ARTICLE pubs.acs.org/JPCC
Magnetic Properties and Dielectrical Relaxation Dynamics in CoFe2O4@PU Nanocomposites )
Natalia Frickel,† Anna Gutina Greenbaum,‡ Moshe Gottlieb,§ and Annette M. Schmidt*,|| Department Chemie, Universit€at zu K€ oln, Luxemburger Strasse 116, D-50939 K€oln, Germany Institut f€ur Organische Chemie und Makromolekulare Chemie, Heinrich-Heine-Universit€at D€usseldorf, Universit€atsstrasse 1, D-40225 D€usseldorf, Germany ‡ Department of Applied Physics, Hebrew University of Jerusalem, Jerusalem, 91904, Israel § Chemical Engineering Department, Ben Gurion University, Beer Sheva 84105, Israel †
ABSTRACT: Organicinorganic nanocomposite materials combine the advantages of organic polymers and inorganic nanoparticles. Due to their high application potential, the investigation of these materials is of great interest. In this study we examine the dielectric response of CoFe2O4@PU nanocomposites subjected to an external electric field. Dielectric properties were investigated in a frequency range of 102 Hz to 1 MHz and over a temperature range of 213 to 333 K, while varying the content of ferromagnetic particles. The experimental results provide insight to the structureproperty relationship of nanocomposites subjected to an alternating electric field. We discuss two relaxation processes (β- and R-processes) that show a dependence on the particle content. For the β-process we observe an Arrhenius-like behavior with temperature, and the related activation energies decrease with increasing particle content. Evidently, the particles' presence facilitates the β-process relaxation in the polymer matrix, resulting in higher dielectric losses. The R-process is attributed to a glass transition of the matrix, and the corresponding transition temperature Tg is compared to Tg obtained from differential scanning calorimetry (DSC).
’ INTRODUCTION Hybrid inorganicorganic nanocomposite materials may offer several advantages over conventional materials, like ceramics, metals, or polymers, as the corresponding properties of the different organic and inorganic components can be combined, or may even result in fully new properties. In this respect, the inclusion of different particles, such as carbon nanotubes and metallic and metalloxidic nanoparticles, into polymer matrices16 can lead to a composite material with superior performance and multifunctionality. One important consideration in the preparation of such hybrid systems is the nanoparticles' impact on the molecular dynamics of the polymer matrix which in turn may affect the mechanical and thermal properties of the polymer. This issue has been investigated systematically for SiO2 nanoparticles embedded into different polymers.79 The changes in polymer dynamics in the presence of nanoparticles lead to an increase in the glass transition temperature (Tg) in the case of an attractive interaction between the nanoparticles and the polymer chains,10,11 while very weak interactions may result in the suppression of the glass transition.12 Starr et al.13 found that Tg can be shifted to either higher or lower temperature by tuning the interactions between polymer and nanoparticles. In yet another study,14 two distinct glass transition temperatures were observed as indicated by the presence of a second tan δ peak in dynamic mechanical r 2011 American Chemical Society
experiments. The first one is associated with polymer chains sufficiently apart from the nanoparticles not to be affected by them whereas the second one is attributed to chain motions in the vicinity of the particles. The inclusion of different conducting or semiconducting metal oxide nanoparticles, in particular In2O3, SnO2, ZnO, TiO2, BaTiO3, SiO2, R-Fe2O3, γ-Fe2O3 and Fe3O4,1525 in order to exploit their unique magnetic and electric properties, have been reported. Magnetic nanocomposites may have promising potential in various applications such as magnetic sensors and transductors, microwave absorption, electromagnetic wave shielding, radio frequency interference shielding and electrostatic dissipation of charges.46,21,2628 CoFe2O4 nanoparticles used in our study possess high cubic magnetocrystalline anisotropy, high coercivity, moderate saturation magnetization, high complex permittivity values at wide frequency range, and good chemical stability. In the present study, composite systems of ferromagnetic CoFe2O4 nanoparticles and polyurethane (PU) have been prepared, varying the content of particles. The CoFe2O4 particles employed exhibit high crystallinity with a volume average core Received: November 29, 2010 Revised: April 14, 2011 Published: May 18, 2011 10946
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The Journal of Physical Chemistry C diameter dc of 14.6 nm, as determined from vibrating sample magnetometry (VSM) experiments. Due to their high magnetocrystalline anisotropy, the blocking temperature (TB) of CoFe2O4 nanoparticles of this size is well above room temperature.29 Consequently, the free rotation of magnetic moments within a particle domain is blocked. The dielectric response of the composites was investigated over a wide frequency and temperature range and shows the change in polymer chain dynamics with increasing particle content.
