Article pubs.acs.org/IECR
Encapsulation of Graphite Nanoflakes for Improving Thermal Conductivity of Mesogenic Epoxy Composites Fubin Luo,†,‡ Kun Wu,*,† Xiaomei Huang,†,§ Wenguang Hu,†,§ and Mangeng Lu† †
Key Laboratory of Cellulose and Lignocellulosics Chemistry, Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100039, P. R. China § Guangdong Provincial Engineering & Technology Research Center for Touch Significant Devices Electronic Materials, Guangzhou 510650, P. R. China ABSTRACT: Graphite nanoflake (GN) is the most prominent filler for thermally conductive polymeric composites. However, the aggregation problem of GN has an adverse effect on the improvement of thermal conductivity of the composites. In this study, GR is decorated with melamineformaldehyde (MF) by in situ polymerization, which successfully solves the well-known aggregation problem of GN. GN@MF is used as nanofillers for improving the thermal conductivity of the biphenyl mesogenic epoxy resin (MEP). Photomicrograph observations of MEP/GN@MF composites exhibit that GN@MF flakes are evenly dispersed in the polymer matrix, which demonstrate that GN@MF has excellent dispersion in the MEP matrix. The homogeneous dispersion of GN@MF in MEP favors the formation of the consecutive thermal conductive paths or networks. As a result, the MEP/GN@MF composite has a higher thermal conductivity than MEP/GN with the same content of loading.
1. INTRODUCTION As miniaturization and power density of modern electronics devices increase, the efficient removal of redundant heat generated by high-power integrated circuits is a critical issue.1−5 Ideal materials for packaging and thermal management in microelectronic devices should have excellent dielectric constant and dielectricloss, high thermal conductivity, and easy processability.6,7 Polymers can offer excellent dielectric properties and remarkable processability. However, the inherent thermal conductivity of polymer materials cannot satisfy the demand of heat diffusion. Carbon-based materials, comprising graphite, carbon nanotube, graphene, graphene oxide (GO) and graphite nanoflake (GN), are intriguing for their excellent chemical stability and high thermal conductivity.8−11 GN is considered to be the most suitable filler for thermally conductive polymeric composites because of its inherent high thermal conductivity, low coefficient of thermal expansion, low cost, lightweight, and ease of feeding for extrusion, and therefore, it has been widely used in polymer composites.12−14 Owing to the high surface energy, the critical issue of 2D nanoflakes materials is the aggregation phenomenon when used as nanofiller for improving thermal conductivity of polymer materials. GN can form an irreversibly precipitated agglomerate, and the aggregated nanoflakes behave no differently than particulate graphite. That is, the advantages of the nanoflakes is lost, which can deleteriously affect the potential applications of the GN.15−17 Recently, to make most of the excellent properties, many researchers have focused on the surface modification and © XXXX American Chemical Society
interface influence of carbon-based materials on the polymer matrix. Gaetano Guerra and his co-workers18,19 had discoveried the catalytic action of different graphite-based nanofillers on the curing of epoxy. Chois20 reported the synthesis of silica-coated graphite by enolization of polyvinylpyrrolidone. The silicacoated graphite/TPEE composites retain the perfectly insulating surface resistivity; however, the in-plane and through-plane thermal conductivity values of the silica-coated graphite/TPEE composites are 67.5% and 86.6% of those of raw graphite/TPEE at 80 phr loading. Wang21 demonstrated the preparation of nanosilica/graphene oxide (m-SGO) hybrid through a sol−gel and surface treatment process. Thermal conductivity of the mSGO1.5/EP nanocomposites is 0.29 W m−1 K−1. Qian22 presents a new class of fillers of alumina-coated graphene sheet (GS@ Al2O3) hybrid fillers via an electrostatic self-assembly route, however, the thermal conductivity of PVDF composites with 10 wt % GS@Al2O3 is below 0.40 W m−1 K−1. Ding23 prepared polyamide-6/graphene by in situ ring-opening polymerization reaction using ε-caprolactam as the monomer, 6-aminocaproic acid as the initiator, and reduced graphene oxide (RGO) as the thermal conductive filler. The generated polyamide-6 chains are covalently grafted onto GO sheets through the “grafting to” strategy with the simultaneous thermal reduction reaction from Received: Revised: Accepted: Published: A
