Pickering suspension polymerization

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 22February 2009

Received in revised form 4June 2009Accepted 5June 2009

Available online 10June 2009Keywords:

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 emulsi?ed in an aqueous dis-persion of Fe 3O 4nanoparticles 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.Fe 3O 4nano-particles 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 Fe

3O 4nanoparticles 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 supracol-loidal structures.

ó2009Elsevier Ltd.All rights reserved.

1.Introduction

Magnetic polymer microspheres have attracted considerable interest due to their extensive application in the ?elds of biosepa-ration and biomedicine,such as cell isolation,targeted drug deliv-ery,protein and enzyme immobilization,immunoassays,and DNA and RNA puri?cation [1–4].The main advantages of magnetic polymer microspheres are the ease with which they can be manip-ulated and the possibilities they provide for automation equipments and mini devices.Fast and cost-ef?cient separation by applying an external magnetic ?eld without ?ltration or centri-fugation 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 poly-mer 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 poly-merization [12],miniemulsion polymerization [13],microemul-sion 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 embed-ded in a polymer shell are mostly fabricated by both the polymer coating method and a polymerization method [3].

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 produc-tion of organized nanostructures [25–29].Pickering emulsion droplets are also used as versatile polymerization vessels to fabri-cate hybrid polymer spheres and capsules with supracolloidal structures [30–47].The solid particles ?rst self-assemble at the li-quid–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 parti-cles 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 hy-brid beads than the conventional emulsion polymerization meth-od.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 Fe 3O 4nanoparticle shells have been prepared by Pickering suspen-sion polymerization;polystyrene (PS)-Fe 3O 4microparticles 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

1381-5148/$-see front matter ó2009Elsevier Ltd.All rights reserved.doi:10.1016/j.reactfunctpolym.2009.06.003

*Corresponding author.Tel./fax:+862087112886.E-mail address:zhywang@https://www.360docs.net/doc/8313632037.html, (C.Wang).

Reactive &Functional Polymers 69(2009)750–754

Contents lists available at ScienceDirect

Reactive &Functional Polymers

j o ur na l h om e pa ge :w w w.e ls e v ie r.c om /lo c at e /re act

by-products produced in the process,and no unwanted contami-nants are left in the polymer[35].The?nal 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 CaCO3microparticles using in situ gelation of Pickering emulsions in our previous work[48,49].We also fabricated ther-mo-sensitive hybrid poly(N-isopropylacrylamide)(PNIPAm) microcapsules with supracolloidal structures using Pickering sus-pension polymerization[45,46]and investigated the growth of lightly cross-linked poly(2-hydroxyethyl methacrylate)(PHEMA) brushes from silica nanoparticles and subsequent capsule forma-tion using PEII-ATRP[47].In the present work,magnetic polymer microspheres with polymer cores and magnetic shells were pre-pared 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 mor-phology 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(FeCl2á4H2O),iron(III)chloride hexahydrate (FeCl3á6H2O),2-propanol(HPLC grade),ammonium hydroxide, and methanol were bought from Guangzhou Chemical Factory, China and were used without further puri?cation.Water used in all experiments was puri?ed by deionization and?ltered with a Millipore puri?cation apparatus to a resistivity higher than 18.0M X cm.

2.2.Preparation of Fe3O4nanoparticles

Superparamagnetic nanoparticles were prepared by a co-pre-cipitation method.A0.5g sample of FeCl3á6H2O was added to 50mL of nitrogen-purged2-propanol.Then,0.25g of FeCl2á4H2O was added while the solution was continuously stirred.The tem-perature of the solution was gradually raised to50°C and8mL ammonium hydroxide was added.The mixture was allowed to react at50°C for30min.To speed up the precipitation of nano-particles,the resulting solution was placed in a refrigerator for 5h.The nanoparticles were subsequently washed with methanol three times and separated by centrifugation at10,000rpm for 5min.Finally,the black precipitate was collected by centrifuga-tion and dispersed in water to obtain superparamagnetic iron oxide nanoparticles having an average particle size of about 8nm.

