that the composites containing GP-DOPO exhibit a better char structures in terms of its yield, thickness, size and number of holes and cracks, etc., than that containing GO-DOPO, suggesting that our GP-DOPO material has stronger catalytic charring effectiveness. Table 3. Element contents of the residues of EP-based composites based on the EDX analyses.

Elements

EP/AgNW

EP/AgNW/GO-DOPO

EP/AgNW/GP-DOPO

Surface

Section

Surface

Section

Surface

Section

C K (at%)

7.36

65.36

39.47

59.03

55.96

74.84

O K (at%)

--

--

5.57

7.50

14.38

9.26

Ag L (at%)

91.82

34.64

53.85

32.89

29.56

15.26


27


Page 28 of 40

Figure 10. Schematic illustration of the flame retardation mechanism of (a) pure EP, (b) EP/AgNW and (c) EP/AgNW/GP-DOPO. It is well known that the melt viscosity of resin during thermal decomposition can significantly affect their flame retardancy by changing the contact area between heat (oxygen) and polymer melts.72 The rheological measurements of the uncured composites (Figure S7 in Supplementary Information) reveal that the viscosity of matrix has been increased by 2-order-ofmagnitude by adding 2 vol% AgNW and 2 wt% GP-DOPO. To simulate the melt state of composites during combustion, the composites were heated to 500 oC for 10 min under nitrogen shielding in a tube furnace. The result (Figure S12 in Supplementary Information) reveals that pure EP and EP/GP-DOPO show an intumescent and spreading char, while the chars of EP/AgNW and EP/AgNW/GP-DOPO can keep the initial size after pyrolysis. These results directly suggest that the melt viscosity of EP can be effectively increased by incorporating AgNW and GP-DOPO, inducing that the composite is hard ignited (prolonged TTI in Table 2) and has no melt dripping during combustion (V-1). Combining to the char residue analyses, the synergistic enhancement mechanism of AgNW and GP-DOPO for the flame retardancy of composite is summarized in Figure 10. Briefly, pure EP shows a very low melt viscosity during thermal decomposition, which causes it to burn sufficiently with little chars (Figure 10a). Although the melt viscosity of pyrolytic EP chains can be effectively increased by incorporating AgNWs (2 vol%), however, poorer flame retardancy of EP/AgNW caused by wicking action and the low specific heat capacity of AgNWs, which leave a thin and poor char layer with many holes and cracks because of the lacking organic chars as binder (Figure 10b). For EP/AgNW/GPDOPO, a large number of organic chars were produced by both the catalytic carbonization of DOPO and the barrier effect of graphene. The produced organic chars not only protect the


28

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AgNWs against melting and wicking action,71 but also connect the AgNWs network to form a protective char layer with compact and robust structure (Figure 10c). The resulted char layer acts as a barrier to prevent the transfer of heat, oxygen and flammable volatile products, thereby improving the flame retardancy of PTCs (EP/AgNW/GP-DOPO).73 4. CONCLUSIONS In this work, we have developed a facile and novel “branch-like” strategy to functionalize RGO, i.e., polymer (PGMA) chains were firstly grafted onto graphene by free radical polymerization, then flame retardant molecules (DOPO) were wrapped into the side chain of PGMA, to improve the flame retardant grafting amount and interfacial compatibility with matrix. The resultant GP-DOPO, combining to the high aspect ratio’s AgNWs, was used to figure out the low thermal conductivity and fire hazard of polymer-based TIMs at low filler loading. The obtained ternary composite (EP/AgNW/GP-DOPO) exhibited a satisfactory improvement both in thermal conductivity and flame retardancy at only 4 vol% AgNW and 2 wt% GP-DOPO level. The morphology and model analyses revealed that the improving thermal conductivity was caused by the synergetic effect of AgNWs and GP-DOPO in terms of improving the AgNWs dispersion, bridging adjacent AgNWs and reducing interfacial thermal resistance by adding GPDOPO. The char analysis confirmed that the char’s yield and quality (integrality and compact degree) were increased importantly by incorporating GP-DOPO into EP/AgNW due to its strong catalytic charring effect. The increasing organic chars not only protect the AgNWs against melting and wicking action, but also connect the AgNWs network to form a protective char layer with compact and robust structure, which acted as a barrier to prevent the transfer of heat, oxygen and flammable volatile products between the inside and outside of polymer melt, and thus improving the flame retardancy of EP/AgNW/GP-DOPO composite.


29


Page 30 of 40

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: The following files are available free of charge. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Y. Ye) and [email protected] (X. Zhou) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support for this work by the National Science Foundation of China (Grant Nos. 51273073 and 51673076), and the analytical and testing assistance from the Analysis and Testing Center of HUST.

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BRIEFS. The significant fire resistance and thermal conductivity of polymer-based thermal interface materials (TIM) are simultaneously improved by the synergistic effect of silver nanowire and “branch-like” flame retardant-functionalized graphene. SYNOPSIS. Table of Content


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Synergetic Improvement in Thermal Conductivity and Flame

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