He-Xin Zhangab,
Yan-Ming Hua,
Dong-Ho Leeb,
Keun-Byoung Yoon*b and
Xue-Quan Zhang*a
aKey Lab. of Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, China. E-mail: xqzhang@ciac.ac.cn
bDepartment of Polymer Science and Engineering, Kyungpook National University, Daegu, Korea. E-mail: kbyoon@knu.ac.kr
First published on 8th March 2016
In the present article, an efficient and thermally stable vanadium (V)-based Ziegler–Natta catalyst supported on graphene oxide (GO) was synthesized. The resultant catalyst exhibited highly dispersed active sites along the surface, superior catalytic activity toward ethylene polymerization, and enhanced thermal stability, in contrast to the conventional VOCl3 catalyst. Interestingly, the resultant polyethylene (PE)/GO nanocomposite exhibited a higher thermal stability and better mechanical properties than PE obtained using VOCl3. Transmission electron microscopy reveals graphene homogeneously dispersed within the PE matrix.
Vanadium (V) catalysts have played a critical role in producing high-molecular-weight PE with a narrow molecular weight distribution, as well as ethylene/α-olefin or cyclo-olefin copolymers with high co-monomer incorporations.5 Several studies have reported that vanadium catalysts with alcohol or phenol ligands exhibited a higher activity toward olefin co-polymerization over their ligand-free counterparts.6 The hydroxyl and carboxyl functional groups of GO can serve as electron donating ligands that stabilize the valence state of V.7 This implies that well-dispersed PE/GO nanocomposites can be directly and efficiently produced by in situ olefin polymerization with a GO modified V-based Ziegler–Natta catalyst.
The morphology of pristine GO and GO-supported catalyst were examined by SEM and TEM. As shown in Fig. 1a and b, the sheet structure of the graphene is retained subsequent to anchoring of a VOCl3 catalyst. In order to investigate the distribution of V element in the resultant catalyst, an EDX analysis was performed. Fig. 1c shows the corresponding EDX elemental mapping images of C, O, V, and Cl, further revealing that elemental V is uniformly distributed in whole GO sheets. In other words, the active site was well dispersed in the GO-supported V-based catalyst. The resulting GO-supported catalyst had a V content of 1.8 wt%, as determined by ICP. TEM analysis of cast film samples at 1.0 mg mL−1 in n-hexane was performed to further characterize the structure of the obtained GO-supported catalyst. The TEM image of a typical GO-supported V-based catalyst (Fig. 1d) shows that the V was well dispersed along the surface of the GO layers.
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Fig. 1 SEM images of (a) GO and (b) GO-supported V-based catalyst; (c) elemental mapping images of GO-supported V-based catalyst; (d) TEM image of GO-supported V-based catalyst. |
The XPS analysis on GO and GO-supported V-based catalyst was conducted to investigate the surface chemical properties corresponding to the nature and reaction of the sample. The XPS spectra presented in Fig. 2a shows that the C, O, V, and Cl elements appear on GO-supported V-based catalyst, while only C and O are detected on pristine GO. The XPS data shows V peaks at 516.8 eV and 524.2 eV, Cl peaks at 197.9 eV (V–Cl bond), and 200.2 eV (C–Cl bond) for GO-supported V-based catalyst, indicating successful immobilization of the VOCl3 onto GO. This also suggests VOCl3 incorporation as opposed to surface absorption on the GO (Scheme 2).9 The C 1s peak region in the XPS spectrum of GO was deconvoluted into five peaks related to CC, C–C, C–O, C
O, and O
C–O.10 With the introduction of VOCl3, the intensity of the functional peaks drastically declined. This phenomenon could be correlated to the chemical reaction of VOCl3 with the functional group on the GO surface, once again proving the chemical incorporation of VOCl3 on the GO surface. With respect to coordination polymerization with VOCl3, the catalyst is normally activated by an EASC co-catalyst. Thus, the XPS structure of GO and GO-EASC was investigated. As given in Fig. 2a, the Al (74.7 eV) and Cl peak (198.8 eV) are observed for EASC treated GO and GO-VOCl3. Additionally, a lower count of functional groups was measured after the reaction, corresponding to the chemical reaction occurred between GO and EASC.
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Fig. 2 XPS spectra of (a) survey area of GO, GO-VOCl3, GO-EASC, and GO-VOCl3-EASC and corresponding (b) C 2s spectra; (c) Cl 2p spectrum of GO-VOCl3; (d) Raman spectra of GO and GO-EASC. |
The composition of GO, GO-VOCl3 and GO-VOCl3-EASC was further characterized by elemental analysis. As presented in Table 2, the C/O ratio in the present research is 1.2, 1.1 and 1.1 for GO, GO-VOCl3 and GO-VOCl3-EASC respectively.
