- Email: [email protected]
High electrical conductivity in polydimethylsiloxane composite with tailored graphene foam architecture
Journal Pre-proof High electrical conductivity in polydimethylsiloxane composite with tailored graphene foam architecture Bo Han, Hongyun Chen, Te Hu, Huijian Ye, Lixin Xu PII:
S0022-2860(19)31525-X
DOI:
https://doi.org/10.1016/j.molstruc.2019.127416
Reference:
MOLSTR 127416
To appear in:
Journal of Molecular Structure
Received Date: 10 August 2019 Revised Date:
12 November 2019
Accepted Date: 12 November 2019
Please cite this article as: B. Han, H. Chen, T. Hu, H. Ye, L. Xu, High electrical conductivity in polydimethylsiloxane composite with tailored graphene foam architecture, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.127416. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
High electrical conductivity in polydimethylsiloxane composite with tailored graphene foam architecture Bo Han, Hongyun Chen, Te Hu, Huijian Ye*, Lixin Xu* College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, China * To whom correspondence should be addressed. Tel: 86-571-88871519; E-mail: Huijian Ye: [email protected] Lixin Xu: [email protected] Abstract: Graphene foam has huge potential applications in thermal management owing to their unique three-dimensional (3D) interconnected structure. It’s still a challenge to construct graphene foam architecture via facile and efficient method to reach superior property with low loading of nanofillers. Here, we report a polymer-template-assisted assembly strategy to develop 3D graphene architecture that is incorporated into polydimethylsiloxane (PDMS) composite with high electrical conductivity and thermal property. A free-standing foam structure has been assembled with graphene nanosheets, which was exfoliated in low-boiling-point chloroform with assistance of hyperbranched polyethylene as polymer stabilizer against aggregation via CH-π noncovalent interactions. The resultant graphene foam/PDMS composite exhibits high thermal conductivity of 0.22 W·m-1·K-1 and high electrical conductivity of 9.1 × 10-3 S·cm-1 at relatively low loading as 0.7 wt%, which is ascribed to the efficient transfer of charge carrier and thermal phonon along the graphene skeleton. This work demonstrates that the PDMS composite incorporated with graphene foam is a promising candidate for thermal interface materials with high electrical conductivity. Keywords: liquid-phase exfoliation; hyperbranched polyethylene; graphene foam; polymer composite; thermal property; electrical conductivity
1
1. Introduction The requirements in miniaturization and integration of electronic devices motivate the rapid development of thermal interface material with high thermal conductivity that is normally applied between the heat source and the heat sink to transfer heat from inside efficiently [1-4]. Polymer-based thermal interface materials are widely used in electronics due to light weight, processability and low cost [5-7]. However, the common polymer tends to exhibit low thermal conductivity, which limits the capability in heat dissipation [8,9]. Therefore, the development of composite with high thermal conductivity has been investigated extensively to improve the efficiency of heat transfer. The thermal interface materials are usually composed of polymer matrix and high thermal conductive fillers, including ceramics [10,11], metal oxides [12,13], and carbon-based nanomaterials [9,14-17]. Graphene, as a two-dimensional nanoplate with excellent in-plane thermal conductivity around 5300 W·m-1·K-1, has attracted enormous attentions for promising candidate in thermal management applications [15,18-20]. The graphene/epoxy composite showed thermal conductivity under room temperature with the maximum value of 4.9 W·m-1·K-1 at 30 wt% loading [16]. Large size of graphene sheets and surface functionalization would further reduce the thermal resistance of the interfaces between of graphene and epoxy resin. Moreover, the high thermal conductivity of composite could be achieved by tuning the orientation of graphene nanofillers. The magnetically aligned graphene/PDMS composite had high anisotropy in thermal conductivity, in which the thermal conductivity of 3 wt% composite was increased by 49% compared with isotropic composite [21]. This enhancement was due to that the magnetic field re-arranged the particles in chain-like structures, which facilitated the effective longitudinal thermal transportation inside the composite. However, the preparation of highly orientation for nanosheets was complicated because high magnetic field was required to drive the heterogenous redistribution of nanosheets. It’s suggested that three-dimensional graphene foam (GF) exhibits efficient motion of matter and energy based on its unique interconnected structure [22-27]. The thermal 2
conductivity of GF consisting of few-layer graphene and ultrathin graphite was increased from 0.26 to 1.70 W·m-1·K-1 by using different etchants [22]. The thermal conductivity of poly(methyl methacrylate) (PMMA) composite with 0.4 wt% GF reached 0.27 W·m-1·K-1 and the local mechanical strength of composite was also enhanced by the addition of GF due to the in-situ polymerization method [28]. The polymer-template-assisted assembly strategy has been developed to prepare a novel class of GF with facile technology. The obtained GF/polymer composite exhibited a high thermal conductivity of 1.52 W·m-1·K-1 at relatively low graphene content of 5.0 wt% [29]. In the GF/polymer composites, the unique interconnected structure of GF enables heat transfer along the GF struts and also avoids the common dispersion issue of nanosheets in composite. Furthermore, GF would provide efficient paths for heat conduction with a very low graphene loading [28]. It’s still a challenge to prepare GF architecture via an efficient technique for polymer-based thermal interface materials. In our previous work, the natural graphite was exfoliated into few-layer graphene in low-boiling-point solvent with assistance of hyperbranched polymer stabilizer, which was adsorbed on the surface of nanosheets against aggregation through CH-π noncovalent interactions [30]. In this study, we report a novel strategy for the preparation of graphene foam by self-assembly method of two-dimensional graphene nanosheets on a polymer skeleton. The resultant GF/PDMS composite presents a high thermal conductivity of 0.22 W·m-1·K-1 and high electrical conductivity of 9.1 × 10-3 S·cm-1 at relatively low GF loading as 0.7 wt%. Meanwhile, the thermal stability and mechanical property of GF/PDMS composite have also been improved. This work demonstrates that the PDMS composite incorporated with graphene foam is a promising candidate for thermal interface material with high electrical conductivity. 2. Experimental 2.1 Materials Chloroform (AR) and natural graphite were purchased from Aldrich, and the PDMS resin (Sylgard 184) was obtained from Dow Corning. Commercial melamine resin 3
foam (Yongkang Jingshang Industry and Trade Co., Ltd.) for kitchen supplies was utilized in this work. The hyperbranched polyethylene (HBPE) was synthesized via chain-walking mechanism according to the typical procedure [30]. 2.2 Preparation of graphene dispersion Graphene dispersion was prepared by liquid-phase exfoliation method with HBPE as polymer stabilizer. A mixture of 800-mg graphite and 320-mg HBPE in 80-mL chloroform was employed for the large-scale production of graphene nanosheets. During the sonication process, the temperature of the sonication bath was maintained constant by a continuous flow of water. The resulting mixture was subsequently centrifuged at 4000 rpm for 45 min and supernatant dispersion was then collected as stable graphene dispersion. 2.3 Preparation of graphene foam/PDMS composite Graphene foam was prepared by the following procedure. Firstly, the commercial melamine resin foam was immersed in graphene solution with 2 mg·mL-1 for 1 h. Then, the surface of melamine resin foam was assembled with graphene nanosheets after drying process. Free-standing 3D graphene architecture with melamine resin foam skeleton was paralyzed at 550 °C for 2 min at air atmosphere to decompose the polymer completely. The foam sample was obtained after adsorption and pyrolysis process. The PDMS matrix was prepared by mixing of prepolymer and curing agent with a ratio of 10:1 by weight, which was infused into 3D graphene architecture. Finally, GF immersed in PDMS matrix was cured at 80 °C for 2 h. The sample was cooled down to the room temperature and then tailored into different shapes for further characterizations. 2.4 Characterizations The morphology of graphene was measured by transmission electron microscope (TEM, Philips-FEI) and atomic force microscope (AFM, Nanoscope 3D, Bruker). The morphology of cross-sectional fractured foam sample and GF/PDMS composite was investigated by scanning electron microscopy (SEM, Nano nova 450, FEI). The 4
