Experimental and theoretical evaluations of the interfacial interaction between carbon nanotubes and carboxylated butadiene nitrile rubber: Mechanical and damping properties

Journal Pre-proof Experimental and theoretical evaluations of the interfacial interaction between carbon nanotubes and carboxylated butadiene nitrile rubber: Mechanical and damping properties Xun Wang, Duoli Chen, Wensheng Zhong, Lin Zhang, Xiaoqiang Fan, Zhenbing Cai, Minhao Zhu PII:

S0264-1275(19)30756-7

DOI:

https://doi.org/10.1016/j.matdes.2019.108318

Reference:

JMADE 108318

To appear in:

Materials & Design

Received Date: 17 September 2019 Revised Date:

26 October 2019

Accepted Date: 28 October 2019

Please cite this article as: X. Wang, D. Chen, W. Zhong, L. Zhang, X. Fan, Z. Cai, M. Zhu, Experimental and theoretical evaluations of the interfacial interaction between carbon nanotubes and carboxylated butadiene nitrile rubber: Mechanical and damping properties, Materials & Design (2019), doi: https:// doi.org/10.1016/j.matdes.2019.108318. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Credit Author Statement The work was seriously completed by the authors, and the submitted content has not been published previously and is not under consideration for publication elsewhere, the submission of the article is approved by all authors. Submission implies that, if accepted, it will not be published elsewhere in the same form, in English or in any other language, without the written consent of the publisher. Author Contributions: Xiaoqiang Fan and Lin Zhang developed the idea for this work; Xun Wang and Duoli Chen performed all measurements and characterization. Wensheng Zhong, Zhenbing Cai and Minhao Zhu discussed the results. Xun Wang prepared the manuscript, and Xiaoqiang Fan reviewed and commented on the manuscript. We declare no competing financial interest. We hereby declare and guarantee those are true, we claimed. Sincerely yours, Xun Wang, Duoli Chen, Wensheng Zhong, Lin Zhang, Xiaoqiang Fan, Zhenbing Cai, Minhao Zhu

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Graphical Abstract

Experimental and theoretical evaluations of the interfacial interaction between carbon nanotubes and carboxylated butadiene nitrile rubber: mechanical and damping properties Xun Wanga,b, Duoli Chenc, Wensheng Zhonga, Lin Zhangb,c,*, Xiaoqiang Fanb,c,*, Zhenbing Caic, Minhao Zhub,c a

State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu

610031, China b

Key Laboratory of Advanced Technologies of Materials (Ministry of Education),

School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China c

Tribology Research Institute, School of Mechanical Engineering, Southwest Jiaotong

University, Chengdu 610031, China ABSTRACT The success of carbon nanotubes (CNTs) as fillers in improving the physicochemical and mechanical properties of polymer is attributed to their unique structure and performance characteristics. The interfacial interaction between fillers and matrix is of significant influence in agglomeration of fillers and mechanical/damping properties of as-prepared composites. Here, polydopamine (PDA), (3-Aminopropyl) triethoxysilane (KH550) and ionic liquid (1-aminoethyl-3methylimidazolium bis ((trifluoromethyl) sulfonyl) imide)) were used to not only functionalize CNTs for suppressing their agglomeration, but also regulate the interfacial interaction between CNTs and carboxylated butadiene nitrile rubber (XNBR) for achieving excellent mechanical properties and better damping properties. The storage modulus of composites rose by 80% (from 1392 to 2488MPa) with the addition amount of 2.2wt% CNTs-KH550 and

