A novel approach to enhance the thermal conductivity of epoxy nanocomposites using graphene core–shell additives

Accepted Manuscript A Novel Approach to Enhance the Thermal Conductivity of Epoxy Nanocomposites Using Graphene Core-Shell Additives Osman Eksik, Stephen F. Bartolucci, Tushar Gupta, Hafez Fard, Theodorian BorcaTasciuc, Nikhil Koratkar PII:

S0008-6223(16)30083-5

DOI:

10.1016/j.carbon.2016.01.095

Reference:

CARBON 10715

To appear in:

Carbon

Received Date: 4 December 2015 Revised Date:

25 January 2016

Accepted Date: 28 January 2016

Please cite this article as: O. Eksik, S.F. Bartolucci, T. Gupta, H. Fard, T. Borca-Tasciuc, N. Koratkar, A Novel Approach to Enhance the Thermal Conductivity of Epoxy Nanocomposites Using Graphene CoreShell Additives, Carbon (2016), doi: 10.1016/j.carbon.2016.01.095. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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A Novel Approach to Enhance the Thermal Conductivity of Epoxy Nanocomposites Using Graphene Core-Shell Additives

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Osman Eksik1, Stephen F. Bartolucci2 , Tushar Gupta1, Hafez Fard1, Theodorian Borca-Tasciuc1, and Nikhil Koratkar1* 1

Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy NY 11280

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U.S. Army Armaments Research Development and Engineering Center, Benet Laboratories, Watervliet, NY, 12189, United States

*Corresponding Author

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Nikhil Koratkar; [email protected] Ph: 518-26-2630; Fax: 518-276-2623

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2

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Abstract

We report on a novel technique to enhance the thermal conductivity of epoxy nanocomposites using graphene coated poly-methyl-methacrylate (PMMA) balls. PMMA balls with a diameter in the range of 200-300 nm were synthesized using suspension polymerization.

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These balls were coated with chemically reduced graphene oxide to form a core-shell additive

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and dispersed in epoxy. Thermal conductivity measurements of bulk samples of graphene-coated PMMA (GPMMA) epoxy nanocomposite were carried out and compared with baseline samples comprised of graphene nanosheets (not in the core-shell form) dispersed in the epoxy resin. Results show that the addition of 1% (by weight) GPMMA balls increases thermal conductivity by 7-fold. By contrast, the addition of 1% (by weight) of graphene nanosheets (not in the core-

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shell form) only improves thermal conductivity of the nanocomposite by approximately 3-fold. We attribute this improvement in thermal performance to more uniform dispersion and improved

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phonon conduction pathways for the GPMMA core-shell additive.

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Introduction

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Thermally conductive polymers have become increasingly important in high-density electronic devices for heat dissipation applications, due to their light weight, good chemical resistance and low-cost fabrication [1, 2]. Unfortunately, the thermal conductivity of

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conventional polymer materials is low, which restricts their applications. Thus, developing polymer-based composites with high thermal conductivity and low fabrication cost is of

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paramount importance [3]. With the aim of producing polymers with higher thermal conductivity, various fillers such as carbon black, aluminum oxide (Al2O3), silicon dioxide (SiO2), aluminum nitride (AlN), boron nitride (BN), copper (Cu), nickel (Ni) and zinc oxide (ZnO) powders have been added to polymers [4-11]. Typically, high thermal conductivity can be obtained by adding a

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high loading of the filler (volume fraction > 50%). However, the high filler content results in significant weight penalty and poor mechanical properties of the polymer [12-14].

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A variety of nano-carbon materials, such as multi-walled carbon nanotubes (MWCNT), single-walled carbon nanotubes (SWCNT), fullerenes, carbon black, and graphite nano-powder

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have been investigated as thermally conductive fillers, due to their low specific gravity and high thermal conductivity [15-22]. Pak et al. investigated the thermal conductivity of polyphenylene sulfide (PPS) composites with a mixture of boron nitride (BN) powder and MWCNT [20] and reported that the thermal conductivity of PPS with 50 wt% BN was 1.00 W/(m-K), while that of PPS composites with 50 wt% BN and 1 wt% MWCNT was increased to 1.45 W/(m-K). Similarly Teng et al. reported [22] that epoxy nanocomposites containing 25 vol.% modified AlN and 1 vol.% functionalized MWCNTs could achieve a thermal conductivity of 1.21 W/(m-K), which is

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comparable to that of epoxy composites containing 50 vol.% untreated AlN (1.25 W/(m-K)), which translates to a 50% reduction in the amount of AlN filler used.