thermal diffusivity coefficient of GF/PDMS composite with the diameter of 25.4 mm was evaluated by a laser-flash diffusivity instrument (LFA 467 MicroFlash, NETZSCH). The thermal stability was examined on a dynamic thermogravimetric analysis (TGA, Q600 SDT, TA) under the nitrogen atmosphere from room temperature to 1000 °C with a heating rate of 10 °C/min. Dynamic thermomechanical analysis (DMA, Q800, TA) was carried out from -140 °C to 30 °C at 3 °C/min with the tensile configuration of 1 Hz. The electrical conductivity of the GF/PDMS composite was measured by four-probe conductivity meter (RTS-8, China) from 20 °C to 80 °C. 3. Results and discussion
Figure 1. The schematically illustration for the liquid-phase exfoliation of few-layer graphene and the preparation of graphene foam/PDMS composite. The preparation process of graphene nanosheets and GF/PDMS composite is schematically illustrated in Figure 1. The graphene dispersion with concentration of 0.16 mg·mL-1 was prepared by the liquid-phase exfoliation method in a low-boiling-point chloroform, in which the hyperbranched HBPE acting as a polymer stabilizer remains stable dispersion of nanosheets. Then, the melamine resin foam was immersed in the concentrated graphene dispersion to attach the graphene nanosheets 5
on the three-dimensional polymer backbone. A complete three-dimensional graphene foam was received by pyrolysis process of removing the polymer skeleton. Finally, the mixture of PDMS prepolymer and curing agent with a weight ratio of 10:1 was filled in graphene foam to obtain a flexible composite after curing process. The two-dimensional nanosheets were successfully assembled into three-dimensional graphene foam via simple technology, which broadens the application of graphene obtained by liquid-phase exfoliation, and also provides a new insight for the construction of polymer composite with sterically architecture. (a)
(c)
(b)
5 layers 500 nm
(d)
50 nm
5 nm
(e)
(f)
(g)
(h)
Figure 2. (a) and (b) TEM images of graphene nanosheets, (c) the edge of a five-layer graphene sample, (d) AFM image of graphene nanosheets, (e) the enlarged AFM image of nanosheets in the dotted box of Figure 2d, (f) 3D AFM image for graphene in Figure 2e, (g) the statistical lateral sizes of graphene nanosheets with sample set N = 120, (h) the thickness statistics of graphene nanosheets with sample set N = 50. 6
The HBPE was applied to exfoliate the few-layer nanosheets from natural graphite as polymer stabilizer in low-boiling solvent, and a stable graphene dispersion was received, which is ascribed to the attached stabilizer against the aggregation of nanosheets [31]. The TEM images of nanosheets are displayed in Figure 2a and 2b, from which the graphene nanosheets are electrically transparent, and some also have wrinkles. The edge of the nanosheet is employed to distinguish the layer number. The morphology of graphene with edge is also shown in Figure 2c, which clearly identifies 5-layer structure for the exfoliated nanosheets. The TEM images of graphite are also shown in Figure S4, which confirms that graphite has been exfoliated into few-layer graphene. AFM technology is an effective way to characterize the thickness of graphene nanosheets, and the results are shown in Figure 2d. The enlarged 2D AFM image and 3D AFM image in the dotted frame are presented in Figure 2e and 2f, respectively. Figure 2g shows the statistical results of the lateral size for randomly selected 120 flakes, and about 88% of resultant nanosheets has the lateral dimension between 100 nm and 500 nm, which is consistent with the lateral dimension of few-layer graphene prepared by liquid-phase exfoliation [31-33]. As displayed in Figure 2f, the thickness of received graphene is less than 1.8 nm. Meanwhile, the thickness of nanosheets with sample set N=50 was estimated statistically in Figure 2h, from which the thickness of the resulting graphene is in the range of 1.0 to 2.2 nm [34,35]. The comparison in thickness and lateral size of graphene nanosheets prepared by liquid-phase exfoliation in the literature is shown in Table S1. Moreover, the XPS spectrum presented in Figure S1 indicates the low defects of nanosheets. The low binding energy at 283.7 eV corresponding to the graphitic C-C/C=C vibration is obviously the dominant peak for nanosheets. As typically observed for carbon materials, the shift towards higher binding energy was detected at 284.3 eV in graphene curve, which is assigned to C-O bonding. These results confirm that the few-layer nanosheets have been obtained by liquid-phase exfoliation from natural graphite in chloroform with assistance of HBPE as the stabilizer. The stable dispersion of graphene nanosheets in solution is mainly due to the interaction between graphene and hyperbranched polyethylene. It is also suggested that the stable 7
existence of substances can be improved by the hydrophobic interactions between molecules [36,37]. The noncovalent CH-π interactions between graphene and HBPE were yielded due to its large number of branched ends to be attached on the surface of nanosheets. It has been reported that the CH-π interaction is a weak attraction interaction between the C-H bond of an alkyl or aryl group and the π system [20]. The free-standing 3D foam architecture was assembled with assistance of commercial melamine resin sponge after pyrolysis process. The SEM images of received 3D graphene sponge and GF/PDMS composite are illustrated in Figure 3. We chose the
Figure 3. (a) and (d) SEM images of melamine resin foam, (b) and (e) SEM images of 3D graphene foam, (c) and (f) SEM images of frozen-fractured surface for GF/PDMS composite. porous melamine resin foam with an interconnected structure as a template for coating with graphene nanosheets. In Figure 3a and 3d, the pure melamine resin foam exhibits a 3D cross-linked structure with smooth surface. These porous are irregular shapes and the macro-porous diameter is around hundreds of micrometers. After soaking and pyrolysis process, the melamine resin foam was assembled with graphene nanosheets on the skeleton surface. Figure 3b and 3e present SEM images of the free-standing foam, in which nanosheets are attached on the surface of melamine resin foam with continuous macropore. The morphology of resultant GF is similar to melamine resin foam, but the skeleton of the network exhibits a rough texture associated with the presence of flexible nanosheets. Meanwhile, the whole architecture of the graphene 8
foam has a slight shrinkage compared with the pure melamine resin foam. The SEM images of freeze-fractured surface of GF/PDMS composite are displayed in Figure 3c and 3f. The approximate pentagon as foam skeleton reveals that GF structure is interconnected through the matrix. It also suggests excellent permeability inside the GF due to the large surface area and the presence of interconnected macropore. It’s reported that the modification of graphene nanosheets with tailored molecules could enhance their compatibility with the polymer matrix [21,38-40]. The problem of compatibility of the two-dimensional graphene filler with the polymer matrix is solved in the GF filled polymer composites due to the interconnected three-dimensional framework structure, which enables efficient transfer of thermal phonon along the graphene skeleton, and thus, improves the thermal conductivity of GF/PDMS composite. In graphene foam composites, the dispersion uniformity of graphene nanosheets is also important [41,42].
(a)
(c)
(b)
(d)
9
Storage modulus (MPa)
(e) 4000 PDMS GF/PDMS 3000
2000
1000
0 -120
-80
-40
0
40
Temperature (ºC)
Figure 4. The digital images of (a) pure PDMS and (b) GF/PDMS composite as the electrical conductor to drive a LED light, (c) thermal conductivity of GF/PDMS composite as a function of temperature, (d) TGA curves, (e) storage modulus of sample as a function of temperature. The GF loading was 0.7 wt%. As shown in Figure 4a and 4b, the digital images of conductive GF/PDMS composite as the conductor to drive a LED light, which indicates that the electrical conductivity of GF/PDMS composite is superior to pure PDMS. The high electrical conductivity of GF/PDMS composite is attributed to the well interconnected GF frameworks. The electrical conductivity of 0.7 wt% GF/PDMS composite shown in Figure S2 reaches 9.1×10-3 S·cm-1 compared with PDMS of 10-14 S·cm-1 as an insulator [43]. This significantly
improved
electrical
conductivity
is
mainly
ascribed
to
the
interconnection of nanosheets along 3D foam structure, which verifies that the interconnected GF provides effective paths for electron transfer inside the polymer. The charge carriers could diffuse efficiently along the graphene network as the GF/PDMS composite retains 3D interconnected network. The transport behavior of electrons in three-dimensional graphene foam composite is different from composite with graphene nanosheets. For the two-dimensional graphene/composite, the transmission of electrons becomes difficult due to the large interfacial resistance between the nanosheets and the polymer matrix. Therefore, the conductivity of the whole composite is very low. For the graphene foam structure, the nanosheets are connected to each other throughout the composite, and the electrical resistance between the graphene nanosheets turns small. The electrons are preferentially
10
transported in the graphene foam and can be transported throughout the polymer matrix, which leads to the high conductivity of the composite with foam architecture. This high electrical conductivity of PDMS composite evidences that the three-dimensional graphene foam structure has been successfully constructed in the system. The thermal conductivity of GF/PDMS composite film at different temperature is shown in Figure 4c. The transient laser flash method was employed to estimate the thermal conductivity of the GF/PDMS composite. Thermal conductivity of polymer composite was calculated from the equation of λ = ϲ·ρ·α, where λ, ϲ, ρ, and α represent thermal conductivity, specific heat, bulk density and thermal diffusivity, respectively. The thermal conductivity of pure PDMS at room temperature is 0.18 W·m-1·K-1. After the addition of graphene foam, the thermal conductivity of 0.7 wt% composite reaches 0.22 W·m-1·K-1 at room temperature, which is 22% higher than that of pure PDMS. The interconnected architecture of GF endows the composite with an effective heat conduction network, which is due to that the heat travels preferentially along graphene struts and leads to high thermal conductivity [22,23]. Meanwhile, the thermal conductivity of GF/PDMS composite exhibits a temperature-dependent effect, that is, the thermal conductivity varies with increasing environmental temperature. This linear temperature-dependent behavior may be related to the phonon distribution of the two-dimensional system such as graphite, indicating that the temperature-dependent thermal conductivity of GF/PDMS composite obeys a similar mechanism of graphite [27]. The thermal transport performance of composite mainly depends on the density of hierarchical framework, which was verified in the graphene/carbon nanotube aerogel system [3]. It’s reasonable to deduce that the transmission path for the thermal phonons becomes long with the large pore size in the foam architecture. The curves of weight loss for pure PDMS and GF/PDMS composite are presented in Figure 4d. The TGA curve of GF/PDMS composite obviously shifts towards higher temperature above 500 °C. Compared with pure PDMS, the addition of graphene 11
foam leads to an increase of the thermal stability. For example, the corresponding temperatures T20% (the temperature at 20 wt% loss of whole weight) of the GF/PDMS composite is higher than that of pure PDMS, which strengthens that the thermal stability of the composite is improved after the addition of foam. This phenomenon is probably ascribed to the addition of graphene with high thermal stability, which is incorporated in the PDMS composite as an interconnected structure. The storage modulus (E’) curves under variable temperatures are demonstrated in Figure 4e. In the glassy region, the incorporation of graphene foam into PDMS yields an increase of 10.4% for E’ at -138 ˚C. The storage modulus E’ decreases dramatically with the increasing temperature from -138 to -110 ˚C as the consequence of the glass transition. Nevertheless, the decrease of E’ is compensated by the presence of foam structure, which illustrates an improvement of thermal stability.
Figure 5. The schematically view of heat transfer in the PDMS composite with graphene foam structure. The electrical conductivity and thermal conductivity of the composite are improved due to the construction of the three-dimensional graphene foam. The thermal conductivity of polymer composite depends on many factors, including the thermal conductivity of matrix, the thermal conductivity of fillers, the volume fraction and thickness of fillers, and the interface thermal resistance between fillers and matrix. These factors determine the establishment of a thermal phonon path in the composite. Conventional two-dimensional graphene fillers are dispersed randomly in polymer 12
matrix, which is disadvantageous for the transportation of thermal phonon and electron under low loading of nanofillers. Therefore, we assemble few-layer graphene obtained by liquid-phase exfoliation method into a three-dimensional foam structure, which is more conducive to the transfer of electrons and thermal phonon along the graphene skeleton as illustrated in Figure 5. The graphene foam has a good interfacial action in the PDMS composite and also forms an interconnected network structure, which has great advantages compared with the two-dimensional nanofillers in thermal interface composite. This facile polymer-template method provides new strategy for the construction of graphene/polymer composite with high electrical conductivity. 4. Conclusions In summary, we demonstrate an effective way to prepare graphene foam by coating nanosheets on melamine resin as polymer template to construct the efficient route for charge carrier and thermal phonon in the composite. The graphite was exfoliated into few-layer graphene with low defects, which is attributed to HBPE polymer adsorbed on nanosheets surface via the CH-π noncovalent interaction. The thermal conductivity of GF/PDMS composite with 0.7 wt% graphene nanosheets is 0.22 W·m-1·K-1, which is 22% higher than that of pure PDMS matrix due to the interconnected 3D architecture. The electrical conductivity of GF/PDMS composite reaches 9.1 × 10-3 S·cm-1, which increases by 11 orders of magnitude than pure PDMS. This PDMS composite incorporated with graphene foam architecture is a promising candidate for applications in thermal management and electrical devices. Acknowledgements The financial support from the National Natural Science Foundation of China (21474091, 51707175) and Natural Science Foundation of Zhejiang Province (LTZ20E070001, LY18B040005) is greatly appreciated. This work is also supported by China Postdoctoral Science Foundation (2018M640572). References [1] A.P. Yu, P. Ramesh, M.E. Itkis, E. Bekyarova, R.C. Haddon, Graphite nanoplatelet-epoxy 13
