Design and Preparation of a Unique Segregated Double Network with

Feb 6, 2017 - It is still a challenge to fabricate polymer-based composites with excellent thermal conductive property because of the well-known diffi...
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Design and Preparation of a Unique Segregated Double Network with Excellent Thermal Conductive Property Kai Wu,† Chuxin Lei,† Rui Huang,† Weixing Yang,† Songgang Chai,‡ Chengzhen Geng,§ Feng Chen,*,† and Qiang Fu*,† †

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China ‡ Guangdong Shengyi Technology Limited Corporation, Dongguan 523039, China § Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, China S Supporting Information *

ABSTRACT: It is still a challenge to fabricate polymer-based composites with excellent thermal conductive property because of the well-known difficulties such as insufficient conductive pathways and inefficient filler−filler contact. To address this issue, a synergistic segregated double network by using two fillers with different dimensions has been designed and prepared by taking graphene nanoplates (GNPs) and multiwalled carbon nanotubes (MWCNT) in polystyrene for example. In this structure, GNPs form the segregated network to largely increase the filler−filler contact areas while MWCNT are embedded within the network to improve the network-density. The segregated network and the randomly dispersed hybrid network by using GNPs and MWCNT together were also prepared for comparison. It was found that the thermal conductivity of segregated double network can achieve almost 1.8-fold as high as that of the randomly dispersed hybrid network, and 2.2-fold as that of the segregated network. Meanwhile, much higher synergistic efficiency (f) of 2 can be obtained, even greater than that of other synergistic systems reported previously. The excellent thermal conductive property and higher f are ascribed to the unique effect of segregated double network: (1) extensive GNPs−GNPs contact areas via overlapped interconnections within segregated GNPs network; (2) efficient synergistic effect between MWCNT network and GNPs network based on bridge effect as well as increasing the network-density. KEYWORDS: thermal conductivity, segregated double network, synergy, graphene nanoplates, multiwalled carbon nanotubes resistance.3 So far, several techniques have been reported to reduce the thermal resistance by enhancing the interface between polymer and filler or forming filler conductive network including surface modification of filler to enhance the interfacial interaction,11,13,14 utilizing two types of conductive filler together via synergistic effect,15−18 selectively localization of filler to form a more conductive network,19−22 orientation of filler along a certain direction,23,24 and so on. Among these methods, the synergistic effect through combining two or more fillers together seems to be a simple and significant way. It needs no chemical modification and few intricate steps. For example, Haddon and co-workers reported that enhanced TC of epoxy nanocomposites can be achieved through combining single-walled carbon nanotubes (SWCNT) and graphene nanoplates (GNPs) together via synergistic effect.15 Similar

1. INTRODUCTION The increasing thermal energy generated in electronics, optoelectronics, and batteries emphasizes the research in thermal conductive materials and attracts numerous researchers to focus on developing polymer-based materials with enhanced capability of heat dissipation.1 Generally, the majority of polymers have low thermal conductivity (TC) of 0.1−0.5 W m−1 K−1.2,3 Therefore, improved TC of polymer is usually obtained by introducing various thermal conductive fillers such as graphene,4−7 graphite,8,9 and boron nitride.10−12 However, the conventional polymer-based thermal conductive composites with randomly distributed filler particles via melt mixing, solution mixing, or in situ polymerization method require a high content to attain the high TC, which inevitably leads to reduced mechanical properties, high cost, and increased processing difficulty. It is believed that the interface between polymer and filler or filler and filler plays the important role in determining the final TC of the polymer composites owing to the existing thermal © 2017 American Chemical Society

Received: December 24, 2016 Accepted: February 6, 2017 Published: February 6, 2017 7637

DOI: 10.1021/acsami.6b16586 ACS Appl. Mater. Interfaces 2017, 9, 7637−7647

Research Article

ACS Applied Materials & Interfaces

Figure 1. Preparation procedures of (a) PS/MWCNT/GNPs nanocomposites with the randomly dispersed hybrid network; (b) PS@(GNPs/ MWCNT) nanocomposites with the segregated network; (c) (PS/MWCNT)@GNPs nanocomposites with the segregated double network.

