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Massive enhancement in the thermal conductivity of polymer composites by trapping graphene at the interface of a polymer blend
Composites Science and Technology 129 (2016) 160e165
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Massive enhancement in the thermal conductivity of polymer composites by trapping graphene at the interface of a polymer blend Jinrui Huang a, *, Yutian Zhu b, **, Lina Xu a, Jianwen Chen b, Wei Jiang b, Xiaoan Nie a, c, *** a Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry; National Engineering Laboratory for Biomass Chemical Utilization; Key and Open Laboratory on Forest Chemical Engineering, State Forestry Administration; Key Laboratory of Biomass Energy and Material, Nanjing, Jiangsu 210042, China b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China c Institute of New Technology of Forestry, Chinese Academy of Forestry, Beijing 100091, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2016 Received in revised form 19 April 2016 Accepted 25 April 2016 Available online 27 April 2016
Trapping graphene at the interface of a poly(ε-caprolactone) (PCL)/poly(lactic acid) (PLA) blend is achieved through the adsorption-desorption of polymer chains on the graphene surface. The resulting composite exhibits remarkably high thermal conductivity due to the graphene sheets being controlled at the interface of the polymer blend. At 0.53 vol%, when a good co-continuous structure is formed, the thermal conductivity of the graphene composite is nearly 4 times higher than that of the pure PCL/PLA blend with co-continuous structure. Moreover, the blend achieves an extremely low thermal percolation threshold (0.11 vol%), the lowest to date, because most of the graphene sheets are selectively located at the interface of the blend with co-continuous structure. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Polymer-matrix composites (PMCs) Thermal conductivity Interface Electrical properties
1. Introduction Continued miniaturization of electronic devices requires increasingly efficient heat dissipation for reliable performance in modern electronic systems [1]. Research interest has been sparked in recent years in the potential of thermally conductive polymer composites (TCPCs) to replace thermally conductive materials (e.g., metals) as heat removers in a variety of applications (e.g., electric motors, electric generators, power electronics, heat exchangers) [2]. However, very high loading of conventional thermally conductive fillers (>30 vol%) is usually needed to obtain appropriate thermal conductivity due to the interfacial thermal resistance between the filler and the polymer matrix [2e5]. Clearly, high filler loading reduces product quality through diminished mechanical properties,
* Corresponding author. Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, Nanjing 210042, China. ** Corresponding author. Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. *** Corresponding author. Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, Nanjing 210042, China. E-mail addresses: [email protected] (J. Huang), [email protected] (Y. Zhu), [email protected] (X. Nie). http://dx.doi.org/10.1016/j.compscitech.2016.04.029 0266-3538/© 2016 Elsevier Ltd. All rights reserved.
increased weight, reduced flexibility, and increased fragility. Furthermore, high loading increases the difficulty and cost of production. Thus, a method of TCPC fabrication with high thermal conductivity at low filler loading is needed. Graphene (GE), discovered by Novoselov in 2004 [6], has received great attention owing to its exceptional mechanical, electrical, and thermal properties [7]. The extraordinary thermal conductivity (5300 W m1 K1) [8] and extremely high surface area (theoretical limit 2630 m2/g) [9] of GE have made it the most promising candidate for fabrication of high performance TCPCs [10e12]. Attention has focused recently on composites with high thermal conductivity at low GE loading. For example, long-term thermal annealing was used to improve the thermal conductivity of polycarbonate/GE composite [13]. Hybrid GE fillers and other fillers such as carbon nanotube [14], fullerene [15], silica [16], silicon carbide [17], and boron nitride [18], have been used to improve composite's thermal conductivity. Unfortunately, with relatively low filler loading, these materials did not show high thermal conductivity. Although considerable efforts have been made to design high performance TCPCs, doing this through the addition of small amounts of GE remains a challenge. In electrically conductive polymer composites (ECPCs), polymer blends are often used to improve electrical conductivity [19e22].
J. Huang et al. / Composites Science and Technology 129 (2016) 160e165
The electrical properties of composites are significantly improved when two immiscible polymer phases form a co-continuous structure [23e25]. However, this strategy is rarely applied in the design of TCPCs [26e28]. In the previously mentioned studies, filler loading was relatively high (the lowest filler content was 10 vol% for a composite with 1.3 W m1 K1 thermal conductivity) because the fillers were selectively located in just one phase of the cocontinuous polymer blend. It is expected that thermal conductivity can be further improved by distributing fillers at the continuous interface of a co-continuous polymer blend. In this way, the filler needed to build up a percolated thermally conductive network is minimized [29e31]. In the current study, adsorption-desorption of polymer chains on the GE surface was used to trap GE at the interface of an immiscible poly(ε-caprolactone) (PCL)/poly(lactic acid) (PLA) blend. Thermal conductivity improved substantially when 0.53 vol% GE was added to the PCL/PLA blend. Moreover, a very low thermal percolation threshold (0.11 vol%) was obtained. 2. Theoretical background of graphene location in the immiscible polymer blend The location of GE in an immiscible polymer blend can be predicted by classical thermodynamics [32]. The location of GE in the immiscible polymer blend can be predicated by the wetting coefficient ua [33].
ua ¼
gGEB gGEA gAB
(1)
where gGEB is the interfacial tension between GE and polymer B, gGEA is the interfacial tension between GE and polymer A, and gAB is the interfacial tension between polymers A and B. If ua < 1, GE is located in polymer B (Fig. 1a). If 1 1, GE is preferentially located in polymer A (Fig. 1c). The interfacial tension between two components,g12, can be calculated using the harmonic mean equation
g12
gp gp gd gd ¼ g1 þ g2 4 d 1 2 d þ p 1 2 p g1 þ g2 g1 þ g2
!
qffiffiffiffiffiffiffiffiffiffiffi
gd1 gd2 þ
gdi gdl þ
qffiffiffiffiffiffiffiffiffiffiffi
gpi gpl
(4) p
where gl is the surface tension of the liquid and gdl and gl are the dispersive and polar portions, respectively, of the liquid's surface tension. We selected distilled water (H2O) and ethylene glycol (C2H6O2) as representative liquids to measure the contact angles of the liquids on the sample surfaces. The dispersive and polar components of the surface tension are 26.00 and 46.80 dyn cm1 for water and 26.30 and 21.30 dyn cm1 for ethylene glycol, respectively [35]. Contact angles were measured using a DSA100 (Kruss Co Ltd., Germany). The contact angles of H2O on the surfaces of PCL, PLA, and GE were 79.2 , 74.2 , and 92.4 , respectively. The C2H6O2 contact angles on the surfaces of PCL, PLA, and GE were 56.1, 52.2 , and 72.4 , respectively. The surface energies were calculated according to Equation (4); these values are shown in Table 1. The interfacial tensions between each of the components are then calculated according to Equations (2) and (3), and shown in Table 2. Finally, wetting coefficient data for the PCL/PLA blend filled with GE were calculated with Equation (1), and shown in Table 3. The strongest thermodynamic interaction was between GE and PCL, as shown by the wetting coefficient data. Thus, we predict that GE tends to be selectively located in the PCL phase. 3. Experimental section 3.1. Materials Graphene (JCCG-1-5, diameter: 100 nme10 mm, thickness: <1 nm, density: 2.20 g/cm3) was purchased from JCNANO Co. Ltd., China. The PCL used in the present work (CAPA6800, density: 1.10 g/ cm3, Mw: 8.00 104 g/mol) was a commercial product of Solvay Co. Ltd., Belgium. The PLA (Ingeo4032D, density: 1.24 g/cm3, Mw: 1.60 105 g/mol) was obtained from Nature Works Co. Ltd., USA. 3.2. Composites preparation
qffiffiffiffiffiffiffiffiffiffiffi
gp1 gp2
qffiffiffiffiffiffiffiffiffiffiffi
(2)
or the geometric mean equation
g12 ¼ g1 þ g2 2
gl ð1 þ cos qÞ ¼ 2
161
(3)
where gi is the surface tension of component i, gdi is the dispersive p portion of the surface tension of component i, and gi is the polar portion of the surface tension of component i. p Surface tension, gi, is the sum of gdi and gi , and can be calculated from contact angle q. The relationship between q and gi is described using Owens-Wendt [34].
