Ultralow-Threshold and Lightweight Biodegradable Porous PLA

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Ultra-low threshold and lightweight biodegradable porous PLA/MWCNT with segregated conductive networks for high-performance 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, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14111 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 11, 2017

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ACS Applied Materials & Interfaces

Ultra-low threshold and lightweight biodegradable porous PLA/MWCNT with segregated conductive networks for high-performance 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*,‡ †

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

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ABSTRACT: Lightweight, biodegradable, thermal insulation 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 ultra-low threshold and biodegradable porous PLA/MWCNT for high-performance thermal insulation and EMI shielding applications. Thanks to the unique structure with microporous PLA matrix embedded by conductive 3D MWCNT networks, the lightweight porous PLA/MWCNT with a density of 0.045 g/cm3 possess 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 the high SE values and the absorption-dominated shielding feature. More importantly, its specific EMI SE is as high as 1010 dB·cm3·g-1, which is superior to 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 high-performance 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 seriously 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 electronic devices, the electromagnetic pollution arising from electromagnetic interference (EMI) has become another major concern of our society, because it not only affects the functionality of electronic devices but also affects 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 conductivities, easy to process, as well as low cost, which make them promising candidates for electromagnetic interference (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 composite conductive or suitable for EMI shielding, a typically high conductive filler content is required due to 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 reflection-dominant EMI shielding feature makes CPCs unfeasible in the areas that need EMI shielding, but also generate 3



electromagnetic radiation at the same time such as the 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 due to the introduction of light and adiabatic air phase.16–18 Second, foaming can improve the processability of CPCs with a high filler content due to 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 due to the segregated structure created by foaming.22–24 Moreover, the multi-interface structure of porous CPCs can help to enhance EMI shielding effectiveness by introducing multiple reflections, scattering and absorptions.25,26 Notably, the enhanced EMI shielding effectiveness 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 absorption-dominant EMI shielding feature. This is much more preferred than the reflection-dominant EMI shielding feature of compact CPCs in case that reflection should be forbidden to ensure undisturbed functioning of other electronic devices or circuits.30–32 Due to 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 4


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fillers to improve polymer foam’s thermal insulation performance 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 material’s density but also decrease the percolation threshold, thus leading to significantly enhanced specific EMI shielding effectiveness (SE).35–38 It was reported that 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/stainless-steel fiber (1.1 vol%) composite foam exhibits a specific EMI SE of 75 dB·cm3·g-1 which is higher than most of the CPC foams reported in literature, yet the density reduction is just less than 35%.9 According to the data in literature,9,30 the maximum specific EMI SE of CPC foams achieved is 258 dB·cm3·g-1, which is obtained by foaming polycaprolactone/MWCNT (0.25 vol%) nanocomposites with supercritical carbon dioxide. Herein, we report an environmentally friendly, scalable, facile and versatile methodology to fabricate ultra-low threshold and lightweight biodegradable polylactide 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 by far in the literature.35 Thanks to the three-dimensional conductive MWCNT networks embedded within the fine microcellular PLA matrix, the segregated PLA/MWCNT foam exhibits excellent EMI shielding performance 5



with the EMI SE of up to 45 dB and with the specific EMI SE of as high as 1010 dB·g-1·cm3, which, to our knowledge, is the highest value available so far for polymer composites. Moreover, the fabricated ultra-lightweight (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·K, much better than the current commercial polymer foams without special insulation gases.

RESULTS AND DISCUSSIONS The fabrication process of the PLA/MWCNT foam with segregated conductive networks is

