Synergetic Improvement in Thermal Conductivity ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 21628−21641

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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 2018.10:21628-21641. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/25/19. For personal use only.

†

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering and ‡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 S Supporting Information *

ABSTRACT: The significant fire hazards on the polymerbased 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 (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 “branch-like” strategy with a polymer as the backbone and flame-retardant molecule as the branch was first 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 the EP/AgNW composites. As expected, the incorporation of GP-DOPO (2 wt %) can increase the thermal conductivity to 1.413 W/(m K) at a very low AgNW loading (4 vol %), which is 545 and 56% increments compared to pure EP and EP/AgNW, respectively. The prominent improvement in thermal conductivity was put down to the synergetic effect of AgNW and GP-DOPO, 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, total heat release and total smoke production reduced by 27.0, 32.4, and 30.9% reduction compared to EP/AgNW, respectively. KEYWORDS: thermal conductivity, flame retardancy, synergistic effect, flame-retardant functionalized graphene, silver nanowires high filler loading (>50%) to achieve thermal conductivity (Kc) > 1 W/(m K) 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 graphenebased PTC by various strategies, such as aligning fillers vertically/horizontally11,12 or distributing fillers selectively in matrix,13,14 constructing segregated structure15,16 or filler network in matrix17 and introducing hybrid fillers with a synergetic effect in matrix.18−21 In view of the preparation

1. INTRODUCTION With high degree of miniaturization, integration, and multifunctionalization 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 essential ingredients of thermal management to avoid 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 TIMs based on their characters of light weight, flow processing, and high sealing.3−5 Nevertheless, the essential fire hazards of polymers, reinforced 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 © 2018 American Chemical Society

Received: March 31, 2018 Accepted: June 1, 2018 Published: June 1, 2018 21628

DOI: 10.1021/acsami.8b05221 ACS Appl. Mater. Interfaces 2018, 10, 21628−21641

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Schematic Diagram of the Two-Step Synthetic Route of Branch-Like Flame-Retardant Functionalized Graphene (GP-DOPO)

believed to be one of the most promising and ecofriendly modifiers for improving the flame-retardant efficiency of graphene, which has been widely studied by Hu and his coworkers.32−35 Nevertheless, by limiting 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 which often led to a limited improvement in the RGO’s compatibility in liquid solvent and polymer matrices, is adverse to its application in polymer nanocomposites. One-dimensional silver nanowire (AgNW) with moderate aspect ratio (∼100), intrinsically high Kc (∼400 W/(m K)), and excellent mechanical property can be regarded as a promising nanofiller for PTCs. Although the excellent processability of the composites reinforced by a relatively small amount of AgNW can be achieved, the enhanced compatibility between polymer and metal phases are required in the premise of reasonable-level AgNW dispersion within the polymer matrix, which limits the practical application of this material.37 Very recently, we found that a synergetic effect in one-dimensional silica-1D AgNWs interconnection inhibits the agglomeration nanofillers, which endows the resulting epoxybased PTCs with excellent processability as well as enhanced Kc.37,38 In comparison to nano-SiO2, 2D graphene is expected to have more potential to synergistically improve Kc for incorporated AgNW composites under a small loading because of its much higher intrinsic Kc and aspect ratio. In view of the flame-retardant effect of graphene, we here demonstrate the rational design and fabrication of PTCs with high Kc and flame retardancy simultaneous by the introduction of a new type nanohybrid 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 we employed a facile and novel “branch-like” strategy with two steps, as shown in Scheme

process complexity, generating a synergetic effect by the hybridization of nano-sized and/or microfillers in a polymer matrix to create a thermally conductive network, seems to be a simple but effective approach to mass-produce PTCs in industry.22,23 But even so, a relatively high microfiller 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 graphenebased PTC showed a Kc of 1.53 W/(m K) at only 10 wt % filler loading.25 Therefore, introducing the concept of synergetic effect, the hybridization of two-dimensional (2D) graphene with different dimensional nanofillers, such as one-dimensional (1D) carbon nanotubes (CNTs), show a significant synergetic enhancement in the Kc of PTCs under low filler loading, which mainly originates from the constructing of thermal conductive filler network by bridging these nanofillers together.22,26 Unfortunately, owing to the presence of lattice defects in the commonly reduced graphene oxide (RGO),27 relatively high filler loading is still required to achieve the desired Kc, which would severely limit their processability due to very high viscosity in the resultant nanohybrid mix slurry caused by their enormous aspect ratio and specific surface.22 Thus, it is essential to explore a new type of nanohybrid materials with a synergistic effect in a polymer matrix to obtain a thermal percolation threshold value superior to the currently used nanohybrid materials under low filler loading. For the issue of flame retardancy of PTCs, the RGO functionalized with a flame retardant has been regarded as promising flame-retardant additives to enhance their charring capacity for polymers, thereby enhancing the flame retardancy of the resultant nanocomposites without deteriorating the mechanical and thermal properties, whereas minimizing the requirement of flame retardant loading to achieve a high flame retardancy.28−31 Phosphorus-containing compounds have been 21629


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Figure 1. AFM images and cross-sectional height profiles of (a) GO, (b) G-PGMA, and (c) GP-DOPO. TEM images of (d) G-PGMA, (e, f) GPDOPO, and (g) GO-DOPO. SEM images of (h) GO-DOPO, (i, j) GP-DOPO, and (k) EDX spectrum of GP-DOPO.

S1 in the Supporting Information, wherein poly(glycidyl methacrylate) (PGMA) chains were first grafted onto the RGO surface via grafting-through approach (step 1) and then flame-retardant molecules (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, DOPO) were decorated on the PGMA chains (step 2), to prepare branch-like flame-retardant functionalized 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 nanohybrid to have a surfactant-like characteristic at the matrix−AgNW interface, (2) the residual unreacted epoxy groups from PGMA present the effective gluing of nanofillers to polymer matrix via covalent bonding, which can reduce the thermal resistance between the nanohybrid and polymer matrix, (3) a large amount of

decorated DOPO molecules around the RGO provide the maximum enhancement in the flame retardancy for the composites, and (4) a unique branch-like structure in graphene provides excellent compatibility and an interfacial interaction with matrix induces the formation of an excellent char layer. Therefore, as a synergistic nanofiller with AgNW, the resulting GP-DOPO is expected to improve Kc and fire resistance of PTCs simultaneously.

2. EXPERIMENTAL SECTION 2.1. Materials. Diglycidyl ether of bisphenol-F epoxy (DGEBF, YDF-170) was supplied by KUKDO Chemical Co., Ltd. 2-Ethyl-4methylimidazole (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. Poly(vinylpyrrolidone) (PVP, Mw = 360 000 g/mol) was provided by TCI Development Co., Ltd. N,N-Dimethylformamide (DMF), ethylenediamine (EDA), ethylene glycol (EG), 1-methyl-2-pyrrolidi21630


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Figure 2. (a) FTIR, (b) XPS, (c) Raman, and (d) X-ray diffraction (XRD) spectra of GO, G-NH2, G-PGMA, GP-DOPO, and GO-DOPO. none (NMP), sodium chloride (NaCl), silver nitrate (AgNO3), acetone, and ethanol with analytically reagent were supported by Sinopharm Chemical Reagent Co., Ltd. The 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, the GO sheets were chemically reduced by EDA to prepare amino-functionalized 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 °C with argon shield and continuous stirring for 4 h to graft some of GMA monomers onto the surface of G-NH2, followed by the addition of AIBN into the system for other reaction time of 20 h to obtain PGMA-grafted graphene (GPGMA). After that, a DMF solution containing equimolar DOPO was injected into the above system with a further reaction time of 24 h at 80 °C 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 a branch-like structure, wherein the wrapped PGMA chains and its side chain DOPO molecules, was used as a flame-retardant additive for composites. As a reference, the DOPO molecules were directly grafted into GO according to the previous literature.36 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 °C. After forming a homogeneous solution, 120 μL of NaCl 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 at a rate of 9.4 mL/min. After that, standing the solution for 15 min resulted in

the color change from brownish-red to grayish-green (glistening) and release of gas. The products (AgNWs) were purified by filtration with ethanol several times and saved in ethanol. 2.4. Preparation of Epoxy/AgNW/GP-DOPO Composites. The epoxy-based composites containing AgNW and GP-DOPO sheets were prepared by solution and mechanical blending and program-controlled curing. Typically, the GP-DOPO sheets were ultrasonically dispersed in ethanol for 1 h, followed by the addition of 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 °C. 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 of the structures and properties of GP-DOPO and EP-based composites are shown in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Characterizations of GP-DOPO. In this work, PGMA chains are grafted onto the 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 increasing its grafting amount of flame retardant to the maximum. The surface morphology of the obtained GP-DOPO with a branch-like structure was characterized by using atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) technologies. GO and G-NH2 sheets show a transparent morphology with some flexible wrinkles (Figure S1 in the Supporting Information) and present 21631


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ACS Applied Materials & Interfaces only one-atom thickness (0.69 nm, Figure 1a).42 After grafting the 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 the DOPO molecules onto the side chain of grafted PGMA and decorating the side chain of the grafted PGMA by DOPO molecules via a ring-opening reaction cause the flat PGMA chains up to form a branch-like structure. The obtained GP-DOPO sheets are found to be thicker (4.51 nm) and darker on the surface, as shown in Figure 1c,e, respectively. The amplified image (Figure 1f) reveals the formation of a branch-like structure, which is in contrast to the surface morphology of the GO functionalized with DOPO directly (Figure 1g) in which only heterogeneous dark spots can be observed. Such a 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 the 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 energy-dispersive X-ray (EDX) spectrum of GP-DOPO (Figure 1j,k) also confirms the presence of phosphorus element corresponding to decorated DOPO molecules on its surface. All of the morphology results mean that such a two-step method is capable of realizing the branch-like functionalization for RGO with a PGMA backbone and a flame-retardant (DOPO) lateral branch. The chemical structures of GP-DOPO were confirmed by using Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) spectroscopic analyses. As exhibited in their FTIR spectra (Figure 2a), the characteristic peaks of the oxygen-containing groups of GO were drastically weakened 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, combined with the disappearing amino group, mean the successful grafting of PGMA onto the G-NH2 surface by a covalent bond. Further decorating DOPO to the side chain of the grafted PGMA chains would result in a decrease in the epoxy groups, the increase of hydroxyl groups, and the appearance of DOPO 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 the GP-DOPO spectrum. Moreover, the elimination of the reactive P−H characteristic peak (DOPO, 2432 cm−1) in the GP-DOPO spectrum directly confirms the covalent grafting of DOPO onto RGO. Besides, a steric hindrance effect arises from the surface of G-PGMA replaced by DOPO gradually, resulting in lots of residual epoxy groups still existing in the resultant GP-DOPO sample. In the XPS spectra (Figure 2b), the peaks at 285 and 529 eV corresponding to C 1s and O 1s, respectively, as well as low C/ O atomic ratio of 2.26 can be found in the original GO. After the reduction by EDA, a significant increase in the C/O atomic ratio (9.43) and a new peak corresponding to N 1s at 400.1 eV can be observed in the G-NH2 sample. Grafting of G-NH2 with PGMA chain, which is composited by a high ratio of oxygen atoms, leads to a noticeable decrease in the C/O ratio (3.74), whereas further decorating with DOPO molecules results in the increase in the C/O ratio (5.03) due to the introduction of the high ratio of carbon atoms by the biphenyl structure from the

DOPO molecules. In contrast to GP-DOPO, GO-DOPO prepared by directly grafting DOPO onto GO36 exhibits a very weak reduction degree (Figure S2a,b in the Supporting Information) and a 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 C 1s core-level spectra of functionalized graphene (Figure S2c−e in the Supporting 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 the 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 the grafted PGMA chains, which is consistent with the previous FTIR result. The structural changes in 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 increases from 0.94 to 1.16 due to the destruction of the crystal structure and the size of 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 carbon atoms; however, its ID/IG ratio decreased to 1.23 by further decorating the DOPO molecules due to the DOPO molecule mainly composed of the biphenyl structure of the sp2 carbon. By contrast, the GODOPO exhibits a similar ID/IG ratio with GO due to the weak reduction of the DOPO molecules. In the XRD pattern (Figure 2d), the sharp peak at 2θ = 11.8° corresponding to the (002) diffraction peak of graphitic lattice is shown in GO with a dspacing of 7.5 Å.48 The EDA reduction can shift the (002) diffraction peak to ∼23.2° (broad peak), resulting from the aggregation induced by reduction. Another broad peak is also formed at ∼10.2° due to the intercalation of EDA molecules in graphene layers. A covalently grafted organic layer on the RGO surface effectively suppresses the aggregation of RGO and increases the d-spacing based on the extended and low-angle broad peak in G-PGMA and GP-DOPO pattern. These results combined with previous FTIR and XPS results indicate the successful functionalization of branch-like flame retardant. The thermogravimetric analysis (TGA) and elemental analysis (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 RGO. As expected, a slight decrease in 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, GNH2 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 °C due to the thermal decomposition of grafted PGMA. Through a quantitative analysis of the residual weight between G-NH2, PGMA, and G-PGMA, the grafting ratio of the PGMA chains can be calculated to be ca. 29.8 wt %. It should be noted that GP-DOPO exhibits a higher thermal stability than G-PGMA, including thermal degradation temperature and char yield, 21632


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TGA result of G-PGMA, the total grafting ratio is about 41.6 wt % for GP-DOPO. Based on the hypothesis of the all-carbon structure of G-NH2 or GO, the grafting density of the 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. 3.2. Morphology and Structure of EP/AgNW/GPDOPO Composites. As mentioned above, the residual epoxy groups in the grafted PGMA chains and the branchlike structure are expected to increase the compatibility of graphene in the matrix. The XRD results (Figure S5 in the Supporting 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 in EP/GO-DOPO with aggregation size of 10−20 μm (Figure 4b), whereas 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 the Supporting Information). Moreover, the high-magnification SEM image (Figure 4d) reveals the strong interfacial interaction between GP-DOPO and EP matrix. For EP/AgNW composite, AgNWs exhibit a severe aggregating behavior in matrix (Figure 4e), which would deteriorate the final properties of the 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 the matrix when incorporating GP-DOPO into EP/AgNW composite (Figure 4g). The phosphorus EDX mapping image (Figure S6c,d in the Supporting Information) and the high-magnification SEM image (Figure 4h) of EP/

Figure 3. TGA curves of GO, G-NH2, G-PGMA, and GP-DOPO under nitrogen atmosphere with a heating rate of 10 °C/min.

Table 1. Elemental Analysis Results of GO-DOPO and GPDOPO samples

C wt %

H wt %

O wt %

N wt %

P wt %

GO-DOPO GP-DOPO

57.690 68.340

2.529 3.647

38.986 20.648

7.365

0.795 2.420

suggesting that decorating DOPO onto the PGMA’s side chain would induce an improvement in its thermal stability. However, the final char yield of G-PGMA becomes unpredictable when it is decorated with DOPO molecules because an increase in the DOPO grating ratio increased both char yield and thermal stability of the resultant materials. Therefore, the total graft amount is hard to calculate just based on the TGA results. For this purpose, we employ EA to correct elements’ contents, especially phosphorus. As shown in Table 1, it is 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 the branch-like grafting strategy. In the quantitative analysis, combined with the

Figure 4. SEM images of the fracture surfaces of (a) pure EP, (b) EP/GO-DOPO, (c, d) EP/GP-DOPO, (e) EP/AgNW, (f) EP/AgNW/GODOPO, and (g, h) EP/AgNW/GP-DOPO with 2 vol % AgNW and/or 2 wt % graphene. 21633


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Figure 5. (a) Kc of EP/AgNW/GP-DOPO as a function of 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 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 effective medium approximation (EMA) model, respectively).

