Density-tunable lightweight polymer composites with dual-functional ability of efficient EMI shielding and heat dissipation

Seung Hwan Lee ab, Seunggun Yu ac, Faisal Shahzad ad, Woo Nyon Kim ab, Cheolmin Park ad, Soon Man Hong a and Chong Min Koo *ade
aMaterials Architecturing Research Center, Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea. E-mail: koo@kist.re.kr
bDepartment of Chemical and Biological Engineering, Korea University, Seoul, 136-713, Republic of Korea
cDepartment of Materials Science and Engineering, Yonsei University, Seoul, 120-749, Republic of Korea
dNanomaterials Science and Engineering, University of Science and Technology, 217, Gajung-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
eKU-KIST Graduate School of Science and Technology, Korea University, Anam-ro 145, Seongbuk-gu, Seoul 02841, Republic of Korea

Received 12th April 2017 , Accepted 15th June 2017

First published on 16th June 2017


Lightweight dual-functional materials with high EMI shielding performance and thermal conductivity are of great importance in modern cutting-edge applications, such as mobile electronics, automotive, aerospace, and military. Unfortunately, a clear material solution has not emerged yet. Herein, we demonstrate a simple and effective way to fabricate lightweight metal-based polymer composites with dual-functional ability of excellent EMI shielding effectiveness and thermal conductivity using expandable polymer bead-templated Cu hollow beads. The low-density Cu hollow beads (ρ ∼ 0.44 g cm−3) were fabricated through electroless plating of Cu on the expanded polymer beads with ultralow density (ρ ∼ 0.02 g cm−3). The resulting composites that formed a continuous 3D Cu network with a very small Cu content (∼9.8 vol%) exhibited excellent EMI shielding (110.7 dB at 7 GHz) and thermal conductivity (7.0 W m−1 K−1) with isotropic features. Moreover, the densities of the composites are tunable from 1.28 to 0.59 g cm−3 in accordance with the purpose of their applications. To the best of our knowledge, the resulting composites are the best lightweight dual-functional materials with exceptionally high EMI SE and thermal conductivity performance among synthetic polymer composites.


Introduction

Lightweight dual-functional materials with high EMI shielding performance and thermal conductivity are urgently required for highly miniaturized portable electronics and state-of-the-art lightweight high technological devices in various industrial sectors, such as mobile electronics, automotive, aerospace, and military.1–3 Electromagnetic (EM) waves are now pervasive in our daily lives. However, the rapid development of highly integrated electronic devices and the necessity to design precise signal transmission devices have led to the emergence of electromagnetic interference (EMI). EMI does not only cause signal distortion, leading to malfunctions of electronic devices, but also causes harmful effects on human health.4–8 Moreover, such highly integrated electronic components also suffer from the accumulation of undesired thermal energy, which causes degradation of their performance and lifetime.9 Furthermore, the decrease of the weight of EMI shielding and heat dissipation materials has been the foremost issue in several sectors, including mobile electronic devices, automobiles, and aerospace vehicle applications.10

The incident EM wave, being imagined as a self-propagating transverse oscillating wave of electric and magnetic fields, is reflected from the surface of shielding materials by impedance mismatching. The unreflected part of the EM wave follows an absorption phenomenon due to the interactions with the electric and magnetic dipoles of the material.4 Thus, high electrical conductivity or magnetic permeability is the basic prerequisite for the efficient EMI shielding. Heat dissipation occurs through free electrons and a lattice vibration called phonon in the materials.11 Metals, such as Al and Cu, have high thermal conductivity due to the abundant free electrons. Thus, metal-based polymer composites have been considered as an obvious choice for dual-functioning materials with high EMI shielding and thermal conductivity since very early stages of the new technology era.

However, metals have very high density; for instance, 8.94 g cm−3 for Cu and 10.5 g cm−3 for Ag. Moreover, conventional polymer composites require high percolation concentrations to form an electrically conductive network of metal fillers, resulting in a large density of composites and additional difficulty in processing.12,13 In the past several decades, overcoming the intrinsic problem of polymer composites was proved to be a daunting task in the lightweight composite applications.