illustrated in Figure 1. Initially, solid PLA pellets with size in 60 to 120 mesh were foamed by 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 with an expansion ratio of up to 30, as shown in Figure 2a. To improve the PLA’s foaming ability and thus EPLA’s cellular structure,39–41 1 wt% chain extender was added to significantly enhance the PLA’s melt strength (Figure S2). Further, the EPLA beads were simply immersed in the prepared MWCNT solution. Thanks to the presence of sodium benzenesulfonate (SDBS) in the solution and the combined mechanical stirring with ultrasonic vibration, MWCNTs with a length of 30–40 µm and with a diameter of 10–15 nm disperse very uniformly in the solution (Figure 2b, c). After immersing, the surfaces of 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 distributing between EPLA pellets was fabricated (Figure 2e, f).43 Thanks to the large void fraction of EPLA beads, the density of the fabricated bulk porous PLA/MWCNT is as low as 0.045 g/cm3. To tune MWCNT content in PLA/MWCNT foam and further to tailor its electrical conductivity, the concentration of MWCNT solution was varied from 0.25 wt% to 2.5 wt%. 6


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Consequently, the measured weight percent of MWCNT in the final bulk porous PLA/MWCNT product increases from 0.022 wt% to 0.216 wt%, and the volume content varies from 0.00055 vol% to 0.0054 vol%.

Figure 1. Schematic for the fabrication procedure of the porous PLA/MWCNT with segregated MWCNT networks.

Figure 2. a) Representative cellular morphology of EPLA. b) TEM image of MWCNT 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. Compared with the regular CPC foam fabrication process reported in literature, the process reported in this study exhibits several superiorities. First, the blending process of polymer and conductive nanoparticles is eliminated. Notably, to improve the uniformity of nanoparticle distribution in polymer matrix, a solution blending method is generally needed in conventional 7



process, which is thus very time-consuming and cannot be easily scaled up. Second, since 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 resulted from processing preferentially distributes 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 MWCNTs are directly in contact with each other, a high electrically 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 to reduce the reflectance while enhance the absorption of electromagnetic waves, thus leading to an absorption-dominated electromagnetic shielding material. Furthermore, the large void fraction of 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 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 (ߪୈେ ) 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 it indicates the formation of percolating conductive networks. It is noticed 8


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in Figure 3a that the transition from nonconductor to 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. Insertion in Figure 3a shows σ versus (φ-φc) in the double logarithmic presentation. As shown in the insertion, the best-fit line with the coefficient of determination of 0.94 corresponds to the least square 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 literature.38 The extremely small percolation threshold is due to the following facts: a) MWCNTs preferably distribute at the boundaries of the EPLA beads directly contact with each other 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 disperses very uniformly without any obvious aggregation (Figure 2b), which helps to 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 herein used to sinter MWCNT-wrapped EPLA beads together can help to enhance MWCNT network’s electrical conductivity by compressing 9



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 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 electromagnetic wave theory, this high electrical conductivity combined with the porous structure can effectively block electromagnetic wave transmission, which can endow the low-density material with excellent thermal insulation and EMI shielding performance. a

1.0E+02 1.0E+00 1.0E-02 1.0E+02

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1.0E-08

1.0E-02 1.0E-04

φc = 0.00094 vol% β = 4.04 R2 = 0.9384

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Figure 3. a) Electrical conductivity of PLA/MWCNT foams versus MWCNT content. The insertion plots σ versus (φ-φc) in the double logarithmic presentation. b) Schematic diagram of the segregated 3D conductive networks composed of MWCNTs at the boundaries of EPLA beads. Thermal conductivities of the porous PLA/MWCNT were measured using the guarded hot plate method according to 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 absence of MWCNTs presents a thermal conductivity of as high as 34.6 mW·m-1·K-1. As the MWCNT content increases, the porous material’s thermal conductivity reduces gradually until it reaches a plateau, and the minimum value achieved is approximately 27.5 mW·m-1·K-1 at the MWCNT volume fraction of 0.0054 vol%. To our knowledge, this is by 10


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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 a

b

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Transmitted wave

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Figure 4. a) The thermal conductivity of PLA/MWCNT foam as a function of MWCNT content. b) Schematic diagram of the heat transfer in PLA/MWCNT foam. c) Various heat transfer components as a function of MWCNT content. d) Schematic of an electromagnetic wave transferring through a thin slab. As illustrated in Figure 4b, the heat transfer in 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 MWCNT 11



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Ultralow-Threshold and Lightweight Biodegradable Porous PLA

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