Origin of Increasing Dielectric Constant at Lower Percolation

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Origin of Increasing Dielectric Constant at Lower Percolation Threshold through Controlling Spatial Distribution of Carbon Nanotubes in Epoxy Resin with Microwave Assisted Thermal Curing Technique Chuan Liu, Longhui Zheng, Li Yuan, Qingbao Guan, Aijuan Gu, and Guozheng Liang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10567 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016

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The Journal of Physical Chemistry

Origin of Increasing Dielectric Constant at Lower Percolation Threshold through Controlling Spatial Distribution of Carbon Nanotubes in Epoxy Resin with Microwave Assisted Thermal Curing Technique

Chuan Liu, Longhui Zheng, Li Yuan, Qingbao Guan, Aijuan Gu*, Guozheng Liang*

State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application Department of Materials Science and Engineering College of Chemistry, Chemical Engineering and Materials Science Soochow University, Suzhou, 215123, China

Author information Chuan Liu, e-mail: [email protected] Longhui Zheng, e-mail: [email protected] Li Yuan, e-mail: [email protected] Qingbao Guan, e-mail: [email protected] Aijuan Gu: Address: No. 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou, Jiangsu, China. Tel: +86 512 65880967. Fax: +86 512 65880089. Current institution e-mail: [email protected] Guozheng Liang: Address: No. 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou, Jiangsu, China. Tel: +86 512 65880967. Fax: +86 512 65880089. Current institution e-mail: [email protected]

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ABSTRACT How to fabricate polymer composites with higher dielectric constant at lower content of conductors based on commercial compositions is still an interesting topic with great challenge. Herein, based on multi-walled carbon nanotubes (CNTs) and epoxy resin (EP), new high dielectric constant (high-k) composite (m-CNT/EP) with much higher permittivity and lower percolation threshold (fc) was prepared by a microwave-assisted thermal curing technology. Results show that the spatial structure and performances of CNT/EP composites are dependent on the curing process used. CNTs are orientated along the Z direction in m-CNT/EP composites, while those are randomly permutated in the composite (t-CNT/EP) produced with traditional thermal curing procedure. Accordingly, t-CNT/EP composites have isotropic dielectric properties, and m-CNT/EP composites exhibit anisotropic dielectric properties. fc values of m-CNT/EP composites in X, Y and Z directions are 0.29, 0.29 and 0.24 wt%, respectively, while those of t-CNT/EP composites are equal to 0.39 wt%. When the loading of CNTs is 0.25 wt%, the dielectric constant in Z direction of m-CNT0.25/EP is as high as 673, about 20.7 times of that of t-CNT0.25/EP. The origin behind these interesting results were discussed from building the relationship between CNT distribution and dielectric properties using finite element and simulating equal circuits.

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1. INTRODUCTION Owing to high energy storage

1, 2

and the ability of homogenizing electric field,3

polymer matrix composites with high dielectric constant (high-k) show wide applications in producing capacitors,4-6 electronic storage devices,7,8 electrical engineer equipment,9,10 and so on. Inorganic conductor/polymer composite is an important type of high-k composites, and has the advantage of maintaining good processing and mechanical properties of polymers11 due to its low percolation threshold (fc < 1 wt%).12,13 However, inorganic nano-conductors generally aggregate in organic resin,14, 15 while dielectric properties are sensitive to the structure of materials, these bring the difficulty of obtaining stable and desirable dielectric property. In order to alleviate the bad influence of the aggregation of conductors on dielectric properties, a lot of studies have been carried out, and the surface modification of conductors, such as grafting or coating,16, 17 has been regarded as an effective method. However, the surface modification of conductors tends to increase the fc of the composite,17, 18 although they can improve the degree of dispersion in resins. Therefore, under the premise of common conductors and polymers, fabricating composites with higher dielectric constant at lower conductor content is still an interesting but difficult topic. It is well known that the achievement of high-k for conductor/polymer composites is based on the percolation theory, so the spatial distribution of conductors is the main factor that affects dielectric properties of composites. Typically, one dimensional nano-conductors, such as multi-walled carbon nanotubes (CNTs), are usually randomly dispersed in a polymer matrix, so the resultant composite could not reach percolation structure at low content of conductors. On the other hand, when the 3