enhancement was also observed for not fibrous but planar boron nitride in PS/boron nitride/GNPs system.16 This is ascribed to the commonly accepted “bridge effect”. The onedimensional SWCNT or two-dimensional boron nitride could serve as a bridge to link the separated GNPs together to form the more percolated combined network in the matrix to reduce the interfacial thermal resistance. However, to the best of our knowledge, the synergistic effect was all achieved through directly melt or solvent mixing fillers and polymer together. These methods always resulted in the randomly dispersed hybrid network, where fillers can only contact with each other through point−point or line−line interconnections. Hence, the synergy may be associated with the more contact points or contact lines when percolated combined network was constructed. These small contact areas can only reduce the interfacial thermal resistance to some extent, which results in the relatively finite synergistic efficiency (f).25,26 Moreover, the large interfacial thermal resistance still exists, which leads to the relatively limited enhancement of TC. In comparison with the point−point or line−line interconnections for randomly dispersed fillers, it is believed that the extensive filler−filler overlap interconnections are more effective, especially for the two-dimensional fillers, such as GNPs, expanded graphite, and boron nitride. For this reason, segregated structure, where aggregated filler network was excluded from the polymer phase, was designed and prepared previously to enhance not only the electrical conductivity, but also the TC.8,20,27−29 In this continuous network, filler−filler overlap can largely reduce the contact resistance. The preparation of segregated filler network usually includes three steps: (1) fabrication of micron-sized polymer particles by melt mixing and crushing; (2) coating of conductive filler on the surface of polymer particles; and (3) compression molding of the filler coated polymer particles at the high temperature. Since the nonthermal conductive micron-sized polymer particles pose limitation to efficient phonon transfer, the enhancement of thermal conductive property of aggregated filler network is still limited. Hence, design and preparation of new or unique aggregated filler network are highly needed.

To further enhance the TC of polymer composites, in this study, a synergistic segregated double network was designed and prepared, in which a conductive filler is pre-embedded in the polymer particles before being coated with another conductive filler. To do this, we took the (polystyrene/ multiwalled carbon nanotubes)@graphene nanoplates ((PS/ MWCNT)@GNPs) nanocomposites as an example. First, MWCNT were melt mixed with PS, and then the composites were smashed into micron-sized particles. Second, collected composites particles were coated directly by two-dimensional GNPs through π−π interaction, and then these ternary particles were compression molded at the high temperature. For comparison, the conventional hybrid network (PS/MWCNT/ GNPs) with randomly dispersed fillers and the segregated network formed by locating two fillers at the interfaces (PS@(GNPs/MWCNT)) were also fabricated. Scanning electron microscope (SEM) and energy dispersive X-ray spectrometry (EDS) mapping results confirmed the successful fabrication of segregated double network, where GNPs−GNPs overlap interconnections increased the contact areas to the largest extent, and embedded MWCNT network gave rise to much denser conductive network. With increasing of MWCNT content, a largely increased TC as well as f was achieved at the low MWCNT content, and then f became saturated when percolated MWCNT network was constructed, while only slightly increased TC in the randomly dispersed hybrid network and more surprisingly gradually decreased TC in the segregated network were obtained, which indicated the importance of not only the filler−filler contact areas, but also the density of the conductive network. The efficient role of segregated double network was concluded as (1) extremely low thermal resistance achieved via overlap interconnections within robust GNPs network; and (2) synergistic effect between MWCNT network and GNPs network based on bridge effect as well as increasing the network-density. We believe that our work could shed some light on the preparation of polymer composites with excellent heat-conducting property. 7638

DOI: 10.1021/acsami.6b16586 ACS Appl. Mater. Interfaces 2017, 9, 7637−7647

Research Article

ACS Applied Materials & Interfaces

Figure 2. SEM images of (a, b) neat PS, (c, d) PS@1 vol % GNPs, and (e, f) PS@ 3.5 vol % GNPs micron-sized particles. The resolutions from top to bottom become increasingly higher. nanocomposites were prepared using a Leica Ultrathin UCT ultramicrotome with a knife made of diamond. The electrical conductivity of PS/MWCNT and PS@GNPs nanocomposites with the thickness of 4 mm was measured by a Keithley 6487 picoammeter under 1 V. To eliminate the influence of contact resistance, silver paint was brushed at the both sides of specimen. Two-dimensional wideangle X-ray diffraction of GNPs was measured by a Bruker DISCOVER D8 diffractometer at the scanning speed of 0.5°/min from 10−60°. Raman spectra of GNPs were tested on Instron 5567 machine at the laser wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos Co., UK) of GNPs was performed to determine the C/O ratio and other contained elements using focused Al Kα radiation (15 kV). The transient place source (TPS) method was utilized to measure the TC of PS nanocomposites with the diameter of 2 mm and the thickness of 4 mm by a Hot Disk thermal analyzer (Hot Disk 2500-OT, Uppsala, Sweden). Before characterization, P2400 SiC paper was applied to smooth the surface for good thermal contact. Melt rheology of all the PS/MWCNT, PS/ MWCNT/GNPs, and (PS/MWCNT)@GNPs nanocomposites was characterized in a strain-controlled dynamic rheometer Bohlin Gemini 200, Malvern Instruments, United Kingdom. The specimens for characterization were fabricated with the diameter of 25 mm and the thickness of 1.8 mm. The mechanical properties of PS nanocomposites were measured on Instron 5567 machine with 1 kN load cell at 25 °C. The cross-head speed was set as 10 mm/min. Thermal gravimetric analysis (TGA, TA Instruments, Q500, USA) was applied to study the thermal stability of PS nanocomposites. The temperature was increased from room temperature to 600 °C at the rate of 10 °C/ min in the nitrogen atmosphere.