2.3.Preparation of magnetic PS microspheres by Pickering suspension polymerization

Styrene(1mL)with BPO(0.1g)was emulsi?ed into an aque-ous dispersion(10mL)of a given weight of Fe3O4nanoparticles (3–30mg)by agitation using an IKA Ultra Turrax T25basic instrument at11,500rpm for3min with a30s pause after every minute.The resulting stable Pickering emulsion was polymerized at70°C for8h after having argon bubbled through for15min. The resulting magnetic PS microspheres were washed three times with methanol and water and dried under vacuum at room temperature.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 counting200emulsion droplets.

Fourier transform infrared spectroscopy(FTIR)spectra were re-corded using a Bruker Vector-33FTIR spectrometer under ambient conditions.The samples were grounded with KBr and then com-pressed into pellets.The spectrum was taken from400to 4000cmà1.Typically,64scans at a resolution of4cmà1were accu-mulated to obtain one spectrum.

Thermogravimetric analysis(TGA)and differential scanning cal-orimetry(DSC)were carried out with a TA-Q5000instrument. Samples were heated from20to700°C at a heating rate of 10°C/min in air.The total amount of Fe3O4uptake 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 at40°C using tetrahydrofuran(THF)as the solvent and narrowly distributed polystryene(PS)as the stan-dard.Magnetic PS microspheres were dissolved in THF for48h with stirring.The solution was centrifuged and the supernatant was used for measurement.

X-ray photoelectron spectroscopy(XPS)was acquired on a Kra-tos Axis Ultra(DLD)X-ray photoelectron spectrometer using a monochromatic Al K a X-ray source(1486.6eV photons).The bind-ing energy was calibrated using the C1s hydrocarbon peak (284.6eV).The X-ray source ran at a power of150W(15kV and 10mA)and the pressure in the analysis chamber was maintained at1.33?10à6Pa.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 K a radiation). Typically,the diffractogram was recorded in a2h range of5–90o C.

Scanning electron microscopy(SEM)was carried out with a Phi-lips XL30electron microscope equipped with a?eld emission elec-tron 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 local-ize 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 Fe3O4nanoparticles is 16.3mV,which is suitable for the preparation of Pickering o/w emulsions.Typical micrographs of Pickering emulsions stabilized by various amounts of Fe3O4nanoparticles are shown in Fig.1.Four kinds of emulsions were stable for more than1week.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 Fe3O4nanoparticles used to stabilize emulsions https://www.360docs.net/doc/8313632037.html,ing3.0mg and6.8mg Fe3O4nanoparticles, the styrene droplets were spherical and?nely dispersed in water. With14.2mg Fe3O4nanoparticles,some styrene droplets bound together;the30.0mg Fe3O4nanoparticles formed styrene droplets where some magnetic particles were not adsorbed.

C.Wang et al./Reactive&Functional Polymers69(2009)750–754751

3.2.Preparation and characterization of magnetic PS microspheres Ensuring the formation of stable Pickering emulsions is neces-sary for the subsequent suspension polymerization to make func-tional microspheres.Polymerization can be easily carried out and needs no stirring after generation of a robust Pickering emulsion.Fe 3O 4nanoparticles act as effective stabilizers during polymeriza-tion and as building blocks for creating the organic–inorganic hy-brid nanocomposite after polymerization [30].We prepared magnetic polymer microspheres with PS cores and shells of Fe 3O 4nanoparticles using the Pickering suspension polymerization illustrated in Scheme 1.

In this study,as shown in Table 1,four batches of varying weights of Fe 3O 4nanoparticles (batch 1:3mg;2: 6.8mg;3:14.2mg;4:30mg)were used to prepare magnetic nanocomposite PS microspheres.GPC of the PS in the nanocomposites is shown in Fig.2.The molecular weight (M n )and the polydispersity (PDI)of the PS are also listed in Table 1.The M n values of PS were similar from 8000to 10,000with similar PDI values from 2.0to 2.4for four batches,which is typical for suspension polymerization of styrene.Fig.3shows TGA curves of the magnetic PS microspheres.The total amount of Fe 3O 4uptake is 0.7wt%,1.1wt%,5.8wt%and 7.8wt%for batches 1–4,respectively,increasing with increasing weight of the magnetic nanoparticles used to stabilize the emulsions.The theoretical Fe 3O 4uptake in the nanocomposites calculated from the ratio of the reactants is 0.3wt%,0.75wt%,1.5wt%and

3.2wt%.The Fe 3O 4uptake from TGA is a little higher than the the-oretical Fe 3O 4uptake 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 polymer-ization conversion was less than 100%[32,33].DSC curves of the nanocomposites are shown in Fig.

4.T g from DSC listed in Table 1is 100.7°C,10

5.9°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

Table 1

Processing parameters

of Pickering suspension polymerization.Batch

Fe 3O 4

PS in products Fe 3O 4content (%)c

M n a

PDI a T g (°C)b 138000 2.4100.70.72 6.89000 2.0105.9 1.1314.29000 2.2111.3 5.84

30

10,000

2.3

112.0

7.8

Conditions:1mL styrene,0.1g BPO,10mL water,polymerization at 70°C for 8h.a

From GPC.b

From DSC.c

From TGA.

Fig.1.Light micrographs of Pickering emulsions stabilized with Fe 3O 4nanoparticles of (a)3mg,(b)6.8mg,(c)14.2mg and (d)30mg.

752 C.Wang et al./Reactive &Functional Polymers 69(2009)750–754

change in heat enthalpy at about300°C results from the crystal transformation of magnetic nanoparticles,and the change at about 400°C from heat degradation of PS.

Fe3O4nanoparticles,pure PS microspheres from emulsion polymerization and Fe3O4-PS nanocomposite made using Fe3O4 nanoparticles from batch2were analyzed by FTIR spectroscopy (Fig.5).The peak at580cmà1is the Fe–O vibration of Fe3O4. The bands at698and752cmà1can be attributed to?exural vibrations(d C–H)of the benzene ring and those at1450,1490,and1596cmà1can be attributed to benzene ring vibrations (m C–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 Fe3O4nanoparticles and polystyrene in the product; the presence of strong chemical bonding would shift the vibra-tional frequencies of the polymer.

The XRD results are shown in Fig.6.No obvious peaks from the Fe3O4nanoparticles were found in the XRD curve of the Fe3O4-PS nanocomposite made using Fe3O4nanoparticles from batch2.A possible reason for this is the small Fe3O4content of the batch.

We used XPS to analyze the surface elements of magnetic PS microspheres made using Fe3O4nanoparticles from batch4and the result is plotted in Fig.7.The binding energy(BE)of Fe2P is 710.8eV.The BE of O1S is529.8eV and that of C1S is283.8eV. The ratio of Fe to C atoms is1:6.4from the peak areas of XPS.If the magnetic nanoparticles are homogeneously dispersed within the polymer matrix,the theoretical ratio of Fe to C atoms calcu-lated from the ratio of the reactants is1:182.The higher Fe content in the surface of the product indicates that Fe3O4nanoparticles were covered on the microsphere surface and the core-shell struc-ture was successfully formed.

Fig.8shows SEM micrographs of the magnetic PS microspheres made using Fe3O4nanoparticles from batch4.The nanocomposites are spherical and have a size distribution as wide as that of the ori-ginal Pickering emulsion,indicating that the high stabilization ef?-ciency of the nanoparticles enabled successful suspension polymerization with controlled morphology.The microsphere sur-face is very rough due to the presence of entrapped magnetic nanoparticles.

C.Wang et al./Reactive&Functional Polymers69(2009)750–754753

4.Conclusion

In summary,nanocomposite microspheres with PS cores and shells of Fe 3O 4nanoparticles were fabricated by Pickering suspen-sion polymerization of styrene stabilized by Fe 3O 4nanoparticles.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 Foun-dation of China (20574023and 20874030),the Scienti?c and Tech-

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Fig.8.SEM micrographs of Fe 3O 4/PS nanocomposite microspheres made using Fe 3O 4nanoparticles from batch 4with (a)low and (b)high magni?cations.

754 C.Wang et al./Reactive &Functional Polymers 69(2009)750–754