The PLA/GE and PCL/GE composites were prepared by solution mixing, followed by compression molding. The GE was dispersed in N,N-dimethylformamide (DMF) at 0.1 mg/ml with the aid of sonication [36,37]. Polymer particles with predetermined weight were then added to the suspension. After stirring for 2 h at 85 C, the
Table 1 The surface energy data of components. Components
gd (mN m1)
gp (mN m1)
g (mN m1)
PCL PLA GE
21.87 17.78 19.49
8.02 13.15 3.27
29.89 30.93 22.76
Fig. 1. Scheme of the GE distribution in the immiscible polymer blends based on the wetting coefficient ua ¼ gGEB gGEA =gAB : (a) ua < 1; (b) 1 < ua < 1; (c) ua > 1.
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J. Huang et al. / Composites Science and Technology 129 (2016) 160e165 Table 2 Interfacial tensions g12 between different components calculated from harmonic and geometric mean equations. Component couple
GE-PCL GE-PLA PCL-PLA
g12 (mN m1) Based on the harmonic mean equation
Based on the geometric mean equation
2.14 6.02 1.67
1.17 3.34 0.84
Table 3 Wetting coefficient ua as calculated using the harmonic and geometric mean equation. Materials
ua calculated from the harmonic mean equation
ua calculated from the geometric mean equation
The location of GE
PCL/PLA/GE
2.32
2.58
PCL
mixture was coagulated in a large volume of methanol. The precipitate was filtered and dried in a vacuum oven at 70 C for 24 h. Finally, the composites were compressed into disks (50 mm in diameter, 3 mm thick) by hot-pressing (180 C and 10 MPa for 10 min) for thermal conductivity measurements. From the theoretical calculations, we determined that GE has greater affinity for the PCL phase. However, filler location in an immiscible polymer blend is governed by more than thermodynamics; it is also governed by factors such as the adsorption and desorption of polymer chains on the filler surface [38]. This adsorption-desorption on the filler surface has been used to control fillers at the interface of immiscible polymer blends [38e40]. In the present work, this method is used to trap GE at the interface of the PCL/PLA blend. In the present work, a PLA/GE master batch was prepared by solution mixing. The PLA/GE master batch was prepared in the same way as the PLA/GE composites. During this process, the PLA chains are adsorbed to the GE surface (Fig. 2a). Both the PLA/GE master batch and the predetermined amount of PCL were then dissolved in the DMF at 85 C. This was followed by rapid mixing under vigorous stirring. Subsequently, the mixture was coagulated in a large volume of methanol. During this process, the adsorbed PLA chains may be desorbed from the surface of the GE to be replaced by the more favorable PCL chains [41,42], as illustrated in Fig. 2b. Because both PLA and PCL chains are adsorbed onto the GE surface, GE may be trapped at the interface between the PCL and PLA phases (Fig. 2c). 3.3. Characterization The thermal conductivities of composites were measured at room temperature on disk samples using a TCi Thermal Conductivity Analyzer (C-Therm Company) by the modified transient plane source method [43]. The internal microstructures of the composites were characterized using transmission electron microscopy (TEM). The TEM measurements were performed using a Technai G2 F20STwin transmission electron microscope (FEI) at an acceleration voltage of 150 kV. The composite morphologies were characterized
by scanning electron microscopy (SEM) on a Hitachi 3400N-I instrument. To prepare the TEM samples (50 nm thickness), the composites were ultra-microtomed in liquid nitrogen using a microtome (RMC POWERTOME XL) equipped with a glass knife. To prepare the SEM samples, the specimens were first fractured in liquid nitrogen and then sputter-coated with a thin layer of gold. For the PCL/PLA/GE composite (PCL/PLA ¼ 50w/50w), the PCL phase of the fractured sample was etched using acetic acid at room temperature for 48 h to obtain a clear SEM morphology of the composite. 4. Results and discussion The selective distribution of GE in the composites was visualized using TEM (Fig. 3). In the PCL/PLA/0.053 vol% GE composite with a droplet-matrix structure (Fig. 3a), GE sheets were located at the blend interface and in the PCL phase near the interface. In the PCL/ PLA/0.053 vol% GE composite (PCL/PLA ¼ 50w/50w) (Fig. 3b), the GE sheets were aligned along the interface between the PCL and PLA phases. Thus, the TEM images confirm that GE can be trapped at the interface through adsorption-desorption of polymer chains on the GE surface. Clearly, controlling the GE sheets at the interface of an immiscible polymer blend can significantly improve the composite's thermal conductivity. The improvement is similar to that of electrical properties in ECPCs [29e31]. We fabricated a series of PCL/ PLA/0.53 vol% GE composites with varying PLA content. The dependence of the composite's thermal conductivity on PLA content is shown in Fig. 4. Composite's thermal conductivity increases substantially after the introduction of PLA into the PCL/GE composite. Maximum thermal conductivity is achieved at a PLA concentration of 50 wt%. It is worth noting that the maximum thermal conductivity of the composite (1.28 W m1 K1) is about four times higher than that of the PLA/0.53 vol% GE composite (0.34 W m1 K1). To our knowledge, this is the highest thermal conductivity of any TCPC at 0.53 vol% filler. As PLA concentration increases, thermal conductivity declines.
Fig. 2. Scheme of the adsorption-desorption process of polymer chains on the GE surface for tapping GE at the interface of the PCL/PLA blend.
J. Huang et al. / Composites Science and Technology 129 (2016) 160e165
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Fig. 3. TEM images of PCL/PLA/0.053 vol% GE composite with different PCL/PLA weight ratios: (a) 20/80; (b) 50/50. PLA phase: bright; PCL phase: gray region. The black arrows point to GE.
The internal structures of PCL/PLA/0.53 vol% GE composites at various PCL/PLA weight ratios were determined using TEM (Fig. 5). The TEM images show that the GE sheets located at the PCL/PLA interfaces and in the PCL phase (gray region). It should be noted that the location of GE in the PCL phase is because the space of the interface is too limited to accommodate such a large amount of GE (0.53 vol%) [31]. The morphologies of composites with various PCL/ PLA weight ratios were characterized using SEM (Fig. 6). At 20 wt%, PLA forms a large number of small domains (Fig. 6a); at 50 wt%, a good co-continuous phase structure is observed (Fig. 6b); and at 80 wt%, phase inversion occurs (i.e., PCL forms small phase domains throughout the PLA matrix) (Fig. 6c). Because GE sheets are selectively located in the PCL phase and at the PCL/PLA interface, increased PLA content (i.e. decreased PCL content) leads to an increase in GE concentration in the PCL phase. Thus, introduction of PLA to the composite can increase thermal conductivity. At 50 wt% PLA, composite thermal conductivity is 4 times higher than that of the PLA/GE composite. This striking improvement in the thermal conductivity is attributed to the double percolation phenomenon, which has already been widely used in ECPCs [19,23e25,29e31]. Firstly, selective distribution of GE sheets at PCL/PLA interface and in the PCL phase, as PCL content decreases from 100 wt% to 50 wt%,
Fig. 4. Thermal conductivities of PCL/PLA/0.53 vol% GE composites at different PLA contents.