2. EXPERIMENTAL SECTION 2.1. Materials. PS (PG-383) was provided by Zhenjiang Qimei Chemical Co. Ltd., China. MWCNT (NC7000) with the length of 1.5 μm and the diameter of 9.5 nm were supplied by Nanocyl S.A, Belgium. GNPs (XTG-P-0762) were obtained from the Deyang Carbonene Science Co. Ltd., China. 2.2. Preparation of Pre-embedded Micron-Sized Particles. PS with the different content of MWCNT was melt mixed in the internal mixer (Haake RC-90, Germany) at 60 rpm/min for 10 min at 190 °C. Then the composites were smashed at the high speed by a functional grinder (BJ-150, 20000 rpm/min) for 5 min. After that, the collected powders were sifted (Shanghai Huadong sifter factory), and the smallest PS or PS/MWCNT nanocomposites particles were collected. 2.3. Preparation of PS Nanocomposites with Three Different Network Structures. According to the fabricated steps in Figure 1, PS nanocomposites with the three different structures were prepared via different methods. For distinguish, randomly dispersed hybrid network, segregated network, and segregated double network were, respectively, labeled as PS/x% MWCNT/y% GNPs, PS@(x% GNPs/y % MWCNT), and (PS/x% MWCNT)@y% GNPs. Conventional randomly dispersed hybrid network of PS/MWCNT/GNPs nanocomposites was fabricated through melt mixing PS, MWCNT, and GNPs directly at 190 °C for 10 min. Different from PS/MWCNT/ GNPs nanocomposites, PS@(GNPs/MWCNT) and (PS/MWCNT) @GNPs nanocomposites were, respectively, prepared via encapsulating PS micron-sized particles with both GNPs and MWCNT or encapsulating PS/MWCNT micron-sized particles with the different content of GNPs via π−π interaction in the functional grinder at 10 000 rpm/min for 5 min. Then compression molding method was utilized to prepare the relevant specimens for the following characterizations. For comparison, PS nanocomposites filled with the randomly distributed GNPs or MWCNT were also prepared via a direct melt mixing method, and they are, respectively, labeled as PS/ GNPs and PS/MWCNT. 2.4. Characterization. The size of the smashed particles was confirmed by SEM (Inspect F, FEI Company, USA). EDS mapping was conducted to confirm the segregated GNPs network within PS@ GNPs nanocomposites. The cryo-fracture surface of PS nanocomposites was also characterized by SEM for observation of the dispersion and network-morphology of fillers embedded in the PS matrix. To further confirm the connections between MWCNT network and GNPs network, transmission electron microscope (TEM, JEF-2100F, jeol) was also used. Ultrathin specimens of PS

3. RESULTS AND DISCUSSION 3.1. Morphology of the Encapsulated PS Particles and Corresponding Nanocomposites. The commercial PS resin was first smashed into micron-sized powder, and the smallest was collected to prevent the further decrease in size during the next encapsulation step. As is shown in Figure S1 (Supporting Information), the size of the collected micron-sized particles has a narrow distribution. The statistical size is equalized as 260 μm. Then these micron-sized particles were coated by GNPs via π−π interaction. To confirm the favorable coating effect, morphology of the encapsulated PS particles was characterized. According to the SEM images in Figure 2, the surface of neat PS particle is smooth. Once coated by 1 vol % GNPs, the 7639

DOI: 10.1021/acsami.6b16586 ACS Appl. Mater. Interfaces 2017, 9, 7637−7647

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ACS Applied Materials & Interfaces

Figure 3. (a) Electrical conductivity of PS@GNPs nanocomposites with different volume content of GNPs; (b) SEM and (c, d) EDS mapping images of the cross-section of [email protected] vol % GNPs nanocomposites: O (red), Fe (blue).

contiguous regions.30,31 Therefore, the authentic continuous thermal conductive network lags behind the Φc slightly. Hence, it can be considered that the GNPs pathways within PS@1 vol % GNPs nanocomposites are discontinuous. However, for 3.5 vol % GNPs, the situation is different. Electrical conductivity of PS@ 3.5 vol % GNPs nanocomposites is much higher than that of PS@1 vol % GNPs nanocomposites. To confirm the continuous GNPs network, SEM combined with the EDS mapping characterization was measured for the cross-section of [email protected] vol % GNPs nanocomposites. The SEM image in Figure 3b exhibits the rough observation of the segregated GNPs network (the white paths) when the cryo-fracture surface was not sprayed by gold. For clear observation, EDS mapping analysis was conducted on this region. According to the XPS results in Figure S3c (Supporting Information), GNPs used in this study contain a small quantity of residual Fe and O element. Hence, Fe or O element was selected to distinguish between PS matrix and GNPs. One can find that the GNPs network, which is in the form of O (red) or Fe (blue) distribution, matches well with the SEM observation. It shows that the continuous GNPs network is excluded from PS phase (the black region), and the PS phase is surrounded completely by GNPs network. Discussions above suggest that segregated structure with the continuous and robust filler network can be fabricated at the GNPs content of 3.5 vol %. In addition, if MWCNT are pre-embedded in the micron-sized PS particles, segregated double network can also be achieved. Since the processing procedures experienced no melt mixing steps,

