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A new strategy to prepare polyethylene nanocomposites by using a late-transition-metal catalyst supported on AlEt3-activated organoclay
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 1727–1733 www.elsevier.com/locate/compscitech
A new strategy to prepare polyethylene nanocomposites by using a late-transition-metal catalyst supported on AlEt3-activated organoclay Fu-An He, Li-Ming Zhang *, Hong-Liu Jiang, Li-Shan Chen, Qing Wu, Hai-Hua Wang Laboratory for Polymer Composite and Functional Materials, Institute of Optoelectronic and Functional Composite Materials, School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou 510275, China Received 14 August 2005; received in revised form 18 June 2006; accepted 26 June 2006 Available online 1 September 2006
Abstract A new strategy for in situ polymerization was developed to prepare the polyethylene nanocomposites, in which a nickel a-diimine late-transition-metal catalyst supported on the AlEt3-activated organoclay was adopted to initiate the ethylene polymerization and to provide the reinforcement materials after the polymerization. It was found that the polymerization conditions such as reaction time, Al/Ni molar ratio, and reaction temperature affected the polymerization activity and the clay loading. The nanoscale dispersion of the clay layers in the polyethylene matrix was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). From the thermogravimetric analyses (TGA), it was found that the decomposition temperature of the nanocomposite with the organoclay of 11.91 wt% could be 46.8 °C higher than that of pure polyethylene when 30% weight loss was selected as a measuring point, showing enhanced thermal oxidation stability of this kind of polyolefin nanocomposites. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Polyethylene; Organoclay; Nanocomposites; Supported catalyst; In situ polymerization
1. Introduction In recent years, the nanocomposites composed of polyolefin and layered silicates have received considerable attention because of their unique advantages, such as increased thermal stability, enhanced mechanical property, reduced flammability and improved gas barrier [1–12]. Among various approaches to the preparation of polyolefin nanocomposites, in situ polymerization strategy has been considered to be most effective. Tudor et al. [13] first treated the layered silicate with a large amount of methylaluminoxane (MAO) and then immobilized the metallocene catalyst on the MAO-modified silicates, resulting in a polyethylene nanocomposite with superior mechanical properties; Heinemann et al.
*
Corresponding author. Tel.: +86 20 84112354; fax: +86 20 84112245. E-mail address: [email protected] (L.-M. Zhang).
0266-3538/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.06.012
[14] prepared polyethylene nanocomposites by means of melt compounding and ethylene homo- and copolymerization, catalyzed with MAO-activated zirconocene (MBI), nickel (DMN) and palladium (DMPN) catalysts, in the presence of layered silicates which were rendered organophilic via ion exchange with various quaternary alkyl ammonium cations; Rong et al. [15] prepared a organic/ inorganic nanocomposite of polyethylene through in situ coordination polymerization, in which the Ziegler–Natta catalyst was first supported on the surface of silicate nanowhiskers to subsequently initiate the polymerization of ethylene on the surface of these nanowhiskers; very recently, He et al. [16] combined a traditional Ziegler–Natta catalyst with monoalkylimidazolium-modified montmorillonite to prepare exfoliated polypropylene nanocomposites; Mariott and Chen [17] used the intercalated metallocene catalysts for the synthesis of in situ polymerized nanocomposites comprising delaminated silicate nanoplatelets dispersed in the atactic, isotactic, or syndiotactic PMMA matrixes.
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In the present work, we attempt to develop a new strategy for the in situ preparation of polyethylene (PE) nanocomposites by using a nickel a-diimine late-transitionmetal catalyst supported covalently on the AlEt3-activated organoclay, as shown in Fig. 1. As a new kind of catalysts for olefin polymerization after Ziegler–Natta and MAObased catalysts, late-transition-metal catalysts have attracted much interest in academic and industry [18]. To our acknowledge, however, their combination with organically modified clay have been scarcely used to prepare polyolefin nanocomposites. 2. Experimental 2.1. Materials The organoclay used in this work was kindly supplied by Southern Clay Products Inc. (Gonzales,Texas) under the trade name of Claytone APA. It was received as fine particle powder. According to Salahuddin and Shehata [19], the modified silicate was produce by a cation exchange reaction between the silicate and dimethyl benzyl hydrogenated tallow ammonium chloride, and the percentage of organic content (dimethyl benzyl hydrogenated tallow ammonium chloride) was 24%. Ethylene was provided by Beijing Qianjing Chemical Factory (China), and used after the treatment with molecular sieves. Diethylaluminum chloride (AlEt2Cl) and triethylaluminum (AlEt3) were purchased from Shanghai Petrochemical Co. (China), and used as 400 g/L solutions in n-heptane, respectively. Toluene was purchased from Guangzhou Chemical Co. (China), and dried over sodium under reflux for 24 h. 4,4 0 -Methylenebis(2,6-diisopropylaniline) was obtained from Acros Co.,
and used as received. All other chemicals were purchased commercially, and used without further purification. 2.2. Preparation of late-transition-metal catalyst (NiLBr2) The late-transition-metal catalyst used in this study is a nickel a-diimine catalyst, namely bis(4,4 0 -methylene-bis(2,6-diisopropylimino))acenaphthene nickel dibromide complex (abbreviated as NiLBr2). Its preparation route was shown in Fig. 2. For the synthesis of the ligand (L), acenaphthenequinone (0.54 g) and 4,4 0 -methylene-bis-(2,6diisopropylaniline) (3.0 g) were suspended in toluene (50 mL), and added into a flask equipped with a side port and a reflux condenser. The system was reflux heated for 14 h. After the reaction, the toluene as the solvent was removed by evaporation in vacuum. Then the crude product was purified by flash chromatography (silica, ethyl acetate/petroleum ether, 1:9), resulting in yellow ligand (L). For the synthesis of NiLBr2, the ligand (L, 0.6 g) and Ni (DME) Br2 (0.3 g) were dissolved in dichloromethane (30 mL), and then the resulting solution was refluxed for 14 h. After the removal of the solvent by evaporation in vacuum, the crude product was purified three times by washing with ethyl ether, affording brown NiLBr2. The ligand (L). Yield: 1.2 g; 1H NMR (500 MHz, CDCl3, ppm): 7.89 (d,2H), 7.37 (t,2H), 7.01 (s,4H), 6.87 (s,4H), 6.74 (s,2H), 3.84 (d,4H), 3.53 (s,4H), 2.19 (s,12H), 2.06 (s,12H); 13C NMR (500 MHz, CDCl3, ppm): 161.016, 145.358, 140.787, 137.938, 136.773, 135.174, 132.586, 131.640, 129.680, 128.694, 127.716, 124.569, 123.235, 123.013, 41.354, 28.586, 27.975, 23.406, 23.170, 22.504; Anal. Calcd (%): C, 84.68; H, 8.96; N, 6.37; Found (%): C, 84.80; H, 8.67; N, 6.34. NiLBr2. Yield: 0.6 g; Anal.
