Structural and Morphological Evolution of Nascent Polyethylene

Jul 26, 2016 - Fe3O4@SiO2 with 3D core–shell nanoparticles were used to immobilize Fe(acac)3/2,6-bis(1-(2-isopropylanilinoethyl)) in order to polyme...
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Structural and Morphological Evolution of Nascent Polyethylene during Ethylene in Situ Polymerization within Fe3O4@SiO2 Nanoparticles Wei Li,* Huaqin Yang, Mengying Shang, Tao Chen, and Wenqin Wang Department of Polymer Science and Engineering, School of Material Science and Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, P. R. China S Supporting Information *

ABSTRACT: Fe3O4@SiO2 with 3D core−shell nanoparticles were used to immobilize Fe(acac)3/2,6-bis(1-(2-isopropylanilinoethyl)) in order to polymerize ethylene in a confined geometry. Structural and morphological evolution of nascent polyethylene is particularly studied. The dependence of molecular weight, molecular weight distribution, condensed structure, and dispersion of nanoparticles on the polymerization time are investigated quantitatively. Both the molecular weight and the molecular weight distribution of synthesized polymers gradually increase as the polymerization proceeds. It is found that the nascent polymer contains a monoclinic phase and a weakly entangled state due to the confined polymerization environment and the low temperature of polymerization. This is the first time that the monoclinic phase is observed in the nascent polyethylene with Mw less than 1 × 105 g/mol. Magnetic property of the polyethylene is presented, as is expected. Finally, fragmentation of Fe3O4@SiO2 nanoparticles during the polymerization is discussed.

1. INTRODUCTION Olefin polymerization catalysts can be processed by being dispersed on solid surfaces (heterogeneous catalysts) or dissolved in a reaction solvent (homogeneous catalysts).1 Heterogeneous catalysts are widely used to prepare polyethylene and polyethylene nanocomposites in industry and academia.2 Polymerization kinetics, particle morphology, and dispersion of support depend greatly on the polymerization process.3 The polymerization process of heterogeneous catalysts can be separated into three clearly distinguishable levels, such as the micro-, meso-, and macroscale, which corresponds to the growing polymer particles, particle aggregation, and reactor scale.4 The most important processes are operating on the microscale level, meaning inside and at the surface of growing polymer particles.4,5 At this level, monomer transfer, chain propagation, and chain crystallization occur simultaneously. This will determine the structure and property of nascent polyethylene.5 In situ polymerization is one of the examples of heterogeneous polymerization. A catalyst for ethylene polymerization can be introduced into the pore and/or the interspace of the nanoparticles by anchoring with functional groups.6,7 Nanoparticles are then dispersed directly into a reaction media during ethylene in situ polymerization. It is an effective way to achieve uniformed dispersion of nanofiller in the polyethylene matrix.8,9 It shall be mentioned that the confined geometry of nanofiller will make a great difference on the monomer transfer, chain propagation, and crystallization behavior, compared with that of a commercially porous silica support.2,3 Thus, it is of utmost importance to study the © 2016 American Chemical Society

structural and morphological evolution of nascent polyethylene occurring in this confined environment. Ren et al.10 used multifunctional montmorillonites (MMT) with a 2D-layer structure to support a metallocene catalyst. An intercalative polymerization occurred in the interspace of the MMT monolayers. A monoclinic crystal was identified in the nascent polymer due to the confined environment of chain growth within the interspace of MMT. However, the structure of the monoclinic phase could not be studied in detail due to its low fraction. Osichow et al.11 and Zhang et al.12 used a watersoluble catalyst (Ni based catalyst) to polymerize ethylene in aqueous emulsion. The catalyst became lipophilic upon activation for polymerization. Polyethylene chain growth resulted in immediate formation of a surfactant-stabilized particle. The crystallization of the growing chains will be confined in each of the surfactant-stabilized particles, hindering the chain overlap behaviors. The nascent polymer presented a fully disentangled state. 10% of monoclinic phase was present in the polymers.11 In this work, SiO2 coated Fe3O4 nanoparticles (Fe3O4@ SiO2) are synthesized and then used to immobilize Fe(acac)3/ 2,6-bis(1-(2-isopropylanilinoethyl)). The Fe(acac)3/2,6-bis(1(2-isopropylanilinoethyl)) is chosen because of its considerable activity at low polymerization temperature.13,14 We aim to study the structural and morphological evolution of the nascent Received: Revised: Accepted: Published: 8719

April 6, 2016 June 30, 2016 July 26, 2016 July 26, 2016 DOI: 10.1021/acs.iecr.6b01312 Ind. Eng. Chem. Res. 2016, 55, 8719−8725

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Industrial & Engineering Chemistry Research

amounts of MAO. Twenty mg of catalyst was mixed with 10 mL of toluene. Then, the Cat-Fe/Fe3O4@SiO2 was introduced into the reactor under the protection of nitrogen. The polymerization took place under a continuous ethylene flow to meet 10 bar at a stirring rate of 300 rpm. After the desired time, the polymerization was quenched with ethanol. The obtained polymer was precipitated and washed with ethanol, filtered, and dried at 50 °C under vacuum. 2.5. Characterization and Analysis. Average molecular weight (Mw) and molecular weight distribution (MWD) were determined using gel permeation chromatography (GPC) at 150 °C in a PL-GPC-220 instrument (Polymer Laboratories, UK), with 1,2,5-trichlorobenzene as solvent. The calibration was made by polystyrene. Differential scanning calorimetry (DSC) was performed with a DSC-7 instrument (PerkinElmer Corp., USA) to measure the melting point and crystallinity of nascent polyethylene. Samples (ca. 8 mg) were first heated to 160 °C at a rate of 10 °C/min and then cooled to 50 °C at the same rate. The second heating cycle was used to acquire DSC thermograms at a heating rate of 10 °C/min. Melting temperatures were taken at the peak of the endotherm. Crystallinity was calculated by comparison with the heat of fusion of a perfectly crystalline polyethylene, i.e., 289 J/ g. Morphology of polymers and Fe3O4@SiO2 were monitored by employing a scanning electron microscope (SEM, Hitachi S4700, Japan) and transmission electron microscope (TEM, Tecnai F20, USA), respectively. The polymers were sputtercoated with Pd before the measurement with SEM. The polymers were first embedded and then cut into trivially before the measurement with TEM. All the characterizations were made at room temperature. For Brunauer−Emmett−Teller (BET) analysis, the specific surface areas of Fe3O4@SiO2 and Fe3O4 were conducted using BET instrument ASAP2020-HD88 (Micromeritics Instrument Corp. USA). The data were obtained by the BET method using BELSORP analysis software. The surface areas of Fe3O4 and Fe3O4@SiO2 were calculated as 15.78 and 7.86 m2/g, respectively. Measurement of the Fe and Si content in the Fe3O4@SiO2 and Cat-Fe/Fe3O4@SiO2 was conducted using inductively coupled plasma elemental analysis with a Varian 730-ES, USA. Ten mg of the support was dissolved in 20 mL of pure hydrofluoric acid for 3 days. The amount of Fe and Si in the Fe3O4@SiO2 was 1.21 and 43.96 wt %, respectively, while the amount of Fe in the Cat-Fe/Fe3O4@SiO2 was 1.92 wt %. Thus, the amount of active Fe (Cat-Fe) in the catalyst was calculated as 0.71 wt %. Thermal gravimetric analyses (TGA) was conducted on a SDT 2960 Simultaneous TG-DTA Instruments under N2 flow (100 mL/min) with a heating rate of 10 °C/min. Rheological studies were performed on a strain-controlled rheometer, HAAKE III instrument. A disk of 8 mm diameter was compressed under 20 MPa at 120 °C for 30 min and was used in all rheological studies.16,17 The disk between the parallel plates of the rheometer was heated to 170 °C under nitrogen atmosphere. After waiting for the thermal stabilization at 170 °C (∼5 min), the rheology experiments were initiated. A dynamic time sweep test was performed to follow the entanglement formation at a fixed frequency of 1 rad/s and strain in the linear viscoelastic regime of the polymer. X-ray diffraction (XRD) measurements were carried out on a Bruker GADDS diffractometer with an area detector operating

polyethylene in the confined environment (generated by the core−shell Fe3O4@SiO2 nanoparticles). The fragmentation behavior of the Fe3O4@SiO2 nanoparticles is discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. All the manipulations of air and moisture sensitive products were carried out under a dry nitrogen atmosphere using standard Schlenk line techniques or glovebox. All solvents were purified prior to use. FeCl3·6H2O, trisodium citrate, NaAc, ethanol, and tetraethyl orthosilicate were purchased from J&K Company (Shanghai, P.R. China) and were used without further purification. 2,6-Bis[1-(2isopropylanilinoethyl)] pyridine was synthesized according to our previous work.14 Fe(acac)3 was purchased from J&K Company. Polymerization-grade ethylene and nitrogen were obtained from Fangxin Corp. (Ningbo, China) and purified by filtering through Mn zeolite and subsequent zeolite of 5 Å. Methylaluminoxane (MAO, 10 wt % solution in toluene) was purchased from Albemarle Chemical Inc. (USA). Toluene (Ningbo Chemical Reagents Co., China) was purified over sodium/benzophenone ketyl and distilled prior to use. 2.2. Preparation of Fe3O4 and SiO2 Coated Fe3O4 (Fe3O4@SiO2).15 Typically, FeCl3·6H2O (1.08 g, 4.0 mmol) and trisodium citrate (0.20 g, 0.68 mmol) were first dissolved in ethylene glycol (20 mL), and afterward, NaAc (1.20 g) was added under constant stirring. The mixture was stirred vigorously for 30 min and then sealed in a Teflon-lined stainless-steel autoclave (50 mL capacity). The autoclave was heated at 200 °C and maintained for 10 h and then allowed to cool to room temperature. The solid was washed with ethanol and distilled water for several times. The synthesized Fe3O4 (0.1 g) was mixed with ethanol (40 mL) and distilled water (8 mL). The mixture was sonicated for 10 min at room temperature. Then, ammonia (2 mL) and tetraethyl orthosilicate (1 mL) were added. This mixture was refluxed at 30 °C for 3 h. During the reflux, the mixture was sonicated 2 min every 15 min at room temperature. Afterward, the product was washed 5 times with ethanol and then was absorbed by a magnet. Fe3O4@SiO2 particles were obtained after the products were dried under vacuum. 2.3. Preparation of Fe3O4@SiO2 Combined with Fe(acac)3/2,6-bis[1-(2-isopropylanilinoethyl)] Pyridine Catalyst. Fe(acac)3 bearing 2,6-bis[1-(2isopropylanilinoethyl)]pyridine ligand was synthesized in a molar ratio of 1:1. The compounds were mixed at room temperature to achieve homogeneous Cat-Fe catalyst with a concentration of 0.021 mmol/L in toluene. The synthesized 1.0 g of Fe3O4@SiO2 was mixed with 30 mL of toluene. Then, the mixture was sonicated at room temperature for 30 min. Subsequently, 10 mL of homogeneous Cat-Fe catalyst was added into the Fe3O4@SiO2/toluene mixture drop by drop. The mixture was slowly stirred for 24 h at room temperature to allow the active species to evenly diffuse into the interspace of nanoparticles. The mixture was dried under vacuum until the solid was obtained. The catalyst was named Cat-Fe/Fe3O4@ SiO2. 2.4. Ethylene Polymerization. Slurry ethylene polymerization was carried out in a 100 mL stainless steel autoclave reactor, equipped with a magnetic stirrer and oil bath. The reactor was heated to 120 °C for 2 h and repeatedly pressurized with nitrogen, purged, and evacuated before polymerization. The reaction temperature was set to 30 °C. The reactor was charged with 30 mL of toluene and injected with appropriate 8720

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chains after a long time of polymerization. These growing chains can be crystallized which will generate a strong monomer-transfer resistance, accelerating the deactivation of the catalyst.19,20 Furthermore, the loading of Fe3O4@SiO2 in the polymer matrix decreases at longer time of polymerization. It offers a simple way to control the magnetic filler loading inside the polymer matrix. 3.3. Evolution of Polymer Microstructure. Table 1 shows the melting point and crystallinity of the synthesized polymer measured by DSC. The DSC curves are shown in Figure S2. The polymer shows a high melting point (around 130 °C), indicating a linear structure.14 The crystallinity and melting point of the synthesized polymer slightly increase with the increment of polymerization time. The gradually decreased loading of the Fe3O4@SiO2 may be suitable for chain folding, increasing the crystallinity.16 Furthermore, the melting point and crystallinity of the second heating scan are slightly higher than those of the first scan. This indicates that the crystallinity can be increased through the melting and recrystallizing procedure.10 Table 1 shows the molecular weight and molecular weight distribution of the synthesized polymers. GPC curves are shown in Figure S3. The synthesized polymers have a broad MWD. This is due to the multiple active centers of Cat-Fe.13,14 This broad MWD may lead to a better balance between the mechanical and rheological properties, resulting in an improvement of processability of final products.20 The Mw and MWD are increased obviously with growing polymerization time. A larger fraction of the high Mw part is synthesized at longer time of polymerization. This is due to the fact that the confined environment within the core−shell support does generate more resistance to monomer diffusion. The chain termination procedure can be hindered.3,13 Further discussion of the Mw and MWD will be presented in the following section. 3.4. Evolution of Condensed Structure. 3.4.1. XRD Characterization. Figure 2 shows XRD patterns of polyethylene and SiO2@Fe3O4. It can be found that the pure SiO2@ Fe3O4 has a typical reflection at 25.2° and 35.5°. The peak at 25.2° corresponds to the characteristic peak of the silica, and the shoulder peak at 35.5° is the characteristic reflection of Fe3O4.18 The XRD curves of polyethylene nanocomposites in Figure 2a show that more characteristic peaks can be observed besides orthorhombic and amorphous phases. In total, the XRD curves can be decomposed into six peaks which are shown in Figure 2b. Details about the analysis are given in Figure S4. The reflection at 21.6° and 24.0° corresponds to the orthorhombic unit cell structure of the (110) and (200) reflection planes of the polyethylene. The reflection at 19.5° with narrow width, 23.2°, and 25.2° corresponds to the monoclinic unit cell structure of the (010), (200), and (210) reflection planes,

under 40 kV and 40 mA, using Cu Kα radiation (λ = 0.154 nm). Nascent polymers were used for the measurement. The XRD curves were decomposed into six components using Origin 8.5 software. Lorentz functions were used for fitting the curves. Magnetic property measurements of the polymer were carried out in a 9 T physical properties measurement system (PPMS) by Quantum Design at 30 °C.

3. RESULTS AND DISCUSSION 3.1. Morphology of Fe3O4@SiO2 Support. Figure 1 shows SEM and TEM images of Fe3O4 and Fe3O4@SiO2

Figure 1. SEM and TEM images of Fe3O4 and Fe3O4@SiO2. (a, b) The SEM and TEM morphology of Fe3O4, respectively. (c, d) The SEM and TEM morphology of Fe3O4@SiO2, respectively.

support. Figure 1a shows that the surface of pure Fe3O4 is rough. However, the surface of Fe3O4@SiO2 becomes smooth (Figure 1c). A clearly spherical morphology is presented in the Fe3O4@SiO2 support which indicates that the silica is coated on the surface of Fe3O4. The morphology of the particles was further investigated by TEM (Figure 1b,d). It is shown that the Fe3O4@SiO2 particles can achieve core−shell structure with several hundred nanometers of diameter. The obviously decreased specific surface area and smooth surface of Fe3O4@SiO2 compared with that of Fe3O4 indicates that the SiO2 was coated more compactly on the Fe3O4.18 The Fouriertransform infrared spectroscopy and particle size distribution of the Fe3O4 and Fe3O4@SiO2 are further shown in the the discussion of Figure S1. 3.2. Ethylene in Situ Polymerization. Table 1 summarizes the results of ethylene polymerization with Cat-Fe/ Fe3O4@SiO2. The catalysts show a high activity in the initial stage of polymerization, and fast deactivation occurs in the following stage. Active centers can be covered by growing

Table 1. Ethylene Polymerization Results of Cat-Fe/Fe3O4@SiO2a melting point run

time, min

yield, g

1 2 3 4 5

2 5 10 20 30

0.78 1.21 1.64 1.84 1.96

−1

activity, 10 gPE·mol[Fe] ·h 6

6.2 3.8 2.6 1.5 1.0

−1

Tm , °C

Tm , °C

129.4 130.6 131.0 131.1 131.5

131.6 133.1 133.9 133.5 134.1

1

2

crystallinity Xc1,

%

51.8 54.4 55.7 57.8 59.2

Xc2, %

Mw, 104 g/mol

MWD

loss weight, wt %b

52.6 55.3 57.3 59.8 61.5

4.7 6.1 8.1 11.2 16.1

9.5 11.6 12.2 14.5 18.7

3.7 2.4 2.0 1.8 1.5

Polymerization conditions: 30 ± 3 mg of catalyst, 10 bar, 30 °C, 1 mL of MAO, 50 mL of toluene. bThe loading of Fe3O4@SiO2 was determined by TGA analysis with the heating temperature ranging from 50 to 600 °C and a heating rate of 10 °C/min. a

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Figure 2. XRD patterns of polyethylene and SiO2@Fe3O4. (a) The XRD pattern of polymers. (b) A typical fitting curve of XRD data.

Table 2. Fitting Results of XRD Curves monoclinic

a

1

samples

crystallinity Xc, DSC , %

orthorhombic Xo, XRD, %

monoclinic Xm, XRD, %

crystallinity Xc, XRD, %

n25.3°, nm

n23.4°, nm

n23.4°, nm

PE-2 min PE-5 min PE-10 min PE-20 min PE-30 min Fe3O4@ SiO2

51.8 54.4 55.7 57.8 59.2

68.9 74.6 75.3 76.1 76.8

18.8a 11.6 11.7 12.1 12.5

87.7a 86.2 87.0 88.2 89.3

23.0a 16.7 14.7 18.6 22.9 28.9

10.2 10.4 11.2 11.4 12.1

10.7 11.8 15.0 18.3 21.7

These values contain the contribution of SiO2 from the catalyst.

Figure 3. (a) Dynamic time sweep test at 170 °C for PE and PE nanocomposites at a constant frequency of 10 rad/s and strain of 1%. GNt is the actual storage modulus normalized by G′max (maximum plateau modulus in the modulus buildup curves). (b) The normalized data of (a).

that of the chain growing rate at this temperature. This will allow the polymer chain to crystallize as soon as they are growing out.16,17 The confined environment of polymerization in the core−shell nanoparticles will impose strong stress on the growing chains during the fragmentation of support, thus enhancing the formation of a metastable crystalline phase.10,22 Gradually decreased catalyst activity will further favor the formation of the metastable crystalline phase at low temperature.16 Therefore, a gradually increased concentration and lattice sizes of the monoclinic phase are observed in the samples synthesized at longer time of polymerization (see Table 2). It shall be mentioned that the reflection peak of SiO2 at 25.2° is overlapped with the reflection peak of the (210) plane of the monoclinic phase. This makes the concentration and lattice size of “PE-2 min” obviously higher than those of the other samples. It also shows an opposite tendency with the DSC results. However, this influence is diminished as the

respectively. The reflection at 19.5° with broad width corresponds to the amorphous phase.10,11 The lattice parameters reported for the orthorhombic and monoclinic phase are as follows: the orthorhombic phase has a cell structure of a = 7.40 Å, b = 4.94 Å, c = 2.53 Å, and β = 90.0°, while the monoclinic phase has a cell structure of a = 8.09 Å, b = 2.53 Å, c = 4.79 Å, and β = 107.0°.21 Up to this study, the monoclinic PE has only been observed after mechanical treatment or with high molecular weights.11,21 In our work, the synthesized polyethylene still shows a considerable amount of the monoclinic phase (i.e., 11.6%; see Table 2), although its Mw is less than 105 g/mol and its MWD is broad. To the best of the authors’ knowledge, this is not reported yet for a comparable system. The formation of the monoclinic phase may be due to the low temperature polymerization conditions and confined environment of support.10,11 The chain crystallization rate will be faster than 8722

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Figure 4. Morphologies of polyethylene obtained at different polymerization times: (a) 2 min, (b) 5 min, (c) 10 min, (d) 20 min, and (e) 30 min.

are synthesized at longer time of polymerization which indicates that the environments for chain propagation become more confined. Figure 5 shows TEM images of the synthesized polymers. It can be seen from Figure 5a,b that an obvious aggregation of

polymerization proceeds. This indicates that the core part in the catalyst is fragmented due to the hydrodynamic forces generated by the chain growth. XRD curves of the annealed sample are shown in Figure S5 for further clarification. 3.4.2. Rheological Characterization. The high concentration of the monoclinic phase probably indicates a less entangled amorphous phase in the nascent polyethylene.10,11 Figure 3a shows dynamic sweep curves of the synthesized polyethylene. An increment of the modulus with time suggests a less entangled state of the nascent polyethylene.16,17 Figure 3b plots the buildup data of normalized storage modulus. Generally, chains approach the maximum number of entanglement at G′max in a thermodynamically stable melt.16 Therefore, GNt indicates the present number of entanglements. It is a fraction of the total number of entanglements in the thermodynamically stable melt.16 Thus, a higher value of GNt suggests a higher entanglement density in the nascent polymer. The GNt is gradually decreasing upon increasing the polymerization time. It indicates that the polymer turns toward a less entangled state with the increment of polymerization time. At low temperature, the crystallization rate of growing chains is fast which can favor the formation of a less entangled structure.16,17 However, the catalyst activity decays fast in the initial stage of polymerization. It can increase the probability of chain overlapping as soon as the chains are growing out.23 Thus, the synthesized polymer is more entangled. Further increment of the polymerization time will result in a decrement of catalyst activity. This is favorable for the formation of a less entangled state of the polymers.24,25 3.5. Evolution of Polymer Morphology. Figure 4 shows morphology of the synthesized polyethylene. Many petal structures can be found in the nascent powders. A similar morphology was reported in the work of Rastogi and coworkers where the synthesized UHMWPE was found in a less entangled state.16 These petal structures exhibit folded chain crystals of polyethylene, suggesting the anisotropy of chain propagation is very strong.2,24 This is caused by the strong monomer-transfer resistance, generated by the confined environment of the Fe3O4@SiO2 nanoparticles.3 Moreover, the petal structures become more obvious when the samples

Figure 5. TEM images of polyethylene nanocomposites. (a, b) The images of PE-2 min with different magnification. (c, d) The images of PE-30 min with different magnification.

nanoparticles occurs in the polymer matrix already during the initial period of polymerization (2 min). The core−shell structure can still be observed in each of the Fe3O4@SiO2 particles (Figure 2b). This indicates that the Fe3O4@SiO2 can aggregate before polymerization due to the van der Waals forces.3 During the first 2 min of polymerization, the active centers located at the interspace of the aggregation are first activated. Polymerization allows the growing chains to separate the large aggregated zones without destroying the core−shell structure of [email protected] At this stage, the Mw is not high and the MWD is not yet broad since monomer-transfer resistance, generated by the aggregation, is not obvious.3,26,27 However, Figure 5c,d shows that the aggregation of Fe3O4@SiO2 has essentially disappeared for a long time of polymerization. The core−shell structure of Fe3O4@SiO2 is destroyed as well, only leaving several dark shadows with diameters of some tens of nanometers in the polyethylene matrix (Figure 2d). This 8723

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morphological evolution during the in situ polymerization. The above results suggest that the growth of polymer chains takes place in two stages. The first stage is the short time of polymerization. During this stage, most of the catalysts supported at the interspace of Fe3O4@SiO2 aggregations are activated. Hydraulic force generated by the chain propagation can conquer the van der Waals force between the magnetic fillers.2,3 The propagating chains can gradually destroy the aggregation of Fe3O4@SiO2 without destroying the particle structure itself. The low temperature polymerization favors the chain crystallization as soon as the chains grow out, resulting in the formation of a monoclinic phase.16,17 However, almost 50% of the catalyst activity decays in this stage. The fast chain growth rate favors the formation of entangled structure. As a result, the polymer synthesized in the initial stage of polymerization is more entangled. The second stage is the long time of polymerization. During this stage, the active centers supported inside the Fe3O4@SiO2 nanoparticles are gradually activated. The confined environment of the Fe3O4@ SiO2 particles can enhance the monomer-transfer resistance which can increase the Mw and MWD of the synthesized polymer and also decrease the catalyst activity. This strong transfer resistance induces the synthesized polyethylene with many petal structures. The remarkable decreased activity and confined environment of supports will enhance the formation of the monoclinic phase and the less entangled state.10,11 Furthermore, the SiO2 part is fragmented because of the hydrodynamic force generated by the chain propagation. Aggregations of Fe3O4 particles can be destroyed totally, resulting in a uniformed dispersion of Fe3O4 particles in the polymer matrix. Finally, the uniformed dispersion of Fe3O4 particles can endow the polyethylene with superparamagnetic characteristics.

suggests that the silica part of the Fe3O4@SiO2 is fragmented during the longer time of polymerization, which in turn indicates that the catalysts immobilized inside the Fe3O4@SiO2 particles, especially in the Fe3O4 part, can be activated after 30 min of polymerization. Hydraulic force generated by the chain propagation can even destroy the structure of the Fe3O4 core.3 The monomer-transfer resistance, induced by the SiO2 shell and Fe3O4 core, is strong compared with that of the Fe3O4@ SiO2 aggregation. The different monomer-transfer environments can make the Mw and MWD increase upon increasing the polymerization time. As a consequence, a higher proportion of the monoclinic phase with a less entangled state of polymer is obtained. 3.6. Magnetic Property of Polyethylene Nanocomposites. Figure 6 shows the saturation magnetization vs applied

Figure 6. Saturation magnetization vs applied magnetic field of the synthesized polymer.

4. CONCLUSIONS Fe3O4@SiO2 with 3D core−shell nanoparticles are used to immobilize Fe(acac)3/2,6-bis(1-(2-isopropylanilinoethyl)) in order to polymerize ethylene in a nanoscale confinement. The structural and morphological evolutions of the nascent polyethylene are studied during the polymerization. Our results demonstrate that the confined environment of the Fe3O4@SiO2 nanoparticles greatly influences the chain propagation and crystallization during the polymerization. It leads to a gradual increase of both the Mw and the MWD of synthesized polymers as the polymerization continues. Low temperature polymerization favors chain crystallization. This can make the chains crystallize as soon as they grow out. In consequence, the nascent polymer possesses a weakly entangled state with a large fraction of the monoclinic phase. Two stages of polymerization can be observed which corresponds to the fragmentation of Fe3O4@SiO2 aggregations and the core−shell structure of the Fe3O4@SiO2 particle, respectively. The magnetic Fe3O4 can achieve uniformed dispersion.

magnetic field at room temperature for the synthesized polyethylene nanocomposites. It can be seen that all the synthesized polyethylene nanocomposites present negligible coercivity and remanence, indicating a typical super-paramagnetic characteristic.28,29 The values of saturation magnetization are decreased gradually as the polymerization time increases, which is caused by the decrement of filler loading. 3.7. Proposed Mechanism for Structural and Morphological Evolution during the in Situ Polymerization. Figure 7 shows the proposed mechanism for structural and

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b01312. The particle size distribution and FTIR of Fe3O4 and Fe3O4@SiO2 nanoparticles; DSC, XRD, and GPC curves of the synthesized polyethylene (PDF)

Figure 7. Proposed mechanism for structural and morphological evolution during the in situ polymerization. 8724

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Industrial & Engineering Chemistry Research

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding from the Project of Natural Science Foundation of China (No. 21206078) and Natural Science Foundation of Ningbo (2016A610048) and sponsorship by the K. C. Wong Magna Fund in Ningbo University, Ningbo Key Laboratory of Specialty Polymers 2014A22001, are gratefully acknowledged.

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DOI: 10.1021/acs.iecr.6b01312 Ind. Eng. Chem. Res. 2016, 55, 8719−8725