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Building on the success of carbon nanotube fillers, recent years have seen intense activity directed towards the use of graphene nanosheets as a thermally conductive additive in polymer nanocomposites [23-25]. Graphene possesses large surface area, high thermal conductivity and

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has a high aspect ratio 2D structure [25], which facilitates the formation of bridges or percolating networks in polymers, creating a thermally conducting chain. The formation of random bridges or

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networks can facilitate phonon transfer and improve the thermal conductivity of polymer composites. In this work, we present an alternative approach in which the graphene additive is not directly dispersed in the polymer, rather it is coated onto PMMA balls forming a core-shell additive (GPMMA). These additives are then uniformly dispersed in the polymer matrix. The

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rationale behind this approach is to address the problems of filler dispersion and phonon scattering at graphene-polymer interfaces. Although graphene has very high intrinsic thermal conductivity, it tends to agglomerate, leading to poor dispersion in polymer matrices and high

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resistance to photon transport at the graphene-polymer interfaces. Here we demonstrate that organic PMMA particles coated with graphene can be employed to achieve good dispersion.

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Further, the graphene skin coating on the GPMMA particle enables the establishment of longrange thermally-conductive pathways that reduce phonon scattering, resulting in a significant improvement in the thermal conductivity of the epoxy nanocomposite.

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Materials

The epoxy resin (Epon 862: diglycidyl ether of bisphenol-F) was provided by Miller-Stephenson

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Chemical Co., Inc. Expandable graphite (Dixon) was provided by Asbury Carbon. Methyl methacrylate (MMA, Aldrich) was purified by being washed three times with 10% (w/w) sodium hydroxide solution (MMA to NaOH (v/v) = 10 : 1), followed by repeated washing with deionized

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water (MMA to water (v/v)= 5 : 1) until the pH of the water dropped to 7. Concentrated sulfuric acid (H2SO4), potassium persulfate (K2S2O8), phosphorus pentoxide (P2O5) potassium

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permanganate (KMnO4), hydrochloric acid (HCl), and hydrogen peroxide (H2O2) and 2,2Azobis(2-methylpropionamidine) dihydrochloride were purchased from Sigma Aldrich. These chemicals were used as received.

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Experimental

Preparation of GO Dispersion

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Graphite oxide (GO) was prepared by modified Hummers method: Graphite powder (325 mesh) was put into a mixture of concentrated H2SO4, K2S2O8 and P2O5. The solution was heated to 90

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°C while being stirred. Next, the mixture was diluted with deionized water and cooled to room temperature overnight. The pre-oxidized graphite was then re-oxidized by Hummers method. Pretreated graphite powder was put into concentrated H2SO4, followed by the gradual addition of KMnO4 under stirring. Successively, the mixture was stirred and then diluted with deionized water, followed by adding H2O2 drop by drop. The mixture was filtered and washed with HCl aqueous solution to remove metal ions, followed by ~1 L of deionized water to remove the acid.

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The resulting solid was dried in air and diluted in deionized water to make graphene oxide dispersion. The synthesized GO was dispersed in deionized water by sonicating in an ultrasonic

sonication, a clear brown dispersion of graphene oxide was formed.

Preparation of Positively Charged PMMA

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bath (Sonics Vibra-cell VC 750 tip sonicator, power ~750 W, frequency ~ 20 kHz). After

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Positively charged PMMA beads were prepared by surfactant-free emulsion polymerization. To obtain nano-sized positively charged PMMA, we first added ~87.5 mL of deionized water to the

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reactor, followed by deoxygenating by extensive bubbling with nitrogen for ~30 min. Subsequently, ~10 g of MMA were added into the reactor. The mixture was stirred at ~350 rpm and bubbled with nitrogen again for ~30 min. The temperature was increased to ~70 °C, followed by the addition of ~0.15 g of 2,2-Azobis(2-methylpropionamidine) dihydrochloride dissolved in

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~25 mL of deionized water. The polymerization was carried out under stirring for ~6 h.

Assembly of PMMA-Reduced Graphene Oxide Composite

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Positively charged PMMA (~10 g) and latex were gradually added, to the desired volume, to the negatively charged graphene oxide (GO) dispersion (~1 mg/mL) under vigorous stirring for ~30

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min. The colloidal suspension PMMA-GO composite was reduced with hydrazine (GO/hydrazine 1:10 w/w) at ~80 °C for ~3 h. The obtained colloidal suspension of GPMMA material was filtered, washed with methanol 3 times, and dried at ~80 °C for ~12 h. GPMMA material was then crushed into a fine powder using a mortar and pestle. Samples with various weight fractions of graphene, relative to the PMMA, were fabricated (0, 1, 2, 4, 8 and 16 wt% graphene). This process is illustrated in Figure 1.