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Scheme 1. Synthesis Scheme of Cobalt Ferrite Nanoparticles
’ EXPERIMENTAL SECTION Materials. CoCl2 3 6H2O, FeCl2 3 4H2O and NH4OH (p.a., 25%), were purchased from Fluka. HCl 1 M (p.a., Riedel-de-Ha€en), NaOH 1 M (Gr€ussing) and tetramethylammonium hydroxide (TMAH, Aldrich, 25%) were used as received. Fe(NO3)3 was purchased from Fluka. Citric acid monohydrate was obtained from Gr€ussing GmbH (99.5%). 3-Methacryloxypropyl trimethoxysilane (MTS, 98%) was purchased from Lancaster. Aliphatic polyisocyanate (hexamethylene diisocyanate (HDI) trimer) Desmodur N 3390 BA (90% by weight in butyl acetate) and polyacrylate-polyol Desmophen A 665 BA (70% by weight in butyl acetate) were provided from Bayer Material Science. Baysilone Paint Additive OL 17 (10% by weight in butyl acetate) was purchased from Borchers GmbH. Butyl acetate was obtained from Aldrich. Ethanol was obtained in technical grade and used after distillation. Instrumentation. TEM images were obtained using a Philips EM 208S. High resolution TEM-images were obtained using a Tecnai 12 TWIN TEM (FEI) instrument. X-ray diffraction (XRD) was performed on a Huber Guinier (System 600). Thereby a monochromatic Cu KR radiation with a wavelength of λ = 1.5406 Å was used. The diffraction intensities were measured at a rate of 2θ = 3° per minute. Quasi-static magnetometry was implemented on an ADE Magnetics vibrating sample magnetometer (VSM) EV7. Dielectric measurements were performed on a Novocontrol Broadband Alpha Impedance Analyzer (frequency range: 0.01 Hz 1 MHz) in the temperature range of 213333 K. For these measurements the surface of the sample was coated with two gold electrodes. Subsequently, a sinusoidal alternating voltage with low amplitude was applied to the sample inducing an alternating current of similar frequency. Differential scanning calorimetry (DSC) measurements were performed using a Mettler Toledo TC 15 TA Instruments apparatus in a temperature range between 223 and 473 K employing a heating rate of 10 K per minute. Synthetic Procedures. Preparation of CoFe2O4 Nanoparticles. The preparation of cobalt ferrite nanoparticles is performed under nitrogen atmosphere by alkaline precipitation of cobalt and ferric chloride (molar ratio1:2), based on the method of Massart and Cabuil,30 and subsequent electrostatic stabilization in aqueous solution by the addition of a 0.01 M solution of citric acid until the particles flocculate, and redispersion by the subsequent addition of tetramethylammonium hydroxide solution.31 Surface Modification of CoFe2O4 Nanoparticles with MTS32. A portion of the electrostatically stabilized, water-based cobalt ferrite was added to dry ethanol at a concentration of 2.0 mg 3 mL1. Because of coagulation of the particles in pure ethanol, ammonium hydroxide (0.07 mol per 1 L of ethanol) was added to the solution. Afterward, a moderate excess (3.6 mmol per 1 g of cobalt ferrite, if not stated otherwise) of the respective alkoxysilane was added dropwise over a period of 10 min, and the
Figure 1. X-ray diffractogram of bare cobalt ferrite nanoparticles.