leads to a remarkable increase in GE concentration (Fig. 5b). On the other hand, the co-continuous phase structure ensures the continuity in the PCL/PLA interface and PCL phase, in which GE is selectively located in (Fig. 6b). In other words, GE sheets can form a percolated, thermally conductive network throughout the composite. Further increases in PLA (to 80 wt%) substantially decrease thermal conductivity due to loss of continuity in the PCL/PLA interface and PCL phase, that is, the inability of GE to form a percolated, thermally conductive network. Thus, it can be seen that the PCL/PLA blend forms a good cocontinuous structure at 50w/50w, the optimum for high thermal conductivity. Subsequently, a series of PCL/PLA/GE (PCL/PLA ¼ 50w/ 50w) composites with varied GE content were fabricated. The thermal conductivities of the resulting PCL/PLA/GE (PCL/ PLA ¼ 50w/50w) composites with different GE contents are shown in Fig. 7. For comparison, we also prepared a series of PCL/GE and PLA/GE composites with varied GE content. The relationship between thermal conductivity and GE content in the PCL/GE and PLA/ GE composites is also shown in Fig. 7. Significant improvement was observed in the thermal conductivities of PCL/PLA/GE composites (PCL/PLA ¼ 50w/50w) as GE content increased. However, the thermal conductivity of PCL/GE and PLA/GE composites increased only slightly with higher GE content. Fig. 7 shows that 0.11 vol% is a critical level for PCL/PLA/GE composites. When the GE content is below 0.11 vol%, thermal conductivity increased slightly with increased GE; this is because the GE sheets are unable to contact one another to form a percolated, thermally conductive pathway. With further increases in GE content, thermal conductivity in PCL/ PLA/GE composites increases remarkably. This is attributed to the formation of a three-dimensional heat conduction network. Thus, the thermal percolation threshold of PCL/PLA/GE composites is ~0.11 vol%. Obviously, this value is much lower than reported in the literature for TCPCs where fillers are selectively located in one phase of an immiscible polymer blend [26e28]. This is because only a very small amount of GE is needed to form the percolated thermal conduction network at the continuous interface of the PCL/PLA blend. To further confirm formation of a conducting network, the electrical properties of PCL/PLA/GE composites were measured (Figs. S1eS3). The electrical percolation threshold of the composite was estimated to be 0.105 vol%. Thus, a percolated conductive network can be formed when GE is 0.11 vol% in the PCL/PLA blend with a co-continuous structure. The thermal stabilities of the PCL/PLA/GE (PCL/PLA ¼ 50w/50w)
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Fig. 5. TEM images of PCL/PLA/0.53 vol% GE composites with different weight ratios of PCL to PLA: (a) 80/20; (b) 50/50; (c) 20/80. PLA phase: bright; PCL phase: gray. The black arrows in (a) and (c) point to GE. The black lines in (b) are GE sheets.
Fig. 6. SEM images of PCL/PLA/0.53 vol% GE composites with different weight ratios of PCL to PLA: (a) 80/20; (b) 50/50; (c) 20/80. (b) PCL phase was etched by acetic acid.
PCL/PLA/GE composite. Because of double percolation, this structure offers a significant improvement in thermal conductivity. This composite, with a co-continuous structure, has a very low thermal percolation threshold (0.11 vol%). To the best of our knowledge, it is the lowest TCPC thermal percolation threshold yet. The current study demonstrates that TCPCs with high thermal conductivity and low filler loading can be produced by controlling filler at the interface of an immiscible polymer blend with a good cocontinuous phase structure. Acknowledgements
Fig. 7. Thermal conductivities of the PCL/PLA/GE (PCL/PLA ¼ 50w/50w), PCL/GE and PLA/GE composites as a function of the GE content. The dashed circle indicates the thermal percolation threshold.
This work was financially supported by Natural Science Foundation of Jiangsu Province for Youth Science Funds (BK20150073), the Fundamental Research Funds for the Central Non-profit Research Institution of CAF (CAFYBB2016QA012), National Natural Science Foundation of China for General Program (51373172), National Natural Science Foundation of China for Major Program (51433009). Appendix A. Supplementary data
composites with various GE content were characterized using thermogravimetric analysis (TGA) (Fig. S4, Table S1). It is clear that the addition of GE to the PCL/PLA blend improves thermal stability. We attribute this to the fact that graphene adsorbed by polymer chains can lower the degradation rate [3e5,12]. 5. Conclusions We demonstrate a simple, effective strategy to trap GE at the interface of an immiscible PCL/PLA blend utilizing adsorptiondesorption of polymer chains on the GE surface. The cocontinuous structure has been shown to be the optimal morphological structure for obtaining high thermal conductivity for the
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.compscitech.2016.04.029. References [1] S.V. Garimella, A.S. Fleischer, J.Y. Murthy, A. Keshavarzi, R. Prasher, C. Patel, et al., Thermal challenges in next-generation electronic systems, IEEE Trans. Compon. Packag. Technol. 31 (2008) 801e815. [2] Z.D. Han, A. Fina, Thermal conductivity of carbon nanotubes and their polymer nanocomposites: a review, Prog. Polym. Sci. 36 (2011) 914e944. [3] J.W. Gu, Q.Y. Zhang, J. Dang, C. Xie, Thermal conductivity epoxy resin composites filled with boron nitride, Polym. Adv. Technol. 23 (2012) 1025e1028. [4] J.W. Gu, Y.Q. Guo, Z.Y. Lv, W.C. Geng, Q.Y. Zhang, Highly thermally conductive POSS-g-SiCp/UHMWPE composites with excellent dielectric properties and thermal stabilities, Compos. Part A 78 (2015) 95e101. [5] J.W. Gu, X.T. Yang, Z.Y. Lv, N. Li, C.B. Liang, Q.Y. Zhang, Functionalized graphite
J. Huang et al. / Composites Science and Technology 129 (2016) 160e165
[6]
[7] [8]
[9] [10] [11]
[12]
[13]
[14]
[15] [16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
nanoplatelets/epoxy resin nanocomposites with high thermal conductivity, Int. J. Heat Mass Transf. 92 (2016) 15e22. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al., Electric field effect in atomically thin carbon films, Science 306 (2004) 666e669. H. Kim, A.A. Abdala, C.W. Macosko, Graphene/polymer nanocomposites, Macromolecules 43 (2010) 6515e6530. A.A. Balandin, S. Ghosh, W.Z. Bao, I. Calizo, D. Teweldebrhan, F. Miao, et al., Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (2008) 902e907. M.D. Stoller, S.J. Park, Y.W. Zhu, J.H. An, R.S. Ruoff, Graphene-based ultracapacitors, Nano Lett. 8 (2008) 3498e3502. K.M.F. Shahil, A.A. Balandin, Grapheneemultilayer graphene nanocomposites as highly efficient thermal interface materials, Nano Lett. 12 (2012) 861e867. 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 graphene flakes with non-covalent functionalization, Adv. Mater. 25 (2013) 732e737. Q. Li, Y.F. Guo, W.W. Li, S.Q. Qiu, C. Zhu, X.F. Wei, et al., Ultrahigh thermal conductivity of assembled aligned multi layer graphene/epoxy composite, Chem. Mater. 