IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 18 (2007) 335704 (8pp) doi:10.1088/0957-4484/18/33/335704

Flexible high-loading particle-reinforcedpolyurethane magnetic nanocompositefabrication throughparticle-surface-initiated polymerizationZhanhu Guo1, Sung Park1, Suying Wei1, Tony Pereira1,Monica Moldovan2, Amar B Karki2, David P Young2 andH Thomas Hahn1

1 Multifunctional Composites Lab, Mechanical & Aerospace Engineering Department andMaterials Science & Engineering Department, University of California Los Angeles,Los Angeles, CA 90095, USA2 Department of Physics and Astronomy, Louisiana State University, Baton Rouge,LA 70803, USA

E-mail: zhanhu@seas.ucla.edu

Received 1 May 2007, in final form 25 June 2007Published 25 July 2007Online at stacks.iop.org/Nano/18/335704

AbstractFlexible high-loading nanoparticle-reinforced polyurethane magneticnanocomposites fabricated by the surface-initiated polymerization (SIP)method are reported. Extensive field emission scanning electron microscopic(SEM) and atomic force microscopic (AFM) observations revealed a uniformparticle distribution within the polymer matrix. X-ray photoelectronspectrometry (XPS) and differential thermal analysis (DTA) revealed a strongchemical bonding between the nanoparticles and the polymer matrix. Theelongation of the SIP nanocomposite under tensile test was about four timesgreater than that of the composite fabricated by a conventional direct mixingfabrication method. The nanocomposite shows particle-loading-dependentmagnetic properties, with an increase of coercive force after the magneticnanoparticles were embedded into the polymer matrix, arising from theincreased interparticle distance and the introduced polymer–particleinteractions.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Magnetic nanoparticles (NPs) have attracted much interestdue to their special physicochemical properties, such asenhanced magnetic moment [1] and larger coercivity [2],which are different from their bulk and atomic counterparts [3].Incorporation of the inorganic nanoparticles into a polymermatrix has extended the particle applications (for example,as high-sensitivity chemical gas sensors [4]). This isprimarily due to the advantages of polymeric nanocompositespossessing high homogeneity, flexible processability andtunable physicochemical properties, such as improved

mechanical, magnetic and conductive properties [5–9]. Highparticle loading and flexibility of the composites are requiredfor certain applications, such as electromagnetic waveabsorbers [10, 11], photovoltaic cells [12], photodetectors andsmart structures [13–15]. However, high particle loadingalways causes brittleness and rigidity of the nanocomposite,and even introduces artificial defects, such as air voids, whichlimits its applications.

The reported methods for incorporating inorganic nanopar-ticles into polymeric matrices include ex situ methods, i.e. dis-persion of the synthesized nanoparticles into an aqueous or or-ganic polymeric solution [16–18], in situ monomer polymer-

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Nanotechnology 18 (2007) 335704 Z Guo et al

ization methods in the presence of the nanoparticles [19–23]or in situ nanoparticle formation in the presence of the poly-mer [6]. The interactions between the polymer and thenanoparticles for the ex situ formed composites are normallysteric interaction forces, van der Waals forces or Lewis acid–base interactions. However, in situ synthetic methods can cre-ate strong chemical bonding within the nanocomposites and areexpected to produce more-stable and higher-quality nanocom-posites [16–18]. Nevertheless, the former has the advantage ofbeing able to utilize a large variety of nanoparticles that havebecome available recently.

The existing challenge in the composite fabricationis to provide a high tensile strength due to local stresswithin the nanocomposite. In other words, the responseof a material to an applied stress is strongly dependenton the nature of the bonds within its microstructure.The interfacial interactions between nanoparticles and thepolymer matrix play a crucial role in determining thequality and properties of the nanocomposites [16]. Thepoor bonding linkage between the fillers and the polymermatrix such as the composites made by simple mixing [16]will introduce artificial defects, which consequently resultin a deleterious effect on the mechanical properties of thenanocomposites [24]. Introducing good linkages betweenthe nanoparticles and the polymer matrix is still a challengefor specific composite fabrication [16–18]. However,appropriate chemical functionalization of the nanofiller surfaceby introducing proper functional groups could improve boththe strength and toughness, with an improved compatibilitybetween the nanofillers and the polymer matrix [25], andmake the nanocomposites stable in harsh environments aswell [26]. Thus, surface functionalization of nanoparticleswith a surfactant or a coupling agent is important not only tostabilize the nanoparticles [27] during processing but also torender them compatible with the polymer matrix.

In this paper, the nanoparticle surface-initiated polymer-ization (SIP) method [28–30] was adopted to fabricate high-loading Fe2O3 nanoparticle-reinforced polyurethane magneticnanocomposites. This method utilized the physicochemical ad-sorption of an initiator onto the iron-oxide (Fe2O3) nanopar-ticle surface for urethane polymerization in a tetrahydrofuran(THF) solution. Strong chemical bonding between nanoparti-cles and polymer matrix was formed without any additionalsurfactant or coupling agent. The obtained nanocompositesexhibit high flexibility as compared with the brittle and rigidcounterparts obtained from the conventional direct mixing(DM) method used for the micron particle (iron carbonyl) re-inforced polyurethane composites. The particle loading can betuned up to 65 wt%.

2. Experimental details

2.1. Materials

The polymeric matrix used was a commercial, clearpolyurethane coating (CAAPCOAT FP-002-55X, manufac-tured by the CAAP Co., Inc.), which contains two-part ure-thane monomers, i.e. 80 wt% diisocyanate and 20 wt%diol; the chemical structures are shown in chart 1. The liq-uid resin has a density of 0.83 g cm−3. Polyurethane cat-alyst (a liquid containing ∼20–65 wt% aliphatic amine, 1–50 wt% parachlorobenzotrifluoride and 10–35 wt% methyl

C

O

R NN C

O

diisocyanate

OH R OH

diol

Chart 1. Chemical formulae of the two-part polyurethane monomersused.

propyl ketone) and accelerator (polyurethane STD-102, con-taining 1 wt% organotitanate and 99 wt% acetone) were pro-vided by CAAP Co. Inc. Iron oxide (γ -Fe2O3, NanophaseTechnologies) nanoparticles with an average diameter of 23 nmand a specific surface area of 45 m2 g−1 were used asnanofillers for the nanocomposite fabrication. All the chem-icals were used as-received without further treatment.

2.2. Nanocomposite preparation

The nanocomposites were fabricated by two different methods.One was the conventional direct mixing (DM) used currentlyfor the micron particle (iron carbonyl) reinforced polyurethanecomposites and the other was based on the surface-initiatedpolymerization (SIP) approach. The fabrication procedureswere as follows. In the DM method, 7.7 g monomers,1.03 g catalyst and 1.42 g accelerator were added into 30 mltetrahydrofuran (THF) with ultrasonic stirring for half an hour.Then nanoparticles were added into the above solution andultrasonically stirred for 10 min. The suspended solution waspoured out into a mould container for curing and evaporationof solvent at room temperature.

Surface-initiated polymerization (SIP) was used toprovide physicochemical adsorption of the initiator onto theiron-oxide (Fe2O3) nanoparticle surface in a tetrahydrofuran(THF) solution. This method utilizes physicochemicallyadsorbed moisture (as seen from the FT-IR spectrum infigure 4) on the nanoparticles as a linking site between thenanoparticle and polymer chain. Figure 1 illustrates the entireSIP process. The nanoparticles were added into the catalystand THF promoter solution. The above nanoparticle suspendedsolution was then sonicated for about 30 min (figure 1, step 1).The sonication energy should enhance the reactivity of thesurface and promote the adsorption of catalyst and promoteronto the nanoparticle surface while evaporating the physicallyadsorbed moisture. The monomers were then introducedinto the above solution dropwise within half an hour and thepolymerization was continued for another 6 h (figure 1, step 2).The final solution was poured into a mould and the solventallowed to evaporate naturally at ambient conditions. Theobtained composite was then pressed at 127 ◦C and 10 psi for10 min in a hot press (figure 1, step 3).

2.3. Characterization

The physicochemical attachment of catalyst–accelerator (CA)mixture onto the nanoparticle surface was initially verifiedby thermo-gravimetric analyses (TGA, Perkin Elmer). Thesample was prepared by dispersing the nanoparticles intoTHF, adding the catalyst and accelerator, stirring for half anhour and washing the precipitated nanoparticles with excessiveTHF to remove the extra CA. TGA was carried out from

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Step 2.

Ultrasonic bath: 6 Hours

7.7g monomers

P

Ultrasonic bath: 30min

Step 1.

5.1 g

+NP

Physicochemicaladsorption of catalystand promoter

NP

NP

Physicallyadsorbed moisture

C: 1.03g

P:1.42 g

NP

Fe2O3NP

Step 3.

Hot press: 130 oC10 minutes

Fe2O3

P/PUSIP N

THF

Aluminum mold:3 days

NPNP/PU

composite

Figure 1. Scheme of the surface-initiated polymerization process.

25 to 600 ◦C with an argon flow rate of 50 ccpm (cubiccentimetres per minute) at a heating rate of 10 ◦C min−1. Thethermal stability of the nanocomposites with different loadingswas characterized by TGA. Polyurethane chemically boundonto the nanoparticle was determined by TGA and derivativethermal gravimetric (DTG). The samples were polymerizednanocomposites in solution after excessive THF washing.

Fourier transform infrared (FT-IR) spectra were recordedin a FT-IR spectrometer (Jasco, FT-IR 420) in transmissionmode under dried nitrogen flow of 10 ccpm. The FT-IRsamples were prepared by mixing with powder KBr, grindingand compressing into a pellet.

The as-received nanoparticles, nanoparticles treated withCA and the Fe2O3/PU nanocomposites underwent surfaceanalysis with an Axis 165 x-ray photoelectron spectrometerusing a monochromatized Al Kα (1486.6 eV) x-ray sourcewith a power of 150 W. Both survey and high-resolutionspectra were obtained using pass energies of 40 eV. Curve-fitting of the high-resolution spectra of all the samplesinvolved in the Fe2O3/PU nanocomposite fabrication wasused to investigate the binding between the polymer and thenanoparticle surface. The curve fitting was performed usinga sum of Gaussian–Lorentzian profile peak shapes (GL(30))after subtraction of a linear background.

Atomic force microscope (AFM) (multimode, DigitalInstruments, Veeco Company) in tapping mode was utilizedto characterize the dispersion quality of nanoparticles inthe polyurethane matrix. The probes used were tapping-mode etched silicon probes with resonant frequency of about300 kHz and spring constant of around 40 N m−1. Thesamples were prepared by embedding the nanocomposite ina cured vinyl-ester-resin tab and polishing the nanocompositecross section. Particle dispersion in polyurethane was furthercharacterized with a scanning electron microscope (SEM,JEOL field emission scanning electron microscope, JSM-6700F). The SEM sample was the same as that used in theAFM analysis except that a thin gold coating was sputteredto improve the electrical conductivity needed for high-qualityimages. Transmission electron microscopy (TEM, JEOL 2000)was used to characterize the morphology of the nanoparticles.The samples were prepared by dropping the nanoparticle-suspended THF solution onto a holey carbon coated coppergrid and drying naturally.

The mechanical properties of the fabricated nanocompos-ites were evaluated by tensile tests following an American So-ciety for Testing and Materials standard (ASTM, 2002, stan-dard D 412-98a). The samples were prepared according to thestandard. A crosshead speed of 15 mm min−1 was used andstrain (mm mm−1) was calculated by dividing the crosshead

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Figure 2. TGA analysis of the as-received Fe2O3 nanoparticles,mixture of the catalyst and accelerator (CA), and the CA-treatednanoparticles.

Figure 3. As-received Fe2O3 nanoparticles remaining on the PETvial wall (left) and composite nanoparticles almost completelyremoved from the PET vial wall (right); chemical structure of PET.

displacement by the gauge length. The effect of the polymermatrix on the magnetic properties of the nanoparticles was in-vestigated in a 9 T physical properties measurement system(PPMS) by Quantum Design.

3. Results and discussion

Figure 2 shows the thermal behaviour of the as-receivednanoparticles, mixture of the catalyst and the accelerator (CA),and the nanoparticles treated with CA. The CA mixture wasobserved to get completely decomposed at around 200 ◦Cand the as-received nanoparticles show continuous weightloss arising from the dehydration of the physicochemicallyadsorbed moisture. However, the CA-treated nanoparticlesshowed a similar weight loss as the CA mixture by itself.The delayed complete decomposition temperature in the CAattached onto the nanoparticle surface indicates the chemicaladsorption of CA mixture on the nanoparticle surface, and thatthe presence of the nanoparticles increased the stability of CAwith a higher complete-decomposition temperature.

When the as-received nanoparticle suspended THFsolution was removed from a polyethylene terephthalate (PET)vial, the wall turned from translucent white to brown as a result

Figure 4. FT-IR spectra of the as-received Fe2O3 nanoparticles, neatPU and the Fe2O3/PU nanocomposite.

of the nanoparticles remaining on the wall as shown in figure 3.However, the 6 h cured nanocomposite-particle-suspendedTHF solution turns red and has no effect on the vial. In otherwords, the polymerization turned the brown nanoparticles intored and the wall still kept translucent white with a trace ofred residue when the vial was emptied. The colour changeafter polymerization is an indicator of the interaction betweenthe nanoparticles and the polymer matrix. The hydrogenbonding between the moisture on the as-received nanoparticlesurface (NP–OH) and the PET wall (C=O) favours the stableattachment of nanoparticles onto the wall. On the other side,the polyurethane coating on the particle has a very weakhydrogen bond and favours affinity to the THF rather thanto the PET wall. The different interactions between thenanoparticle and the different polymers can also explain thequick sedimentation of as-received nanoparticles in THF andthe stable suspension of the nanocomposite particles in THF.This is also consistent with the polymer serving as a stabilizerfor certain nanoparticles in some solvents.

Figure 4 shows the FT-IR spectra of the as-receivedFe2O3 nanoparticles, neat cured polyurethane and the preparedFe2O3/polyurethane nanocomposites. The peak at 3429 cm−1

corresponds to the OH stretching band, indicating that the as-received nanoparticles were partially hydrolyzed, consistentwith the TGA observation. The density of the hydrolyzedOH group was estimated to be 16 μmol m−2 based on theweight loss from TGA and the particle-specific surface area.The estimated value is of the same magnitude as that ofnanoparticles with a comparable size and specific surface areaof ZrO2 (∼20 μmol m−2) and Al2O3 (∼10 μmol m−2) [31].The spectrum of the Fe2O3/PU nanocomposite was observedto have the peaks characteristic of both Fe2O3 (e.g. polyhedralFe3+–O2− stretching vibrations [32] in the range of 450and 800 cm−1 in both the as-received nanoparticles and theprepared nanocomposites as seen in figure 4) [33] and ofpolyurethane (e.g. amide II bands in the range of 1570–1515 cm−1 due to the CO–NH motion, which is seen inboth the neat polyurethane and the nanocomposites). Therelative intensity of the peaks of polyurethane and iron-oxide nanoparticles was observed to be different from the

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Nanotechnology 18 (2007) 335704 Z Guo et al

Figure 5. XPS O 1s spectra of the as-received Fe2O3 NP, CA-treatedFe2O3 nanoparticles and the final PU–Fe2O3 nanocomposite.

Table 1. Deconvolution data of O 1s high-resolution spectra of theas-received Fe2O3 nanoparticles, CA-treated Fe2O3 nanoparticlesand the PU–Fe2O3 nanocomposite. The relative sensitivity factor(RSF) is 0.78 for O 1s.

PeakPosition(eV) RSF

Massconc. (%)Fe2O3 NPs

Massconc. (%)Fe2O3 NPs AC

Massconc. (%)PU–Fe2O3

1 O 1s 527.5 0.78 11.70 63.01 29.362 O 1s 529.9 0.78 58.54 29.86 50.243 O 1s 531.5 0.78 29.76 7.13 20.40

corresponding spectra of polyurethane and iron oxide. Thisindicates a physicochemical interaction between the twocomponents, iron-oxide nanoparticles and neat polyurethane.

Figure 5 shows the high-resolution XPS spectra andtable 1 summarizes quantification data for O 1s of the as-received Fe2O3 nanoparticles, CA-treated Fe2O3 nanoparticlesand the final Fe2O3/PU nanocomposite, respectively. Threeoxygen components were observed in all three spectra withvarying ratios. The oxygen high-resolution spectrum canbe deconvolutionalized into three components. The O 1s at531.5 eV is from the adsorbed moisture (H2O) or carbonylgroups (C=O) after the polymerization, O 1s at 529.9 eVoriginates from hydrolyzed OH groups and O 1s at 527.5 eVcomes from the Fe–O–Fe group [34–36]. In the spectra ofthe as-received nanoparticles, the peaks at 531.5 and 529.5 eVreflect the presence of the partially hydrolyzed nanoparticlesand physically adsorbed moisture. The spectra still have threecharacteristic peaks after the nanoparticles were treated withCA under ultrasonication. However, the intensity (countsper second, cps) of different peaks is completely differentfrom the as-received nanoparticles as shown in table 1. Themoisture portion at 531.5 eV in the nanoparticles decreased(O 1s = 531.5 eV, reduced from 29.76% to 7.13%) due tothe ultrasonic stimulated natural evaporation. The hydrolyzed–OH (at 529.9 eV) amount was also observed to decreasedramatically due to the condensation of the hydrolyzed groupsfrom within the nanoparticles. The evaporation of the mostphysically adsorbed moisture and the condensation of thehydrolyzed FeO–OH was due to the local hot-spots arising

Figure 6. TGA of pure polyurethane and composites made byconventional and surface-initiated polymerization methods measuredat different sample locations (centre and edge); left inset showscorresponding DTG and right inset shows the bright-field TEMmicrostructures of the treated nanoparticles (scale bar 190 nm).

from the ultrasonic stirring. As a result, the intensity of thepeak at 527.5 eV for the Fe–O–Fe contribution was observedto increase dramatically. The absorbed catalyst accelerator(CA) by the nanoparticles was observed to have a significanteffect on the thermal stability of the nanoparticles as shownin figure 1 of the TGA analysis. The intensity of the Fe–O–Fe decreased after the polymerization in solution, which isdue to the polyurethane coating on the nanoparticle surface.The corresponding peaks at 529.9 and 531.5 eV were observedto be relatively higher. The –OH and moisture parts wereobserved to have relative higher intensities arising from thepresence of diols. The enhanced peak at 531.5 eV is due tothe polyurethane carbonyl groups (C=O).

The transmission electron microscope (TEM) bright-field microstructure (inset of figure 6) of the nanoparticlesafter polymerization in THF for 6 h showed a size of26 nm, similar to the reported as-received nanoparticles(Nanophase Technologies), indicating that no Oswald ripeningphenomena [37] happened during the composite fabrication.Figure 6 depicts the weight percentage loss from differentlocations of the nanocomposite sample (centre and edge)in TGA. The uniform particle loading was indicated in theSIP samples as observed by the almost overlapping weightpercentage variation. However, a dramatic difference wasobserved in the DM samples. This shows that the SIP methodimproves particle dispersion as compared with the compositesfabricated with the DM method.

Different thermal behaviours of pure polyurethane andcomposites fabricated by SIP and DM, respectively, werefurther noticed in the derivative thermal gravimetric (DTG)curves shown in the inset of figure 6. The differencewas due to the present status of the polymer, i.e. freechains in the pure polyurethane, weak interaction in theDM Fe2O3/PU nanocomposite and strong interaction in theSIP Fe2O3/PU nanocomposite. Only one peak is observedin pure polyurethane, and the existence of nanoparticlespromotes the decomposition of bulk polyurethane with a lower

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Nanotechnology 18 (2007) 335704 Z Guo et al

C

O

R NN C

O

OH R OH

OH

OHOH

OH

OHOH

C

O

R NN C

OH

O R OO

HnOH

OH

OH OH

+

diisocyanate diol

polyurethane

+NP

NP

Scheme 1. Nanocomposite formation by the SIP method.

Figure 7. AFM tapping mode phase images of the 65 wt% composites fabricated by (a) the DM method and (b) the SIP method.

decomposition temperature as observed in the DM sample.However, SIP composites show three different peaks. The firsttwo peaks are from the bulk matrix and the intermediate phase,and the latter one is from polyurethane, chemically attachedto the nanoparticle surface, respectively. This observationindicates that the SIP method further favours good bondingbetween nanoparticles and the polyurethane matrix except fora more uniform particle distribution, as indicated by the XPSand TGA analysis.

All the above analyses indicate the physicochemicaladsorption of catalyst and accelerator (CA) on the nanoparticlesurface and the subsequent surface-initiated polymerizationfor composite fabrication. Scheme 1 shows the reactionof the composite formation. Diisocyanate monomers withthe aid of the catalyst attached to the particle surfacereacted with physicochemically attached moisture and furthercopolymerized with diols to form a composite unity.

Particle dispersion within the polymer matrix was furtheranalysed by microscopic observation. Figure 7 shows thetypical atomic force microscope (AFM) phase image undertapping mode of a composite with 65 wt% loading. Clearagglomeration is observed in the composite fabricated by theconventional DM method, similar to the reported extrusionprocess [14]. However, discrete nanoparticles without obviousagglomeration are observed in the SIP composite, indicatinggood particle dispersion consistent with the TGA analysis.Here, scanning electron microscopy was further utilized tostudy the particle dispersion quality. Similar uniform particle

dispersion quality shown in figures 8(a) and (c) was observedwith extensive SEM examinations. The observed sphericalparticles in the nanoscale within the polymer matrix are a littlelarger than the iron-oxide nanoparticles due to the polymerwrapping. As shown in figures 8(b) and (d), larger clusterswith agglomerated nanoparticles wrapped with a continuouspolymer matrix were observed for the nanocomposite by theconventional DM method. This indicates that the SIP methodfavours the high-quality nanocomposite fabrication due to thepolymer layer chemically bound to the nanoparticle during theSIP process in solution, which prevented agglomeration via thesteric hindrance forces.

Large cracks were observed in the nanocompositesfabricated by a conventional DM method as shown in the insetof figure 9, whereas no sign of cracking were observed in thepure polyurethane and Fe2O3/PU nanocomposites fabricatedby the SIP method, especially in the high particle loading. Thelatter two specimens were much more flexible than the first.The flexibility of high-loading nanocomposites was furtherquantitatively evaluated by the tensile test. Figure 9 showsthe tensile stress–strain curves of the Fe2O3/PU compositesfabricated from the DM and the SIP method, respectively. TheYoung’s moduli and tensile strengths are almost the same forboth composites. However, the elongation is about four timesgreater than that of the DM composite. The strong chemicalbonding between nanoparticles and polyurethane, polymerstructure variation [14] with particle addition, and uniform

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Nanotechnology 18 (2007) 335704 Z Guo et al

Figure 8. Scanning electron microscope micrographs of the composites by the SIP method: (a) magnification 220 000 and (c) 44 000; and theDM method: (b) low magnification 22 000 and (d) high magnification 44 000.

Figure 9. Tensile stress–strain curves of 65 wt% nanocompositesfabricated by the DM and SIP methods; insets show the opticalmicrographs of composites by DM (left), SIP (right) and purepolyurethane (middle).

particle distribution within the polymer matrix contribute to theobserved flexible behaviour in the SIP composites.

The effect of the polymer matrix on the magnetic propertyof the nanoparticles was investigated in a 9 T physicalproperties measurement system by Quantum Design. Figure 10shows the hysteresis loops of the pure as-received Fe2O3

nanoparticles and the SIP Fe2O3/PU nanocomposites with aloading of 65 wt%, respectively. The observed non-zero valuesof the coercive force (coercivity, Hc) in both the as-receivednanoparticles and the polymeric nanocomposite indicate theferromagnetic state. Coercivity was used to classify thematerials into soft (less than 100 Oe) and hard (larger than

Figure 10. Magnetic hysteresis loop of the nanocomposite by SIPand pure Fe2O3 nanoparticles (inset shows the enlarged loop at lowfield).

100 Oe) materials [38]. With the magnetic nanoparticlesuniformly dispersed into the polyurethane matrix, Hc increasedas compared with the pure nanoparticles assembly, whilemaintaining the same saturation magnetization (Ms). Thisindicates that the incorporation of nanoparticles into thepolymer matrix in the polymer nanocomposites induced thematerials much harder. The coercivity is reported to bestrongly dependent on the size of particles and to be the largestin the nanoparticles with a single-domain size (166 nm forthe γ -Fe2O3) [38]. In other words, the coercivity decreaseswhen the nanoparticles are larger or smaller than this single-domain size. The increased coercivity as shown in the insetof figure 10 is attributed to the decreased interparticle dipolar

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Nanotechnology 18 (2007) 335704 Z Guo et al

interaction arising from the increased interparticle distancewithin the single-domain as compared with the close contactof the pure nanoparticles, and also attributed to the polymer–particle interfacial effect [39] due to the large portion of theatoms exposed to the polymer.

4. Conclusion

In conclusion, flexible magnetic Fe2O3/PU nanocompositeswith high particle loading were fabricated by utilizinga surface-initiated polymerization (SIP) method. Surfaceengineering the nanoparticles with the SIP method effectivelyimproves the quality of the high particle loading Fe2O3/PUnanocomposite with a uniform distributed particle and strongchemical bonding. As a result, high flexibility was observedin the SIP nanocomposites, which renders possible coatingindustrial applications in the areas of electromagnetic waveabsorbers and communication systems. The reported SIPnanocomposite fabrication method is general and has thepotential to be used in other nanoparticle and polymersystems. Flexible nanocomposites filled with higher saturationmagnetization zero-valence metallic nanoparticles, such asmetallic iron, have been achieved. The initial results showfairly well selective electromagnetic reflection loss in theSIP fabricated nanocomposite [40, 41] and make them goodcandidate materials for giant magnetoresistance (GMR) sensorfabrication [15].

Acknowledgments

This work was supported by the Air Force Office ofScientific Research through grant FA0550-05-1-0318 managedby Dr B Les Lee. DPY kindly acknowledges the support fromthe National Science Foundation under grant no. DMR 04-49022.

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