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conductor content is close to fc, the insulating material exhibit conductive feature, and then the dielectric constant increases dramatically.19 Therefore, the orientation of CNTs is beneficial to endow corresponding composites with low fc . At present, the orientation of CNTs can be obtained by some techniques such as magnetic field,20 electric field,21 tensile and shear,22 electrostatic spinning23 and microwave curing.24 However, each of them has its limits. For example, tensile shear and electrospinning are not suitable for thermosetting composites, while electric and magnetic methods require high electric and magnetic fields, and the material should have special dimensions. Our research group found that under the condition of microwave curing, and CNTs had good dispersion with some degree of orientation,24 but the preliminary investigation not only did not study the detail distribution of CNTs; but the dielectric property and mechanism in different directions were also not discussed. In addition, the third problem of microwave curing was that the composite cured under microwave had larger fc than that cured under traditional thermal procedure. Our investigation reported herein aimed to intensively study the spatial distribution of CNTs and its influence on the dielectric properties under microwave curing condition, the structure and dielectric properties of composites prepared with thermal curing (t-CNT/EP) were also investigated for comparison and revealing the origin of dielectric properties. 2. EXPERIMENTAL SECTION 2.1 Materials CNTs used were multi-walled carbon nanotubes with a purity of more than 97 % (the average out diameter is 7-15 nm, and the length is greater than 5 µm) were bought from Shenzhen Nanoport Company, China. 2-Cymene-4-ethylic imidazole 4


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was purchased from Kailer Chemical Plant, China. Diglycidyl ether of bisphenol A (EP) with an epoxide equivalent weight of 196 g/mol was purchased from Wuxi Resin Plant, China. 2.2 Preparation of CNT/EP composites Appropriate amounts of EP and CNTs were thoroughly blended at 65 ℃ for 15 min

with

vigorous

stirring

under

sonication,

and

then

pre-weighted

2-cymene-4-ethylic imidazole was added with stirring for about 10 min to form a prepolymer. The prepolymer was cast into a mold, and placed in the vacuum drying oven to remove the bubbles for 20 min. After that the mold was put into a microwave oven (700 W) for an intermittent irradiation (the total time was 10 min, each cycle contained an irradiation of 3-4 s succeeded with an interval of 10-15 s) followed by postcuring according to the procedure of 80 ℃/2 h+100 ℃/2 h+120 ℃/2 h + 150 ℃ /4 h in a thermal oven. The resultant composite was coded as m-CNTw/EP composite, where w was the mass fraction of CNTs in the composite. When discussing the performance in X, Y, or Z directions, the composite was denoted as m-XXw, m-YYw, or m-ZZw for convenience. Appropriate amounts of EP and CNTs were thoroughly blended at 65 ℃ for 15 min with vigorous stirring under sonication to form a mixture, into which pre-weighted 2-cymene-4-ethylic imidazole was added with stirring for 10 min to get a prepolymer. The prepolymer was cast into a mold, and placed in the vacuum drying oven to remove the bubbles for 20 min, and then cured and postured according to the procedure of 80 ℃/2 h + 100 ℃/2 h +120 ℃/2 h + 150 ℃/4 h in an thermal oven, successively. The resultant composite was coded as t-CNTw/EP composite, where w was the mass fraction of CNTs in the composite. When discussing the performance in X, Y, or Z direction the composite was denoted as t-XXw, t-YYw, or t-ZZw for 5



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convenience. 2.3 Characterizations A scanning electron microscope (SEM, Hitachi S-4700, Japan) was employed to observe the morphologies of fracture surfaces of composites. Polarized Raman spectra were recorded using a confocal Raman microscope spectrometer (Renishaw inVia, the United Kingdom). The laser wavelength was selected as 633 nm. Dielectric properties and electric conductivities were measured on a broadband dielectrics spectrometer (Novocontrol Concept 80, Hundsangen, Germany) at room temperature at a frequency ranging from 1 to 107 Hz. The dimensions of the bulk sample were (10 ± 0.1)×(10 ± 0.1)×(10 ± 0.1) mm3. Dielectric properties were tested through putting electrodes on different sides. The sides were coded as X, Y or Z direction according to the direction in microwave oven as shown in Figure 1.