Fig. 1. A new route to the in situ preparation of polyethylene nanocomposites.
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duced into the reactor in succession. The polymerization was controlled at certain temperature, with an ethylene pressure of 0.1 MPa for a given times and then terminated with acidified ethanol. The polymerization product was washed with ethanol several times and dried in vacuum at 60 °C for 6 h. When the supported catalyst was used, the polyethylene (PE) composite was obtained. When the unsupported catalyst was used, pure PE was obtained. 2.5. Characterization
Fig. 2. The preparation of the late-transition-metal catalyst (NiLBr2) used in this study.
Calcd (%): C, 67.82; H, 7.17; N, 5.10; Found (%): C, 67.12; H, 6.72; N, 4.92. 2.3. Activation of organoclay and its combination with NiLBr2 For the activation of the organoclay, the clay (Claytone APA, 3.0 g) was first introduced into a glass vessel equipped with a magnetic stirrer, and then 15 ml heptane was added. After that, 8 mL of AlEt3 solution (in heptane, 400 g/L) was added to the mixture at 0 °C and stirred for 20 h. Considering that AlEt3 is unstable in air, these operations were carried out under an atmosphere of nitrogen using standard Schlenk techniques. The treated organoclay was separated by decantation, washed up with heptane and dried under vacuum at 60 °C for 2 h. To combine with NiLBr2, 0.2 g of NiLBr2 was dissolved in 10 ml of dichloromethane and then added to the suspension composed of above AlEt3-activated organoclay and 15 ml of heptane, which was stirred at room temperature overnight. After decantation, the solid was washed up with heptane for several times, and then dried in vacuum. Thus, the nickel a-diimine catalyst supported on the AlEt3-activated organoclay was obtained as yellow powder. 2.4. Ethylene polymerization The polymerization was carried out in a 100 ml glass reactor equipped with a magnetic stirrer. After heating and evacuating alternately for 30 min, the reactor was filled with ethylene to a pressure of 0.1 MPa. Toluene (50 mL) and required amounts of the cocatalyst (AlEt2Cl) and the supported catalyst or unsupported catalyst were intro-
The Ni content in the supported catalyst was determined by an Inductively Coupled Plasma-atomic Emission Spectrometry (TJA Co., USA). The interlayer structure of the clay in the composites was analyzed by a Rigaku D/maxRB X-ray diffractometer (XRD) with Cu-Ka radiation (40 kV, 100 mA). For this purpose, the samples were prepared by a compression-molding method. At first, the prepared PE composite was heated at 180 °C for 5 min under 10 MPa, and then molded to the sheets at ambient temperature under the same pressure. The dispersion and morphology of the clay in the composites were investigated by a JEM-100CX II transmission electron microscopy (TEM) with an acceleration voltage of 100 kV. For this purpose, a solution method was used, in which 0.01 g of as-prepared composite was dispersed in 10 ml of toluene and then placed a drop of the dispersion onto a carboncoated TEM copper grid. Meanwhile, the compressionmolded sample was also analyzed by TEM for a comparison. In this case, the ultrathin slide was obtained by sectioning the compression-molded sample along a direction perpendicular to the compression. Thermogravimetric analysis (TGA) was carried out using Netzsch TG-209 thermogravimetric analyzer in air at a purge rate of 50 mL/min, with the scanning temperature in the range from 50 to 700 °C. To determine the clay loading, the thermogravimetric experiments for the nanocomposite samples were carried out in nitrogen, and the percentage of residual ashes was taken as the clay loading. 3. Results and discussion 3.1. Polymerization activity and clay loading For the in situ polymerization using NiLBr2 supported on the AlEt3-activated organoclay, it was found from Figs. 3–5 that the Al/Ni molar ratio, polymerization temperature and time affected the polymerization activity and the clay loading. With the increase of Al/Ni molar ratio from 150 to 900, the activity increased first and then decreased, while the opposite held true for the clay loading (Fig. 3). The highest activity was observed when the Al/Ni molar ratio is equal to 600. On one hand, the enhancement in the activity at higher Al/Ni molar ratio could be associated with greater opportunity for reducing nickel diimine complexes to form cationic, catalytically active species, and the decrease in the activity at the Al/Ni molar ratio
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Fig. 3. Effects of Al/Ni molar ratio on the polymerization activity and the clay loading (Conditions: time, 90 min; temperature, 30 °C).
of 900 may be due to the chain transfer competing with chain-propagation [20]. On the other hand, low polymerization activity resulted in the formation of small amount of polyethylene (PE), which increased the relative amount of the clay in the composites (clay loading), and high polymerization activity led to the formation of large amount of PE, which decreased the clay loading. As far as the effects of polymerization temperature are considered, increasing the temperature from 0 to 50 °C caused an increase in the activity and a decrease in the clay loading, and further increase to 70 °C caused a decrease in the activity and an increase in the clay loading (Fig. 4). In general, an elevated temperature would result in a higher chain-propagation rate that could be expected to increase the catalytic activity. However, the accelerated deactivation rate of the catalyst and the reduction of ethylene solubility in toluene at higher temperature might decrease the catalytic activity [21,22]. The combination of these effects is likely to account for the different dependence of the catalytic activity on the temperature. In addition, the polymerization activity and the clay loading decreased with the increase of polymerization time (Fig. 5). 3.2. XRD characteristics
Fig. 4. Effects of polymerization temperature on the polymerization activity and the clay loading (Conditions: Al/Ni molar ratio, 600; time, 90 min).
Fig. 5. Effects of polymerization time on the polymerization activity and the clay loading (Conditions: Al/Ni molar ratio, 600; temperature, 30 °C).
The XRD patterns in the range of 2h = 1.5–10° for original organoclay, Claytone APA, and the compressionmolded samples of PE composites with various clay loadings are shown in Fig. 6. The basal spacing of Claytone APA is determined to be 1.87 nm from the d001 diffraction peak at 2h = 4.71°. This characteristic peak disappeared completely in the XRD patterns of the samples containing respectively the clay loading of 5.05 and 8.35 wt%, suggesting that the stacking layers of the clay in these two samples were fully separated and an exfoliated nanostructure was formed. For the sample with the clay loading of 11.91 wt%, however, it is interesting to found that the XRD pattern showed a broad peak (from 5° to 7°) centered
Fig. 6. X-ray diffraction patterns of original organoclay and the PE composites with various clay loadings.
F.-A. He et al. / Composites Science and Technology 67 (2007) 1727–1733
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at 6.1°, indicating a decrease rather than a increase in the basal spacing. Similar observations were reported for the nanocomposites composed of polypropylene (PP), anhydride-grafted PP and organically modified montmorillonites. Kim et al. [23] attributed this phenomenon to the transformation of interlayer structure from bilayer arrangement to monolayer arrangement, which made the surface character of the organoclay less organophilic and easier to stacking recovery. In fact, the peak at 2h = 4.7° corresponded to the basal spacing for the bilayer arrangement of alkylammonium chains, while the peak at 2h = 6.0° corresponded to that of monolayer arrangement, as confirmed by Yoon et al. [24]. According to Vaia and Giannelis [25], the broadened peak resulted from the disorderedly dispersion of organoclay in the polymer matrix after melt intercalation. It seems that the compressionmolding method of the sample for XRD measurement significantly affected the original dispersion of the clay layers in the polyethylene matrix. 3.3. Morphological structure To further confirm the dispersion states of the organoclay, TEM analysis was carried out at first for the PE composite sample with the clay loading of 11.91 wt%, which was prepared by the solution method. As shown in Fig. 7, the exfoliation of the clay into nanolayers including single clay plates was clearly observed, which were dispersed disorderedly and uniformly in the PE matrix. This verified that the exfoliated PE nanocomposite could be formed by the in situ polymerization using NiLBr2 supported on AlEt3-activated organoclay, even at a high clay loading of 11.91 wt%. For a comparison, the compres-
Fig. 8. TEM images at various magnifications for the compressionmolded sample of the PE composite with the clay loading of 11.91 wt%: (a) X10000; (b) X27000; and (c) X100000.
Fig. 7. TEM image for the PE composite sample prepared by the solution method (clay loading, 11.91 wt%).