smooth morphology of the surface has changed obviously. It is found that some lamellar crack-shaped sheets are firmly absorbed on it. It suggests that PS powder can be coated by GNPs under the intense mechanical stirring via π−π interaction, and segregated structure is likely to be constructed if these PS nanocomposites particles experience no intense mixing steps. Moreover, for 1 vol % GNPs, images of the Figure 2c and d exhibit that all the GNPs are totally absorbed. There are no redundant sheets observed among PS particles. However, 3.5 vol % GNPs are superabundant. It can be seen that a some of GNPs still exist among micron-sized particles (Figure 2e). It can be considered that 3.5 vol % GNPs may be enough to construct the continuous segregated network in the matrix. To confirm this assumption, the electrical conductivity and the sectional morphology of the corresponding PS nanocomposites were respectively characterized. As is shown in Figure 3a, the electrical conductivity of PS@ GNPs nanocomposites exhibits the sharp increase with several orders of magnitude at the range of 0.5−2 vol %. The fitting result is based on the equation log σ = log σ0 + t log (Φ − Φc), where σ is the electrical conductivity of the PS composites, Φ is the volume content of the filler, σ0 is a constant, and t is the critical exponent. Result in the Figure S2a (Supporting Information) indicates that 1 vol % is near the percolation threshold (Φc). For electrical conductivity, this Φc corresponds to the content where at least one electrical conductive path embedded in the polymer matrix takes shape. According to the electrical conductive theory, electrons are able to flow across an insulating barrier via quantum mechanical tunneling between 7640

DOI: 10.1021/acsami.6b16586 ACS Appl. Mater. Interfaces 2017, 9, 7637−7647

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Figure 4. (a) Thermal conductivity of PS nanocomposites prepared via different fabricating methods as a function of the total volume content of fillers. (b) Synergistic efficiency of PS ternary nanocomposites as a function of the volume content of MWCNT. The variation tendency of the (c) thermal conductivity and (d) synergistic efficiency as a function of different GNPs/MWCNT ratio for both PS/MWCNT/GNPs and (PS/ MWCNT)@GNPs nanocomposites. Comparison of the (e) thermal enhancement and (f) synergistic efficiency of the synergistic systems ever reported.

segregated filler would contact directly without being hindered by any absorbed PS layer. 3.2. Thermal Conductivity. Figure 4a depicts the experimental TC of PS nanocomposites with three different kinds of network-morphologies, including randomly dispersed hybrid network, segregated network, and segregated double network, as a function of the total fillers content. First, in terms of the randomly dispersed hybrid network, the TC of PS/ MWCNT/GNPs nanocomposites exhibits gradual increase

with no sharp enhancement as increase of MWCNT content at the low MWCNT loading (below 1 vol %). Close inspection of the increasing tendency shows that the slope is almost the same to that of PS/MWCNT nanocomposites. The same increasing slope combined with the calculated f of 1 in the Figure 4b (calculation method is provided in the Supporting Information) suggests that no obvious synergistic effect exists between GNPs and MWCNT at the low MWCNT content. Once the content of MWCNT exceeds the turning point of 1 7641

DOI: 10.1021/acsami.6b16586 ACS Appl. Mater. Interfaces 2017, 9, 7637−7647

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ACS Applied Materials & Interfaces

Figure 5. (a) Electrical conductivity of PS/MWCNT nanocomposites as a function of MWCNT content. Rheological storage modulus of (b) PS/ MWCNT, (c) PS/MWCNT/3.5 vol % GNPs, and (d) (PS/MWCNT)@3.5 vol % GNPs nanocomposites filled with different contents of MWCNT.