Figure 1. The definition of different direction for a CNT/EP composite in a microwave

3. RESULTS AND DISCUSSION 3.1 Spatial distribution of CNTs in composites SEM images of fracture surfaces in three directions of t-CNT0.3/EP and 6


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m-CNT0.3/EP composites are shown in Figure 2 and Figure 3, respectively. The numbers of CNTs (Table 1) within an area of 1 µm2 (each yellow box) in different regions were counted, and summarized in Table 1. It can be seen that there are obvious aggregation of CNTs in three directions of t-CNT0.3/EP composite (Figure 2), and the numbers of CNTs in different directions are almost the same, suggesting that CNTs with poor dispersion are randomly distributed in t-CNT/EP composites.

Figure 2. SEM images of fracture surfaces in three directions of t-CNT0.3/EP composites (a-d: X direction; e-h: Y direction; i-l: Z direction).

Differently, CNTs in three directions of m-CNT0.3/EP have good dispersion, but their numbers are obviously different (Figure 3). Only a small amount of "spots" (CNTs) can be seen in X (Figure 3a) and Y (Figure 3e) directions, while many scattered "spots" are found in Z direction section (Figure 3i). From the enlarged view of cross-sections, only a small amount of CNTs can be seen in the cross-section of X direction (Figure 3b-3d), while a large amount of CNTs appear in the Z direction (Figure 3j-3l), the number of CNTs in the Y direction (Figure 3f-3h) falls in between those in X and Z directions. These results reflect that CNTs in m-CNT/EP are not randomly distributed, instead, they have a certain orientation in the Y-Z direction, and 7



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tend to be arranged along the Z direction.

Figure 3. SEM images of fracture surfaces in three directions of m-CNT0.3/EP composites (a-d: X direction; e-h: Y direction; i-l: Z direction). Table 1. Numbers of CNTs within 1 µm2 in different regions of composites Composite

Direction Figure X

m-CNT0.3/EP

Y

Z

X

t-CNT0.3/EP

Y

Z

Region 1

Region 2

Region 3

Region 4

a

7

9

8

8

b

6

10

6

7

c

7

11

9

9

d

24

28

31

30

e

15

18

21

22

f

30

34

27

31

g

44

57

59

63

h

38

42

47

51

i

41

47

55

65

a

17

16

15

19

b

18

14

15

17

c

15

14

20

13

d

19

14

16

13

e

14

14

15

17

f

15

16

16

19

g

13

15

14

18

h

16

17

19

12

i

18

17

16

13

8


Number 7-9

19-30

45-56

16-17

15-17

15-16

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It is well known that microwave is a kind of electromagnetic wave.25, 26 The magnetic susceptibility and polarization rate in the direction parallel to the axis of CNT are different from those in the direction perpendicular to the axis of CNT,27 so a CNT tends to orient along the direction of the electromagnetic field (Figure 4). In other words, microwave can make CNTs orientation. However, for CNTs that are dispersed in a thermosetting resin, the orientation degree is also affected by the viscosity of the resin, especially, the resin will be cured under microwave, so CNTs cannot be completely oriented along the electromagnetic field direction but with an angle.

Figure 4. Schematic mechanism about the orientation of CNTs induced by the action of microwave radiation

Polarized Raman spectra can be used to characterize the orientation of CNTs in composites. When the angle between the polarization direction of the laser and the axis of CNT changes, the intensity of the polarized Raman spectrum for CNTs that have a certain degree of orientation will be varied,28, 29 however, that for CNTs with random distribution almost does not changes.30 Figure 5 gives polarized Raman spectra of m-CNT0.3/EP and t-CNT0.3/EP composites, in which 0°, 30°, 60°, and 90° are polarization angles. The G-band31 of CNTs appears at 1610 cm-1 in each spectrum, however, the intensity of G-band is increased and then decreased as the polarization 9



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angle increases in the spectra of m-CNT0.3/EP composites. When the polarization angle is 60°, the intensity of G-band shows the maximum value, meaning that CNTs in m-CNT0.3/EP composites have a certain orientation; when the polarization angle is 90°, the intensity of G-band decreases, suggesting that the orientation of CNTs is not completely along the vertical direction (Z direction). Above phenomenon does not appear in t-CNT0.3/EP composites. With the increase of the polarization angle, the peak intensity of G-band is almost constant, indicating that CNTs are randomly distributed in t-CNT0.3/EP composites. This is consistent with the results of SEM photos (Figure 2).

Figure 5. Polarized Raman spectra of m-CNT0.3/EP and t-CNT0.3/EP composites.