26 (2014) 4459e4465. J.R. Potts, S. Murali, Y.W. Zhu, X. Zhao, R.S. Ruoff, Microwave-exfoliated graphite oxide/polycarbonate composites, Macromolecules 44 (2011) 6488e6495. X.Y. Huang, C.Y. Zhi, P.K. Jiang, Toward effective synergetic effects from graphene nanoplatelets and carbon nanotubes on thermal conductivity of ultrahigh volume fraction nanocarbon epoxy composites, J. Phys. Chem. C 116 (2012) 23812e23820. K. Zhang, Y. Zhang, S.R. Wang, Effectively decoupling electrical and thermal conductivity of polymer composites, Carbon 65 (2013) 105e111. L. Huang, P.L. Zhu, G. Li, D.Q. Lu, R. Sun, C.P. Wong, Core-shell SiO2@RGO hybrids for epoxy composites with low percolation threshold and enhanced thermo-mechanical properties, J. Mater. Chem. A 2 (2014) 18246e18255. W. Dai, J.H. Yu, Y. Wang, Y.Z. Song, F.E. Alam, K. Nishimura, et al., Enhanced thermal conductivity for polyimide composites with a three-dimensional silicon carbide nanowire@graphene sheets filler, J. Mater. Chem. A 3 (2015) 4884e4891. M.H. Tsai, I.H. Tseng, J.C. Chiang, J.J. Li, Flexible polyimide films hybrid with functionalized boron nitride and graphene oxide simultaneously to improve thermal conduction and dimensional stability, ACS Appl. Mater. Interfaces 6 (2014) 8639e8645. €tschke, Electrical, rheological S. Bose, A.R. Bhattacharyya, A.R. Kulkarni, P. Po and morphological studies in co-continuous blends of polyamide 6 and acrylonitrile-butadiene-styrene with multiwall carbon nanotubes prepared by melt blending, Compos. Sci. Technol. 69 (2009) 365e372. D.F. Wu, Y.S. Zhang, M. Zhang, W. Yu, Selective localization of multiwalled carbon nanotubes in poly(epsilon-caprolactone)/polylactide blend, Biomacromolecules 10 (2009) 417e424. X.Y. Qi, D. Yan, Z.G. Jiang, Y.K. Cao, Z.Z. Yu, F. Yavari, et al., Enhanced electrical conductivity in polystyrene nanocomposites at ultra-low graphene content, ACS Appl. Mater. Interfaces 3 (2011) 3130e3133. G. Vleminckx, S. Bose, J. Leys, J. Vermant, M. Wübbenhorst, A.A. Abdala, et al., Effect of thermally reduced graphene sheets on the phase behavior, morphology, and electrical conductivity in poly(alpha-methyl styrene)-co(acrylonitrile)/poly(methyl-methacrylate) blends, ACS Appl. Mater. Interfaces 3 (2011) 3172e3180. €tschke, A.R. Bhattacharyya, A. Janke, Morphology and electrical resistivity P. Po of melt mixed blends of polyethylene and carbon nanotube filled polycarbonate, Polymer 44 (2003) 8061e8069. Z.H. Xu, Y.Q. Zhang, Z.G. Wang, N. Sun, H. Li, Enhancement of electrical conductivity by changing phase morphology for composites consisting of polylactide and poly(epsilon-caprolactone) filled with acid-oxidized multiwalled
165
carbon nanotubes, ACS Appl. Mater. Interfaces 3 (2011) 4858e4864. [25] C. Mao, Y.T. Zhu, W. Jiang, Design of electrical conductive composites: tuning the morphology to improve the electrical properties of graphene filled immiscible polymer blends, ACS Appl. Mater. Interfaces 4 (2012) 5281e5286. [26] D. Yorifuji, S. Ando, Enhanced thermal conductivity over percolation threshold in polyimide blend films containing ZnO nano-pyramidal particles: advantage of vertical double percolation structure, J. Mater. Chem. 21 (2011) 4402e4407. [27] J.P. Cao, J. Zhao, X.D. Zhao, F. You, H.Z. Yu, G.H. Hu, et al., High thermal conductivity and high electrical resistivity of poly(vinylidene fluoride)/polystyrene blends by controlling the localization of hybrid fillers, Compos. Sci. Technol. 89 (2013) 142e148. [28] J.P. Cao, X.D. Zhao, J. Zhao, J.W. Zha, G.H. Hu, Z.M. Dang, Improved thermal conductivity and flame retardancy in polystyrene/poly(vinylidene fluoride) blends by controlling selective localization and surface modification of SiC nanoparticles, ACS Appl. Mater. Interfaces 5 (2013) 6915e6924. [29] J. Chen, Y.Y. Shi, J.H. Yang, N. Zhang, T. Huang, C. Chen, et al., A simple strategy to achieve very low percolation threshold via the selective distribution of carbon nanotubes at the interface of polymer blends, J. Mater. Chem. 22 (2012) 22398e22404. [30] Y.Q. Tan, L.J. Fang, J.L. Xiao, Y.H. Song, Q. Zheng, Grafting of copolymers onto graphene by miniemulsion polymerization for conductive polymer composites: improved electrical conductivity and compatibility induced by interfacial distribution of graphene, Polym. Chem. 4 (2013) 2939e2944. [31] J.R. Huang, C. Mao, Y.T. Zhu, W. Jiang, X.D. Yang, Control of carbon nanotubes at the interface of a co-continuous immiscible polymer blend to fabricate conductive composites with ultralow percolation thresholds, Carbon 73 (2014) 267e274. , Uneven distribution of nanoparticles [32] F. Fenouillot, P. Cassagnau, J.C. Majeste in immiscible fluids: morphology development in polymer blends, Polymer 50 (2009) 1333e1350. [33] M. Sumita, K. Sakata, S. Asai, K. Miyasaka, H. Nakagawa, Dispersion of fillers and the electrical conductivity of polymer blends filled with carbon black, Polym. Bull. 25 (1991) 265e271. [34] D.K. Owens, R.C. Wendt, Estimation of the surface free energy of polymers, J. Appl. Polym. Sci. 13 (1969) 1741e1747. [35] A.M. Pinto, S. Moreira, I.C. Gonçalves, F.M. Gama, A.M. Mendes, F.D. Magalh~ aes, Biocompatibility of poly(lactic acid) with incorporated graphene-based materials, Colloids Surf. B 104 (2013) 229e238. [36] Y.W. Cao, J.C. Feng, P.Y. Wu, Preparation of organically dispersible graphene nanosheet powders through a lyophilization method and their poly(lactic acid) composites, Carbon 48 (2010) 3834e3839. [37] E. Murray, B.C. Thompson, S. Sayyar, G.G. Wallace, Enzymatic degradation of graphene/polycaprolactone materials for tissue engineering, Polym. Degrad. Stab. 111 (2015) 71e77. [38] A.C. Baudouin, C. Bailly, J. Devaux, Interface localization of carbon nanotubes in blends of two copolymers, Polym. Degrad. Stab. 95 (2010) 389e398. [39] A.E. Zaikin, R.R. Karimov, V.P. Arkhireev, A study of the redistribution conditions of carbon black particles from the bulk to the interface in heterogeneous polymer blends, Colloid J. 63 (2001) 53e59. [40] A.C. Baudouin, J. Devaux, C. Bailly, Localization of carbon nanotubes at the interface in blends of polyamide and ethylene-acrylate copolymer, Polymer 51 (2010) 1341e1354. [41] D. Baskaran, J.W. Mays, M.S. Bratcher, Noncovalent and nonspecific molecular interactions of polymers with multiwalled carbon nanotubes, Chem. Mater. 17 (2005) 3389e3397. [42] Y.Y. Liang, S. Yang, X. Jiang, G.J. Zhong, J.Z. Xu, Z.M. Li, Nucleation ability of thermally reduced graphene oxide for polylactide: role of size and structural integrity, J. Phys. Chem. B 119 (2015) 4777e4787. [43] A. Harris, S. Kazachenko, R. Bateman, J. Nickerson, M. Emanuel, Measuring the thermal conductivity of heat transfer fluids via the modified transient plane source (MTPS), J. Therm. Anal. Calorim. 116 (2014) 1309e1314.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Massive enhancement in the thermal conductivity of polymer composites by trapping graphene at the interface of a polymer blend Jinrui Huang a, *, Yutian Zhu b, **, Lina Xu a, Jianwen Chen b, Wei Jiang b, Xiaoan Nie a, c, *** a Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry; National Engineering Laboratory for Biomass Chemical Utilization; Key and Open Laboratory on Forest Chemical Engineering, State Forestry Administration; Key Laboratory of Biomass Energy and Material, Nanjing, Jiangsu 210042, China b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China c Institute of New Technology of Forestry, Chinese Academy of Forestry, Beijing 100091, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2016 Received in revised form 19 April 2016 Accepted 25 April 2016 Available online 27 April 2016