vol %, f improved gradually, which is accompanied with the gradually enhanced TC. Unfortunately, such enhancement is still limited and exhibits the lower TC in comparison with PS filled with highly thermal conductive GNPs at the same volume content because GNPs exhibits much effective impact on enhancing TC compared to the same volume content of MWCNT. The effect of 1 vol % GNPs on enhancing the TC is approximately equal to that of 3.5 vol % MWCNT. This difference is ascribed to the reasons as follows. On the one hand, heat conduction is much more beneficial along axial direction for fibrous MWCNT than along almost heatinsulating radial direction. However, intertwined MWCNT could not exploit the advantage of axial direction to the full in the PS nanocomposites.3,32 On the other hand, the GNPs used in this study have the high C/O ratio of 13.70 and few defects according to the XPS, XRD, and Raman spectra (Figure S3, Supporting Information); hence, large lateral span for contact and well-retained conjugated structure make them superior for heat conduction. Second, for segregated network, it is unusual and surprising to find that the TC of PS@(GNPs/MWCNT) nanocomposites gradually decreases as increase of segregated MWCNT content. As a result, the TC of PS@(GNPs/ MWCNT) nanocomposites becomes the lowest in comparison with that of other ternary nanocomposites, even closer to the TC of PS/MWCNT nanocomposites. This phenomenon is very abnormal, and the mechanism will be discussed later. Finally, for segregated double network, the trend is extremely

different from the two structures just discussed above. Herein, two different situations of segregated double network, including both discontinuous and continuous segregated GNPs network embedded in the PS matrix, were respectively studied. When the volume content of GNPs is below or near the percolation threshold, for example, 1 vol %, the GNPs network is discontinuous. Further addition of MWCNT results in the gradual increase of TC with no obvious leap. It also suggests that no synergistic effect takes place in this system. Hence, the line of (PS/MWCNT)@1 vol % GNPs nanocomposites in Figure 4b exhibits almost the stable f (∼1). When the loading of GNPs is 3.5 vol %, the segregated GNPs network is continuous. Introduction of small amount of MWCNT leads to the sharp increase of TC. The turning point takes place at the MWCNT volume content of 1.5%, where a little bit slower trend is observed than that at the low volume content. Calculated f also shows a same tendency. Before 1.5 vol % content, addition of MWCNT results in a rapid enhancement of the f until ∼2. After this turning point, almost constant and saturated value of ∼2 is kept. Moreover, according to Figure S4 in the Supporting Information, the TC of (PS/MWCNT)@ GNPs nanocomposites with the segregated double network is even much higher than that of PS@GNPs nanocomposites although part of MWCNT are replaced by GNPs, which have much better thermal conductive property. From what has been discussed above, segregated double network seems to be much more beneficial to enhance the TC of PS nanocomposites. 7642

DOI: 10.1021/acsami.6b16586 ACS Appl. Mater. Interfaces 2017, 9, 7637−7647

Research Article

ACS Applied Materials & Interfaces However, in terms of the filler−filler synergy, the filler A/ filler B ratio was believed to be important to determine the final TC and f of the ternary composites15,16,18,33 because suitable filler A/filler B ratio will result in denser and more percolated conductive network via filler−filler synergy. In this study, the variation tendencies of TC and f as functions of different GNPs/MWCNT ratio were also studied on the premise of fixing total fillers content as 5 vol %. It aims at comparing the TC of randomly dispersed hybrid network and segregated double network to exclude the influence of GNP/MWCNT ratio. The relevant results are plotted in Figure 4c and d. For PS/MWCNT/GNPs nanocomposites, the higher GNPs loading, the higher TC. It exhibits almost a stable increased TC as increase of GNPs content, which indicates no obvious association between enhanced TC and GNPs/MWCNT ratio. According to the calculated f in Figure 4d, weak synergistic effect exists between GNPs and MWCNT except at the critical point where GNPs content is 3.5 vol % and MWCNT content is 1.5 vol %. However, in terms of (PS/MWCNT)@GNPs nanocomposites, the situation is different. One can find that the irregular enhanced TC of segregated double network is always higher than that of the randomly dispersed hybrid network, especially when GNPs/MWCNT ratio is in the range from 3.25:1.75 to 4:1. At the MWCNT content of 1.5 vol % and GNPs content of 3.5 vol %, the highest TC of (PS/MWCNT) @GNPs nanocomposites is achieved, almost 1.8-fold that of PS/MWCNT/GNPs nanocomposites. At this critical GNPs/ MWCNT ratio, f plotted in Figure 4d exhibited an extremely high value of ∼2. To the best of our knowledge, the high f of 2 is even greater than that of other synergistic systems reported previously (Figure 4f and Table S1 (Supporting Information)).16,17,25,33−38 Hence, the enhancement and the final TC are relatively much more efficient at this lower filler content of 5 vol % (Figure 4e). Overall, we find that at the same volume content of GNPs and MWCNT, (PS/MWCNT)@3.5 vol % GNPs with the 3-D segregated double network has the highest TC compared to other preparation methods including randomly dispersed hybrid network and segregated network. The excellent thermal conductive properties and the high f of segregated double network at the critical content are very attractive and never been reported yet. Hence, next characterizations were conducted for the purpose of figuring out the mechanism behind the large enhancement. 3.3. Meaning of These Turning Points. To figure out the meaning of these turning points in the first place, electrical conductivity and rheological properties were measured in Figure 5 to reflect the condition of fillers network in PS nanocomposites. First, PS/MWCNT binary nanocomposites were characterized. According to Figure 5a, an abrupt change with several orders of magnitude in electrical conductivity happens at the range from 1.0−1.75 vol %. The fitting result plotted in Figure S2b (Supporting Information) states that 1.18 vol % is the percolation threshold of MWCNT for PS/ MWCNT nanocomposites. It suggests that at this critical content, fibrous MWCNT start interconnecting together as an electrical conductive network, and at least one conductive pathway has been established. However, it is believed that continuous thermal conductive network lags behind the electrical percolation threshold slightly. Considering the relatively higher electrical conductivity of PS/1.5−1.75 vol % MWCNT nanocomposites, it is deemed that 1.5−1.75 vol % are enough to construct the thermally percolated network in PS

matrix. Further demonstration may be provided by the rheological properties because when MWCNT or GNPs is added into the PS matrix, the majority of PS chains are adhered to the fillers, which create a powerful barrier to the reptation motion and result in an obvious increase in the relaxation time and following a shift of a terminal relaxation to very low frequencies. Hence, the flatting tendency is gradually obvious as there is an increase of fillers, saturating at a certain filler content, which is called rheological threshold where the filler network is formed.39,40 Therefore, rheological storage modulus of PS/MWCNT nanocomposites was supplied in Figure 5b. One can find that when MWCNT content is lower than 1.5 vol %, the rheology curves of PS/MWCNT nanocomposites show a sharp increase as increase of the shear frequency in the low frequency region, indicating that no MWCNT network has formed in the PS nanocomposites. However, the curves of PS/ MWCNT nanocomposites filled with more than 1.5 vol % MWCNT exhibit the trend of flatting in the low frequency region but no obvious platform, which suggests that MWCNT network is possible to be built at this content, but this network is not too robust to resist the mild shear. Hence, 1.5 vol % is considered to be the critical content where the thermal conductive MWCNT network is constructed in PS matrix. After the significance of 1.5 vol % MWCNT in PS matrix was made clear, the rheological storage modulus of synergistic systems (PS/MWCNT/GNPs and (PS/MWCNT)@GNPs nanocomposites) was also studied. The results are shown in Figure 5c and d. For the randomly dispersed PS/MWCNT/ GNPs nanocomposites, platform at the low frequency region only occurs when MWCNT content achieves 1 vol %. Accordingly, 1 vol % happens to be the turning point where TC and f increase rapidly. It suggests that at this critical point, these MWCNT are just able to contact with each other to construct continuous network to link separate GNPs. However, 1 vol % is a little bit lower than 1.5 vol % because in the PS/ MWCNT/GNPs nanocomposites, volume exclusive effect of fluffy GNPs, may lead to the reduced percolation threshold of MWCNT. In terms of the segregated double network (Figure 5d), we can first find that the filler network of [email protected] vol % GNPs nanocomposites is loosened owing to the much lower storage modulus under the effect of shear force. When small amount of MWCNT was introduced into the PS phases, obviously increased storage modulus can be observed, which indicates the largely enhanced network-density via addition of MWCNT. Moreover, when the content of MWCNT achieves 1.5 vol %, the platform in the low frequency obviously occurs, which also indicates that thermally percolated MWCNT network in PS particles comes in to being at this critical content. This is also consistent with the conclusion in the PS/ MWCNT system, which has been just discussed above. Hence, 1.5 vol % is believed to be the critical content where continuous segregated double network is formed in the (PS/MWCNT)@ 3.5 vol % GNPs nanocomposites. 3.4. Morphology of the Ternary PS Nanocomposites with Three Different Network Structures. It is believed that the network-morphology plays the important role in determining the final TC of the polymer composites. Hence, the cross-sections of PS@(GNPs/MWCNT), PS/MWCNT/ GNPs, and (PS/MWCNT)@GNPs nanocomposites were, respectively, characterized by SEM and TEM. For ternary nanocomposites with the segregated network, the morphology is almost similar to the observations discussed in section 3.1. According to Figure S5 (Supporting Information), small-sized 7643

DOI: 10.1021/acsami.6b16586 ACS Appl. Mater. Interfaces 2017, 9, 7637−7647

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ACS Applied Materials & Interfaces

Figure 6. Typical SEM images of (a, b) PS@(GNPs/MWCNT), (c, d) PS/MWCNT/GNPs, and (e, f) (PS/MWCNT)@GNPs nanocomposites; TEM images of (g) PS/MWCNT/GNPs and (h) (PS/MWCNT)@GNPs nanocomposites. The volume content of GNPs and MWCNT in the PS nanocomposites was kept the same as, respectively, 3.5% and 1.5%.

Figure 7. Structural diagrams of (a) PS@(GNPs/MWCNT), (b) PS/MWCNT/GNPs, (c) (PS/MWCNT)@1 vol % GNPs, and (d) (PS/ MWCNT)@3.5 vol % GNPs nanocomposites. In the diagrams, PS matrix, MWCNT, and GNPs are, respectively, painted as blue, green, and black.

After hot-compression, one can find that continuous MWCNT/GNPs network is completely excluded from the PS regions, and the pure polymer phases make the embedded

MWCNT tend to be anchored on the surface of the large-sized GNPs. In addition, GNPs/MWCNT hybrid fillers can also be absorbed on the surface of PS particles via π−π interaction. 7644

DOI: 10.1021/acsami.6b16586 ACS Appl. Mater. Interfaces 2017, 9, 7637−7647

Research Article

ACS Applied Materials & Interfaces filler network loosened. However, the image in Figure 6b also shows the difference. The fibrous MWCNT, which insert between adjacent GNPs, inevitably hinder the direct GNPs− GNPs contact. In terms of the morphology of the randomly dispersed hybrid network, it is now commonly accepted that combining GNPs and MWCNT together could promote the dispersion of both the two fillers in the polymer matrix.18,41 A similar phenomenon was also observed in the PS/MWCNT/GNPs nanocomposites as good dispersion and homogeneity of MWCNT are exhibited in the Figure 6c, d, and g. This homogeneous MWCNT network also bridges adjacent GNPs together as the randomly dispersed hybrid network. In this case, more conductive pathways are interconnected in the PS matrix. For the segregated double network, after fibrous MWCNT were pre-embedded into the PS micron-sized particles, the segregated double network cooperated by dense MWCNT network, and continuous GNPs network came into being (Figure 6e,f). On the one hand, for the segregated GNPs network, GNPs can contact with each other directly. On the other hand, for the pre-embedded MWCNT network, one can find that melt mixing method will generate some MWCNT aggregates. Although the generation of such MWCNT aggregates is not satisfactory, these MWCNT aggregates are still able to form the percolated MWCNT network and make the conductive network much denser than that of PS@(GNPs/ MWCNT) nanocomposites. Meanwhile, since the sample was prepared via molten compression, the TEM image provided in the Figure 6h exhibits that MWCNT at the edge of the PS particles can contact with adjacent GNPs through π−π interaction. Hence, it is also considered that MWCNT network can serve as the bridge to interconnect segregated GNPs together. 3.5. Network Morphology-Dominated Synergy. After the latent meaning of these turning points and the networkmorphology of each ternary nanocomposite were figured, a mechanism of network morphology-dominated synergy was proposed from the perspective of filler−filler interconnections as well as network-density. Figure 7a illustrates the structural diagram of PS@(GNPs/MWCNT) nanocomposites with the segregated network. Although aggregated filler network is constructed in the excluded regions, further added MWCNT, which locate between adjacent GNPs, hinder the GNP−GNP overlap interconnections. It is apparent that GNPs−GNPs overlap interconnections have much larger contact area than GNPs−MWCNT overlap interconnections because the surface−surface contact is more efficient than surface−line contact. Moreover, we think that PS@GNPs powder is much easier to be compressed than PS@(GNPs/MWCNT) powder during the hot-compression procedure because GNPs−GNPs contact is tight, while GNPs−MWCNT contact is fluffy. In this case, the largely decreased contact areas for PS@(GNPs@MWCNT) nanocomposites resulted in the higher contact resistance than PS@GNPs nanocomposites. According to the EMT model,42 thermal resistance plays an important role in determining the final TC of polymer composites. The thermal resistance with several times of change will result in obviously variational TC according to the previous literature, especially when the thermal resistance is very small.25,35,42 Meanwhile, added MWCNT are mainly located at the interfaces. It could not alter the loosened network-morphology of the segregated network. Hence, a part of thermal energy transferring in the form of lattice vibration in the PS matrix would still be impeded. As a result, the segregated

network exhibited gradually decreased TC as increase of MWCNT content. It is believed that not synergistic effect, but inhibitive effect is played when hybrid fillers are utilized for morphology of the segregated network. In terms of the ternary nanocomposites with the randomly dispersed hybrid network (Figure 7b), one can see that MWCNT are too short to bridge adjacent GNPs together. Hence, synergistic effect only occurs when MWCNT form their thermally percolated network. Therefore, the stable increase of TC is observed at the low MWCNT content for PS/MWCNT/ 3.5 vol % GNPs nanocomposites (Figure 4a). Moreover, it is found that there are numerous MWCNT located among separate GNPs, but only a part of them can form the continuous network to serve as the bridge at the critical content. Therefore, more MWCNT are introduced, and more conductive pathways between adjacent GNPs are fabricated. It results in gradually reduced thermal resistance as well as gradually enhanced TC and f (Figure 4b). It is believed that the synergy in the randomly dispersed hybrid network is mainly associated with the more filler−filler contact points based on the bridge effect to reduce the thermal resistance. However, these limited contact areas can only reduce the interfacial thermal resistance to some extent, which result in the relatively finite f. In terms of the ternary nanocomposites with the segregated double network, two different structures of respectively discontinuous and continuous GNPs network were studied and discussed. First, for (PS/MWCNT)@1 vol % GNPs nanocomposites (Figure 7c), this content of GNPs could not afford to build the continuous GNPs network in the excluded areas. Interfaces between GNPs and PS matrix are abundant and result in large interfacial thermal resistance. Further added MWCNT could only reduce the thermal resistance to a certain extent based on more contact points, and no network−network synergy has happened in this case. As a result, a gradual and stable increase of TC is observed in Figure 4a as the number of MWCNT increases. Then once the GNPs loading is high enough to be able to construct a continuous and robust segregated GNPs network in PS matrix, for example, 3.5 vol % (Figure 7d), aggregated GNPs, which are excluded from PS particles, can form overlap interconnections to largely reduce the thermal resistance. However, still limited enhancement in TC has been obtained for PS@GNPs nanocomposites compared to PS/GNPs nanocomposites (Figure 4a), which highlights the significance of not only filler−filler contact, but also network-density. Indeed, heat transmission is not the same as electron transport. It is a continuous process and has directionality. In segregated structure, the vast majority of thermal energy is hindered in the nonconductive PS phase. Hence, as more and more MWCNT are pre-embedded within segregated GNPs network, higher and higher network-density is achieved, which exhibits largely increased TC and f at the low MWCNT content. Once the MWCNT form their thermally percolated network, one can see that all GNPs are interconnected. Hence, at this critical content, bridge role of MWCNT can be played to the full. Moreover, once the MWCNT network is constructed, further addition of MWCNT could not largely enhance the network-density again. As a result, saturated and constant f is maintained after this critical content of 1.5 vol % (Figure 4b). In conclusion, we believe that it is the segregated double network morphology-promoted synergy that results in such largely enhanced TC and the much higher f. The unique role of segregated double network can be 7645

DOI: 10.1021/acsami.6b16586 ACS Appl. Mater. Interfaces 2017, 9, 7637−7647

ACS Applied Materials & Interfaces

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ACKNOWLEDGMENTS We would like to express our sincere thanks to the National Natural Science Foundation of China for financial support (Grant Nos. 51573102 and 51421061).

summed up as (1) extremely low thermal resistance achieved via overlap interconnections within robust GNPs network; and (2) synergistic effect between MWCNT network and GNPs network based on bridge effect as well as increasing the network-density. In addition to the excellent capability of heat conduction, mechanical properties are also provided in Figure S6 (Supporting Information). One can find that in comparison with the conventional melt mixing method, these (PS/ MWCNT)@GNPs nanocomposites with a 3D segregated double network maintain the approximate elongation and exhibit slightly decreased tensile strength. Also, according to thermal properties plotted in Figure S7 (Supporting Information), the thermal decomposition temperature is almost the same to that of the melt mixing methods, and exhibits better thermal stability than neat PS. All these properties suggest that this novel and environmentally friendly method of constructing segregated double network will present an easy and highly industrialized procedure to fabricate excellent thermal conductive composites.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b16586. SEM images and statistical distribution; fitting results of percolation threshold; XRD, Raman spectra, and XPS results; thermal conductivity; enhancement of thermal conductivity and synergistic efficiency; SEM images; tensile stress−strain curves; TGA (PDF)

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REFERENCES

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4. CONCLUSIONS In conclusion, the novel structure with a robust and continuous 3D segregated double network is constructed in PS nanocomposites, and it exhibits the most outstanding capability of heat dissipation in comparison with other structures including randomly dispersed hybrid network and segregated network. Note that the synergistic efficiency of segregated double network can ultimately achieve ∼2, greater than that of other synergistic systems reported previously. This excellent thermal conductive property and higher synergistic efficiency are nicely explained by the unique role of segregated double network: (1) extremely low thermal resistance via GNP−GNP overlap interconnections; and (2) efficient synergy based on bridge effect of MWCNT network as well as largely increasing the network-density. We expect that our study is helpful for others to comprehend synergistic effect more deeply, and we believe that this novel and environmentally friendly method will help to fabricate extraordinary thermal conductive materials.

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-28-85460690. *E-mail: [email protected]. Phone: +86 28 8546 1795. Fax: +86 28 8546 1795. ORCID

Qiang Fu: 0000-0002-2346-1171 Notes

The authors declare no competing financial interest. 7646

DOI: 10.1021/acsami.6b16586 ACS Appl. Mater. Interfaces 2017, 9, 7637−7647

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DOI: 10.1021/acsami.6b16586 ACS Appl. Mater. Interfaces 2017, 9, 7637−7647