3.2 Conductivities and fc values of composites Figure 6 displays the frequency dependence of AC conductivity (σ) in three directions of m-CNT/EP and t-CNT/EP composites. The order of the conductivity in each direction of m-CNT/EP is σzz＞σyy＞σxx (Figure 6a-6c), indicating that CNTs are easy to form conductive path or tunneling effect in the Z direction. This result is consistent with the SEM images of cross-sections in three directions of m-CNT/EP (Figure 3). For t-CNT/EP composites, at a fix f, the conductivities of X, Y and Z directions are basically equal (Figure 6d-6f), this result is consistent with the random 10


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distribution of CNTs in the composites (Figure 2).

Figure 6. Frequency dependence of AC conductivity (σ) in three directions of m-CNT/EP and t-CNT/EP composites (a, d: X direction; b, e: Y direction; c, f: Z direction).

Figure 7 gives AC conductivities at 1 Hz in three directions of m-CNT/EP and t-CNT/EP composites. When f increases from 0.1 to 0.3 wt%, the electrical conductivity in three directions of m-CNT/EP increases four orders of magnitude 11



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(Figure 6a-6c), reflecting that the composites have a percolation phenomenon within this range of CNT loading. Based on the percolation theory (Eqn. 1, 2),32, 33 the fc in X, Y and Z directions of m-CNT/EP are simulated to be 0.29 wt% (Figure 7a), 0.29 wt% (Figure 7b) and 0.24 wt% (Figure 7c), respectively. The fc values in three directions of t-CNT/EP are equal to be 0.39 wt% (Figure 7d-7f). Obviously, these different results between m-CNT/EP and t-CNT/EP composites are attributed to the difficulty degree in forming conductive pathway or tunneling effect, this is related to the orientation of CNTs. σ ∝（f－fc）q，

when

f＞fc

（1）

σ ∝（fc－f）-s，

when

f＜fc

（2）

where q and s are the critical parameters of the conductive phase. Compared with t-CNT/EP composites, m-CNT/EP composites with the same f not only have higher electrical conductivities in all three directions, but also show lower fc in three directions. These results are attributed to the difference in conductive network induced by different spatial distributions of CNTs in the two kinds of composites. Importantly, the lower fc of m-CNT/EP is attractive and also contrast to that obtained from in the previous research,24 which is attributed to the building of new processing parameter. Specifically, compared to the previous work, each microwave radiation time is shortened to be 1/3 length of that reported, so that CNTs have sufficient time for orientation, and thus forming a conductive network at lower fc. This result further proves that orientation is the significant factor that affects the electrical conductivity of composites.

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Figure 7. log-log plots of conductivity at 1 Hz vs. (f - fc) in three directions of m-CNT/EP and t-CNT/EP composites (a, d: X direction; b, e: Y direction; c, f: Z direction).

3.3 Dielectric properties and mechanism of composites Figure 8 displays the frequency dependence of dielectric constant in three directions of m-CNT/EP and t-CNT/EP composites. The curves of m-CNT/EP in three directions are completely different (Figure 8a-8c). At the same f, the dielectric 13



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constants in three directions of m-CNT/EP are different. When f ≤ 0.25 wt%, the order of the dielectric constant in three directions are εxx＜εyy＜εzz, this should be attributed to CNT orientation. In the same direction, with the increase of f, the dielectric constant reaches the maximum value at fc and then decreases, this is a typical percolation phenomenon.34 When f＝0.3 wt%, the dielectric constants of m-CNT/EP composites in X and Y direction reach the maximum values, about 412 and 487, respectively (Figure 8a, 8b); when f ＝0.25 wt%, the dielectric constant in Z direction of m-CNT/EP composites has the maximum value (672) (Figure 8c). These results show that dielectric constants of m-CNT/EP in three directions are not only different, but the maximum value is also different, indicating that there are other factors that contribute to the dielectric constant.

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Figure 8. Frequency dependence of dielectric constant in three directions of m-CNT/EP and t-CNT/EP composites (a, d: X direction; b, e: Y direction; c, f: Z direction).

The curves in three directions of t-CNT/EP are basically coincident (Figure 8d-8f). When f＝0.4 wt%, the dielectric constants in X, Y and Z direction reach the maximum values, about 359, 389 and 395, respectively (Figure 8d-8f), further 15



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proving that the distribution of CNTs in the composite is completely random. When f

Origin of Increasing Dielectric Constant at Lower Percolation

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