epoxy composite for thermal conductivity enhancement

Polymer 183 (2019) 121834

Contents lists available at ScienceDirect

Polymer journal homepage: http://www.elsevier.com/locate/polymer

Co-curable poly(glycidyl methacrylate)-grafted graphene/epoxy composite for thermal conductivity enhancement Hyunwoo Oh, Youjin Kim, Jooheon Kim * School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul, 156-756, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Polymer-matrix composites (PMCs) Curing Thermal properties

Thermally conductive epoxy composite is prepared by directly incorporated graphene into the matrix. The 3methacryloyloxypropyltriethoxysilane (MPTES) functionalized reduced graphene oxide (MRGO) is synthesized by a sol-gel reaction followed by chemical reduction, leading to the introduction of methacrylate groups with carbon-carbon double bonds. Then, radical copolymerization is employed to afford poly(glycidyl methacrylate) grafted MRGO, which can react with amine groups in the curing agent, 4.40 -diaminodiphenylmethane. The exothermic peaks were observed for the epoxy composites revealed that p-MRGO is co-cured with the curing agent and incorporated into the matrix. The thermal conductivity of the p-MRGO/epoxy composites reaches 0.75 W m 1 K 1 at p-MRGO content of 7 wt%, corresponding to a thermal conductivity enhancement of 249% Furthermore, the crosslinking density is calculated to confirm the co-curing effect of p-MRGO. Our results demonstrated that the covalent bond between the filler and matrix is an advantageous approach for improving the thermal conductivity of epoxy composites.

1. Introduction In recent years, with rapid growth of technology, electronic devices (including light-emitting devices, printed circuit boards and automobile components) have been developed high-level integration and minia­ turization [1–4]. Unfortunately, as the performance of the electronic equipment is improved, excess heat is a critical issue, leading to the device malfunction and substrate deterioration. Thus, there is an in­ crease in the demand for thermally conductive materials that can effectively dissipate heat. Typically, thermal interface materials (TIMs), which are inserted between an integrated circuit and a heat sink, are used to assist with the dissipation of heat in electronic device packaging [5–7]. In particular, polymer-based TIMs have been widely used as electronic components due to their lightweight nature, good processability, and cost effec­ tiveness; however, their use is limited by low thermal conductivity [8–10]. To improve the heat dissipation capacity, thermally conductive fillers, including metals (viz. copper, aluminum, silver [11–13]), ce­ ramics (viz. aluminum oxide, aluminum nitride, silica, silicon carbide, hexagonal-boron nitride (BN) [14–22]) and carbon-based materials (viz. graphite, carbon nanotubes, carbon fibers, graphene [23–27]) have been introduced in the polymer matrix.

Among these aforementioned materials, graphene comprising sp2hybridized carbon arranged in a honeycomb lattice exhibits superior thermal conductivity (~5000 W m 1 K 1); hence, graphene has attrac­ ted enormous interest in the industry [28]. Hence, graphene as a filler is incorporated into the polymer composite for high thermal conductivity. Despite the outstanding performance of the graphene, the thermal conductivity enhancement of the composite is ineffective due to the following reasons. Typically, graphene is synthesized by reduction starting from graphene oxide. However, during reduction, the graphene undergoes facile aggregation easily due to the van der Waals forces and its large surface area, which is restraint the heat transfer in the com­ posite [29–32]. The desired dispersion of fillers in thermally conductive polymer composites is key for developing a heat transport network. Therefore, several methods such as, ball mill mixing, roll milling, and shear mixing have been reported to improve the dispersion of graphene [33–37]. Although these attempts produced composites comprised well-dispersed graphene, these methods cannot complement the low compatibility with the matrix due to only simple physical treatment. The poor interfacial adhesion induced enormous voids that serve as a phonon scattering sites, which in turn can function as a critical resis­ tance to heat transfer. To overcome this issue, previously reported studies have reported the surface functionalization of graphene [38–41].

* Corresponding author. E-mail address: [email protected] (J. Kim). https://doi.org/10.1016/j.polymer.2019.121834 Received 30 July 2019; Received in revised form 17 September 2019; Accepted 22 September 2019 Available online 17 October 2019 0032-3861/© 2019 Elsevier Ltd. All rights reserved.

H. Oh et al.

Polymer 183 (2019) 121834

Scheme 1. Illustration of fabrication procedure for p-MRGO/epoxy composite.

Generally, organic materials exhibiting favorable compatibility with the matrix are introduced for the functionalization of the graphene surface. Functionalized graphene has the capability of reducing the voids due to improved interface condition. However, a majority of the surface func­ tionalization methods are focused on the improvement of the interfacial adhesion, supported by the relatively weak physical interactions such as hydrogen bonding; dipole-dipole interactions; π-π interactions; and hy­ drophobic interactions. Hence, a new approach should be developed to increase the interaction between the filler and matrix. In this study, p-MRGO/epoxy composites were prepared by a cocuring reaction between the filler and the matrix to improve the ther­ mal conductivity. First, 3-methacryloyloxypropyltriethoxysilane (MPTES) functionalized reduced graphene oxide (MRGO) was synthe­ sized by a sol-gel reaction of graphene oxide (GO) with MPTES, followed by chemical reduction for the introduction of methacrylate groups. The incorporated MPTES chains exhibited two effects, viz. (1) The chains prevented the aggregation of RGO particles, leading to form welldispersed state in the organic solvent, and (2) the methacrylate groups containing carbon-carbon double bonds are capable to be copoly­ merized with poly(glycidyl methacrylate) (PGMA) by radical polymer­ ization. Then, PGMA-grafted-MRGO (p-MRGO), which exhibited reactivity with amine groups, was incorporated into the epoxy com­ posite. The co-curing effect was confirmed by the measurement of the heat flow in the reaction, and the fabricated p-MRGO/epoxy composite exhibited high thermal conductivity, corresponding to a thermal con­ ductivity enhancement of 0.75 W m 1 K 1. Moreover, the viscoelastic properties of the epoxy composites were confirmed to investigate the crosslinking densities.

After the reaction was completed, the mixture was cooled to RT and poured into ice-cold DI water (400 mL) containing 30% H2O2 (3 mL) with stirring for 4 h. Next, the mixture was washed with 20% HCl (300 mL) and excess DI water and subjected to centrifugation to remove the metal ions and remaining acid. Finally, the resultant aqueous dispersion was dried in a convection oven at 60 � C for 48 h, affording GO papers. 2.2. Synthesis of MPTES-functionalized reduced graphene oxide (MRGO) First, dried GO papers were dispersed in the DI water (2 mg mL 1) by sonication at low power output (80 W) for 1 h, leading to a stable dispersion. First, MPTES (97%, Alfa Aesar) was introduced into the GO dispersion (0.0025 mL for mg of GO) under stirring for 6 h at 75 � C. After the reaction, hydrazine monohydrate (0.1 mL for 100 mg of GO) was continuously added to the mixture, and the flask was heated to 95 � C for 12 h under stirring. Finally, the mixture was cooled to RT and washed with excess water by filtration. The resulting dark powder was dried overnight at 80 � C. 2.3. Preparation of poly(glycidyl methacrylate) (PGMA)-grafted graphene (p-MRGO) p-MRGO was synthesized by free-radical copolymerization. First, MRGO (0.3 g) was dispersed in N,N-dimethylformamide (DMF) (150 mL) by sonication at low power output (80 W) for 1 h. Second, glycidyl methacrylate (GMA, 13 mL) was introduced into homoge­ neously dispersed MRGO, and the mixture was subjected to sonication at low power output (80 W) for 1 h. Next, the mixture was purged with N2 for 10 min to remove dissolved air. Then, 2,20 -azobisisobutyronitrile (0.148 g) was added to the mixture and stirred for 10 h at 65 � C. The product was cooled to RT and precipitated using excess methanol. The precipitates were dissolved in 200 mL of tetrahydrofuran, and the so­ lution was subjected to centrifugation at 12000 rpm for 1 h. This centrifugation step was repeated several times, and the black solid was washed with acetone at last time. Finally, the products were filtered and dried overnight in a convection oven at 50 � C.

2. Experimental 2.1. Synthesis of GO papers GO was prepared by using graphite powder (Graphene Supermarket) according to the modified Hummers’ method [42]. For the improved method, graphite powder (3 g) was first added to a 9:1 mixture of concentrated H3PO4/H2SO4 (40:360 mL) for 4 h with stirring in an ice bath. Second, under vigorous agitation, KMnO4 (18 g) was slowly introduced into the graphite-dispersed mixture over 1 h in an ice bath. Third, the mixture was subsequently heated to 50 � C and stirred for 12 h. 2

H. Oh et al.

Polymer 183 (2019) 121834

Fig. 1. HR-TEM images (a) RGO, (b) MRGO, (c) EDS spectra of MRGO, (d) EDS elemental mapping of MRGO.

2.4. Preparation of graphene/epoxy composites

patterns of the material crystal structure were recorded at 40 mA and 40 kV using a scan rate of 1� /s by using Cu-Kα radiation (λ ¼ 0.154056 nm). Material binding energies were examined by X-ray photoelectron spectroscopy (XPS, VG-Microtech, ESCA2000) by using a monochromatic Al-Kα X-ray source (1486 eV). Thermogravimetric analysis (TGA, TGA-2050, TA Instruments) was employed to investigate the thermal degradation of materials at a heating rate of 20 � C/min under N2. Material morphologies were observed by field-emission scanning electron microscopy (FE-SEM, Sigma, Carl Zeiss). The exothermic behavior and specific heat of the materials were investigated by differential scanning calorimetry (DSC, PerkinElmer Co., DSC-7 sys­ tem). The thermal diffusivity of the samples was investigated by using a laser flash apparatus (LFA, Netzsch Instruments Co., Nanoflash LFA 467) at RT. The mechanical properties of the samples were investigated by dynamic mechanical analysis (DMA; Triton Instrument, Triton DMTA) at a frequency of 1 Hz at a heating rate of 10 � C/min with a temperature range of 25–240 � C.

In this study, epoxy composites were fabricated by a casting method. Before fabricating the epoxy composites, graphene was first dispersed in acetone for 1 h by sonication at low power output (80 W). Second, bisphenol A (DGEBA, epoxy equivalent weight ¼ 186.8 g/eq, Kukdo Chemical) (10 g) was heated to 100 � C for 15 min to reduce the viscosity, followed by the addition of the graphene suspension. Next, the mixture was increased to 150 � C to evaporate the solvent. Then, 4,40 -dia­ minodiphenylmethane (DDM, �97%, Sigma-Aldrich) (4 g) as the curing agent was introduced into the mixture, and the mixture was placed into a vacuum oven at 90 � C for 20 min to remove the residual solvent and air bubbles. Finally, the mixture was poured into a self-manufactured pol­ ydimethylsiloxane mold and subjected to curing at 160 � C for 2 h. All the procedure of fabrication for p-MRGO/epoxy composite is illustrated in Scheme 1. 2.5. Characterization

3. Results and discussion

Material microstructures were observed by field-emission trans­ mission electron microscopy (FE-TEM, JEM-2100F). Fourier transform infrared (FTIR; PerkinElmer Spectrum One) spectra of the materials were recorded at a resolution of 4.0 cm in a frequency range from 4000 cm 1 to 400 cm 1. Raman spectra (DXR2xi, Thermo, USA) of the materials were recorded using an Ar laser with an excitation wavelength of 514 nm. X-ray diffraction (XRD, New D8-Advance/Bruker-AXS)

The morphologies of GO, MPTES functionalized RGO (MRGO) were investigated by FE-TEM, EDS and AFM images. As shown in Fig. 1a, GO showed a typical transparent, wrinkled structure due to the thin thick­ ness of sheet. After the sol-gel reaction and chemical reduction, more opaque and mottled images were observed for the MRGO sheet. The dark image for MRGO was distinguished to the GO sheet, but the

