Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 1195−1203
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Ultralow-Threshold and Lightweight Biodegradable Porous PLA/ MWCNT with Segregated Conductive Networks for HighPerformance Thermal Insulation and Electromagnetic Interference Shielding Applications Guilong Wang,*,†,‡ Long Wang,‡ Lun Howe Mark,‡ Vahid Shaayegan,‡ Guizhen Wang,§ Huiping Li,∥ Guoqun Zhao,*,† and Chul B. Park*,‡
ACS Appl. Mater. Interfaces 2018.10:1195-1203. Downloaded from pubs.acs.org by KENT STATE UNIV on 01/07/19. For personal use only.
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Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan, Shandong 250061, P. R. China ‡ Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario M5S 3G8, Canada § Key Laboratory of Chinese Education Ministry for Tropical Biological Resources, Hainan University, Haikou, Hainan 570228, P. R. China ∥ School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, P. R. China S Supporting Information *
ABSTRACT: Lightweight, biodegradable, thermally insulating, and electrically conductive materials play a vital role in achieving the sustainable development of our society. The fabrication of such multifunctional materials is currently very challenging. Here, we report a general, facile, and eco-friendly way for the large-scale fabrication of ultralow-threshold and biodegradable porous polylactic acid (PLA)/multiwalled carbon nanotube (MWCNT) for high-performance thermal insulation and electromagnetic interference (EMI) shielding applications. Thanks to the unique structure of the microporous PLA matrix embedded by conductive 3D MWCNT networks, the lightweight porous PLA/MWCNT with a density of 0.045 g/cm3 possesses a percolation threshold of 0.00094 vol %, which, to our knowledge, is the minimum value reported so far. Furthermore, the material exhibits excellent thermal insulation performance with a thermal conductivity of 27.5 mW·m−1·K−1, which is much lower than the best value of common thermal insulation materials. Moreover, it also shows outstanding EMI shielding performance characterized by its high shielding effectiveness (SE) values and absorption-dominated shielding feature. More importantly, its specific EMI SE is as high as 1010 dB·cm3·g−1, which is superior to those of other shielding materials reported so far. Thus, this novel multifunctional material and its general fabrication methodology provide a promising way to meet the growing demand for highperformance multifunctional materials in sustainable development. KEYWORDS: PLA foam, MWCNT network, percolation threshold, thermal insulation, EMI shielding
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INTRODUCTION Nowadays, the world’s excessive fossil energy consumption has led to serious environmental and ecological problems. As fossil fuels dominate primary energy consumption, increasing energy efficiency and reducing wasted energy are of paramount importance. Moreover, with the rapidly increasing usage of © 2017 American Chemical Society
electronic devices, the electromagnetic (EM) pollution arising from EM interference (EMI) has become another major Received: September 17, 2017 Accepted: December 5, 2017 Published: December 5, 2017 1195
DOI: 10.1021/acsami.7b14111 ACS Appl. Mater. Interfaces 2018, 10, 1195−1203
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic for the fabrication procedure of the porous PLA/MWCNT with segregated MWCNT networks.