sion-molded sample of PE composite with the clay loading of 11.91 wt% was also observed by TEM at various magnifications. From the TEM images shown in Fig. 8, it is difficult to find the exfoliated clay layers similar to those appeared in Fig. 7. On the contrary, the aggregation of the silicates layers occurred and some multiplayer stacks with a nanometer size were relatively orderly dispersed in the PE matrix, indicating that the obtained composite has
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a stacking recovery. This may be due to the fact that the elevated temperature and high pressure during the compression-molding of the sample make it easier for the clay layers to aggregate and orient along the compression direction. TEM result is consistent with the above-mentioned XRD result. Indeed, the compression method has a great influence on the original distribution of the clay layer in the PE matrix. 3.4. Thermal oxidation stability Fig. 9 illustrates the TGA curves of pure PE and the PE nanocomposites with various clay loadings. The degradation rate for pure PE becomes very quick after 350 °C, while the PE nanocomposites investigated show much slower degradation rates before 400 °C. Different from the PE nanocomposites, pure PE has almost no residues above 560 °C. When 30% weight loss was selected as a point of comparison, the thermal decomposition temperatures for pure PE and the PE nanocomposites with the respective clay loading of 5.05, 8.35 and 11.91 wt% are determined as 398.3, 427.0, 442.8 and 445.1 °C, respectively. Especially for the PE nanocomposite with the clay loading of 11.91 wt%, its decomposition temperature could be 46.8 °C higher than that of pure polyethylene. Compared with the pure PE, the PE nanocomposites investigated are characteristic of enhanced thermal oxidation stability. Moreover, the thermal property of the PE nanocomposite becomes better with the increase of the clay loading. Such behavior has already been reported for other nanocomposites composed of layered silicates and the polymer matrix such as nylon-6 [26], polystyrene [27], poly(L-lactide) [28] or polypropylene [29] filled with various types of organo-modified montmorillonites. Obviously, the molecular dispersion of the clay nanolayers in the PE matrix could contribute to the enhanced thermal stability of the exfoliated PE nanocomposites.
Fig. 9. TGA profiles for: (A) pure PE; (B) PE nanocomposite with the clay loading of 5.05 wt%; (C) PE nanocomposite with the clay loading of 8.35 wt%; and (D) PE nanocomposite with the clay loading of 11.91 wt%.
4. Conclusions The PE nanocomposites with superior thermal property could be prepared successfully by a new strategy for in situ polymerization, in which a nickel a-diimine late-transitionmetal catalyst supported on the AlEt3-activated organoclay was adopted to initiate the ethylene polymerization and to provide the reinforcement materials after the polymerization. The polymerization activity and the clay loading could be adjusted by controlling reaction time, Al/Ni molar ratio, and reaction temperature. XRD and TEM examinations provided direct evidence for the formation of the PE nanocomposite. Due to the nanometric scale reinforcement of the clay platelets in the PE matrix, the resulting nanocomposite exhibits enhanced thermal oxidation stability. Acknowledgements This work was supported by NSFC (20273086; 30470476), NSFG (021769; 039184), Department of Science and Technology of Guangdong Province (2004B33101003), and NCET Program in Universities as well as SRF for ROCS, SEM, China. References [1] Jeon HG, Jung HT, Lee SW, Hudson SD. Poly Bull 1998;41:107. [2] Zhao C, Qin H, Gong F, Feng M, Zhang S, Yang M. Polym Degrad Stab 2005;87:183. [3] Gopakumar TG, Lee JA, Kontopoulou M, Parent JS. Polymer 2002;43:5483. [4] Zanetti M, Bracco P, Costa L. Polym Degrad Stab 2004;85:657. [5] Zhao C, Feng M, Gong F, Qin H, Yang M. J Appl Polym Sci 2004;93:676. [6] Shin SYA, Simon LC, Soares JBP, Scholz G. Polymer 2003;44:5317. [7] Wang J, Liu ZY, Guo CY, Chen YJ, Wang D. Macromol Rapid Commun 2001;22:1422. [8] Alexandre M, Dubois P, Sun T, Garces JM, Jerome R. Polymer 2002;43:2123. [9] Yang F, Zhang X, Zhao H, Chen B, Huang B, Feng Z. J Appl Polym Sci 2003;89:3680. [10] Jin YH, Park HJ, Im SS, Kwak SY, Kwak S. Macromol Rapid Commun 2002;23:135. [11] Wei LM, Tang T, Huang BT. J Polym Sci A 2004;42:941. [12] Kuo SW, Huang WJ, Huang SB, Kao HC, Chang FC. Polymer 2003;44:7709. [13] Tudor J, Willington L, O’Hare D, Royan B. Chem Commun 1996:2031. [14] Heinemann J, Reichert P, Thomann R, Mulhaupt R. Macromol Rapid Commun 1999;20:423. [15] Rong J, Jing Z, Li H, Sheng M. Macromol Rapid Commun 2001;22:329. [16] He A, Hu H, Huang Y, Dong JY, Han CC. Macromol Rapid Commun 2004;25:2008. [17] Mariott WR, Chen EYX. J Am Chem Soc 2003;125:15726. [18] Ittel SD, Johnson LK. Chem Rev 2000;100:1169. [19] Salahuddin N, Shehata M. Polymer 2001;42:8379. [20] Xue XH, Yang X, Xiao YZ, Zhang QX, Wang HH. Polymer 2004;45:2877. [21] Gate DP, Svejda SA, Onate E, Killian CM, Johnson LK, White PS, et al. Macromolecules 2000;33:2320. [22] Ye ZB, Alsyouri H, Zhu SP, Lin YS. Polymer 2002;44:969. [23] Kim KN, Kim H, Lee JM. Polym Eng Sci 2001;41:1963.
F.-A. He et al. / Composites Science and Technology 67 (2007) 1727–1733 [24] [25] [26] [27]
Yoon JT, Jo WH, Lee MS, Ko MB. Polymer 2001;42:329. Vaia RA, Giannelis EP. Macromolecules 1997;30:8000. Liu L, Qi Z, Zhu X. J Appl Polym Sci 1999;71:1133. Kim TH, Lim ST, Lee CH, Choi HJ, Jhon MS. J Appl Polym Sci 2003;87:2106.
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[28] Paul MA, Dege’e P, Henrist C, Rulmont A, Dubois P. Polymer 2003;44:443. [29] Zanetti M, Camino G, Reichert P, Mulhaupt R. Macromol Rapid Commun 2001;22:176.
A new strategy to prepare polyethylene nanocomposites by using a late-transition-metal catalyst supported on AlEt3-activated organoclay Fu-An He, Li-Ming Zhang *, Hong-Liu Jiang, Li-Shan Chen, Qing Wu, Hai-Hua Wang Laboratory for Polymer Composite and Functional Materials, Institute of Optoelectronic and Functional Composite Materials, School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou 510275, China Received 14 August 2005; received in revised form 18 June 2006; accepted 26 June 2006 Available online 1 September 2006
Abstract A new strategy for in situ polymerization was developed to prepare the polyethylene nanocomposites, in which a nickel a-diimine late-transition-metal catalyst supported on the AlEt3-activated organoclay was adopted to initiate the ethylene polymerization and to provide the reinforcement materials after the polymerization. It was found that the polymerization conditions such as reaction time, Al/Ni molar ratio, and reaction temperature affected the polymerization activity and the clay loading. The nanoscale dispersion of the clay layers in the polyethylene matrix was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). From the thermogravimetric analyses (TGA), it was found that the decomposition temperature of the nanocomposite with the organoclay of 11.91 wt% could be 46.8 °C higher than that of pure polyethylene when 30% weight loss was selected as a measuring point, showing enhanced thermal oxidation stability of this kind of polyolefin nanocomposites. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Polyethylene; Organoclay; Nanocomposites; Supported catalyst; In situ polymerization
1. Introduction In recent years, the nanocomposites composed of polyolefin and layered silicates have received considerable attention because of their unique advantages, such as increased thermal stability, enhanced mechanical property, reduced flammability and improved gas barrier [1–12]. Among various approaches to the preparation of polyolefin nanocomposites, in situ polymerization strategy has been considered to be most effective. Tudor et al. [13] first treated the layered silicate with a large amount of methylaluminoxane (MAO) and then immobilized the metallocene catalyst on the MAO-modified silicates, resulting in a polyethylene nanocomposite with superior mechanical properties; Heinemann et al.
*
Corresponding author. Tel.: +86 20 84112354; fax: +86 20 84112245. E-mail address: [email protected] (L.-M. Zhang).
0266-3538/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.06.012
[14] prepared polyethylene nanocomposites by means of melt compounding and ethylene homo- and copolymerization, catalyzed with MAO-activated zirconocene (MBI), nickel (DMN) and palladium (DMPN) catalysts, in the presence of layered silicates which were rendered organophilic via ion exchange with various quaternary alkyl ammonium cations; Rong et al. [15] prepared a organic/ inorganic nanocomposite of polyethylene through in situ coordination polymerization, in which the Ziegler–Natta catalyst was first supported on the surface of silicate nanowhiskers to subsequently initiate the polymerization of ethylene on the surface of these nanowhiskers; very recently, He et al. [16] combined a traditional Ziegler–Natta catalyst with monoalkylimidazolium-modified montmorillonite to prepare exfoliated polypropylene nanocomposites; Mariott and Chen [17] used the intercalated metallocene catalysts for the synthesis of in situ polymerized nanocomposites comprising delaminated silicate nanoplatelets dispersed in the atactic, isotactic, or syndiotactic PMMA matrixes.
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In the present work, we attempt to develop a new strategy for the in situ preparation of polyethylene (PE) nanocomposites by using a nickel a-diimine late-transitionmetal catalyst supported covalently on the AlEt3-activated organoclay, as shown in Fig. 1. As a new kind of catalysts for olefin polymerization after Ziegler–Natta and MAObased catalysts, late-transition-metal catalysts have attracted much interest in academic and industry [18]. To our acknowledge, however, their combination with organically modified clay have been scarcely used to prepare polyolefin nanocomposites. 2. Experimental 2.1. Materials The organoclay used in this work was kindly supplied by Southern Clay Products Inc. (Gonzales,Texas) under the trade name of Claytone APA. It was received as fine particle powder. According to Salahuddin and Shehata [19], the modified silicate was produce by a cation exchange reaction between the silicate and dimethyl benzyl hydrogenated tallow ammonium chloride, and the percentage of organic content (dimethyl benzyl hydrogenated tallow ammonium chloride) was 24%. Ethylene was provided by Beijing Qianjing Chemical Factory (China), and used after the treatment with molecular sieves. Diethylaluminum chloride (AlEt2Cl) and triethylaluminum (AlEt3) were purchased from Shanghai Petrochemical Co. (China), and used as 400 g/L solutions in n-heptane, respectively. Toluene was purchased from Guangzhou Chemical Co. (China), and dried over sodium under reflux for 24 h. 4,4 0 -Methylenebis(2,6-diisopropylaniline) was obtained from Acros Co.,
and used as received. All other chemicals were purchased commercially, and used without further purification. 2.2. Preparation of late-transition-metal catalyst (NiLBr2) The late-transition-metal catalyst used in this study is a nickel a-diimine catalyst, namely bis(4,4 0 -methylene-bis(2,6-diisopropylimino))acenaphthene nickel dibromide complex (abbreviated as NiLBr2). Its preparation route was shown in Fig. 2. For the synthesis of the ligand (L), acenaphthenequinone (0.54 g) and 4,4 0 -methylene-bis-(2,6diisopropylaniline) (3.0 g) were suspended in toluene (50 mL), and added into a flask equipped with a side port and a reflux condenser. The system was reflux heated for 14 h. After the reaction, the toluene as the solvent was removed by evaporation in vacuum. Then the crude product was purified by flash chromatography (silica, ethyl acetate/petroleum ether, 1:9), resulting in yellow ligand (L). For the synthesis of NiLBr2, the ligand (L, 0.6 g) and Ni (DME) Br2 (0.3 g) were dissolved in dichloromethane (30 mL), and then the resulting solution was refluxed for 14 h. After the removal of the solvent by evaporation in vacuum, the crude product was purified three times by washing with ethyl ether, affording brown NiLBr2. The ligand (L). Yield: 1.2 g; 1H NMR (500 MHz, CDCl3, ppm): 7.89 (d,2H), 7.37 (t,2H), 7.01 (s,4H), 6.87 (s,4H), 6.74 (s,2H), 3.84 (d,4H), 3.53 (s,4H), 2.19 (s,12H), 2.06 (s,12H); 13C NMR (500 MHz, CDCl3, ppm): 161.016, 145.358, 140.787, 137.938, 136.773, 135.174, 132.586, 131.640, 129.680, 128.694, 127.716, 124.569, 123.235, 123.013, 41.354, 28.586, 27.975, 23.406, 23.170, 22.504; Anal. Calcd (%): C, 84.68; H, 8.96; N, 6.37; Found (%): C, 84.80; H, 8.67; N, 6.34. NiLBr2. Yield: 0.6 g; Anal.
