Connection-Improved Conductive Network of Carbon Nanotubes in a

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Functional Nanostructured Materials (including low-D carbon)

Connection-improved conductive network of carbon nanotubes in the rubber crosslink network Lin Gan, Ming Dong, Ying Han, Yanfang Xiao, Lin Yang, and Jin Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03081 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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ACS Applied Materials & Interfaces

Connection-improved conductive network of carbon nanotubes in the rubber crosslink network Lin Gan=1, Ming Dong=1, Ying Han 1, Yanfang Xiao 1, Lin Yang 1, Jin Huang* 1 1

School of Chemistry and Chemical Engineering, Southwest University, No. 2,

Tiansheng Road, Beibei District, Chongqing 400715, China. Corresponding author * Jin Huang, Email: [email protected].

Abstract: Conductive rubber composites usually suffer a large filler content and relatively low conductivity because the uniform dispersion of conductive nanofillers in rubbers is probably inhibited by the crosslink networks. However, by establishing a double-network model of crosslink and conductive networks, we found the connection of 1D nanofillers could be improved by crosslink networks, which stabilized the conductive network. The percolation value of nanofillers could reduce to 0.06 wt% in experiments, using carbon nanotubes (CNTs) with 9.5 nm diameter and 1.5 µm length as nanofillers and polydimethylsiloxane as the matrix. Moreover, the conductive network owned a critical exponent of 5.63, which was higher than that of conventional conductive networks (ca. 2). This feature proved that the connection between CNTs was improved by the PDMS crosslink network. This work subverted

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the fundamental conception that crosslink networks in rubbers should make fillers aggregate, and we believed it would conduce to the development of sensors and flexible devices of rubber composites. Keywords: conductive network; crosslink network; rubber; polydimethylsiloxane; carbon nanotubes

2


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1

Introduction

Conductive rubber composites, as an active branch of conductive composites, could be used as sensors1-5, thermistors6-8, integrated circuit chips9-11, electromagnetic shielding12, 13, etc. due to their high elasticity. However, the crosslink network, where the elasticity comes from, limited the movements of conductive fillers in rubbers and further their uniform dispersion14. This feature leads to the fact that the loading level of those fillers is usually high, which may lower the sensitivity, mechanical properties or other performance of rubbers15, 16. The reason explaining the limit of crosslink networks on dispersion of conductive fillers was much studied and found to be closely related to the size of fillers17, 18. Specifically, the average distance between crosslink sites is too short to introduce filler into the crosslink network. The conductive nanofillers, whose size is as small as a nanometer scale at least one dimension, are thus used to improve the filler dispersion. Many carbon-based and metal-based fillers have been much studied, such as carbon nanotubes19, 20, graphene5, 15, 21, noble metal nanospheres and nanowires22-24, etc. Among them, carbon nanotubes (CNTs), a high aspect ratio one-dimensional conductive filler, has been widely used in conductive nanocomposites, due to its high mechanical properties and low density. Using PDMS (poly dimethyl silicon), a widely-used rubbers in electronics, as the matrix, the conventional percolation value of the conductive network based on CNTs can be as low as 0.3 wt%25-28. We then found that percolation value should be resulted from the conventional volume concluded networks the fillers normally connect29. However, by establishing a model of crosslink/conductive double network, we believed that the percolation value 3



can be lowered by one order of magnitudes by adjusting the preparing methods. In such a model, the connecting sites of CNTs in PDMS were fixed in the PDMS crosslink network (as shown in Scheme 1.). The effect of the diameter of CNTs and the crosslink density of PDMS on their dispersion in PDMS was investigated. This was because the diameter must be smaller than half of the average distance between crosslink sites to ensure the CNT connection. The chemical structure of CNTs was then adjusted to study whether the reaction between conductive nanofillers and matrix during the thermal crosslink process of PDMS can affect the percolation value of conductive networks. This work could conduce to the application conductive rubber composites with low filler content in the field of sensors and flexural electron devices.

2 The percolation value of conductive CNT networks in the crosslink network of PDMS As shown in scheme 1., the CNTs were assumed to penetrate the crosslink network of PDMS. In such a network, PDMS chains could construct boxes with crosslink sites as vertexes. The average side length () of those boxes should be the average distance between crosslink sites and was calculated with Equation (1)

Scheme 1.

Schematic of one-dimensional conductive fillers that establish a 4


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conductive network in a cross-linked network

=

1 (1)

Where the NA and Dc referred to the Avogadro’s constant and crosslink density, respectively. However, the actual side length varied and could be calculated with Equation (2)

() = (1 − )

(2)

Where the N referred to the number of PDMS repeating units between 2 adjacent crosslink sites, and preferred to the probability of one repeating unit to form a crosslink site.

The p should be calculated with Equation (3).

= ⁄ = (3) Where the x referred to the length of the PDMS repeating unit and equal to 2λsin(α/2). The λ was the length of the Si-O single bond (1.60Å) and α was the Si-O-Si bond angle (108°)30. x was equal to 2.59Å. The probability that the a was equal to or greater than Nx should be:

= ( ≥ ) = −

(1 − ) (4) ln(1 − )

As shown in Scheme, the CNTs could only connected each other in the condition that at least three a of the PDMS chain box were greater than the twice diameter (2R) of CNTs. Assumed that the diameter was equal to ξx, the probability of the above condition : = ( ≥ 2!) = −

(1 − )" (5) #ln (1 − )$

However, when the size of the box was too large, 1-D filler is less likely to interact 5



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with each other because of long distance between them in the box. Since the maximum bending angle of CNT could be assumed to be 2π/χ 31, where χ was aspect ratio of CNTs. Therefore, their maximum radial span was:

S'() =

* *! ,- = = *! (6) + ,- ,-

The P2R3 should thus be amended to be Pfree: P0122 = − ∙∙4

=−

⁄)

51 − 6

, 751 − 6

⁄)

9:;51 − 6
!/2 = ,- ) = − (9) :;(1 − C ) D

A

So the average distance between two connecting sites of CNTs in the conductive network was LN=R/2PN. Furthermore, the volume fraction of CNTs that played a role to connect the network was

V % =

3*! × 100% (10) 4

At the same time, there were also many unconnected CNTs in the system, which were immobilized in the boxes that own two side length longer than R. The probability that those boxes exist was P(N)2/3. The volume contribution of these CNTs was 6


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! J % = ∙ (11) 3

Therefore, the percolation value of the conductive network formed by the CNT was

Vc%=V1%+V2%. However, when the was longer than the PDMS segment length, PDMS molecular chain can be considered to be able to move. The conductive network of CNTs should be constructed by directly connecting CNTs in three-direction and the percolation value of CNTs should be calculated with:

J % = KLMCN

3*! (12) 4,-

Where, Clink was the correction index from the CNT entanglement, which increased the percolation value. Since the CNTs should entangle each other in a parallel direction, the Clink may be 6 because cylinders, which were similar with the morphology of CNTs, owned a coordination number of 6 in the closely packing. Finally, the percolation threshold of conductive network constructed by CNTs in PDMS was

R3*! + ∙ ! , ( ≤ V2W'2CX ) P 4 3 V % = (13) 4.5*! Q , ( ≤ V2W'2CX ) P ,O

In addition, the segment length of PDMS can be calculated using its equivalent free segment length, Kuhn length32.

LV2W'2CX =

K[ \ 1 − cos _ ∙ (14) sin _⁄2 1 + cos _

For PDMS, C∞ was 6.132. Therefore, the length of PDMS segments was 22.8 Å. 3. Materials and methods 7



