The Preparation of Nanosized Polyethylene Particles via Carbon

We sincerely thank the National Natural Science Foundation of China (No. 21174011), the Natural Science Foundation of Beijing, China (No. 2102036) and...
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The Preparation of Nanosized Polyethylene Particles via Carbon Sphere Nanotemplates Jing Wang,† Mengshan Yu,† Wanhe Jiang,† Yang Zhou,† Fengjiao Li,† Lu Cheng,† Jianjun Yi,‡ Qigu Huang,*,† Yunfang Liu,*,† and Wantai Yang† †

State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ‡ Lab for Synthetic Resin Research Institution of Petrochemical Technology, China National Petroleum Corporation, Beijing 100083, People’s Republic of China ABSTRACT: A novel type of nano template was prepared by loading a Ziegler−Natta catalyst on nanosized carbon spheres for ethylene polymerization to form nanosized polyethylene (PE) particles; triethylaluminum (AlEt3) was used as a co-catalyst. The results revealed that the nano templates were spherical and their average diameter was ∼180 nm. The nano template had a high titanium content of 4.5 wt %, determined by inductively coupled plasma (ICP) emission spectroscopy and high catalytic activity (up to 7.68 × 105 gPE/((mol Ti) h) for ethylene polymerization. Polyethylene particles 310 nm in diameter were obtained. The morphology of the polymer particles was spherical and the size distribution is uniform, as confirmed by scanning electron microscopy (SEM).

1. INTRODUCTION

and the nano templates as the catalyst is efficient for ethylene polymerization.

Polyethylene (PE) is a commodity polymer that has become ubiquitous in our life and in industries over the past decades, because of its low price and good mechanical properties.1 However, PE has hardly played any role in the field of nanotechnology, because nanosized PE particles are obtained, unless under rigorous conditions. It was demonstrated that ethylene was polymerized in emulsion systems by nonmetallocene catalysts, which opened the way for the creation of nanostructures made from PE.2−4 Mecking5−7 reported that ethylene polymerization was catalyzed by phosphorus−oxygen chelated nickel(II) complexes in aqueous emulsion to obtain the polymer particles 26−118 nm in diameter. Klapper8 reported that a nonaqueous emulsion formed by toluene for ethylene and propylene polymerization. Nanosized polymer particles ∼30 nm in diameter were obtained. However, the obstacle for this technology is that few catalysts can withstand the emulsion polymerization conditions. Although it is possible to obtain nanosized PE particles by emulsion polymerization, the surfactant that influences the properties of the obtained polymer is difficult to remove completely. However, Mecking9 reported that salicylaldiminatosubstituted titanium(IV) complexes have been immobilized on modified silica nanoparticles for ethylene polymerization to form polyethylene particles ∼200 nm in size. However, a small report has been opened on nanosized polyethylene particles synthesized by this supported Ziegler−Natta catalyst, where the active component (TiCl4) was supported on the nanosized carbon spheres as new support. Herein, we describe a novel method about making the nano template for ethylene polymerization to produce spherical and nanosized PE particles which experience dielectric properties that will be used in various fields without further purification. This work showed that the use of oxidized nanosized carbon spheres as the support © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. All the manipulations were conducted under a nitrogen atmosphere using Schlenk tube techniques. n-Hexane was further purified by refluxing over sodium metal under nitrogen for 48 h and distilled before use. TiCl4 and polymerization-grade ethylene were not further purified. 2.2. Characterization. The titanium content was determined using a Shimadzu ICPS-5000-type inductively coupled plasma (ICP) emission spectrometer. The average molecular weight and molecular weight distribution was measured by a PL-GPC220 instrument, using standard polystyrene as a reference and 1,2,4-trichlorobenzene as a solvent at 150 °C. The X-ray photoelectron spectroscopy (XPS) survey spectra were obtained using an ESCALAB Model 250 X-ray photoelectron instrument. Raman spectroscopic measurements were generated using a Renishaw InVia Raman microscope. The excitation lines were 514 nm with a power of 10 mW. Transmission electron microscopy (TEM) was performed on electron microscopes (Hitachi, Model H-800, and JEOL, Model JEM-2100). Scanning electron microscopy (SEM) was performed on a SUPRA 55/55VP system. The particle size was determined via the use of a laser granulometer (Mastersizer, Model 2000). 2.3. Preparation of Oxidized Nanosized Carbon Spheres. Two grams (2.0 g) of nanosized carbon spheres and 80 mL of concentrated nitric acid were added into a 300-mL Schlenk flask. The mixture was subjected to ultrasonic washing for 5 min Received: Revised: Accepted: Published: 17691

July 22, 2013 November 4, 2013 November 23, 2013 November 23, 2013 dx.doi.org/10.1021/ie4022946 | Ind. Eng. Chem. Res. 2013, 52, 17691−17694

Industrial & Engineering Chemistry Research

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2.5. Preparation of Nanosized PE Particles. The preparation of nanosized PE particles was carried out in a 500-mL glass flask with a mechanical stirrer. Three hundred milliliters (300 mL) of hexane, along with the desired amount of nano template and Et3Al, were added in the order. After preactivation for 10 min, and heating to 70 °C, ethylene was charged into the reactor and its pressure was kept at 0.2 MPa for a desired time. Nanosized PE particles were obtained after filtration, washing with alcohol and water, and drying overnight in a vacuum oven at 50 °C.

3. RESULTS AND DISCUSSION 3.1. Properties of the Nano Template. After the nanosized carbon particles were treated with TiCl4, the nano templates were formed (Figure 1). The titanium content (4.5 wt %) was determined by ICP on the surface of the nano templates, implying that the active species TiCl4 was loaded comfortably on the nano templates. We can note from Figure 1 that nano templates were spherical and homogeneous. The particle average diameter was 80 nm. The results indicated that the active species were dispersed on the surface of the nanosized carbon particle homogenously. It was helpful for synthesizing spherical PE particles.10−14 To analyze the elemental composition as well as the chemical bonding environment, XPS measurement was carried out for the nano template. The XPS survey spectra are depicted in Figure 2. From Figure 2a, we can find that the nano template exhibited five peaks at 199.32, 284.99, 456.69, 532.81, and 1302.42 eV, which were assigned to Cl 2p, C 1s, Ti 2p3, O 1s, and Mg 1s, respectively. The C 1s and O 1s peaks were attributed to the sp2-hybridized carbon bonds from the graphite layer of nano carbon sphere and oxygen-containing functional groups.15,16 The existence of Ti 2p3 and C 1s peaks demonstrated that the active species TiCl4 was successfully immobilized on the surface of the nano templates through Grignard synthesis, and the Mg 1s peak corresponded to the few byproducts (MgCl2) remaining after the Grignard reaction. With regard to the obtained polyethylene, as Figure 2b showed, no atoms other than C and O atoms were detected in

Figure 1. Transmission electron microscopy (TEM) image showing the morphology of the particles that comprise the nanosized templates.

with an ultrasonic washer and then was stirred for 5 h with a stirring bar at 80 °C. The obtained residue after filtrating was washed twice with distilled water and anhydrous ethanol, respectively, and then was further purified by vacuum to obtain black bulk solid, oxidized nanosized carbon spheres with a yield of 1.8 g. 2.4. Preparation of the Nano Template. One gram (1.0 g) of oxidized nanosized carbon spheres and 80 mL of n-hexane were added into a 250-mL Schlenk flask. The mixture was stirred in high speed with a dripping of 7.0 mL of CH3MgCl (0.02 mol). After 2 h of reaction at room temperature, the reactant was filtrated and the residue was washed twice with 40 mL of n-hexane to remove the remnant Grignard reagent. The obtained solid and 50 mL of n-hexane were added into a 250-mL Schlenk flask, followed by a dripping of 9.0 mL TiCl4 at 0 °C. The reaction system then was stirred for 1 h at the same temperature. The mixture was then warmed to 60 °C slowly, and the mixture was maintained at that temperature for 2 h. After that, the reaction system was filtrated, washed with n-hexane (30 mL × 4) at 50 °C, and dried under vacuum for 2 h to get a black powder product with a yield of 0.61 g. The titanium content of the catalyst was 4.5 wt %, as confirmed by ICP analysis.

Figure 2. X-ray photoelectron spectroscopy (XPS) curves of (a) nano templates and (b) the obtained polyethylene. 17692

dx.doi.org/10.1021/ie4022946 | Ind. Eng. Chem. Res. 2013, 52, 17691−17694

Industrial & Engineering Chemistry Research

Research Note

Figure 3. Raman spectra of (a) oxidized nanosized carbon spheres and (b) nano templates.

the surface layer of the obtained polymer; this observation was ascribed to the thick covering efficiency, which implied that the internal nano template core was not probed due to the thicker PE coating. For the study of carbon materials, Raman spectroscopy is a powerful tool and provides direct evidence for covalent sidewall functionalization. As shown in Figure 3, the D- and G-bands of the nanosized carbon spheres at ca. 1350 cm−1 and 1580 cm−1, attributed to the defects and disorder-induced modes and in-plane E2g zone-center mode, the functionalized groups −COOH and −OH from the oxidized nanosized carbon spheres were clearly observed. For the nano template, it was obviously that the D- to G-band intensity ratios (ID/IG) are slightly higher than that of the oxidized nanosized carbon spheres. Increasing in these ratios was indicative of structural defects in the oxidized nanosized carbon spheres’ surface due to covalent bonding with titanium tetrachloride. On the other hand, the larger area ratio in the disorder (D) mode band probably corresponds to the conversion of the hybridization of the C atoms on the nanosized carbon spheres from sp2 to sp3, which indicates covalent side-wall functionalization.17 In addition, Figure 3b shows that four special absorption peaks were also found, at 153, 202, 393, and 619 cm−1. It was reported that the bands at 153, 393, and 619 cm−1 belong to the characteristic absorbance of TiO2,18 which may be formed through the reaction of titanium halide and moisture during sample preparation. However, another characteristic band of TiO2 at 514 cm−1 was not observed in Figure 3b. In our opinion, this observation can be explained by the theory that the partial active species TiCl4 was reacted with atmospheric moisture, followed by the formation of a new O−Ti−Cl covalent bond. The strong extra band, at 202 cm−1, demonstrated this point and also confirmed the higher content of Ti loading in the oxidized nanosized carbon spheres. 3.2. Morphology of Nanosized PE particles. Nanosized PE particles were obtained through the nano templates. PE particles 300 nm in diameter were obtained when the polymerization lasted for 5 min at 70 °C (see Figure 4a). The particle size was enlarged to 700 nm in diameter (Figure 4b) when the polymerization time was extended to 20 min. If the reaction time was increased to 60 min, the size of the particles grew to ∼2 μm in diameter (see Figure 4c). The gel permeability

Figure 4. Scanning electron microscopy (SEM) images of the resulting polyethylene particles with polymerization times of (a) 5, (b) 20, and (c) 60 min.

chromatography (GPC) results show that the PE particles exhibit high molecular weight (Mw = 4.4 × 105 g/mol for 5 min of polymerization and Mw = 5.7 × 105 g/mol for 60 min of polymerization). It indicates that the size of the PE particles is controlled by the polymerization time.

4. CONCLUSIONS A nanosized template of carbon particles was prepared by treating nanosized carbon particles with TiCl4. The template weas formed from particles that had a regular spherical morphology and uniform size distribution. The polyethylene (PE) particles obtained via the template methodology were 300−700 nm in diameter, and the particle size was controlled by the polymerization time.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 86 1064433856. Fax: 86 1064433856. E-mail: huangqg@ mail.buct.edu.cn. Notes

The authors declare no competing financial interest. 17693

dx.doi.org/10.1021/ie4022946 | Ind. Eng. Chem. Res. 2013, 52, 17691−17694

Industrial & Engineering Chemistry Research

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Research Note

ACKNOWLEDGMENTS We sincerely thank the National Natural Science Foundation of China (No. 21174011), the Natural Science Foundation of Beijing, China (No. 2102036) and the Petroleum China Innovation Fund (Grant No. 2011D-5006-0502).

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REFERENCES

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