Enhanced Electrical and Electromagnetic Interference Shielding

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Applications of Polymer, Composite, and Coating Materials

Enhanced Electrical and Electromagnetic Interference Shielding Properties of Polymer-Graphene Nanoplatelet Composites Fabricated via Supercritical-fluid Treatment and Physical Foaming Mahdi Hamidinejad, Biao Zhao, Azadeh Zandieh, Nima Moghimian, Tobin Filleter, and Chul B Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10745 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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ACS Applied Materials & Interfaces

Enhanced Electrical and Electromagnetic Interference Shielding Properties of Polymer-Graphene Nanoplatelet Composites Fabricated via Supercritical-fluid Treatment and Physical Foaming Mahdi Hamidinejad a, b, Biao Zhao a, Azadeh Zandieh a, Nima Moghimian c, Tobin Filleter b*, and Chul B. Park a* a

Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Canada M5S 3G8 b Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto M5S 3G8, Canada c NanoXplore Inc., 25 Boul. Montpellier, Saint-Laurent, QC, H4N 2G3 *

Corresponding Authors’ Information: E-mail: [email protected]; [email protected]

Abstract Lightweight high-density polyethylene (HDPE)-graphene nanoplatelet (GnP) composite foams were fabricated via a supercritical-fluid (SCF) treatment and physical foaming in an injectionmolding process. We demonstrated that the introduction of a microcellular structure can substantially increase the electrical conductivity and can decrease the percolation threshold of the polymer-GnP composites. The nanocomposite foams had a significantly higher electrical conductivity, a higher dielectric constant and a higher electromagnetic interference (EMI) shielding effectiveness (SE) and a lower percolation threshold compared to their regular injectionmolded counterparts. The SCF treatment and foaming exfoliated the GnPs in situ the fabrication process. This process also changed the GnP’s flow-induced arrangement by reducing the melt viscosity and cellular growth. Moreover, the generation of a cellular structure rearranged the GnPs to be mainly perpendicular to the radial direction of the bubble growth. This enhanced the GnP’s interconnectivity and produced a unique GnP arrangement around the cells. Therefore, the through-plane conductivity increased up to a maximum of nine orders of magnitude and the percolation threshold decreased by up to 62%. The lightweight injection-molded nanocomposite foams of 9.8 vol.% GnP exhibited a real permittivity of ε'=106.4, which was superior to that of 1 ACS Paragon Plus Environment

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their regular injection-molded (ε'=6.2). A maximum K-band EMI SE of 31.6 dB was achieved in HDPE−19 vol. % GnP composite foams, which was 45% higher than that of the solid counterpart. In addition, the physical foaming reduced the density of the HDPE-GnP foams by up to 26%. Therefore, the fabricated polymer-GnP nanocomposite foams in this study pointed towards the further development of lightweight and conductive polymer-GnP composites with tailored properties. Keywords: Polymer-Graphene nanoplatelet composites, Physical foaming, Microcellular structure, Electrical conductivity, Electromagnetic interference shielding effectiveness, Dielectric permittivity

1.

Introduction Polymer composites have shown impressive potential as a highly desirable class of advanced

functional materials for use in various applications such as capacitors (dielectric materials 1), electromagnetic interference (EMI) shielding

2,3

, electro-static dissipation

4

, and energy

conversion (bipolar plates of fuel cells 5,6). Polymer composites offer tailorable electrical, thermal, and mechanical properties. They are also low cost, offer ease of processing, and their chemical resistance is superior to their metallic and ceramic counterparts

6–8

. The recent advances in

conducive nanofillers such as graphene have significantly increased the opportunities to develop polymer nanocomposites with tailored functionalities 1,2,9. Graphene provides a unique combination of exceptional electrical, thermal, and mechanical properties. Notably, the electrical conductivity of monolayer graphene has been reported as ∼6,000 S/cm 8. One major class of graphene-based polymer nanocomposites are those which take advantage of the electron transport characteristics of graphene for applications such as EMI shielding, where the focus has been on achieving a higher electrical conductivity at lower 2 ACS Paragon Plus Environment

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graphene concentrations 10. EMI shielding of radio frequency radiation is a serious concern in our technological society and graphene has attracted great attention for the fabrication of efficient EMI shields 11–14. Polymer-graphene nanocomposites also exhibit promise for use as dielectric materials with high dielectric permittivity (ε') and low dielectric loss (tan δ) for high-performance capacitors 1. The high electrical conductivity of graphene, when compared to that of the polymer matrix, results in interfacial polarization and, consequently, improved the dielectric permittivity

15,16

.

However, the dielectric properties of percolative polymer nanocomposites change significantly near the percolation threshold. The dielectric loss abruptly increases due to the formation of conductive paths throughout the composite system. Therefore, the dielectric properties of the percolative polymer composites need to be optimized within an “adjustable window” near the percolation threshold, where the dielectric constant can be enhanced while the dielectric loss is still limited

17

. This is, however, extremely challenging

18

. In addition, reaching graphene’s

potential to improve the polymer-graphene nanocomposites’ electrical conductivity, EMI shielding performance, and dielectric properties involves highly complex processes. There are challenges associated with exfoliation, homogeneous dispersion, and the microscopic arrangement of the graphene platelets within the polymer 19. Different methods have been used to develop more efficient graphene-based polymer composites with enhanced electrical and EMI shielding properties. These have included modifying the graphene platelets’ surfaces

1,20

, exploiting the synergistic behavior of the hybrid

nanomaterials 21,22, and in-situ polymerization 1,23. Zhao et al. 2 fabricated hybrid poly-(vinylidene fluoride)-5 wt.% carbon nanotube/10 wt.% GnP thin films of 0.1 mm thickness, using solution casting followed by hot pressing with the EMI SE of 27.58 dB. Wu et al. shielding

graphene

foam

(GF)/poly(3,4-

14

developed EMI

ethylenedioxythiophene):poly(styrenesulfonate) 3

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(PEDOT:PSS) composites by drop coating of the PEDOT:PSS on the cellular structure of the freestanding GFs. The fabricated composites exhibited EMI SE of 91.9 dB. Yousefi et al.20 fabricated self-aligned reduced graphene oxide (rGO)-polymer nanocomposites by dispersing monolayer graphene in epoxy using an aqueous casting method through the in-situ reduction of graphene oxide (GO) 20. They achieved a very low percolation threshold of 0.12 vol% 20. Kim et al. 22 fabricated a hybrid polymer nanocomposite through the chemical vapor deposition of carbon nanotubes onto rGO oxide platelets, followed by solution mixing. They reported a dielectric constant of 32 with a dielectric loss of 0.051 at 0.062 wt% loading of hybrid fillers and 1×102 Hz 22

. Soliman et al.

24,25

developed porous-organic polymers (POPs)-GnP with enhanced electrical

conductivity. They utilized the POP-GnP interactions and homogeneous in-situ coating of the POP atop GnP through a bottom-up assembly on the dispersed GnPs. Unlike the batch-type synthesis methods

1,14,20–23

injection molding is an economically viable

and continuous method to manufacture polymer composites. When it is combined with physical foaming, another layer of flexibility is added, which can tailor the polymer composites’ functional properties. In addition to weight reductions, supercritical fluid SCF treatment and physical foaming can enhance the fillers’ dispersion orientation within the polymer matrix

26

18,29,30

and exfoliation

, their thermal conductivity

33–37

29

, and can re-arrange their

. Foaming can also enhance various polymer

composite functionalities, including their electrical conductivity 18,32

27–29

7,31

, their dielectric performance

, and their electromagnetic interference shielding effectiveness

. However, to the best of our knowledge, no research has been published on the electrical

properties of injection-molded graphene-polymer nanocomposite foams. In this study, we have presented a facile manufacturing platform to decrease the percolation threshold and to enhance the electrical properties and the EMI SE of high-density-polyethylene (HDPE)-graphene nanoplatelet (GnP) composites. Herein, we have demonstrated that the 4 ACS Paragon Plus Environment

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generation of a microcellular structure can substantially enhance the electrical conductivity and reduce the percolation threshold of the GnP based polymer composites. The microcellular HDPEGnP composite foams were fabricated using melt mixing, SCF-treatment and, finally, foaming in an injection-molding process. The generated microcellular structure re-orientated and changed the arrangement of well exfoliated GnPs within the polymer matrix. The HDPE-GnP nanocomposites foams had a lower percolation threshold, enhanced the electrical conductivity, the EMI SE, and the dielectric constant, which made them superior to the regular injection-molded and compression-molded nanocomposites.

2.

Experimental Section

2.1.

Materials and sample preparation

An HHM 5502BN Marlex® grade HDPE (MFI:0.35 dg min−1 230 °C/2.16 kg) with a density of 0.955 g cm−3 was loaded with GnP powder provided by NanoXplore Inc. (heXo-g-V20 with a density of 2.2 g.cm-3, a surface area per unit mass of 30 m2/g) to make a HDPE-35 wt.% GnP masterbatch. The HDPE-35 wt.% GnP masterbatch was produced by melt compounding using a TDS-20 twin-screw extruder with a 22 mm screw diameter and a 40 L/D ratio. The temperature profile was set to 180°C - 220°C. A rotational speed of 45 rpm and a throughput of 5 kg.min-1 were used. HDPE-GnP composites with a different GnP loading content were then obtained by diluting the HDPE-35 wt.% GnP masterbatch with neat HDPE and mixing them in a twin-screw extruder (with a diameter of 27 mm and L/D: 40). Nitrogen (N2), supplied by Linde Gas, Canada, was used as the SCF. A 50-ton Arburg Allrounder 270/320C injection-molding machine (Lossburg, Germany), with a 30-mm diameter screw equipped with MuCell® technology (Trexel, Inc., Woburn, Massachusetts) was used to fabricate the HDPE-GnP composites. The following two types of 5 ACS Paragon Plus Environment


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HDPE-GnP composite samples were fabricated: injection-molded solid (Solid), injection-molded foam (Foam). The degrees of foaming in the foamed samples were controlled by partially filling the mold volume. The degree of foaming is another term for the void fraction in the injectionmolded foam samples. Details regarding the manufacturing of the solid and the foamed samples were reported in our previous work and in the Supporting Information (Table S1)

29

. The solid

and foamed samples were cut from the injection-molded nanocomposites at a distance of 100 mm from the gate. The schematic of the injection-molded parts has been reported in our previous work 29 and Figure S1.

2.2.

Characterization

Scanning electron microscopy (SEM) imaging was performed using a Quanta EFG250. The SEM samples were prepared through cryofracture and subsequently sputter-coating with gold. Transmission electron microscopy (TEM) imaging was performed using a FEI Tecnai-20 TEM to investigate the level of GnP’s exfoliation within the polymer matrix. The TEM samples were prepared by cryo-ultramicrotomy (Leica EM FCS). The through-plane electrical conductivity, the dielectric constant, and the dielectric loss of the samples with a 20 mm diameter × 3 mm thickness, were measured using an Alpha-A high performance dielectric impedance analyzer (Novocontrol Technologies GmbH & Co. KG). The broadband electrical properties of the HDPE-GnP composites were analyzed at frequencies that ranged from 1×10-1 Hz to 3×10+5 Hz. The electrical conductivity was measured at a frequency of 0.1 Hz and was reported as the direct current (DC) conductivity (σDC)

7,31,35

. The comparative

analyses of the dielectric properties were conducted at a frequency of 1×10+3 Hz 17,38. The EMI SE values of the HDPE-GnP composites with dimensions of 10.6 mm×4.3 mm×3.0 mm were measured over a frequency range of 18−26.5 GHz (K-band) using the waveguide 6 ACS Paragon Plus Environment

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method via the Agilent N5234A vector network analyzer. The power coefficient of the reflection (R), transmission (T), and absorption (A) were calculated from the S-parameters (that is, the S11 and S21) based on the following Equations 39–42: R = |S11|2

(1)

T = |S21|2

(2)

A=1˗R˗T

(3)

Thus, the total EMI shielding (SET), including the shielding by absorption (SEA) and the reflection (SER), can be described by the following Equations 40,41,43,44: SET = SER + SEA

(4)

SER = −10logଵ଴ (1 − ܴ)

(5)

SEA = −10logଵ଴ (

் ଵିோ

)

3.

Results and Discussion

3.1.

Microstructure and morphology of the HDPE-GnP composites

(6)

Figure 1 shows the microstructure of the core and skin regions of the solid and foamed HDPE9.8 vol.% GnP composites. As was expected, the solid samples’ structure was completely solid. The GnPs were highly oriented in the flow direction in the skin (about 500 µm on each side) region of the solid samples. This was due to the high shear stresses caused during injection molding

45

. However, in the core region of the solid samples, the GnPs had a relatively more

random orientation. The foamed nanocomposites had a microcellular structure with a non-homogeneous cell morphology. The average cell size of the HDPE-GnP composites foams with a 16% degree of foaming was 20±11µm. This non-homogeneous microcellular structure was a result of the structure’s heterogeneities, which were caused by the dispersed GnPs, where the lower activation 7 ACS Paragon Plus Environment


energy for cell nucleation is required

46–48

. Moreover, in the foamed samples, the GnPs’

orientation in both of these regions was random. This was mainly attributed to (i) the nanocomposites’ lower melt viscosity due the SCF’s dissolution and (ii) the growth of cells during the physical foaming. The growth of bubbles caused the rotation and displacement of the GnPs and oriented them mainly perpendicular to the radial direction of the cell growth 49,50. This re-arranged the GnPs’ flow-induced orientation, and thus increased the opportunities for their interconnectivity

31,51

. In Figure 1b, the schematic diagram shows the GnPs’ arrangement and

interconnectivity in the solid and foamed HDPE-GnP composites.

Figure 1. (a) SEM micrographs of the skin and core regions of the solid and foamed (16 % degree of foaming) HDPE-GnP composites at 9.8 vol % GnP content, and (b) Ideal conceptualization of the GnPs’ arrangement in the solid and foamed samples. The arrow shows the melt’s flow direction in the injection-molding process.

3.2.

The effect of physical foaming on the GnP’s exfoliation and dispersion

Following the SCF-treatment and physical foaming the thick and agglomerated GnPs in the solid samples were further exfoliated into thinner layers. This process was discussed in detail in 8 ACS Paragon Plus Environment

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our previous study

29

. Figure S2 shows more analysis of the HDPE-4.5 vol.% GnP composites

using wide-angle X-ray diffraction (WAXD) (Figure S2a) and transmission electron microscopy (TEM) (Figure S2b-c). The intensity reduction at the diffraction peak of the (002) plane indicated the GnPs’ exfoliation 28,29,52,53. Once the HDPE-GnP melt had received the SCF-treatment, the SCF was dissolved within the polymer matrix, and then it was diffused between the GnPs’ layers. Due to the rapid depressurization in the mold cavity, the SCF experienced phase transformation. The SCF’s expansion during its transformation into a gaseous state exfoliated the graphene layers

29,54

.

Moreover, the nucleated bubble growth near the GnPs further dispersed the GnPs within the polymer matrix 27,29.

3.3.

The electrical conductivity of the polymer-GnP composites

Figure 2a shows the broadband conductivity of the nanocomposites across a frequency range of 1×10-1 Hz to 1×10+5 Hz. The solid samples had a 7 to 12.6 vol.% GnP content. The foamed samples were fabricated using the corresponding solid precursor, which contained 7, 9.8 and 12.6 vol.% of the GnP. The broadband electrical conductivity of all the solid samples (containing 7, 9.8 and 12.6 vol.% GnP) followed a frequency-dependent behavior across the whole frequency range. The frequency-dependency of the electrical conductivity is one of the typical characteristics of insulating polymer composites

17,55

. This indicates that the GnPs were distributed within the

polymer matrix without forming conductive channels. And this behavior is defined by σ = σDC + σAC, where the σDC is the frequency-independent part and the σAC (alternative current (AC) conductivity) is the frequency-dependent part of the total electrical conductivity. The frequency below which the electrical conductivity shows a frequency-independent behavior is known as the 9 ACS Paragon Plus Environment


critical frequency 17,55. The frequency-dependent conductivity of the solid samples containing 9.8 vol.% GnP was decreased from 1.1×10-8 S.cm-1 to 2.0×10-14 when the frequency was decreased from 1×10+5 to 1×10-1 Hz.

10

-7

Foam 9.8vol.% GnP

10-8 10-9 10-10 10-11

7 lid So

10-12 10-13

12 vol.% 9 vol.% 7 vol.%

10-14 10-1

100

101

102

ol.% .0v

P Gn

Solid Solid Solid

103

Foam Foam Foam

104

10

-5

10

-6

10-7

(b) Foam (GnP content with respect to total volume) Foam (GnP content with respect to polymer volume) Solid

10-8 10-9 10-10

Foam

10

-6

Foam 12.6vol.% GnP

m

10

-5

10-4

Foa

(a)

10-4

Conductivity,σDC (S.cm-1)

Conductivity,σAC (S.cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10-11 10-12 10-13 10

Solid

-14

10-15

105

0

2

4

6

8

10

12

14

16

18

GnP content (vol.%) Frequency (Hz) Figure 2. (a) The AC conductivity of the solid, and foamed HDPE-GnP composite; and (b) The DC conductivity of the solid, and foamed HDPE-GnP composite measured at 0.1 Hz (DF means degree of foaming)

However, the physical foaming transformed the frequency-dependent behavior of the solid samples (containing 9.8 vol.% GnP) into the frequency-independent behavior at frequency ranges of below 2×10+3. By increasing the GnP content to 12.6 vol.%, the foamed samples exhibited a frequency-independent behavior across the entire frequency range from 1×10-1 Hz to 1×10+5 Hz. Moreover, foaming enhanced the electrical conductivity of the solid HDPE-12.6 vol.% GnP composites by eight orders of magnitude at frequency ranges of below 1×100. Figure 2b shows the variation of the DC conductivity of the solid and foamed HDPE-GnP composite as a function of the GnP loading. The HDPE-GnP composite’s electrical conductivity was significantly affected by the physical foaming. This occurred through two different mechanisms, which included the following: (i) The foaming actions, such as bubble growth which affected the GnPs’ arrangement and interconnectivity 29; and (ii) The volume exclusion effect of 10 ACS Paragon Plus Environment

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foaming which resulted in the GnPs’ localization within the struts and cell walls

7,29,31

. To focus

solely on how the foaming actions affected the electrical conductivity, the GnP content was considered in relation to the polymer volume. In other words, the GnP content in the foamed samples was reported the same as their solid precursors. To include how density reduction in the foaming affected the electrical conductivity, the GnP content was calculated in relation to the total volume of the nanocomposite foams. The conductivity of all the HDPE-GnP composites showed a clear insulation-conduction transition behavior. The abrupt insulation–conduction transition of the foamed samples began at a much lower GnP content than that of their solid counterparts. Thus, the percolation threshold of the foamed samples was found to be around 9.8 vol.% GnP (that is, in relation to the polymer volume). This outcome was far superior to the 19 vol.% GnP that was found in the solid samples. Moreover, by taking a 16 vol.% degree of foaming into account, the percolation threshold of the foamed samples was further decreased from 9.8 vol.% to 8.2 vol.% GnP. In other words, the generation of a microcellular structure within the injection-molded samples decreased the percolation threshold for the nanocomposites by more than 2.3-fold. Meanwhile, to achieve the same level of electrical conductivity in the given volume of the samples, the required GnP content (in relation to the total volume) for the foamed nanocomposites was much lower than it had been for the solid ones. For example, the foamed samples with a GnP content of 8.2 vol.% had the same electrical conductivity, which had been achieved with 19 vol.% GnP, in the solid nanocomposites. The GnPs’ flow-induced orientation in the solid nanocomposites (discussed in Section 3.1) significantly deteriorated their interconnectivity and the formation of a conductive network. And, consequently, the through-plane electrical conductivity was inferior. This resulted in a high

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percolation threshold and in the very slow increase of the electrical conductivity in the solid samples with an increased GnP content. The higher through-plane electrical conductivity and the lower percolation threshold of the foamed samples, as compared to the solid counterparts, were mainly attributed to the changes in the microstructures. This had been induced by the introduction of foaming, which operated in several ways and included the following actions: (a) a higher level of GnPs’ exfoliation and dispersion in the polymer; (b) a decreased flow-induced orientation of GnPs due to the foaming actions and reduced viscosity; (c) enhanced local interconnectivity of GnPs due to the cell growth during foaming; and (d) reduction in the skin layer’s thickness. It is also believed that the GnPs’ aspect ratio is higher with foaming, due to the lower melt viscosity and the lower fillers’ mechanical breakdown 31,35.

3.3.1. The effect of the foaming degree on the electrical conductivity Figure 3a shows the variations of the σDC with the foaming degree in the foamed nanocomposites with various GnP contents (in relation to the polymer volume). Below the percolation threshold of the solid nanocomposites (that is, 9.8, 12.6 and 15.6 vol.% GnP in Figure 2b), the generation of a 7% foaming degree caused the formation of conductive percolative networks and resulted in a sharp increase in the σDC from 6 to 9 orders of magnitude. Around the solid samples’ (19 vol.% GnP) percolation threshold, the conductivity enhancement due to the foaming was less pronounced and increased only by 3 orders of magnitude. This was attributed to the percolative networks that had already formed within the solid nanocomposites at 19 vol.% GnP.


Page 12 of 27

10-4

(a)

-5

10

-6

10

19

l.% vo

15.6 vol.% GnP

G


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 13 of 27

nP

12.6 vol.% GnP

-7

10

10-8

9.8 vol.% Gn P

-9

10

10-10 10-11 10-12 10-13

7.0 vol.% GnP

10-14 5

10

15

20

(b) Foam (7% DF) Foam (16% DF) Foam (26% DF) Solid

10-5 -6

10

10-7 10-8 10-9 10-10 10-11 10-12

Solid

10-13 10-14 10-15

4.5 vol.% GnP

0

10-4

0

25

2

4

6

8

10

12

14

16

18

Degree of foaming (%) GnP content (vol.%) Figure 3. (a) Variations of the foaming degree on the electrical conductivity of the HDPE-GnP composites; (b) The evolution of the percolation threshold with the foaming degree

To further investigate how the foaming degree affected the electrical conductivity, the σDC of the solid and foamed nanocomposites were plotted as a function of the GnP content in Figure 3b. Notably, the percolation threshold was decreased by the increased foaming degree. The percolation threshold sharply dropped from 19 to 9.1 vol.% GnP when a 7% degree of foaming was generated. The percolation threshold was further decreased from 9.1 to 7.2 vol.%, when the degree of foaming was increased to 26%. Therefore, the generation of the microcellular structure decreased the percolation threshold by up to 62%. The decrease in the percolation threshold that was obtained by the increase in the foaming degree from 7% to 26% was mainly attributed to the volume exclusion effect induced in the gaseous phase.

3.4.

The dielectric properties of polymer-GnP composites

The dielectric permittivity presents in a complex function, which is composed of a real part ε' and an imaginary part ε''. The real part is related to the charge displacement, which is governed by the polarization within the material. Interfacial polarization is the most common type of polarization that occurs across frequency ranges of less than 1 MHz Wagner–Sillars (MWS) effect

56

15

. Based on the Maxwell–

, charges are accumulated at the interface of the polymer and 13 ACS Paragon Plus Environment


filler. The imaginary part of the dielectric permittivity (ε'') is used to quantify the dielectric loss (tan δ), which is defined as the ratio of the imaginary part to the real part of the dielectric permittivity. Figure 4a-b exhibits the dialectic constant and loss of the solid and foamed (16% degree of foaming) nanocomposites as a function of the GnP content. The dielectric constant (ε') in all of the samples was enhanced by increasing the GnP content. The higher GnP content increased the polymer-GnP interface area, which resulted in a higher interfacial polarization. Moreover, the polymer-GnP nanocomposites can be considered as nanoscale parallel-plate capacitors, where the GnPs act like electrodes, and the polymer matrix is considered to be dielectric

17,18

. Therefore,

increasing the GnP content increased the number of nanocapacitors and decreased the interspatial distances between the adjacent GnPs, thus leading to a higher real permittivity.

Dielectric loss, (tan δ)

200

Foam (GnP content with respect to total volume) Foam (GnP content with respect to polymer volume)

Solid

150

Foam

250

(a)

F oam

300

100

lid

Real permittivity, (ε')

50

So

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

103

(b)

102

(GnP content with respect to total volume) Foam (GnP content with respect to polymer volume)

Foam

101

Solid

10

0

10-1 10

Solid

-2

10-3

0 0

2

4

6

8

10

12

14

16

0

18

2

4

6

8

10

12

14

16

18

GnP content (vol.%) GnP content (vol.%) Figure 4. (a) Real dielectric permittivity (ε'); and (b) The dielectric loss (tan δ) of the solid and foamed (16% degree of foaming) nanocomposites as a function of the GnP content measured at 1×10+3 Hz. (GnP vol.% is reported in relation to the polymer volume)

However, with the same GnP content, the dielectric constant of the foamed samples was considerably higher than that of their solid counterparts. For instance, at 9.8 vol.% GnP, the real permittivity of the solid nanocomposites was 6.2. However, the introduction of the microcellular 14 ACS Paragon Plus Environment

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structure substantially increased the real permittivity of the foamed nanocomposites to 106.4 with a 9.8 vol.% GnP content (Figure 4a). In other words, the real permittivity of the foamed samples with a 9.8 vol.% GnP content was more than one order of magnitude higher than that of their solid counterparts. Figure 4b shows that the dielectric loss was increased by the increased GnP content in both solid and foamed samples. The increased GnP content enlarged the number of the charge carriers and the nanocapacitors which, respectively, resulted in a higher Ohmic and polarization loss 18,32. The foamed nanocomposites had a higher dielectric loss than the solid samples, mainly due to the more random distribution of the fillers in the polymer matrix 18,56. And this led to the formation of GnP conductive networks and, thereby, a higher Ohmic loss

18,32

. On the other hand, the

introduction of foaming increased both the dielectric permittivity and the dielectric loss of the nanocomposites18.

However, it is interesting to note that the dielectric loss of the foamed

samples, around the percolation threshold, was still relatively low. For instance, the real permittivity and the dielectric loss of the foamed samples with a 9.8 vol.% GnP was 106.4 and 0.4, respectively. The increased real permittivity of the foamed samples, when compared with that of the solid nanocomposites, was mainly attributed to the unique GnP parallel-plates arrangement in the cell walls due to the cellular growth that occurred between the adjacent GnPs

32

18,32

. This led to a highly effective interface area

. Moreover, a higher level of GnP exfoliation: (a) increased the

number of nanoscale capacitors; (b) raised the polymer-GnP interfaces; and (c) decreased the interspatial distances between the adjacent GnPs, which enhanced the real permittivity 32. Figure 5 shows the broadband real permittivity (ε') and the dielectric loss (tan δ) of the solid and foamed (with 16 % degree of foaming) nanocomposites with different GnP contents. The broadband real permittivity of all the solid samples followed a relatively frequency-independent 15 ACS Paragon Plus Environment


behavior across the whole frequency range (Figure 5a). On the other hand, the generation of the microcellular structure not only substantially increased the real permittivity, but it also changed the frequency-independent behavior of the real permittivity in the solid nanocomposites, which contained 9.8 and 12.6 vol.% GnP, into the frequency-dependent behavior found in their foamed counterparts (Figure 5b). This frequency-dependent behavior of the dielectric constant is a characteristic of the conductive composites

7,56

. It indicated that conductive paths had formed

103

(a)

Solid 12.6 vol% Gr 9.8 vol% Gr

Broadband real permittivity (ε')

Broadband real permittivity (ε')

within the foamed samples 31.

7.0 vol% Gr 4.5 vol% Gr

102

10

1

100

101

102

103

104

103

(b)

Foam 9 .8

(c)

2

10

7.0 vol% GnP 4.5 vol% GnP

101 100 10-1

12 v ol.% GnP

10

4.5 vol.% GnP

100 103 10

2

10

1

101

102

10-3

102

103

103

104

105

104

10-3

105

(d)

Foam 12

9v ol. %

vo l.%

Gn P

Gn P

100

10-2

4.5 vol.% GnP

101

% GnP

7.0 vol.% GnP

10-1

10-2

100

12.6 v ol.

Frequency (Hz)

Solid 12.6 vol% GnP 9.8 vol% GnP

Gn P

1

105

Broadband dielectric loss (tan δ)

103

vol. %

102

Frequency (Hz) Broadband dielectric loss (tan δ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

7 vol.% GnP

4.5 vol.% GnP

100

101

102

103

104

105

Frequency (Hz)

Frequency (Hz)

Figure 5. Broadband dielectric permittivity of (a) The solid samples, and (b) The foamed 9.8 vol.% HDPE-GnP composites. Broadband dielectric loss of (c) The solid samples, and (d) The foamed 9.8 vol.% HDPE-GnP composites 16 ACS Paragon Plus Environment

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Figure 5c-5d shows that, beyond the percolation threshold, the broadband dielectric loss of the foamed nanocomposites was higher than in the solid counterparts. The higher dielectric loss of the foamed samples was attributed mainly to their higher Ohmic loss, which was related to the σDC. The total dielectric loss consisted of the Ohmic loss and polarization loss of the space charges 57,58

. However, in the current polymer-GnP system, the Ohmic loss was the major contributor to

the total dielectric loss 57,58. The higher σDC and, consequently, the higher Ohmic loss, caused the frequency-dependency of the dielectric loss in the foamed nanocomposites with a 9.8 and 12.6 vol.% GnP content.

3.5.

The EMI shielding effectiveness (SE) of the polymer-GnP composites

The EMI’s shielding effectiveness represented the material’s ability to reduce the electromagnetic waves’ intensity.

The shielding performance for a given electromagnetic

radiation is defined as SE=10log (Pi/Pt), where Pi is the incident power and Pt is the transmitted power in decibels (dB) 35,39. For instance, a material with a SE of 40 dB can block 99.99% of the incident wave. Figure 6 shows the EMI SE of the solid and foamed HDPE-GnP composites over the K-band frequency range (between 18 GHz and 26.5 GHz). The EMI SE values were greater at a higher GnP content in both the foamed and solid samples.



Foam

Solid

(a)

4.5 vol.% GnP 7.0 vol.% GnP 12.6 vol.% GnP

30 25 20

P 19vol.% Gn

15

(b)

35

15.6 vol.% GnP 19.0 vol.% GnP

15.6vol.% GnP

12.6vol.% GnP

10

EMI SE (dB)

35

EMI SE (dB)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

7.0vol.% GnP

19vol.% GnP

30 4.5 vol.%GnP 7.0 vol.%GnP 12.6 vol.%GnP

25 20 15 10

15.6 vol.%GnP 19.0 vol.% GnP 15.6vol.% GnP

12.6vol.% GnP

7.0vol.% GnP

5

5

4.5vol.% GnP

4.5vol.% GnP

0

0 18

20

22

24

18

26

Frequency (GHz)

20

22

24

26

Frequency (GHz)

Figure 6. K-band EMI SE of (a) the solid; and (b) the foamed HDPE-GnP composites with various GnP content.

As shown in Figure 7a, at a given GnP content, the foamed samples had higher SE values than their solid counterparts. The grand average of the three sample replications’ measured values over the K-band frequency range were plotted as the EMI SE shown in Figure 7. At a 19 vol% GnP, the EMI SE of the foamed samples reached 31.6 dB, which corresponded to a 99.93% blockage of the incident EMI wave. With the same GnP content, the solid samples had an EMI SE of 21.8 dB. Figure 7a also presents the foamed samples’ EMI SE as a function of the GnP content, which was calculated in relation to the nanocomposite foams’ total volume. It is notable that to attain a certain EMI SE value in a given nanocomposite volume, the GnP content required for the foamed nanocomposites was considerably lower than it was for their solid counterparts. For instance, to reach an EMI SE of about 21 dB, the final GnP vol.% was, respectively, 19 and 14 for the solid and foamed nanocomposites. This corresponded to a 26% reduction in the GnP usage when foaming was done.


Foa

(GnP content with respect to polymer volume)

Solid

So lid

20 15 10

EMI SE (dB)

Foam

25

Foam-Absorption Solid-Absorption

25

Foam-Reflection Solid-Reflection

20 15

Absorption

lid

(GnP content with respect to total volume)

(b) Fo a m

30 Foam

Foa m

30

(a) m

35

EMI SE (dB)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


So

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10 5

5

Reflection

0

0 0

2

4

6

8

10

12

14

16

18

0

2

4

6

8

10

12

14

16

18

GnP content (vol.%)

GnP content (vol.%)

Figure 7. (a) The K-band EMI SE of the solid and foamed HDPE-GnP composites as a function of their GnP content; (b) The contributions of the reflection and absorption mechanisms to the total K-band EMI SE of the solid and foamed HDPE-GnP composites as a function of their GnP content; (c) schematic diagrams of the scattering and multiple reflections of the electromagnetic waves

The wave reflection (SER) and the absorption (SEA) are the main electromagnetic attenuation mechanisms

39–42

. To further demonstrate the shielding mechanisms in both the solid and foamed

nanocomposites, Figure 7b shows the contributions of the wave reflection and the absorption to the total EMI SE (SET). The contribution of the reflection to the total shielding in both the solid and foamed nanocomposites was similar, and it reached ∼3.5 dB around the percolation threshold region. However, the absorption mechanism clearly dominated the shielding mechanism, and it was 19 ACS Paragon Plus Environment


Page 20 of 27

continuously increased by the addition of GnP in both the foamed and solid nanocomposites. For example, the absorption mechanism contributed, respectively, 84% and 88% of the total shielding in the solid and foamed HDPE-19 vol.% GnP composites. It was also notable that the foamed samples’ SEA was higher than the solid counterparts’ with the same GnP content. This gave the foamed nanocomposites a higher SET. The reflection mechanism is related to the impedance mismatch between the shielding composite and the air. The presence of the charge carriers (that is, the electrons and holes) and/or the surface charge are mainly assumed to govern the reflection mechanism

2,59,60

absorption mechanism originates from the Ohmic and polarization losses

61

. However, the

. The Ohmic loss

results in energy attenuation via the current flow through the conduction and tunneling mechanisms. The polarization loss is correlated to the interfacial polarization’s density and is thereby transferred to the absorber’s real permittivity 2,4. The foamed samples’ enhanced SE was mainly attributed to three factors. The first of these is the electromagnetic wave’s multiple reflections on various surfaces (that is, of the cell-composite matrix surface area), which created another shielding mechanism

13,31,35,36

. The electromagnetic

waves entering the nanocomposites foams were reflected and scattered in the microcellular structure numerous times. Therefore, the adequate wave absorption capability of the composite matrix combined with the multiple reflections inside the cells to further enhance the shielding properties of the electromagnetic waves. Thus, the foamed nanocomposites’ SET was improved. Figure 7c shows schematic diagrams of the scattering and multiple reflections of the electromagnetic waves in both the solid and foamed nanocomposites. The second factor was the GnPs’ increased interconnectivity and, hence, the samples’ resultant higher conductivity and permittivity. It has been reported that higher conductivity and permittivity (ε') result in a higher SE 2,31,62. The third factor was a higher level of GnP exfoliation caused by the SCF treatment and 20 ACS Paragon Plus Environment

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foaming processes. The higher level of GnP exfoliation would contribute to the enhancement of the electrical conductivity and the dielectric permittivity of the foamed samples (as discussed in Sections 3.3 and 3.4) and, thereby, would result in a higher EMI SE in the foamed nanocomposites 2,31,62.

4.

Summary & Conclusions Herein, we have demonstrated that SCF-treatment and physical foaming can substantially

increase the electrical conductivity and reduce the percolation threshold of the polymer-GnP composites. This facile technique at once enhanced the electrical conductivity, the dielectric constant and the EMI shielding performance of the HDPE-GnP composites and decreased their percolation thresholds. The lightweight HDPE-GnP composite foams were prepared by melt compounding followed by foaming in an injection molding process. The SCF-treatment and physical foaming were found to exfoliate the GnPs and change their flow-induced orientation by reducing the viscosity and bubble growth. The generation of a microcellular structure re-arranged the GnPs so that they were mainly perpendicular to the radial direction of the cellular growth within the cell walls. This enhanced the GnPs’ interconnectivity which resulted in a significantly higher conductivity and a lower percolation threshold. For example, in addition to 26% density reduction, the percolation threshold of 19 vol.% GnP in the solid samples was sharply decreased to 7.2 vol.% GnP with the introduction of a 26% degree of foaming. Foaming substantially enhanced the real permittivity of the foamed samples. The real permittivity of the foamed samples with a 9.8 vol.% GnP was 106.4 while that of their solid counterparts was 6.2. Moreover, the introduction of a microcellular structure enhanced the EMI shielding performance of the HDPEGnP composites. A maximum EMI SE of 31.6 dB was achieved in HDPE−19 vol. % GnP composite foams, which was superior to 21.8 dB of the solid counterparts. 21 ACS Paragon Plus Environment


These research results show that SCF-treatment and physical foaming in an injection-molding process offers a facile, cost-effective, and industrially viable method by which to develop lightweight conductive polymer-GnP nanocomposites.

5.

Associated content

Supporting Information: The method of sample preparation, the processing parameters of injection molding, the XRD spectra of neat HDPE, GnP powder, solid and foamed nanocomposites, and the TEM analysis of solid and foamed samples. 6.

Acknowledgments The authors gratefully acknowledge NanoXplore Inc.’s financial support and their donation of

materials for this study. We also appreciate the Natural Sciences and Engineering Research Council of Canada’s (NSERC) financial support. M.H. would like to acknowledge funding from the NSERC Alexander Graham Bell Canada Graduate Scholarship Program and the Ontario Graduate Scholarship (OGS).

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Enhanced Electrical and Electromagnetic Interference Shielding

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