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Reinforcement of liquid ethylene-propylene-dicyclopentadiene copolymer based elastomer with vinyl functionalized multiwalled carbon nanotubes.

INTRODUCTION

The research of rubber composites with multiwall carbon nanotubes (MWCNTs) had attracted increasing attention due to their enhanced mechanical property, electrical performance, and so on [1-7]. To achieve reinforcement of polymer with MWCNTs, two problems should be solved. The first was the dispersion of MWCNTs in the matrices and the second was the interfacial adhesion of MWCNTs to the host polymer [1, 8]. Because of the entanglement of MWCNTs and the lack of functional groups on their smooth surface, substantial reinforcement of polymers by MWCNTs was seldom observed. For this reason, quite a few interests were concentrated on the surface treatment of MWCNTs including chemical and physical methods [9-17]. The treatment with acids constituted the most effective way to induce oxygenous groups on the surface of MWCNTs, which could provide reactive sites for chemical linkages with polymer matrix. In most cases, however, the acid induced functional groups occurred at a low density and were not sufficient to produce an effective bonding to polymer matrix. To increase the density of reactive sites, an approach of poly(acryloyl chloride) (PACl) grafting was proposed by this research group [18]. The acryloyl chloride groups on the grafted PACl could readily be further transformed to other functional groups and thus provided potential linkage sites suitable for different types of polymer matrixes [19].

In this article, a methodology for enhancing mechanical properties of liquid ethylene-propylene-dicyclopenta-diene copolymer (liquid-EPDM) based elastomers with MWCNTs was proposed. Because of the low molecular weight of the precursor, the obtained elastomer possessed poor mechanical properties and needed reinforcement in some special applications [20-21]. Among the abundance of reinforcers, MWCNTs constituted a potential one worth trying. As the first consideration, there should be chemical linkages between MWCNTs and the cured EPDM. In this methodology, vinyl groups were introduced indirectly onto the surface of MWCNTs, which were covulcanized with the double bonds on the liquid-EPDM. The resulted chemical linkages promoted the interfacial interaction between MWCNTs and EPDM matrix and thus produced effective reinforcement of the elastomer.

EXPERIMENTAL

Materials

MWCNTs (purity [greather than or equal to] 95%, average outer diameter 10-20 nm) were provided by Shenzhen Nanotech Port, China. Ethylene-propylene-dicylopentadiene copolymer (Trilene[R] 65, MW = 7000 g/mol) was purchased from Uniroyal Chemical Company. Concentrated sulfuric acid (98%), concentrated nitric acid (70%), tetrahydrofuran (THF, 99%), 1, 4-dioxane (99%), and 2,2'-azoisobutyrontrile (AIBN) were purchased from Vas Chemical of China. Acryloyl chloride (97%) was purchased from Acros Organic. Hydroxy ethyl acrylate (HEA) was purchased from Beijing Eastern Acrylic Chemical Technology Co. Dicumyl peroxide (DCP, (greather than or equalto)50%) was purchased from Sinopharm Chemical Reagent Co. 1,4-dioxane, and THF were distilled for purification before using. AIBN and DCP were purified by recrystallization from ethanol before using.

Acid Oxidization of MWCNTs

The pristine MWCNTs were suspended and refluxed in a mixture of concentrated [H.sub.2][SO.sub.4]/[HNO.sub.3] (20/20 mL) at 140[degrees]C for 1 h. Subsequently the acid-treated MWCNTs were retrieved and washed repeatedly with deionized water, and dried in vacuum.

Synthesis of PACl

PACl was prepared via free radical polymerization of acryloyl chloride [22]. In a 20 mL dry Schlenk tube equipped with a magnetic stirring bar, a solution of acryloyl chloride (1.5 mL) and AIBN (25 mg) in anhydrous dioxane (1.5 mL) was stirred for 48 h at 60[degrees]C under dry nitrogen atmosphere.

Grafting of MWCNTs with PACl

The solution of PACl obtained earlier was introduced into a suspension of acid-treated MWCNTs (50 mg) in anhydrous 1,4-dioxane (100 mL), then the mixture was kept vigorously stirring at 80[degrees]C for 48 h to allow the grafting of PACl onto the surface of MWCNTs to occur [18]. The solid product was collected and washed five times with anhydrous THF, and finally was placed in dry nitrogen atmosphere. All the procedures were carried out under nitrogen protection.

Esterification of PACl-Grafted MWCNTs

Into a solution of HEA (2.5 mL) and triethylamine (3.0 mL), a suspension of PACl-grafted MWCNTs (80 mg) in 100 mL anhydrous dioxane was introduced dropwise with magnetic stirring at 60[degrees]C under nitrogen atmosphere. After the addition, the system was kept stirring at 60[degrees]C for 24 more hours to allow the esterification between the acyl chloride groups on PACl and the hydroxyl groups on HEA to occur. The black product was retrieved and washed with THF several times. The modified MWCNTs bearing vinyl groups will be denoted as vinyl-MWCNTs later. The reactions mentioned earlier were shown in Fig. 1.

[FIGURE 1 OMITTED]

Preparation of the Composite

Into a suspension of vinyl-MWCNTs in THF (0.25 mg/mL), a THF solution of liquid-EPDM (2.0 g/mL) and DCP (0.08 g/mL) was introduced. After stirring at room temperature for 12 h, the mixture was poured onto a flat-bottomed pan (100 mm in diameter by 20 mm in height) to form a uniform film about 1 mm in thickness. The film was dried in a vacuum oven for 7 days at 80[degrees]C (below the cure temperature of liquid-EPDM) to completely remove THF. The dried film was subjected to curing at 170[degrees]C for 1.5 h in [N.sub.2] atmosphere, the obtained sample was used for various characterizations and mechanical testing. For purpose of comparison, composites from EPDM and pristine MWCNTs were also prepared with earlier procedures.

Characterization

Fourier transform infrared spectrometer (FTIR, Nexus 670) was used to detect the functional groups on the surface of grafted MWCNTs, which was measured as pellets with KBr.

The morphological study of samples was carried out using transmission electron microscopy (TEM, JEM100CX). A few drops of vinyl-MWCNTs suspension in THF were placed onto a carbon-coated copper grid, followed by evaporating off solvent, the remaining solid species were viewed.

Thermogravimetric analysis (TGA, TASC 414/4) was conducted at 10[degrees]C/min from room temperature to 800[degrees]C under argon atmosphere.

Gel contents were measured by the Soxhlet extraction technique using THF as solvent. After extracting at 50[degrees]C for 48 h, the remaining of a sample was dried in air before keeping it in a vacuum oven at 66[degrees]C until constant weight. The test was duplicated twice for every sample, and the average value was taken [23, 24]. The gel content was defined as follows:

Gel content(%) = ([W.sub.2] - [W.sub.CNT])/([W.sub.1] - [W.sub.CNT]) x 100%

where [W.sub.1] and [W.sub.2] were the weights of the samples before and after extraction, respectively. [W.sub.CNT] denoted the weight of the MWCNTs introduced, either modified or pristine.

Tensile tests were performed with an Instron TM testing machine at room temperature at a deformation rate of 20 mm/min, using a load cell of 50 N for good sensitivity. Five tests were made for each sample type.

The dried solid films of various products were viewed by scanning electron microscope (SEM) (Hitachi S-4700). The sample films were mounted on an aluminum stub using electric adhesive tape paste, followed by spraying gold panicles on sample surface for observation.

RESULTS AND DISCUSSION

Fourier Transform Infrared Spectrometer

The infrared spectra of the PACI-grafted MWCNTs (a) and vinyl-MWCNTs (b) are shown in Fig. 2. The grafting of PACI onto MWCNTs had been confirmed in our previous work [18]. From the features in Fig. 2, one may see that HEA molecules were esterified with acryloyl chloride groups. Of course, only a small amount of acryloyl chloride groups were consumed for the previous grafting of PACI onto MWCNTs, the major part of acryloyl chloride groups remained intact, which was used to introduce vinyl groups indirectly onto the surface of MWCNTs (Fig. 3).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Transmission Electron Microscopy

Figure 4 compares the TEM micrographs of the morphology of the acid-treated MWCNTs (a) and vinyl-MWCNTs (b). It could be observed that the acid-oxidized MWCNTs showed a smooth and clean surface and entangled loosely. However, they still had an intact structure, which was beneficial to reinforced matrices [19]. After grafting, all the MWCNTs were wrapped and "glued" with polymeric species. When the vinyl-MWCNTs were soaked in THF for a long time (several weeks), it was found that its weight was almost unchanged, which indicated that the polymer layer was indeed covalently linked to the carbon nanotubes [18]. One may expect that when the vinyl-MWCNTs were blended with polymer precursors, an easy dispersion could be achieved.