Fig. 2. (a) FT-IR analysis of GO, RGO and MRGO (b) Raman spectra of GO, RGO and MRGO (c) XRD patterns of graphite, GO, RGO, MRGO. 3

H. Oh et al.

Polymer 183 (2019) 121834

Fig. 3. (a) XPS wide scan spectra of GO, RGO, MRGO and p-MRGO, C1s XPS spectra of (b) GO, (c) RGO, (d) MRGO, (e) p-MRGO.

presence of MPTES chains are not well apparent. However, the EDS mapping images of MRGO appeared three atoms, with the uniform distribution of Si atoms uniformly covering the graphene sheet. In addition, the EDS spectra of MRGO also revealed a new peak, corre­ sponding to Si. Hence, the morphology analysis for MRGO shows suc­ cessful functionalization of MPTES. For the further analysis of materials, FTIR, and Raman spectra, as well as XRD patterns, were recorded to investigate MRGO via silaniza­ tion. In Fig. 2a, FTIR spectrum of GO presented characteristic vibrational spectral peaks corresponding to the hydroxyl (O–H, C–OH) stretching vibrations at 3402 cm 1, 1395 cm 1, 1215 cm 1, epoxy (C–O–C) – O) stretching vibration stretching vibration at 1040 cm 1, carboxyl (C– at 1739 cm 1 [43]. After reduction of GO by hydrazine, a majority of the characteristic peaks were removed except for the conjugated carbon – C) peak observed at 1570 cm 1 and the hydroxyl (C–OH) stretching (C– vibration observed at 1215 cm 1. However, drastically different peaks from RGO were confirmed from MRGO; characteristic vibrational peaks – O) were observed at of siloxyl (Si–O), silyl (Si–C), and methacrylate (C– 1104 cm 1, 1296 cm 1, and 1715 cm 1 corresponding to the hydrolysis and condensation between MPTES and the hydroxyl groups [44]. Raman spectroscopy is the useful technique for characterizing graphitic structure materials. As shown in Fig. 2b, the G and D peaks were observed at 1350 cm 1 and 1600 cm 1, respectively, in all the Raman spectra for graphene materials. The G peak corresponded to the E2g vibration mode of conjugated carbon atoms related to the degree of graphitization, while the D peak corresponded to the symmetry A1g mode related to the small domains of aromatic rings, reflecting the amount of sp3 carbons [45]. Therefore, the intensity ratios of the G peak to the D peak (ID/IG) are expected to decrease after reduction by the restoration of aromatic rings. However, after the GO reduction, ID/IG ratios for RGO and MRGO increased from 1.31 to 1.50 and 1.45, respectively, indicative of an unexpected result. This phenomenon has also been reported previously: Stankovich et al. suggested that the for­ mation of a number of small and isolated domains of conjugated carbon atoms can occur upon reduction and it induces the increase of D peak

after reduction [45]. Moreover, Tung et al. have argued that the increase of ID/IG ratio can be generated from a large number of residual amor­ phous carbon or unreduced epoxides and hydroxyl groups [46]. This explanation represents the limitation of the chemical reduction. The XRD patterns of GO, RGO and MRGO are shown in Fig. 2c. The characteristic (002) diffraction peak of natural graphite was observed at 26.4� and the peak of GO shifted to 11.3� , indicating d-spacing of 0.78 nm, corresponding to the oxygen-containing functional groups [47]. After the reduction of GO, a broad peak was located at 24.6; hence, the d-spacing of RGO is decreased to 0.36 nm due to the aggregation of the graphene sheets induced by the removal of the functional groups. In the case of MRGO, as in RGO, the most of (002) diffraction peak dis­ appeared, but the peak shifts to the lower angle at 22.4� , corresponding d-spacing of 0.41 nm. By the silanization of GO, MPTES chains were anchored onto the surface and the covalently linked chains prevented the aggregation of the sheets during chemical reduction. The synthesized MRGO was incorporated with GMA monomers by radical co-polymerization using carbon-carbon double bonds in MPTES. In the Fig. S1, FTIR spectrum of p-MRGO was compared with that of PGMA and a majority of the peaks showed good agreement. Moreover, the characteristic vibration peaks of epoxy ring (C–O–C) observed at 914 cm 1, 848 cm 1, and 759 cm 1 were a crucial key evidence for the grafted PGMA chains. And as shown in Fig. 3, XPS spectra were analyzed for the more precise confirmation of the graphene materials. Fig. 3a shows the XPS wide-scan spectra of GO, RGO, MRGO, and p-MRGO. Compared to GO and RGO, the peaks corresponding to Si atoms were observed at ~100 eV in the XPS spectra of MRGO and p-MRGO. In addition, the C1s peaks for all graphene materials were deconvoluted in detail. In Fig. 3b, GO presents typical six chemically shifted peaks, viz. – C (284.3 eV), C–C (284.8 eV), C–OH (285.5 eV), C–O–C (286.6 eV), C– – C–O (288.5 eV), respectively [48]. After the C–OH (285.5 eV), and O– reduction, the oxygen-containing functional group content of RGO, viz. – O (287.8 eV), O– – C–O C–OH (285.8 eV), C–O–C (286.6 eV), C– – C component (284.6 eV) of (288.8 eV), clearly decreased, while the C– RGO increased. Also, peak of π–π* (291.1 eV) was generated for RGO 4

H. Oh et al.

Polymer 183 (2019) 121834

related to the introduced silane moiety was confirmed in MRGO. Furthermore, p-MRGO presents an additional weight loss between 230 � C and 420 � C, which is corresponded to the grafted PGMA. From the char residues, in p-MRGO, the calculated content of grafted PGMA and RGO was confirmed to be 13.75 wt% and 53.3 wt%, respectively (see Fig. 5). Grafted PGMA can react with the curing agent; hence, the curing reaction of RGO and p-MRGO with epoxy matrix was evaluated by DSC under N2 atmosphere. In Fig. S2, epoxy composites presented similar exothermic peaks with increasing filler contents. Neat epoxy exhibited peaks at 159.4 � C and the other RGO/epoxy composites showed similarshaped peaks with only slight temperature changes despite the addition of the filler. However, for p-MRGO/epoxy composites as shown in Fig. 5, the variation of the exothermic peaks was clearly observed. By using 0.5–7 wt.% of p-MRGO, the peak temperatures of the epoxy composites shifted from 157.2 � C to 146.5 � C. In addition, with the increase in the filler content, these peaks became broader. To investigate this phe­ nomenon, the heat flow of a mixture with p-MRGO and DDM as the curing agent was examined in Fig. S2. With temperature increasing, an endothermic peak which is the melting of DDM first occurred, followed by a broad exothermic peak observed at 143.1 � C, corresponding to the curing reaction occurring between epoxide groups in grafted PGMA and amine groups in DDM. Thus, the capability of participation of p-MRGO in the curing reaction of the epoxy composites was confirmed. Within the composites, the reaction sites between curing agent and epoxy groups were increased with adding p-MRGO. Hence, p-MRGO acceler­ ated the curing reaction of the composites by the catalytic effect and accordingly the exothermic peak migrated toward the lower tempera­ ture. In addition, broader peaks were interpreted as a network interlock effect. The curing agent reacted with the epoxy matrix and filler to simultaneously generate two networks, which served as hindrance and retarded the curing rate [50]. From the above results, the direct effect of p-MRGO for the co-curing reaction is confirmed. To investigate the dispersion and interface state of the filler in the matrix, FE-SEM images of the freeze-fractured structures of the epoxy composites were recorded. Fig. 6 shows the FE-SEM images of the composite morphologies with 5 wt% of RGO and p-MRGO under different magnifications. At low magnification, all the composites exhibited a crumpled rough surface as crack propagation patterns. In Fig. 6a, considerable RGO aggregates were observed in the matrix caused by absence of surface functionalization to hinder the interaction between sheets. Moreover, in Fig. 6c, several voids were located on the surface, which were induced by the weak interfacial bonding between the filler and matrix. In comparison, the p-MRGO/epoxy composite exhibited more crumpled structures, and clear clusters were not observed for the matrix, indicative of a highly dispersed filler.

Fig. 4. TGA curves of GO, RGO, MRGO, p-MRGO and PGMA.

corresponding to the restoration of the delocalized conjugated carbon – C–O (288.8 eV) component in groups. As shown in Fig. 3d, The O– MRGO increased, indicative of the side groups of the attached MPTES chains. Moreover, in the XPS spectrum of p-MRGO, the presence of grafted PGMA chains is clearly confirmed. Fig. 3e showed the increased –O content of several components, including C–O–C (286.5 eV), C– – C–O (288.8 eV). In particularly, functionalized (287.3 eV), and O– epoxide groups in graphene was the essential component for direct incorporation into the matrix by the curing reaction during fabrication of epoxy composite. TGA analysis is a useful tool for examining the amount of function­ alized PGMA for RGO. In Fig. 4, GO showed thermally unstable stage a temperature of less than 110 � C with a 12% weight loss that is assigned the evaporation of adsorbed water molecules, followed by the main weight loss stage up to 300 � C which is attributed to the decomposition of labile oxygen-containing functional groups. Next, a gradual stage is observed due to the pyrolysis of residual char [49]. And after intro­ ducing MPTES, a clearly different weight loss between RGO and MRGO was observed in the TGA curve. Although RGO exhibited gradual weight loss due to the removal of functional groups, a decomposition stage

Fig. 5. DSC thermogram of (a) p-MRGO/epoxy composites with 0, 0.5, 1, 3, 5, 7 wt% filler content. (b) MRGO/epoxy composites with 0, 0.5, 1, 3, 5, 7 wt% filler content. 5

H. Oh et al.

Polymer 183 (2019) 121834

Fig. 6. FE-SEM images of cross-sectional structures of (a–c) 5 wt% RGO/epoxy composite (The white arrows indicate RGO sheets in Fig. 6a) (d-f) 5 wt% p-MRGO/ epoxy composite.

Fig. 7. (a) Thermal conductivities of RGO and p-MRGO/epoxy composites (b) TCEF of RGO and p-MRGO/epoxy composites.

Furthermore, at high magnification, p-MRGO sheets were well embedded in the matrix due to the prevention of the aggregation of pMRGO sheets by the copolymerized MPTES and PGMA chains and the reaction of the grafted-PGMA-containing epoxy chains with the amine groups during the fabrication of the composites, which were covalently linked to the matrix. This uniform dispersion and strong interfacial adhesion between the filler and matrix were advantageous to form an efficient heat transport network. Thermally conductive polymer composites are filled with several carbon-based fillers to achieve high thermal conductivity [51–53]. As

shown in Fig. 7, the thermal conductivities of the epoxy composites continuously improved with increasing the loading of the filler content, starting from 0.21 W m 1 K 1 for neat epoxy and reaching 0.33 W m 1 K 1 with 7 wt% RGO, reflecting the thermal conductivity enhancement factor (TCEF) of 54%, according to the following equation: TCEF ¼