*Corresponding Author. Tel: +86 028 87600128. Fax: +86 028 87600128. E–mail address: [email protected] (X.Q. Fan), [email protected] (L. Zhang). 1

the tensile strength rose by 110% (from 0.32 to 0.68 MPa) with 3.0 wt% CNTs-IL. In addition, the damping degradation of composites caused by the agglomeration of fillers was resolved. The results of molecular dynamics (MD) simulation show that the strong interfacial interaction of CNTs-PDA and CNTs-KH550 is mainly attributed to hydrogen bond interaction, while the interaction of CNTs-IL is determined by hydrogen bond interaction and van der Waals' (vdW) force. The functionalized CNTs with excellent interfacial interaction and dispersion have bright application prospects in the field of composite. Keywords: Functionalized carbon nanotubes, Composite, Molecular dynamics simulation, Mechanical/Damping properties. 1. Introduction Carbon nanotubes have been widely used in composite materials due to their unique structure, excellent chemical stability, low density and outstanding mechanical properties [1-3]. However, the addition of unfunctionalized carbon nanotubes could not improve the mechanical properties of composites significantly or even reduce its damping properties [4,5]. This is mainly due to the high aspect ratio of carbon nanotubes and van der Waals’ force between nanotubes, which leads to agglomeration and uneven dispersion in the matrix material [6]. In addition, the design of interface between CNTs and polymer must be considered. The excellent interface interactions are beneficial for efficient mechanical and damping behavior [7]. Proper surface functionalization of carbon nanotubes is an effective way to solve these problems. However, the direct functionalization of CNTs is subject to great obstacle due to its chemical inertness, so in most cases strong oxidants are used for oxidative pretreatment [8,9]. In this study, the carboxylate carbon nanotubes were directly used to omit the oxidation process. Although the oxidation of carbon nanotube can lead into some oxygen-containing functional groups and slightly solve the agglomerate problem, the groups are difficult to achieve high adaptability to matrix material [10]. Therefore, in order to obtain excellent mechanical properties and higher damping properties of the

2

composite rubber, further functionalization is indispensable. Polydopamine can be coated on the surface of carbon materials by π-π interaction, the presence of terminal hydroxyl groups after grafting make it a suitable modifier for these nano-fillers [11]. In recent years, the surface modification of CFs, GO and CNTs by polydopamine has been achieved. These modified products have been applied to a variety of different polymers to improve their properties [12-14]. The functional groups after modification are advantageous for their interaction with XNBR matrix, but the research on this respect is few. Therefore, polydopamine is selected as a modifier in this experiment. The silane coupling agent as surface modifier of nanomaterials has been also commonly used, different silane coupling agents have been used to achieve the silanization of carbon nanotubes [15,16]. KH550 can generate hydroxyl and amino groups after grafting, which is the best choice to improve the dispersion of carbon nanotubes in the matrix and enhance the interfacial interaction between them. KH550 is also selected as a modifier for this study. Ionic liquids are also a surface modifier that can improve the dispersibility of carbon nanotubes in matrix materials and this functionalization can facilitate the bonding of carbon nanotubes to the rubber [17]. A new type of ionic liquid was chosen for functionalization in this experiment. The C=C bonds in the ionic liquid and the amide groups produced during grafting process are considered to be the key to improving the dispersion of CNTs [18]. So, three different modifiers are selected to achieve the surface functionalization of carbon nanotubes and regulate the interfacial interaction between functionalized CNTs and matrix. Carboxylated butadiene nitrile rubber (XNBR) is an elastomeric ionomer, which contains unsaturated C=C bonds and carboxyl groups. Compared with nitrile rubber, XNBR has better mechanical and damping properties, but it still cannot meet the needs of industrial applications [19]. Therefore, improving the performance of XNBR is one of the important goals of this work. The preparation methods of CNTs/rubber composites are also an important part. At present, the main preparation methods are latex compounding, solution blending and mechanical melt mixing [20-22]. Among