CPCs in the case that reflection should be forbidden to ensure the undisturbed functioning of other electronic devices or circuits.30−32 Because of the superior features of porous CPCs, many efforts have been devoted to develop high-performance CPC foams for advanced thermal insulation and/or EMI shielding applications in recent years. It has been demonstrated that carbon-based materials including carbon black, activated carbon, graphite, carbon nanofiber, carbon nanotube, and graphene are very effective fillers to improve the thermal insulation performance of polymer foams by reducing radiative heat transfer.16,33,34 Regarding EMI shielding applications, some works have demonstrated that the porous structure created by foaming can not only reduce the density of materials but also decrease the percolation threshold, thus leading to a significantly enhanced specific EMI SE.35−38 It was reported that the polyetherimide/graphene (10 wt %) nanocomposite foam with a density of around 0.3 g/cm3 exhibits a specific EMI SE of 44 dB·cm3·g−1.36 The polystyrene/graphene (30 wt %) nanocomposite foam with a density of 0.45 g/cm3 presents a specific EMI SE of as high as 64 dB·cm3·g−1, which is the highest value at that time.37 While most of the efforts have been focused on batch-scale systems that cannot be easily scaled up, some researchers have recently tried to fabricate CPC foams using scalable and facile methods, such as microcellular injection molding.9,10 The fabricated polypropylene/stainlesssteel fiber (1.1 vol %) composite foam exhibits a specific EMI SE of 75 dB·cm3·g−1, which is higher than those of most of the CPC foams reported in the literature, yet the density reduction is just less than 35%.9 According to the data in the literature,9,30 the maximum specific EMI SE of CPC foams achieved is 258 dB·cm3·g−1, which is obtained by foaming polycaprolactone/ multiwalled carbon nanotube (MWCNT) (0.25 vol %) nanocomposites with supercritical carbon dioxide. Herein, we report an environmentally friendly, scalable, facile, and versatile methodology to fabricate ultralow-threshold and lightweight biodegradable polylactic acid (PLA)/MWCNT foams with segregated conductive networks for high-performance thermal insulation and EMI shielding applications. The percolation threshold of the segregated PLA/MWCNT foam is as low as 0.00094 vol %, which is much lower than the value available so far in the literature.35 Thanks to the threedimensional conductive MWCNT networks embedded within the fine microcellular PLA matrix, the segregated PLA/ MWCNT foam exhibits excellent EMI shielding performance with an EMI SE of up to 45 dB and a specific EMI SE of as high as 1010 dB·cm3·g−1, which, to our knowledge, is the highest value available so far for polymer composites. Moreover, the fabricated ultralightweight (0.045 g/cm3) porous PLA/ MWCNT also shows remarkable thermal insulation performance with a thermal conductivity of as low as 27.5 mW·m−1· K−1, which is much better than those of the current commercial polymer foams without special insulation gases.
concern of our society because it not only affects the functionality of electronic devices but also affects the human health.1−3 In this context, it becomes increasingly urgent to develop low-cost, versatile, environmentally friendly, lightweight, thermally insulating, and electrically conductive materials that can be widely used for thermal insulation and EMI shielding applications. Conductive polymer composites (CPCs) are such kind of multifunctional materials which have many attractive properties including lightweight, chemical stability, flexibility, widely tunable electrical conductivity, easy processing, and low cost, which make them promising candidates for EMI shielding applications.4−8 However, CPCs typically have a much high thermal conductivity compared with the current thermally insulating materials, such as mineral wool and polymer foams, which makes them unsuitable for insulation applications. Moreover, to make polymer composites conductive or suitable for EMI shielding, a typically high conductive filler content is required because of the severe agglomeration and poor filler− matrix bonding, which adversely affects the processability, mechanical properties, flexibility, weight reduction, thermal insulation, and economic feasibility of the product.9−11 Furthermore, the primary EMI shielding mechanism of the compact CPCs is reflection due to the strong impedance mismatch between air and CPCs.12−15 This reflectiondominant EMI shielding feature makes CPCs unfeasible in the areas that