Trapping graphene at the interface of a poly(ε-caprolactone) (PCL)/poly(lactic acid) (PLA) blend is achieved through the adsorption-desorption of polymer chains on the graphene surface. The resulting composite exhibits remarkably high thermal conductivity due to the graphene sheets being controlled at the interface of the polymer blend. At 0.53 vol%, when a good co-continuous structure is formed, the thermal conductivity of the graphene composite is nearly 4 times higher than that of the pure PCL/PLA blend with co-continuous structure. Moreover, the blend achieves an extremely low thermal percolation threshold (0.11 vol%), the lowest to date, because most of the graphene sheets are selectively located at the interface of the blend with co-continuous structure. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Polymer-matrix composites (PMCs) Thermal conductivity Interface Electrical properties
1. Introduction Continued miniaturization of electronic devices requires increasingly efficient heat dissipation for reliable performance in modern electronic systems [1]. Research interest has been sparked in recent years in the potential of thermally conductive polymer composites (TCPCs) to replace thermally conductive materials (e.g., metals) as heat removers in a variety of applications (e.g., electric motors, electric generators, power electronics, heat exchangers) [2]. However, very high loading of conventional thermally conductive fillers (>30 vol%) is usually needed to obtain appropriate thermal conductivity due to the interfacial thermal resistance between the filler and the polymer matrix [2e5]. Clearly, high filler loading reduces product quality through diminished mechanical properties,
* Corresponding author. Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, Nanjing 210042, China. ** Corresponding author. Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. *** Corresponding author. Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, Nanjing 210042, China. E-mail addresses: [email protected] (J. Huang), [email protected] (Y. Zhu), [email protected] (X. Nie). http://dx.doi.org/10.1016/j.compscitech.2016.04.029 0266-3538/© 2016 Elsevier Ltd. All rights reserved.
increased weight, reduced flexibility, and increased fragility. Furthermore, high loading increases the difficulty and cost of production. Thus, a method of TCPC fabrication with high thermal conductivity at low filler loading is needed. Graphene (GE), discovered by Novoselov in 2004 [6], has received great attention owing to its exceptional mechanical, electrical, and thermal properties [7]. The extraordinary thermal conductivity (5300 W m1 K1) [8] and extremely high surface area (theoretical limit 2630 m2/g) [9] of GE have made it the most promising candidate for fabrication of high performance TCPCs [10e12]. Attention has focused recently on composites with high thermal conductivity at low GE loading. For example, long-term thermal annealing was used to improve the thermal conductivity of polycarbonate/GE composite [13]. Hybrid GE fillers and other fillers such as carbon nanotube [14], fullerene [15], silica [16], silicon carbide [17], and boron nitride [18], have been used to improve composite's thermal conductivity. Unfortunately, with relatively low filler loading, these materials did not show high thermal conductivity. Although considerable efforts have been made to design high performance TCPCs, doing this through the addition of small amounts of GE remains a challenge. In electrically conductive polymer composites (ECPCs), polymer blends are often used to improve electrical conductivity [19e22].
J. Huang et al. / Composites Science and Technology 129 (2016) 160e165
The electrical properties of composites are significantly improved when two immiscible polymer phases form a co-continuous structure [23e25]. However, this strategy is rarely applied in the design of TCPCs [26e28]. In the previously mentioned studies, filler loading was relatively high (the lowest filler content was 10 vol% for a composite with 1.3 W m1 K1 thermal conductivity) because the fillers were selectively located in just one phase of the cocontinuous polymer blend. It is expected that thermal conductivity can be further improved by distributing fillers at the continuous interface of a co-continuous polymer blend. In this way, the filler needed to build up a percolated thermally conductive network is minimized [29e31]. In the current study, adsorption-desorption of polymer chains on the GE surface was used to trap GE at the interface of an immiscible poly(ε-caprolactone) (PCL)/poly(lactic acid) (PLA) blend. Thermal conductivity improved substantially when 0.53 vol% GE was added to the PCL/PLA blend. Moreover, a very low thermal percolation threshold (0.11 vol%) was obtained. 2. Theoretical background of graphene location in the immiscible polymer blend The location of GE in an immiscible polymer blend can be predicted by classical thermodynamics [32]. The location of GE in the immiscible polymer blend can be predicated by the wetting coefficient ua [33].
ua ¼
gGEB gGEA gAB
(1)
where gGEB is the interfacial tension between GE and polymer B, gGEA is the interfacial tension between GE and polymer A, and gAB is the interfacial tension between polymers A and B. If ua < 1, GE is located in polymer B (Fig. 1a). If 1
g12
gp gp gd gd ¼ g1 þ g2 4 d 1 2 d þ p 1 2 p g1 þ g2 g1 þ g2
!
qffiffiffiffiffiffiffiffiffiffiffi
gd1 gd2 þ
gdi gdl þ
qffiffiffiffiffiffiffiffiffiffiffi
gpi gpl
(4) p
where gl is the surface tension of the liquid and gdl and gl are the dispersive and polar portions, respectively, of the liquid's surface tension. We selected distilled water (H2O) and ethylene glycol (C2H6O2) as representative liquids to measure the contact angles of the liquids on the sample surfaces. The dispersive and polar components of the surface tension are 26.00 and 46.80 dyn cm1 for water and 26.30 and 21.30 dyn cm1 for ethylene glycol, respectively [35]. Contact angles were measured using a DSA100 (Kruss Co Ltd., Germany). The contact angles of H2O on the surfaces of PCL, PLA, and GE were 79.2 , 74.2 , and 92.4 , respectively. The C2H6O2 contact angles on the surfaces of PCL, PLA, and GE were 56.1, 52.2 , and 72.4 , respectively. The surface energies were calculated according to Equation (4); these values are shown in Table 1. The interfacial tensions between each of the components are then calculated according to Equations (2) and (3), and shown in Table 2. Finally, wetting coefficient data for the PCL/PLA blend filled with GE were calculated with Equation (1), and shown in Table 3. The strongest thermodynamic interaction was between GE and PCL, as shown by the wetting coefficient data. Thus, we predict that GE tends to be selectively located in the PCL phase. 3. Experimental section 3.1. Materials Graphene (JCCG-1-5, diameter: 100 nme10 mm, thickness: <1 nm, density: 2.20 g/cm3) was purchased from JCNANO Co. Ltd., China. The PCL used in the present work (CAPA6800, density: 1.10 g/ cm3, Mw: 8.00 104 g/mol) was a commercial product of Solvay Co. Ltd., Belgium. The PLA (Ingeo4032D, density: 1.24 g/cm3, Mw: 1.60 105 g/mol) was obtained from Nature Works Co. Ltd., USA. 3.2. Composites preparation
qffiffiffiffiffiffiffiffiffiffiffi
gp1 gp2
qffiffiffiffiffiffiffiffiffiffiffi
(2)
or the geometric mean equation
g12 ¼ g1 þ g2 2
gl ð1 þ cos qÞ ¼ 2
161
(3)
where gi is the surface tension of component i, gdi is the dispersive p portion of the surface tension of component i, and gi is the polar portion of the surface tension of component i. p Surface tension, gi, is the sum of gdi and gi , and can be calculated from contact angle q. The relationship between q and gi is described using Owens-Wendt [34].