kc

km km

where kc and km are the thermal conductivity of the composite and 6

H. Oh et al.

Polymer 183 (2019) 121834

vectors as a reservoir to impede heat transfer. However, in the p-MRGO/epoxy composite, the copolymerized MPTES-PGMA chains prevented the aggregation between the sheets, and the well-dispersed filler constructed an effective heat conduction path in the matrix. Above all, with the increase in the content of p-MRGO, the TCEF of composites was increased sharply. This finding could be confirmed by the percolation threshold effect [56]. The heat is transported depending on the filler; hence, at low concentrations, as the isolated filler is similar to an island, inefficient conduction occurs. However, with increasing concentration, an efficient heat transfer network is formed by the con­ tact between fillers, related to the high improvement in the TCEF of the composites. As shown in Table 1, the p-MRGO/epoxy composite with 7 wt% (4 vol %) was compared with other thermally conductive polymer composites. Although the p-MRGO/epoxy composite did not exhibit the highest thermal conductivity, it showed a highly competitive TCEF relative to the amount of fillers. Table 1 compares the thermal con­ ductivity of graphene-based polymer composites reported in the litera­ ture. All the composites show increasing tendency of the thermal conductivity with adding graphene. F. Kargar et al. obtained high thermal conductivity of 11.4 W m 1 K 1 with high loading of graphene [57]. Also, they found that the thicker few-layered graphene/epoxy composite exhibits high thermal conductivity of 8 W m 1 K 1 and more effective to increase TCEF better than the thinner fillers [58]. J. S. Lewis et al. reported graphene-BN/epoxy composite which shows thermal conductivity of 6.5 W m 1 K 1 with synergistic effect [59]. M. Saadah et al. presented the graphene based TIMs which exhibit the thermal conductivity of 1.2 W m 1 K 1 [60]. Wang et al. improved thermal conductivity of graphene/epoxy composite to 0.46 W m 1 K 1 using sonication followed by three-roll milling [61]. Jarosinski et al. devel­ oped thermal conductive graphene/epoxy composite which possesses 0.43 W m 1 K 1 with high-shear mixer [62]. Among the results, p-MRGO/epoxy composite shows high thermal conductivity, which in­ dicates the effective co-curable filler system. Although our result did not exhibit the highest thermal conductivity, it has high TCEF relative to the

Table 1 Comparison of thermal conductivities of graphene/epoxy composites in re­ ported literature and in this work. Filler

Content

K (W/m

Graphene Graphene BN/Graphene Graphene Graphene Graphene Graphene Graphene p-MRGO

45 vol% 55 wt% 43.6 vol% 4 wt% 5 wt% 4 wt% 5 wt% 7 wt% 7 wt% (3.97 vol%)

11.4 8 6.5 1.2 0.46 0.43 0.56 0.58 0.75

1

K 1)

TCEF (%)

Ref

5082 3536 2854 120 115 132 155 138 249

58 59 60 61 65 66 67 68 This work

polymer matrix. Although RGO has intrinsic high thermal conductivity, it was apparently ineffective for the epoxy composite. However, pMRGO was more effective for improving the thermal conductivity of the epoxy composite. The thermal conductivity of p-MRGO/epoxy com­ posite was increased to 0.75 W m 1 K-1 with 7 wt% (3.97 vol %) pMRGO, corresponding to TCEF values of 249%. This phenomenon was related to several reasons. First, the interfacial adhesion effect between the filler and matrix is the main factor. In polymer composites, the transport of heat conduction occurs mainly via the vibrations of acoustic phonons, in which phonon–boundary scattering occurs owing to the mismatch between the rigid filler and soft matrix, leading to resistance [54]. As confirmed by the morphology images, multitudinous voids were observed for the RGO/epoxy composite. By contrast, p-MRGO was apparently well embedded in the matrix, with a majority of the voids removed. The p-MRGO containing PGMA induced a co-curing reaction with the epoxy matrix, diminishing the scattering of phonons by the covalent linkage of the filler and matrix [55]. Then, further investigation about thermal boundary resistance of the composites was confirmed by theoretical calculation in Fig. S5. In addition, aggregated RGO was observed in the matrix, suggesting the incidence of reciprocal phonon

Fig. 8. IR images and surface temperature profile with heating and cooling time of neat epoxy and 7 wt% p-MRGO/epoxy composite. 7

H. Oh et al.

Polymer 183 (2019) 121834

Fig. 9. (a) Storage moduli of p-MRGO/epoxy composites with 0, 0.5, 3, 7 wt% (b) tan δ of p-MRGO/epoxy composites with 0, 0.5, 3, 7 wt% (c) crosslinking densities of RGO and p-MRGO/epoxy composites.

weight fraction of the filler. Hence, p-MRGO synthesized herein exhibits an advantageous effect for thermally conductive composites. To investigate the thermal management capability, the temperature distribution images of neat epoxy and the p-MRGO composites with 7 wt % were recorded using am IR camera. As shown in Fig. 8, the samples were heated from 15 � C to 80 � C on a hot-plate. After heating for 300 s, the surface temperature of the p-MRGO/epoxy composite reached 71 � C, which was similar to the hot plate temperature, and this temperature was greater than that of neat epoxy (60 � C). Then, the samples were immediately transferred on a cold plate. Similarly, the intrinsic high thermal conductivity of p-MRGO/epoxy composite led to more rapid heat radiation to out of the surface. and it was confirmed by the lowest surface temperature about 29.2 � C. According to the above results, pMRGO/epoxy composite demonstrates potential for thermal manage­ ment application. Fig. 9 shows the viscoelastic properties as a function of temperature, including storage modulus and tan δ characterized by DMA for the pMRGO/epoxy composites with different loading of the filler (Fig. S3 shows the DMA results of RGO/epoxy composites). Storage moduli of the composites gradually improved from 2.1 GPa to 2.8 GPa with adding p-MRGO. The increased storage modulus is attributed to the nanofiller reinforcement effect, which has been confirmed for previously reported polymer composites [63,64]. Moreover, the dispersibility of the filler and the strong interfacial adhesion with the matrix contributed to more effective improvement. However, more p-MRGOs resulted in a decrease in the storage modulus of the composite, which can be explained by the change in crosslinking density. With increasing temperature, the tran­ sition of the polymer chains started from a glassy state to a rubbery state during which the storage moduli sharply decreased owing to the energy dissipation. From the following results, tan δ, is typically utilized to determine Tg. In Fig. 9b, As the filler was added, the Tg values of p-MRGO/epoxy composites increased but shifted to a lower temperature at 7 wt% p-MRGO. Although p-MRGO improved the interfacial inter­ action with the matrix, the flexible MPTES-PGMA chains grafted onto the filler surface caused a decrease in Tg [63]. In addition, the cross­ linking density affected to the polymer structure was calculated by the following equation [65]:

ρ¼

4. Conclusion In this study, p-MRGO/epoxy composites are fabricated as a result of the co-curing effect to improve the thermal conductivity. By using a pMRGO loading of 7 wt%, the thermal conductivity of the composites is 0.75 W m 1 K 1, corresponding to a TCEF of 249%. MRGO is synthe­ sized by a sol–gel reaction using MPTES for the introduction of meth­ acrylate groups. Then, PGMA chains are grafted onto MRGO by radical polymerization to afford p-MRGO, which can be co-cured with DDM as the curing agent in the matrix. Hence, p-MRGO linked by covalent bonds to the matrix exhibits increased interfacial adhesion with the matrix as well as a well-dispersed state. In addition, shear moduli are investigated to obtain the crosslinking density of the p-MRGO/epoxy composites, which increases by the addition of p-MRGO but decreases with a loading of greater than 3 wt% due to saturation. Hence, the approach for incorporating the filler into the matrix by the co-curing reaction is effective for improving the thermal conductivity of composites. Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2017R1A2A2A05069858). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.polymer.2019.121834. References [1] P. Tao, W. Shang, C. Song, Q. Shen, F. Zhang, Z. Luo, et al., Bioinspired engineering of thermal materials, Adv. Mater. 27 (2015) 428–463. [2] J. Yang, Y. Yang, S.W. Waltermire, X. Wu, H. Zhang, T. Gutu, et al., Enhanced and switchable nanoscale thermal conduction due to van der Waals interfaces, Nat. Nanotechnol. 7 (2012) 91. [3] P.E. Hopkins, M. Baraket, E.V. Barnat, T.E. Beechem, S.P. Kearney, J.C. Duda, et al., Manipulating thermal conductance at metal–graphene contacts via chemical functionalization, Nano Lett. 12 (2012) 590–595. [4] C. Zweben, Ultrahigh-thermal-conductivity packaging materials, in: 21st IEEE SEMI-THERM Symp. San Jose (California, USA): 21st IEEE SEMI-THERM Symp. Proc, 2005, pp. 168–174. [5] K.M. Shahil, A.A. Balandin, Graphene–multilayer graphene nanocomposites as highly efficient thermal interface materials, Nano Lett. 12 (2012) 861–867. [6] A. Yu, P. Ramesh, M.E. Itkis, E. Bekyarova, R.C. Haddon, Graphite nanoplatelet epoxy composite thermal interface materials, J. Phys. Chem. C 111 (2007) 7565–7569. [7] V. Goyal, A.A. Balandin, Thermal properties of the hybrid graphene-metal nanomicro-composites: applications in thermal interface materials, Appl. Phys. Lett. 100 (2012) 073113. [8] N. Burger, A. Laachachi, M. Ferriol, M. Lutz, V. Toniazzo, D. Ruch, Review of thermal conductivity in composites: mechanisms, parameters and theory, Prog. Polym. Sci. 61 (2016) 1–28. [9] D. Chung, Materials for thermal conduction, Appl. Therm. Eng. 21 (2001) 1593–1605. [10] H. Chen, V.V. Ginzburg, J. Yang, Y. Yang, W. Liu, Y. Huang, et al., Thermal conductivity of polymer-based composites: fundamentals and applications, Prog. Polym. Sci. 59 (2016) 41–85.

G’ 3RT

where ρ, G’, R and T are the crosslinking density (mol/m3), shear modulus (Pa) which is storage modulus at Tgþ30, universal gas constant (8.314 J mol 1 K 1) and absolute temperature that was shear modulus determined. In Fig. 9c, by the addition of the filler, the crosslinking densities of the p-MRGO/epoxy composites increased. As confirmed by the DSC results, p-MRGO reacted with the amine groups during the curing reaction to form a crosslinked structure within the matrix. However, by the incorporation of greater than 3 wt% of p-MRGO, the stoichiometry between the epoxide and amine groups was off balance; thus, the crosslinking density of the composites decreases. As a result, pMRGO is confirmed to be an effective filler in the co-cured epoxy composite system, and it is successfully incorporated into the composite. 8

H. Oh et al.

Polymer 183 (2019) 121834

[11] K. Ahn, K. Kim, J. Kim, Thermal conductivity and electric properties of epoxy composites filled with TiO2-coated copper nanowire, Polymer 76 (2015) 313–320. [12] I. Tavman, Thermal and mechanical properties of aluminum powder-filled highdensity polyethylene composites, J. Appl. Polym. Sci. 62 (1996) 2161–2167. [13] H. Jiang, K.-s. Moon, Y. Li, C. Wong, Surface functionalized silver nanoparticles for ultrahigh conductive polymer composites, Chem. Mater. 18 (2006) 2969–2973. [14] Y. Hu, G. Du, N. Chen, A novel approach for Al2O3/epoxy composites with high strength and thermal conductivity, Compos. Sci. Technol. 124 (2016) 36–43. [15] T.M.L. Dang, C.-Y. Kim, Y. Zhang, J.-F. Yang, T. Masaki, D.-H. Yoon, Enhanced thermal conductivity of polymer composites via hybrid fillers of anisotropic aluminum nitride whiskers and isotropic spheres, Compos. B Eng. 114 (2017) 237–246. [16] W. Dai, J. Yu, Y. Wang, Y. Song, F.E. Alam, K. Nishimura, C.-T. Lin, N. Jiang, Enhanced thermal conductivity for polyimide composites with a three-dimensional silicon carbide nanowire@ graphene sheets filler, J. Mater. Chem. A 3 (2015) 4884–4891. [17] J. Han, G. Du, W. Gao, H. Bai, An anisotropically high thermal conductive boron nitride/epoxy composite based on nacre-mimetic 3D network, Adv. Funct. Mater. (2019) 1900412. [18] X. Huang, C. Zhi, P. Jiang, D. Golberg, Y. Bando, T. Tanaka, Polyhedral oligosilsesquioxane-modified boron nitride nanotube based epoxy nanocomposites: an ideal dielectric material with high thermal conductivity, Adv. Funct. Mater. 23 (2013) 1824–1831. [19] A. Rybak, K. Gaska, C. Kapusta, F. Toche, V. Salles, Epoxy composites with ceramic core–shell fillers for thermal management in electrical devices, Polym. Adv. Technol. 28 (2017) 1676–1682. [20] A. Rybak, K. Gaska, Functional composites with core–shell fillers: I. Particle synthesis and thermal conductivity measurements, J. Mater. Sci. 50 (2015) 7779–7789. [21] A. Rybak, J. Nieroda, Aluminosilicate-epoxy resin composite as novel material for electrical insulation with enhanced mechanical properties and improved thermal conductivity, Polym. Compos. 40 (2019) 3182–3188. [22] A. Rybak, L. Jarosinski, K. Gaska, C. Kapusta, Graphene nanoplatelet-silica hybrid epoxy composites as electrical insulation with enhanced thermal conductivity, Polym. Compos. 39 (2018) E1682–E1691. [23] J. Gu, X. Yang, Z. Lv, N. Li, C. Liang, Q. Zhang, Functionalized graphite nanoplatelets/epoxy resin nanocomposites with high thermal conductivity, Int. J. Heat Mass Transf. 92 (2016) 15–22. [24] I. Kholmanov, J. Kim, E. Ou, R.S. Ruoff, L. Shi, Continuous carbon nanotube–ultrathin graphite hybrid foams for increased thermal conductivity and suppressed subcooling in composite phase change materials, ACS Nano 9 (2015) 11699–11707. [25] W.-b. Zhang, Z.-x. Zhang, J.-h. Yang, T. Huang, N. Zhang, X.-t. Zheng, et al., Largely enhanced thermal conductivity of poly (vinylidene fluoride)/carbon nanotube composites achieved by adding graphene oxide, Carbon 90 (2015) 242–254. [26] Y.-H. Zhao, Y.-F. Zhang, S.-L. Bai, X.-W. Yuan, Carbon fibre/graphene foam/ polymer composites with enhanced mechanical and thermal properties, Compos. B Eng. 94 (2016) 102–108. [27] J. Yang, E. Zhang, X. Li, Y. Zhang, J. Qu, Z.-Z. Yu, Cellulose/graphene aerogel supported phase change composites with high thermal conductivity and good shape stability for thermal energy storage, Carbon 98 (2016) 50–57. [28] A.A. Balandin, Thermal properties of graphene and nanostructured carbon materials, Nat. Mater. 10 (2011) 569. [29] J. Tong, H.-X. Huang, M. Wu, Simultaneously facilitating dispersion and thermal reduction of graphene oxide to enhance thermal conductivity of poly (vinylidene fluoride)/graphene nanocomposites by water in continuous extrusion, Chem. Eng. J. 348 (2018) 693–703. [30] S. Jiang, Z. Gui, C. Bao, K. Dai, X. Wang, K. Zhou, et al., Preparation of functionalized graphene by simultaneous reduction and surface modification and its polymethyl methacrylate composites through latex technology and melt blending, Chem. Eng. J. 226 (2013) 326–335. [31] X. Wang, H. Bai, Z. Yao, A. Liu, G. Shi, Electrically conductive and mechanically strong biomimetic chitosan/reduced graphene oxide composite films, J. Mater. Chem. 20 (2010) 9032–9036. [32] M. Fang, K. Wang, H. Lu, Y. Yang, S. Nutt, Covalent polymer functionalization of graphene nanosheets and mechanical properties of composites, J. Mater. Chem. 19 (2009) 7098–7105. [33] L.-C. Tang, Y.-J. Wan, D. Yan, Y.-B. Pei, L. Zhao, Y.-B. Li, et al., The effect of graphene dispersion on the mechanical properties of graphene/epoxy composites, Carbon 60 (2013) 16–27. [34] H. Wu, W. Zhao, H. Hu, G. Chen, One-step in situ ball milling synthesis of polymerfunctionalized graphene nanocomposites, J. Mater. Chem. 21 (2011) 8626–8632. [35] S. Chatterjee, J. Wang, W. Kuo, N. Tai, C. Salzmann, W. Li, et al., Mechanical reinforcement and thermal conductivity in expanded graphene nanoplatelets reinforced epoxy composites, Chem. Phys. Lett. 531 (2012) 6–10. [36] F. Wang, L.T. Drzal, Y. Qin, Z. Huang, Mechanical properties and thermal conductivity of graphene nanoplatelet/epoxy composites, J. Mater. Sci. 50 (2015) 1082–1093. [37] C. Liu, G. Hu, Highly efficient reduction of graphene oxide by sub/supercritical water and their application for thermal interface materials, Appl. Therm. Eng. 90 (2015) 193–198. [38] S.H. Song, K.H. Park, B.H. Kim, Y.W. Choi, G.H. Jun, D.J. Lee, et al., Enhanced thermal conductivity of epoxy–graphene composites by using non-oxidized

[39] [40] [41] [42] [43]

[44] [45] [46] [47] [48] [49] [50] [51]

[52] [53] [54]

[55] [56] [57]

[58] [59] [60] [61] [62] [63] [64] [65]

9

graphene flakes with non-covalent functionalization, Adv. Mater. 25 (2013) 732–737. Y.-J. Wan, L.-C. Tang, L.-X. Gong, D. Yan, Y.-B. Li, L.-B. Wu, et al., Grafting of epoxy chains onto graphene oxide for epoxy composites with improved mechanical and thermal properties, Carbon 69 (2014) 467–480. T. Kuila, S. Bose, C.E. Hong, M.E. Uddin, P. Khanra, N.H. Kim, et al., Preparation of functionalized graphene/linear low density polyethylene composites by a solution mixing method, Carbon 49 (2011) 1033–1037. M. Fang, K. Wang, H. Lu, Y. Yang, S. Nutt, Covalent polymer functionalization of graphene nanosheets and mechanical properties of composites, J. Mater. Chem. 19 (2009) 7098–7105. D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, et al., Improved synthesis of graphene oxide, ACS Nano 4 (2010) 4806–4814. S. Zhang, P. Liu, X. Zhao, J. Xu, Preparation of poly (vinyl alcohol)-grafted graphene oxide/poly (vinyl alcohol) nanocomposites via in-situ low-temperature emulsion polymerization and their thermal and mechanical characterization, Appl. Surf. Sci. 396 (2017) 1098–1107. C. Cha, S.R. Shin, X. Gao, N. Annabi, M.R. Dokmeci, X. Tang, A. Khademhosseini, Controlling mechanical properties of cell-laden hydrogels by covalent incorporation of graphene oxide, Small 10 (2014) 514–523. S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, et al., Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon 45 (2007) 1558–1565. V.C. Tung, M.J. Allen, Y. Yang, R.B. Kaner, High-throughput solution processing of large-scale graphene, Nat. Nanotechnol. 4 (2009) 25. J. Lin, P. Zhang, C. Zheng, X. Wu, T. Mao, M. Zhu, et al., Reduced silanized graphene oxide/epoxy-polyurethane composites with enhanced thermal and mechanical properties, Appl. Surf. Sci. 316 (2014) 114–123. Y.-J. Wan, L.-C. Tang, D. Yan, L. Zhao, Y.-B. Li, L.-B. Wu, J.-X. Jiang, G.-Q. Lai, Improved dispersion and interface in the graphene/epoxy composites via a facile surfactant-assisted process, Compos. Sci. Technol. 82 (2013) 60–68. M. Khan, A.H. Al-Marri, M. Khan, N. Mohri, S.F. Adil, A. Al-Warthan, et al., Pulicaria glutinosa plant extract: a green and eco-friendly reducing agent for the preparation of highly reduced graphene oxide, RSC Adv. 4 (2014) 24119–24125. M.-S. Lin, S.-T. Lee, Curing behaviour of fully and semi-interpenetrating polymer networks based on epoxy and acrylics, Polymer 36 (1995) 4567–4572. J. Tong, H.-X. Huang, M. Wu, Simultaneously facilitating dispersion and thermal reduction of graphene oxide to enhance thermal conductivity of poly (vinylidene fluoride)/graphene nanocomposites by water in continuous extrusion, Chem. Eng. J. 348 (2018) 693–703. F.E. Alam, W. Dai, M. Yang, S. Du, X. Li, J. Yu, N. Jiang, et al., In situ formation of a cellular graphene framework in thermoplastic composites leading to superior thermal conductivity, J. Mater. Chem. A 5 (2017) 6164–6169. S. Chatterjee, J. Wang, W. Kuo, N. Tai, C. Salzmann, W. Li, et al., Mechanical reinforcement and thermal conductivity in expanded graphene nanoplatelets reinforced epoxy composites, Chem. Phys. Lett. 531 (2012) 6–10. M.-C. Hsiao, C.-C.M. Ma, J.-C. Chiang, K.-K. Ho, T.-Y. Chou, X. Xie, et al., Thermally conductive and electrically insulating epoxy nanocomposites with thermally reduced graphene oxide–silica hybrid nanosheets, Nanoscale 5 (2013) 5863–5871. A. Rybak, in: S. Kenig (Ed.), Processing Influence on Thermal Conductivity of Polymer Nanocomposites, Process. Of Polym. Nanocompos, Hanser, 2019, pp. 463–487. C. Yu, W. Gong, W. Tian, Q. Zhang, Y. Xu, Z. Lin, et al., Hot-pressing induced alignment of boron nitride in polyurethane for composite films with thermal conductivity over 50 W m 1 K 1, Compos. Sci. Technol. 160 (2018) 199–207. F. Kargar, Z. Barani, R. Salgado, B. Debnath, J.S. Lewis, E. Aytan, R.K. Lake, A. A. Balandin, Thermal percolation threshold and thermal properties of composites with high loading of graphene and boron nitride fillers, ACS Appl. Mater. Interfaces 10 (2018) 37555–37565. F. Kargar, Z. Barani, M. Balinskiy, A.S. Magana, J.S. Lewis, A.A. Balandin, Dualfunctional graphene composites for electromagnetic shielding and thermal management, Adv. Electron. Mater. 5 (2019) 1800558. J.S. Lewis, Z. Barani, A.S. Magana, F. Kargar, A.A. Balandin, Thermal and electrical conductivity control in hybrid composites with graphene and boron nitride fillers, Mater. Res. Express 6 (2019) 085325. M. Saadah, E. Hernandez, A. Balandin, Thermal management of concentrated multi-junction solar cells with graphene-enhanced thermal interface materials, Appl. Sci. 7 (2017) 589. F. Wang, L.T. Drzal, Y. Qin, Z. Huang, Mechanical properties and thermal conductivity of graphene nanoplatelet/epoxy composites, J. Mater. Sci. 50 (2015) 1082–1093. L. Jarosinski, A. Rybak, K. Gaska, G. Kmita, R. Porebska, C. Kapusta, Enhanced thermal conductivity of graphene nanoplatelets epoxy composites, Mater. Sci. 35 (2017) 382–389. W. Liu, K.L. Koh, J. Lu, L. Yang, S. Phua, J. Kong, et al., Simultaneous catalyzing and reinforcing effects of imidazole-functionalized graphene in anhydride-cured epoxies, J. Mater. Chem. 22 (2012) 18395–18402. Y.-J. Wan, L.-C. Tang, L.-X. Gong, D. Yan, Y.-B. Li, L.-B. Wu, et al., Grafting of epoxy chains onto graphene oxide for epoxy composites with improved mechanical and thermal properties, Carbon 69 (2014) 467–480. H. Miyagawa, A.K. Mohanty, M. Misra, L.T. Drzal, Thermo-physical and impact properties of epoxy containing epoxidized linseed oil, 1, Macromol. Mater. Eng. 289 (2004) 629–635.