Effects of GO oxidation degree on GO/BuMgCl-supported Ti-based Ziegler–Natta catalyst performance and nanocomposite properties

Abstract

In the present research, three graphene oxide (GO)/n-butyl-magnesium chloride (BuMgCl)-supported Ti-based Ziegler–Natta catalysts were synthesized by the reaction of Grignard reagent with GO at various oxidation degrees, followed by anchoring of TiCl₄. The effects of GO oxidation degree and of the reaction conditions on in situ ethylene polymerization were investigated. The obtained PE/reduced GO (rGO) product was subsequently used as a masterbatch for melt-blending with commercial PE. The resultant PE/rGO nanocomposites, even at low rGO loads, due to the good dispersion and to the good interface adhesion of the graphene sheet and the PE matrix, exhibited significant increase in heat distortion temperature, thermal decomposition temperature, tensile strength and modulus, in addition to acceptable reduction in elongation at break.

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1. Introduction

In comparison to traditional composites, polymer nanocomposites exhibit remarkable changes in some properties at very low loadings of nanofillers, such as exfoliated nano-silicate layers, carbon nanotubes, and graphite nanoplatelets.¹ However, the properties conferred by these fillers can only be achieved when they are homogeneously dispersed in the matrix and there is strong interfacial adhesion between the fillers and the polymer matrix.²

In recent years, graphene has attracted a lot of attention due to its excellent mechanical, electrical, and thermal properties.³ As a precursor to graphene, GO should be preferred over graphene in reinforcing polymers, because the functional groups on the surface of GO sheets can enhance the compatibility between GO and the polar polymer matrix. GO contains graphitic domains and oxidation regions, with the epoxide and hydroxyl groups located on the basal planes and the carbonyl and carboxyl groups at the edges.⁴ The presence of these functional groups endows GO with strong hydrophilicity and allows enhanced interactions with polar polymers. Although the preparation of graphene nanocomposites based on polar polymers has been successfully achieved,⁵ the synthesis of polyolefin/graphene nanocomposites, resulting in well-dispersed graphene fillers remained a challenge.

Polyethylene (PE) is one of the more widely used polymers, with several commercial applications, because of its excellent combination of chemical and physical properties, along with the low cost, the superior processability, and the good recyclability.⁶ Concerning PE/graphene nanocomposites, several groups, starting from graphene and graphite, attempted to produce a polyolefin-based nanocomposite, yet resulting in minor property improvement,⁷ due to the poor dispersion of fillers or the weak adhesion. Mülhaupt et al.⁸ reported a one-step mechanochemical process to synthesize PE/graphene nanocomposites; however the fillers exhibited a remarkable improvement on modulus only at very high graphene loading (5 wt%), together with significant losses of elongation at break. Ramazani S. A. et al. also reported similar results,⁹ when PE/GO was prepared by in situ polymerization with TiCl₄/Mg(OEt)₂–GO or TiCl₄/GO catalyst. The resultant PE/GO presented an improvement in Young's modulus and tensile strength, but led to ∼50% reduction in elongation at break value. Developing polymer nanocomposites with high level of stiffness and toughness balance is always a demand for high-performance plastics for engineering purposes.

In this research, three GO/BuMgCl-supported Ti-based Ziegler–Natta catalysts were synthesized with GO at various oxidation degrees. The effects of catalyst compositions and of the reaction conditions on in situ ethylene polymerization behaviours and polymer properties were investigated. The resultant PE/rGO product was subsequently used as a masterbatch for melt-blending with commercial PE. The physical properties of the composed PE/rGO nanocomposites were studied. This research provides a way to optimize the stiffness/toughness behavior of PE, by incorporating very low amounts of rGO sheets.

2. Experimental

2.1. Materials

Expanded graphite (EG) powder <100 μm, 99.9% (Timcal Graphite & Carbon, Switzerland), sulfuric acid (H₂SO₄, 95%), hydrogen chloride (HCl, 37%), hydrogen peroxide (H₂O₂, 28%, Duk San Chemical Co.), sodium nitrate (NaNO₃, Dae Jung Co.), n-butyl-magnesium chloride (BuMgCl in THF, Sigma-Aldrich, USA), triethyl-aluminum (TEA, Tosoh Akzo, Japan), titanium tetrachloride (TiCl₄, Sigma Aldrich, USA) and potassium permanganate (KMnO₄ > 99.0%, Sigma Aldrich, USA) were used as received. Polymerization grade ethylene was provided by Korea Petrochemical Ind. Co., Ltd., Korea. n-Hexane and THF were distilled from sodium/benzophenone under nitrogen atmosphere prior to use.

2.2. Preparation of GO

GO was prepared from EG powder following Hummers oxidation method.¹⁰ In accordance to the typical procedure, 4 g of expanded graphite powder and 2 g of NaNO₃ were added to 100 mL of H₂SO₄, in a 1000 mL beaker cooled down to 0 °C. 24 g of KMnO₄ were added and the suspension was stirred for 2 h. The temperature was raised to 35 °C and the mixture for stirred for another 0.5 h. After that, the solution was diluted in 500 mL of distilled water, and treated with 30% H₂O₂, to ensure the reduction of residual permanganate into soluble manganese ions. Then the mixture was centrifuged (10 000 rpm for 0.5 h) and the supernatant was decanted. The solid was washed with water followed by centrifugation using 250 mL of 10% HCl until pH 7. The solid was filtrated under vacuum and dried overnight at 40 °C. The resulting composite was named as GO1. GO2 and GO3 were prepared using the same method, modifying the amount of KMnO₄ added, to 8 g and 4 g respectively.

2.3. Synthesis of GO-supported Ziegler–Natta catalysts

BuMgCl (1.25 mol) in THF was added dropwise to a THF suspension (1.5 L) of GO (1.0 g). After 12 h reflux at 80 °C, the excess of Grignard reagent was removed by filtration, and the solid was washed five times with THF and n-hexane. The resultant GO/BuMgCl was suspended in 500 mL of n-hexane, under ultrasonic vibration for 10 min. TiCl₄ (20 mL) was added dropwise to the suspension of GO/BuMgCl at room temperature, before the temperature was increased to 80 °C, and then the mixture was stirred for 4 h. The mixture was filtered to remove the unreacted TiCl₄, and a new quantity of TiCl₄ (20 mL) was charged into the reactor. The reaction was completed after stirring for 4 h at 80 °C. The reaction mixture was filtered, and the solid was washed several times with hot n-hexane. The obtained powder was dried under vacuum at 60 °C for 3 h. Three catalysts named as GO-1, GO-2, and GO-3 were synthesized using GO1, GO2, and GO3 suspensions in THF, respectively, at the initial step of the procedure. The contents of Mg and Ti of the produced catalysts were determined by inductively coupled plasma (ICP) method.

2.4. In situ polymerization and masterbatch method

The polymerization was performed in a 300 mL glass reactor equipped with a magnetic stirring bar. The reactor was backfilled thrice with nitrogen and was charged with the required amount of n-hexane. At the stipulated temperature, the reaction solution was stirred under pressure 1 atm of ethylene for the desired period of time, and then the cocatalyst (TEA) was added to the reactor. After cocatalyst addition, the catalyst was injected, and polymerization with a continuous feed of ethylene was initiated. The ethylene pressure remained constant during polymerization, using a bubbler. After 1 h, the polymerization was terminated by adding 10% HCl/CH₃OH solution. The mixture was poured into methanol (500 mL) to precipitate the polymer and then dried under vacuum at 60 °C until constant weight. The products are named as PE/rGO masterbatches. In the final step, the PE/rGO masterbatch was blended with commercial PE using a twin-screw mixer (Plasticorder PLE331, Brabender) for 5 min at 100 rpm and temperature 190 °C.

2.5. Characterization

The Mg and Ti contents of the catalyst were determined using Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES, PerkinElmer, Optima 7300DV). The chemical structure of the catalyst was examined using Fourier Transformation Infrared Spectroscopy (FT-IR, Jasco 4100, Japan).

The melting point (T_m) of the obtained polymer was determined using differential scanning calorimetry (DSC, DSC131evo, Setaram) at a heating rate of 10 °C min⁻¹. The sample was heated to 200 °C and was maintained in the molten state for 3 min, to eliminate the influence of previous thermal history. The molten polymer was cooled to 30 °C at a rate of 10 °C min⁻¹. A second scan was performed for the determination of the melting point. Decomposition temperature analysis was conducted under nitrogen atmosphere, using a Thermogravimetric Analyzer (TGA, Setaram Labsys evo) with heating rate of 10 °C min⁻¹ from 30 to 800 °C. Tensile mechanical properties of PE and PE/rGO nanocomposites were measured using Universal Testing Machine (Instron M4465) based on ASTM D882. The sample sizes during the tensile measurements were 5.0 × 75.0 × 1.0 mm³. The sample gauge length was 20.0 mm and the crosshead speed was 10.0 mm min⁻¹. The dispersion state of rGO in the nanocomposites was studied using a scanning electron microscope (SEM, JEOL JSM-6380LV).

3. Results and discussion

Fig. 1 shows the TGA curves of pure EG and GO, which were prepared by Hummers method, using different amounts of KMnO₄. The samples were heated to 750 °C, at a heating rate of 10 °C min⁻¹ under N₂ atmosphere. Pure EG curve is linear, without any steps, indicating a uniform weight loss of the physisorbed impurities. On the contrary, GO is thermally unstable and begins to lose mass upon heating even below 100 °C, presenting three distinct steps. At the first step (<150 °C), weight loss is due to the evaporation of water; at the second step (150–300 °C), weight loss is caused by the decomposition of labile oxygen-containing functional groups (carboxylic, anhydride, or lactone groups) forming CO and/or CO₂; and at the third step (>300 °C), weight loss is attributed to the pyrolysis of the carbon skeleton of GO.¹¹ Thus, it can be assumed that the oxidation degree of GO had the following order: GO1 > GO2 > GO3, in relation to the quantity of KMnO₄ added, 24 g, 8 g, and 4 g respectively. The samples were also characterized by FT-IR, and the spectra are presented in Fig. S1.† It was found that the intensity of functional groups peaks (C=O, C–O–C, C–OH) increased significantly with the KMnO₄ content increasing.

Fig. 1 TGA curves of EG and GO.

The composition of EG and GO was further characterized by elemental analysis. As presented in Table 1, the O content of carbon materials in the present research is 0 wt%, 42.0 wt%, 35.2 wt% and 18.8 wt% for EG, GO1, GO2, and GO3 respectively. The higher oxidation degree, was demonstrated by the higher O content and the lower C/O ratio, in agreement with the TGA results. Therefore, the results clearly indicate that the degree of oxidation strongly depends on KMnO₄ concentration.

Table 1 Composition of EG and GO

	C (wt%)	H (wt%)	O (wt%)	N (wt%)	C/O ratio
EG	98.1	0.1	0.0	0.1	—
GO1	45.3	2.4	42.0	0.3	1.4
GO2	50.6	2.0	35.2	0.2	1.9
GO3	70.7	1.2	18.8	0.2	5.0

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According to the literature,¹² the GO bearing hydroxyl, epoxide, and carboxyl polar groups can be efficiently treated with the Grignard reagent (BuMgCl), without any surface modification, in order to synthesize the GO/BuMgCl-supported Ti-based Ziegler–Natta catalyst. After the reaction with Grignard reagent, considerable amount of small graphitic domains are formed through –O–Mg–Cl bonds. In other words, the GO is reduced during the reaction with BuMgCl.¹³ Therefore, the PE/rGO masterbatches could be directly obtained through in situ ethylene polymerization with GO/BuMgCl-supported Ti-based Ziegler–Natta catalyst.¹³ After anchoring of TiCl₄, the catalysts composition was studied using ICP-MS and the results are shown in Table 2. The contents of Mg and Ti of GO-1, GO-2 and GO-3 catalysts were 12.6, 3.5, 2.2 wt% and 16.1, 6.5, 4.0 wt%, respectively. The contents of the Mg supported to the GO gradually increased, following the increase of GO functional groups. This could be a result of the increased oxygen-containing functional groups, providing more reaction sites for Grignard reagent (BuMgCl). Simultaneously, the increase of Mg content provides more active sites for TiCl₄ anchoring.

Table 2 Mg and Ti Contents of GO-1, GO-2 and GO-3 catalysts

	Mg content (wt%)	Ti content (wt%)
GO-1 catalyst	12.6	16.1
GO-2 catalyst	3.5	6.5
GO-3 catalyst	2.2	4.0

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The behaviour of GO/BuMgCl-supported catalysts during ethylene polymerization was evaluated, after activation with TEA cocatalyst. GO-2 catalyst, with medium GO oxidation degree, exhibits the highest catalyst activity. GO-1 and GO-3 catalysts, show similar activity. As shown in Fig. 2a, the catalyst activity significantly increased with increasing polymerization temperature to 40 °C. However, further increase of the polymerization temperature leads to an enormous decrease in activity. This can be attributed to the reduced solubility of the ethylene monomer at higher polymerization temperature. The effect of cocatalyst concentration ([Al]/[Ti]) on the catalyst activity was also studied under polymerization temperature of 40 °C. As presented in Fig. 2b, the catalyst activity significantly increased with cocatalyst concentration increasing to [Al]/[Ti] = 100, while further increase in cocatalyst concentration to [Al]/[Ti] = 400 exhibits almost no effect on the catalyst activity. Thus, polymerization temperature of 40 °C and [Al]/[Ti] = 100 were used to scale up the synthesis of PE/rGO masterbatch. The total volume of n-hexane required is 500 mL and the required polymerization time is 10 min. Three PE/rGO masterbatches were synthesized containing 6.0 wt%, 6.5 wt% and 7.0 wt% of GO1, GO2 and GO3, respectively.

Fig. 2 Effects on catalyst activity of (a) temperature, under standard cocatalyst concentration ([Al]/[Ti] = 100) and (b) cocatalyst concentration [Al]/[Ti], under standard temperature 40 °C (polymerization conditions: 40 mg catalyst, 1 atm ethylene, 1 h).

The PE/rGO masterbatches were subsequently melt-blended with commercial PE to fabricate the PE/rGO nanocomposites with rGO loadings of 0.1 wt%, 0.5 wt% and 1.0 wt%. For comparison, PE nanocomposites containing dispersed EG were also prepared with the same method. The effect of rGO and EG on the crystallization of PE was investigated using DSC and the typical DSC curves are given in Fig. 3. As shown in Table 3, the T_m of pure PE was 135.4 °C. Addition of EG or rGO, the T_m almost not changed, while the X_c slight decreased for 1.0 wt% addition and a significantly increases in X_c for PE/rGO nanocomposites were observed. Non-isothermal crystallization temperature, T_c was slightly increased by the addition of EG or rGO, compared to that of pure PE, indicating that the EG or rGO nanoplatelets can act as a nucleating agent. However, there was no significant difference in T_m, T_c, and X_c between PE/rGO-1, PE/rGO-2 and PE/rGO-3. Liu et al.¹⁴ and Shevchenko et al.¹⁵ have also reported similar behavior for PE/graphite oxide and PP/graphene nanocomposites prepared by in situ polymerization with metallocene catalyst. The overall homogeneity of the PE/rGO nanocomposites is also reflected by the DSC traces where smooth curves with relatively sharp endothermic peaks were recorded.

Fig. 3 DSC (a) cooling and (b) heating curves of PE, PE/0.1 wt% EG, PE/0.1 wt% rGO-1, PE/0.1 wt% rGO-2 and PE/0.1 wt% rGO-3 nanocomposites.

Table 3 Thermal properties of the PE/EG and PE/rGO nanocomposites at various contents

Sample		EG or rGO (wt%)	Tc (°C)	Tm (°C)	Xc (°C)	Td5% (°C)	Char yield (wt%)
Virgin PE		—	119.4	135.4	67.3	345.2	0
PE/EG		0.1	119.5	135.2	67.0	369.5	5.3
		0.5	120.2	134.9	67.4	381.4	6.3
		1.0	120.4	134.9	64.5	377.3	10.4
PE/rGO-1		0.1	120.9	135.8	75.5	374.3	1.1
		0.5	121.3	135.5	77.6	389.5	1.9
		1.0	120.7	135.9	75.2	396.0	6.3
PE/rGO-2		0.1	120.6	136.0	76.0	371.9	2.6
		0.5	121.1	135.7	74.3	387.4	7.1
		1.0	121.6	135.6	72.2	403.9	12.1
PE/rGO-3		0.1	120.8	135.5	73.5	368.1	1.3
		0.5	121.2	135.7	73.8	388.2	2.5
		1.0	121.5	135.3	70.8	390.9	7.1

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Heat distortion temperature is an important value with regard to the temperature resistance of plastics. As shown in Table 4, compared to virgin PE, the higher heat distortion temperatures of the PE/rGO nanocomposites can be observed. The increase in the heat distortion temperature could be ascribed to the increased modulus of PE/rGO nanocomposites at high temperatures.

Table 4 Effect of rGO-2 contents on the heat distortion temperature of PE/rGO nanocomposites

	rGO-2 content (wt%)	Heat distortion temperature (°C)
Virgin PE	—	77.7
PE/rGO-2	0.1	78.5
	0.5	83.6
	1.0	85.1

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Thermal stability is another very important parameter for polymeric materials as it is usually the limiting factor during both processing and end-use applications. Therefore, thermal degradation of pure PE, PE/EG, and PE/rGO nanocomposites with different weight fractions of EG and rGO was investigated by TGA, under nitrogen atmosphere. The 5% weight loss temperatures (T_d5%) and the char yields (%) were obtained from Fig. 4, and the results are summarized in Table 3. TGA curves suggest that there is a single degradation process. As shown in Fig. 4, compared to pure PE, the degradation temperatures of PE/rGO nanocomposites, are monotonously shifted up to higher temperatures with increasing filler loading, inferring a significant improvement of the thermal oxidation stability of PE. With regards to degradation temperature at 5 wt% loss of PE/EG and PE/rGO nanocomposites, the addition of 1.0 wt% EG or rGO results to an enhance of 32.1 °C, 50.8 °C, 58.7 °C, and 45.7 °C for EG, rGO-1, rGO-2 and rGO-3, respectively, compared to pure PE. The improvement of the thermal stability of PE in the presence of rGO could be attributed to the well dispersion of rGO in the PE matrix, which may act as an insulator between the heat source and the surface area of the polymer, where the combustion occurs. Additionally, rGO layers may hinder the diffusion of volatile decomposition products within the PE/rGO nanocomposites by promoting char formation.¹⁶ As presented in Table 3, the char yield of PE/rGO nanocomposites containing 1.0 wt% rGO was 6.3 wt%, 12.1 wt% and 7.1 wt% for rGO-1, rGO-2, and rGO-3 respectively, while the char yield for PE/EG nanocomposites containing 1.0 wt% EG was 10.4 wt%. The char formed layer acts as a mass transport barrier, that retards the escape of the volatile products generated, as PE decomposes.¹⁷ The char yields of the nanocomposites increase with increasing contents of rGO, due to rGO's planet sheet structure that can promote the carbonization on the polymer's surface. Furthermore, unburned filler together with the high heat resistance exerted by the filler, also contribute to higher char residues.

Fig. 4 TGA curves of (a) PE/EG, (b) PE/rGO-1, (c) PE/rGO-2 and (d) PE/rGO-3 nanocomposites, with various contents of rGO.

In order to investigate the dispersion of rGO sheet in PE matrix, the PE/rGO nanocomposites were further hot-pressed to films. As shown in Fig. 5(a), (d), (f) and (h), rGO was highly compatible with PE matrix, so that the entire surface seemed flat and homogeneous without any aggregates for rGO at the lower loading (0.1 wt%); quite similar to the surface of pure PE film. At higher rGO loading (1.0 wt%), the aggregations of rGO gradually become visible in the film surface. The aggregation size of rGO was in the following order: rGO-3 > rGO-2 > rGO-1. Thus, the higher degree of oxidation leads to a more homogeneous dispersion of rGO in PE matrix. However, when the filler selected is EG, a significant aggregation was observed, even at the lower EG loading (0.1 wt%). As a result, PE/rGO nanocomposites are expected to exhibit better mechanical properties than PE/EG nanocomposites.

Fig. 5 Optical micrographs of (a) PE, (b) PE/EG 0.1 wt%, (c) PE/EG 1.0 wt%, (d) PE/rGO-1 0.1 wt%, (e) PE/rGO-1 1.0 wt%, (f) PE/rGO-2 0.1 wt%, (g) PE/rGO-2 1.0 wt%, (h) PE/rGO-3 0.1 wt%, and (i) PE/rGO-31.0 wt% nanocomposites.

The mechanical properties of PE/rGO nanocomposites as a function of EG and rGO loading are presented in Fig. 6 and Table 5. The tensile strength and modulus of the resultant PE nanocomposites is significantly enhanced with the incorporation of EG and rGO, even at very low filler loading. In addition, PE/rGO nanocomposites present higher tensile strength and modulus than PE/EG nanocomposites, at the same filler loading. Feeding of EG filler reduces dramatically the elongation at break value, up to ∼82% and ∼98% after loading 0.1 wt% and 1.0 wt% of EG respectively. Interestingly, elongation at break value of PE/rGO-1 and PE/rGO-2 nanocomposites, was only slightly reduced for 0.1 wt% rGO loading. However, further increase of rGO loading leads to a reduction in elongation at break value. The degree of reduction in elongation at break value does correlate with GO oxidation degree that decreased in the following order: rGO-3 > rGO-2 > rGO-1, suggesting that the higher oxidation degree results to well dispersion of rGO in the PE matrix. It is not surprising that rGO tends to form larger aggregates at lower degree of oxidation (Fig. 5e, g, and i). The above results indicate that PE/rGO nanocomposites obtained by combined in situ polymerization with masterbatch method present a remarkable stiffness–toughness balance.

Fig. 6 Effect of filler (EG and rGO) content on (a) tensile strength, (b) modulus, and (c) elongation at break.

Table 5 Mechanical properties of the PE/EG and PE/rGO composites at various EG and rGO contents

Sample	EG or rGO (wt%)	Tensile strength (MPa)	Modulus (MPa)	Elongation at break (%)
Virgin PE	—	30.0	590 ± 15	1800 ± 70
PE/EG	0.1	30.3	640 ± 15	330 ± 45
	0.5	31.3	690 ± 15	50 ± 20
	1.0	30.5	690 ± 30	40 ± 20
PE/rGO-1	0.1	31.5	710 ± 10	1600 ± 60
	0.5	31.6	710 ± 25	1500 ± 55
	1.0	32.0	700 ± 30	1400 ± 70
PE/rGO-2	0.1	32.0	720 ± 25	1600 ± 60
	0.5	32.4	720 ± 25	1300 ± 75
	1.0	32.6	730 ± 20	1200 ± 50
PE/rGO-3	0.1	32.0	700 ± 20	1100 ± 70
	0.5	31.7	720 ± 25	400 ± 55
	1.0	31.8	740 ± 20	200 ± 45

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Further clarification of the reinforcement mechanism of the fillers to the polymer matrix, was achieved by the observation of the micro-morphology of fracture surface of the samples through SEM images. As shown in Fig. 7, the surface of pure PE (Fig. 7a) is relatively smooth and flat, and becomes rougher with the loading of EG (Fig. 7b and c). Moreover, numerous and large aggregated EG sheet layers, can be observed in the fracture surface with the increase of EG content in PE/EG nanocomposites, indicating poor dispersion of EG in the PE/EG nanocomposites. On the contrary, the surfaces of PE/rGO nanocomposites are smooth and flat like pure PE and no aggregation of the rGO sheet could be observed. Thus, the PE/rGO nanocomposites exhibited a significant improvement in mechanical properties. It is concluded that the excellent physical properties of PE/rGO nanocomposites could be attributed to the well dispersion and the strong interfacial adhesion between rGO and PE matrix.

Fig. 7 SEM images of (a) PE, (b) PE/EG 0.1 wt%, (c) PE/EG 1.0 wt%, (d) PE/rGO-1 0.1 wt%, (e) PE/rGO-1 1.0 wt%, (f) PE/rGO-2 0.1 wt%, (g) PE/rGO-2 1.0 wt% (h) PE/rGO-3 0.1 wt%, and (i) PE/rGO-31.0 wt% nanocomposites.

4. Conclusions

PE/rGO nanocomposites with well-dispersed rGO nanofillers were successfully fabricated through the combination of in situ polymerization with masterbatch method. Among the catalysts, GO-2/BuMgCl-supported catalyst with medium oxidation degree exhibits the highest catalyst activity towards ethylene polymerization. The incorporated rGO sheets can act as a nucleating agent in the PE/rGO nanocomposites, proved by a higher non-isothermal crystallization temperature and the crystallization degree. After blending with commercial PE, the resultant PE/rGO nanocomposites exhibited a remarkable improvement in tensile strength and modulus, together with slight reduction on elongation at break value, even at very low amount of rGO loading (0.1 wt%). The thermal oxidative stability of PE is significantly improved by the addition of rGO sheets due to the barrier effect of rGO's planet sheet structure. Thus, the present work provides way for scaled up production of high performance PE with good thermal stability, stiffness and toughness balance.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (NRF-2015R1D1A1A0161012). The authors would also like to acknowledge the financial support from Natural Science Foundation of China (No. U1462124). Dr H. X. Zhang would like to thank the China Scholarship Council (201504910334) for the financial support on his visit at KNU.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00141f

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