3

them, the mechanical melt mixing method is the most convenient and practical. It is widely used in industrial production, so this method is adopted in this experiment. Finally, the structure and composition of different functionalized carbon nanotubes were characterized in detail. The microstructure and properties of different CNTs/rubber composites were discussed according to the interfacial interactions, thereby proposing the reinforced mechanism of composites. 2. Materials and experimental procedures 2.1. Materials Carboxylated Multiwall Carbon Nanotubes (CNTs) are produced by Suzhou TANFENG graphene Tech Inc. Hydroxyphenethylamine hydrochloride (dopamine, 98%),

tris-(hydroxy-methyl)

aminomethane

(TRIS,

99%)

and

dicyclohexyl-

carbodiimide (DCC, 99%) are purchased from Aladdin Co. Ltd. (3-Aminopropyl) triethoxysilane (KH550) and Dimethylformamide (DMF, 99.5%) are obtained from Chengdu Kelong Chemicals Co. 1-aminoethyl-3methylimidazolium bis((trifluoromethyl) -sulfonyl)imide (IL) is supplied by Lanzhou Institute of Chemical Physics. Carboxylated butadiene nitrile rubber (XNBR, Krynac, Polysar, 27 wt% acrylonitrile) is used for matrix material in this study. All chemical reagents and solvents adopted in this study are without further purification. 2.2. Modification of CNTs by Poly (dopamine) The carboxylated CNTs (100mg) were dispersed in deionized water (150ml) under sonication for 30 min, the dopamine (60mg) was added into the suspensions under magnetic stirring for 30 min. Then the TRIS, which was used as pH regulators, was poured into the mixture solution to adjust pH to 8.5. Adjust the temperature to 60

and

keep the reaction last for 8 h to obtain the CNTs which coated by PDA (Fig. 1a). Finally, the CNTs-PDA were washed with deionized water until the filtrate was transparent and dried by freeze-drying for 48 h. 2.3. Modification of CNTs by KH550 Similar to the preparation of CNTs-PDA, 100mg carboxylated CNTs were dispersed

4

in 150ml deionized water. Then added 3.75ml KH550 into the suspensions under magnetic stirring and reaction at 60

for 8 hours. As illustrated in Fig. 1b, KH550

would be grafted onto surface of CNTs. The CNTs-KH550 was also washed with deionized water for several times and the resulting black powder was dried by freeze-drying for 48 h. 2.4. Modification of CNTs by IL Fig. 1c shows a schematic of the process how to prepare CNTs-IL. The carboxylated CNTs (100mg), IL (200mg) and DCC (200mg) were mixed in DMF (150ml) under sonication for 30 min, then held at 50

for 12 h under vigorous agitation. Finally, the

CNTs-IL was washed with deionized water for several times and dried by freeze-drying for 48 h.

Fig. 1. The process of surface functionalization of CNTs by (a) PDA, (b) KH550, and (c) IL (dotted line represents the position of chemical bond fracture and formation during modification). (d) The dispersion mechanism of functionalized carbon nanotube (dotted line represents hydrogen bonds). 2.5. Preparation of nanocomposites The CNTs (unmodified and modified) were mixed with XNBR by a two-roll mill to 5

prepare CNTs/XNBR composites. The composites were placed in a mold, which is made of stainless steel and has a channel with 2 mm thickness and hot-pressed at 160 °C for 20min under a pressure of 10 MPa. The addition amount of CNTs (unmodified and modified) was controlled in 0.6, 1.4, 2.2 and 3.0 wt%. 2.6. Characterization To measure the chemical composition of the modified CNTs, X-ray photoelectron spectroscopy measurements (XPS, ESCALAB 250Xi, Thermo Scientific) were carried out on modified CNTs. Fourier transform infrared (FTIR, Nicolet 6700, America) spectroscopy was used to investigate the organic functional groups of modified CNTs in the mid infrared region (4000−500 cm−1). In order to determine thermal stability of different CNTs, thermogravimetric analyzer (TGA, STA 449C, Germany) was used to evaluate it under nitrogen atmosphere at a heating rate of 10 °C·min-1 from 30 to 800°C. Transmission electron microscopy (TEM, JEM-2100F, Japan) was applied to investigate structure of modified CNTs. The mechanical properties were measured by dynamic thermomechanical analyzer (DMA, Q800, America) in stretch mode over a temperature range from -70 to 40 °C with a heating rate of 3 °C·min-1. The frequency was 1 Hz and the amplitude was 20 µm. Besides, the tensile machine (RGM-2040, China) was also used to test the mechanical properties of composite rubber. The morphologies of fracture surfaces for XNBR/CNTs composites were observed by Scanning electron microscopy (SEM, JSM-7800F, Japan). 3. Results and Discussion 3.1. Surface Functionalization and Characterization of CNTs The preparation procedures of CNTs-PDA via self-polymerization of dopamine are described in Fig. 1a. Under oxidation and alkaline conditions, the polymerization of dopamine is spontaneous process, and the product has high adhesion that can easily adhere to solid surface. In addition, the benzene ring in PDA can form the π-π interaction with carbon nanotubes [23]. Both points make it possible to coat PDA on the surface of carbon nanotubes. As illustrated in Fig. 1b, the triethoxy (OC2H5)3 from