September 10, 2016 December 20, 2016 December 22, 2016 December 22, 2016 DOI: 10.1021/acs.iecr.6b03506 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
stirring after the PH was adjusted to 8−9 with sodium carbonate solution. The resultant product was MF prepolymer. Decoration of GN with MF via in situ polymerization: GN (1.00 g) was dispersed in 750 mL of N-methyl-2-pyrrolidone (NMP) by sonication for 30 min. After that, half of the prepared MF prepolymer was poured into the flask. The mixture was stirred at 80 °C for 5 h. Subsequently, the suspension of the product was filtrated and dried. The resultant product was named GN@MF. 2.4. Preparation of MEP Resin and Nanocomposites. The synthesized MEP was mixed with DDM with stoichiometric ratio of 2:1. An appropriate amount (0%, 2%, 5%, and 8%) of GN or GN@MF was respectively added into the mixture. The powder was milled in mortar. After that the mixture was cast into a mold for curing and postcuring via the procedures of 105 °C/4 h+, 155 °C/3 h, and 195 °C/1 h, respectively. The resultant nanocomposites were coded as MEP, MEP/GN-n, MEP/GN@ MF-n, respectively, where n represents the weight percent of fillers in the resultant nanocomposites (n = 2, 5, 8). 2.5. Characterization. Fourier transform infrared spectrometry (FTIR) was carried out using Bruker TENSOR 27 FTIR spectrophotometer with KBr pellets for solid samples. Scanning electron microscopy (SEM) was performed on a Hitachi S3400N scanning electron microscope. The samples were sprayed with gold. Transmission electron microscopy (TEM) was performed on a JEM-1230 transmission electron microscope. The powder samples for TEM were were diluted with alcohol until invisible, and then ultrasonicated for 20 min, the samples were prepared by mounting a drop of the micelle solution (approximate 0.05 mL) on a copper grid. Laser Raman spectroscopy was performed by using Renishaw invia Raman spectrometer excitated at 632.8 nm. The X-ray diffraction (XRD) patterns were collected using a Rigaku D/MAX-1200 X-ray diffractometer with Cu Ka radiation (k = 0.154 nm) at room temperature in the range of 1.5−80°. The dynamic mechanical properties were investigated with a Netzsch DMA242 instrument; all the samples were heated from room temperature to 200 °C, at a frequency of 1 Hz in tensile mode. The sample dimensions were 30 × 8 × 2 mm3. Thermal conductivities of the composites were measured by hot-wire thermal conductivity instrument (Xiatech TC3000E) with dimensions of 30 × 30 × 2 mm3. The dielectric property was
GO to RGO. The thermal conductivity of the composite is increased to 0.416 W m−1 K−1 with 10 wt % GO sheets loading. In this work, GN is encapsulated with polymer resin by in situ polymerization other than surface modification; the compact coverage can lower the surface energy efficiently. This method is expected to successfully solve the well-known aggregation phenomenon of GN. The biphenyl mesogenic epoxy resin (MEP) was synthesized and chosen as the matrix resin because of its promising application in many cutting-edge fields, which urgently require improved thermal conductivity.24−26 The resultant GN is demonstrated to be a more effective nanofiller for enhancing thermal conductivity of MEP compared with raw GN.
2. EXPERIMENTAL SECTION 2.1. Materials. Melamine (AR) and formaldehyde were obtained from Tianjing Kemiou Chemical Reagent Co. Ltd. Graphite nanoflake was purchased from Suzhouhengqiu Technological Co., Ltd. 4,4-Diaminodiphenylmethane (DDM) was supplied by Aladdin Industrial Corporation. 2.2. Preparation of Biphenyls Mesogenic Epoxy. Biphenyls mesogenic epoxy (MEP) named 3,3′,5,5′-tetramethyl-4,4′ biphenyl diglycidyl ether was synthesized in our laboratory according to our earlier reports.27−29 Chemical structure of the resultant synthesized epoxy used in this study was depicted in Scheme 1. The chemical structure and liquid crystallinity were studied in our previous work.27,28 Scheme 1. Molecular Formula of the Synthesized Epoxy
2.3. Synthesis of GN@MF. Preparation of melamine− formaldehyde (MF) prepolymer: Melamine (10.0 g) was mixed with 50.0 mL of distilled water in a flask. Formaldehyde (17.9 mL, 37%) was dropped into the suspension. The reaction process was proceeded at 80 °C for 30 min with mechanical
Figure 1. FTIR (a) and Raman (b) spectra of GN and GN@MF. B
DOI: 10.1021/acs.iecr.6b03506 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. SEM micrographs of (a-1, a-2, a-3) GN and (b-1, b-2, b-3) GN@MF.
Figure 3. TEM micrographs of (a) GN and (b) GN@MF.
After decoration, the GN@MF flakes are bondless because of the presence of polymer shell and the aggregation phenomenon cannot be observed anymore. To further identify the decoration of GN, the morphology of GN and GN@MF is observed by TEM. The micrographs are presented in Figure 3. The raw GN is translucent and exhibits a typically smooth multilayered structure. While The TEM micrograph of GN@MF exhibits that the nanosheet is covered by a black domain, reflecting that dense MF resin is deposited on the surface of GN as a layer of protective shell. The SEM and TEM results demonstrate that GN is decorated by MF materials. 3.3. XRD Patterns of GN and GN@MF. The XRD spectra are measured in the same condition of GN and GN@MF. The diffraction peaks are depicted in Figure 4. Raw GN shows a strong 002 peak at 26.44°, which is the typical XRD diffraction of raw GN, corresponding to the d spacing of 0.34 nm based on the Bragg equation.32−34 As for GN@MF, it also has the characteristic diffraction peak located at 26.44°. However, the relative intensity of the peak for GN@MF is much weaker than that of GN. Approximately, the intensity of the peak for GN@MF is 8% that of GN. The sharply decrement intensity of the peak of GN is aroused by the decoration. The XRD results demonstrate that GN is decorated by the MF resin during the preparation process rather than the simple surface modification. According to the intensity decrement of diffraction peaks, it can be speculated that the vast majority of flakes of GN are successfully encapsulated. 3.4. Thermal Conductivity of the Nanocomposites. The thermal conductivity coefficient of neat MEP is 0.26 W m−1 K−1. As Figure 5 shows, raw GN can enhance the thermal conductivity of MEP. The thermal conductivity coefficient of MEP/GN-2, MEP/GN-5, MEP/GN-8 is 0.30, 0.41, and 0.43 W m−1 K−1, respectively. By comparison, it can found that MEP/GN@MF-n
recorded by Agilent E4981A. Optical microscope images were recorded by an optical microscope (Orthoglan, LEITZ, Germany); samples were prepared by rapid slice, the thickness of the film were about 1 um.
3. RESULTS AND DISCUSSION 3.1. FTIR and Raman Characterization of GN and GN@ MF. The MF resin was prepared and investigated by FTIR and Raman. The spectra of MF, GN, and GN@MF are shown in Figure 1a,b. The typical bands of MF are located at 3350, 1635, 1513, 1330, 1191, and 1120, which can be assigned to the absorption of N−H stretch, bending vibration of N−H of primary amine, stretching vibration of CN, and the stretching vibration of symmetric C−O−C− of − CH2−O−CH2− between melamine groups.30,31 Raw GN shows no typical absorption, while GN@MF presents the same characteristic absorption peaks as MF resin. In the Raman spectra, the D-band around 1322 cm−1 (corresponding the defects or edges) and Gband around 1575 cm−1 (corresponding to the first-order scattering of the E2g mode) were presented in Figure 1b, indicating that GN has a graphene structure. For GN@MF, an additional peak can be observed, which is aroused by the MF coating. The above results suggest that the shell of GN is MF polymer resin. 3.2. SEM and TEM Observation of GN and GN@MF. Figure 2 depicts the SEM micrographs of GN and GN@MF. Obviously, GN easily agglomerates because of its high surface energy. Most of the flakes of GN naturally form a variety of aggregations as Figure 2 shows. The aggregation phenomenon of GN would lead to its poor dispersion in a polymer matrix. It can be found that every GN@MF flake is decorated by a layer of shell. C
DOI: 10.1021/acs.iecr.6b03506 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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the MEP/GN@MF-8 matrix. Isolated GN@MF nanoflakes are homogeneously dispersed in epoxy matrix and almost no large agglomerates or bundles can be observed. By comparison, the distribution of GN in MEP is uneven, GN aggregation can be found in the MEP matrix and some tiny cracks exist between the interface of matrix and GN flakes. The tiny cracks might be ascribed to the poor compatibility of GN in MEP. The dispersion mode of MF and GN@MF can be speculated. The dispersion of the additives in MEP matrix is observed directly using optical microscope. The optical microscope images of three random samples of MEP/GN-8 and MEP/GN@MF-8 are recorded, respectively, which are shown in Figure 7. Apparently, GN@MF has superior dispersiveness compared with GN in MEP. Abundant clusters can be found of GN in the matrix. The poor dispersion of GN in the epoxy matrix corresponds to that reported in the literature.35 By contrast, few GN@MF additives are aggregated in MEP. The dispersion modes of the additives are shown in Figure 7 panels a and b. The observations correspond to the results seen in Figure 2 and Figure 6. In addition, because of the superior dispersiveness of GN@MF in MEP, much more thermal conductive paths can be formed in the composites, resulting in the thermal conductivity of MEP/GN@MF being higher than that of MEP/GN. 3.6. Mechanical and Dielectric Properties of the Nanocomposites. The thermomechanical property of MEP and the nanocomposites are studied by DMA. The curves are shown in Figure 8. The storage modulus (E′ at 30 °C) of MEP is 2268 MPa, that of MEP/GN@MF-8 and MEP/GN-8 is 2394 and 2213 MPa, respectively. The E′ of MEP/GN@MF-8 is increased by 126 MPa compared to that of neat MEP. The MDA results imply that GN@MF can enhance the mechanical property of MEP. However, due to poor compatibility and dispersion, GN exhibits a negative effect on the mechanical property of MEP. For embedded passives such as resistors, capacitors, and inductors in integrated circuits to be smaller, faster, and more flexible, and possess higher performance, high dielectric constant polymer materials with high thermal conductivity are required.21,36 The dielectric constant and dielectric loss of MEP, MEP/GN@MF-8, and MEP/GN-8 are displayed in Table 1. It can be found that the dielectric constant and dielectric loss is increased due to the addition of nanofillers. It can be seen that dielectric loss of MEP/GN@MF-8 and MEP/GN-8 are both 0.47, which is a slight increase of 20%. The results imply that the decoration of GN would not increase the dielectric loss of its MEP composite. However, the dielectric constant of MEP/ GN@MF-8 is as high as 10.98, which is 268% of that of MEP and 141% of that of MEP/GN-8. The increase dielectric constant of MEP/GN@MF-8 can mainly attributed to the formation of mini-capacitors in the composite because of the excellent dispersion of GN@MF in the MEP matrix.37
Figure 4. XRD patterns of GN and GN@MF.
Figure 5. Thermal conductivity of MEP nanocomposites with varied fillers content and the enhancement coefficient compared with neat MEP.
has higher thermal conductivity coefficient than that of MEP/ GN-n. Thermal conductivity coefficient of MEP/GN@MF-8 is 0.52 W m−1 K−1, which is enhanced by 100% compared with that of neat MEP. However, the enhancement coefficient of MEP/ GN-8 is only 65.4% compared with neat MEP, which is 34.6% lower than that of MEP/GN@MF-8. The above results indicate that GN@MF is more efficient for improving the thermal conductivity of MEP. This phenomenon can be attributed to the excellent dispersion of GN@MF. Raw GN easily aggregates and hardly evenly disperses in the MEP matrix. Most likely, GN would present in matrix as clusters as the SEM images depict. 3.5. Morphologies of the Fracture Surface of the Nanocomposites. The morphologies of the fracture surface of MEP, MEP/GN-8, and MEP/GN@MF-8 are observed by SEM, and the images are displayed in Figure 6. It can be found that fracture surface of MEP is smooth and relatively flat. Neat striation can be found on the surface. In contrast, owing the presence of fillers, the fracture surface of MEP/GN-8 and MEP/ GN@MF-8 is quite rough. Abundant nanoflakes can be observed in the fracture surface. GN@MF nanoflakes are dispersed well in
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CONCLUSION GN is decorated by MF resin via in situ polymerization. The decoration of GN solves the aggregation phenomenon which is the critical issue for its commercial application. The resultant GN@MF exhibits excellent natural dispersibility. GN@MF is used as nanofillers for improving the thermal conductivity, dielectric property, and mechanical properties of the biphenyl mesogenic epoxy resin (MEP). The results indicate that MEP/ GN@MF composite obtains a higher storage modulus than MEP/GN composites, and the dielectric constant of MEP/ GN@MF-8 is enhanced by 168% compared with neat MEP. The D
DOI: 10.1021/acs.iecr.6b03506 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 6. Morphologies of the fracture surface of (a-1, a-2, a-3) MEP, (b-1, b-2, b-3) MEP/GN-8, and (c-1, c-2, c-3)MEP/GN@MF-8.
Figure 7. Optical microscope image of MEP/GN-8 (a-1, a-2, a-3) and MEP/GN@MF-8 (b-1, b-2, b-3).
Table 1. the Dielectric Constant and Dielectric Loss of MEP, MEP/GN@MF-8, and MEP/GN-8 (1 MHz) samples
dielectric constant
dielectric loss
MEP MEP/GN-8 MEP/GN@MF-8
4.09 7.79 10.98
0.39 0.47 0.47
However, the enhancement efficiency of MEP/GN-8 is only 65.4% compared with neat MEP, which is 34.6% lower than that of MEP/GN@MF-8. The above results indicate that GN@MF is more efficient for improving the thermal conductivity of MEP. This phenomenon can be attributed to the excellent dispersion of GN@MF, resulting in a much more thermal conductive path formed in the composites.
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AUTHOR INFORMATION
Corresponding Author
*Tel (Fax): +86-20-85231925. E-mail:
[email protected].
Figure 8. Dynamic mechanical properties curves of MEP, MEP/GN@ MF-8, and MEP/GN-8.
ORCID
Kun Wu: 0000-0001-6494-8851 thermal conductivity coefficient of MEP/GN@MF-8 is 0.52 W m−1 K−1, which is enhanced by 100% compared with neat MEP.
Notes
The authors declare no competing financial interest. E
DOI: 10.1021/acs.iecr.6b03506 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS The financial supports from Guangdong Natural Science Foundation, China (No. 2015A030313798, 2016A030313161) and Guangdong Special Support Program-Youth Top-notch Talent (No. 2014TQ01C400) are acknowledged.
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