presented in Figure 5b, both EP/AgNW and EP/AgNW/GPDOPO show a continuous increase in Kc values as a function of AgNW content from 1 to 4 vol %. The thermal conductivity enhancements (compared to pure EP) reach 313 and 545% at 4 vol % AgNW, respectively. A synergistic enhancement in Kc is easily found by comparing the results of EP/AgNW and EP/ AgNW/GP-DOPO composites, e.g., Kc of the composites with 4 vol % AgNW increases from 0.90 to 1.41 W/(m K), 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/GP-DOPO and Kc0 is the Kc of EP/AgNW, we show the results in Figure 5b. It is found that the synergistic effect strength increase with AgNWs loading due to increasing interparticle 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 a 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 of Kc of homogeneous composites in previous reports is shown in Figure 5c. It can be easily found that a high filler loading was required to achieve the desired Kc for the composites containing microfillers (hBN), whereas 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 the desired Kc and the viscosity of

AgNW/GP-DOPO confirm the uniform distribution and strong interfacial interaction of GP-DOPO in the matrix. Such an improvement in AgNWs’ dispersion is mainly ascribed to the inhibiting settlement action of the 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 the Supporting Information) further prevented the combination of AgNWs during the curing process.50 3.3. Thermal Conductivity of EP/AgNW/GP-DOPO Composites. In this work, 1D AgNWs with mean length of 10 μm and mean diameter of 90 nm (Figure S4 in the Supporting Information), synthesized by a two-step-injection polyol method,41 was used as a thermal conductive nanofiller due to its high intrinsic Kc (∼400 W/(m K)) and high aspect ratio. Combination of 2D graphene (GP-DOPO) with 1D AgNW inducing a synergetic effect to endow the composites with a high Kc is a critical composite concept for the first time. Figure 5a shows the 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 Kc of the composites is a positive correlation with the GP-DOPO content and reaches 0.88 W/(m K) at 2 wt % GP-DOPO content, which increases by ∼34% compared to the composites without GP-DOPO. The GP-DOPO content is fixed at 2 wt % in the next section to investigate the synergistic effects on Kc of composites. As 21634


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Figure 6. High-magnification SEM and TEM images of the fracture surfaces of (a) EP/AgNW and (b, c) EP/AgNW/GP-DOPO. Proposed thermally conductive models of (d) EP/AgNW and (e) EP/AgNW/GP-DOPO.

The morphology analyses (Figure 4e) reveal that the AgNWs show a poor dispersion in the EP/AgNW composite. The highmagnification SEM image of its fractured surface (Figure 6a) further reveals that most of the 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 monodispersion distribution in EP/AgNW/GP-DOPO (Figure 6b). Moreover, its TEM image (Figure 6e) reveals that the segregated AgNWs (yellow arrows) are connected by GPDOPO (red arrows), where a connective thermally conductive path is presented as a dashed line. Such a bridging effect by GPDOPO can effectively weaken the effect of gaps on Kc. Based on the morphological analyses, the heat transfer mechanisms of the incorporated AgNW and AgNW/GP-DOPO nanohybrid composites are proposed in Figure 6d,e, respectively. The synergistic enhancement in Kc of our EP/AgNW/GP-DOPO composite mainly by (1) the presence of surfactant-like GPDOPO sheets in the mixture prevents the agglomeration of AgNWs during curing process and (2) the incorporated GPDOPO sheets improves the matrix−AgNW interface as well as bridges AgNWs to form a highly effective heat-conductive network in the final composites. A combination of improved dispersion of AgNWs in the matrix and decreased Rb analyzed by the EMA model plays 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

resultant mix slurry. The comprehensive comparison reveals that the incorporated AgNW/GP-DOPO nanohybrid composite in this work exhibits the 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 the effective medium approximation (EMA) theory, proposed by Garnett64 and developed by Nan et al.,65 to predict Rb. For the randomly onedimensional fillers, the normalized Kc of Kc/Km for composites can be simplified as follow by Huang et al.66 Kc fp K p/K m =1+ 3 p + 2R bK m K p Km d K

m

(1)

where Kc, Km, and Kp are Kc of composites, matrix, and fillers, respectively. f and p are the volume fraction and aspect ratio of the fillers. In this work, AgNWs with a high p of ∼110 and Kp of ∼400 W/(m K) were used filler, and other parts of composites (EP in EP/AgNW, EP and GP-DOPO in EP/ AgNW/GP-DOPO) were regarded as matrix with Km of 0.219 and 0.240 W/(m K), 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), it is worth noting that the introduction of GPDOPO is capable of weakening Rb greatly, which may be ascribed to the improvement of modulus mismatch between matrix and AgNWs by GP-DOPO.67 21635


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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.

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.

graphene to form an effective barrier layer in the composites, restraining the release of flammable volatile products and preventing the transfer of oxygen and heat during thermal decomposition.69,70 Such a decomposition process, similar to the combustion process, is beneficial to analyze the flame retardation mechanism of the composites. In contrast, GPDOPO has a higher flame-retardant grafting amount and reactive surfaces as well as abundant interfacial bonding with better compatibility in the matrix, which induce the production of more and better char layers, further increasing its thermal stability. 3.5. Flame Retardancy of EP/AgNW/GP-DOPO Composites. The synergetic effect of GP-DOPO and AgNWs on

be observed in their TGA curves under nitrogen and air shielding, corresponding to the pyrolysis of epoxy chains and its chars, respectively. By introducing graphene (GO-DOPO and GP-DOPO) into the EP matrix or the EP/AgNW composite, four facts can be observed in their TGA and derivative thermogravimetry (DTG) results: reduced onset thermal decomposition temperature (Tonset), weakened weight loss rate, increased char yield, and improved char’s oxidation stability, all of 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 in char’s yield; these char combine with 21636


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ACS Applied Materials & Interfaces Table 2. Combustion Data of Pure EP and Its Composites samples

TTI (s)

PHRR (kW/m2)

THR (MJ/m2)

TSP (m2)

FIGRA (kW/(m2 s))

LOI (vol %)

UL-94 rating

pure EP EP/GO-DOPO EP/GP-DOPO EP/AgNW EP/AgNW/GO-DOPO EP/AgNW/GP-DOPO

67 62 57 74 86 86

1137.6 997.3 853.5 1053.7 950.4 769.1

81.6 72.0 63.2 92.0 84.1 62.2

62.4 44.2 40.7 55.4 47.1 38.3

8.8 8.7 8.1 8.1 7.3 5.9

25.0 25.6 26.8 26.2 27.3 28.7

NR NR V-2 NR V-2 V-1

Figure 9. SEM images of the residues’ surface and section after CC test at different magnifications: (a, b, a′, b′) EP/AgNW, (c, d, c′, d′) EP/AgNW/ GO-DOPO, and (e, f, e′, f′) EP/AgNW/GP-DOPO.

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, respectively. The 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 the 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 a high DOPO grafting amount not only improves the interfacial compatibility between RGO and matrix but also reinforces its catalytic carbonization in polymer chains.24,68 The introduction of GPDOPO 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) in the heat release rate curves disappears after adding GO-DOPO or GPDOPO. This result indicates an increase in 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/GP-DOPO, 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. 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 % GO-DOPO in EP/AgNW results in a 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), corresponding to 32.4, 23.8, and 38.6% diminution in comparison to pure EP and 27.0, 32.4, and 30.9% reduction 21637


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ACS Applied Materials & Interfaces compared to EP/AgNW. Besides, the fire growth rate (FIGRA) index of EP/AgNW/GP-DOPO also demonstrates a drastic decrease in contrast to pure EP and EP/AgNW (Table 2), meaning that the flashover time of fire is delayed.33 Such considerable reductions in the above-mentioned combustion parameters reveal a significant inhibition in the heat and smoke hazards of flammable EP/AgNW by GP-DOPO. Additionally, pure EP and EP/AgNW with low LOI of 25.0 and 26.2% do not pass the UL-94 vertical burning test (Table 2), and a meltdripping phenomenon was observed simultaneously. Incorporating GP-DOPO in EP or EP/AgNW matrix can observably increase the LOI value, but make them 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 small amount branch-like flameretardant functionalized graphene (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 the CC test were investigated. It is found that the char yields (Figure S8 in the Supporting Information) and quality (integrality and compact degree, Figure S9 in the Supporting Information) after the CC test are improved significantly after incorporating GO-DOPO or GPDOPO 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/GODOPO (Figure S10 in the Supporting Information). This is mainly ascribed to the better catalytic carbonization of GPDOPO due to its higher DOPO grafting ratio and a stronger interfacial interaction than that of GO-DOPO in the 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 the Supporting Information and Figure 9a), which are regarded to be the transmission channels for volatile products,68 were left after the burning of EP/AgNW. Also, the melting AgNW structure (Figure 9b) with less organic components (Table 3) on the char’s surface was also found in

found. When incorporating 2 wt % GO-DOPO or GP-DOPO into the EP/AgNW composite, the chars’ surfaces show remarkable changes including increased char’s thickness, reduced number density of holes and cracks, and weakened melting behavior of surficial AgNWs (Figure 9c−f). Moreover, the unbroken AgNWs structure (Figure 9c′−f′) and higher carbon and oxygen content (Table 3 and Figure S11 in the Supporting 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 the EP matrix, formed by combining the catalytic carbonization of functionalized graphene and the incomplete melting of AgNWs network. It is worth noting that the composites containing GP-DOPO exhibit better char structures in terms of yield, thickness, size, and number of holes and cracks, etc., than that containing GO-DOPO, suggesting that our GP-DOPO material has a stronger catalytic charring effectiveness. 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 the Supporting Information) reveal that the viscosity of the matrix is increased by 2 orders of magnitude by adding 2 vol % AgNW and 2 wt % GP-DOPO. To simulate the melt state of the composites during combustion, they were heated to 500 °C for 10 min under nitrogen shielding in a tube furnace. The result (Figure S12 in the Supporting Information) reveals that pure EP and EP/GPDOPO show an intumescent and spreading char, whereas 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, implying that the composite is hard ignited (prolonged TTI in Table 2) and has no melt dripping during combustion (V-1). Combining the char residue analyses, we summarize the synergistic enhancement mechanism of AgNW and GP-DOPO for the flame retardancy of the composite 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 %), a poorer flame retardancy of EP/AgNW caused by wicking action and a low specific heat capacity of AgNWs 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 catalytic carbonization of DOPO and barrier effect of graphene. The produced organic chars not only protect the AgNWs against melting and wicking action71 but also connect the AgNWs network to form a protective char layer with a compact and robust structure (Figure 10c). The obtained 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

Table 3. Element Contents of the Residues of EP-Based Composites Based on the EDX Analyses EP/AgNW

EP/AgNW/GO-DOPO

EP/AgNW/GP-DOPO

elements

surface

section

surface

section

surface

section

C K (atom %) O K (atom %) Ag L (atom %)

7.36

65.36

91.82

34.64

39.47 5.57 53.85

59.03 7.50 32.89

55.96 14.38 29.56

74.84 9.26 15.26

the high-magnification SEM image and EDX spectrum (Figure S11 in the Supporting Information). Such a poor char layer in the only incorporated AgNW composite indicates that AgNW only has insufficient ability to protect the matrix under the char layer, where the melting AgNW morphology (Figure 9a′,b′) and the less carbon and oxygen content in the residue’s section (Figure S11 in the Supporting Information and Table 3) were

4. CONCLUSIONS In this work, we have developed a facile and novel branch-like strategy to functionalize RGO, wherein polymer (PGMA) chains were first grafted onto graphene by free radical polymerization and then flame-retardant molecules (DOPO) were wrapped into the side chain of PGMA, to improve the 21638


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Figure 10. Schematic illustration of the flame-retardation mechanism of (a) pure EP, (b) EP/AgNW, and (c) EP/AgNW/GP-DOPO.

■

flame-retardant grafting amount and the interfacial compatibility with the matrix. The resultant GP-DOPO, combining the high aspect ratio of 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 improved 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 the interfacial thermal resistance by adding GP-DOPO. 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 increased 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 a 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 the polymer melt, thus improving the flame retardancy of EP/AgNW/GP-DOPO composite.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.Y.). *E-mail: [email protected] (X.Z.). ORCID

Yunsheng Ye: 0000-0002-2351-1845 Hu Liu: 0000-0003-3840-8135 Xiaolin Xie: 0000-0001-5097-7416 Author Contributions

The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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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ASSOCIATED CONTENT

REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05221. Characterizations using atomic force microscopy, morphologies of GO and G-NH2, structure of functionalized graphene, morphology of AgNWs, structure and morphologies of composites, char analysis (PDF) 21639


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