One popular approach to mitigate the high percolation and large density issues has been to adopt anisotropic fillers, such as metal nanowires,14 metal filaments,15 carbon fibers,16 carbon nanotubes (1D),17 and graphene (2D).18 Shape anisotropy of fillers does alleviate the percolation threshold concentration. However, the issues such as the increase in density, difficulty in processing, and cost increment could not be fully eliminated in the polymer composites, because the fillers have a larger density than the matrix polymer. Moreover, anisotropic fillers affect the property owing to the likely orientation along the in-plane direction. Such a preference in the orientation produces large thermal conductivity in the in-plane direction, but weak thermal conductivity in the out-of-plane direction. Introduction of a foam structure to the polymer composites has also been considered useful not only for reducing the density increment and percolation concentration, but also for achieving the isotropic attributes.19–21 However, the foamed composite suffers from challenges in the fabrication process. The foamed composites also need to be very thick, generally above 2 mm, to satisfy the practical EMI shielding requirements. Moreover, the foam structure deteriorates thermal conductivity because of the presence of thermal insulating air spaces entrapped inside the structure.

Herein, we demonstrate a simple and effective way to fabricate the lightweight metal-based polymer composites with dual-functional performance of excellent EMI shielding and thermal conductivity, using expandable polymer bead-templated low density Cu hollow beads. The Cu hollow beads were fabricated using the extremely low density expanded polymer bead (EB) templates. The resulting composites exhibited a tunable low density from 1.28 to 0.59 g cm−3 while providing dual-functional ability of high EMI shielding and thermal conductivity.

Results and discussion

Fig. 1a and b show representative optical microscopy (OM) and scanning electron microscopy (SEM) micrographs of EBs. The EB foam particles were fabricated through the thermal expansion process of expandable polymer beads consisting of a viscoelastic acrylonitrile-based copolymer shell and a volatile isobutane core working as a blowing agent (Fig. S1, ESI). The inset in Fig. 1b shows the TEM micrograph of the EB particles. The EB particles revealed a wide size distribution with an average diameter of 11.8 μm and a polymer shell thickness of 0.07–0.90 μm (Fig. S2, ESI). After Cu electroless plating, the surface color of the EB changed from white to brown (Fig. 1c). The surface of the Cu-coated EB (EBCu) was uniformly covered with a Cu layer, which had remarkably smooth roughness (Fig. 1d and Fig. S3, ESI). The inset in Fig. 1d shows that the hollow polymer core/Cu shell structure was intact after the electroless plating process. In the X-ray diffraction (XRD) analysis of EBCu (Fig. S4, ESI), three peaks at the 2θ values of 43.5°, 50.7°, and 74.3° corresponding to the (111), (200), and (220) planes of Cu were indexed to the FCC structure of Cu.22 The thickness of the Cu shell on EBCus was varied through controlling the electroless Cu plating time, as shown in Fig. 1e–g (Fig. S5, ESI). The EBCu particles with Cu shell thicknesses of 0.10 ± 0.01, 0.29 ± 0.05, and 0.67 ± 0.10 μm were denoted as EBCuA, EBCuB, and EBCuC, respectively (Table S1, ESI). Fig. 1h–j show photographic images of EBCu samples in water. EBCuA and EBCuB particles with a density of 0.44 and 0.75 g cm−3, respectively, float in water, whereas EBCuC (density of 1.34 g cm−3) settles down at the bottom of the vial (Fig. 1k). On the other hand, the density of Cu of 8.94 g cm−3 was approximately 20 times larger than that of EBCuA particles. This lightweight characteristic of EBCu is ascribed to the EB beads with ultralow density (ρ ∼ 0.02 g cm−3), making it float in water (Fig. S6, ESI).
image file: c7nr02618h-f1.tif
Fig. 1 Lightweight characteristics of EBCu particles. (a, b) OM and SEM micrographs of EBs. The inset shows the TEM micrograph of the thin polymer shell in EB. (c, d) OM and SEM micrographs of EBCus. The inset shows the SEM micrograph of fractured EBCu with a hollow structure. SEM micrographs in the cross-sectional view of: (e) EBCuA, (f) EBCuB, and (g) EBCuC, respectively. Photographs of aqueous solution with the beads of: (h) EBCuA, (i) EBCuB, and (j) EBCuC, respectively. (k) Density plots of EB, EBCus, and solid Cu beads.

The EBCu/PCL composites were prepared through the solution blending of EBCu and poly(ε-caprolactone) (PCL) followed by hot-press molding. The morphologies of the as-prepared EBCuB/PCL composites with various EBCuB contents are shown in Fig. 2a–c. The EBCuB2.2/PCL composite with a Cu content of 2.2 vol% exhibited isolated EBCuBs in the polymer matrix. As the Cu content gradually increased, the distance between the EBCuB particles reduced to contact each other, giving rise to the continuous network structure of Cu shells with full interconnection morphology in the EBCuB6.9/PCL composite. Moreover, through a careful morphological examination of the EBCuB/PCL composite, a transition from point-to-point contact to area contact of deformable Cu shells as a function of Cu content was observed at the interface between CuEBs in the composites, as shown in micrographs in the insets of Fig. 2a–c. For comparison, the morphology of the PCL composite with a solid Cu bead content of 16.1 vol% (SCu16.1/PCL) was examined (Fig. 2d). The SCu beads with a diameter of approximately 20 μm were fully isolated in the polymer matrix even though it contained two times more Cu volume fraction as compared with the EBCuB6.9/PCL composite.


image file: c7nr02618h-f2.tif
Fig. 2 Lightweight characteristics of EBCu/PCL composites. SEM micrographs of: (a) EBCu2.2/PCL, (b) EBCu4.0/PCL, (c) EBCu6.9/PCL, and (d) SCu7.2/PCL composites. The existence of solid Cu beads in the SCu7.2/PCL composite was highlighted with red color overlapped by the EDS mapping image. The inset images magnify the selected area and reveal that interconnections between the Cu shells of neighboring EBCus evolve with the Cu content and filler types. (e) Density plots of EBCu/PCL and SCu/PCL composites as a function of Cu content. (f) Porosity of EBCuB/PCL composites as a function of Cu content. Photographs of: (g) PCL, (h) EBCuB6.9/PCL, and (i) SCu7.2/PCL composites in water-containing vials. The red arrows point to the locations of the samples.

The densities of EBCu/PCL and SCu/PCL composites were determined (Fig. 2e). The EBCu/PCL sample revealed a very wide range of density tunability, from 1.34 to 0.59 g cm−3. The densities of EBCuA/PCL and EBCuB/PCL composites decreased with the increase in Cu content, whereas the density of the EBCuC/PCL composite increased when the Cu content was increased. In particular, the EBCuA/PCL sample exhibited the smallest density value of approximately 0.59 g cm−3, which is generally found in conventional cellular materials.23 Simultaneously, the porosity of the EBCu/PCL composite gradually increased with the increase of Cu content. For example, the porosity of the EBCuB/PCL composite gradually increased from 30.4% to 79.8% (Fig. 2f). The conventional SCu/PCL composite always reveals the density that linearly increases with the solid Cu content. Thus, the EBCuB/PCL composite can float in water, whereas SCu/PCL and PCL settle down at the bottom (Fig. 2g–i).

The electrical conductivities of EBCu/PCL and SCu/PCL composites with various Cu contents are graphed in Fig. 3a. The pristine PCL is an insulator with an electrical conductivity value of 10−12 S cm−1.24 EBCu/PCL provides a very small value of electrical conductivity with low Cu contents that rapidly increases at the percolation threshold concentration. After that, the electrical conductivity is nearly saturated with the further increase of Cu content. For example, EBCuB/PCL composites exhibited electrical conductivity values of 1.0 × 10−7 and 5.9 × 10−7 S cm−1 with small Cu contents of 2.2 and 3.2 vol%, respectively. At the percolation threshold between 3.2 and 5.6 vol%, the conductivity value stunningly increased by 9 orders of magnitude to 6.2 × 102 S cm−1. Then, electrical conductivity showed a slow increment with the further increase of Cu content.


image file: c7nr02618h-f3.tif
Fig. 3 Electrical conductivity and EMI shielding effectiveness of EBCu/PCL composites. (a) Electrical conductivities of EBCu/PCL and SCu/PCL composites as a function of Cu content. The inset shows a plot of log σ vs. log(ϕϕc), where σ and ϕc are electrical conductivity and the percolation threshold concentration, respectively. (b–d) Total EMI shielding effectiveness, reflection contribution, and absorption contribution of various EBCuB/PCL and SCu16.1/PCL composites, respectively. The legends in (c) and (d) graphs are the same as in graph (b).

The percolation threshold of the polymer composite is calculated using the power law, expressed as eqn (1)25

 
σ = σ0(ϕϕc)t(1)

where σ is the electrical conductivity of the composite, σ0 is the scaling factor, ϕ is the volume fraction of the filler, ϕc is the percolation threshold volume fraction, and t is related to the dimensionality of the system. Small t values (t < 1.5) are observed in aggregated composites25 or 2D systems,26 whereas it varies between 1.5 and 2 in 3D systems.27 The inset in Fig. 3a shows linear fitting of the power law equation using the electrical conductivity data of EBCuB/PCL composites. The best fitting values for ϕc and t were, respectively, 0.034 and 1.7. Similarly, values of 0.015 and 1.9 for EBCuA/PCL and 0.072 and 1.7 for EBCuC/PCL, respectively, were obtained through the curve fitting method. The EBCu/PCL composites revealed very low percolation concentrations that depended on the type of EBCu. The EBCuA sample with a thinner Cu shell formed an electrically conductive percolation network with a smaller Cu content (ϕc = 1.5 vol%), whereas EBCuC with a thicker Cu shell revealed a greater percolation concentration (ϕc = 7.2 vol%). Additionally, all EBCu/PCL composites exhibited t values larger than 1.5, indicating that EBCu forms a continuous 3D Cu network structure that allows electrons to transfer easily above the percolation concentration. On the other hand, SCu/PCL composites failed to reach the percolation threshold until the Cu content of 24 vol%.

EMI shielding effectiveness (SE) is the ability to shield devices from the EM wave source and is expressed as a logarithmic ratio of the incident and transmitted power.28

 
image file: c7nr02618h-t1.tif(2)

Where PI and PT are the incident and transmitted power components of the EM waves, respectively. Total EMI SE consists of reflection (SER), absorption (SEA), and multiple reflection (SEM) components

 
SET = SER + SEA + SEM(3)

The component SER is related to the impedance mismatching between the free space and shielding material, whereas the SEA term is associated with the energy loss such as the current induced in the shielding material.29 SEM occurs between the two interfaces and is usually ignored when the SET value increases beyond 15.0 dB. However, in the case of some composites, for instance, with multilayers or hollow structures, the SEM value is significant. The contribution from multiple internal reflection is merged in the absorption, as the re-reflected waves are absorbed or dissipated in the form of heat.4 The values of SER and SEA are experimentally determined using scattering parameters (S11, S12, S21, and S22) obtained from the vector network analyzer30

 
image file: c7nr02618h-t2.tif(4)
 
image file: c7nr02618h-t3.tif(5)
where A, R, and T denote absorptivity, reflectivity, and transmissivity, which are related as A + R + T = 1. SET can be deduced from eqn (4) and (5) as
 
SET = SER + SEA = 20log(S21)(6)

Examinations of total values of EMI SE for EBCuB/PCL and SCu/PCL composites revealed an increase in EMI SE as the EBCuB filler content was increased (Fig. 3b). The EMI SE values of EBCuB6.3/PCL and EBCuB6.8/PCL composites reached 61.7 and 88.0 dB at 7 GHz, respectively, whereas EBCuB4.1/PCL and EBCuB2.2/PCL composites showed values smaller than 15.0 dB for this property. According to eqn (2), the value of 88.0 dB for EBCuB6.8/PCL indicates that more than 99.9999998% of incident waves were blocked. The EBCuC9.8/PCL composite revealed the largest EMI SE value of 110.7 dB at 7 GHz (Fig. S7, ESI). In contrast, the SCu16.1/PCL composite exhibited 3.0 dB at the same frequency for EMI SE. This weak performance is because the SCu/PCL composite fails to establish an electrical percolation network.

The study of the reflection and absorption parts of shielding for EBCuB/PCL composites indicated absorption as the major shielding mechanism (Fig. 3c and d). For instance, SER and SEA were obtained as 10.9 and 46.3 dB for the EBCuB5.6/PCL composite and 16.8 and 71.5 dB for the EBCuB6.9/PCL composite at 7.0 GHz, respectively. Theoretically, electrical conductivity contributes in both the reflection and absorption parts. In the case of absorption, the EM wave is attenuated exponentially in the form of ohmic losses, heating up the material. The distance at which the strength of the wave is diminished to 1/e times the original value in the shielding material is defined as the skin depth, δ31

 
image file: c7nr02618h-t4.tif(7)
where ω is the frequency, μ is the permeability, t is the thickness of the sample, and σ is the electrical conductivity. According to this equation, it is easily expected that the skin thickness decreases with the increase in conductivity. EBCu/PCL composites exhibited remarkably thinner skin when EBCu contents were increased (Fig. S8, ESI). This observation is attributed to the formation of a conductive Cu network and the resulting large electric conductivity. Consequently, the incident EM wave is more absorbed in the short distance, resulting in a high EMI absorption value.

Thermal conduction behaviors of PCL and the composites of SCu/PCL and EBCu/PCL were examined with various Cu contents (Fig. 4a and b). The pristine PCL exhibited a thermal conductivity value of 0.23 W m−1 K−1, whereas that of the SCu/PCL solid composite was measured to be 0.30 W m−1 K−1 with the Cu content of 7.20 vol%. The thermal conductivity was hardly increased even when the solid Cu content was significantly increased to 23.00 vol%. Meanwhile, the thermal conductivity of the EBCu/PCL composites remarkably increased with the increase in Cu content. With the same Cu content, the thermal conductivity of the composite increased as the thickness of the Cu shell decreased. The sample of the EBCuA/PCL composite with the thinnest Cu shell revealed the largest thermal conductivity. The EBCuC/PCL composites exhibited the thermal conductivity value of 7.0 W m−1 K−1 with the Cu content of 9.80 vol%, which is approximately 14 times greater than that of the SCu/PCL composite. It also far exceeded the thermal conductivity value (3.7 W m−1 K−1) of the SCu/PCL composite with the solid Cu content of 60 vol% in the literature.32


image file: c7nr02618h-f4.tif
Fig. 4 Heat transport behaviors of EBCu/PCL composites. (a) Thermal conductivities of EBCu/PCL and SCu/PCL composites as a function of Cu content. (b) Comparison between the experimental thermal conductivity values of EBCuB/PCL composites and the calculated values from the Hashin–Shtrikman (H–S) model at upper and lower bound boundary conditions. (c,d) Temperature variation at the top surfaces of neat PCL, SCu16.1/PCL, and EBCuB6.9/PCL composites as a function of time upon isothermal heating at 60 °C.

The significant increase of the thermal conductivity value of the EBCuB/PCL composites was further understood using the Hashin and Shtrikman (H–S) model to estimate the lower and higher bounds of thermal conductivity as33

 
image file: c7nr02618h-t5.tif(8)

where km and kf are the thermal conductivities of the polymer matrix and filler, respectively, and ϕ is the volume fraction of the filler. In this model, the lower and higher bounds represent the thermal conductivities in two extreme boundary conditions where all fillers are entirely isolated from each other or are fully connected in the matrix, respectively. In Fig. 4b, the thermal conductivity of the EBCuB/PCL composite at various Cu contents is plotted with the upper (red) and lower (green) bounds predicted using the H–S model. Above the percolation concentration, the thermal conductivity of the EBCuB/PCL composite reached approximately 28% of the H–S upper bound. Considering the previously reported Ag-based polymer composite system composed of highly fused Ag networks in an epoxy matrix reaching 14% of H–S upper bound; 28% value of H–S upper bound of the EBCuB/PCL composite indicates that the EBCuB/PCL composite formed more concrete and advanced 3D network conduction pathways to transfer heat compared with the conventional composite systems ever reported.34–36

During the isothermal heating of the bottom surface of samples at 60 °C, the temperature distribution of the top surfaces of PCL, SCu/PCL, and EBCuB/PCL was monitored using an infrared (IR) camera. The results are presented in Fig. 4c and d (Fig. S9, ESI). The EBCuB/PCL composite revealed faster heat conduction with a temperature increase rate of 1.71 °C s−1 than the SCu/PCL composite with a rate of 0.48 °C s−1. As the results reveal, PCL exhibited the slowest temperature increase rate. Additionally, the EBCuB/PCL composite exhibited isotropic features in thermal conductivity. The in-plane thermal conductivity of 4.21 W m−1 K−1 was comparable to the through-plane thermal conductivity of 4.65 W m−1 K−1 with the Cu content of 6.2 vol%.

Thermal conductivity and EMI SE values for EBCu/PCL are compared with those for conventional composite materials at various densities in Fig. 5a. In most cases, except for the foam materials, EMI SE and thermal conductivity increase as the density of the material increases. This behavior is mainly because the fillers have a much larger density than the polymer matrix, mostly 1.5 to 2.0 g cm−3 for carbon-based fillers and 2.0 to 10.0 g cm−3 for metallic fillers. As mentioned previously, the foam materials do have weak EMI shielding performance at low thickness and small thermal conductivity. On the other hand, our simple approach to use a lightweight EBCu filler achieved not only improvement in EMI SE and thermal conductivity but also a decrease in density or weight. The EBCu/PCL composite provided excellent dual-functioning properties for an EMI SE of 110.7 dB at 7 GHz and a thermal conductivity of 7.0 W m−1 K−1. The possible mechanism for the excellent EMI shielding and heat conduction behavior with low Cu content in EBCu composites is shown in Fig. 5b. Because of the low density of Cu hollow beads, EBCu particles were fabricated to obtain the ultralow-density template of the expanded polymer bead (ρ ∼ 0.02 g cm−3). As a result, the EBCu/PCL composite can form a very concrete 3D Cu network structure at a very low density (ρ ∼ 0.59 g cm−3). Electrons and heat can easily be transferred through the 3D network so that the electrical conductivity, thermal conductivity, and EMI shielding efficiency could be improved. In addition, the hollow structural feature of the composite improves the EMI shielding performance. When incident EM waves strike the surface of the shielding material, they are partly reflected back due to the highly conducting nature of the polymer composite, whereas the remaining EM waves create induced currents in the polymer composite, which in turn develop eddy current losses and the wave energy is dissipated in the form of heat.28 We recently reported 2D transition metal carbides (MXenes) for EMI shielding application,4 where multiple internal reflection plays a significant role in the mitigation of EM waves when there are additional reflecting surfaces available inside the material. Likewise, in the EBCu/PCL composite, there are numerous hollow sites located between very thin Cu metal shells, which may be responsible for the multiple internal reflection. In these hollow sites, the EM wave is likely reflected many times, losing its intensity. More importantly, the EBCu/PCL composite showed density tunable characteristics, implying that specific target density and EMI SE are achievable through simply changing the thickness of the Cu shell and content of the hollow Cu beads. We believe that the structure of the EBCu/PCL composite can be applied to any applications that simultaneously require high EMI SE, thermal conductivity, and lightweight properties.


image file: c7nr02618h-f5.tif
Fig. 5 (a) Comparison of thermal conductivity and EMI shielding effectiveness between EBCu/PCL composites and conventional composites reported in the literature. EBCuA/PCL (red square), EBCuB/PCL (orange circle), and EBCuC/PCL composites (green triangle) are compared with other polymer composites in the literature (grey colored) as a function of density. Open grey-colored symbols: Cu/PP,37 Ni/PVC,38 Al/HDPE,39 AlN/Epoxy,40 Graphene/Epoxy,41 Al/PES,42 MWCNT/PES/PEEK,43 GNP/PEDOT:PSS,44 MWCNT/WPU.45 (b) Schematic illustration of the EMI shielding and thermal conduction mechanism of the lightweight EBCuC/PCL composite with continuous 3D Cu networks.

Experimental

Materials

Expandable polymer beads (Expancel 461 DU 40, Akzo Nobel, Sweden) used in this study were composed of a thin viscoelastic shell of poly(vinylidene chloride-co-acrylonitrile-co-methyl methacrylate) with a glass transition temperature of approximately 97.2 °C and a volatile isobutane gas core with a low boiling point (b.p. ∼ −11.7 °C). The expandable beads had diameters in the range of 1–15 μm. Tetrahydrofuran (99.5 wt%), hydrochloric acid solution (35.0 wt%), and sulfuric acid (98.0 wt%) were provided by Daejung Chemicals and Metals, Republic of Korea. All electroless plating reagents, including the cleaner, activator, and electroless Cu plating solutions, were purchased from PI Tech, Republic of Korea. Polycaprolactone (PCL, Sigma Aldrich, U.S.A.) with an average molecular mass of 14[thin space (1/6-em)]000 g mol−1 was used as the polymer matrix. Copper solid beads (Sigma Aldrich, U.S.A.) with diameters of 14–25 μm were used as the control system.

Preparation of Cu-coated expanded polymer beads (EBCu)

Expanded polymer beads (EBs) were prepared through the thermal treatment of the expandable beads in an oven at 110 °C for a few minutes (Fig. S1, ESI). The prepared EBs had an average diameter of 20 μm. The EBs were sanitized in 1 M NaOH aqueous solution for 5 min followed by neutralizing them with 0.2 M HCl solution. These EBs were treated with tin/palladium colloid solution for 5 min followed by with 0.2 M H2SO4 solution for 5 min. After that, the mixtures were put into the CuSO4 aqueous solution to achieve the Cu shell on the EBs. The shell thickness was precisely controlled by changing the electroless plating time from 30 s to 7 min to 14 min for EBCuA, EBCuB, and EBCuC, respectively (Fig. S5, ESI). The fabricated EBCu was rinsed with distilled water and then treated with a commercial antioxidant agent (AT-208C, MSC Co., Ltd), and dried in a vacuum for 18 h, and stored in an Ar-charged glovebox to prevent oxidation.

Preparation of 3D hollow structural EBCu/PCL composites

This process was performed in several steps. First, PCL was dissolved in THF under sonication for 20 min. EBCu particles were then mixed with PCL solution and stirred for 10 min. This mixture was dried in a vacuum oven overnight at room temperature followed by forming with a compression molding machine (Auto series, Carver Inc., U.S.A.) in a stainless-steel mold under 5.0 MPa at 65 °C. The solid Cu (SCu)/PCL composite was used for comparison as the control system.

Characterization

SEM and EDS examinations were performed on an Inspect F50 field-emission scanning electron microscope (FEI Company, Hillsboro, U.S.A.) to observe the morphology. The size distribution of EBs was analyzed by using ImageJ software. To examine the cross section of CuEB, the composites were fractured cryogenically. XRD (D8 Advance Sol-X, Bruker, Germany) measurements were carried out from 10° to 90° at 2° min−1 rate to examine crystal structures. Thermogravimetric analysis (TGA, Q-50, TA Instruments Inc., U.S.A.) was performed under a N2 atmosphere to analyze the content of the polymer in EBCu. The density and porosity of EBCu were examined with known constants of PCL density and the weight fraction of CuEB and PCL in the composite. The electrical conductivity values of the composites were determined using a four-pin probe (MCP-TP06P PSP, Mitsubishi Chemical, Japan) method with a low resistivity Loresta GP meter (MCP-T610, Mitsubishi Chemical, Japan). Electromagnetic interference shielding effectiveness (EMI SE) values of the samples were measured using a vector network analyzer (E5071C, Keysight Technologies, U.S.A.) with a coaxial airline sample holder from 100 MHz to 12 GHz. Toroidal-shaped composite specimens (ϕout = 7.0 mm, ϕin = 3.0 mm) with a thickness of 1 mm were used for the EMI SE measurements. The laser flash method (LFA-447, Netzsch, Germany) was used to measure the thermal conductivity of the samples at 25 °C. The specific heat capacity and the diffusivity were measured in comparison with the pyroceramic reference sample. Thermal conductivity was calculated from the equation k = Td × ρ × Cp, where k, Td, ρ, and Cp are thermal conductivity (W m−1 K−1), thermal diffusivity (mm2 s−1), density (g cm−3), and specific heat capacity (J g−1 K−1), respectively. The diameter and thickness of the samples for electrical and thermal conductivities were observed to be 12.7 mm and 1 mm, respectively. Thermal imaging analysis was performed using a conventional forward looking infrared radiometer camera T360 (FLIR Systems, Wilsonville, USA).

Conclusions

Lightweight polymer composites with dual-functional ability of excellent EMI shielding effectiveness and thermal conductivity were fabricated using expandable polymer bead-templated low density Cu hollow beads. The density of the prepared expanded composite was found to be tunable as small as 0.59 g cm−3. These composites provided not only high EMI SE (110.7 dB at 7.0 GHz) but also large thermal conductivity (7.0 W m−1 K−1). Furthermore, all these properties are isotropic and direction-independent. Such results are unique and exceptional, holding great promise for the future development of dual-functional materials in EMI shielding and thermal conduction applications.

Acknowledgements

This work was supported by the Fundamental R&D Program for Core Technology of Materials and the Industrial Strategic Technology Development Program funded by the Ministry of Trade, Industry and Energy, Republic of Korea. This work was also partially funded by the Korea Institute of Science and Technology through the Young Fellow program. This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning.

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Footnotes

Electronic supplementary information (ESI) available: OM, EDS, XRD, and EMI SE data. See DOI: 10.1039/c7nr02618h
These authors contributed equally.

This journal is © The Royal Society of Chemistry 2017