6

KH550 is easily hydrolyzed to form a trisilanol, then it dehydrates with the hydroxyl groups on the surface of carbon nanotubes to achieve the coating [24]. And as shown in Fig. 1c, the amino group from the cationic portion of IL undergoes amination reaction with the carboxyl group on the surface of the carboxylated CNTs to obtain effective modification [25]. XPS spectra were recorded to measure the chemical composition of the modified CNTs. Wide scan XPS spectra and C 1s core-level spectra of different modified CNTs are displayed in Fig. 2 and Fig. 3, respectively. For carboxylated CNTs, its wide scan spectrum has two peaks corresponding to C 1s and O 1s, as shown in Fig. 2a, and other peaks have not been observed. Its C 1s XPS spectrum in Fig. 3a is distinguished into two carbon bonds, including C–C (284.6 eV) and O–C=O (285.4 eV). It can be stated that the carboxylated CNTs used are free from impurities.

Fig. 2. XPS wide-scan of (a) carboxylated CNTs, (b) CNTs-PDA, (c) CNTs-KH550 and (d) CNTs-IL.

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In wide scan spectrum of CNTs-PDA (Fig. 2b) and CNTs-KH550 (Fig. 2c), the C 1s peak and O 1s peak are also clearly observed, but the N 1s peak of former significantly increases, while the latter shows two strong Si (2s, 2p) peaks, both of them are derived from modifiers [26]. For C 1s XPS spectrum of CNTs-PDA (Fig. 3b) and CNTs-KH550 (Fig. 3c), compared with carboxylated CNTs, the C–N (285.5 eV) bond and C-O (286.5 eV) bonds have already appeared, and the latter shows C−Si bonds at 282.6 eV, illustrating the successful modifications via using the modifiers. Compared with Fig. 2c and 2d, the wide scan spectrum of CNTs-IL shows the new peaks of N 1s and F 1s, its C 1s XPS spectrum shows the new bonds of C−N (285.5 eV), C=N (285.8 eV) and C−F (286.7 eV) [27]. The presence of these bonds further confirms the successful grafting reaction between IL and CNTs.

Fig. 3. XPS C 1s core-level spectra of (a) carboxylated CNTs, (b) CNTs-PDA, (c) CNTs-KH550 and (d) CNTs-IL. In order to further detect the groups in modified carbon nanotubes, FTIR spectra were

8

analyzed and the results are shown in Fig. 4. In all FTIR spectra, there is a stretching vibration peak of -OH at near 3446 cm-1 and a stretching vibration peaks of C=O at 1638 cm-1. However, comparing the spectrum of carboxylated CNTs and CNTs-PDA, it can be found that the peak at 3446 cm-1 is significantly stronger and deeper, which is caused by -OH and -NH from the polydopamine coated on the carbon nanotube after modification. For the spectrum of CNTs-KH550, the new absorption peak at 1032 cm−1 is assigned to C−O−Si stretch vibration from the grafting treatment of KH550, which is well consistent with XPS results. In the FTIR spectrum of CNTs–IL, the peaks at 1620 and 1638 cm−1 can be assigned to the N-H band bending vibration and the carboxylic C=O band stretching vibration, respectively. The formation of amide linkage between carboxylated CNTs and IL was strongly confirmed by the peak appearing at 1620 cm−1 [28].

Fig. 4. FTIR spectra of carboxylated CNTs, CNTs-PDA, CNTs-KH550 and CNTs-IL. Thermal decomposition curves of carboxylated CNTs, CNTs-PDA, CNTs- KH550 and CNTs-IL are shown in Fig. 5. When the temperature is below 140

, the weight

loss is mainly attributed to the volatilization of absorbed water. At this stage, the weight loss of CNTs-PDA was the fastest and the most, which was caused by the higher content of PDA and their more absorption water [29]. When the temperature increased from 140 to 800

, the weight loss rate remained unchanged for CNTs, which was mainly caused 9

by the elimination of residual oxygen-containing groups [30]. For CNTs-PDA, the weight loss at this stage was much more than CNTs due to the decomposition of PDA chain. It can be observed from the illustration that KH550 and IL lose weight rapidly at 470 and 445

, respectively, which represents the decomposition of KH550 and IL

[30,31]. For these modified carbon nanotubes, their weight loss at the temperature range from 30 to 800

was 6.8% (carboxylated CNTs), 14.5% (CNTs-PDA), 10.4%

(CNTs-KH550) and 7.8% (CNTs-IL), respectively. The increased weight loss of modified carbon nanotubes is the content of graft products, which are estimated 7.7% (PDA), 3.6% (KH550) and 1% (IL), respectively. The different weight losses provide strong evidence for the success of the modification.

Fig. 5. Thermogravimetric analysis results of carboxylated CNTs, CNTs-PDA, CNTs-KH550 and CNTs-IL. TEM images of carboxylated CNTs, CNTs-PDA, CNTs-KH550 and CNTs-IL are shown in Fig. 6. The transparent center of the CNTs is marked by red lines. The arrows indicate the beginning and end of the lattice streaks and their orientation. As displayed in Fig. 6a, the lattice fringes extend from the surface of the CNTs to the transparent center and the CNTs surface is relatively smooth. For CNTs-PDA in Fig. 6b, its lattice fringes become blurred and a layer of amorphous regions with the thickness about 11.4 nm is observed due to the coating of PDA. After being modified by KH550, some amorphous regions with the thickness of 3.5 nm are also observed clearly (Fig. 6c), 10

confirming the grafting of KH550 on the CNTs. Similar to the former, an amorphous region with the thickness of 1.4 nm appears on the surface of the CNTs-IL and the surface of CNTs is no longer smooth. The emergence of these amorphous regions is an important evidence for the realization of modifications. The difference in thickness of these amorphous regions can also reflect the difference in grafting efficiency and is consistent with the thermogravimetric results. Taken together, all these results verify that different modifications of carbon nanotubes can be achieved by the experimental operations.

Fig. 6. TEM images of (a) carboxylated CNTs, (b) CNTs-PDA, (c) CNTs-KH550 and (d) CNTs-IL. 3.2. Application of Functionalized CNTs in Polymer Composites In order to examine the effect of modified carbon nanotubes, four kinds of carbon nanotubes were added to the XNBR in different proportions and subjected to DMA test. The storage modulus (E') of XNBR-based composites are shown in Fig. 7, it can be seen clearly that the composite materials have higher E' than pure XNBR. Among them, the unfunctionalized carbon nanotubes have the lowest E' and the worst reinforcement 11