[FIGURE 4 OMITTED]

Thermogravimetric Analysis

Figure 5 shows TGA traces of the three species: pristine MWCNTs, vinyl-MWCNTs, and HEA esterified PACl. The pristine MWCNTs (trace a) kept stable without detectable weight loss until 800[degrees]C. HEA esterified PACl (trace c) lost about 84% between 400[degrees]C and 460[degrees]C. In almost the same temperature range, the vinyl-MWCNTs (trace b) exhibited a weight loss about 76%. Through a comparison among the three traces, the amount of polymer species grafted on the MWCNTs can be estimated [25]. Typically, the weight fraction of polymeric species in vinyl-MWCNTs was about 90%.

[FIGURE 5 OMITTED]

Gel Content of the Composites

The gel content of composites was determined to evaluate the efficiency of crosslinking. Figure 6 shows the gel contents of composites with various MWCNTs loadings. Whereas the gel content in pristine MWCNTs/EPDM was approximately constant, those in vinyl-MWCNTs/EPDM composites increased steadily with increasing vinyl-MWCNTs loading. This was obviously attributed to the vinyl groups attached to the surface of carbon nanotubes. During the curing process, the double bonds on the liquid-EPDM chains were initiated to produce free radicals and the system underwent a crosslinking leading to a gel structure. The vinyl groups grafted on the carbon nanotubes could also easily be initiated and participate in the free radical polymerization among the liquid-EPDM chains. It was expected that higher degrees of crosslinking may result in higher tensile strengths (Fig. 7).

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Tensile Properties of the Composites

Figure 7 presents the tensile strengths of the composite elastomers filled with pristine- and vinyl-MWCNTs. While the pristine MWCNTs provided marginal reinforcement, vinyl-MWCNTs caused a substantial reinforcement. With the same content of MWCNTs, the tensile strength of vinyl-MWCNTs based elastomer was about two times that of pristine MWCNTs based one. Obviously, the greater reinforcement was attributed to the stronger interfacial interaction (chemical bond) between the MWCNTs and the matrix. The matrix may effectively transfer the external load via chemical bonds to the MWCNTs, resulting in a higher strength.

Scanning Electron Microscopy

From the SEM microstructures in Figure 8, the obvious morphological change could be observed between pristine MWCNTs/EPDM composite and vinyl-MWCNTs/EPDM composite. The micrographs represented by Fig. 8a and b reveal that both pristine MWCNTs and vinyl-MWCNTs were drawn out from the matrix. But all the outcrops of pristine MWCNTs were long and smooth, which seemed to be slid and pulled out from the matrix, and most outcrops of vinyl-MWCNTs were short, which seemed to embedded and held tightly with the matrix [19, 26]. It was indicated that there was a strong interfacial bonding between the vinyl-MWCNTs and the EPDM molecular chains in the composite. Therefore, the vinyl-MWCNTs could transfer stress effectively throughout the matrix under load, and play a valid reinforcement role in the EPDM composite.

[FIGURE 8 OMITTED]

CONCLUSION

The polymer chains containing vinyl groups were grafted covalently on the surface of MWCNTs, which was confirmed with FTIR, TEM, and THF extraction experiments. TGA showed that the weight fraction of the polymers encapsulating the MWCNTs was about 90 wt%. The surface vinyl groups of vinyl-MWCNTs participated in the curing reaction of liquid-EPDM resulting in higher gel contents, enhanced interfacial interaction between the MWCNTs and rubber matrix, and thus an effective reinforcement.

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Hongfu Zhou, (1) Chen Zhang, (1) Hangquan Li, (1) Zhongjie Du, (1) Congju Li(2)

(1) Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology Beijing 100029, People's Republic of China

(2) Beijing Key Laboratory of Clothing Material R&D and Assessment, Beijing Institute of Fashion Technology, Beijing 100029, People's Republic of China

Correspondence to: Zhongjie Du; e-mail: duzj@mail.buct.edu.cn or Congju Li; e-mail: congjuli@sina.com

Contract grant sponsor: 863 Project; contract grant number: 2007AA021900; Contract grant sponsor: 13th CHINA-JAPAN S & T Cooperation; contract grant number: S2010GR0533; Contract grant sponsor: PHR (IHLB).

Published online in Wiley InterScience (www.interscience.wiley.com).

[C] 2010 Society of Plastics Engineers

DOI 10.1002/pen.21684
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Author:Zhou, Hongfu; Zhang, Chen; Li, Hangquan; Du, Zhongjie; Li, Congju
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Jul 1, 2010
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