The GO and GO-EASC was further studied by Raman analysis, and the spectra are given in Fig. 2d. The band appearing at 1360 cm−1 is assigned to the D-band, which originates from lattice distortion in sp2-hybridized carbon. The band observed at 1600 cm−1 is assigned to the G-band, which reflects the structural intensity of sp2-hybridized carbon atoms. The G-band represents the presence of crystalline graphitic carbon, while the D-band suggests a disordered graphite structure was present. The intensity ratio of the D and G bands (ID/IG) can be used to measure the degree of disorder of the samples.11 The ID/IG for GO was calculated to be ∼0.81 based on the data shown in Fig. 3, while the ratio for EASC-reacted GO was 0.59. After the introduction of EASC, the defects on the graphene carbon skeleton decreased, thus reducing the extent of distortion by the introduction of EASC. Dong et al.12 also observed the similar phenomenon for graphite oxide that reacted with n-BuMgCl reagent.
In Fig. 3, the XRD patterns of GO, GO-VOCl3 and GO-VOCl3-EASC are compared. The XRD pattern of GO shows a sharp, tall peak at approximately 12.1°, which is the characteristic peak of GO, corresponding to a d-spacing of approximately 0.73 nm. The larger interlayer spacing than that of graphite (∼0.34 nm) is attributable to the presence of functional groups. After treatment with VOCl3 and EASC, the intensity of the peak decreased markedly, indicating that the functional groups of GO was chemical reacted with VOCl3 and EASC.13
The performance of the synthesized GO-supported V-based catalyst in ethylene polymerization was evaluated after activation with an EASC co-catalyst and summarized in Table 1. The catalytic activities were in the range of 286–1200 kg PE per mol V per h per atm depending on the catalyst feed, polymerization time, and temperature. The catalyst with an ETCA activator performed with an activity of 743 kg per PE mol V per h per atm (run 3), which was over 2 times that of the ETCA free system (run 1). This phenomenon could be ascribed to the continuous oxidation of low oxidation state V (inactive or less active) to a higher oxidation state (highly active) by ETCA.14 Thus, detailed investigations have been carried out by employing ETCA as an activator. In general, the polymer yield increased remarkably with increasing polymerization time and catalyst feed amount. For comparison, polymerization was also conducted using VOCl3. In contrast to the VOCl3 catalyst, the GO-supported catalyst exhibited relatively higher activity on the same catalyst feed (run 4, 6, 7 vs. 8, 9, 10). Such a phenomenon can be explained by the electron-donating properties of the GO, which could increase the electron density of V and stabilize the active species. These are favorable for the coordination polymerization of ethylene, therefore resulting in a better activity. Generally, the electron-donating ligand is beneficial for the thermal stability of the transition metal catalyst. Thus, polymerization at higher temperature was also investigated.
Run | Cat. (mg) | t (min) | Temp. (°C) | Yield (g) | Activity (kg per mol V per h per atm) | GO content (wt%) | Tm (°C) | Tc (°C) | Mw (×10−4) | Mw/Mn |
---|---|---|---|---|---|---|---|---|---|---|
a Polymerization conditions: 100 mL n-hexane, EASC co-catalyst, [Al]/[V] = 50, [ETCA]/[V] = 3, 1 atm.b ETCA free.c 7 μmol VOCl3. | ||||||||||
1b | 20 | 30 | 20 | 1.2 | 343 | 1.7 | 134.4 | 120.2 | 27.2 | 2.4 |
2 | 20 | 15 | 20 | 2.1 | 1200 | 1.0 | 134.9 | 120.3 | 20.1 | 2.6 |
3 | 20 | 30 | 20 | 2.6 | 743 | 0.8 | 134.9 | 119.8 | 24.6 | 2.5 |
4 | 20 | 60 | 20 | 3.4 | 486 | 0.6 | 134.5 | 120.0 | 26.9 | 2.6 |
5 | 40 | 60 | 20 | 6.1 | 436 | 0.7 | 134.9 | 119.7 | 25.8 | 2.7 |
6 | 20 | 60 | 40 | 3.8 | 514 | 0.5 | 133.5 | 119.0 | 22.9 | 2.5 |
7 | 20 | 60 | 60 | 2.0 | 286 | 1.0 | 131.2 | 119.8 | 16.5 | 2.5 |
8c | — | 60 | 20 | 2.9 | 414 | — | 134.8 | 118.5 | 25.1 | 2.4 |
9c | — | 60 | 40 | 1.7 | 243 | — | 134.0 | 118.3 | 21.6 | 2.3 |
10c | — | 60 | 60 | 0.9 | 129 | — | 132.5 | 118.0 | 15.7 | 2.5 |
C (wt%) | O (wt%) | C/O ratio | |
---|---|---|---|
GO | 45.8 | 38.1 | 1.2 |
GO-VOCl3 | 38.5 | 36.9 | 1.1 |
GO-VOCl3-EASC | 35.4 | 32.4 | 1.1 |
Regarding the VOCl3 catalyst, catalyst deactivation was observed at elevated polymerization temperatures from 20 °C to 60 °C (runs 8–10). For GO-supported catalyst, the catalytic activity initially increased, then decreased with increasing polymerization temperature. Thus, we could conclude that the thermal stability of the VOCl3 was enhanced by the introduction of the GO support. Adjustment of the polymerization parameters produced PE/GO nanocomposites with a GO content range of 0.5–1.7 wt%.
All the resultant PE/GO nanocomposites were characterized by DSC and GPC, and the results listed in Table 1. It was found that the melting temperature (Tm) of PE/GO nanocomposites was identical to that of PE itself. In addition, the DSC traces (Fig. 4a) were generally smooth curves with relatively sharp endothermic peaks, reflecting the overall homogeneity of the matrix PE. This implied that the GO-supported VOCl3 catalyst was homogeneous in terms of active site distribution in the GO as a whole. Non-isothermal crystallization temperature, Tc, was slightly increased by the addition of GO, compared to that of pure PE, indicating that the GO nanoplatelets can act as a nucleating agent. By extracting the neat PE from their GO containing raw products, the PE was revealed to have a slightly higher Mw than the PE obtained by a VOCl3 catalyst. As expected, the Mw of the resultant PE gradually decreased with increasing polymerization temperature (both VOCl3 and GO supported V-based catalyst). This may be due to a stronger chain transfer reaction at higher polymerization temperature. The GPC curves of the PE (extracted from PE/GO nanocomposites) obtained at different polymerization temperatures are given in Fig. 4b. The GPC traces are unimodal with narrow Mw/Mn. This also suggested that the active sites of the resultant catalyst are homogeneously distributed on the GO surface.
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Fig. 4 (a) DSC and (b) GPC curves of PE/GO nanocomposites obtained at different polymerization temperatures. |
Furthermore, the morphology of the resultant PE/GO nanocomposite was investigated by TEM. Fig. 5 shows the TEM micrograph of the PE/GO nanocomposites. The relatively dark ripple lines can be observed, being the cross sections of GO layers that were in a disordered but well-dispersed state in the PE matrix.
TGA of the PE/GO nanocomposites was carried out to assess its degradation temperature and thermal stability. Fig. 6 displays TGA curves for PE (run 9) and PE/GO nanocomposites with 0.5 wt% (run 6), 1.0 wt% (run 2) and 1.7 wt% (run 1) GO. One can see that all the TGA curves exhibit a single degradation. The degradation temperature at 5 wt% loss of the PE/GO nanocomposites are 433 °C, 437 °C and 439 °C for 0.5 wt%, 1.0 wt% and 1.7 wt% GO, respectively. With a degradation temperature for 5 wt% loss of pure PE at 416 °C, the enhancement in thermal stability of PE in the presence of GO could be attributed to the high dispersion of GO, which may act as an insulator between the heat source and polymer surface where combustion occurs. Additionally, the GO layers may hinder the diffusion of volatile decomposition products within the PE/GO nanocomposites by promoting char formation.15 As shown in Fig. 6, the char yield of PE and PE/GO nanocomposites with 0.5 wt%, 1.0 wt% and 1.7 wt% GO are 0.2 wt%, 2.6 wt%, 3.7 wt% and 5.7 wt%, respectively. The char formed layer acts as a mass transport barrier that retards the escape of the volatile products generated as the PE decomposes.
The mechanical properties of PE and PE/GO (run 5) nanocomposites are presented in Table 3. The tensile strength and modulus of the resultant PE/GO nanocomposites is significantly enhanced with the incorporation of GO, even at only 0.7 wt% GO loading. Interestingly, elongation at break value of PE/GO nanocomposites was also improved. The better mechanical properties of PE/GO nanocomposites could be attributed to the well dispersion and the strong interfacial adhesion between GO and PE matrix.
Sample | Tensile strength (MPa) | Modulus (MPa) | Elongation at break (%) |
---|---|---|---|
PE | 24.5 | 670 ± 15 | 620 ± 50 |
PE/0.7 wt% GO | 27.5 | 690 ± 15 | 750 ± 50 |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00746e |
This journal is © The Royal Society of Chemistry 2016 |