effect. When the weight fraction of filler exceeds 1.4 wt%, the E' of composites decreases rapidly, possible due to the agglomeration of carbon nanotubes. Compared with E' of XNBR/CNTs (Fig. 7a) and XNBR/CNTs-PDA (Fig. 7b), although both materials with 1.4 wt% filler have the maximum E', the decrease in E' of the latter is much less than that of the former with the increase of fillers content. This is because the hydroxyl groups from the PDA generate hydrogen bonds with the nitrile and carboxyl groups in the XNBR, thereby weakening the interaction between the carbon nanotubes and preventing the agglomeration [29]. The XNBR/CNTs-KH550 (2488 MPa) with the addition amount of 2.2 wt% has the highest E' (rises up by to 80% than pure XNBR (1392 MPa)) in Fig. 7c, which is attributed to better dispersion of carbon nanotubes. The terminal hydroxyl groups in KH550 simultaneously generate hydrogen bonds with the nitrile in the rubber to enhance the interface interaction between the filler and the matrix, thereby restricting the slippage of chains and preventing agglomeration [32]. As shown in Fig. 7d, the E' of the XNBR/CNTs-IL (1830 MPa) increases with the increase of the filler content, a maximum of E' (36% higher than pure XNBR) was obtained as adding 3 wt% filler, suggesting that more CNTs-IL can be uniformly dispersed in the matrix with a good dispersion. This result may be caused by the hydrogen bond interaction between the amide group from the IL and the carboxyl group in the XNBR, the plasticization effect of ionic liquid is also a factor [33].

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Fig.

7.

Temperature-dependent

Storage

modulus

of

(a)

XNBR/CNTs,

(b)

XNBR/CNTs-PDA, (c) XNBR/CNTs-KH550 and (d) XNBR/CNTs-IL. In order to reflect the mechanical properties of composites intuitively, the stress-strain curves of pure XNBR and four composites are shown in Fig. 8a. Compared with pure XNBR, the tensile strength of composite rubbers is obviously improved. The tensile strength of XNBR/CNTs-IL is the highest with the increasing strain (Fig. 8b). When the strain reached 600%, the tensile strength of XNBR/CNTs-IL (0.68 MPa) is 110% higher than that of pure XNBR (0.32 MPa). The improvement of tensile strength is attributed to the dispersion of the filler and strong interfacial interactions between modified filler and matrix [34]. The dispersion of the filler can reduce the defects in the composite, while the strong interfacial interactions between modified filler and matrix can limit the slip of the rubber chains [35]. Similarly, the addition of CNTs-KH550 can also greatly improve the tensile strength of XNBR (from 0.32 to 0.54 MPa). However, the tensile strength of XNBR/CNTs and XNBR/CNTs-PDA was not significantly improved. This is

13

due to the serious agglomeration phenomenon (Fig. 10) when the addition amount of the two fillers reaches 3.0 wt%. It not only destroyed the uniform dispersion of CNTs, but also invalidated the high specific surface area of the nanomaterials, thus destroying the interfacial interaction between the filler and the matrix. The interfacial interaction between the modified filler and matrix will be discussed in 3.3. The result on tensile strength matches well with the storage modulus from DMA measurements (Fig. 7).

Fig. 8. Mechanical properties of XNBR with 3 wt% fillers: (a) stress–strain curves, (b) tensile strength at different strains. Fig. 9 gives the tan δ peaks of composites with different functionalized CNTs, it can be concluded that a single glass transition temperature (Tg) of pure XNBR at -1.4 XNBR/CNTs (-0.8

), XNBR/CNTs-PDA (1

) and XNBR/CNTs-KH550 (0.9

higher Tg than pure XNBR, while XNBR/CNTs-IL (-2.1

.

) have

) are the opposite. The main

reason for the decrease in the Tg of XNBR/CNTs-IL is the plasticization of IL [33], which is expected to broaden the range of rubber operating temperature (Tg~Tf). For the composites with elevated Tg, this might be due to the filler-rubber interactions, which improves the stiffness of rubber chains. The addition of nanomaterials with weak interfacial interaction will result in the agglomeration of fillers in the rubber [36]. The agglomeration of fillers makes it difficult to slide and rivet the polymer chains around them [37]. This results in the reduction of internal friction and energy dissipation during the deformation of the composites, thereby decreasing the damping properties of composites. The tan δ peak of the XNBR/CNTs in Fig. 9 becomes shorter and narrower, 14

while the other peaks become wider or even higher. This indicates that the addition of unfunctionalized carbon nanotubes deteriorates the damping properties of the XNBR, while functionalization can avoid the damage of damping properties and even increase it to some extent. The hydrogen bond interaction between the functionalized carbon nanotubes and XNBR has two main functions. On the one hand, this interaction prevents the agglomeration of fillers and makes fillers slip easier. On the other hand, it increases the energy dissipation by breaking the hydrogen bond as it slips. The decrease in damping properties of composites is restrained.

