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Facile fabrication of nanocomposite microspheres with polymer cores and magnetic shells by Pickering suspension polymerization
Reactive & Functional Polymers 69 (2009) 750–754
Contents lists available at ScienceDirect
Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Facile fabrication of nanocomposite microspheres with polymer cores and magnetic shells by Pickering suspension polymerization Chaoyang Wang *, Chengjin Zhang, Yu Li, Yunhua Chen, Zhen Tong Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 22 February 2009 Received in revised form 4 June 2009 Accepted 5 June 2009 Available online 10 June 2009 Keywords: Magnetic polymer microspheres Nanocomposites Pickering emulsion Suspension polymerization PS
a b s t r a c t Pickering suspension polymerization was used to prepare magnetic polymer microspheres that have polymer cores enveloped by shells of magnetic nanoparticles. Styrene was emulsified in an aqueous dispersion of Fe3O4 nanoparticles using a high shear. The resultant Pickering oil-in-water (o/w) emulsion stabilized solely by magnetic nanoparticles was easily polymerized at 70 °C without stirring. Fe3O4 nanoparticles act as effective stabilizers during polymerization and as building blocks for creating the organic– inorganic hybrid nanocomposite after polymerization. The fabricated magnetic nanocomposites were characterized by FTIR, XRD, TGA, DSC, GPC, XPS and SEM. The structures of the polymer core and the nanoparticle shell were analyzed. We investigated the effects on the products of the weight of Fe3O4 nanoparticles used to stabilize the original Pickering emulsions. Pickering suspension polymerization provides a new route for the synthesis of a variety of hybrid nanocomposite microspheres with supracolloidal structures. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Magnetic polymer microspheres have attracted considerable interest due to their extensive application in the fields of bioseparation and biomedicine, such as cell isolation, targeted drug delivery, protein and enzyme immobilization, immunoassays, and DNA and RNA purification [1–4]. The main advantages of magnetic polymer microspheres are the ease with which they can be manipulated and the possibilities they provide for automation equipments and mini devices. Fast and cost-efficient separation by applying an external magnetic field without filtration or centrifugation makes magnetic microspheres even more useful [5,6]. Many approaches have been employed to prepare magnetic polymer microspheres. The conventional method is to coat the magnetic particles with a linear polymer to form magnetic polymer microspheres by phase separation [7], solvent evaporation [8,9] and sol–gel transition [10,11]. However, polymerization methods [1,2] are more widely used and include emulsion polymerization [12], miniemulsion polymerization [13], microemulsion polymerization [14], dispersion polymerization [15], suspension polymerization [16–20], seed polymerization [21], and the two-step swelling method [2,22–24]. Magnetic polymer microspheres with a core-shell structure of magnetic cores embedded in a polymer shell are mostly fabricated by both the polymer coating method and a polymerization method [3].
* Corresponding author. Tel./fax: +86 20 87112886. E-mail address: [email protected] (C. Wang). 1381-5148/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2009.06.003
Recently, self-assembly of solid particles at the liquid–liquid interface to stabilize so-called Pickering emulsion has been well documented and offers a straightforward pathway for the production of organized nanostructures [25–29]. Pickering emulsion droplets are also used as versatile polymerization vessels to fabricate hybrid polymer spheres and capsules with supracolloidal structures [30–47]. The solid particles first self-assemble at the liquid–liquid interface and act as effective stabilizers during the polymerization process, eliminating the need for any conventional stabilizers. After the polymerization process is complete, the particles are captured at the surface of the resultant polymer beads where they are most effective for subsequent applications. Such a surfactant-free emulsion polymerization process, called Pickering emulsion polymerization, is more attractive for preparation of hybrid beads than the conventional emulsion polymerization method. Polymerizations based on Pickering emulsion include Pickering miniemulsion polymerization, Pickering suspension polymerization, Pickering dispersion polymerization and Pickering emulsion interface-initiated atom transfer radical polymerization (PEII-ATRP) [30–47]. Magnetic microparticles with poly(methyl methacrylate) (PMMA) [35] or polyaniline (PANI) [44] cores and Fe3O4 nanoparticle shells have been prepared by Pickering suspension polymerization; polystyrene (PS)-Fe3O4 microparticles have been produced by Pickering dispersion polymerization [36]. The method based on the Pickering emulsion technique has many advantages [35]. The nanoparticles are added as both a component and a stabilizer during the polymerization process. There is no need to use conventional organic stabilizers. There are also no
C. Wang et al. / Reactive & Functional Polymers 69 (2009) 750–754