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Epoxy Nanocomposite Fabrication The composites, containing nanoparticles of GPMMA or graphene platelets (GPL), were prepared similarly to a method previously published [26]. The nanoparticles were dispersed in

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acetone by ultrasonication at room temperature to break up any agglomerates. The nanoparticles were dispersed in epoxy resin by tip sonication. Afterwards, the solvent was evaporated in a vacuum oven and the composite was subsequently mixed with hardener via shear force mixing.

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The mixture was cast into aluminum molds treated with a release agent. Finally, the composite was cured at ~80 °C for ~8 hours. Samples with ~0.1, ~0.5, and ~1.0 wt% total graphene content

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were fabricated with GPMMA as well as regular graphene platelet (GPL) additives.

Figure 1. Schematic fabrication of epoxy nanocomposite

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Scanning Electron Microscopy The GPMMA balls were examined using an FEI Helios Nanolab 600i Dual Beam FIB/FE-SEM system operating at ~5 kV.

Composite samples were cryo-fractured and the surface was

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examined using scanning electron microscopy. The fracture surface of the GPMMA/epoxy composite was examined using a Carl Zeiss Ultra 1540 Dual Beam FIB/SEM system operating

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at ~3.00 kV with 0° tilt in low vacuum conditions.

Thermogravimetric Analysis (TGA)

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The thermal stability of the GPMMA materials was studied using a Perkin-Elmer TGA 7 with a platinum pan. The samples were approximately 10 mg each and were run from ambient to ~800 °C at a heating rate of ~10 °C/min in air.

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Thermal Conductivity Measurement

To measure the thermal conductivity of the nanocomposites, a steady-state one-dimensional heat conduction method [25] was used. The experimental setup consists of an electrical heater, a heat

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sink and two thermocouples to measure the temperature gradient (Figure 2). To minimize the interfacial thermal resistance, fine diameter electrically insulated thermocouples were embedded

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into two soft indium layers to measure the temperature at both sides of a thin cylindrical sample. Pressure is applied using a screw mechanism that is thermally insulated from the sample by a thick Teflon block. The heat losses in the experimental setup were calibrated using glass samples of known conductivity.

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Figure 2. Experimental setup for thermal conductivity measurement

Thermal Stability

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Result and Discussion

The incorporation of varying amounts of graphene on the surfaces of the PMMA balls

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enhances the thermal stability of the resultant material, as seen in Figure 3. In all the samples, one-step degradation was observed. We compare thermal stability of the various samples (Table

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1) by comparing the onset temperature and peak degradation in the temperature range of 280–356 °C. It is clear that on incorporation of graphene around the PMMA balls, the thermal stability is enhanced at each weight fraction of graphene, with the best performance being achieved with ~1 wt% graphene. At higher graphene loading, the degradation temperature gradually decreases and approaches the performance of the neat PMMA.

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It should be noted that in terms of the thermal degradation performance of the GPMMA, there are two competing effects at play. First, the graphene sheets create a barrier effect, slowing the diffusion of oxygen (air) to the PMMA, thereby slowing the thermal degradation. However, at

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the very high graphene content, the thermal conductivity becomes much higher, allowing heat to diffuse to the PMMA more rapidly and uniformly, resulting in enhanced degradation. This explains why the thermal stability of the GPMMA begins to deteriorate at the high graphene

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loadings.

Figure 3. TGA plot of neat PMMA and GPMMA materials with various graphene contents (wt%) heated in air at ~10 ˚C/min.

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Table 1: Onset degradation and peak degradation rate temperature 10° C / min Materials

20° C / min

Peak °C

Onset °C

Peak °C

PMMA-0%

289

320

299

349

GPMMA-1%

344

396

356

GPMMA-2%

329

381

345

GPMMA-4%

325

380

336

GPMMA-8%

321

376

335

392

GPMMA-16%

303

347

312

362

416 393

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Morphological Characterization

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Onset °C

In order to investigate the morphology of neat PMMA and GPMMA materials, and to

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look into finer details of the microstructure, sanning electron microscopy (SEM) was performed. Figure 4a-f shows SEM images of PMMA and GPMMA materials (1, 2, 8 and 16 wt% graphene). The neat PMMA (Fig. 4a) shows a smooth surface. In comparison, GPMMA materials

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(Fig. 4b, c, d and e) show a relatively rough texture due to the wrinkled and crumpled graphene sheets that decorate the surfaces of the PMMA balls. For preperation of epoxy nanocomposites

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we selected the 16wt% GPMMA additive (Figure 4e-f) due to its high graphene content. Transmisison electron microscopy (TEM) imaging of the 16wt% GPMMA additive was also carried out as shown in Fig. 4g; these images clearly indicate the “core-shell” structure of the graphene-encapsulated PMMA. We also studied the dispersion of the GPMMA additives in the epoxy matrix by SEM imaging. For this, the epoxy nanocomposite samples were freeze-fractured and we characterized the fracture surfaces. Figure 5 shows the typical microstructure for the ~1 wt% GPMMA-epoxy

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nanocomnposite showing spherical GPMMA balls that are dispersed in the epoxy matrix. Note that for the ~1 wt% of GPMMA-epoxy composite, the loading fraction of graphene sheets is ~1 wt% with respect to the total composite mass and is ~16 wt% with respect to the PMMA

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mass alone.

