Organic Functionalized Carbon Nanotubes Covalently Bonded with PE by Grafting Co
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Abstract. Grafting copolymerization method was used to synthesize copolymer of polyethylene with single-walled carbon nanotubes (SWNTs). Firstly, SWNTs were functionalized along their side walls with 1-alkene organic groups using the 1, 3-dipolar cycloaddition reaction. Then, a metallocene catalyst C2H4[(tert-H)Ind]ZrCl2 was used to synthesize polyethylene grafted from SWNTs (PE-g-SWNTs) copolymers by copolymerization with ethylene. SWNTs can be used to compound PE-g-SWNTs nanocomposites by a grafting copolymerization approach. The results show that PE covalently grafted from organic functionalized SWNTs forming a strong interface between the SWNTs and the PE matrix. The microstructure of the obtained copolymers was characterized. The SWNTs were dispersed well in the copolymers due to covalent linkage of functionalized SWNTs with polyethylene. Thermal stability of PE-g-SWNTs is better than those of pure PE and PE/SWNTs physical mixture. It indicates that the addition of SWNTs by the grafting copolymerization approach is mainly due to the delay of the thermal decomposition temperature in PE matrix.
I. INTRODUCTION
Since they were discovered1, single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) have been the most actively studied materials in the past few years due to their unique electronic and mechanical properties. They have been investigated for a wide range of applications such as molecular electronics, sensors, field-emission devices, and components in high-performance composites. Functionalization of SWNTs through surface modification has attracted significant interest recently. Carbon nanotubes have been encased in a wide range of functional groups have been reported by some coworkes in the past few years. In addition, proteins and DNA have also been demonstrated to modify carbon nanotubes. The major investigations focused on polymeric/SWNTs along with the progresses on the synthesis, purification and chemical modification of SWNTs. Several reports about wrapping and coating by polymers and surfactants have been reported. There are two main methods in fabricating polymeric/SWNTs: (1) directly mixing of SWNTs in dissolved polymer, (2) in situ polymerization of SWNTs with polymer monomer mixture.2-8 Though in situ polymerization is a better way of obtaining a homogeneous dispersion, because of the SWNTs insolubility in most common solvents, especially the bad dispersion and the weak interaction between the polymer and the SWNTs in polymeric/SWNTs composite, the applications of SWNTs in composites were greatly limited.
In this paper, we report the preparation of PE covalently grafted onto organic functionalized SWNTs (f-SWNTs) by a “grafting” copolymerization approach. According to our information, the preparation of SWNTs bonded with polyolefin by catalytic copolymerization or simple 1-alkenes with organic SWNTs containing 1- alkenes comonomer has not been reported. And the “grafting” approach provides a route to fabricate polyolefin (such as pe, PP) /SWNTs.
Experimental Part
Materials
Single-walled carbon nanotubes (SWNTs) were purchased from Shenzhen Nanotech Port Co., Ltd (Shenzhen, China). Both undecylenic aldehyde and sarcosine are from Acros. The metallocene catalyst C2H4[(tert-H)Ind]ZrCl2 (M-Cat) was purchased from Boulder Scientific Co. MAO in toluene (1.3M) was from Ethyl Corp. Polymerization grade ethylene (Liaoyang Chemical Corp. ) was used without further treatment. Toluene was dried over 4 molecular sieves for 10 days and then refluxed over Na for 12 hours.
Preparation of functionalized SWNTs (f-SWNTs)
SWNTs (0.6g) and undecylenic aldehyde (3.6g) were suspended in 300 ml of DMF. The mixture was heated at 130℃ while sarcosine 4g in DMF (100 ml) was added in portions (4×25 ml every 24 h) and the reaction was continued for 5 days. Then the organic phase was obtained by centrifugation and extracting with CH2Cl2/H2O. The organic phase was washed with CH3OCH3 and concentrated to dryness giving. 1-alkene organic functional groups grafted onto SWNTs (f-SWNTs) were obtained as a brown solid.
Polyethylene grafted onto SWNTs via copolymerization catalyzed by M-Cat
The copolymerization was carried out under mechanical stirring in a 100-mL Schlenk-type reactor at 55℃ with [Zr] = 3.2×10-6 mol/L. After dispersing the f-SWNTs in toluene via ultrasonication, M-Cat and the f-SWNTs could be dissolved together in toluene (50 ml) solvent. By our experiments, the M-Cat was found to be unable to catalyze homopolymerization of the f-SWNTs. Then 1.2 atm of ethylene was led for 0.5 h. An acidified ethanol solution was quickly injected to terminate the polymerization. The precipitated product was collected after being washed with ethanol and dried in vacuo at 60℃ for 24 h.
Characterization
Field emission scanning electron microscope (FSEM, A TMX 840) was used to observe morphology of the samples. Atomic Force Microscopy (AFM) images were recorded on SPA 300HV with an SPI3800 controller in the tapping mode. A silicon microcantilever with an etched conical tip was used for the scanning. Molecular weight and molecular weight distribution of the samples were determined by GPC with a Waters 410. The contents of grafted PE and SWNTs were measured by a PerkinElmer TGA-7 Thermal Analysis System Series at a heating rate of 20℃/min from 25 to 900℃ under N2 atmosphere.
The general strategy for grafting polyethylene onto SWNTs via catalytic copolymerization is described in Scheme 1. There are two main steps: (1) synthesis of 1-alkene organic functional groups grafted onto SWNTs (f-SWNTs) and (2) preparation of polyethylene covalently bonded with the f-SWNTs comonomer (PE-g-SWNTs) by the “grafting” copolymerization approach.
Scheme 1 general strategy for grafting polymers onto the SWNTs
The organic functionalization progress is based on the 1, 3-dipolar cycloaddition of azometheme vlides, a common reaction which has been widely applied to the organic modification of fullerene C60 9-11and been used in SWNTs for few years 12. It is a simple method to connect 1-alkene organic groups to s. By the method compound f-SWNTs was prepared by reaction of pristine SWNTs (purified) with undecylenic aldehyde and sarcosine. The 1H NMR result showed that we obtained the f-SWNTs successfully. The f-SWNTs could soluble in toluene solvent for a long time while purified SWNTs insoluble in it. It indicated that about 0.08mol(1-alkene organic groups)/mol(C) in f-SWNTs, which was measured by thermogravimetric analysis (TGA).
Figure1. TEM images of (a) pristine SWNTs,(b) f-SWNTs.
Another piece of evidence for f-SWNTs was measured by the transmission electron microscopy (TEM). The TEM images showed pictures of the pristine SWNTs (in Figure 1a) and f-SWNTs (in Figure 1b). Comparing images Figure 1a with Figure 1b, difference due to organic functionalization could be observed. There were small branches grafted onto the SWNTs in Figure 1b. It was true that 1-alkene organic groups had been grafted onto almost each individual CNT in Figure 1b.
The SWNTs containing 1-alkene organic groups also have azomethane groups which could affect the transition-metal based Luis acid catalyst in the olefin polymerization. Whereas the long space between the amine and the 1-alkene groups on the SWNTs can minimize these problems. In the catalytic copolymerization, the productivities were not adversely affected to a significant extent by the f-SWNTs comonomer, which could be seen in Table 1.
The copolymerization was carried out under stirring in a 100-mL Schlenk-type reactor at 55℃ in toluene solvent and 1.2 bar ethylene pressure with [Zr]=3.2×10-6 mol/L. After dispersing the f-SWNTs in toluene via ultrasonic sound, the metallocene catalyst and the f-SWNTs comonomer could be dissolved together in toluene before ethylene addition because CAT/MAO was found to be unable to affect homepolymerization of the f-SWNTs comonomer by our experiments. To prove the covalent linkage of PE onto SWNTs two parallel experiments were conducted. The first one was obtaining pure PE by the same catalyst. The second one was affording PE/SWNTs blend by solution mixing method with 4.4% pristine SWNTs weight content. The copolymerization results of ethylene bonded with SWNTs were showed in Table 1. The productivities of the copolymerization are from 4.0~10.0×105 kg(PE) mol-1 (Zr) h-1 atm-1.And the different weight contents of the organic SWNTs in the samples were measured by TGA for the f-SWNTs and the PE-g-SWNTs copolymers. GPC analyses showed that the Mw of the copolymer was around 2.3×105,and the MWD of PE-g-SWNTs copolymers ranged from 2.8 to 4.0. The amount of the grafting is dependent on the amount of f-SWNTs in the reactions and the reaction time.
Table 1. Copolymerization of f-SWNTs with Ethylene Using C2H4[(tert-H)Ind]ZrCl2/MAOa
a conditions: [Zr]=3.2×10-6mol/L, with MAO cocatalyst , [Al]/[Zr]= 2000, P(ethylene)=1.2 bar, toluene solvent 55℃
b measured by GPC.
c measured by TGA.
The direct evidence for PE-g-SWNTs was obtained by 1H NMR spectroscopy (Figure 2, C6D4Cl2 solvent 130℃) which showed that the two peaks of the 1-alkene groups (δ5.8 and 4.9 ppm) grafted to SWNTs, were disappeared. The resonances of NCH2 (δ4.8 and 4.7 ppm) groups, and NMe (δ3.6 ppm) were well resolved. The 1H NMR spectroscopy can give direct evidence for the conclusion that SWNTs is covalently bonded with PE. There was a characteristic peak in about 5.4 ppm because of the -CH=CH- groups at the end of the copolymer. And we also know that polymerization of PP can have the peak by the β-H transition in polymerization 13-14. Because of the structure of the f-SWNTs containing 1-alkene is the same like the propylene structure so it could have the same peak.
Figure2. 1H NMR spectrum of the PE-g-SWNTs copolymer (8.7wt% of the f-SWNTs.C2D2Cl4 solvent,130℃)
Figure3. SEM image of the solution crystallized films PE-g-SWNTs copolymers (8.7wt% of the f-SWNTs)
The morphologies of the samples were detected with scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM image (Figure 3) suggested that the PE-g-SWNTs either as small ropes or as individual tubes. Figure 3 shows a SEM image of coated tubes from PE-g-SWNTs films each is of the same thickness thus any PE-g-SWNTs samples with anomalously large diameters are undoubtedly due to coating of small ropes of a few of SWNTs. One such coated rope may be seen in Figure 3. Interestingly, when magnify the SEM image (on the right side in Figure 3) we can see that the copolymer of PE-g-SWNTs looked like “shish-kebab”. The beads were uniform in size
A thin film of the PE-g-SWNTs with 8.7 wt% of f-SWNTs was prepared by spin-coating of dilute xylene solution on a mica surface, and its microstructure was analyzed by tapping mode AMF (Figure 4). No large SWNTs bundles were found. Thus almost all of the tubes are individual or very small bundles. Meanwhile AFM image showed the structure of “shish-kebab” in the PE-g-SWNTs sample. When magnifying the picture (in Figure 4) “shish-kebab” could be seen clearly.
Figure5. TGA weight loss curves (for heating under N2 atomosphere) for pure PE, PE-g-SWNTs copolymers with different f-SWNTs contents and PE/SWNTs blend (4.4wt% of pristine SWNTs).
The thermal stability of the copolymer was assessed by TGA. The samples were subjected to TGA under nitrogen atmosphere from room temperature to 900℃( about PE to 700℃) at 20℃/min.TGA weight loss curves of the PE-g-SWNTs copolymer, PE/SWNTs mixture and pure PE are shown in Figure 5. The thermal decomposition temperature has been found goes up with the addition of carbon nanotubes. And the thermal decomposition temperature of the copolymers is higher than pure PE and PE/SWNTs blend. Comparing pure PE (in Figure 5-0) with the PE/SWNTs blend (in Figure 5-3), the solution mixing method seemed to have little effect upon TGA measurements. This may be partly due to poor dispersion of SWNTs in the PE matrix by solution mixing method. It showed that the PE-g-SWNTs copolymers prepared by copolymerization can improve the thermal stability of PE matrix. It indicates that the addition of SWNTs by the “grafting” copolymerization approach is mainly due to the delay of the thermal decomposition temperature in PE matrix.
In conclusion, we have demonstrated that PE may be grafted onto individual SWNTs by the “grafting” copolymerization. The nanotubes are well dispersed in the polyolefin matrix by the “grafting” copolymerization. It provides a simple and effective means to produce polyolefin /SWNTs nanocomposites. Furthermore, based on TGA measurement it is clear that the copolymers obtained by the approach can improve the thermal stability. Further studies of the relationship between the mechanical properties and the interaction between PE and SWNTs interface at the copolymer matrix are now in press.
Acknowledgement
The present work was supported by the Science Foundation of Jiangsu University of Science and Technology (Project No. 35061005)
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Key words: Grafting copolymerization; polyethylene; single-walled carbon nanotubes