by-products produced in the process, and no unwanted contaminants are left in the polymer [35]. The final product has a polymer core with a nanoparticles shell, which can be used in catalysis, electronics, or sensing technologies. We prepared microcapsules with alginate gel cores and shells of porous CaCO3 microparticles using in situ gelation of Pickering emulsions in our previous work [48,49]. We also fabricated thermo-sensitive hybrid poly(N-isopropylacrylamide) (PNIPAm) microcapsules with supracolloidal structures using Pickering suspension polymerization [45,46] and investigated the growth of lightly cross-linked poly(2-hydroxyethyl methacrylate) (PHEMA) brushes from silica nanoparticles and subsequent capsule formation using PEII-ATRP [47]. In the present work, magnetic polymer microspheres with polymer cores and magnetic shells were prepared using a novel approach based on Pickering suspension polymerization that differs from conventional polymerization, which produces microspheres consisting of magnetic cores embedded in a polymer shell. Polystyrene (PS) was chosen to be the model system. The polymerization factors and the morphology of the fabricated magnetic PS microspheres were analyzed. 2. Materials and methods 2.1. Materials Styrene (Guanghua Chemical Industries Co., China) was distilled and benzoyl peroxide (BPO) was recrystallized before use. Iron (II) chloride tetrahydrate (FeCl24H2O), iron (III) chloride hexahydrate (FeCl36H2O), 2-propanol (HPLC grade), ammonium hydroxide, and methanol were bought from Guangzhou Chemical Factory, China and were used without further purification. Water used in all experiments was purified by deionization and filtered with a Millipore purification apparatus to a resistivity higher than 18.0 MX cm. 2.2. Preparation of Fe3O4 nanoparticles Superparamagnetic nanoparticles were prepared by a co-precipitation method. A 0.5 g sample of FeCl36H2O was added to 50 mL of nitrogen-purged 2-propanol. Then, 0.25 g of FeCl24H2O was added while the solution was continuously stirred. The temperature of the solution was gradually raised to 50 °C and 8 mL ammonium hydroxide was added. The mixture was allowed to react at 50 °C for 30 min. To speed up the precipitation of nanoparticles, the resulting solution was placed in a refrigerator for 5 h. The nanoparticles were subsequently washed with methanol three times and separated by centrifugation at 10,000 rpm for 5 min. Finally, the black precipitate was collected by centrifugation and dispersed in water to obtain superparamagnetic iron oxide nanoparticles having an average particle size of about 8 nm. 2.3. Preparation of magnetic PS microspheres by Pickering suspension polymerization Styrene (1 mL) with BPO (0.1 g) was emulsified into an aqueous dispersion (10 mL) of a given weight of Fe3O4 nanoparticles (3–30 mg) by agitation using an IKA Ultra Turrax T25 basic instrument at 11,500 rpm for 3 min with a 30 s pause after every minute. The resulting stable Pickering emulsion was polymerized at 70 °C for 8 h after having argon bubbled through for 15 min. The resulting magnetic PS microspheres were washed three times with methanol and water and dried under vacuum at room temperature.
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2.4. Characterization Pickering emulsions were observed with an optical microscope (Carl Zeiss, German). The size and size distribution of Pickering emulsions were estimated by counting 200 emulsion droplets. Fourier transform infrared spectroscopy (FTIR) spectra were recorded using a Bruker Vector-33 FTIR spectrometer under ambient conditions. The samples were grounded with KBr and then compressed into pellets. The spectrum was taken from 400 to 4000 cm 1. Typically, 64 scans at a resolution of 4 cm 1 were accumulated to obtain one spectrum. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out with a TA-Q5000 instrument. Samples were heated from 20 to 700 °C at a heating rate of 10 °C/min in air. The total amount of Fe3O4 uptake was obtained in a TGA measurement from the residual weight percentage at 700 °C. Gel permeation chromatography (GPC) was carried out with a Waters GPC instrument at 40 °C using tetrahydrofuran (THF) as the solvent and narrowly distributed polystryene (PS) as the standard. Magnetic PS microspheres were dissolved in THF for 48 h with stirring. The solution was centrifuged and the supernatant was used for measurement. X-ray photoelectron spectroscopy (XPS) was acquired on a Kratos Axis Ultra (DLD) X-ray photoelectron spectrometer using a monochromatic Al Ka X-ray source (1486.6 eV photons). The binding energy was calibrated using the C 1s hydrocarbon peak (284.6 eV). The X-ray source ran at a power of 150 W (15 kV and 10 mA) and the pressure in the analysis chamber was maintained at 1.33 10 6 Pa. The surface element content was determined from the ratios of the spectral peak areas after calibrating with the experimentally determined sensitivity factors. Powder X-ray diffraction (XRD) patterns were recorded using a Rigaku D/max-3Ainstrument (monochromated Cu Ka radiation). Typically, the diffractogram was recorded in a 2h range of 5–90 oC. Scanning electron microscopy (SEM) was carried out with a Philips XL30 electron microscope equipped with a field emission electron gun. Samples were sputter-coated with gold prior to measurement. 3. Results and discussion 3.1. Formation of Pickering emulsions stabilized by Fe3O4 nanoparticles It is well known that colloidal particles can spontaneously localize to liquid–liquid interfaces to act as stabilizers for Pickering emulsions [25–29]. Hydrophilic particles tend to form oil-in-water (o/w) emulsions whereas hydrophobic particles form water-in-oil (w/o) emulsions [25]. However, Pickering emulsions cannot be formed if particles are highly hydrophilic. Here, we use Fe3O4 nanoparticles as stabilizers to generate styrene-in-water (o/w) Pickering emulsions. The zeta-potential of Fe3O4 nanoparticles is 16.3 mV, which is suitable for the preparation of Pickering o/w emulsions. Typical micrographs of Pickering emulsions stabilized by various amounts of Fe3O4 nanoparticles are shown in Fig. 1. Four kinds of emulsions were stable for more than 1 week. The styrene droplets had a wide size distribution from several micrometers to hundreds of micrometers. The average size of the styrene droplets decreased as the weight of Fe3O4 nanoparticles used to stabilize emulsions increased. Using 3.0 mg and 6.8 mg Fe3O4 nanoparticles, the styrene droplets were spherical and finely dispersed in water. With 14.2 mg Fe3O4 nanoparticles, some styrene droplets bound together; the 30.0 mg Fe3O4 nanoparticles formed styrene droplets where some magnetic particles were not adsorbed.
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Fig. 1. Light micrographs of Pickering emulsions stabilized with Fe3O4 nanoparticles of (a) 3 mg, (b) 6.8 mg, (c) 14.2 mg and (d) 30 mg.
3.2. Preparation and characterization of magnetic PS microspheres
Table 1 Processing parameters of Pickering suspension polymerization.
Ensuring the formation of stable Pickering emulsions is necessary for the subsequent suspension polymerization to make functional microspheres. Polymerization can be easily carried out and needs no stirring after generation of a robust Pickering emulsion. Fe3O4 nanoparticles act as effective stabilizers during polymerization and as building blocks for creating the organic–inorganic hybrid nanocomposite after polymerization [30]. We prepared magnetic polymer microspheres with PS cores and shells of Fe3O4 nanoparticles using the Pickering suspension polymerization illustrated in Scheme 1. In this study, as shown in Table 1, four batches of varying weights of Fe3O4 nanoparticles (batch 1: 3 mg; 2: 6.8 mg; 3: 14.2 mg; 4: 30 mg) were used to prepare magnetic nanocomposite PS microspheres. GPC of the PS in the nanocomposites is shown in Fig. 2. The molecular weight (Mn) and the polydispersity (PDI) of the PS are also listed in Table 1. The Mn values of PS were similar from 8000 to 10,000 with similar PDI values from 2.0 to 2.4 for four batches, which is typical for suspension polymerization of styrene. Fig. 3 shows TGA curves of the magnetic PS microspheres. The total amount of Fe3O4 uptake is 0.7 wt%, 1.1 wt%, 5.8 wt% and 7.8 wt% for batches 1–4, respectively, increasing with increasing weight of the magnetic nanoparticles used to stabilize the emulsions. The theoretical Fe3O4 uptake in the nanocomposites calculated from the ratio of the reactants is 0.3 wt%, 0.75 wt%, 1.5 wt% and
Batch
Fe3O4
1 2 3 4
3 6.8 14.2 30
Fe3O4 content (%)c
PS in products M na
PDIa
Tg (°C)b
8000 9000 9000 10,000
2.4 2.0 2.2 2.3
100.7 105.9 111.3 112.0
0.7 1.1 5.8 7.8
Conditions: 1 mL styrene, 0.1 g BPO, 10 mL water, polymerization at 70 °C for 8 h. a From GPC. b From DSC. c From TGA.
a
c
b d
23
24
25
26
27
28
29
30
Retention time (min) Fig. 2. GPC of PS in magnetic nanocomposite microspheres prepared with Fe3O4 nanoparticles of (a) 3 mg, (b) 6.8 mg, (c) 14.2 mg and (d) 30 mg.
Nanoparticles
Water
Polymerization Styrene BPO
Pickering emulsion
70 oC
PS
Magnetic PS microspheres
Scheme 1. Mechanism of the nanoparticle-stabilized suspension polymerization of styrene with BPO as the initiator.
3.2 wt%. The Fe3O4 uptake from TGA is a little higher than the theoretical Fe3O4 uptake for all four batches. The experimental data suggests that the majority of the nanoparticles in the solution were harvested during the suspension polymerization and the polymerization conversion was less than 100% [32,33]. DSC curves of the nanocomposites are shown in Fig. 4. Tg from DSC listed in Table 1 is 100.7 °C, 105.9 °C, 111.3 °C and 112.0 °C for batches 1–4, respectively, increasing with increasing weight of the magnetic nanoparticles used to stabilize the emulsions. This trend can be attributed to the inorganic shells protecting the PS cores. The
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100
Weight (%)
80 60
Fe3O4/PS nanocomposite
40
Fe3O4
20
d c ab
0 0
100
200
300
400
500
600
700
10
20
30
40
Fig. 3. TGA of magnetic PS microspheres prepared with Fe3O4 nanoparticles of (a) 3 mg, (b) 6.8 mg, (c) 14.2 mg and (d) 30 mg.
0.5
Heat flow (mW/mg)
0.0
a -0.5
b c
-1.0
d
-1.5 -2.0 -2.5 -3.0 100
200
300
400
50
60
70
80
90
o
2θ ( )
o
Temperature ( C)
500
600
700
o
Temperature ( C) Fig. 4. DSC of magnetic PS microspheres prepared with Fe3O4 nanoparticles of (a) 3 mg, (b) 6.8 mg, (c) 14.2 mg and (d) 30 mg.
change in heat enthalpy at about 300 °C results from the crystal transformation of magnetic nanoparticles, and the change at about 400 °C from heat degradation of PS. Fe3O4 nanoparticles, pure PS microspheres from emulsion polymerization and Fe3O4-PS nanocomposite made using Fe3O4 nanoparticles from batch 2 were analyzed by FTIR spectroscopy (Fig. 5). The peak at 580 cm 1 is the Fe–O vibration of Fe3O4. The bands at 698 and 752 cm 1 can be attributed to flexural vibrations (dC–H) of the benzene ring and those at 1450, 1490,
Fe3O4
PS
Fig. 6. XRD patterns of (a) Fe3O4 nanoparticles, (b) Fe3O4/PS nanocomposite microspheres made using Fe3O4 nanoparticles from batch 2.
and 1596 cm 1 can be attributed to benzene ring vibrations (mC–C) of polystyrene. All the major characteristic bands of Fe3O4-PS nanocomposite are present and there is no difference from the infrared spectrum of the polystyrene standard. This indicates that there is no or only very weak chemical bonding between the Fe3O4 nanoparticles and polystyrene in the product; the presence of strong chemical bonding would shift the vibrational frequencies of the polymer. The XRD results are shown in Fig. 6. No obvious peaks from the Fe3O4 nanoparticles were found in the XRD curve of the Fe3O4-PS nanocomposite made using Fe3O4 nanoparticles from batch 2. A possible reason for this is the small Fe3O4 content of the batch. We used XPS to analyze the surface elements of magnetic PS microspheres made using Fe3O4 nanoparticles from batch 4 and the result is plotted in Fig. 7. The binding energy (BE) of Fe 2P is 710.8 eV. The BE of O 1S is 529.8 eV and that of C 1S is 283.8 eV. The ratio of Fe to C atoms is 1:6.4 from the peak areas of XPS. If the magnetic nanoparticles are homogeneously dispersed within the polymer matrix, the theoretical ratio of Fe to C atoms calculated from the ratio of the reactants is 1:182. The higher Fe content in the surface of the product indicates that Fe3O4 nanoparticles were covered on the microsphere surface and the core-shell structure was successfully formed. Fig. 8 shows SEM micrographs of the magnetic PS microspheres made using Fe3O4 nanoparticles from batch 4. The nanocomposites are spherical and have a size distribution as wide as that of the original Pickering emulsion, indicating that the high stabilization efficiency of the nanoparticles enabled successful suspension polymerization with controlled morphology. The microsphere surface is very rough due to the presence of entrapped magnetic nanoparticles.
Fe 2p
O 1s C 1s
Fe3O4/PS nanocomposite
500
1000
1500
2000
2500
3000
3500
4000
-1
Wavenumber (cm ) Fig. 5. FTIR of (a) Fe3O4 nanoparticles, (b) PS microspheres and (c) Fe3O4/PS nanocomposite microspheres made using Fe3O4 nanoparticles from batch 2.
700
600
500
400
300
200
Binding energy (eV) Fig. 7. XPS of Fe3O4/PS nanocomposite microspheres made using Fe3O4 nanoparticles from batch 4.
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nologic Program of Guangzhou Municipality (2007J1-C0351) and NCET-07-0306. References
Fig. 8. SEM micrographs of Fe3O4/PS nanocomposite microspheres made using Fe3O4 nanoparticles from batch 4 with (a) low and (b) high magnifications.
4. Conclusion In summary, nanocomposite microspheres with PS cores and shells of Fe3O4 nanoparticles were fabricated by Pickering suspension polymerization of styrene stabilized by Fe3O4 nanoparticles. The morphologies of magnetic PS microspheres are tunable and can be controlled via the method(s) by which the original Pickering emulsions are prepared due to their high stabilization during the suspension polymerization procedure. Suspension polymerization based on Pickering emulsion droplets opens up a new route to making a variety of hybrid nanocomposite microspheres with supracolloidal structures, allowing the synthesis of magnetic beads that have potential applications in bioseparation and biomedicine. Acknowledgements This work was supported by the National Natural Science Foundation of China (20574023 and 20874030), the Scientific and Tech-
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Contents lists available at ScienceDirect
Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Facile fabrication of nanocomposite microspheres with polymer cores and magnetic shells by Pickering suspension polymerization Chaoyang Wang *, Chengjin Zhang, Yu Li, Yunhua Chen, Zhen Tong Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 22 February 2009 Received in revised form 4 June 2009 Accepted 5 June 2009 Available online 10 June 2009 Keywords: Magnetic polymer microspheres Nanocomposites Pickering emulsion Suspension polymerization PS
a b s t r a c t Pickering suspension polymerization was used to prepare magnetic polymer microspheres that have polymer cores enveloped by shells of magnetic nanoparticles. Styrene was emulsified in an aqueous dispersion of Fe3O4 nanoparticles using a high shear. The resultant Pickering oil-in-water (o/w) emulsion stabilized solely by magnetic nanoparticles was easily polymerized at 70 °C without stirring. Fe3O4 nanoparticles act as effective stabilizers during polymerization and as building blocks for creating the organic– inorganic hybrid nanocomposite after polymerization. The fabricated magnetic nanocomposites were characterized by FTIR, XRD, TGA, DSC, GPC, XPS and SEM. The structures of the polymer core and the nanoparticle shell were analyzed. We investigated the effects on the products of the weight of Fe3O4 nanoparticles used to stabilize the original Pickering emulsions. Pickering suspension polymerization provides a new route for the synthesis of a variety of hybrid nanocomposite microspheres with supracolloidal structures. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Magnetic polymer microspheres have attracted considerable interest due to their extensive application in the fields of bioseparation and biomedicine, such as cell isolation, targeted drug delivery, protein and enzyme immobilization, immunoassays, and DNA and RNA purification [1–4]. The main advantages of magnetic polymer microspheres are the ease with which they can be manipulated and the possibilities they provide for automation equipments and mini devices. Fast and cost-efficient separation by applying an external magnetic field without filtration or centrifugation makes magnetic microspheres even more useful [5,6]. Many approaches have been employed to prepare magnetic polymer microspheres. The conventional method is to coat the magnetic particles with a linear polymer to form magnetic polymer microspheres by phase separation [7], solvent evaporation [8,9] and sol–gel transition [10,11]. However, polymerization methods [1,2] are more widely used and include emulsion polymerization [12], miniemulsion polymerization [13], microemulsion polymerization [14], dispersion polymerization [15], suspension polymerization [16–20], seed polymerization [21], and the two-step swelling method [2,22–24]. Magnetic polymer microspheres with a core-shell structure of magnetic cores embedded in a polymer shell are mostly fabricated by both the polymer coating method and a polymerization method [3].
* Corresponding author. Tel./fax: +86 20 87112886. E-mail address: [email protected] (C. Wang). 1381-5148/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2009.06.003
Recently, self-assembly of solid particles at the liquid–liquid interface to stabilize so-called Pickering emulsion has been well documented and offers a straightforward pathway for the production of organized nanostructures [25–29]. Pickering emulsion droplets are also used as versatile polymerization vessels to fabricate hybrid polymer spheres and capsules with supracolloidal structures [30–47]. The solid particles first self-assemble at the liquid–liquid interface and act as effective stabilizers during the polymerization process, eliminating the need for any conventional stabilizers. After the polymerization process is complete, the particles are captured at the surface of the resultant polymer beads where they are most effective for subsequent applications. Such a surfactant-free emulsion polymerization process, called Pickering emulsion polymerization, is more attractive for preparation of hybrid beads than the conventional emulsion polymerization method. Polymerizations based on Pickering emulsion include Pickering miniemulsion polymerization, Pickering suspension polymerization, Pickering dispersion polymerization and Pickering emulsion interface-initiated atom transfer radical polymerization (PEII-ATRP) [30–47]. Magnetic microparticles with poly(methyl methacrylate) (PMMA) [35] or polyaniline (PANI) [44] cores and Fe3O4 nanoparticle shells have been prepared by Pickering suspension polymerization; polystyrene (PS)-Fe3O4 microparticles have been produced by Pickering dispersion polymerization [36]. The method based on the Pickering emulsion technique has many advantages [35]. The nanoparticles are added as both a component and a stabilizer during the polymerization process. There is no need to use conventional organic stabilizers. There are also no
C. Wang et al. / Reactive & Functional Polymers 69 (2009) 750–754
by-products produced in the process, and no unwanted contaminants are left in the polymer [35]. The final product has a polymer core with a nanoparticles shell, which can be used in catalysis, electronics, or sensing technologies. We prepared microcapsules with alginate gel cores and shells of porous CaCO3 microparticles using in situ gelation of Pickering emulsions in our previous work [48,49]. We also fabricated thermo-sensitive hybrid poly(N-isopropylacrylamide) (PNIPAm) microcapsules with supracolloidal structures using Pickering suspension polymerization [45,46] and investigated the growth of lightly cross-linked poly(2-hydroxyethyl methacrylate) (PHEMA) brushes from silica nanoparticles and subsequent capsule formation using PEII-ATRP [47]. In the present work, magnetic polymer microspheres with polymer cores and magnetic shells were prepared using a novel approach based on Pickering suspension polymerization that differs from conventional polymerization, which produces microspheres consisting of magnetic cores embedded in a polymer shell. Polystyrene (PS) was chosen to be the model system. The polymerization factors and the morphology of the fabricated magnetic PS microspheres were analyzed. 2. Materials and methods 2.1. Materials Styrene (Guanghua Chemical Industries Co., China) was distilled and benzoyl peroxide (BPO) was recrystallized before use. Iron (II) chloride tetrahydrate (FeCl24H2O), iron (III) chloride hexahydrate (FeCl36H2O), 2-propanol (HPLC grade), ammonium hydroxide, and methanol were bought from Guangzhou Chemical Factory, China and were used without further purification. Water used in all experiments was purified by deionization and filtered with a Millipore purification apparatus to a resistivity higher than 18.0 MX cm. 2.2. Preparation of Fe3O4 nanoparticles Superparamagnetic nanoparticles were prepared by a co-precipitation method. A 0.5 g sample of FeCl36H2O was added to 50 mL of nitrogen-purged 2-propanol. Then, 0.25 g of FeCl24H2O was added while the solution was continuously stirred. The temperature of the solution was gradually raised to 50 °C and 8 mL ammonium hydroxide was added. The mixture was allowed to react at 50 °C for 30 min. To speed up the precipitation of nanoparticles, the resulting solution was placed in a refrigerator for 5 h. The nanoparticles were subsequently washed with methanol three times and separated by centrifugation at 10,000 rpm for 5 min. Finally, the black precipitate was collected by centrifugation and dispersed in water to obtain superparamagnetic iron oxide nanoparticles having an average particle size of about 8 nm. 2.3. Preparation of magnetic PS microspheres by Pickering suspension polymerization Styrene (1 mL) with BPO (0.1 g) was emulsified into an aqueous dispersion (10 mL) of a given weight of Fe3O4 nanoparticles (3–30 mg) by agitation using an IKA Ultra Turrax T25 basic instrument at 11,500 rpm for 3 min with a 30 s pause after every minute. The resulting stable Pickering emulsion was polymerized at 70 °C for 8 h after having argon bubbled through for 15 min. The resulting magnetic PS microspheres were washed three times with methanol and water and dried under vacuum at room temperature.
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2.4. Characterization Pickering emulsions were observed with an optical microscope (Carl Zeiss, German). The size and size distribution of Pickering emulsions were estimated by counting 200 emulsion droplets. Fourier transform infrared spectroscopy (FTIR) spectra were recorded using a Bruker Vector-33 FTIR spectrometer under ambient conditions. The samples were grounded with KBr and then compressed into pellets. The spectrum was taken from 400 to 4000 cm 1. Typically, 64 scans at a resolution of 4 cm 1 were accumulated to obtain one spectrum. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out with a TA-Q5000 instrument. Samples were heated from 20 to 700 °C at a heating rate of 10 °C/min in air. The total amount of Fe3O4 uptake was obtained in a TGA measurement from the residual weight percentage at 700 °C. Gel permeation chromatography (GPC) was carried out with a Waters GPC instrument at 40 °C using tetrahydrofuran (THF) as the solvent and narrowly distributed polystryene (PS) as the standard. Magnetic PS microspheres were dissolved in THF for 48 h with stirring. The solution was centrifuged and the supernatant was used for measurement. X-ray photoelectron spectroscopy (XPS) was acquired on a Kratos Axis Ultra (DLD) X-ray photoelectron spectrometer using a monochromatic Al Ka X-ray source (1486.6 eV photons). The binding energy was calibrated using the C 1s hydrocarbon peak (284.6 eV). The X-ray source ran at a power of 150 W (15 kV and 10 mA) and the pressure in the analysis chamber was maintained at 1.33 10 6 Pa. The surface element content was determined from the ratios of the spectral peak areas after calibrating with the experimentally determined sensitivity factors. Powder X-ray diffraction (XRD) patterns were recorded using a Rigaku D/max-3Ainstrument (monochromated Cu Ka radiation). Typically, the diffractogram was recorded in a 2h range of 5–90 oC. Scanning electron microscopy (SEM) was carried out with a Philips XL30 electron microscope equipped with a field emission electron gun. Samples were sputter-coated with gold prior to measurement. 3. Results and discussion 3.1. Formation of Pickering emulsions stabilized by Fe3O4 nanoparticles It is well known that colloidal particles can spontaneously localize to liquid–liquid interfaces to act as stabilizers for Pickering emulsions [25–29]. Hydrophilic particles tend to form oil-in-water (o/w) emulsions whereas hydrophobic particles form water-in-oil (w/o) emulsions [25]. However, Pickering emulsions cannot be formed if particles are highly hydrophilic. Here, we use Fe3O4 nanoparticles as stabilizers to generate styrene-in-water (o/w) Pickering emulsions. The zeta-potential of Fe3O4 nanoparticles is 16.3 mV, which is suitable for the preparation of Pickering o/w emulsions. Typical micrographs of Pickering emulsions stabilized by various amounts of Fe3O4 nanoparticles are shown in Fig. 1. Four kinds of emulsions were stable for more than 1 week. The styrene droplets had a wide size distribution from several micrometers to hundreds of micrometers. The average size of the styrene droplets decreased as the weight of Fe3O4 nanoparticles used to stabilize emulsions increased. Using 3.0 mg and 6.8 mg Fe3O4 nanoparticles, the styrene droplets were spherical and finely dispersed in water. With 14.2 mg Fe3O4 nanoparticles, some styrene droplets bound together; the 30.0 mg Fe3O4 nanoparticles formed styrene droplets where some magnetic particles were not adsorbed.
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Fig. 1. Light micrographs of Pickering emulsions stabilized with Fe3O4 nanoparticles of (a) 3 mg, (b) 6.8 mg, (c) 14.2 mg and (d) 30 mg.
3.2. Preparation and characterization of magnetic PS microspheres
Table 1 Processing parameters of Pickering suspension polymerization.
Ensuring the formation of stable Pickering emulsions is necessary for the subsequent suspension polymerization to make functional microspheres. Polymerization can be easily carried out and needs no stirring after generation of a robust Pickering emulsion. Fe3O4 nanoparticles act as effective stabilizers during polymerization and as building blocks for creating the organic–inorganic hybrid nanocomposite after polymerization [30]. We prepared magnetic polymer microspheres with PS cores and shells of Fe3O4 nanoparticles using the Pickering suspension polymerization illustrated in Scheme 1. In this study, as shown in Table 1, four batches of varying weights of Fe3O4 nanoparticles (batch 1: 3 mg; 2: 6.8 mg; 3: 14.2 mg; 4: 30 mg) were used to prepare magnetic nanocomposite PS microspheres. GPC of the PS in the nanocomposites is shown in Fig. 2. The molecular weight (Mn) and the polydispersity (PDI) of the PS are also listed in Table 1. The Mn values of PS were similar from 8000 to 10,000 with similar PDI values from 2.0 to 2.4 for four batches, which is typical for suspension polymerization of styrene. Fig. 3 shows TGA curves of the magnetic PS microspheres. The total amount of Fe3O4 uptake is 0.7 wt%, 1.1 wt%, 5.8 wt% and 7.8 wt% for batches 1–4, respectively, increasing with increasing weight of the magnetic nanoparticles used to stabilize the emulsions. The theoretical Fe3O4 uptake in the nanocomposites calculated from the ratio of the reactants is 0.3 wt%, 0.75 wt%, 1.5 wt% and
Batch
Fe3O4
1 2 3 4
3 6.8 14.2 30
Fe3O4 content (%)c
PS in products M na
PDIa
Tg (°C)b
8000 9000 9000 10,000
2.4 2.0 2.2 2.3
100.7 105.9 111.3 112.0
0.7 1.1 5.8 7.8
Conditions: 1 mL styrene, 0.1 g BPO, 10 mL water, polymerization at 70 °C for 8 h. a From GPC. b From DSC. c From TGA.
a
c
b d
23
24
25
26
27
28
29
30
Retention time (min) Fig. 2. GPC of PS in magnetic nanocomposite microspheres prepared with Fe3O4 nanoparticles of (a) 3 mg, (b) 6.8 mg, (c) 14.2 mg and (d) 30 mg.
Nanoparticles
Water
Polymerization Styrene BPO
Pickering emulsion
70 oC
PS
Magnetic PS microspheres
Scheme 1. Mechanism of the nanoparticle-stabilized suspension polymerization of styrene with BPO as the initiator.
3.2 wt%. The Fe3O4 uptake from TGA is a little higher than the theoretical Fe3O4 uptake for all four batches. The experimental data suggests that the majority of the nanoparticles in the solution were harvested during the suspension polymerization and the polymerization conversion was less than 100% [32,33]. DSC curves of the nanocomposites are shown in Fig. 4. Tg from DSC listed in Table 1 is 100.7 °C, 105.9 °C, 111.3 °C and 112.0 °C for batches 1–4, respectively, increasing with increasing weight of the magnetic nanoparticles used to stabilize the emulsions. This trend can be attributed to the inorganic shells protecting the PS cores. The
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100
Weight (%)
80 60
Fe3O4/PS nanocomposite
40
Fe3O4
20
d c ab
0 0
100
200
300
400
500
600
700
10
20
30
40
Fig. 3. TGA of magnetic PS microspheres prepared with Fe3O4 nanoparticles of (a) 3 mg, (b) 6.8 mg, (c) 14.2 mg and (d) 30 mg.
0.5
Heat flow (mW/mg)
0.0
a -0.5
b c
-1.0
d
-1.5 -2.0 -2.5 -3.0 100
200
300
400
50
60
70
80
90
o
2θ ( )
o
Temperature ( C)
500
600
700
o
Temperature ( C) Fig. 4. DSC of magnetic PS microspheres prepared with Fe3O4 nanoparticles of (a) 3 mg, (b) 6.8 mg, (c) 14.2 mg and (d) 30 mg.
change in heat enthalpy at about 300 °C results from the crystal transformation of magnetic nanoparticles, and the change at about 400 °C from heat degradation of PS. Fe3O4 nanoparticles, pure PS microspheres from emulsion polymerization and Fe3O4-PS nanocomposite made using Fe3O4 nanoparticles from batch 2 were analyzed by FTIR spectroscopy (Fig. 5). The peak at 580 cm 1 is the Fe–O vibration of Fe3O4. The bands at 698 and 752 cm 1 can be attributed to flexural vibrations (dC–H) of the benzene ring and those at 1450, 1490,
Fe3O4
PS
Fig. 6. XRD patterns of (a) Fe3O4 nanoparticles, (b) Fe3O4/PS nanocomposite microspheres made using Fe3O4 nanoparticles from batch 2.
and 1596 cm 1 can be attributed to benzene ring vibrations (mC–C) of polystyrene. All the major characteristic bands of Fe3O4-PS nanocomposite are present and there is no difference from the infrared spectrum of the polystyrene standard. This indicates that there is no or only very weak chemical bonding between the Fe3O4 nanoparticles and polystyrene in the product; the presence of strong chemical bonding would shift the vibrational frequencies of the polymer. The XRD results are shown in Fig. 6. No obvious peaks from the Fe3O4 nanoparticles were found in the XRD curve of the Fe3O4-PS nanocomposite made using Fe3O4 nanoparticles from batch 2. A possible reason for this is the small Fe3O4 content of the batch. We used XPS to analyze the surface elements of magnetic PS microspheres made using Fe3O4 nanoparticles from batch 4 and the result is plotted in Fig. 7. The binding energy (BE) of Fe 2P is 710.8 eV. The BE of O 1S is 529.8 eV and that of C 1S is 283.8 eV. The ratio of Fe to C atoms is 1:6.4 from the peak areas of XPS. If the magnetic nanoparticles are homogeneously dispersed within the polymer matrix, the theoretical ratio of Fe to C atoms calculated from the ratio of the reactants is 1:182. The higher Fe content in the surface of the product indicates that Fe3O4 nanoparticles were covered on the microsphere surface and the core-shell structure was successfully formed. Fig. 8 shows SEM micrographs of the magnetic PS microspheres made using Fe3O4 nanoparticles from batch 4. The nanocomposites are spherical and have a size distribution as wide as that of the original Pickering emulsion, indicating that the high stabilization efficiency of the nanoparticles enabled successful suspension polymerization with controlled morphology. The microsphere surface is very rough due to the presence of entrapped magnetic nanoparticles.
Fe 2p
O 1s C 1s
Fe3O4/PS nanocomposite
500
1000
1500
2000
2500
3000
3500
4000
-1
Wavenumber (cm ) Fig. 5. FTIR of (a) Fe3O4 nanoparticles, (b) PS microspheres and (c) Fe3O4/PS nanocomposite microspheres made using Fe3O4 nanoparticles from batch 2.
700
600
500
400
300
200
Binding energy (eV) Fig. 7. XPS of Fe3O4/PS nanocomposite microspheres made using Fe3O4 nanoparticles from batch 4.
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nologic Program of Guangzhou Municipality (2007J1-C0351) and NCET-07-0306. References
Fig. 8. SEM micrographs of Fe3O4/PS nanocomposite microspheres made using Fe3O4 nanoparticles from batch 4 with (a) low and (b) high magnifications.
4. Conclusion In summary, nanocomposite microspheres with PS cores and shells of Fe3O4 nanoparticles were fabricated by Pickering suspension polymerization of styrene stabilized by Fe3O4 nanoparticles. The morphologies of magnetic PS microspheres are tunable and can be controlled via the method(s) by which the original Pickering emulsions are prepared due to their high stabilization during the suspension polymerization procedure. Suspension polymerization based on Pickering emulsion droplets opens up a new route to making a variety of hybrid nanocomposite microspheres with supracolloidal structures, allowing the synthesis of magnetic beads that have potential applications in bioseparation and biomedicine. Acknowledgements This work was supported by the National Natural Science Foundation of China (20574023 and 20874030), the Scientific and Tech-
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