The PLA/GE and PCL/GE composites were prepared by solution mixing, followed by compression molding. The GE was dispersed in N,N-dimethylformamide (DMF) at 0.1 mg/ml with the aid of sonication [36,37]. Polymer particles with predetermined weight were then added to the suspension. After stirring for 2 h at 85 C, the
Table 1 The surface energy data of components. Components
gd (mN m1)
gp (mN m1)
g (mN m1)
PCL PLA GE
21.87 17.78 19.49
8.02 13.15 3.27
29.89 30.93 22.76
Fig. 1. Scheme of the GE distribution in the immiscible polymer blends based on the wetting coefficient ua ¼ gGEB gGEA =gAB : (a) ua < 1; (b) 1 < ua < 1; (c) ua > 1.
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J. Huang et al. / Composites Science and Technology 129 (2016) 160e165 Table 2 Interfacial tensions g12 between different components calculated from harmonic and geometric mean equations. Component couple
GE-PCL GE-PLA PCL-PLA
g12 (mN m1) Based on the harmonic mean equation
Based on the geometric mean equation
2.14 6.02 1.67
1.17 3.34 0.84
Table 3 Wetting coefficient ua as calculated using the harmonic and geometric mean equation. Materials
ua calculated from the harmonic mean equation
ua calculated from the geometric mean equation
The location of GE
PCL/PLA/GE
2.32
2.58
PCL
mixture was coagulated in a large volume of methanol. The precipitate was filtered and dried in a vacuum oven at 70 C for 24 h. Finally, the composites were compressed into disks (50 mm in diameter, 3 mm thick) by hot-pressing (180 C and 10 MPa for 10 min) for thermal conductivity measurements. From the theoretical calculations, we determined that GE has greater affinity for the PCL phase. However, filler location in an immiscible polymer blend is governed by more than thermodynamics; it is also governed by factors such as the adsorption and desorption of polymer chains on the filler surface [38]. This adsorption-desorption on the filler surface has been used to control fillers at the interface of immiscible polymer blends [38e40]. In the present work, this method is used to trap GE at the interface of the PCL/PLA blend. In the present work, a PLA/GE master batch was prepared by solution mixing. The PLA/GE master batch was prepared in the same way as the PLA/GE composites. During this process, the PLA chains are adsorbed to the GE surface (Fig. 2a). Both the PLA/GE master batch and the predetermined amount of PCL were then dissolved in the DMF at 85 C. This was followed by rapid mixing under vigorous stirring. Subsequently, the mixture was coagulated in a large volume of methanol. During this process, the adsorbed PLA chains may be desorbed from the surface of the GE to be replaced by the more favorable PCL chains [41,42], as illustrated in Fig. 2b. Because both PLA and PCL chains are adsorbed onto the GE surface, GE may be trapped at the interface between the PCL and PLA phases (Fig. 2c). 3.3. Characterization The thermal conductivities of composites were measured at room temperature on disk samples using a TCi Thermal Conductivity Analyzer (C-Therm Company) by the modified transient plane source method [43]. The internal microstructures of the composites were characterized using transmission electron microscopy (TEM). The TEM measurements were performed using a Technai G2 F20STwin transmission electron microscope (FEI) at an acceleration voltage of 150 kV. The composite morphologies were characterized
by scanning electron microscopy (SEM) on a Hitachi 3400N-I instrument. To prepare the TEM samples (50 nm thickness), the composites were ultra-microtomed in liquid nitrogen using a microtome (RMC POWERTOME XL) equipped with a glass knife. To prepare the SEM samples, the specimens were first fractured in liquid nitrogen and then sputter-coated with a thin layer of gold. For the PCL/PLA/GE composite (PCL/PLA ¼ 50w/50w), the PCL phase of the fractured sample was etched using acetic acid at room temperature for 48 h to obtain a clear SEM morphology of the composite. 4. Results and discussion The selective distribution of GE in the composites was visualized using TEM (Fig. 3). In the PCL/PLA/0.053 vol% GE composite with a droplet-matrix structure (Fig. 3a), GE sheets were located at the blend interface and in the PCL phase near the interface. In the PCL/ PLA/0.053 vol% GE composite (PCL/PLA ¼ 50w/50w) (Fig. 3b), the GE sheets were aligned along the interface between the PCL and PLA phases. Thus, the TEM images confirm that GE can be trapped at the interface through adsorption-desorption of polymer chains on the GE surface. Clearly, controlling the GE sheets at the interface of an immiscible polymer blend can significantly improve the composite's thermal conductivity. The improvement is similar to that of electrical properties in ECPCs [29e31]. We fabricated a series of PCL/ PLA/0.53 vol% GE composites with varying PLA content. The dependence of the composite's thermal conductivity on PLA content is shown in Fig. 4. Composite's thermal conductivity increases substantially after the introduction of PLA into the PCL/GE composite. Maximum thermal conductivity is achieved at a PLA concentration of 50 wt%. It is worth noting that the maximum thermal conductivity of the composite (1.28 W m1 K1) is about four times higher than that of the PLA/0.53 vol% GE composite (0.34 W m1 K1). To our knowledge, this is the highest thermal conductivity of any TCPC at 0.53 vol% filler. As PLA concentration increases, thermal conductivity declines.
Fig. 2. Scheme of the adsorption-desorption process of polymer chains on the GE surface for tapping GE at the interface of the PCL/PLA blend.
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163
Fig. 3. TEM images of PCL/PLA/0.053 vol% GE composite with different PCL/PLA weight ratios: (a) 20/80; (b) 50/50. PLA phase: bright; PCL phase: gray region. The black arrows point to GE.
The internal structures of PCL/PLA/0.53 vol% GE composites at various PCL/PLA weight ratios were determined using TEM (Fig. 5). The TEM images show that the GE sheets located at the PCL/PLA interfaces and in the PCL phase (gray region). It should be noted that the location of GE in the PCL phase is because the space of the interface is too limited to accommodate such a large amount of GE (0.53 vol%) [31]. The morphologies of composites with various PCL/ PLA weight ratios were characterized using SEM (Fig. 6). At 20 wt%, PLA forms a large number of small domains (Fig. 6a); at 50 wt%, a good co-continuous phase structure is observed (Fig. 6b); and at 80 wt%, phase inversion occurs (i.e., PCL forms small phase domains throughout the PLA matrix) (Fig. 6c). Because GE sheets are selectively located in the PCL phase and at the PCL/PLA interface, increased PLA content (i.e. decreased PCL content) leads to an increase in GE concentration in the PCL phase. Thus, introduction of PLA to the composite can increase thermal conductivity. At 50 wt% PLA, composite thermal conductivity is 4 times higher than that of the PLA/GE composite. This striking improvement in the thermal conductivity is attributed to the double percolation phenomenon, which has already been widely used in ECPCs [19,23e25,29e31]. Firstly, selective distribution of GE sheets at PCL/PLA interface and in the PCL phase, as PCL content decreases from 100 wt% to 50 wt%,
Fig. 4. Thermal conductivities of PCL/PLA/0.53 vol% GE composites at different PLA contents.