Fig.

9.

The

tan

δ

of

pure

XNBR,

XNBR/CNTs,

XNBR/CNTs-PDA,

XNBR/CNTs-KH550 and XNBR/CNTs-IL. Fig. 10 is the cross-sectional SEM images of four composites with 3 wt% filler. As shown in Fig. 10a, a large amount of agglomerated CNTs can be clearly seen, indicating that the unmodified CNTs have poor dispersion in XNBR. The large cracks are observed in Fig. 10a, which symbolizes a decrease in the toughness of the material and explains why the material damping decreases. For CNTs-PDA, both size and amount of agglomeration in rubber matrix are decreased (Fig. 10c). This can be attributed to hydrogen bonding between CNTs-PDA and matrix [37]. The CNTs-KH550 (Fig. 10e) and the CNTs-IL (Fig. 10g) are well dispersed in the rubber without large-scale agglomeration. There are no large cracks in the cross-section of rubber, but small crack 15

in orientation, which indicates that the toughness of rubber is improved. The strong interaction between the modified filler and the matrix prevents agglomeration, which will be described in the next section.

Fig. 10. SEM images of the fracture surfaces for (a,b) XNBR/CNTs-3 wt%, (c,d) XNBR/CNTs-PDA-3

wt%,

(e,f)

XNBR/CNTs-KH550-3

XNBR/CNTs-IL-3 wt%. 3.3. Molecular Dynamics (MD) simulation strategies 16

wt%

and

(g,h)

Molecular Dynamics (MD) simulation is an important tool for observing and understanding the interaction between microscopic particles [39,40]. In this work, MD simulation was used to analyze the interaction between modified filler and rubber chain. First, several initial composite models were built by using the Amorphous Cell module in Materials studio under the COMPASS force field. Then, in order to bring the system to equilibrium, the system is separately optimized for energy and geometry. Subsequently, the optimized system was annealed between 200K and 400K to prevent local energy accumulation [41]. Finally, 100 ps of NVT simulation at 298 K and 200 ps of NPT simulation at 0.1 MPa are performed to relax the polymer structure and achieve system equilibrium. The equilibrium of temperature and energy are two criteria for judging system equilibrium. The system is in equilibrium when the temperature and energy fluctuations are within 10% [42]. As shown in Fig. 11, the temperature fluctuations of the three different composite systems are all within 30 K and the energy fluctuations are within 600 kcal/mol, indicating that the composite system is already in equilibrium and can be used for subsequent calculations.

Fig. 11. Plot of (a) temperature and (b) energy vs. simulation time for composite rubber.

