Materials Letters 76 (2012) 141–143

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Materials Letters

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Preparation of magnetic polyacrylonitrile core–shell nanospheres by theminiemulsion polymerization method

Aslam Khan ⁎, Ahmed Mohamed El-Toni, Mohamad Alsalhi, Abdullah S. Aldwayyan, Mansour AlhoshanKing Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia

⁎ Corresponding author. Tel.: +966 1 4678369.E-mail addresses: aslamkhan@ksu.edu.sa, aslampoly

0167-577X/$ – see front matter © 2012 Elsevier B.V. Aldoi:10.1016/j.matlet.2012.02.089

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 January 2012Accepted 21 February 2012Available online 25 February 2012

Keywords:Magnetic nanospheresMiniemulsionPolyacrylonitrileCore–shell

In this paper, the miniemulsion polymerization method was adopted to prepare nanospheres of magneticnanoparticles (Mag NPs) encapsulated in polyacrylonitrile. In this approach, ~9 nm diameter Mag NPswere first synthesized via the thermal-decomposition method using oleic acid as a capping agent.Subsequently, the core–shell structures of aggregated Mag NPs-cores within polyacrylonitile-shells weresuccessfully formed by the polymerization of acrylonitrile in the presence of Mag NPs, i.e. via the miniemulsionpolymerization method. The morphology, shape, size, and elemental composition of these as-prepared MagNPs-polyacrylonitrile composite particles were determined by field-emission scanning electron microscopy,X-ray diffraction, and Fourier-transform infrared spectroscopy. The encapsulation of aggregatedMag NPs withinthe polymer matrix to form core–shell structures was further confirmed by transmission electron microscopy.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

There have been immense efforts directed toward the synthesis oforganic/inorganic nanocomposites [1] in attempt to exploit newhybrid properties derived from various components. Because oftheir small size, nanoparticles offer unique properties. For instance,magnetic nanoparticles based on iron oxide composites or hybridmaterials, particularly in the form of core–shell systems, are earninggreat interest in the field of biomedical applications [2]. The combinationof magnetic nanoparticles with polymer to achieve a colloidal or stablecomposite system has attracted much interest. A polymer shell can actas a compatibilizer interacting with the environment and can befunctionalized with biologically or catalytically active sites [3].Furthermore, the magnetic properties of the nanoparticles allow thephysical manipulation of the system [4].

Many polymerization techniques have been developed to preparemagnetic/polymer composite particles, such as conventionalemulsion, soapless emulsion [5], atomic transfer radical [6],miniemulsion [7], and inverse emulsion [8]. However, the existenceof free polymer particles, i.e. particles without encapsulated magneticcores, in the products makes it difficult to control the magneticcontent in the composite particles and somewhat limits theirapplications in the field of biotechnology and biomedicine [9].Among the many techniques, miniemulsion polymerization hasbeen considered to be one of the most advantageous encapsulation

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methods because of its ability to nucleate all the monomer dropletscontaining the dispersed hydrophobic magnetic nanoparticles [10].

In this paper, we report a simple method to synthesize core–shellstructures of Mag NPs-polyacrylonitrile composite particles by theminiemulsion polymerization method. The crystalline structures andmorphology of the as-prepared composite particles were alsoinvestigated.

2. Experimental section

Acrylonitrile (Aldrich), benzyl ether (Aldrich), 1,2-hexadecanediol(Aldrich), iron(III) acetylacetonate (Fe(acac)3; Aldrich), oleic acid(Aldrich), oleylamine (Aldrich), sodium dodecyl sulfate (SDS;Merck), sodium bicarbonate (Merck), cyclohexane (Merck), ethanol(Merck), potassium peroxodisulfate (KPS; Fluka) were used as-received from the indicated suppliers. Water was purified usingNANOpure water system (Thermo Scientific).

Mag NPs (Fe3O4) were synthesized [11] in 100 mL of benzyl etherin which 12.92 g of 1,2-hexadecanediol and 3.22 g of Fe(acac)3 werefirst dissolved until a clear solution was obtained. Then, 8.5 g ofoleic acid and 8.0 g of oleylamine were added to this solution. Next,the solution was heated to 300 °C. After cooling, particles wereprecipitated by adding ethanol to achieve a 3:1 volume ratio ofethanol to solution and using a permanent magnet.

To form the oil phase, 10 mg of the as-prepared Mag NPs wasdispersed in a mixture of 1 mL of acrylonitrile and 2.8 mL of cyclohexaneby ultrasonic agitation (Nexul Ultrasonic, Kodo). Additionally, 0.08 g ofSDS and 0.008 g of sodium bicarbonate were dissolved in 25 mL ofwater to form the water phase. These two phases were thenultrasonicated in an ice-cooled bath for 10 min to form a miniemulsion

142 A. Khan et al. / Materials Letters 76 (2012) 141–143

and then transferred to a 100 mL three-necked flask equipped with acondenser, a nitrogen inlet, and a stirrer. To initiate the polymerizationof acrylonitrile, KPS (0.5 mol%) dissolved in 0.5 mL of water was addedwhile the solution was kept at 70 °C and continuously stirred at250 rpm. After 18 h, the Mag NPs-polyacrylonitrile composite particleswere obtained. To remove the unreacted monomers, the dispersedsolution was centrifuged and washed with distilled water several timesby repeated magnetic decantation, thus preparing the nanoparticles forfurther studies.

Transmission electron microscopy (TEM) images were obtainedusing a JEOL 2100 microscope operated at 200 kV accelerationvoltage. Field emission scanning electron microscopy (FESEM) studieswere performed on a JSM 7600F microscope. Dynamic light scattering(DLS) was conducted using a Malvern Nano ZS particle-size analyzer.X-ray diffraction (XRD) analysis was performed using a X'Pert PROPANalytical X-ray diffractometer (Cu Kα, 40 kV, 35 mA). Finally, theFTIR spectra of the powder samples were recorded with attenuatedtotal reflectance FTIR, using a Bruker Vertex-80 spectrometer.

3. Results and discussion

Fig. 1b shows a TEM image of the oleic-acid-modified Mag NPs.The Mag NPs were spherical in shape and were in the range of 8 to11 nm in diameter. Information on mean size and standard deviation(SD) was calculated from measurements of over 100 NPs in randomfields of view. The mean±SD of Mag NPs was 9.4±2.5 nm. Fig. 1a

4 5 6 7 8 9 10 11 12 13 140

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Fig. 1. (a) DLS data of Mag NPs dispersed in cyclohexane. (b) TEM image of Mag NPs. (c) FES(e–g) TEM images of Mag NPs-polyacrylonitrile nanospheres showing core–shell structurethe spheres of aggregated Mag NPs. (h) Corresponding X-ray energy-dispersive spectrosco

represents the average hydrodynamic diameter of the Mag NPs dis-persed in cyclohexane, which was determined by DLS to be 9 nm insize.

Fig. 1e–g shows TEM images of Mag NPs-polyacrylonitrilecomposites prepared by the method described above. TEM imagesdisplay both the polydispersity and spherical shape of the core–shellparticles. The dark spherical area in the core region of the particlesin the TEM image corresponds to Mag NPs within a thin layer of~6 nm polymer shell of polyacrylonitrile. Fig. 1g shows a higher-magnification view of the previous images in which the thin layer ofpolymer shell (as indicated by the arrow) surrounds the aggregatesof Mag NPs to form a core–shell structure. To observe the surfacemorphology of the Mag NPs-polyacrylonitrile composite particles, adrop of colloidal solution was drop-casted onto a silicon wafer, airdried, and observed under FESEM (see Fig. 1c–d). These SEM imagesshow that nanometer-sized (approximately 100 nm) sphericalshaped core–shell particles composed of aggregated Mag NPs-coreswith thin layer of polymer-shells were observed. This result is ingood agreement with that from TEM analysis.

Fig. 2A (a) shows a comparison of the FTIR spectra of the Mag NPsand the Mag NPs-polyacrylonitrile composite. It was reported previouslythat the characteristic absorption band of Fe\O bond of bulk Fe3O4 wasat 570 and 375 cm−1 [12]. However, when the Fe3O4 particles werereduced to nanoscale dimensions, the force constant of the surfacebond increased due to the breaking of a large number of bonds at thesurface atoms, which resulted in the rearrangement of non-localized

c

f

EM image of Mag NPs-polyacrylonitrile composite particles. (d) Magnified image of (c).s (under different magnifications). The arrow indicates the polymer layer surroundingpy (EDS) results of the as-prepared Mag NPs-polyacrylonitrile composite particles.

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633 cm1431 cm

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Fig. 2. (A) FTIR spectra of (a) Mag NPs, and (b) Mag NPs-polyacrylonitrile compositeparticles. (B) Powder XRD pattern of as-prepared Mag NPs-polyacrylonitrile compositeparticles.

Fig. 3. Photographs of the separation (a to b) and redispersion (b to a) of the MagNPs-polyacrylonitrile composite particles: (a) without external magnetic field, (b) withexternal magnetic field (the magnetic field strength of the magnet is 2000 G). A colorchange from ochre brown to transparent was observed when an external magnetic fieldwas applied.

143A. Khan et al. / Materials Letters 76 (2012) 141–143

electrons on the particle surface [13]. Therefore, the FTIR spectrum ofMag NPs and their composite particles was expected to exhibit a blueshift. Accordingly, as shown in Fig. 2A (a) and (b), the characteristicabsorption bands of the Fe\O bondwere shifted to higher wavenumbersof approximately 590 cm−1, confirming the predominant phase of ironoxide in both samples. The IR spectrum of Mag NPs-polyacrylonitrilecomposite particles shows the stretching vibration peak of methylene(\CH2-) group at approximately 2935 cm−1. The peaks at 2877 cm−1

were due to the symmetric stretching of the —CH3 group. In additionto these peaks, the stretching vibration of nitrile groups (\CN-) wasobserved at 2245 cm−1 [14] in the composite sample, indicating thatpolymerization had occurred. The above results revealed that polyacrylo-nitrile was successfully coated onto the Mag NPs.

The crystallinity of Mag NPs-polyacrylonitrile composite particleswas investigated by powder XRD as shown in Fig. 2B. The resultsshow that the composites prepared by the miniemulsion polymerizationhad six diffraction peaks at 2θ of 30.1°, 35.4°, 42.9°, 52.7°, 57.5° and 62.7°,representing the corresponding indices of (220), (311), (400), (422),(511) and (440), respectively, of iron oxide. In addition to these peaks,a diffraction peak at 2θ of 22° was observed, indicating the crystallinenature of the polyacrylonitrile.

The superparamagnetic property of the Mag NPs-polyacrylonitrilecomposite particles is critical for their application in biomedical andbioengineering fields, as it prevents composites of NPs from aggrega-tion and enables them to redisperse rapidly when the magnetic fieldis removed [15]. The variation of magnetization with an applied

magnetic field provides the information on the magnetic propertiesof the composite particles. Fig. 3 illustrates the separation andredispersion process of the Mag NPs-polyacrylonitrile compositeparticles. In the absence of an external magnetic field, the dispersionof the composite particle was ochre brown and homogeneous(Fig. 3a). When the external magnetic field was applied, the MagNPs-polyacrylonitrile composite particles were locally enriched onthe side of the centrifuge tube closer to the magnet, leading to anincreased transparency of the remaining dispersion (Fig. 3b). Theas-prepared composite particles dispersed in aqueous solutions arestable for months; the same dispersed solution can be repeatedlyenriched by an external magnet and redispersed in the solution.This process can be repeated many times and is reproducible.

4. Conclusions

A straightforward and effective technique to prepare nanospheres ofmagnetic particles coatedwith polyacrylonitrile was demonstrated. Theas-prepared composite particle was highly dispersible and shows goodsign of stability in water when stored at room temperature for months.The composite particleswere ~100 nm in size and had a core–shell typemorphology consisting of nanospheres of aggregated Mag NPs in thecore surrounded by a polymer shell. The resulting core–shell structuresMag NPs maybe used as novel materials for various applications.

Acknowledgment

This work was financially supported by the King Abdul Aziz Cityfor Science & Technology (KACST) under the National Plan for Scienceand Technology (NPST) grant no. 10-NAN1008-02.

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