need EMI shielding but generate EM radiation at the same time, such as electronic circuits. This is because the secondary reflection can adversely affect the functionality of other electronic devices and circuits. Porous CPCs fabricated by foaming technology provide ideal alternatives to address the shortcomings of compact CPCs. First, porous CPCs have much lower density and much better thermal insulation performance than the compact ones because of the introduction of a light and adiabatic air phase.16−18 Second, foaming can improve the processability of CPCs with a high filler content because of the involved blowing agents with a low viscosity.19,20 Because the blowing agent, especially the physical blowing agent, serves as a plasticizer and effectively reduces the viscosity of the polymer, the material can be molded at a much lower temperature under much lower force, less energy, and shorter molding cycle.21 Third, porous CPCs have much lower apparent percolation threshold required to generate the conductive network than the compact CPCs because of their segregated structure created by foaming.22−24 Moreover, the multiinterface structure of porous CPCs can help enhance the EMI shielding effectiveness (SE) by introducing multiple reflections, scatterings, and absorptions.25,26 Notably, the enhanced EMI SE also contributes to thermal insulation because radiative heat transfer can become very significant for low-density porous polymers.27−29 Furthermore, porous CPCs generally exhibit an absorptiondominant EMI shielding feature. This is much more preferred than the reflection-dominant EMI shielding feature of compact 1196
DOI: 10.1021/acsami.7b14111 ACS Appl. Mater. Interfaces 2018, 10, 1195−1203
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Representative cellular morphology of EPLA. (b) Transmission electron microscopy (TEM) image of MWCNTs from a solution containing 1 wt % MWCNT. (c) TEM image of a single MWCNT. (d) Representative surface morphology of EPLA wrapped with MWCNTs. (e) Morphology of the sintered EPLA beads wrapped with MWCNTs. (f) Morphology at the boundaries of the sintered EPLA beads.
Figure 3. (a) Electrical conductivity of PLA/MWCNT foams vs MWCNT content. The inset plots σ vs (φ − φc) in the double logarithmic presentation. (b) Schematic diagram of the segregated 3D conductive networks composed of MWCNTs at the boundaries of the EPLA beads.
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RESULTS AND DISCUSSION The fabrication process of the PLA/MWCNT foam with segregated conductive networks is illustrated in Figure 1. Initially, solid PLA pellets with a size of 60−120 mesh were foamed by the bead foaming process to obtain expanded PLA (EPLA) beads (Figure S1), which have a fine uniform microcellular structure with an average cell size of about 5.4 μm and an expansion ratio of up to 30, as shown in Figure 2a. To improve the foaming ability of PLA and thus the cellular structure of EPLA,39−41 1 wt % chain extender was added to significantly enhance the melt strength of PLA (Figure S2). Further, the EPLA beads were simply immersed in the prepared MWCNT solution. Thanks to the presence of sodium dodecylbenzenesulfonate (SDBS) in the solution and the combined mechanical stirring with ultrasonic vibration, MWCNTs with a length of 30−40 μm and a diameter of 10−15 nm disperse very uniformly in the solution (Figure 2b,c). After immersion, the surfaces of the EPLA pellets were wrapped with a thin layer of MWCNTs (Figure 2d). Finally, the MWCNT-wrapped EPLA pellets were sintered together by the steam-chest molding process;42 thus, a bulk PLA/MWCNT foam product with 3D segregated MWCNT networks distributed between the EPLA pellets was fabricated (Figure 2e,f).43 Thanks to the large void fraction of the EPLA beads, the density of the fabricated bulk porous PLA/MWCNT is as low as 0.045 g/cm3. To tune the MWCNT content in the PLA/ MWCNT foam and further to tailor its electrical conductivity, the concentration of MWCNT solution was varied from 0.25 to 2.5 wt %. Consequently, the measured weight percent of MWCNT in the final bulk porous PLA/MWCNT product
increases from 0.022 to 0.216 wt %, and the volume content varies from 0.00055 to 0.0054 vol %. Compared with the regular CPC foam fabrication process reported in the literature, the process reported in this study exhibits several superiorities. First, the blending process of the polymer and conductive nanoparticles is eliminated. Notably, to improve the uniformity of nanoparticle distribution in the polymer matrix, a solution-blending method is generally needed in the conventional process, which is thus very time-consuming, and cannot be easily scaled up. Second, because foaming is conducted without the presence of conductive fillers which act as resistances for foaming, porous polymers with a much lower density can be achieved.44 Third, the conductive fillers without any breakage that resulted from processing preferentially distribute at the interfaces of adjacent EPLA beads in the new process, which can lead to an extremely low percolation threshold to create 3D electrically conductive networks.38 Because the MWCNTs are directly in contact with each other, a high electrical conductivity can be achieved with the formed 3D conductive networks.45 Moreover, theoretically, the created unique composite structure with electrically insulated microporous beads embedded in 3D electrically conductive networks can help reduce the reflectance while enhance the absorption of EM waves, thus leading to an absorption-dominated EM shielding material. Furthermore, the large void fraction of the EPLA beads combined with 3D electrically conductive networks, which can effectively block radiative heat transfer, leads to an excellent thermal insulation performance.16,27 Thanks to the excellent electrical conductivity of MWCNTs combined with their large aspect ratio (>2000) (Figure S3), it 1197
DOI: 10.1021/acsami.7b14111 ACS Appl. Mater. Interfaces 2018, 10, 1195−1203
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) Thermal conductivity of the PLA/MWCNT foam as a function of MWCNT content. (b) Schematic diagram of the heat transfer in the PLA/MWCNT foam. (c) Various heat transfer components as a function of MWCNT content. (d) Schematic of an EM wave transferring through a thin slab.
threshold is due to the following facts: (a) MWCNTs, which are directly in contact with each other, preferably distribute at the boundaries of the EPLA beads to generate segregated conductive networks (Figure 3b), (b) the used MWCNTs have an extremely large aspect ratio (>2000) and thus can create a conductive network with low filler contents,46 (c) MWCNTs disperse very uniformly without any obvious aggregation (Figure 2b), which helps reduce the critical MWCNT content required to form a conductive network, (e) foaming in the current process does not affect the formation of a conductive interconnected nanofiber network throughout the porous material, which is significantly different from the conventional foaming-based process used to fabricate porous CPC materials, and (f) the steam-chest molding process used herein to sinter MWCNT-wrapped EPLA beads together can help enhance the electrical conductivity of MWCNT networks by compressing MWCNTs more closely (Figure S3). Moreover, we noticed that the electrical conductivity of the foam can be as high as 6.3 S/m, which is higher than those of most of either porous or solid CPC materials,4,6,9,38 particularly considering the extremely low filler concentration of 0.0054 vol %. According to the EM wave theory, this high electrical conductivity combined with the porous structure can effectively block EM wave transmission, which can endow the low-density material with excellent thermal insulation and EMI shielding performance. Thermal conductivities of the porous PLA/MWCNT were measured using the guarded hot plate method according to the European Standards EN 12664 and EN 12667. Figure 4a plots the measured thermal conductivity as a function of MWCNT volume fraction. It is observed that the pure porous PLA in the absence of MWCNTs presents a thermal conductivity of as high as 34.6 mW·m−1·K−1. As the MWCNT content increases, the thermal conductivity of the porous material reduces
is expected that the prepared PLA/MWCNT foam with the segregated 3D conductive networks composed of mutually contacted MWCNTs will exhibit good electrical properties. The electrical conductivity measurement was performed using a dielectric spectrometer (Alpha-A, Novocontrol Technologies) according to ASTM D257. Figure 3a plots the direct-current electrical conductivity (σdc) of the PLA/MWCNT foam at various MWCNT contents. It is observed that the electrical conductivity increases gradually with the increase of the volume fraction of MWCNT (φ) in the foam. It is found that the electrical conductivity increases sharply by many orders of magnitude as the MWCNT content exceeds a certain threshold concentration (φc), and this indicates the formation of percolating conductive networks. It is noticed in Figure 3a that the transition from a nonconductor to a conductor takes place at an extremely