leads to a remarkable increase in GE concentration (Fig. 5b). On the other hand, the co-continuous phase structure ensures the continuity in the PCL/PLA interface and PCL phase, in which GE is selectively located in (Fig. 6b). In other words, GE sheets can form a percolated, thermally conductive network throughout the composite. Further increases in PLA (to 80 wt%) substantially decrease thermal conductivity due to loss of continuity in the PCL/PLA interface and PCL phase, that is, the inability of GE to form a percolated, thermally conductive network. Thus, it can be seen that the PCL/PLA blend forms a good cocontinuous structure at 50w/50w, the optimum for high thermal conductivity. Subsequently, a series of PCL/PLA/GE (PCL/PLA ¼ 50w/ 50w) composites with varied GE content were fabricated. The thermal conductivities of the resulting PCL/PLA/GE (PCL/ PLA ¼ 50w/50w) composites with different GE contents are shown in Fig. 7. For comparison, we also prepared a series of PCL/GE and PLA/GE composites with varied GE content. The relationship between thermal conductivity and GE content in the PCL/GE and PLA/ GE composites is also shown in Fig. 7. Significant improvement was observed in the thermal conductivities of PCL/PLA/GE composites (PCL/PLA ¼ 50w/50w) as GE content increased. However, the thermal conductivity of PCL/GE and PLA/GE composites increased only slightly with higher GE content. Fig. 7 shows that 0.11 vol% is a critical level for PCL/PLA/GE composites. When the GE content is below 0.11 vol%, thermal conductivity increased slightly with increased GE; this is because the GE sheets are unable to contact one another to form a percolated, thermally conductive pathway. With further increases in GE content, thermal conductivity in PCL/ PLA/GE composites increases remarkably. This is attributed to the formation of a three-dimensional heat conduction network. Thus, the thermal percolation threshold of PCL/PLA/GE composites is ~0.11 vol%. Obviously, this value is much lower than reported in the literature for TCPCs where fillers are selectively located in one phase of an immiscible polymer blend [26e28]. This is because only a very small amount of GE is needed to form the percolated thermal conduction network at the continuous interface of the PCL/PLA blend. To further confirm formation of a conducting network, the electrical properties of PCL/PLA/GE composites were measured (Figs. S1eS3). The electrical percolation threshold of the composite was estimated to be 0.105 vol%. Thus, a percolated conductive network can be formed when GE is 0.11 vol% in the PCL/PLA blend with a co-continuous structure. The thermal stabilities of the PCL/PLA/GE (PCL/PLA ¼ 50w/50w)
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J. Huang et al. / Composites Science and Technology 129 (2016) 160e165
Fig. 5. TEM images of PCL/PLA/0.53 vol% GE composites with different weight ratios of PCL to PLA: (a) 80/20; (b) 50/50; (c) 20/80. PLA phase: bright; PCL phase: gray. The black arrows in (a) and (c) point to GE. The black lines in (b) are GE sheets.
Fig. 6. SEM images of PCL/PLA/0.53 vol% GE composites with different weight ratios of PCL to PLA: (a) 80/20; (b) 50/50; (c) 20/80. (b) PCL phase was etched by acetic acid.
PCL/PLA/GE composite. Because of double percolation, this structure offers a significant improvement in thermal conductivity. This composite, with a co-continuous structure, has a very low thermal percolation threshold (0.11 vol%). To the best of our knowledge, it is the lowest TCPC thermal percolation threshold yet. The current study demonstrates that TCPCs with high thermal conductivity and low filler loading can be produced by controlling filler at the interface of an immiscible polymer blend with a good cocontinuous phase structure. Acknowledgements
Fig. 7. Thermal conductivities of the PCL/PLA/GE (PCL/PLA ¼ 50w/50w), PCL/GE and PLA/GE composites as a function of the GE content. The dashed circle indicates the thermal percolation threshold.
This work was financially supported by Natural Science Foundation of Jiangsu Province for Youth Science Funds (BK20150073), the Fundamental Research Funds for the Central Non-profit Research Institution of CAF (CAFYBB2016QA012), National Natural Science Foundation of China for General Program (51373172), National Natural Science Foundation of China for Major Program (51433009). Appendix A. Supplementary data
composites with various GE content were characterized using thermogravimetric analysis (TGA) (Fig. S4, Table S1). It is clear that the addition of GE to the PCL/PLA blend improves thermal stability. We attribute this to the fact that graphene adsorbed by polymer chains can lower the degradation rate [3e5,12]. 5. Conclusions We demonstrate a simple, effective strategy to trap GE at the interface of an immiscible PCL/PLA blend utilizing adsorptiondesorption of polymer chains on the GE surface. The cocontinuous structure has been shown to be the optimal morphological structure for obtaining high thermal conductivity for the
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.compscitech.2016.04.029. References [1] S.V. Garimella, A.S. Fleischer, J.Y. Murthy, A. Keshavarzi, R. Prasher, C. Patel, et al., Thermal challenges in next-generation electronic systems, IEEE Trans. Compon. Packag. Technol. 31 (2008) 801e815. [2] Z.D. Han, A. Fina, Thermal conductivity of carbon nanotubes and their polymer nanocomposites: a review, Prog. Polym. Sci. 36 (2011) 914e944. [3] J.W. Gu, Q.Y. Zhang, J. Dang, C. Xie, Thermal conductivity epoxy resin composites filled with boron nitride, Polym. Adv. Technol. 23 (2012) 1025e1028. [4] J.W. Gu, Y.Q. Guo, Z.Y. Lv, W.C. Geng, Q.Y. Zhang, Highly thermally conductive POSS-g-SiCp/UHMWPE composites with excellent dielectric properties and thermal stabilities, Compos. Part A 78 (2015) 95e101. [5] J.W. Gu, X.T. Yang, Z.Y. Lv, N. Li, C.B. Liang, Q.Y. Zhang, Functionalized graphite
J. Huang et al. / Composites Science and Technology 129 (2016) 160e165
[6]
[7] [8]
[9] [10] [11]
[12]
[13]
[14]
[15] [16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