Figure 4. SEM images of (a) Pure PMMA, (b) GPMMA material (1 wt% graphene), (c) GPMMA material (2 wt% graphene), (d) GPMMA material (8 wt% graphene), (e) GPMMA material (16 wt% graphene), (f) higher magnification of 16 wt% graphene material, (g) transmission electron micrographs of GPMMA (16 wt% graphene) showing core-shell structure.

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Figure 5: SEM image of the fracture surface of 1 % by weight GPMMA/epoxy composite. The

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total graphene content in the composite is 1 wt%. The scale bar is 400 nm.

Figure 6. Measured thermal conductivities of the GPMMA/epoxy and GPL/epoxy composites.

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Thermal Conductivity Graphene nanoplatelets (GPL) and GPMMA core-shell particles were dispersed in an epoxy polymer (Epon 862: diglycidyl ether of bisphenol-F) at loading fractions of up to ~1

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weight %. Note that loading fraction refers to the total graphene content (weight fraction) in the composite. To compare the effect of graphene, we maintain the amount of graphene in each composite (i.e. GPL/epoxy and GPMMA/epoxy), to be the same. To measure the thermal

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conductivity of these composites, a steady-state one-dimensional heat conduction method [25] was used. The thermal conductivity of the neat epoxy measured with this setup (~0.20 W/m-K)

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matches the value reported in the literature [25].

The thermal transport characterization results are shown in Figure 6. As expected, the thermal conductivity of the epoxy nanocomposites are considerably enhanced by the presence of graphene and graphene-coated PMMA balls. It can be seen that the addition of 1% (by weight) of

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GPMMA balls increased thermal conductivity by ~7-fold, while the addition of 1% (by weight) of GPL (i.e. not in core-shell form) only improves thermal conductivity by ~ 3-fold. It should be noted that since PMMA has a low thermal conductivity (similar to the epoxy), the PMMA is not

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responsible for the increased thermal conductivity of the composite. The enhanced thermal conductivity can be attributed to two key effects: (1) the GPMMA balls are well dispersed and do

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not aggregate in the epoxy polymer (Figure 5) and (2) the GPMMA balls provide a path of lower resistance for phonons to travel. We have illustrated this schematically in Figure 7b, where the graphene core-shell particles create a more contiguous phonon conduction pathway, as compared to the conventional case (Figure 7a), where phonon scattering at graphene-polymer interfaces limits the phonon transport. The mean-free path (Λ) of phonons in a material is affected by various scattering processes that can occur during phonon conduction. The heat-carrying acoustic phonons, are scattered by, among other things, interfaces. The interface between two materials

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has a thermal boundary resistance (TBR), or Kapitza resistance [27], resulting in a temperature drop when a heat flux flows through the boundary. This can be due to phonon scattering, which is a result of mismatched acoustic properties. A graphene-graphene conduction path is expected to

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lead to a lower thermal resistance than the graphene-polymer interface, due to lower mismatch in acoustic properties across the interface. Given the higher number of graphene-graphene conduction paths for the GPMMA-epoxy composite, we hypothesize that this leads to a lower

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TBR, and hence, higher thermal conductivity for this composite.

(b)

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(a)

Figure 7. Schematic representation of the mechanism by which the GPMMA/epoxy composite (b) has improved thermal conductivity relative to the GPL/epoxy composite (a) for the same graphene content.

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Conclusions

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In this study, we fabricated conventional epoxy nanocomposites with graphene additives as well as epoxy nanocomposites with novel graphene core-shell additives (i.e. graphene skin on PMMA balls). We showed that adding the graphene-coated PMMA balls to the epoxy polymer

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matrix significantly boosts thermal conductivity. A composite with 1% (by weight) GPMMA balls increased thermal conductivity by ~7-fold, while the incorporation of 1% (by weight) of

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graphene platelets improved the thermal conductivity of the composite by ~ 3-fold. This shows that the organic PMMA nanoparticles coated with graphene could be employed to promote welldispersed conditions (with reduced phonon scattering at the graphene-polymer interfaces). Our results indicate that GPMMA are an effective additive for enhancing the thermal conductivity of

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thermal interface materials.

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epoxy thermal adhesive materials and has the potential to be applied to other types of polymers or

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