small volume fraction of MWCNT, approximately equal to 0.001 vol %. To determine the percolation threshold accurately, the statistical percolation theory power law, σ = σ0(φ − φc)β, was used to evaluate the correlation between electrical conductivity and filler content, where σ is the electrical conductivity of the foam, σ0 is a factor related to the intrinsic electrical conductivity of MWCNTs, φ is the filler volume fraction, φc is the percolation threshold, and β is the critical exponent related to filler dispersion and dimensionality. The inset in Figure 3a shows σ versus (φ − φc) in the double logarithmic presentation. As shown in the inset, the best-fit line with the coefficient of determination of 0.94 corresponds to the least-squares equation of the experimental data with φc = 0.00094 vol % and β = 4.04. This large value of β indicates a broad tunneling distance distribution and 3D conductive networks. Notably, this is by far the lowest φc achieved among the CPC materials, much lower than the minimum value (0.0054 vol %) reported in the literature.38 The extremely small percolation 1198
DOI: 10.1021/acsami.7b14111 ACS Appl. Mater. Interfaces 2018, 10, 1195−1203
Research Article
ACS Applied Materials & Interfaces
Figure 5. (a) Measured total EMI SE of the PLA/MWCNT foams with various MWCNT contents. (b) The specific EMI SE of PLA/MWCNT foams as a function of MWCNT content. (c) Dependences of the total EMI SE, the EMI SE contributed by absorption (SEA), and the EMI SE contributed by reflection (SER) on MWCNT content at 10 GHz.
and thermal radiation is the second major contributor of heat transfer. This unexpected significant radiative heat transfer can be owing to the extremely small cell size of the prepared foam. In our earlier work, it has been clearly demonstrated that there is an optimal cell size for minimizing λeff of polymer foams under a given void fraction.27 Below the optimal cell size, the radiative heat transfer can increase sharply as the cell size reduces. This can be attributed to the dramatically reduced infrared wave reflection of cell walls resulting from the strong film-interference effect. As illustrated in Figure 4d, the incident infrared waves will be partially reflected and absorbed when they transmit through a cell wall. As the cell wall thickness is far less than the infrared wavelength in the case of extremely small cell sizes, strong destructive interferences of the reflected infrared waves from the two interfaces can occur (Figure 4d), which results in a dramatically reduced reflectivity of the cell wall (Figure S4). Although the number of cell walls increases as the cell size reduces, the radiative heat transfer can still increase significantly.27,50 By introducing the 3D electrically conductive MWCNT networks (Figure 4b) which are very effective in blocking infrared waves through reflection and absorption,16,34 the radiative heat transfer can therefore be reduced remarkably with the increase of MWCNT content, as shown in Figure 4c. Thus, it is clearly demonstrated that the improved thermal insulation of the foam by increasing the MWCNT content (Figure 4a) is owing to the reduced radiative heat transfer. Taking into account the unique structure composed of numerous randomly distributed micropores and segregated 3D conductive networks, the fabricated PLA/MWCNT foam can exhibit superior EMI shielding performance because the multiple interfaces of the microcellular structure and the segregated conductive networks both can help block and absorb broadband microwaves.7,9,38 To characterize the EMI SE of the PLA/MWCNT foam, we performed EMI shielding measurement in the X-band frequency range of 8.2−12.4 GHz using a
gradually until it reaches a plateau, and the minimum value achieved is approximately 27.5 mW·m−1·K−1 at a MWCNT volume fraction of 0.0054 vol %. To our knowledge, this is by far the second lowest thermal conductivity for polymer foams in the absence of any special insulation gas and is just slightly higher than the thermal conductivity of the nanocellular poly(methyl methacrylate) foam reported very recently by us.47 As illustrated in Figure 4b, the heat transfer in the PLA/ MWCNT foam includes the following components: (1) thermal conductions through gas (λgas), PLA skeleton (λPLA), and MWCNT network (λCNT), (2) thermal convection of gas (λcon), and (3) thermal radiation (λrad). As the cell size is far less than the critical value of approximately 4 mm, the thermal convection term can be neglected.16,27 Moreover, taking into account the extremely small volume fraction of MWCNTs (