17

After the MD simulation, the three composite models with balanced conformation were achieved, as shown in Fig. 12 (a, c, e). The blue dashed lines in Fig. 12 represent the hydrogen bond generated in the system. Among them, the hydrogen bond between the modified part and the rubber chain is the focus of attention. In Fig. 12a, we can clearly see that a hydrogen bond is formed between the hydroxyl group in the PDA and the nitrile group in the XNBR. Similarly, in Fig. 12c, the terminal hydroxyl group in KH550 also generates hydrogen bonds with the nitrile group in XNBR. And as shown in Fig. 12e, the amide group in the IL can form a hydrogen bond with the carboxyl group in the XNBR. In order to further determine the strength of the generated hydrogen bond and judge the interaction between the two atoms, the radial distribution functions (RDF) calculation is completed. The pair correlation functions of the above hydrogen-forming atoms are calculated by calculation. The pair correlation functions between atoms can reflect the probability of another atom appearing at different distances. If the distance between atoms is 2.5 Å-3.1 Å, the interaction between two atoms belongs to a hydrogen bond and the distance between 3.1 Å-5 Å is a strong van der Waals' (vdW) force [43]. Fig. 12b shows the pair correlation function of H (PDA) and N (XNBR). There is a distinct peak at 2.89 Å, which indicates that the interaction between the two atoms is a hydrogen bond. Similarly, Fig. 12d shows the pair correlation function of H (KH550) and N (XNBR), the position of the peak is 2.73 Å-2.93 Å, which also indicates that the interaction between the two atoms is a hydrogen bond. However, there are two peaks in Fig. 12f at positions 2.6 Å and 3.9 Å, respectively. This indicates that there are both hydrogen bonding interaction and strong vdW force between N (IL) and H (XNBR). Among them, strong vdW force plays a leading role. According to bond length less than 2.5 Å and bond angle larger than 130°, the number of hydrogen bonds in the three composites was counted. The number of hydrogen bonds in an amorphous cell of XNBR/CNTs-PDA, XNBR/CNTs-KH550 and XNBR/CNTs-IL is 10, 13 and 10, respectively. The results can be well verified with RDF calculations.

18

Fig. 12. Models for MD simulation of (a) XNBR/CNTs-PDA, (c) XNBR/CNTs-KH550 and (e) XNBR/CNTs-IL composites (blue sphere represents N atom, red sphere represents O atom, white sphere represents H atom, grey sphere represents C atom, and blue dashed line represents hydrogen bond). Pair correlation functions for (b) H(PDA) and N(XNBR), (d) H(KH550) and N(XNBR) and (f) N(IL) and H(XNBR). The interaction between the modified portion and the rubber chain can increase the interfacial interaction among them and prevent the CNTs from agglomeration. By comparison, it can be found that the hydrogen bond peak of H (KH550) and the strong

19

vdW force peak of N (IL) are higher than the hydrogen bond peak of H (PDA), which is also consistent with the previous experimental phenomenon. The improved interaction between the modified filler and the rubber chain is an important factor affecting the good dispersion of the filler. 4. Conclusion Three different functionalized carbon nanotubes were successfully prepared and their composites were tested for mechanical properties, damping properties, etc. The conclusions can be drawn as follows: (a) The surface functionalization of carbon nanotubes by PDA, KH550 and IL was achieved by π-π interaction, hydrolysis reaction and amide reaction respectively, which improved the dispersibility of the filler in XNBR. The content of graft products was estimated 7.7% (PDA), 3.6% (KH550) and 1% (IL), respectively. (b) The agglomeration of CNTs in XNBR is prevented and the uniform dispersion of CNTs was achieved. MD simulation illustrates that the interaction of modifier and rubber chains is an important factor contributing to this effect. The interfacial interaction between CNTs-PDA/CNTs-KH550 and XNBR is mainly attributed to hydrogen bond interaction. For CNTs-IL, however, vdW force plays a important role. (c) The mechanical properties of the composite rubber have been greatly improved and the damping performance failure has been well suppressed. Among them, the storage modulus of XNBR/CNTs-KH550 (2488 MPa) was maximally improved (rises up to 80% than pure XNBR), and the XNBR/CNTs-IL (0.68 MPa) has the most outstanding tensile properties (rises up to 110% than pure XNBR). Therefore, this work provides a general pathway to specifically functionalize CNTs that is of great interest to the industrial field. Acknowledgment The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No. 51705435 and 51805454); the Key Project of Sichuan Department of Science and Technology (No. 2018JZ0048 and 2019YFG0292). References 20

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Highlights： ：


Prepared three functionalized CNTs by grafting PDA, KH550 and ionic liquid.




Improved the dispersion of CNTs in XNBR through functionalization.




Enhanced the mechanical properties (80%) of XNBR composite and suppressed the decrease in damping properties.




Analyzed the interfacial interaction between filler and matrix by molecular dynamics simulation on the molecular scale.

Declaration of interests

☒The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: