Ultralight Microcellular Polymer-Graphene Nanoplatelet Foams with

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Ultralight Microcellular Polymer-Graphene Nanoplatelet Foams with Enhanced Dielectric Performance Mahdi Hamidinejad, Biao Zhao, Raymond K.M. Chu, Nima Moghimian, Hani E. Naguib, Tobin Filleter, and Chul B Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03777 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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ACS Applied Materials & Interfaces

Ultralight Microcellular Polymer-Graphene Nanoplatelet Foams with Enhanced Dielectric Performance Mahdi Hamidinejad a, b, ⊥, Biao Zhao a, ⊥, Raymond K.M. Chu a, Nima Moghimian c, Hani Naguib d , 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

Nano Mechanics and Materials Lab, Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto M5S 3G8, Canada c

d

NanoXplore Inc., 25 Boul. Montpellier, Saint-Laurent, QC, H4N 2G3

Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto M5S 3G8, Canada ⊥

*

M.H. and B.Z. equally contributed Corresponding Authors’ Information: E-mail: [email protected]; [email protected]

Abstract Dielectric polymer nanocomposites with high dielectric constant (ε'), and low dielectric loss (tan δ) are extremely desirable in the electronics industry. Percolative polymer-graphene nanoplatelet (GnP) composites have shown great promise as dielectric materials for high-performance capacitors. Herein an industrially-viable technique for manufacturing a new class of ultralight polymer composite foams using commercial GnPs with excellent dielectric performance is presented. Using this method, the high-density polyethylene (HDPE)-GnPs composites with a microcellular structure were fabricated by melt mixing. This was followed by supercritical fluid (SCF) treatment and physical foaming in an extrusion process, which added an extra layer of design flexibility. The SCF treatment effectively in-situ exfoliated the GnPs in the polymer matrix. Moreover, the generation of a microcellular structure produced numerous parallel-plate nanocapacitors consisting of GnP pairs as electrodes with insulating polymer as nanodielectrics. This significantly increased the real permittivity and decreased the dielectric loss. The ultralight extruded HDPE-1.08 vol.% GnP composite foams, with a 0.15 g.cm-3 density, had an excellent 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

combination of dielectric properties (ε'=77.5, tan δ=0.003 at 1×105 Hz) which were superior to their compression-molded counterparts (ε'=19.9, tan δ =0.15 and density of =1.2 g.cm-3) and to those reported in the literature. This dramatic improvement resulted from in-situ GnP’s exfoliation and dispersion, as well as a unique GnP parallel-plates arrangement around the cells. Thus, this facile method provides a scalable method to produce ultralight dielectric polymer nanocomposites, with a microscopically-tailored microstructure for use in electronic devices. Keywords: Dielectric permittivity; dielectric loss; graphene nanoplatelets; polymer nanocomposites; physical foaming; microcellular structure

1.

Introduction

High performance dielectric materials are vital to the development of next-generation miniaturized electronic devices. Dielectric materials with high dielectric permittivity (ε') and low dielectric loss (tan δ) have been receiving increasing interest in modern electronics as the capacitors and integrated capacitors

1–4

. Multifunctional, lightweight, and low-cost polymer

nanocomposites show much promise for use as dielectric materials. Their dielectric permittivity and dielectric loss tunability is large; their resistance to chemicals is outstanding; they are easily processed, and they have tailorable thermal and mechanical properties 5,6. Polymers have an extremely low dielectric loss and a high dielectric breakdown strength; however, they suffer from a low dielectric constant (ε'103 Hz). The lower dielectric loss at higher frequencies was proportional to the polarization loss 67,69, This was further restricted after foaming was introduced. Thus, the dielectric loss dropped down to 0.003 at 1×105 Hz in the extruded foam samples. However, the dielectric loss of their SCM counterpart samples was 0.18 at 1×105 Hz, and it sharply increased as the frequency decreased. The reduction in the polarization loss in the extruded foam samples was attributed to a better dispersion of the GnPs and to their better interfacial interaction with the polymer matrix. This was due to the dissolution of the scCO2 in the polymer. Thus insulating layers formed among the GnPs to prevent the migration of

(a)

Broadband dielectric loss (tan δ)

the space charge within the nanocomposites 66.

Broadband real permittivity (ε')

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


Foam

80 60 So l id

40 20 Neat HDPE

0 101

102

103

104

105

102

(b)

101

Sol id

100 10-1

Foam

10-2 10-3 10

Neat HDPE

-4

101

Frequency (Hz)

102

103

104

105

Frequency (Hz)

Figure 7. (a) Broadband dielectric permittivity; (b) Broadband dielectric loss of the SCM HDPE9.8 vol.% GnP composites and their extruded foam (with a density of 0.15 g.cm-3 or ~8 times foam expansion ratio) counterparts.

21 ACS Paragon Plus Environment


3.4.1. Effect of the density on the dielectric properties Figure 8 shows the variation of the real permittivity and the dielectric loss as a function of the density of the extruded foam samples, which were made from solid precursors containing 4.5 vol.% GnP. When the density of the HDPE-GnP composites decreased from 1.07 to 0.08 g.cm-3, the ε' increased from 9.6 to 22.3, and the tan δ decreased from 0.04 to 0.006. A further decrease in the density to 0.05 g.cm-3, however, slightly decreased the ε' to 18.8. Meanwhile, the dielectric loss continued to decrease to as low as 0.004. The optimal behavior of the ε' can be mainly attributed to the changes in the microcellular structures, when their densities (that is, the foam expansion ratios) were varied as was discussed in Section 3.1.

20

100

10-1

15 10-2

10 5

Dielectric loss (tan δ)

Real permittivity Dielectric loss

25

Real permittivity (ε')

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 22 of 32

10-3 0.1

-3

1

Density (g.cm ) Figure 8. Variations in real permittivity and dielectric loss measured at 1×10+3 Hz as a function of density in the extruded HDPE-GnP composite foams made from solid precursors containing 4.5 vol% GnP. The parallel-plates arrangement of the GnPs within the cell walls enhanced the effective interfaces between the adjacent GnPs (Figure 9a), and thus increased the real permittivity. As the density continued to decrease, the cell walls were compressed in the thickness direction due to bubble growth (Figure 9b), which can decrease the interspace distances between the adjacent 22 ACS Paragon Plus Environment

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GnPs. This further enhanced the ε'. However, when the density was decreased to 0.05, the cell wall thickness dropped to approximately 1 µm, and the number of adjacent GnPs in the cell walls decreased dramatically, due to polymer matrix’s excessive compression. Interestingly, almost no GnP can be found in the cell wall for the lower density foams (Figure 9c). This resulted in lower effective interfaces between the adjacent GnPs and in lower real permittivity. During the fast cellular growth process, the solid fillers (e.g. GnPs) could barely flow together with the polymer melt

32

. Therefore, the number of GnPs in a unit area of the cell walls decreased with the

reduction in the density of foam-extruded samples. The decrease in the tan δ when the density dropped can mainly be attributed to the higher foam expansion (up to 21-fold), which further separated the GnPs and, thereby, resulted in a loss of conductive networks. This, in turn, led to a lower Ohmic loss. Figure 9d shows the ideal 2-D conceptualization of the change in the GnP’s alignment with the density. Interestingly, the tailored microcellular structure further enhanced the HDPE-GnP composites’ dielectric performance. The facile sc-CO2-treatment and physical foaming of the HDPE-GnP composites significantly increased their real permittivity (ε') and greatly decreased both their dielectric loss (tan δ) and their density. As a result, an excellent combination of the dielectric properties, together with an ultra-low density, resulted. In the extruded 1.08 vol.% HDPE-GnP composite foam with a density of 0.15 g.cm-3, the tan δ dropped down as low as 0.003 at 1×105 Hz while the ε' reached 77.5. These results were greatly superior to those of the SCM nanocomposites (ε'~19.9, tan δ ~0.15 and density of ~1.2 g.cm-3).



Page 24 of 32

Figure 9. SEM and TEM micrographs of the extruded HDPE-GnP composite foams made from solid precursors containing 4.5 vol% GnP, which show the GnPs’ arrangement at different densities including: (a) 0.13 g.cm-3; (b) 0.08 g.cm-3; and (c) 0.05 g.cm-3. (d) Ideal 2-D conceptualization of GnP’s arrangement in cell walls as the density decreased. Table 1 shows some of the recent advances made in the development of polymer nanocomposites as dielectric materials. And they are compared with the dielectric performance of the HDPE-GnP reported in our study. Most of the presented batch-type studies

3,4,11,16,17,24,25,29,26–28

have

undertaken complex synthesis procedures which are challenging to be scaled up and/or have high material cost. For instance, Wen et al. achieved a very good combination of the real permittivity 24 ACS Paragon Plus Environment

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(ε'=74) and the dielectric loss (tan δ =0.08)

16

, however, their fabrication method was a tedious

multiple-step synthesis process (three-step process for GnP preparation + synthesis of poly(vinylidene fluoride-trifluorethylene) copolymer with internal double-bonds through a dehydrochlorination process). Jin et al. also reported another great combination of dielectric properties (ε'=71.7, and tan δ =0.045) 11 for the poly (vinylidene fluoride) hybrid nanocomposites (containing MWNT/BaTiO3) which were fabricated through a complex miscible-immiscible coagulation method. However, the required nanomaterials loading was rather high which resulted in heavy (density of ~4.6 g.cm-3) and expensive dielectric materials. Table 1. Dielectric performance and density of different polymer nanocomposites Filler content 4.0 vol%

Frequency (Hz) 103

Dielectric permittivity 74

Dielectri c loss 0.08

Fabrication method solution casting, functionalization and crosslinking

Density (g.cm-3) ~1.8

poly (vinylidene fluoride)/ BaTiO3 /BaTiO3 nanofibers

30 vol%/3 vol%

102

27

0.06

emulsion polymerization

~1.1

28

poly (vinylidene fluoride-cohexafluoroprop ylene)/titanium dioxidemodified reduced graphene oxide (rGO)

20 wt.%

102

24.5

0.22

in-situ assembling TiO2 on graphene oxide (GO)+ solution mixing and drop casting

~1.86

4

poly(pphenylene benzobisoxazol e)/ Functionalized Graphene Nanosheets

2.0 wt.%

103

66.27

0.045

polymer chains grafting in GnPs, in-situ polymerization followed by further thermal treatment

~1.57

29

Materials poly(vinylidene fluoridetrifluorethylene )/graphene nanosheets


Ref. 16


Page 26 of 32

poly (vinylidene fluoride)/functi onalized graphene– BaTiO3

1.25/30 vol.%

106

65

0.35

GO synthesis followed by twostep solution mixing and hotpressing

~3

3

polydimethylsil oxane/thermall y expanded graphene nanoplates

2.0 wt%

103

89

1.5

thermal exfoliation of graphene followed by solution mixing and vulcanization

~0.97

17

polyimide /graphene/BaTi O3

1.0/16vol.%

102

31

0.03

Multiple step solution mixing

~3.5

26

cyanoethyl pullulan polymer/carbon nanotubes/rGO

0.062 wt.%

102

32

0.051

carbon nanotubes/rGO fabricated by thermal CVD followed by solution mixing

~1.1

24

poly (vinylidene fluoride) / multiwall carbon nanotubes /BaTiO3 diglycidyl ether of bisphenolA/rGO

3.0/37.1vol. %

103

71.7

0.045

miscibleimmiscible coagulation method followed by hot pressing

~4.6

11

1.0 wt.

103

32

0.08

covalent functionalization and solution mixing and curing

~1.7

27

HDPE/GnP

1.08 vol%*

103

77.5

0.014

melt mixing and foam extrusion

0.15

this work

HDPE/GnP

1.08 vol%*

105

77.1

0.003


0.15

this work

HDPE/GnP

0.8 vol.%*

103

39.7

0.012


0.14

this work

HDPE/GnP

0.5 vol.%*

103

18

0.010


0.13

this work

HDPE/GnP

0.5 vol.%*

103

22.3

0.006


0.08

this work

* the GnP vol.% is reported with respect to the total volume

4.

Conclusion

In this study, a new class of ultralight, microcellular dielectric HDPE-GnP composites was introduced. The composite foams of HDPE with highly exfoliated GnPs were developed using a melt mixing method, followed by a sc-CO2-treatment and physical foaming via an extrusion 26 ACS Paragon Plus Environment

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process. The generation of a microcellular structure provided a unique parallel-plate arrangement of GnPs around the cell walls. This significantly enhanced the real permittivity and greatly decreased the dielectric loss of the HDPE-GnP composites. For example, ultralight extruded HDPE-1.08 vol.% GnP foam with a density of 0.15 g.cm-3 had a high dielectric permittivity of ε'=77.5 and an extremely low dielectric loss of tan δ=0.003 at 1×105 Hz, which made it superior to solid compression-molded samples (ε'=19.9 and the tan δ=0.15 with a density of 1.2 g.cm-3). The extremely low dielectric loss, together with the enhanced real permittivity, of the extruded foam samples provided an excellent combination of dielectric properties. Our study showed that the tailored morphologies existing in the microcellular structure within the HDPE-GnP composites offer a novel, industrially viable and cost-effective method to develop ultralight dielectric materials with high permittivity and low dielectric loss. 5.

Acknowledgments:

The authors gratefully acknowledge NanoXplore Inc.’s financial support and 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.

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