3.1 Materials Polydimethylsiloxane (PDMS) was provided by Dow corning (USA), which consisted two parts: a pre-polymer base (part A) and a cross-linking curing agent (part B). Types of multiwall carbon nanotubes (MWCNTs) were NC7000 from nanocyl (Belgium), TNSM2 from TIME NANO (china), and TNSM3 from TIME NANO (china). The other materials: ethyl alcohol (Chongqing chuandong chemical(group)CO., Ltd., China), acetone (Chongqing chuandong chemical(group)CO., Ltd., China) and methylbenzene (Chongqing chuandong chemical(group)CO., Ltd., China). 3.2 Preparation of PDMS/CNT nanocomposites A certain mass of CNTs was added into absolute ethyl alcohol and dispersed by ultrasonic cell disruptor. The part A of PDMS was then added into the CNT suspension and the evaporation of ethyl alcohol happened on the oil bath at 80oC with stirring. After the ethanol was evaporated completely, part B of PDMS was slowly added with stirring in 10min at room temperature to be mixed uniformly (mass ratio of part A and part B was 10:1). Then, the mixture was transferred to the mold and the curing and forming processes were carried out under hot pressing conditions at a specific temperature to control the crosslink density of PDMS matrix. 3.3 Characterization Measurement of crosslink density. the cross-linking density was measured by DMA (Dynamic Mechanical Analysis) data. DMA (TA instrument, Q800) was carried on the condition of frequency:1Hz, strain:0.05%, temperature range: -140~150oC, heating rate: 3oC/min. 8


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Measurement of conductivity for the PDMS/CNT nanocomposites. Electric conductivity

of

PDMS/CNT

nanocomposites

was

measured

by

a

high resistance instrument (Keithley 6517B, USA) at room temperature. The shape of composites was the rotundity, of which the diameter is 64mm. Measurement of chemical structure of CNTs. Raman spectra were measured by the confocal laser raman spectrometer (RENISHAW). The wave length of excitation light source was 532nm. Measurement of electronic reaction in PDMS/CNT nanocomposites. Electrochemical workstation (CH Instruments, CHI62OD) was carried out under the conditions of potential: -1.5 ~ 1.5V, scanning speed 50mV / min, with gold electrode. Samples were immersed in PBS buffer at 25 °C. Measurement of the size of CNTs. Transmission electron microscopy (TEM) observation of the morphology and size of CNTs was performed on the S-5500 transmission electron microscopy (HITACHI, Japan) at an accelerating voltage of 100 kV. Measurement of morphological fractures. Cross-section SEM (scanning electron microscope) images of the PDMS and PDMS/CNT composites were observed with a JSM-7800F Scanning electron microscope (JEOL Ltd., Japan). The composite samples were quenched in liquid nitrogen and fractured, and the fracture surface was platinum-coated for observation. 4. Results and discussion 4.1 The effect of the CNT diameter on the conductive network. The effect of the CNT diameter on the conductivity of PDMS/CNT nanocomposites 9



was investigated with two kinds of CNTs: NC7000 and TNSM3. The NC7000 owned an average length of 1.5 µm and average diameter of 9.5 nm (Fig. S1a), and TNSM3 owned an average length of 1.5 µm and average diameter of 15 nm (Fig. S1b). The relationship between the conductivity of PDMS/NC7000 nanocomposites and the nanofiller concentration was measured (Fig. 1a) at different PDMS crosslink density, along with the PDMS/TNSM3 nanocomposites (Fig. 1b), in which ρv referred to the volume resistivity. The conductivity-concentration curve of PDMS/NC7000 and PDMS/TNSM3 nanocomposites was fitted by the statistics percolation theory (Equation (15))33 and the critical exponent of those nanocomposites was calculated (Table S1). The results showed that the percolation value of PDMS/TNSM3 nanocomposites varied little with the PDMS crosslink density (Fig. 1c).

σ = cd (e − e, )X (15) where the σ and σ0 referred to the conductivity of the composite and the filler respectively, φ referred to the volume fraction of nanofiller, and φc referred to the percolation threshold and t was represented the critical exponent Meanwhile, the theoretical percolation value from the double-network assumption was obtained from Equation (13), as shown in Fig. 1c. The result indicated that the theoretical percolation value would exceed 100vol% when the average side length was smaller than the segment length of PDMS (the break point in Fig. 1c). This feature may suggest that the large diameter of CNT may lead to a phase-separation of the PDMS/CNT nanocomposites. In this case, the crosslink network could not improve the conductivity of conductive network, which was consistent with the 10


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experimental result of Fig. 1b. Those results suggested that CNTs with a large diameter, like TNSM3, may be not able to establish a conductive network inside the PDMS crosslink network. By contrast, the relationship between the percolation value of PDMS/NC7000 nanocomposites and the PDMS crosslink density was similar to the double-network results from Equation (13), as shown in Fig. 1d. These results indicated that the diameter of CNT should be small enough to establish a conductive network with low percolation value.

Figure 1. (a) the relationship between the conductivity of PDMS/NC7000 nanocomposites and the nanofiller concentration;(b) the relationship between the conductivity of PDMS/TNSM3 nanocomposites and the nanofiller concentration; (c) 11



the relationship between percolation value and density of crosslinking obtained in theory and experiment of PDMS/TNSM3 nanocomposites. (d) the relationship between percolation value and density of crosslinking obtained in theory and experiment of PDMS/NC7000 nanocomposites. The fitting results based on Statistics Percolation Equation also indicated that critical exponent of the conductive network of PDMS/NC7000 nanocomposites was found to be smaller than 2, when the PDMS crosslink density was lower than 141 mol/m3 (Table S1). At those PDMS crosslink density, the average distance between adjacent crosslink sites was larger than that of the PDMS segment from Equation (13). This feature was consistent with the double-network assumption. This was because the conductive network of NC7000 was a conventional volume-concluded network in that assumption, of which critical exponent should be ca. 2. Moreover, the critical exponent of NC7000 conductive network increased to 5.63 when the PDMS crosslink density was 227 mol/m3. This feature proved that the connection between NC7000 in PDMS was improved by the PDMS crosslink network. It should be noted that there was a slightly deviation between the theoretical percolation value and the experimental value, which may be due to the fact that the density of CNTs (1.8g/cm3) used to convert from mass fraction to volume fraction was different from the actual density. 4.2 The effect of the chemical structure of CNTs on the conductive network

12


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Figure 2. (a) the relationship between the conductivity of PDMS/TNSM3 nanocomposites and the nanofiller concentration; (b) the conductivity-concentration relationship to the equation of statistics percolation theory. Interestingly, when we used TNSM2, which owns the same size of NC7000 (as shown in Fig. S1c), instead of NC7000, the conductivity of PDMS-based nanocomposites decreased significantly at different nanofiller concentration, as shown in Fig. 2a. The percolation value of PDMS/TNSM2 nanocomposites was much higher than that of PDMS/NC7000 nanocomposites at different PDMS crosslink density, as shown in Table S1. We also fitted the conductivity-concentration relationship to the Statistics Percolation Equation, as shown in Fig. 2b. The result indicated that the critical exponent of TNSM2 conductive network was lower than 2 at different PDMS crosslink density. This feature suggested that the PDMS crosslink network owned an interaction with the TNSM2 and thus hindered the establishment of TNSM2 conductive network.

13



Figure 3. Raman spectra of (a)NC7000, (b) TNSM2, (c)TNSM3. To study the mechanism of the TNSM2-PDMS interaction, we measured the chemical structure of NC7000 (Fig. 3a), TNSM2 (Fig. 3b), TNSM3 (Fig. 3c) with Raman scattering. The D mode and the G mode of CNTs were observed at ~1350 cm-1 and ~1580 cm-1, respectively. Researches34, 35 had proved that the D and G modes referred to the sp3 and sp2 structure of carbon in CNT, respectively. We found that the ratio between the intensity of D mode peak and G mode peak (ID/IG) was 1.52 for TNSM2. This high ID/IG suggested that the TNSM2 owned many chemical defects, because the sp3-structure carbon should not belong to the chemically perfect CNT. By contrast, the NC7000 owned a low ID/IG of 1.26 (obtained from the Raman scattering spectrum of NC7000. Those results proved that the conductive network of CNT could hardly be established with a low loading level of CNT when the CNT owned too many chemical defects. Moreover, the Raman scattering spectrum of TNSM3 was also measured. The result showed that the TNSM3 owned a relatively low ID/IG of 1.32, which was similar to that of NC7000. This feature proved that the difference in conductivity between NC7000 network and TNSM3 network should be ascribed to the diameter difference of those CNTs. 14


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4.3 The electronic reaction in PDMS/CNT nanocomposites

