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Preparation and properties of poly(acrylic acid) oligomer stabilized
superparamagnetic ferrofluid
Chia-Lung Lin, Chia-Fen Lee
1, Wen-Yen Chiu
∗Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan, Republic of China Received 20 February 2005; accepted 9 May 2005
Available online 11 July 2005
Abstract
Ferrofluids, which are stable dispersions of magnetic particles, behave as liquids that have strong magnetic properties. Nanoparticles of magnetite with a mean diameter of 10–15 nm, which are in the range of superparamagnetism, are usually prepared by the traditional method of co-precipitation from ferrous and ferric electrolyte solution. When diluted, the ferrofluid dispersions are not stable if anionic or cationic surfactants are used as the stabilizer. This work presents an efficient way to prepare a stable aqueous nanomagnetite dispersion. A stable ferrofluid containing Fe3O4nanoparticles was synthesized via co-precipitation in the presence of poly(acrylic acid) oligomer. The
mechanism, microstructure, and properties of the ferrofluid were investigated. The results indicate that the PAA oligomers promoted the nucleation and inhibited the growth of the magnetic iron oxide, and the average diameter of each individual Fe3O4particle was smaller
than 10 nm. In addition, the PAA oligomers provided both electrostatic and steric repulsion against particle aggregation, and the stability of dispersions could be controlled by adjusting the pH value of solution. A small amount of Fe2O3was found in the nanoparticles but the superparamagnetic behavior of the nanoparticles was not affected.
2005 Elsevier Inc. All rights reserved.
Keywords: Fe3O4; Poly(acrylic acid); Ferrofluid; Superparamagnetic; Nanoparticles
1. Introduction
Ferrofluids are colloidal dispersions of small single-domain magnetic particles suspended in a carrier fluid. Ferrofluids characteristically have both magnetic and fluid properties and have found a diverse range of applications, such as in audio devices, inertia dampers, stepper motors, sensors, vacuum seals, electromagnetic shielding, and high-density digital storage[1,2]. In general, finer particles are more surface active; i.e., they have a higher specific sur-face area. On the other hand, it is often difficult to handle fine particles such as colloids. For example, the filtration will be time-consuming or impractical, or else a centrifuge has to be used. But unlike other ultrafine particles,
mag-* Corresponding author. Fax: +886 2 23623259.
E-mail address:[email protected](W.-Y. Chiu).
1 Present address: Department of Cosmetic Science, Chia Nan University
of Pharmacy and Science, Tainan, Taiwan, Republic of China.
netic particles can be handled by an external magnetic field. Therefore, ferrofluids are widely studied for their appli-cations in various fields in biology and medicine such as enzyme and protein immobilization, genes, radiopharma-ceuticals, magnetic resonance imaging MRI, diagnostics, immunoassays, purification, separation, and controlled drug release [3–6]. However, the nanoparticles in the ferrofluids are likely to aggregate. Consequently, how to prevent aggre-gation between the nanoparticles during synthesis and treat-ment has received considerable attention when the applica-tions need well-dispersed magnetic nanoparticles. Although well-dispersed magnetic nanoparticles can be obtained by ball milling[7], high-energy requirements and unavoidable contamination of the product have necessitated the devel-opment of more economical and reliable ways to produce magnetic particles by chemical coprecipitation.
Iron oxide (Fe3O4), the dominant magnetic material in magnetic fluid preparations, can be synthesized through the coprecipitation of Fe(II) and Fe(III) salts by addition of a 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved.
base. Stable ferrofluids can be prepared by adsorption of sta-bilizing agents on the surface of magnetite particles after a synthesis process. The feasibility of preparing magnetic fluids using anionic or nonionic surfactants as dispersing agents has been examined [8–10]. Massart obtained stable aqueous alkaline and acidic magnetic liquids by free pre-cipitation[11,12]. The nature of the counterions and the pH of the suspensions played an important role in stabilizing the charged magnetic particles through interactions between their electrical double layers. The applications of magnetic fluids, which are stabilized entirely by electrostatic repul-sion, are somewhat restricted, since this system is overly sensitive to conditions such as pH and ionic strength and of-fers little flexibility for changing the surface properties of the particles. Stabilization of magnetic fluid suspensions can be achieved by coating the particle surfaces with organic sur-factants. When two such particles approach each other, the surfactants interpenetrate and the particles are subjected to steric repulsion due to an increase in osmotic pressure[8,13]. The traditional methods of preparing ferrofluids in-volve long-term grinding of magnetic materials in a carrier medium containing oleic acid, which is commonly used as a dispersing agent to stabilize dispersions in kerosene and other hydrocarbon dispersion media. This method has prompted the application of the same technique to prepare a water-based magnetic fluid. The latter preparation was antic-ipated to be much simpler, since one usually starts with aque-ous solutions of ferraque-ous and ferric salts. Shimoiizaka et al. proposed the principle of bilayer stabilization in the early 1980s[14]. They first precipitated oleic acid-coated particles and then redispersed them in aqueous solutions of sodium dodecylbenzene sulfonate, poly(oxyethylene) nonylphenyl ethers, and di(2-ethylhexyl)adipate. The colloidal particles are thus doubly coated, first with a chemisorbed monolayer of oleic acid on top of which a second layer of the surfac-tant is physically adsorbed. Following this work, Khalafalla and Reimers [7] and Wooding et al. [15]produced stable aqueous magnetic fluids using various saturated and unsat-urated fatty acids as primary and secondary surfactants. In our previous studies[16,17], we have also synthesized fer-rofluids by adsorbing stabilizing agents, lauric acid, after the co-precipitation process [7,16–18]. However, the fer-rofluids thus prepared were not stable when being diluted. Aggregation of particles occurred when excess quantity of water was added. Applications of magnetite dispersions composed of electrostatic stabilizers and bilayer surfactants are restricted due to pH sensitivity and dispersion stability, respectively[7].
Recently, aqueous magnetic fluids have been reported using polymers as steric stabilizers and polymer tem-plates [19–24]. But it should be noted that many of the reported polymeric stabilizers have not been designed with functional groups to bind to the surface of Fe3O4particles, and thus, stability of ferrofluid has been limited[24]. This study presents a novel and simple procedure to synthesize an extremely stable and water-based Fe3O4ferrofluid via a
co-precipitation method in the presence of poly(acrylic acid) oligomers. The PAA oligomers were chemically bound on the surface of Fe3O4 nanoparticles to suspend the Fe3O4 particles in a fine state of dispersion, and the PAA oligomers provided both electrostatic and steric repulsion against par-ticle aggregation.
2. Experimental
2.1. Materials
Fe(II) chloride tetrahydrate (99%), Fe(III) chloride hexa-hydrate (97%), and ammonium hydroxide (28% NH3 in water, w/w) were purchased from ACROS and used as sup-plied. Poly(acrylic acid) (PAA oligomer: Mw∼ 2000) was purchased from Aldrich. Other chemicals were of analyti-cal grade and used without further purification. Distilled and deionized water was used throughout the work.
2.2. Synthesis of ferrofluid
The stable Fe3O4 ferrofluid was synthesized using the method of coprecipitation of Fe(II) and Fe(III) salts in the presence of PAA oligomer (Mw ∼ 2000). In this process, 1 g of PAA oligomer (Mw∼ 2000), 4.75 g of FeCl3·6H2O, and 1.75 g of FeCl2·4H2O were dissolved in 80 ml of wa-ter with vigorous stirring at 60◦C under N2. Then 7.5 ml of NH4OH (28% (w/w)) was quickly added to the reactor and stirring was continued at 400 rpm for 30 min. Once the ammonium solution was added into the reactor, the color of the mixture turned from orange to black immediately. The ferrofluid thus obtained was designed by the sample code of PAA/MAG= 1/2, in which the weight ratio of PAA oligomers to Fe3O4particles was theoretically calculated as 1/2 from the feed composition.
2.3. Observation of Fe3O4nanoparticles
To observe the morphology of PAA oligomer-stabilized Fe3O4particles, the ferrofluid was diluted in deionized water and observed by using a TEOL JSM-1200 EX II transmis-sion electron microscope (TEM). In addition, the ferrofluid was coated on a glass plate and dried naturally, and then was observed by using a Hitachi S-800 scanning electron micro-scope (SEM).
2.4. Measurement of FTIR
To prepare the samples for the FTIR measurements (Bio-Rad FTS-3000 spectrometer), the PAA oligomers which were used as supplied were grind-blended with KBr and then compressed to form a pellet. In addition, the ferrofluid was dialyzed (molecular weight cut off MWCO= 6000– 8000) against deionic water until the conductivity of the so-lution was stable and then was freeze-dried to form the PAA
oligomers stabilized Fe3O4 particles. In the same way, the sample of PAA oligomers stabilized Fe3O4 particles were grind-blended with KBr and then compressed to form a pel-let for the measurement of FTIR. The transmission spectra were obtained for 64 scans at a resolution of 4 cm−1.
2.5. Measurement of particle size distribution and zeta potential of Fe3O4nanoparticles
The zeta potential and the size distribution of the PAA oligomers stabilized Fe3O4 particles were measured by us-ing laser light scatterus-ing (Malvern Zeta Sizer 3000H) at 25◦C. Before the measurements, the ferrofluids were dia-lyzed for 7 days and diluted with distilled and deionized water.
2.6. Stability of ferrofluid
To investigate the effect of pH value on the stability of fer-rofluid, the pH value of the ferrofluid was measured during the process of titration using 28% (w/w) ammonium solu-tion as a titrant. When the pH value of solusolu-tion was stable for several minutes, the stability of the particles suspended in the fluid was observed visually. Furthermore, the effect of acid medium on the stability of the PAA oligomers stabi-lized Fe3O4particles was studied by using methacrylic acid as a titrant to titrate the ferrofluid, which contained 2 ml 28% (w/w) ammonium solution, and the pH value and stability of the Fe3O4particles during the process of titration were ob-served.
2.7. TGA and XRD analysis
The ferrofluid was dialyzed until the conductivity of the solution was stable. Then the ferrofluid was freeze-dried to produce the PAA oligomer-stabilized Fe3O4 particles. The Fe3O4particles were subjected to TGA (Perkin–Elmer TGA-7). The temperature was kept at 100◦C for 10 min and then was raised to 800◦C at a heating rate of 10◦C/min. Ad-ditionally, X-ray diffraction measurements with monochro-matic CuKα radiation were taken to investigate the crystal
structure of the particles.
2.8. Measurement of magnetization of Fe3O4particles The magnetization of the PAA oligomer-stabilized nano-particles were measured by SQUID magnetometer (su-perconducting quantum interface device, Quantum Design MPMS5) at 298 K and±10,000 G applied magnetic field. Due to the applied magnetic field (H ), the magnetic induc-tion (B) could be measured according to Faraday’s law, and the magnetization (M ) was then obtained by the relationship (1) B= H + 4πM.
The applied magnetic field was increased until the satura-tion magnetizasatura-tion was achieved. Then the applied field was
reduced to zero to measure the remanence (residual magne-tization). When the applied field was further reversed, the coercivity (the applied field that reduces magnetization to zero) and the saturation magnetization in the reverse direc-tion could be obtained. This increasing and decreasing ap-plied field process was repeated five times to get the magne-tization curve (magnemagne-tization (M ) versus applied magnetic field (H )) and examine the magnetic properties of the Fe3O4 particles.
In addition, the important parameters used to character-ize the magnetic properties of solids, magnetic susceptibil-ity (κ), and magnetic permeabilsusceptibil-ity (µ) were also calculated using the following equations:
(2) κ= M/H,
(3) µ= B/M.
3. Results and discussion
3.1. Synthesis of ferrofluid
The traditional method of preparing Fe3O4nanoparticles was to use the chemical co-precipitation of ferric and ferrous salts in alkaline medium:
Fe2++ 2Fe3++ 8OH− Fe(OH)2+ 2Fe(OH)3
→ Fe3O4+ H2O. (4)
However, the Fe3O4nanoparticles synthesized by this tra-ditional method would aggregate easily, as shown inFig. 1a. To prevent aggregation, the PAA oligomers were used to modify the traditional chemical co-precipitation method to synthesize the stable superparamagnetic ferrofluid. The fer-rofluid containing the Fe3O4 nanoparticles could be sus-pended homogeneously and stably in water or monomer so-lution as shown in Fig. 1b. The proposed mechanism was shown inFig. 2.
At first the Fe2+ and Fe3+ ions and the carboxylic acid groups of PAA oligomers formed a complex structure. After the ammonium solution was added, carboxylic acid groups on the PAA oligomer promoted the nucleation of Fe3O4 par-ticles. Because the PAA oligomers acted as the template for Fe3O4nucleation, the growth of particles would be hindered by the oligomers [25,26]and the size of the particles was about 5–10 nm, which was smaller than the traditional prod-uct. Also, a lot of carboxylic acid groups were bound on the surface of Fe3O4nanoparticles to suspend the particles in a fine state of dispersion as discussed later. The Fe3O4 nanoparticles could be synthesized in situ and form a hyper-branch structure as shown inFigs. 3a and 3b. But there is no evidence to explain the formation of a hyperbranch structure. From the chemical equation of Fe3O4 precipitation, Eq. (4), the suitable mole concentration ratio of Fe(II) to Fe(III) should be 1/2 for the synthesis of Fe3O4 nanopar-ticles. In addition, the suitable pH value for the synthesis of Fe3O4 nanoparticles should be higher than 6.93, which
Fig. 1. TEM photographs of (a) Fe3O4 nanoparticles prepared by the
method of co-precipitation in the absence of PAA oligomers; (b) ferro-fluid prepared by the method of co-precipitation in the presence of PAA oligomers.
Fig. 2. Mechanism for the synthesis of ferrofluid: the nucleation of nanopar-ticles.
could be calculated using (4) for Ksp values of Fe(OH)2 and Fe(OH)3 at 25◦C of 8× 10−16 and 4× 10−38. If the mole concentration ratio of Fe(II) to Fe(III) was larger than
Fig. 3. SEM photographs of (a) ferrofluid prepared by the method of co-precipitation in the presence of PAA oligomer; (b) magnification of (a). 1/2, not only Fe3O4 but also Fe(OH)2 would be produced. Moreover, as shown in Eq.(5), Fe(OH)3would be in excess and then converted to Fe2O3 if the mole concentration ra-tio of Fe(II) to Fe(III) were smaller than 1/2 or the pH value were lower than 6.93. In the reaction system we used, the mole concentrations of Fe(II) and Fe(III) were 0.1105 and 0.221 M and the pH value of the ferrofluid after synthesis was 8.8. Therefore, these conditions of reaction were suit-able to form Fe3O4nanoparticles:
2Fe(OH)3→ Fe2O3+ 3H2O. (5)
3.2. FTIR analysis
Since there are a large surface-to-volume atomic ratio, high surface activity, and a large amount of dangling bonds on the nanoparticle surface, the atoms on the surface are apt to adsorb ions or molecules in solution. In the system of Fe3O4nanoparticles dispersed in an alkaline solution of PAA oligomers, the exposed atoms of Fe on the surface of Fe3O4 particles would adsorb OH− and COO− groups of
Fig. 4. FTIR analysis of Fe3O4nanoparticles, PAA oligomers, and the
fer-rofluid prepared by the method of co-precipitation in the presence of PAA oligomers.
PAA oligomers, and the atoms of O on the particle surface would adsorb H+. In addition, the OH-rich surface of Fe3O4 nanoparticles could also react with the residual carboxylic acid groups of PAA oligomers.
Fig. 4shows a comparison between the FTIR spectra of the Fe3O4nanoparticles, the pure PAA oligomers, and the ferrofluid prepared by co-precipitation in the presence of PAA oligomer. Previously, it was reported that the charac-teristic absorption bands of the Fe–O bond of bulk Fe3O4 were at 570 and 375 cm−1 [27]. However, when the size of Fe3O4 particles was reduced to nanoscale dimensions, the surface bond force constant increased due to the ef-fect of finite size of nanoparticles, in which the breaking of a large number of bonds for surface atoms resulted in the rearrangement of nonlocalized electrons on the parti-cle surface [28]. Therefore, the FTIR spectrum of Fe3O4 nanoparticles would exhibit a blue shift and the character-istic absorption bands of the Fe–O bond were shifted to high wavenumbers of about 580 and 434 cm−1, as shown inFig. 4. The strong band at ca. 1710 cm−1 as shown in the FTIR spectrum of PAA oligomer is attributed to the car-bonyl group stretch of the PAA oligomer[29], and the bands at ca. 1400–1450 cm−1are attributed to the stretch of C–O and the deformation vibration of OH[29]. In addition, the bands at ca. 1150–1280 cm−1are attributed to the aliphatic acid[29]. The FTIR spectrum of the ferrofluid shows that the peak at 1710 cm−1shrank and two new peaks at ca. 1540
Fig. 5. The zeta potential of the ferrofluids: (a) the effect of pH values at constant PAA/MAG ratio; (b) the effect of PAA/MAG ratio at constant pH value.
and 1405 cm−1appeared due to the binding of the carboxylic acid groups to the surface of the nanoparticles to form car-boxylate groups [30,31]. The new peaks correspond to the COO− antisymmetric vibration and the COO− symmetric vibration, which indicate the bidentate bonding of the car-bonyl groups to the surface Fe atoms[32,33]. The remain-ing but shrunken peak at 1710 cm−1is due to C=O stretch and indicates that some fraction of the PAA oligomers were bonded to nanoparticles either in monodentate form or as an acid[32,33]. Consequently, the interaction between PAA oligomers and Fe3O4 nanoparticles was through chemical bonding and the mechanism shown inFig. 2was evidenced.
3.3. Zeta potential and light scattering analysis
The negative zeta potential indicates that the PAA oligo-mers are bound to the particle surface. Fig. 5a shows the zeta potentials of sample PAA/MAG= 1/2 in the solutions with different pH values. When the solution became more alkaline, the zeta potential became more negative that may due to the ionization of PAA oligomers. And the ionization of PAA oligomers would provide the electrostatic repulsion against the aggregation between nanoparticles. So Fig. 6a
Fig. 6. The size distribution of ferrofluids (with ultrasonification before the light scattering measurement): (a) the effect of pH values at constant PAA/MAG ratio; (b) the effect of PAA/MAG ratio at constant pH value. shows that the sample in the solution with higher pH value would show small and narrow size distribution.
The zeta potentials of the samples with different PAA/ MAG ratio were not changed drastically if the pH value of solutions were the same, as shown in Fig. 5b. ButFig. 6b shows that the sample with a higher concentration of PAA oligomer showed a small and narrow size distribution. The reason was due to the PAA oligomers that acted as tem-plates and steric stabilizers, forming a wall to protect the particles. So the addition of the PAA oligomers in the reac-tion provides steric repulsion to improve the quality of the dispersion. These effects of polymeric stabilizers in metal-lic nanoparticle preparations were also discussed in previ-ous studies [19–22]. Therefore, the PAA oligomers, which bonded to the surfaces of Fe3O4 particles, not only reduce the intrinsic particle size during their forming but also pro-vide both factors of electrostatic and steric repulsion against the aggregation between Fe3O4particles.
In addition, it is worth mentioning that when there was a high concentration of PAA oligomers in the ferrofluid, the effect of physical crosslinking due to the hydrogen bonding among COOH groups or the entanglement of PAA oligomers
Fig. 7. Effect of ultrasonification before the light scattering measurement on the size distribution of ferrofluid.
would cause a pseudo-aggregation of nanoparticles several weeks after the ferrofluid was synthesized, and therefore the size distribution of Fe3O4 measured by light scattering was much larger than its intrinsic size if ultrasonification was not applied before the measurement.Fig. 7compared the size distribution of the Fe3O4ferrofluid aggregates with or without ultrasonification before the light scattering mea-surement. It indicates that the Fe3O4particles aggregated as clusters with a size range of 150–450 nm in the presence of high concentrations of PAA oligomers. The ultrasonifica-tion then destroyed the pseudo-aggregaultrasonifica-tion in clusters and the size distribution of ferrofluid aggregates was reduced to about 10–40 nm.
3.4. Stability of ferrofluid
Because the ferrofluid was stabilized by PAA oligomers, the stability of Fe3O4nanoparticles in a dispersing medium could be controlled by the ionization of carboxylic acid groups of PAA oligomers. The adjustment of the pH value of the medium solution could control the ionization degree of PAA oligomers and thus the stability of the ferrofluid.
The pKavalue of PAA oligomers (Mw∼ 2000) could be
calculated to be 4.09 by Eqs.(6) and (7). This value is close to the pKavalue of the aliphatic acid (4.0–4.5).
The concentration of carboxylic acid groups is
(6) C0= m/M V and (7) pKa= − log (COO−)(H+) (COOH) = − log (10−pH)(10−pH) C0− 10−pH ,
in which the pH value of the solution was measured to be 2.43 by dissolving 1 g PAA oligomer (m) in 80 g (≈80 L) of deionic water (V ), and M is the molecular weight of the repeating unit of the PAA oligomer (72 g/mol). From fur-ther calculation, the relationship between the pH value and
Fig. 8. The pH value and stability of ferrofluid observed during the process of titration by using 28% (w/w) ammonium solution as a titrant. (The sta-bility of the ferrofluid was observed visually after the pH value was stable for several minutes.)
the degree of ionization of carboxylic acid groups of PAA oligomer could be found as follows:
(8) pKa= − log C0(d)(H+) C0(1− d) = pH − log d 1− d .
The symbol d used here is the ionization degree of acid groups. To simplify the calculation, it was assumed that more than 99.99% of carboxylic acid groups were ionized to provide electrostatic repulsion to form the most stable fer-rofluid. The pH value under this condition was found to be higher than 8.1. In other words, the carboxylic acid groups of PAA oligomers were almost completely ionized and the ferrofluid was stable when the pH value was higher than 8.1. In our experiments, after the addition of ammonium solu-tion into the reacsolu-tion system to synthesize the ferrofluid, the final pH value of the ferrofluid was 8.8 which assured the formation of stable ferrofluids, and the ferrofluids thus syn-thesized would remain stable for several months. If the am-monium solution was added continuously as shown inFig. 8, the ferrofluid was still stable due to the acid groups being almost ionized, and not only steric repulsion but also electro-static repulsion were provided against particle aggregation. However,Fig. 9reveals that the pH value of the ferrofluid decreased gradually during the process of the titration by us-ing methacrylic acid as a titrant. Once the pH value of the ferrofluid was lower than 5, the ferrofluid became unstable within several seconds and the aggregation of particles could be observed visually. It was because the ionization degree of acid groups would be lower than 90% when the pH value of the ferrofluid was lower than 5, and this condition could not provide sufficient electrostatic repulsion to preserve the sta-bility of Fe3O4nanoparticle suspension in ferrofluid. Con-sequently, the stability of ferrofluid could be controlled by adjusting the pH value of the solution, and therefore the fer-rofluid could be applied properly.
Fig. 9. The pH value and stability of the ferrofluid observed during the process of titration by using methacrylic acid as a titrant. (Before the process of titration, 2 ml 28% (w/w) ammonium solution was added into the ferrofluid. The stability of the ferrofluid was observed visually after the pH value was stable for several minutes.)
Fig. 10. XRD pattern of the pure Fe3O4nanoparticles (a), PAA oligomers
(b), and ferrofluid particles (c).
3.5. XRD analysis
To investigate the effect of PAA oligomers on the prepa-ration of Fe3O4nanoparticles, the Fe3O4nanoparticles pre-pared in the absence of PAA oligomers and the ferrofluid prepared in the presence of PAA oligomers were analyzed by XRD and the results are shown inFig. 10. The results show
that the Fe3O4nanoparticles prepared in the absence of PAA oligomers had six diffraction peaks at 2θ = 30.2◦, 35.7◦, 43.4◦, 53.7◦, 57.4◦, and 62.9◦, which were the characteris-tic peaks of standard Fe3O4crystal (isometric-hexoctahedral crystal system). However, the ferrofluid prepared in the pres-ence of PAA oligomers had an additional diffraction peak at ca. 2θ= 32.8◦, which was the characteristic peak of Fe2O3 crystal (trigonal–hexagonal scalenohedral crystal system). These results indicate that some Fe2O3 particles consisted in the ferrofluid because the particles were synthesized in the initial condition of low pH value (in PAA oligomer solution pH 2.43), although the pH value of the final ferrofluid was 8.8 after the addition of ammonium solution. At the begin-ning of the precipitation reaction, the acidic condition would oxidize the Fe3O4or oxidize the Fe2+to form Fe3+partially so that an initial mole concentration ratio of 1/2 in these ions would end up with excess quantities of Fe3+and form Fe2O3as mentioned in Eq.(5). In addition, the broad diffrac-tion peaks shown inFig. 10indicate that the particle size of magnetite was very small according to the Debye–Scherrer equation[34,35],
(9) D= 0.9λ
β cos θ,
where D is the average crystallite size (Å), λ is the wave-length of X-rays (CuKα: λ= 1.5418 Å), θ is the Bragg
diffraction angle, and β is the full width at half maximum (FWHM) (in radians). By using Eq.(9)and the FWHM of the diffraction peak at 2θ= 35.7◦, the crystallite size of the Fe3O4 precipitated in the presence of PAA oligomer could be calculated to be 5.2 nm, which was smaller than the crys-tallite size of Fe3O4(8.3 nm) synthesized in the absence of PAA oligomers. This result also shows that the presence of PAA oligomers reduced the particle size of Fe3O4 dramati-cally due to the fact that the carboxylic acid groups of PAA oligomers not only promoted nucleation but also inhibited growth of the Fe3O4 particles. Furthermore, the broad dif-fraction peak appeared in the range from 10◦to 40◦indicates the existence of amorphous PAA oligomers coated on the surface of Fe3O4particles.
3.6. TGA analysis
Fig. 11 shows the TGA analysis of the pure Fe3O4 nanoparticles, the PAA oligomer (Mw ∼ 2000), and the ferrofluid particles synthesized in the presence of PAA oligomers. The pure Fe3O4nanoparticles showed insignifi-cant weight loss from 100 to 800◦C. But there were three stages during degradation shown in the TGA curve of PAA oligomers. The initial two stages of degradation (250– 300◦C) was due to the dehydration (Eq.(10))[36–38]and decarboxylation (Eq. (11)) [36–38] of the carboxylic acid groups of the PAA oligomers, while the main degradation
Fig. 11. TGA analysis of the pure Fe3O4nanoparticles, the PAA oligomer
(Mw∼ 2000), and the ferrofluid particles synthesized in the presence of
PAA oligomers.
temperature of the PAA oligomer was near 400◦C:
(10)
(11) The TGA curve of ferrofluid particles also showed three-stage decomposition. The first-three-stage decomposition, which occurred near 200◦C, was due to dehydration (Eq. (12)), which was caused by the destruction of polyelectrolyte com-plex between COO−and NH+4 and the formation of amide bondings
(12) Then the temperature was raised to 400◦C to enable the main degradation of PAA oligomer to occur. The interest-ing decomposition which occurred at 700◦C was due to the deoxidization of the Fe2O3particles, which consisted in the ferrofluid as mentioned above, to form Fe3O4at 700◦C un-der N2 atmosphere. Therefore, both the decomposition of PAA oligomers and the deoxidization of Fe2O3contributed to the weight loss of the TGA analysis of ferrofluid parti-cles. The residual weight percentage of ferrofluid particles after the TGA analysis (about 60%) was mainly the con-tribution of the Fe3O4 nanoparticles and a small amount of char yield induced from PAA oligomers. The results in-dicate that the weight ratio of PAA oligomers (∼40%) to Fe3O4nanoparticles (∼60%) was nearly 1:2, where the fer-rofluid was dialyzed before the TGA measurement. Theoret-ically, from the feed composition the weight ratio of PAA oligomers to Fe3O4nanoparticles in the ferrofluid was
cal-Fig. 12. SQUID magnetization curve of the ferrofluid particles synthesized in the presence of PAA oligomers.
culated to be also 1/2. It could be concluded from this re-sult that the PAA oligomers (Mw∼ 2000) were chemically bonded on the surface of the magnetite nanoparticles and could not be removed during the dialysis even though the MWCO (molecular weight cut off) of the dialytic membrane was 6000–8000.
3.7. Squid analysis
It is well known that the Fe3O4 nanoparticles show a superparamagnetic property. But when there was a quan-tity of Fe2O3in the ferrofluid and the PAA oligomers were chemically bonded to nanoparticles, the magnetic property of the ferrofluid particles should be examined. The result of the SQUID analysis is shown inFig. 12. When a mag-netic field was applied, the dipolar particles aligned them-selves with the applied magnetic field, and resulted in a mea-surable magnetization. The saturation magnetization of the ferrofluid was about 35 emu/g (emu per gram of nanopar-ticles (iron oxides) in the ferrofluid, where the amount of nanoparticles was determined by using TGA analysis), which was lower than the saturation magnetization of pure Fe3O4 nanoparticles (50 emu/g), due to the fact that the saturation magnetization reduced significantly when the par-ticle size was smaller than 10 nm[39]. Also, the saturation magnetization of Fe2O3 nanoparticles was lower than that of Fe3O4 nanoparticles [40]. So the saturation magnetiza-tion of nanoparticles in the ferrofluid, which consisted of a small amount of Fe2O3 nanoparticles, was lower than that of pure Fe3O4 nanoparticles. Furthermore, the remanence and coercivity were zero, and there was no magnetic hys-teresis loop as shown inFig. 12. The magnetic susceptibility and permeability were calculated to be 0.024 and 1.302 (in the applied magnetic field from 0 to 100 G), which reveals superparamagnetic behavior[18,41]. These results indicate that the magnetic nanoparticles synthesized in the presence of PAA oligomers were superparamagnetic.
4. Conclusion
In this work, the method of co-precipitation was used to synthesize a water-based, stable ferrofluid that contained Fe3O4nanoparticles in the presence of PAA oligomers. The PAA oligomers promoted the nucleation and inhibited the growth of the magnetic iron oxide so as to produce the smaller Fe3O4nanoparticles. In addition, a lot of carboxylic acid groups of PAA oligomers were bound on the surface of Fe3O4nanoparticles to suspend the Fe3O4particles in a fine state of dispersion. The PAA oligomers provided both elec-trostatic and steric repulsion against particle aggregation, and the stability of dispersions could be controlled by ad-justing the pH value of solution. Although there were some Fe2O3particles formed in the ferrofluid in the presence of PAA oligomers, the remanence and coercivity of the fer-rofluid particles were zero and the magnetic hysteresis loop was not shown. It was concluded that the nanoparticles syn-thesized in this work were superparamagnetic.
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