nanoplatelets/epoxy resin nanocomposites with high thermal conductivity, Int. J. Heat Mass Transf. 92 (2016) 15e22. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al., Electric field effect in atomically thin carbon films, Science 306 (2004) 666e669. H. Kim, A.A. Abdala, C.W. Macosko, Graphene/polymer nanocomposites, Macromolecules 43 (2010) 6515e6530. A.A. Balandin, S. Ghosh, W.Z. Bao, I. Calizo, D. Teweldebrhan, F. Miao, et al., Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (2008) 902e907. M.D. Stoller, S.J. Park, Y.W. Zhu, J.H. An, R.S. Ruoff, Graphene-based ultracapacitors, Nano Lett. 8 (2008) 3498e3502. K.M.F. Shahil, A.A. Balandin, Grapheneemultilayer graphene nanocomposites as highly efficient thermal interface materials, Nano Lett. 12 (2012) 861e867. 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 graphene flakes with non-covalent functionalization, Adv. Mater. 25 (2013) 732e737. Q. Li, Y.F. Guo, W.W. Li, S.Q. Qiu, C. Zhu, X.F. Wei, et al., Ultrahigh thermal conductivity of assembled aligned multi layer graphene/epoxy composite, Chem. Mater. 26 (2014) 4459e4465. J.R. Potts, S. Murali, Y.W. Zhu, X. Zhao, R.S. Ruoff, Microwave-exfoliated graphite oxide/polycarbonate composites, Macromolecules 44 (2011) 6488e6495. X.Y. Huang, C.Y. Zhi, P.K. Jiang, Toward effective synergetic effects from graphene nanoplatelets and carbon nanotubes on thermal conductivity of ultrahigh volume fraction nanocarbon epoxy composites, J. Phys. Chem. C 116 (2012) 23812e23820. K. Zhang, Y. Zhang, S.R. Wang, Effectively decoupling electrical and thermal conductivity of polymer composites, Carbon 65 (2013) 105e111. L. Huang, P.L. Zhu, G. Li, D.Q. Lu, R. Sun, C.P. Wong, Core-shell SiO2@RGO hybrids for epoxy composites with low percolation threshold and enhanced thermo-mechanical properties, J. Mater. Chem. A 2 (2014) 18246e18255. W. Dai, J.H. Yu, Y. Wang, Y.Z. Song, F.E. Alam, K. Nishimura, et al., Enhanced thermal conductivity for polyimide composites with a three-dimensional silicon carbide nanowire@graphene sheets filler, J. Mater. Chem. A 3 (2015) 4884e4891. M.H. Tsai, I.H. Tseng, J.C. Chiang, J.J. Li, Flexible polyimide films hybrid with functionalized boron nitride and graphene oxide simultaneously to improve thermal conduction and dimensional stability, ACS Appl. Mater. Interfaces 6 (2014) 8639e8645. €tschke, Electrical, rheological S. Bose, A.R. Bhattacharyya, A.R. Kulkarni, P. Po and morphological studies in co-continuous blends of polyamide 6 and acrylonitrile-butadiene-styrene with multiwall carbon nanotubes prepared by melt blending, Compos. Sci. Technol. 69 (2009) 365e372. D.F. Wu, Y.S. Zhang, M. Zhang, W. Yu, Selective localization of multiwalled carbon nanotubes in poly(epsilon-caprolactone)/polylactide blend, Biomacromolecules 10 (2009) 417e424. X.Y. Qi, D. Yan, Z.G. Jiang, Y.K. Cao, Z.Z. Yu, F. Yavari, et al., Enhanced electrical conductivity in polystyrene nanocomposites at ultra-low graphene content, ACS Appl. Mater. Interfaces 3 (2011) 3130e3133. G. Vleminckx, S. Bose, J. Leys, J. Vermant, M. Wübbenhorst, A.A. Abdala, et al., Effect of thermally reduced graphene sheets on the phase behavior, morphology, and electrical conductivity in poly(alpha-methyl styrene)-co(acrylonitrile)/poly(methyl-methacrylate) blends, ACS Appl. Mater. Interfaces 3 (2011) 3172e3180. €tschke, A.R. Bhattacharyya, A. Janke, Morphology and electrical resistivity P. Po of melt mixed blends of polyethylene and carbon nanotube filled polycarbonate, Polymer 44 (2003) 8061e8069. Z.H. Xu, Y.Q. Zhang, Z.G. Wang, N. Sun, H. Li, Enhancement of electrical conductivity by changing phase morphology for composites consisting of polylactide and poly(epsilon-caprolactone) filled with acid-oxidized multiwalled
165
carbon nanotubes, ACS Appl. Mater. Interfaces 3 (2011) 4858e4864. [25] C. Mao, Y.T. Zhu, W. Jiang, Design of electrical conductive composites: tuning the morphology to improve the electrical properties of graphene filled immiscible polymer blends, ACS Appl. Mater. Interfaces 4 (2012) 5281e5286. [26] D. Yorifuji, S. Ando, Enhanced thermal conductivity over percolation threshold in polyimide blend films containing ZnO nano-pyramidal particles: advantage of vertical double percolation structure, J. Mater. Chem. 21 (2011) 4402e4407. [27] J.P. Cao, J. Zhao, X.D. Zhao, F. You, H.Z. Yu, G.H. Hu, et al., High thermal conductivity and high electrical resistivity of poly(vinylidene fluoride)/polystyrene blends by controlling the localization of hybrid fillers, Compos. Sci. Technol. 89 (2013) 142e148. [28] J.P. Cao, X.D. Zhao, J. Zhao, J.W. Zha, G.H. Hu, Z.M. Dang, Improved thermal conductivity and flame retardancy in polystyrene/poly(vinylidene fluoride) blends by controlling selective localization and surface modification of SiC nanoparticles, ACS Appl. Mater. Interfaces 5 (2013) 6915e6924. [29] J. Chen, Y.Y. Shi, J.H. Yang, N. Zhang, T. Huang, C. Chen, et al., A simple strategy to achieve very low percolation threshold via the selective distribution of carbon nanotubes at the interface of polymer blends, J. Mater. Chem. 22 (2012) 22398e22404. [30] Y.Q. Tan, L.J. Fang, J.L. Xiao, Y.H. Song, Q. Zheng, Grafting of copolymers onto graphene by miniemulsion polymerization for conductive polymer composites: improved electrical conductivity and compatibility induced by interfacial distribution of graphene, Polym. Chem. 4 (2013) 2939e2944. [31] J.R. Huang, C. Mao, Y.T. Zhu, W. Jiang, X.D. Yang, Control of carbon nanotubes at the interface of a co-continuous immiscible polymer blend to fabricate conductive composites with ultralow percolation thresholds, Carbon 73 (2014) 267e274. , Uneven distribution of nanoparticles [32] F. Fenouillot, P. Cassagnau, J.C. Majeste in immiscible fluids: morphology development in polymer blends, Polymer 50 (2009) 1333e1350. [33] M. Sumita, K. Sakata, S. Asai, K. Miyasaka, H. Nakagawa, Dispersion of fillers and the electrical conductivity of polymer blends filled with carbon black, Polym. Bull. 25 (1991) 265e271. [34] D.K. Owens, R.C. Wendt, Estimation of the surface free energy of polymers, J. Appl. Polym. Sci. 13 (1969) 1741e1747. [35] A.M. Pinto, S. Moreira, I.C. Gonçalves, F.M. Gama, A.M. Mendes, F.D. Magalh~ aes, Biocompatibility of poly(lactic acid) with incorporated graphene-based materials, Colloids Surf. B 104 (2013) 229e238. [36] Y.W. Cao, J.C. Feng, P.Y. Wu, Preparation of organically dispersible graphene nanosheet powders through a lyophilization method and their poly(lactic acid) composites, Carbon 48 (2010) 3834e3839. [37] E. Murray, B.C. Thompson, S. Sayyar, G.G. Wallace, Enzymatic degradation of graphene/polycaprolactone materials for tissue engineering, Polym. Degrad. Stab. 111 (2015) 71e77. [38] A.C. Baudouin, C. Bailly, J. Devaux, Interface localization of carbon nanotubes in blends of two copolymers, Polym. Degrad. Stab. 95 (2010) 389e398. [39] A.E. Zaikin, R.R. Karimov, V.P. Arkhireev, A study of the redistribution conditions of carbon black particles from the bulk to the interface in heterogeneous polymer blends, Colloid J. 63 (2001) 53e59. [40] A.C. Baudouin, J. Devaux, C. Bailly, Localization of carbon nanotubes at the interface in blends of polyamide and ethylene-acrylate copolymer, Polymer 51 (2010) 1341e1354. [41] D. Baskaran, J.W. Mays, M.S. Bratcher, Noncovalent and nonspecific molecular interactions of polymers with multiwalled carbon nanotubes, Chem. Mater. 17 (2005) 3389e3397. [42] Y.Y. Liang, S. Yang, X. Jiang, G.J. Zhong, J.Z. Xu, Z.M. Li, Nucleation ability of thermally reduced graphene oxide for polylactide: role of size and structural integrity, J. Phys. Chem. B 119 (2015) 4777e4787. [43] A. Harris, S. Kazachenko, R. Bateman, J. Nickerson, M. Emanuel, Measuring the thermal conductivity of heat transfer fluids via the modified transient plane source (MTPS), J. Therm. Anal. Calorim. 116 (2014) 1309e1314.