Figure 4. Cyclic voltammetry test of three types of CNTs (a) NC7000, (b) TNSM3, (c) TNSM2. To further investigate the relationship between the chemical defects of CNT and the conductivity of CNT network in PDMS crosslink network, we studied the reaction in PDMS/CNT nanocomposites with cyclic voltammetry. Fig.4 shows the C-V (current-voltage) plots of NC 7000 (Fig. 4a), TNSM3 (Fig. 4b) and TNSM2 (Fig. 4c). The results showed that the NC7000 and TNSM3 could not react electrochemically with the PDMS matrix, which was consistent with the chemical defect measurements. By contrast, an oxidation peak was found at ca. 0.25 V in the PDMS/TNSM2 system, two reduction peaks were also found at ca. 0.25 V and 0.75 V. This feature proved that the TNSM2, which owned many chemical defects, could conduct oxidation-reduction reaction with PDMS matrix. Such a reaction must hinder the uniform dispersion of TNSM2 in PDMS during the preparing process and conductivity-measuring process.

15



4.4 the dispersion of CNTs (NC7000) in PDMS matrix

Figure 5. Cross-section SEM images of pure PDMS (a) and PDMS/CNT nanocomposites with the CNT content of 0.06 wt% (b and c), 0.14 wt % (d and e) and 0.5 wt% (f and g). The blue and red cycles refers to the filler-aggregate particles. The cross-section SEM images of PDMS and PDMS/CNT(NC7000) at the crosslink density of 182mol/m3 were shown in Fig. 5. The contents of 0.06% and 0.14% were chosen because they were the beginning content and end content of the rapid conductivity-increasing region at this crosslink density (as shown in Fig. 1a). At these two contents, not many particles (4 in Fig. 5c and 7 in Fig. 5e), which should belong to filler aggregates, can be found. This indicated that no phase separation occurred before and when the conductive network formed, and the conductive network should be dispersed inside the crosslink network. In contrast, the fractured surface of the PDMS/CNT-0.5% nanocomposite became rough (Fig. 5e and 5f), and many filler-aggregate particles can be found. The serious aggregate suggested that the crosslink network had an inside-dispersion limit for CNT, which was consistent with the fact that the content of 0.5 wt % was in the conductive plateau region at this 16


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crosslink density (shown in Fig. 1b). 5. Conclusion In this work, we took the advantages of NC7000 (a kind of CNT) in small diameter and few chemical defects to prepare a conductive PDMS rubber with an extremely low percolation value of 0.06 wt%. The relationship between the percolation value and PDMS crosslink density was also consistent with our double-network assumption, in which the connection between CNTs could be improved by the PDMS crosslink network. The high critical exponent (5.63) of NC7000 conductive network, which was calculated

with

Statistics

Percolation

Theory,

also

proved

that

the

“connection-improved” point. We also found that the CNT with large diameter (TNSM3) or many chemical defects (TNSM2) could not establish conductive network with low nanofiller content in PDMS due to the diameter effect or filler/matrix reaction, respectively. Since our work suggested that the crosslink network could be conducive,

rather

than

harmful,

to

establishing

conductive

network,

the

connection-improved theory from double-network can certainly be used to prepare high-performance conductive rubbers. ASSOCIATED CONTENT Supporting information The specific data on diameter of carbon nanotubes, the critical exponent and crosslink density of the PDMS/NC7000 nanocomposites were shown in the Supporting Information as Figure S1, Table S1, Figure S2 and Figure S3, respectively. TEM images and diameter statistics graph of (a)NC7000, (b)TNSM2, (C)TNSM3. the critical exponent of the PDMS/NC7000 nanocomposites shown in the table. The 17



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crosslink density measured by DMA and the DMA data was also shown in the supporting information.

AUTHOR INFORMATION Corresponding author * Jin Huang, Email: [email protected]. Author Contributions =

These two authors contributed equally to this work.

ACKNOWLEDGEMENTS This research is financially supported by the National Natural Science Foundation of China (51603171), Fundamental Research Funds for the Central Universities (XDJK2016A017 and XDJK2016C033), Project of Basic Science and Advanced Technology

Research,

Chongqing

Science

and

Technology

Commission

(cstc2016jcyjA0796), Talent Project of Southwest University (SWU115034), and Key Laboratory of Polymeric Composite & Functional Materials of Ministry of Education (PCFM201605). NOTES The authors declare no competing financial interest. Reference 1.

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Connection-Improved Conductive Network of Carbon Nanotubes in a

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