Fig. 1. A new route to the in situ preparation of polyethylene nanocomposites.
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duced into the reactor in succession. The polymerization was controlled at certain temperature, with an ethylene pressure of 0.1 MPa for a given times and then terminated with acidified ethanol. The polymerization product was washed with ethanol several times and dried in vacuum at 60 °C for 6 h. When the supported catalyst was used, the polyethylene (PE) composite was obtained. When the unsupported catalyst was used, pure PE was obtained. 2.5. Characterization
Fig. 2. The preparation of the late-transition-metal catalyst (NiLBr2) used in this study.
Calcd (%): C, 67.82; H, 7.17; N, 5.10; Found (%): C, 67.12; H, 6.72; N, 4.92. 2.3. Activation of organoclay and its combination with NiLBr2 For the activation of the organoclay, the clay (Claytone APA, 3.0 g) was first introduced into a glass vessel equipped with a magnetic stirrer, and then 15 ml heptane was added. After that, 8 mL of AlEt3 solution (in heptane, 400 g/L) was added to the mixture at 0 °C and stirred for 20 h. Considering that AlEt3 is unstable in air, these operations were carried out under an atmosphere of nitrogen using standard Schlenk techniques. The treated organoclay was separated by decantation, washed up with heptane and dried under vacuum at 60 °C for 2 h. To combine with NiLBr2, 0.2 g of NiLBr2 was dissolved in 10 ml of dichloromethane and then added to the suspension composed of above AlEt3-activated organoclay and 15 ml of heptane, which was stirred at room temperature overnight. After decantation, the solid was washed up with heptane for several times, and then dried in vacuum. Thus, the nickel a-diimine catalyst supported on the AlEt3-activated organoclay was obtained as yellow powder. 2.4. Ethylene polymerization The polymerization was carried out in a 100 ml glass reactor equipped with a magnetic stirrer. After heating and evacuating alternately for 30 min, the reactor was filled with ethylene to a pressure of 0.1 MPa. Toluene (50 mL) and required amounts of the cocatalyst (AlEt2Cl) and the supported catalyst or unsupported catalyst were intro-
The Ni content in the supported catalyst was determined by an Inductively Coupled Plasma-atomic Emission Spectrometry (TJA Co., USA). The interlayer structure of the clay in the composites was analyzed by a Rigaku D/maxRB X-ray diffractometer (XRD) with Cu-Ka radiation (40 kV, 100 mA). For this purpose, the samples were prepared by a compression-molding method. At first, the prepared PE composite was heated at 180 °C for 5 min under 10 MPa, and then molded to the sheets at ambient temperature under the same pressure. The dispersion and morphology of the clay in the composites were investigated by a JEM-100CX II transmission electron microscopy (TEM) with an acceleration voltage of 100 kV. For this purpose, a solution method was used, in which 0.01 g of as-prepared composite was dispersed in 10 ml of toluene and then placed a drop of the dispersion onto a carboncoated TEM copper grid. Meanwhile, the compressionmolded sample was also analyzed by TEM for a comparison. In this case, the ultrathin slide was obtained by sectioning the compression-molded sample along a direction perpendicular to the compression. Thermogravimetric analysis (TGA) was carried out using Netzsch TG-209 thermogravimetric analyzer in air at a purge rate of 50 mL/min, with the scanning temperature in the range from 50 to 700 °C. To determine the clay loading, the thermogravimetric experiments for the nanocomposite samples were carried out in nitrogen, and the percentage of residual ashes was taken as the clay loading. 3. Results and discussion 3.1. Polymerization activity and clay loading For the in situ polymerization using NiLBr2 supported on the AlEt3-activated organoclay, it was found from Figs. 3–5 that the Al/Ni molar ratio, polymerization temperature and time affected the polymerization activity and the clay loading. With the increase of Al/Ni molar ratio from 150 to 900, the activity increased first and then decreased, while the opposite held true for the clay loading (Fig. 3). The highest activity was observed when the Al/Ni molar ratio is equal to 600. On one hand, the enhancement in the activity at higher Al/Ni molar ratio could be associated with greater opportunity for reducing nickel diimine complexes to form cationic, catalytically active species, and the decrease in the activity at the Al/Ni molar ratio
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Fig. 3. Effects of Al/Ni molar ratio on the polymerization activity and the clay loading (Conditions: time, 90 min; temperature, 30 °C).
of 900 may be due to the chain transfer competing with chain-propagation [20]. On the other hand, low polymerization activity resulted in the formation of small amount of polyethylene (PE), which increased the relative amount of the clay in the composites (clay loading), and high polymerization activity led to the formation of large amount of PE, which decreased the clay loading. As far as the effects of polymerization temperature are considered, increasing the temperature from 0 to 50 °C caused an increase in the activity and a decrease in the clay loading, and further increase to 70 °C caused a decrease in the activity and an increase in the clay loading (Fig. 4). In general, an elevated temperature would result in a higher chain-propagation rate that could be expected to increase the catalytic activity. However, the accelerated deactivation rate of the catalyst and the reduction of ethylene solubility in toluene at higher temperature might decrease the catalytic activity [21,22]. The combination of these effects is likely to account for the different dependence of the catalytic activity on the temperature. In addition, the polymerization activity and the clay loading decreased with the increase of polymerization time (Fig. 5). 3.2. XRD characteristics
Fig. 4. Effects of polymerization temperature on the polymerization activity and the clay loading (Conditions: Al/Ni molar ratio, 600; time, 90 min).
Fig. 5. Effects of polymerization time on the polymerization activity and the clay loading (Conditions: Al/Ni molar ratio, 600; temperature, 30 °C).
The XRD patterns in the range of 2h = 1.5–10° for original organoclay, Claytone APA, and the compressionmolded samples of PE composites with various clay loadings are shown in Fig. 6. The basal spacing of Claytone APA is determined to be 1.87 nm from the d001 diffraction peak at 2h = 4.71°. This characteristic peak disappeared completely in the XRD patterns of the samples containing respectively the clay loading of 5.05 and 8.35 wt%, suggesting that the stacking layers of the clay in these two samples were fully separated and an exfoliated nanostructure was formed. For the sample with the clay loading of 11.91 wt%, however, it is interesting to found that the XRD pattern showed a broad peak (from 5° to 7°) centered
Fig. 6. X-ray diffraction patterns of original organoclay and the PE composites with various clay loadings.
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at 6.1°, indicating a decrease rather than a increase in the basal spacing. Similar observations were reported for the nanocomposites composed of polypropylene (PP), anhydride-grafted PP and organically modified montmorillonites. Kim et al. [23] attributed this phenomenon to the transformation of interlayer structure from bilayer arrangement to monolayer arrangement, which made the surface character of the organoclay less organophilic and easier to stacking recovery. In fact, the peak at 2h = 4.7° corresponded to the basal spacing for the bilayer arrangement of alkylammonium chains, while the peak at 2h = 6.0° corresponded to that of monolayer arrangement, as confirmed by Yoon et al. [24]. According to Vaia and Giannelis [25], the broadened peak resulted from the disorderedly dispersion of organoclay in the polymer matrix after melt intercalation. It seems that the compressionmolding method of the sample for XRD measurement significantly affected the original dispersion of the clay layers in the polyethylene matrix. 3.3. Morphological structure To further confirm the dispersion states of the organoclay, TEM analysis was carried out at first for the PE composite sample with the clay loading of 11.91 wt%, which was prepared by the solution method. As shown in Fig. 7, the exfoliation of the clay into nanolayers including single clay plates was clearly observed, which were dispersed disorderedly and uniformly in the PE matrix. This verified that the exfoliated PE nanocomposite could be formed by the in situ polymerization using NiLBr2 supported on AlEt3-activated organoclay, even at a high clay loading of 11.91 wt%. For a comparison, the compres-
Fig. 8. TEM images at various magnifications for the compressionmolded sample of the PE composite with the clay loading of 11.91 wt%: (a) X10000; (b) X27000; and (c) X100000.
Fig. 7. TEM image for the PE composite sample prepared by the solution method (clay loading, 11.91 wt%).
sion-molded sample of PE composite with the clay loading of 11.91 wt% was also observed by TEM at various magnifications. From the TEM images shown in Fig. 8, it is difficult to find the exfoliated clay layers similar to those appeared in Fig. 7. On the contrary, the aggregation of the silicates layers occurred and some multiplayer stacks with a nanometer size were relatively orderly dispersed in the PE matrix, indicating that the obtained composite has
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a stacking recovery. This may be due to the fact that the elevated temperature and high pressure during the compression-molding of the sample make it easier for the clay layers to aggregate and orient along the compression direction. TEM result is consistent with the above-mentioned XRD result. Indeed, the compression method has a great influence on the original distribution of the clay layer in the PE matrix. 3.4. Thermal oxidation stability Fig. 9 illustrates the TGA curves of pure PE and the PE nanocomposites with various clay loadings. The degradation rate for pure PE becomes very quick after 350 °C, while the PE nanocomposites investigated show much slower degradation rates before 400 °C. Different from the PE nanocomposites, pure PE has almost no residues above 560 °C. When 30% weight loss was selected as a point of comparison, the thermal decomposition temperatures for pure PE and the PE nanocomposites with the respective clay loading of 5.05, 8.35 and 11.91 wt% are determined as 398.3, 427.0, 442.8 and 445.1 °C, respectively. Especially for the PE nanocomposite with the clay loading of 11.91 wt%, its decomposition temperature could be 46.8 °C higher than that of pure polyethylene. Compared with the pure PE, the PE nanocomposites investigated are characteristic of enhanced thermal oxidation stability. Moreover, the thermal property of the PE nanocomposite becomes better with the increase of the clay loading. Such behavior has already been reported for other nanocomposites composed of layered silicates and the polymer matrix such as nylon-6 [26], polystyrene [27], poly(L-lactide) [28] or polypropylene [29] filled with various types of organo-modified montmorillonites. Obviously, the molecular dispersion of the clay nanolayers in the PE matrix could contribute to the enhanced thermal stability of the exfoliated PE nanocomposites.
Fig. 9. TGA profiles for: (A) pure PE; (B) PE nanocomposite with the clay loading of 5.05 wt%; (C) PE nanocomposite with the clay loading of 8.35 wt%; and (D) PE nanocomposite with the clay loading of 11.91 wt%.
4. Conclusions The PE nanocomposites with superior thermal property could be prepared successfully by a new strategy for in situ polymerization, in which a nickel a-diimine late-transitionmetal catalyst supported on the AlEt3-activated organoclay was adopted to initiate the ethylene polymerization and to provide the reinforcement materials after the polymerization. The polymerization activity and the clay loading could be adjusted by controlling reaction time, Al/Ni molar ratio, and reaction temperature. XRD and TEM examinations provided direct evidence for the formation of the PE nanocomposite. Due to the nanometric scale reinforcement of the clay platelets in the PE matrix, the resulting nanocomposite exhibits enhanced thermal oxidation stability. Acknowledgements This work was supported by NSFC (20273086; 30470476), NSFG (021769; 039184), Department of Science and Technology of Guangdong Province (2004B33101003), and NCET Program in Universities as well as SRF for ROCS, SEM, China. References [1] Jeon HG, Jung HT, Lee SW, Hudson SD. Poly Bull 1998;41:107. [2] Zhao C, Qin H, Gong F, Feng M, Zhang S, Yang M. Polym Degrad Stab 2005;87:183. [3] Gopakumar TG, Lee JA, Kontopoulou M, Parent JS. Polymer 2002;43:5483. [4] Zanetti M, Bracco P, Costa L. Polym Degrad Stab 2004;85:657. [5] Zhao C, Feng M, Gong F, Qin H, Yang M. J Appl Polym Sci 2004;93:676. [6] Shin SYA, Simon LC, Soares JBP, Scholz G. Polymer 2003;44:5317. [7] Wang J, Liu ZY, Guo CY, Chen YJ, Wang D. Macromol Rapid Commun 2001;22:1422. [8] Alexandre M, Dubois P, Sun T, Garces JM, Jerome R. Polymer 2002;43:2123. [9] Yang F, Zhang X, Zhao H, Chen B, Huang B, Feng Z. J Appl Polym Sci 2003;89:3680. [10] Jin YH, Park HJ, Im SS, Kwak SY, Kwak S. Macromol Rapid Commun 2002;23:135. [11] Wei LM, Tang T, Huang BT. J Polym Sci A 2004;42:941. [12] Kuo SW, Huang WJ, Huang SB, Kao HC, Chang FC. Polymer 2003;44:7709. [13] Tudor J, Willington L, O’Hare D, Royan B. Chem Commun 1996:2031. [14] Heinemann J, Reichert P, Thomann R, Mulhaupt R. Macromol Rapid Commun 1999;20:423. [15] Rong J, Jing Z, Li H, Sheng M. Macromol Rapid Commun 2001;22:329. [16] He A, Hu H, Huang Y, Dong JY, Han CC. Macromol Rapid Commun 2004;25:2008. [17] Mariott WR, Chen EYX. J Am Chem Soc 2003;125:15726. [18] Ittel SD, Johnson LK. Chem Rev 2000;100:1169. [19] Salahuddin N, Shehata M. Polymer 2001;42:8379. [20] Xue XH, Yang X, Xiao YZ, Zhang QX, Wang HH. Polymer 2004;45:2877. [21] Gate DP, Svejda SA, Onate E, Killian CM, Johnson LK, White PS, et al. Macromolecules 2000;33:2320. [22] Ye ZB, Alsyouri H, Zhu SP, Lin YS. Polymer 2002;44:969. [23] Kim KN, Kim H, Lee JM. Polym Eng Sci 2001;41:1963.
F.-A. He et al. / Composites Science and Technology 67 (2007) 1727–1733 [24] [25] [26] [27]
Yoon JT, Jo WH, Lee MS, Ko MB. Polymer 2001;42:329. Vaia RA, Giannelis EP. Macromolecules 1997;30:8000. Liu L, Qi Z, Zhu X. J Appl Polym Sci 1999;71:1133. Kim TH, Lim ST, Lee CH, Choi HJ, Jhon MS. J Appl Polym Sci 2003;87:2106.
1733
[28] Paul MA, Dege’e P, Henrist C, Rulmont A, Dubois P. Polymer 2003;44:443. [29] Zanetti M, Camino G, Reichert P, Mulhaupt R. Macromol Rapid Commun 2001;22:176.