composite thermal interface materials, Journal of Physical Chemistry C, 111 (2007) 7565-7569. [2] K.M.F. Shahil, A.A. Balandin, Thermal properties of graphene and multilayer graphene: Applications in thermal interface materials, Solid State Communications, 152 (2012) 1331-1340. [3] P. Lv, X.W. Tan, K.H. Yu, R.L. Zheng, J.J. Zheng, W. Wei, Super-elastic graphene/carbon nanotube aerogel: A novel thermal interface material with highly thermal transport properties, Carbon, 99 (2016) 222-228. [4] K.M.F. Shahil, A.A. Balandin, Graphene-multilayer graphene nanocomposites as highly efficient thermal interface materials, Nano Letters, 12 (2012) 861-867. [5] P.B. Kaul, M.F.P. Bifano, V. Prakash, Multifunctional carbon nanotube-epoxy composites for thermal energy management, Journal of Composite Materials, 47 (2013) 77-95. [6] Q. Li, Y.F. Guo, W.W. Li, S.Q. Qiu, C. Zhu, M.L. Chen, C.J. Liu, S.T. Liao, Y.P. Gong, A.K. Mishra, L.W. Liu, Ultrahigh thermal conductivity of assembled aligned multilayer graphene/epoxy composite, Chemistry of Materials, 26 (2012) 4459-4465. [7] Z.Q. Kuang, Y.L. Chen, Y.L. Lu, L. Liu, S. Hu, S.P. Wen, Y.Y. Mao, L.Q. Zhang, Fabrication of highly oriented hexagonal boron nitride nanosheet/elastomer nanocomposites with high thermal conductivity, Small, 11 (2015) 1655-1659. [8] X.F. Xu, J. Chen, J. Zhou, B.W. Li, Thermal conductivity of polymers and their nanocomposites, Advanced Materials, 30 (2018) 1705544. [9] Z.C. Tao, H.B. Wang, X.F. Li, Z.J. Liu, Q.G. Guo, Expanded graphite/polydimethylsiloxane composites with high thermal conductivity, Journal of Applied Polymer Science, 134 (2017) 44843. [10] X.L. Zeng, Y.M. Yao, Z.Y. Gong, F.F. Wang, R. Sun, J.B. Xu, C.P. Wong, Ice-templated assembly strategy to construct 3D boron nitride nanosheet networks in polymer composites for thermal conductivity improvement, Small, 11 (2015) 6205-6213. [11] M. Loeblein, S.H. Tsang, M. Pawlik, E.J.R. Phua, H. Yong, X.W. Zhang, C.L. Gan, E.H.T. Teo, High-density 3D-boron nitride and 3D-graphene for high-performance nano-thermal interface material, ACS Nano, 11 (2017) 2033-2044. [12] H. Yu, L.L. Li, Y.J. Zhang, Silver nanoparticle-based thermal interface materials with 14
ultra-low thermal resistance for power electronics applications, Scripta Materialia, 66 (2012) 931-934. [13] D. Suh, C.M. Moon, D. Kim, S. Baik, Ultrahigh thermal conductivity of interface materials by silver-functionalized carbon nanotube phonon conduits, Advanced Materials, 28 (2016) 7220-7227. [14] T.X. Ji, Y.Y. Feng, M.M. Qin, W. Feng, Thermal conducting properties of aligned carbon nanotubes and their polymer composites, Composites Part A-Applied Science & Manufacturing, 91 (2016) 351-369. [15] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau, Superior thermal conductivity of single-layer graphene, Nano Letters, 8 (2008) 902-907. [16] B. Tang, G.X. Hu, H.Y. Gao, L.Y. Hai, Application of graphene as filler to improve thermal transport property of epoxy resin for thermal interface materials, International Journal of Heat & Mass Transfer, 85 (2015) 420-429. [17] P. Zhang, J.H. Zeng, S.P. Zhai, Y.Q. Xian, D.G. Yang, Q. Li, Thermal properties of graphene filled polymer composite thermal interface materials, Macromolecular Materials & Engineering, 302 (2017) 1700068. [18] S.H. Xie, Y.Y. Liu, J.Y. Li, Comparison of the effective conductivity between composites reinforced by graphene nanosheets and carbon nanotubes, Applied Physics Letters, 92 (2008) 243121. [19] H.Q. Yao, S.A. Hawkins, H.J. Sue, Preparation of epoxy nanocomposites containing well-dispersed graphene nanosheets, Composites Science & Technology, 146 (2017) 161-168. [20] H.J. Ye, X.H. Zhang, C.F. Xu, B. Han, L.X. Xu, Enhanced dielectric property and energy density in poly(vinylidene fluoride-chlorotrifluoroethylene) nanocomposite incorporated with graphene
functionalized
with
hyperbranched
polyethylene-graft-poly(trifluoroethyl
methacrylate) copolymer, Journal of Materials Chemistry C, 6 (2018) 11144-11155. [21] B. Li, S. Dong, X. Wu, C.P. Wang, X.J. Wang, J. Fang, Anisotropic thermal property of magnetically oriented carbon nanotube polymer composites, Composites Science & Technology, 147 (2017) 52-61. [22] M.T. Pettes, H.X. Ji, R.S. Ruoff, L. Shi, Thermal transport in three-dimensional foam 15
architectures of few-layer graphene and ultrathin graphite, Nano Letters, 12 (2012) 2959-2964. [23] H.X. Ji, D.P. Sellan, M.T. Pettes, X.H. Kong, J.Y. Ji, L. Shi, R.S. Ruoff, Enhanced thermal conductivity of phase change materials with ultrathin-graphite foams for thermal energy storage, Energy & Environmental Science, 7 (2014) 1185-1192. [24] A. Pedrielli, S. Taioli, G. Garberoglio, N.M. Pugno, Mechanical and thermal properties of graphene random nanofoams via molecular dynamics simulations, Carbon, 132 (2018) 766-775. [25] Y.F. Li, Z.F. Chen, XH/pi (X = C, Si) Interactions in graphene and silicene: weak in strength, strong in tuning band structures, Journal of Physical Chemistry Letters, 4 (2013) 269-275. [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, Composites Part B-Engineering, 94 (2016) 102-108. [27] H.Y. Yan, Y.X. Tang, W. Long, Y.F. Li, Enhanced thermal conductivity in polymer composites with aligned graphene nanosheets, Journal of Materials Science, 49 (2014) 5256-5264. [28] C. Zhang, A. Li, Y.H. Zhao, S.L. Bai, Y.F. Zhang, Thermal, electrical and mechanical properties of graphene foam filled poly(methyl methacrylate) composite prepared by in situ polymerization, Composites Part B-Engineering, 135 (2018) 201-206. [29] Z.D. Liu, D.Y. Shen, J.H. Yu, W. Dai, C.Y. Li, S.Y. Du, N. Jiang, H.R. Li, C.T. Lin, Exceptionally high thermal and electrical conductivity of three-dimensional graphene-foam based polymer composites, RSC Advances, 6 (2016) 22364-22369. [30] L.X Xu, J.W. Mcgraw, F. Gao, M. Grundy, Z.B. Ye, Z.Y. Gu, J.L. Shepherd, Production of high-concentration graphene dispersions in low-boiling-point organic solvents by liquid-phase noncovalent exfoliation of graphite with a hyperbranched polyethylene and formation of graphene/ethylene copolymer composites, Journal of Physical Chemistry C, 117 (2013) 10730-10742. [31] M. Lotya, P.J. King, U. Khan, S. De, J.N. Coleman, High-concentration, surfactant-stabilized graphene dispersions, ACS Nano, 4 (2010) 3155-3162. [32] A. O’Neill, U. Khan, P.N. Nirmalraj, J. Boland, J.N. Coleman, Graphene dispersion and 16
exfoliation in low boiling point solvents, Journal of Physical Chemistry C, 115 (2011) 5422-5428. [33] S. Haar, M. Bruna, J.X. Lian, F. Tomarchio, Y. Olivier, R. Mazzaro, V. Morandi, J. Moran, A.C. Ferrari, D. Beljonne, Liquid-phase exfoliation of graphite into single and few layers graphene with α-functionalized alkanes, Journal of Physical Chemistry Letters, 7 (2016) 2714-2721. [34] A.B. Bourlinos, V. Georgakilas, R. Zboril, T.A. Steriotis, A.K. Stubos, Liquid-phase exfoliation of graphite towards solubilized graphenes, Small, 5 (2009) 1841-1845. [35] S. Sampath, A.N. Basuray, K.J. Hartlieb, T. Aytun, S.I. Stupp, J.F. Stoddart, Direct exfoliation of graphite to graphene in aqueous media with diazaperopyrenium dications, Advanced Materials, 25 (2013) 2740-2745. [36] J. Chamani, M. Heshmati, Mechanism for stabilization of the molten globule state of papain by sodium n-alkyl sulfates: Spectroscopic and calorimetric approaches, Journal of Colloid and Interface Science, 322 (2008) 119-127 [37] A.A. Moosavi-Movahedi, S. Hakimelahi, J. Chamani, G.A. Khodarahmi, F. Hassanzadeh, F.T. Luo, T.W. Ly, K.S. Shia, C.F. Yen, M.L. Jain, R. Kulatheeswaran, C.H. Xue, M. Pasdar, G.H. Hakimelahi, Design, synthesis, and anticancer activity of phosphonic acid diphosphate derivative of adenine containing butenolide and its water-soluble derivatives of paclitaxel with high antitumor activity, Bioorganic and Medicinal Chemistry, 11 (2003) 4303-4313 [38] J.J. Tang, W. Yao, W.L. Li, J. Xu, L. Jin, J.D. Zhang, Z.X. Xu, Study on a novel composite coating based on PDMS doped with modified graphene oxide, Journal of Coatings Technology & Research, 15 (2018) 375-383. [39] J. Osicka, M. Ilcikova, M. Mrlik, A. Minarik, V. Pavlinek, J. Mosnacek, The impact of polymer grafting from a graphene oxide surface on its compatibility with a PDMS matrix and the light-induced actuation of the composites, Polymers, 9 (2017) 264. [40] K.T.S. Kong, M. Mariatti, A.A. Rashid, J.J.C. Busfield, Enhanced conductivity behavior of polydimethylsiloxane
(PDMS)
hybrid
composites
containing
exfoliated
graphite
nanoplatelets and carbon nanotubes, Composites Part B-Engineering, 58 (2014) 457-462. [41] H. Sanei, A. Asoodeh, S. Hamedakbari-Tusi, J. Chamani, Multi-spectroscopic investigations of aspirin and colchicine interactions with human hemoglobin: binary and ternary systems, 17
Journal of Solution Chemistry, 40 (2011) 1905-1931. [42] N. Shakibapour, F.D. Sani, S. Beigoli, H. Sadeghian, J. Chamani, Multi-spectroscopic and molecular modeling studies to reveal the interaction between propyl acridone and calf thymus DNA in the presence of histone H1 binary and ternary approaches, Journal of Biomolecular Structure and Dynamics, 37 (2019) 359-371. [43] M.P.
Wolf,
G.B.
Salieb-Beugelaar,
P.
Hunziker,
PDMS
with
designer
functionalities-Properties, modifications strategies, and applications, Progress in Polymer Science, 83 (2018) 97-134.
18
A graphene foam architecture is incorporated into PDMS composite.
Graphene was exfoliated in chloroform with HBPE via CH-π interactions.
The composite exhibits high thermal conductivity with low loading.
The efficient route of thermal phonon is formed along the skeleton.
Author contributions: Huijian Ye and Lixin Xu conceived the entire research aims; Bo Han, Hongyun Chen and Te Hu conducted the experimental investigations and fabricating for the devices; Bo Han, Hongyun Chen and Huijian Ye were responsible for the draft of manuscript; and all authors reviewed the final version of manuscript.
Declaration of interests ☐ √The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0022-2860(19)31525-X
DOI:
https://doi.org/10.1016/j.molstruc.2019.127416
Reference:
MOLSTR 127416
To appear in:
Journal of Molecular Structure
Received Date: 10 August 2019 Revised Date:
12 November 2019
Accepted Date: 12 November 2019
Please cite this article as: B. Han, H. Chen, T. Hu, H. Ye, L. Xu, High electrical conductivity in polydimethylsiloxane composite with tailored graphene foam architecture, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.127416. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
High electrical conductivity in polydimethylsiloxane composite with tailored graphene foam architecture Bo Han, Hongyun Chen, Te Hu, Huijian Ye*, Lixin Xu* College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, China * To whom correspondence should be addressed. Tel: 86-571-88871519; E-mail: Huijian Ye: [email protected] Lixin Xu: [email protected] Abstract: Graphene foam has huge potential applications in thermal management owing to their unique three-dimensional (3D) interconnected structure. It’s still a challenge to construct graphene foam architecture via facile and efficient method to reach superior property with low loading of nanofillers. Here, we report a polymer-template-assisted assembly strategy to develop 3D graphene architecture that is incorporated into polydimethylsiloxane (PDMS) composite with high electrical conductivity and thermal property. A free-standing foam structure has been assembled with graphene nanosheets, which was exfoliated in low-boiling-point chloroform with assistance of hyperbranched polyethylene as polymer stabilizer against aggregation via CH-π noncovalent interactions. The resultant graphene foam/PDMS composite exhibits high thermal conductivity of 0.22 W·m-1·K-1 and high electrical conductivity of 9.1 × 10-3 S·cm-1 at relatively low loading as 0.7 wt%, which is ascribed to the efficient transfer of charge carrier and thermal phonon along the graphene skeleton. This work demonstrates that the PDMS composite incorporated with graphene foam is a promising candidate for thermal interface materials with high electrical conductivity. Keywords: liquid-phase exfoliation; hyperbranched polyethylene; graphene foam; polymer composite; thermal property; electrical conductivity
1
1. Introduction The requirements in miniaturization and integration of electronic devices motivate the rapid development of thermal interface material with high thermal conductivity that is normally applied between the heat source and the heat sink to transfer heat from inside efficiently [1-4]. Polymer-based thermal interface materials are widely used in electronics due to light weight, processability and low cost [5-7]. However, the common polymer tends to exhibit low thermal conductivity, which limits the capability in heat dissipation [8,9]. Therefore, the development of composite with high thermal conductivity has been investigated extensively to improve the efficiency of heat transfer. The thermal interface materials are usually composed of polymer matrix and high thermal conductive fillers, including ceramics [10,11], metal oxides [12,13], and carbon-based nanomaterials [9,14-17]. Graphene, as a two-dimensional nanoplate with excellent in-plane thermal conductivity around 5300 W·m-1·K-1, has attracted enormous attentions for promising candidate in thermal management applications [15,18-20]. The graphene/epoxy composite showed thermal conductivity under room temperature with the maximum value of 4.9 W·m-1·K-1 at 30 wt% loading [16]. Large size of graphene sheets and surface functionalization would further reduce the thermal resistance of the interfaces between of graphene and epoxy resin. Moreover, the high thermal conductivity of composite could be achieved by tuning the orientation of graphene nanofillers. The magnetically aligned graphene/PDMS composite had high anisotropy in thermal conductivity, in which the thermal conductivity of 3 wt% composite was increased by 49% compared with isotropic composite [21]. This enhancement was due to that the magnetic field re-arranged the particles in chain-like structures, which facilitated the effective longitudinal thermal transportation inside the composite. However, the preparation of highly orientation for nanosheets was complicated because high magnetic field was required to drive the heterogenous redistribution of nanosheets. It’s suggested that three-dimensional graphene foam (GF) exhibits efficient motion of matter and energy based on its unique interconnected structure [22-27]. The thermal 2
conductivity of GF consisting of few-layer graphene and ultrathin graphite was increased from 0.26 to 1.70 W·m-1·K-1 by using different etchants [22]. The thermal conductivity of poly(methyl methacrylate) (PMMA) composite with 0.4 wt% GF reached 0.27 W·m-1·K-1 and the local mechanical strength of composite was also enhanced by the addition of GF due to the in-situ polymerization method [28]. The polymer-template-assisted assembly strategy has been developed to prepare a novel class of GF with facile technology. The obtained GF/polymer composite exhibited a high thermal conductivity of 1.52 W·m-1·K-1 at relatively low graphene content of 5.0 wt% [29]. In the GF/polymer composites, the unique interconnected structure of GF enables heat transfer along the GF struts and also avoids the common dispersion issue of nanosheets in composite. Furthermore, GF would provide efficient paths for heat conduction with a very low graphene loading [28]. It’s still a challenge to prepare GF architecture via an efficient technique for polymer-based thermal interface materials. In our previous work, the natural graphite was exfoliated into few-layer graphene in low-boiling-point solvent with assistance of hyperbranched polymer stabilizer, which was adsorbed on the surface of nanosheets against aggregation through CH-π noncovalent interactions [30]. In this study, we report a novel strategy for the preparation of graphene foam by self-assembly method of two-dimensional graphene nanosheets on a polymer skeleton. The resultant GF/PDMS composite presents a high thermal conductivity of 0.22 W·m-1·K-1 and high electrical conductivity of 9.1 × 10-3 S·cm-1 at relatively low GF loading as 0.7 wt%. Meanwhile, the thermal stability and mechanical property of GF/PDMS composite have also been improved. This work demonstrates that the PDMS composite incorporated with graphene foam is a promising candidate for thermal interface material with high electrical conductivity. 2. Experimental 2.1 Materials Chloroform (AR) and natural graphite were purchased from Aldrich, and the PDMS resin (Sylgard 184) was obtained from Dow Corning. Commercial melamine resin 3
foam (Yongkang Jingshang Industry and Trade Co., Ltd.) for kitchen supplies was utilized in this work. The hyperbranched polyethylene (HBPE) was synthesized via chain-walking mechanism according to the typical procedure [30]. 2.2 Preparation of graphene dispersion Graphene dispersion was prepared by liquid-phase exfoliation method with HBPE as polymer stabilizer. A mixture of 800-mg graphite and 320-mg HBPE in 80-mL chloroform was employed for the large-scale production of graphene nanosheets. During the sonication process, the temperature of the sonication bath was maintained constant by a continuous flow of water. The resulting mixture was subsequently centrifuged at 4000 rpm for 45 min and supernatant dispersion was then collected as stable graphene dispersion. 2.3 Preparation of graphene foam/PDMS composite Graphene foam was prepared by the following procedure. Firstly, the commercial melamine resin foam was immersed in graphene solution with 2 mg·mL-1 for 1 h. Then, the surface of melamine resin foam was assembled with graphene nanosheets after drying process. Free-standing 3D graphene architecture with melamine resin foam skeleton was paralyzed at 550 °C for 2 min at air atmosphere to decompose the polymer completely. The foam sample was obtained after adsorption and pyrolysis process. The PDMS matrix was prepared by mixing of prepolymer and curing agent with a ratio of 10:1 by weight, which was infused into 3D graphene architecture. Finally, GF immersed in PDMS matrix was cured at 80 °C for 2 h. The sample was cooled down to the room temperature and then tailored into different shapes for further characterizations. 2.4 Characterizations The morphology of graphene was measured by transmission electron microscope (TEM, Philips-FEI) and atomic force microscope (AFM, Nanoscope 3D, Bruker). The morphology of cross-sectional fractured foam sample and GF/PDMS composite was investigated by scanning electron microscopy (SEM, Nano nova 450, FEI). The 4
thermal diffusivity coefficient of GF/PDMS composite with the diameter of 25.4 mm was evaluated by a laser-flash diffusivity instrument (LFA 467 MicroFlash, NETZSCH). The thermal stability was examined on a dynamic thermogravimetric analysis (TGA, Q600 SDT, TA) under the nitrogen atmosphere from room temperature to 1000 °C with a heating rate of 10 °C/min. Dynamic thermomechanical analysis (DMA, Q800, TA) was carried out from -140 °C to 30 °C at 3 °C/min with the tensile configuration of 1 Hz. The electrical conductivity of the GF/PDMS composite was measured by four-probe conductivity meter (RTS-8, China) from 20 °C to 80 °C. 3. Results and discussion
Figure 1. The schematically illustration for the liquid-phase exfoliation of few-layer graphene and the preparation of graphene foam/PDMS composite. The preparation process of graphene nanosheets and GF/PDMS composite is schematically illustrated in Figure 1. The graphene dispersion with concentration of 0.16 mg·mL-1 was prepared by the liquid-phase exfoliation method in a low-boiling-point chloroform, in which the hyperbranched HBPE acting as a polymer stabilizer remains stable dispersion of nanosheets. Then, the melamine resin foam was immersed in the concentrated graphene dispersion to attach the graphene nanosheets 5
on the three-dimensional polymer backbone. A complete three-dimensional graphene foam was received by pyrolysis process of removing the polymer skeleton. Finally, the mixture of PDMS prepolymer and curing agent with a weight ratio of 10:1 was filled in graphene foam to obtain a flexible composite after curing process. The two-dimensional nanosheets were successfully assembled into three-dimensional graphene foam via simple technology, which broadens the application of graphene obtained by liquid-phase exfoliation, and also provides a new insight for the construction of polymer composite with sterically architecture. (a)
(c)
(b)
5 layers 500 nm
(d)
50 nm
5 nm
(e)
(f)
(g)
(h)
Figure 2. (a) and (b) TEM images of graphene nanosheets, (c) the edge of a five-layer graphene sample, (d) AFM image of graphene nanosheets, (e) the enlarged AFM image of nanosheets in the dotted box of Figure 2d, (f) 3D AFM image for graphene in Figure 2e, (g) the statistical lateral sizes of graphene nanosheets with sample set N = 120, (h) the thickness statistics of graphene nanosheets with sample set N = 50. 6
The HBPE was applied to exfoliate the few-layer nanosheets from natural graphite as polymer stabilizer in low-boiling solvent, and a stable graphene dispersion was received, which is ascribed to the attached stabilizer against the aggregation of nanosheets [31]. The TEM images of nanosheets are displayed in Figure 2a and 2b, from which the graphene nanosheets are electrically transparent, and some also have wrinkles. The edge of the nanosheet is employed to distinguish the layer number. The morphology of graphene with edge is also shown in Figure 2c, which clearly identifies 5-layer structure for the exfoliated nanosheets. The TEM images of graphite are also shown in Figure S4, which confirms that graphite has been exfoliated into few-layer graphene. AFM technology is an effective way to characterize the thickness of graphene nanosheets, and the results are shown in Figure 2d. The enlarged 2D AFM image and 3D AFM image in the dotted frame are presented in Figure 2e and 2f, respectively. Figure 2g shows the statistical results of the lateral size for randomly selected 120 flakes, and about 88% of resultant nanosheets has the lateral dimension between 100 nm and 500 nm, which is consistent with the lateral dimension of few-layer graphene prepared by liquid-phase exfoliation [31-33]. As displayed in Figure 2f, the thickness of received graphene is less than 1.8 nm. Meanwhile, the thickness of nanosheets with sample set N=50 was estimated statistically in Figure 2h, from which the thickness of the resulting graphene is in the range of 1.0 to 2.2 nm [34,35]. The comparison in thickness and lateral size of graphene nanosheets prepared by liquid-phase exfoliation in the literature is shown in Table S1. Moreover, the XPS spectrum presented in Figure S1 indicates the low defects of nanosheets. The low binding energy at 283.7 eV corresponding to the graphitic C-C/C=C vibration is obviously the dominant peak for nanosheets. As typically observed for carbon materials, the shift towards higher binding energy was detected at 284.3 eV in graphene curve, which is assigned to C-O bonding. These results confirm that the few-layer nanosheets have been obtained by liquid-phase exfoliation from natural graphite in chloroform with assistance of HBPE as the stabilizer. The stable dispersion of graphene nanosheets in solution is mainly due to the interaction between graphene and hyperbranched polyethylene. It is also suggested that the stable 7
existence of substances can be improved by the hydrophobic interactions between molecules [36,37]. The noncovalent CH-π interactions between graphene and HBPE were yielded due to its large number of branched ends to be attached on the surface of nanosheets. It has been reported that the CH-π interaction is a weak attraction interaction between the C-H bond of an alkyl or aryl group and the π system [20]. The free-standing 3D foam architecture was assembled with assistance of commercial melamine resin sponge after pyrolysis process. The SEM images of received 3D graphene sponge and GF/PDMS composite are illustrated in Figure 3. We chose the
Figure 3. (a) and (d) SEM images of melamine resin foam, (b) and (e) SEM images of 3D graphene foam, (c) and (f) SEM images of frozen-fractured surface for GF/PDMS composite. porous melamine resin foam with an interconnected structure as a template for coating with graphene nanosheets. In Figure 3a and 3d, the pure melamine resin foam exhibits a 3D cross-linked structure with smooth surface. These porous are irregular shapes and the macro-porous diameter is around hundreds of micrometers. After soaking and pyrolysis process, the melamine resin foam was assembled with graphene nanosheets on the skeleton surface. Figure 3b and 3e present SEM images of the free-standing foam, in which nanosheets are attached on the surface of melamine resin foam with continuous macropore. The morphology of resultant GF is similar to melamine resin foam, but the skeleton of the network exhibits a rough texture associated with the presence of flexible nanosheets. Meanwhile, the whole architecture of the graphene 8
foam has a slight shrinkage compared with the pure melamine resin foam. The SEM images of freeze-fractured surface of GF/PDMS composite are displayed in Figure 3c and 3f. The approximate pentagon as foam skeleton reveals that GF structure is interconnected through the matrix. It also suggests excellent permeability inside the GF due to the large surface area and the presence of interconnected macropore. It’s reported that the modification of graphene nanosheets with tailored molecules could enhance their compatibility with the polymer matrix [21,38-40]. The problem of compatibility of the two-dimensional graphene filler with the polymer matrix is solved in the GF filled polymer composites due to the interconnected three-dimensional framework structure, which enables efficient transfer of thermal phonon along the graphene skeleton, and thus, improves the thermal conductivity of GF/PDMS composite. In graphene foam composites, the dispersion uniformity of graphene nanosheets is also important [41,42].
(a)
(c)
(b)
(d)
9
Storage modulus (MPa)
(e) 4000 PDMS GF/PDMS 3000
2000
1000
0 -120
-80
-40
0
40
Temperature (ºC)
Figure 4. The digital images of (a) pure PDMS and (b) GF/PDMS composite as the electrical conductor to drive a LED light, (c) thermal conductivity of GF/PDMS composite as a function of temperature, (d) TGA curves, (e) storage modulus of sample as a function of temperature. The GF loading was 0.7 wt%. As shown in Figure 4a and 4b, the digital images of conductive GF/PDMS composite as the conductor to drive a LED light, which indicates that the electrical conductivity of GF/PDMS composite is superior to pure PDMS. The high electrical conductivity of GF/PDMS composite is attributed to the well interconnected GF frameworks. The electrical conductivity of 0.7 wt% GF/PDMS composite shown in Figure S2 reaches 9.1×10-3 S·cm-1 compared with PDMS of 10-14 S·cm-1 as an insulator [43]. This significantly
improved
electrical
conductivity
is
mainly
ascribed
to
the
interconnection of nanosheets along 3D foam structure, which verifies that the interconnected GF provides effective paths for electron transfer inside the polymer. The charge carriers could diffuse efficiently along the graphene network as the GF/PDMS composite retains 3D interconnected network. The transport behavior of electrons in three-dimensional graphene foam composite is different from composite with graphene nanosheets. For the two-dimensional graphene/composite, the transmission of electrons becomes difficult due to the large interfacial resistance between the nanosheets and the polymer matrix. Therefore, the conductivity of the whole composite is very low. For the graphene foam structure, the nanosheets are connected to each other throughout the composite, and the electrical resistance between the graphene nanosheets turns small. The electrons are preferentially
10
transported in the graphene foam and can be transported throughout the polymer matrix, which leads to the high conductivity of the composite with foam architecture. This high electrical conductivity of PDMS composite evidences that the three-dimensional graphene foam structure has been successfully constructed in the system. The thermal conductivity of GF/PDMS composite film at different temperature is shown in Figure 4c. The transient laser flash method was employed to estimate the thermal conductivity of the GF/PDMS composite. Thermal conductivity of polymer composite was calculated from the equation of λ = ϲ·ρ·α, where λ, ϲ, ρ, and α represent thermal conductivity, specific heat, bulk density and thermal diffusivity, respectively. The thermal conductivity of pure PDMS at room temperature is 0.18 W·m-1·K-1. After the addition of graphene foam, the thermal conductivity of 0.7 wt% composite reaches 0.22 W·m-1·K-1 at room temperature, which is 22% higher than that of pure PDMS. The interconnected architecture of GF endows the composite with an effective heat conduction network, which is due to that the heat travels preferentially along graphene struts and leads to high thermal conductivity [22,23]. Meanwhile, the thermal conductivity of GF/PDMS composite exhibits a temperature-dependent effect, that is, the thermal conductivity varies with increasing environmental temperature. This linear temperature-dependent behavior may be related to the phonon distribution of the two-dimensional system such as graphite, indicating that the temperature-dependent thermal conductivity of GF/PDMS composite obeys a similar mechanism of graphite [27]. The thermal transport performance of composite mainly depends on the density of hierarchical framework, which was verified in the graphene/carbon nanotube aerogel system [3]. It’s reasonable to deduce that the transmission path for the thermal phonons becomes long with the large pore size in the foam architecture. The curves of weight loss for pure PDMS and GF/PDMS composite are presented in Figure 4d. The TGA curve of GF/PDMS composite obviously shifts towards higher temperature above 500 °C. Compared with pure PDMS, the addition of graphene 11
foam leads to an increase of the thermal stability. For example, the corresponding temperatures T20% (the temperature at 20 wt% loss of whole weight) of the GF/PDMS composite is higher than that of pure PDMS, which strengthens that the thermal stability of the composite is improved after the addition of foam. This phenomenon is probably ascribed to the addition of graphene with high thermal stability, which is incorporated in the PDMS composite as an interconnected structure. The storage modulus (E’) curves under variable temperatures are demonstrated in Figure 4e. In the glassy region, the incorporation of graphene foam into PDMS yields an increase of 10.4% for E’ at -138 ˚C. The storage modulus E’ decreases dramatically with the increasing temperature from -138 to -110 ˚C as the consequence of the glass transition. Nevertheless, the decrease of E’ is compensated by the presence of foam structure, which illustrates an improvement of thermal stability.
Figure 5. The schematically view of heat transfer in the PDMS composite with graphene foam structure. The electrical conductivity and thermal conductivity of the composite are improved due to the construction of the three-dimensional graphene foam. The thermal conductivity of polymer composite depends on many factors, including the thermal conductivity of matrix, the thermal conductivity of fillers, the volume fraction and thickness of fillers, and the interface thermal resistance between fillers and matrix. These factors determine the establishment of a thermal phonon path in the composite. Conventional two-dimensional graphene fillers are dispersed randomly in polymer 12
matrix, which is disadvantageous for the transportation of thermal phonon and electron under low loading of nanofillers. Therefore, we assemble few-layer graphene obtained by liquid-phase exfoliation method into a three-dimensional foam structure, which is more conducive to the transfer of electrons and thermal phonon along the graphene skeleton as illustrated in Figure 5. The graphene foam has a good interfacial action in the PDMS composite and also forms an interconnected network structure, which has great advantages compared with the two-dimensional nanofillers in thermal interface composite. This facile polymer-template method provides new strategy for the construction of graphene/polymer composite with high electrical conductivity. 4. Conclusions In summary, we demonstrate an effective way to prepare graphene foam by coating nanosheets on melamine resin as polymer template to construct the efficient route for charge carrier and thermal phonon in the composite. The graphite was exfoliated into few-layer graphene with low defects, which is attributed to HBPE polymer adsorbed on nanosheets surface via the CH-π noncovalent interaction. The thermal conductivity of GF/PDMS composite with 0.7 wt% graphene nanosheets is 0.22 W·m-1·K-1, which is 22% higher than that of pure PDMS matrix due to the interconnected 3D architecture. The electrical conductivity of GF/PDMS composite reaches 9.1 × 10-3 S·cm-1, which increases by 11 orders of magnitude than pure PDMS. This PDMS composite incorporated with graphene foam architecture is a promising candidate for applications in thermal management and electrical devices. Acknowledgements The financial support from the National Natural Science Foundation of China (21474091, 51707175) and Natural Science Foundation of Zhejiang Province (LTZ20E070001, LY18B040005) is greatly appreciated. This work is also supported by China Postdoctoral Science Foundation (2018M640572). References [1] A.P. Yu, P. Ramesh, M.E. Itkis, E. Bekyarova, R.C. Haddon, Graphite nanoplatelet-epoxy 13
composite thermal interface materials, Journal of Physical Chemistry C, 111 (2007) 7565-7569. [2] K.M.F. Shahil, A.A. Balandin, Thermal properties of graphene and multilayer graphene: Applications in thermal interface materials, Solid State Communications, 152 (2012) 1331-1340. [3] P. Lv, X.W. Tan, K.H. Yu, R.L. Zheng, J.J. Zheng, W. Wei, Super-elastic graphene/carbon nanotube aerogel: A novel thermal interface material with highly thermal transport properties, Carbon, 99 (2016) 222-228. [4] K.M.F. Shahil, A.A. Balandin, Graphene-multilayer graphene nanocomposites as highly efficient thermal interface materials, Nano Letters, 12 (2012) 861-867. [5] P.B. Kaul, M.F.P. Bifano, V. Prakash, Multifunctional carbon nanotube-epoxy composites for thermal energy management, Journal of Composite Materials, 47 (2013) 77-95. [6] Q. Li, Y.F. Guo, W.W. Li, S.Q. Qiu, C. Zhu, M.L. Chen, C.J. Liu, S.T. Liao, Y.P. Gong, A.K. Mishra, L.W. Liu, Ultrahigh thermal conductivity of assembled aligned multilayer graphene/epoxy composite, Chemistry of Materials, 26 (2012) 4459-4465. [7] Z.Q. Kuang, Y.L. Chen, Y.L. Lu, L. Liu, S. Hu, S.P. Wen, Y.Y. Mao, L.Q. Zhang, Fabrication of highly oriented hexagonal boron nitride nanosheet/elastomer nanocomposites with high thermal conductivity, Small, 11 (2015) 1655-1659. [8] X.F. Xu, J. Chen, J. Zhou, B.W. Li, Thermal conductivity of polymers and their nanocomposites, Advanced Materials, 30 (2018) 1705544. [9] Z.C. Tao, H.B. Wang, X.F. Li, Z.J. Liu, Q.G. Guo, Expanded graphite/polydimethylsiloxane composites with high thermal conductivity, Journal of Applied Polymer Science, 134 (2017) 44843. [10] X.L. Zeng, Y.M. Yao, Z.Y. Gong, F.F. Wang, R. Sun, J.B. Xu, C.P. Wong, Ice-templated assembly strategy to construct 3D boron nitride nanosheet networks in polymer composites for thermal conductivity improvement, Small, 11 (2015) 6205-6213. [11] M. Loeblein, S.H. Tsang, M. Pawlik, E.J.R. Phua, H. Yong, X.W. Zhang, C.L. Gan, E.H.T. Teo, High-density 3D-boron nitride and 3D-graphene for high-performance nano-thermal interface material, ACS Nano, 11 (2017) 2033-2044. [12] H. Yu, L.L. Li, Y.J. Zhang, Silver nanoparticle-based thermal interface materials with 14
ultra-low thermal resistance for power electronics applications, Scripta Materialia, 66 (2012) 931-934. [13] D. Suh, C.M. Moon, D. Kim, S. Baik, Ultrahigh thermal conductivity of interface materials by silver-functionalized carbon nanotube phonon conduits, Advanced Materials, 28 (2016) 7220-7227. [14] T.X. Ji, Y.Y. Feng, M.M. Qin, W. Feng, Thermal conducting properties of aligned carbon nanotubes and their polymer composites, Composites Part A-Applied Science & Manufacturing, 91 (2016) 351-369. [15] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau, Superior thermal conductivity of single-layer graphene, Nano Letters, 8 (2008) 902-907. [16] B. Tang, G.X. Hu, H.Y. Gao, L.Y. Hai, Application of graphene as filler to improve thermal transport property of epoxy resin for thermal interface materials, International Journal of Heat & Mass Transfer, 85 (2015) 420-429. [17] P. Zhang, J.H. Zeng, S.P. Zhai, Y.Q. Xian, D.G. Yang, Q. Li, Thermal properties of graphene filled polymer composite thermal interface materials, Macromolecular Materials & Engineering, 302 (2017) 1700068. [18] S.H. Xie, Y.Y. Liu, J.Y. Li, Comparison of the effective conductivity between composites reinforced by graphene nanosheets and carbon nanotubes, Applied Physics Letters, 92 (2008) 243121. [19] H.Q. Yao, S.A. Hawkins, H.J. Sue, Preparation of epoxy nanocomposites containing well-dispersed graphene nanosheets, Composites Science & Technology, 146 (2017) 161-168. [20] H.J. Ye, X.H. Zhang, C.F. Xu, B. Han, L.X. Xu, Enhanced dielectric property and energy density in poly(vinylidene fluoride-chlorotrifluoroethylene) nanocomposite incorporated with graphene
functionalized
with
hyperbranched
polyethylene-graft-poly(trifluoroethyl
methacrylate) copolymer, Journal of Materials Chemistry C, 6 (2018) 11144-11155. [21] B. Li, S. Dong, X. Wu, C.P. Wang, X.J. Wang, J. Fang, Anisotropic thermal property of magnetically oriented carbon nanotube polymer composites, Composites Science & Technology, 147 (2017) 52-61. [22] M.T. Pettes, H.X. Ji, R.S. Ruoff, L. Shi, Thermal transport in three-dimensional foam 15
architectures of few-layer graphene and ultrathin graphite, Nano Letters, 12 (2012) 2959-2964. [23] H.X. Ji, D.P. Sellan, M.T. Pettes, X.H. Kong, J.Y. Ji, L. Shi, R.S. Ruoff, Enhanced thermal conductivity of phase change materials with ultrathin-graphite foams for thermal energy storage, Energy & Environmental Science, 7 (2014) 1185-1192. [24] A. Pedrielli, S. Taioli, G. Garberoglio, N.M. Pugno, Mechanical and thermal properties of graphene random nanofoams via molecular dynamics simulations, Carbon, 132 (2018) 766-775. [25] Y.F. Li, Z.F. Chen, XH/pi (X = C, Si) Interactions in graphene and silicene: weak in strength, strong in tuning band structures, Journal of Physical Chemistry Letters, 4 (2013) 269-275. [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, Composites Part B-Engineering, 94 (2016) 102-108. [27] H.Y. Yan, Y.X. Tang, W. Long, Y.F. Li, Enhanced thermal conductivity in polymer composites with aligned graphene nanosheets, Journal of Materials Science, 49 (2014) 5256-5264. [28] C. Zhang, A. Li, Y.H. Zhao, S.L. Bai, Y.F. Zhang, Thermal, electrical and mechanical properties of graphene foam filled poly(methyl methacrylate) composite prepared by in situ polymerization, Composites Part B-Engineering, 135 (2018) 201-206. [29] Z.D. Liu, D.Y. Shen, J.H. Yu, W. Dai, C.Y. Li, S.Y. Du, N. Jiang, H.R. Li, C.T. Lin, Exceptionally high thermal and electrical conductivity of three-dimensional graphene-foam based polymer composites, RSC Advances, 6 (2016) 22364-22369. [30] L.X Xu, J.W. Mcgraw, F. Gao, M. Grundy, Z.B. Ye, Z.Y. Gu, J.L. Shepherd, Production of high-concentration graphene dispersions in low-boiling-point organic solvents by liquid-phase noncovalent exfoliation of graphite with a hyperbranched polyethylene and formation of graphene/ethylene copolymer composites, Journal of Physical Chemistry C, 117 (2013) 10730-10742. [31] M. Lotya, P.J. King, U. Khan, S. De, J.N. Coleman, High-concentration, surfactant-stabilized graphene dispersions, ACS Nano, 4 (2010) 3155-3162. [32] A. O’Neill, U. Khan, P.N. Nirmalraj, J. Boland, J.N. Coleman, Graphene dispersion and 16
exfoliation in low boiling point solvents, Journal of Physical Chemistry C, 115 (2011) 5422-5428. [33] S. Haar, M. Bruna, J.X. Lian, F. Tomarchio, Y. Olivier, R. Mazzaro, V. Morandi, J. Moran, A.C. Ferrari, D. Beljonne, Liquid-phase exfoliation of graphite into single and few layers graphene with α-functionalized alkanes, Journal of Physical Chemistry Letters, 7 (2016) 2714-2721. [34] A.B. Bourlinos, V. Georgakilas, R. Zboril, T.A. Steriotis, A.K. Stubos, Liquid-phase exfoliation of graphite towards solubilized graphenes, Small, 5 (2009) 1841-1845. [35] S. Sampath, A.N. Basuray, K.J. Hartlieb, T. Aytun, S.I. Stupp, J.F. Stoddart, Direct exfoliation of graphite to graphene in aqueous media with diazaperopyrenium dications, Advanced Materials, 25 (2013) 2740-2745. [36] J. Chamani, M. Heshmati, Mechanism for stabilization of the molten globule state of papain by sodium n-alkyl sulfates: Spectroscopic and calorimetric approaches, Journal of Colloid and Interface Science, 322 (2008) 119-127 [37] A.A. Moosavi-Movahedi, S. Hakimelahi, J. Chamani, G.A. Khodarahmi, F. Hassanzadeh, F.T. Luo, T.W. Ly, K.S. Shia, C.F. Yen, M.L. Jain, R. Kulatheeswaran, C.H. Xue, M. Pasdar, G.H. Hakimelahi, Design, synthesis, and anticancer activity of phosphonic acid diphosphate derivative of adenine containing butenolide and its water-soluble derivatives of paclitaxel with high antitumor activity, Bioorganic and Medicinal Chemistry, 11 (2003) 4303-4313 [38] J.J. Tang, W. Yao, W.L. Li, J. Xu, L. Jin, J.D. Zhang, Z.X. Xu, Study on a novel composite coating based on PDMS doped with modified graphene oxide, Journal of Coatings Technology & Research, 15 (2018) 375-383. [39] J. Osicka, M. Ilcikova, M. Mrlik, A. Minarik, V. Pavlinek, J. Mosnacek, The impact of polymer grafting from a graphene oxide surface on its compatibility with a PDMS matrix and the light-induced actuation of the composites, Polymers, 9 (2017) 264. [40] K.T.S. Kong, M. Mariatti, A.A. Rashid, J.J.C. Busfield, Enhanced conductivity behavior of polydimethylsiloxane
(PDMS)
hybrid
composites
containing
exfoliated
graphite
nanoplatelets and carbon nanotubes, Composites Part B-Engineering, 58 (2014) 457-462. [41] H. Sanei, A. Asoodeh, S. Hamedakbari-Tusi, J. Chamani, Multi-spectroscopic investigations of aspirin and colchicine interactions with human hemoglobin: binary and ternary systems, 17
Journal of Solution Chemistry, 40 (2011) 1905-1931. [42] N. Shakibapour, F.D. Sani, S. Beigoli, H. Sadeghian, J. Chamani, Multi-spectroscopic and molecular modeling studies to reveal the interaction between propyl acridone and calf thymus DNA in the presence of histone H1 binary and ternary approaches, Journal of Biomolecular Structure and Dynamics, 37 (2019) 359-371. [43] M.P.
Wolf,
G.B.
Salieb-Beugelaar,
P.
Hunziker,
PDMS
with
designer
functionalities-Properties, modifications strategies, and applications, Progress in Polymer Science, 83 (2018) 97-134.
18
A graphene foam architecture is incorporated into PDMS composite.
Graphene was exfoliated in chloroform with HBPE via CH-π interactions.
The composite exhibits high thermal conductivity with low loading.
The efficient route of thermal phonon is formed along the skeleton.
Author contributions: Huijian Ye and Lixin Xu conceived the entire research aims; Bo Han, Hongyun Chen and Te Hu conducted the experimental investigations and fabricating for the devices; Bo Han, Hongyun Chen and Huijian Ye were responsible for the draft of manuscript; and all authors reviewed the final version of manuscript.
Declaration of interests ☐ √The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: