Influence of Distribution on the Heating of Superparamagnetic Iron

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Influence of Distribution on the Heating of Superparamagnetic Iron Oxide Nanoparticles in Poly(methyl methacrylate) in an Alternating Magnetic Field Stefan A. Rovers,*,† Carin H. J. T. Dietz,† Leon A. M. v. d. Poel,† Richard Hoogenboom,‡ Maartje F. Kemmere,† and Jos T. F. Keurentjes† Process DeVelopment Group, Department of Chemical Engineering and Chemistry, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, and Dolphys Medical, Den Dolech 2, 5612 AZ EindhoVen, The Netherlands ReceiVed: December 17, 2009; ReVised Manuscript ReceiVed: April 6, 2010

The effect of distribution on the heating of superparamagnetic iron oxide nanoparticles in a polymer matrix has been investigated in an alternating magnetic field. Commercially available particles have been distributed using 30 and 50 wt % loading in a poly(methyl methacrylate) (p(MMA)) matrix by different preparation methods, resulting in different distributions of the particles. Freeze-drying a mixture of ferrofluid and p(MMA) latex followed by compounding is found to diminish particle aggregation, leading to an optimal distribution. Subsequently, the heating of the particles in the nanocomposites has been investigated in an alternating magnetic field of 2850 A m-1 with a 745 kHz frequency. These heating experiments show significantly higher specific absorption rates (SARs) of the incorporated iron oxide particles in the case of the freeze-drying method due to the improved distribution of the particles when compared to direct compounding or solvent casting. Furthermore, the higher particle loading provides faster heating, although the SAR decreases due to the presence of larger aggregates.

( )

1. Introduction

τN ) τ0 e

In recent years, remote heating of a polymer in a magnetic field has been studied by a number of groups for several applications including remote shape changing1,2 and externally triggered drug delivery.3-5 In drug delivery, magnetic particles are often coated with a polymer that exhibits a lower critical solution temperature (LCST). Magnetically heating the particles to above the LCST results in drug release from the polymer coating. Even though this polymer phase transition is reversible, often the majority of the incorporated drug is squeezed out in a single dose.5 For repetitive drug release, the glass transition of a polymer can be used as a reversible switch for externally triggered on-demand drug delivery.6 Below the glass transition temperature (Tg) of the polymer (T < Tg), the diffusion of the incorporated drug is low. By magnetically heating the polymer above the Tg (T > Tg), the diffusion coefficient of the drug is increased significantly. As the drugs are not squeezed out of the polymer and the drug concentration in the polymer matrix is constant due to the presence of drug crystals that replenish the released drug, this system can be used for multiple doses. Moreover, magnetic particles have been incorporated into polymer matrices to distinguish between Ne´el and Brown relaxation of superparamagnetic iron oxide nanoparticles.7-9 Ne´el and Brown relaxation are two processes by which such nanoparticles can be heated in an alternating magnetic field. Ne´el relaxation is the reorientation of the magnetic moment within the particle in order to stay aligned with the changing field direction. Thereby, an anisotropy barrier is crossed, resulting in a temperature increase. The Ne´el relaxation time is given by10 † ‡

Eindhoven University of Technology. Dolphys Medical.

KV kbT

(1)

where τ0 is the exponential prefactor of 10-9 s, K is the anisotropy constant of magnetite of 8 kJ m-3,7,11 V is the volume of the particle core [m3], kb is the Boltzmann constant [J K-1], and T is the temperature [K]. Brown relaxation is the reorientation of the magnetic particle itself in a fluid, resulting in friction and, consequently, heating of the particle.12 By dispersing iron oxide particles in a polymer matrix, Brown relaxation can be excluded and Ne´el relaxation is the only occurring relaxation process. By comparing the heating of the particles in a ferrofluid to that in the polymer matrix, the contribution of both processes in the ferrofluid can be determined. The decreased specific absorption rate of incorporated particles compared to particles in fluid is often accredited to the loss of Brown relaxation.8 However, the effect of the distribution of the particles in the polymer matrix on the heating of such composites has not been discussed in the literature. In the present work, the effect of different preparation methods on the distribution of commercially available iron oxide nanoparticles into a poly(methyl methacrylate) (p(MMA)) matrix has been investigated. Subsequent studies on heating the resulting nanocomposites in an alternating magnetic field have been performed to evaluate the effect of particle distribution on the specific absorption rate (SAR). Using incorporated iron oxide nanoparticles to specifically heat a polymer above its Tg, the particle distribution should be optimum for heating. A maximum specific absorption rate of the particles would minimize the amount of required particles, minimizing potential hindering of drug diffusion. It is expected from literature13 that the agglomeration of iron oxide particles will increase interparticles interactions, decreasing the heating by Ne´el relaxation. Presumably, the iron oxide

10.1021/jp911944y  2010 American Chemical Society Published on Web 04/20/2010

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particles require addition chemical or physical stabilization during processing to prevent significant particle agglomeration. 2. Materials and Methods 2.1. Materials. The iron oxide nanoparticles used in this study were purchased from Ferrotec, Germany. Poly(methyl methacrylate) (p(MMA)) (MW 120 000), tetrahydrofuran (THF) (99+ %), methyl methacrylate (MMA) (99.5+ %), and lauric acid (99.5+ %) were purchased from Sigma Aldrich. Tetramethyl ammonium hydroxide (25% in water) and potassium persulfate (99+ %) were purchased from VWR International. 2.2. Distribution of EMG705 Particles in Polymer Matrix. The water-based iron oxide nanoparticles, EMG705, were distributed in p(MMA) in two ways. In the first method, 2.5 g of p(MMA) was added to a preheated custom built double cone screw compounder with a volume of 5 cm3, set at 190 °C and rotating at 100 rpm. Subsequently, 7.5 mL of EMG705 ferrofluid (33 wt % particles) was added dropwise. After 15 min, cylindrical bars of approximately 10 mm with a diameter of 3 mm were extruded from the compounder. Additionally, EMG705 nanoparticles were distributed by mixing a p(MMA) latex and the EMG705 ferrofluid. Subsequently, this aqueous mixture was freeze-dried using a Labconco Freezone 4.5 instrument in combination with a Chemstar 1402N vacuum pump, operated at 84 × 10-3 mbar. The resulting powder was compounded as described above. The p(MMA) latex was prepared by emulsion polymerization at 80 °C using 50 g of water, 5 g of tetramethyl ammonium hydroxide, 1.8 g of lauric acid, 70 g of methyl methacrylate (MMA), and 1 g of potassium persulfate under argon atmosphere. The resulting p(MMA) particles were measured by dynamic light scattering (DLS) and were found to have a size of 54.8 ( 14.4 nm. 2.3. Distribution of EMG1200 Particles in Polymer Matrix. The iron oxide coated for organic media, EMG1200, was distributed in p(MMA) by solvent casting. For this purpose, EMG1200 particles were suspended in THF for 2 h by mixing in an ultrasound bath. Subsequently, a solution of p(MMA) in THF was added in the appropriate ratio. After mixing for 1 h, the suspension was casted into a Petri dish and covered with perforated aluminum foil to slowly evaporate THF and to prevent the formation of bubbles. Subsequently, the casted film was removed, grinded, and compounded into cylindrical bars, as described above. 2.4. Characterization. The size of the iron oxide nanoparticles was determined by transmission electron microscopy. Images were taken of the EMG705 and EMG1200 particles. TEM samples were prepared by diluting EMG705 ferrofluid (100× w/w) and suspending EMG1200 particles in THF (0.03 wt %) and placing a single drop on a carbon coated copper grid. The presence and size of agglomerates in the EMG705 ferrofluid and EMG1200 nanoparticles suspended in THF were determined using DLS; furthermore, the agglomerates in the EMG705 ferrofluid were confirmed by cryo-TEM images. DLS was performed using a Coulter N4 Plus particle size analyzer measuring three times for 900 s at 20 °C at an angle of 90°, and data were analyzed using the cumulant method. Cryo-TEM samples were prepared on carbon coated copper grids by injection into liquid ethane using a Vitrobot Mark III instrument. In this work, a FEI Tecnai G2 Sphera cryo-TEM apparatus was used, operating at 200 kV. The distribution of the iron oxide in p(MMA) was examined by different microscopy techniques: light (LM) and transmission electron microscopy (TEM). For light microscopy, 2-5 µm thick coupes were made of the composite samples using a Leica

Figure 1. TEM images of (a) EMG705 nanoparticles and (b) EMG1200 particles.

RM2165 rotary microtome. Subsequently, light microscopy analysis was done using a Zeiss Axioplan 2 imaging microscope. Moreover, the particle distribution of EMG705 containing p(MMA) composite prepared by freeze-drying was studied using TEM as well. A Leica RM2165 rotary microtome was used to cut coupes with an approximate thickness of 50 nm, and the samples were placed on a carbon coated copper grid. The viscosity of p(MMA) was measured at a shear rate of 100 s-1 and 190 °C using a TA Instruments AR-G2 rheometer. 2.5. Temperature Measurements. Temperature measurements were performed to determine the amount of heat generated by the superparamagnetic iron oxide nanoparticles in fluid and distributed in the p(MMA), induced by an AC magnetic field. Therefore, the heating profiles of iron oxide suspended in fluid were measured by placing a Luxtron fluoroptic temperature probe in a glass tube with an inner diameter of 6 mm and a height of 40 mm containing 0.5 g of EMG705 ferrofluid or EMG1200 in THF (30 wt %). The heating of the iron oxide imbedded p(MMA) was measured by placing three cylindrical samples with a length of 10 mm and a diameter of 3 mm, around the Luxtron probe. Subsequently, the samples were placed in a custom built setup generating an AC magnetic field of 2850 A m-1 with a frequency of 745 kHz. The amount of heat generated per gram of iron oxide nanoparticles was calculated from the initial heating rate of the samples, the iron oxide content, and the specific heat as measured by DSC analysis, eq 2.

SAR )

Cp dT fw dt

( )

ini

(2)

where the specific absorption rate (SAR) is the amount of heat generated per gram of iron oxide [W g-1 iron oxide], Cp is the specific -1 ], fw is the iron oxide weight heat of the sample [J °C -1 gsample fraction of the sample [-], and (dT/dt)ini is the initial temperature increase [°C s-1]. 3. Results and Discussion 3.1. Characterization of Iron Oxide Nanoparticles. 3.1.1. TEM/DLS. The particle size of the superparamagnetic iron oxide nanoparticles used has been determined by TEM; see Figure 1. Both EMG705 and EMG1200 particles have a size in the superparamagnetic region, with a diameter of 12.1 ( 3.0 nm and 11.4 ( 2.6 nm, respectively.14 Dynamic light scattering analysis has shown that EMG705 nanoparticles in ferrofluid are clustered in agglomerates of 118 ( 48.4 nm. Using cryo-TEM images, these agglomerates have been confirmed; see Figure 2. A size distribution could not be reliably determined from cryo-TEM due to the relatively small number of particles that are visualized. In addition, by analyzing

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Figure 2. Cryo-TEM image of EMG705 nanoparticles.

Figure 3. Heating of iron oxide nanoparticles in fluids.

TABLE 1: Initial Heating Rate and Specific Absorption Rate of Commercially Available Iron Oxide Nanoparticles in Fluid particles

initial heating rate [°C s-1]

specific absorption -1 rate [W giron oxide]

EMG705 in ferrofluid EMG1200 in THF

0.642 0.866

7.82 5.32

a thin slice of several hundreds of nanometers of solution, size discrimination can occur. In the suspension of EMG1200 particles in THF, agglomerates of 175 ( 67.3 nm have been found by DLS. 3.1.2. Heating of EMG Nanoparticles. The heating of iron oxide nanoparticles in fluid has been determined by measuring the temperature increase in an alternating magnetic field of 2850 A m-1 with a frequency of 745 kHz. The initial temperature increase of the water-based EMG705 ferrofluid is significantly lower than that of the EMG1200 dispersed in THF, 0.642 and 0.866 °C s-1, respectively; see Figure 3 and Table 1. However, due to a substantially higher heat capacity of water compared to THF, calculation of the specific absorption rates shows that the SAR of the EMG705 particles is higher than -1 that of the EMG1200 particles, 7.82 and 5.32 W giron oxide, respectively. The increased SAR of the EMG705 particles compared to the EMG1200 particles can be explained by the slightly larger size of the EMG705 particles. As Ne´el relaxation is dependent on the particle size and the optimal size at the frequency used (17.4 nm) is higher than the particles used, the heating rate due to Ne´el relaxation is higher for larger particles.11 3.2. Characterization of EMG Nanoparticles Incorporated in p(MMA). As agglomeration of the iron oxide nanoparticles is expected to decrease the specific absorption rate, the distribution of the iron oxide nanoparticles in the p(MMA) matrix has been studied using several preparation methods. 3.2.1. Effect of Preparation Method on the Distribution of 50 wt % EMG Nanoparticles in p(MMA). The organic media particles, EMG1200, have been incorporated by solvent casting

Figure 4. Light microscopy images of the distribution of 50 wt % EMG nanoparticles in p(MMA) by different preparation methods: (a) EMG705 by direct injection of the ferrofluid in the compounder, (b) EMG1200 by solvent casting, (c) EMG1200 by solvent casting and subsequent compounding, and (d) EMG705 by freeze-drying a mixture of ferrofluid and p(MMA) latex and subsequent compounding.

together with p(MMA) and subsequent compounding. The water-based particles, EMG705, have been distributed by direct injection of the ferrofluid and p(MMA) in the compounder as well as by freeze-drying a mixture of ferrofluid and p(MMA) latex and subsequent compounding. Figure 4 shows the light microscopy images of p(MMA) containing 50 wt % iron oxide nanoparticles. In the case of direct injection of ferrofluid in the compounder prefilled with p(MMA), agglomerates with a large size distribution from 1 to 50 µm have been observed; see Figure 4a. Most likely, these agglomerates are formed during the fast evaporation of the water as the ferrofluid is injected in the compounder operating at 190 °C. During the mixing of the particles in the polymer, the clusters remain as the compounder is not capable of disintegrating the clusters. As an approximation, the size reduction of the agglomerates in the compounder has been estimated as a result of rupture and erosion mechanisms. For rupture, a model derived for agglomerates of small spherical particles in a Newtonian fluid has been used, in which the ratio between the shear force (Fh) and the cohesive force (Fc) is described by15

Fh ) Z sin2 θ sin φ cos φ Fc

(3)

8 ε 24z2d Z ) χµγ 9 1-ε A

(4)

in which

where θ and φ are the Euler angles of orientation, χ is a numerical constant of 12.23,16 µ is the medium viscosity [Pa s], γ is the shear rate [s-1], ε is the porosity [-], assuming 0.26 for the iron oxide agglomerates, z is the physical adsorption separation distance of typically 0.4 nm,15 d is the primary particle

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size [m], and A is the Hamaker constant of 15.2 × 10-20 J for iron oxide in p(MMA).17-19 In order to rupture the agglomerates under shear flow, the force of shear flow is required to be larger than the cohesive force, so that Fh/Fc g 1. Assuming optimum positioning of the agglomerate for rupture, rupture will occur when Z g 2. Using the viscosity of p(MMA) measured to be 5000 Pa s at 190 °C under the maximum shear rate experienced, 100 s-1, Z has been calculated to be 0.6. Therefore, no rupture of the agglomerates is expected due to the shear force. Moreover, the local tensile stress around agglomerates in a viscoelastic matrix has been found to be lower than that in a Newtonian fluid, further suppressing rupture of the agglomerates.20 However, erosion is feasible at significantly lower shear rates than rupture, although this is a considerably slower process.21 The size reduction due to erosion can be described by21

R0 - R(t) k'' ) Waµγt R0 τc

(5)

where R0 and R(t) are the agglomerate radius at time 0 and t, respectively, k′′ is a constant independent of agglomerate and medium, Wa is the work of adhesion, which is square root dependent on the surface tension of the medium, and τc is the cohesivity, where

τc ∝

1-ε A ε 24z2d

(6)

Calculating an average shear rate of 50 s-1 for 15 min, the dimensionless shearing time is 45 000. For agglomerates of titanium oxide in p(DMS), it has been found that the size reduction due to erosion is approximately 0.5% at this shearing time.21 Taking into account the differences in both the agglomerates and the medium, the size reduction due to erosion in the present case has been estimated. For the agglomerates, these hold the Hamaker constant 2.7 × 10-20 J for titanium oxide17,18 as well as the primary particle size and porosity, 167 nm and 0.574, respectively, for the titanium oxide reported. Differences of the medium taken into account are the viscosity, 10 Pa s for p(DMS), and surface tension of the medium, 40 and 18.4 mJ m-2 for p(MMA) and p(DMS), respectively.22 This results in an estimation of the size reduction due to erosion of 1.2%. Therefore, no significant size reduction of the agglomerates is expected in the present case by shear flow in the compounder. This analysis clearly indicates that clustering of the nanoparticles has to be prevented during processing since formed agglomerates cannot be ruptured afterward. When iron oxide nanoparticles are solvent casted into the polymer and subsequently compounded, large agglomerates have been found as well; see Figure 4c. It can be speculated that during the solvent casting process the surfactant is probably able to dissolve in the polymer matrix, thus detaching from the iron oxide surface.23 Therefore, the nanoparticles are not sufficiently stabilized during slow solvent evaporation, and the agglomerates are formed during solvent casting. The high particle loading and the loose packing of the agglomerates in the sample are evident from Figure 4b. As in the case of injection of the ferrofluid, the compounder is not able to fragment these clusters. However, in case that the EMG705 ferrofluid and a p(MMA) latex are premixed and freeze-dried prior to compounding, no

Figure 5. Light microscopy images of the distribution of 30 wt % EMG nanoparticles in p(MMA) by different preparation methods: (a) EMG705 by direct injection of the ferrofluid in the compounder, (b) EMG1200 by solvent casting, (c) EMG1200 by solvent casting and subsequent compounding, and (d) EMG705 by freeze-drying a mixture of ferrofluid and p(MMA) latex and subsequent compounding.

agglomerates are observed by light microscopy; see Figure 4d. Using transmission electron microscopy, however, small sized agglomerates could be observed, as depicted further on in Figure 6b The size of these clusters (∼500 nm) is slightly larger than that for the clusters found in the initial ferrofluid (∼100 nm). It is thought that due to the high iron oxide loading and, thus, the close proximity of particles, the particles might agglomerate to some extent. Nonetheless, since both the latex and iron oxide particles in solution are stabilized by surfactants, the agglomeration is suppressed. During subsequent freeze-drying, the particle position remains unaffected, preventing the formation of agglomerates. As the dried mixture is compounded above the glass transition temperature of the latex, the latex particles flow together, forming a solid matrix between the iron oxide particles. 3.2.2. Effect of Loading on the Distribution of Iron Oxide Nanoparticles Incorporated in a p(MMA) Matrix. To decrease particle agglomeration, the distribution of 30 wt % iron oxide particles in p(MMA) has been investigated using the different preparation methods, identical to the distribution methods used for 50 wt % particles. Similar to the distribution of 50 wt %, the direct injection of ferrofluid into the compounder and solvent casting the particles into p(MMA) and subsequent compounding result in substantial agglomeration of the iron oxide particles; see Figure 5a and c, respectively. Nevertheless, it should be noted that the agglomerates are smaller (up to ∼25 µm). However, freeze-drying a mixture of ferrofluid and p(MMA) latex and subsequent compounding results in an excellent distribution of the particles. Identical to the distribution of 50 wt %, the stabilization of both the latex and iron oxide particles by different surfactants and the lack of movement during the freeze-drying process results in good distribution of the iron oxide particles. As it is not possible to identify any agglomerates using light microscopy, TEM has been used to further investigate the particle distribution. The TEM images show very small agglomerates in the order of 100-200 nm; see Figure 6a. These agglomerates have a similar size as the agglomerates that have been observed in

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Figure 6. TEM images of p(MMA) containing (a) 30 wt % and (b) 50 wt % EMG705 nanoparticles prepared by the freeze-drying method. Figure 8. Heating of iron oxide-p(MMA) composites containing 30 and 50 wt % EMG705 nanoparticles prepared by the freeze-drying method.

TABLE 3: Initial Heating Rate and Specific Absorption Rate of p(MMA) Containing Different Loading of Iron Oxide Nanoparticles Prepared by the Freeze Drying Method

Figure 7. Heating of iron oxide-p(MMA) composites containing 50 wt % of different iron oxide nanoparticles and using different preparation methods.

TABLE 2: Initial Heating Rate and Specific Absorption Rate of Iron Oxide Nanoparticles Incorporated in p(MMA) (50 wt % loading) by Different Preparation Methods preparation method

initial heating rate [°C s-1]

specific absorption rate -1 [W giron oxide]

EMG705 freeze-drying EMG705 direct injection EMG1200 solvent casting

2.051 0.590 0.288

6.53 1.88 0.92

the ferrofluid, measured by DLS. Therefore, it can be concluded that the very small agglomerates have not been formed during the incorporation into the p(MMA) matrix. 3.2.3. Effect of Particle Distribution on Heating Iron Oxide Nanoparticles p(MMA) Nanocomposites. To investigate the effect of iron oxide particle distribution in the p(MMA) matrix on the heating of the particles in the nanocomposites containing 50 wt % in an AC magnetic field, temperature measurements have been performed. For this purpose, three cylindrical bars have been placed around a fluoroptic temperature probe in an alternating magnetic field. The heating of the nanocomposite prepared by freeze-drying the mixture of p(MMA) latex and ferrofluid shows a significantly higher initial temperature increase than that for the samples prepared by the other two methods; see Figure 7 and Table 2. Due to the substantially smaller agglomerates, the Ne´el relaxation of the iron oxide nanoparticles experiences less interparticle interactions, resulting in faster heating of the composite.13 Furthermore, the reduced heating of the composite prepared by solvent casting compared to the heating of the composite prepared by direct injection of the ferrofluid can be explained by the difference in iron oxide particle size as has been observed and described in the heating of the particles in fluid. Moreover, slightly increased agglomeration in the case of solvent casting

iron oxide loading (wt %)

initial heating rate [°C s-1]

specific absorption -1 rate [W giron oxide]

30 50

1.421 2.051

7.83 6.53

prepared composites would be able to explain the reduced heating. Unfortunately, this could not be quantified. 3.2.4. Effect of Loading on the Heating of Iron Oxide Nanoparticles Incorporated in p(MMA). The effect of loading on the heating of iron oxide nanoparticles incorporated in p(MMA) in an AC magnetic field has been determined by a direct comparison of the heating of nanocomposites prepared by the freeze-drying method containing 30 and 50 wt % iron oxide in an alternating magnetic field. As can be expected, the nanocomposite containing 50 wt % iron oxide particles has a higher initial heating rate than the composites containing 30 wt %; see Figure 8 and Table 3. The composite with the higher loading is capable of reaching higher temperatures as well. However, when comparing the specific absorption rates, it has been found that in the composite with a loading of 30 wt % more thermal energy is produced per unit of mass of iron oxide incorporated (Table 3). This can be explained by the improved distribution of the iron oxide particles in the p(MMA) matrix in the case of 30 wt % loading. The specific absorption rate found with 30 wt % loading is identical to that of the initial ferrofluid, demonstrating similar particle agglomeration. Thus, the specific absorption rate due to Ne´el relaxation is identical to the overall specific absorption rate, justifying the conclusion that only Ne´el relaxation contributes to heating the ferrofluid.9 4. Conclusion Different preparation methods of incorporating superparamagnetic iron oxide nanoparticles into a p(MMA) matrix have been investigated. The distribution of the nanoparticles has been examined by light microscopy and transmission electron microscopy. The method of mixing a ferrofluid with a polymer latex solution and subsequent freeze-drying and compounding the resulting powder resulted in none to very little agglomeration of the iron oxide particles in the polymer matrix compared to the nanosized agglomerates in the initial ferrofluid. The absence of agglomeration is expected to be due to the combination of stabilizing both the polymer and iron oxide particles by surfactants and the lack of mobility during freeze-drying.

Heating of EMG Nanoparticles in p(MMA) Furthermore, the effect of the distribution of the particles on the heating thereof has been determined by comparing the heating of nanocomposites with different particle distributions. It has been observed that improvement of the distribution results in a significantly better heating of the composite, that is, the specific absorption rate of the iron oxide nanoparticles increases, due to a decrease in interparticle interactions. Using the iron oxide nanoparticles to specifically heat a polymer for on-demand drug delivery, optimum distribution of the nanoparticles would decrease the amount of particles required for sufficient heating. Moreover, it has been found that a lower loading of the particles results in a decrease in agglomeration as the probability of agglomeration decreases. Therefore, a higher specific absorption rate has been observed in composites with a lower iron oxide loading. Nonetheless, it is worth mentioning that, despite the lower specific absorption rate, the heating rate and the attainable temperature increase with increasing particle loading. As the decrease in specific absorption rate of superparamagnetic iron oxide nanoparticles when incorporated in a polymer matrix is often accredited to the loss in Brown relaxation, this contribution clearly demonstrates that the distribution of the particles in the matrix should be taken into account as well. Acknowledgment. This research has been financially supported by SenterNovem and carried out with the support of the Soft-Matter Cryo-TEM Research Unit, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology. Furthermore, the authors wish to express their gratitude toward Jef Noijen and Klaas Kopinga of the Transport in Permeable Media group, Department of Applied Physics, Eindhoven University of Technology, for the design and development of the alternating magnetic field setup. References and Notes (1) Mohr, R.; Kratz, K.; Weigel, T.; Lucka-Gabor, M.; Moneke, M.; Lendlein, A. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 3540.

J. Phys. Chem. C, Vol. 114, No. 18, 2010 8149 (2) Schmidt, A. M. Macromol. Rapid Commun. 2006, 27, 1168. (3) Schmidt, A. M. J. Magn. Magn. Mater. 2005, 289, 5. (4) Wakamatsu, H.; Yamamoto, K.; Nakao, A.; Aoyagi, T. J. Magn. Magn. Mater. 2006, 302, 327. (5) Mu¨ller-Schulte, D.; Schmitz-Rode, T. J. Magn. Magn. Mater. 2006, 302, 267. (6) Keurentjes, J. T. F.; Kemmere, M. F.; Bruinewoud, H.; Vertommen, M. A. M. E.; Rovers, S. A.; Hoogenboom, R.; Stemkens, L. F. S.; Pe´ters, F. L. A. M. A.; Tielen, N. J. C.; van Asseldonk, D. T. A.; Gabriel, A.; Joosten, B.; Marcus, M. A. E. Angew. Chem., Int. Ed. 2009, 48, 9867. (7) Hergt, R.; Hiergeist, R.; Hilger, I.; Kaiser, W. A.; Lapatnikov, Y.; Margel, S.; Richter, U. J. Magn. Magn. Mater. 2004, 270, 345. (8) Chan, D. C. F.; Kirpotin, D. B.; Bunn, P. A., Jr. In Scientific and clinical applications of magnetic carriers; Ha¨feli, U., Ed.; Plenum Press: New York, 1997. (9) Rovers, S. A.; Hoogenboom, R.; Kemmere, M. F.; Keurentjes, J. T. F. J. Phys. Chem. C 2008, 112, 15643. (10) Ne´el, L. Ann. Geophys. 1949, 5, 99. (11) Hergt, R.; Andra¨, W.; d’Ambly, C. G.; Hilger, I.; Kaiser, W. A.; Richter, U.; Schmidt, H. G. IEEE Trans. Magn. 1998, 34, 3745. (12) Hergt, R.; Hiergeist, R.; Zeisberger, M.; Glo¨ckl, G.; Weitschies, W.; Ramirez, L. P.; Hilger, I.; Kaiser, W. A. J. Magn. Magn. Mater. 2004, 280, 358. (13) Bu¨scher, K.; Helm, C. A.; Gross, C.; Glo¨ckl, G.; Romanus, E.; Weitschies, W. Langmuir 2004, 20, 2435. (14) Dutz, S.; Hergt, R.; Mu¨rbe, J.; Müller, R.; Zeisberger, M.; Andra¨, W.; To¨pfer, J.; Bellemann, M. E. J. Magn. Magn. Mater. 2007, 308, 305. (15) Manas-Zloczower, I. In Mixing in polymer processing; Rauwendaal, C., Ed.; Marcel Dekker, Inc.: New York, 1991. (16) Nir, A.; Acrivos, A. J. Fluid Mech. 1973, 59, 209. (17) Visser, J. AdV. Colloid Interface Sci. 1972, 3, 331. (18) Drummond, C. J.; Georgaklis, G.; Chan, D. Y. C. Langmuir 1996, 12, 2617. (19) Rosensweig, R. E. Ferrohydrodynamics; Cambridge University Press: Cambridge, 1985. (20) Astruc, M.; Vervoort, S. Rheol. Acta 2003, 42, 421. (21) Lee, Y. J.; Feke, D. L.; Manas-Zloczower, I. Chem. Eng. Sci. 1993, 48, 3363. (22) Zisman, W. A. In AdVances in Chemistry Series 43; American Chemical Society: Washington, DC, 1964. (23) Povey, A. C.; Nixon, J. R.; O’Neill, I. K. J. Microencapsulation 1987, 4, 299.

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