reaction mixture was stirred at ambient temperature for 15 h. Condensation of the siloxane groups was promoted by water removal by azeotropic distillation under reduced pressure. The particles were washed repeatedly with wet acetone by magnetic separation to remove any excess of reagent, citric acid, and salts. Afterward, the particles were directly redispersed in ethanol. Preparation of CoFe2O4@PU Magnetic Foils. CoFe2O4 magnetic foils were obtained by the incorporation of methacrylatemodified CoFe2O4 particles into a matrix of PU. At first Desmophen A 665 BA (55.95% by weight), Baysilone OL 17 (0.6% by weight) and butyl acetate (20.82% by weight, to reduce the viscosity of the mixture) were mixed by mechanical stirring. Then a dispersion of MTS modified CoFe2O4 particles in ethanol was added to this solution and stirred until the particles were completely dispersed. Ethanol was removed from the mixture by rotary evaporation at room temperature under reduced pressure of ca. 50 mbar. Afterward, Desmodur N 3390 BA was added to the solution and stirred up to yield a homogeneous dispersion. Foils were coated with a scraper and dried for several days in air. The following seven foil samples with an average thickness of 300 μm were prepared: PU-0P (PU matrix without particles), PU-0.9P (particle content 0.9 mass %), PU-1.9P, PU-3.8P, PU6.1P, PU-7.1P and PU-9.4P.
’ RESULTS AND DISCUSSION In this work we present the magnetic and dielectric response of the cobalt ferrite (CoFe2O4) nanoparticles homogeneously incorporated in a polyurethane (PU) matrix. The influence of CoFe2O4 content on nanocomposite behavior is studied by means of dielectric spectroscopy (DS). The experiments are performed as a function of frequency by different temperatures in order to research the relaxation processes. The glass transition temperature determined by DS Tg,DS is compared with Tg,DSC obtained from differential scanning calorimetry (DSC) experiments. Particle Properties. The particles are obtained by alkaline precipitation following Massart’s method30 (see Scheme 1) and stabilized electrostatically with citric acid and tetramethylammonium 10947
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Figure 2. (a) TEM measurement of CoFe2O4 nanoparticles and respective core size distribution histogram with log-normal distribution fitting. (b) High-resolution TEM (HRTEM) measurement of CoFe2O4 nanoparticles.
Figure 3. Quasi-static magnetic properties of CoFe2O4 ferrofluid.
hydroxide in water31 leading to a stable ferrofluid of CoFe2O4 nanoparticles. X-ray diffraction (XRD) analysis of these particles indicates a CoFe2O4 crystalline phase with expected inverse spinel structure (see Figure 1). Using the Scherrer equation,33 we calculated a mean crystalline size of 12.4 nm from the line broadening of the signals. TEM images show particles with a size distribution between 5 and 15 nm. The core diameter extracted from a log-normal fitting of the histogram (see Figure 2a) corresponds to a mean diameter of dc = 12.2 nm and a standard deviation s = 0.23, and is in close agreement with the crystalline size obtained from X-ray (see above). Furthermore, the particles exhibit high crystallinity as shown in the high-resolution TEM (HRTEM) image (see Figure 2b). The water based CoFe2O4 dispersion shows quasi-superparamagnetic behavior in vibrating sample magnetometry (VSM) experiments, indicated by an absence of hysteresis (see Figure 3). Assuming spherical, single-domain particles we obtain a volume average core diameter dV of 14.6 nm. The close agreement between the latter value and the value for the core diameter obtained from XRD results and TEM experiments indicates that the particles respond nearly individually to the magnetic field. The blocking temperature TB of CoFe2O4 nanoparticles at this diameter size is above room temperature.29 Consequently, below TB, the free rotation of magnetic moments in the particles is
frozen (blocked). Yet, in a fluid dispersion the entire particle is able to rotate by the so-called Brown mechanism. As result, under an external magnetic field the coercive field strength diminishes and the particle ensembles show a pseudo-superparamagnetic behavior (see Figure 3). Magnetic hysteresis of such particles is observed only if they are incorporated and immobilized in a solid matrix. Preparation of CoFe2O4@PU Composites. The ability to obtain a finely dispersed nanocomposite film depends strongly on the compatibility of the particles with the polymer matrix. This can be achieved by modification of the particle surface. In our case, we modified CoFe2O4 nanoparticles with 3-methacryloxypropyl triethoxysilane (MTS).32 The polyurethane (PU) matrix used is a two-component composite consisting of aliphatic polyisocyanate and an acrylate polyol. The polyisocyanate component consists of a hexamethylene diisocyanate (HDI) trimer. The acrylate polyol is composed of a copolymer and polyester component. The copolymer contains styrene, hydroxymethacrylate, butyl acetate and acrylic acid with di-tert-butyl peroxide as a starting group. The polyester components are ethylhexanoic acid, trimethylol propionic acid, adipinic acid and hexahydrophthalic anhydride. A series of composites with CoFe2O4 particle contents between 0 and 9.4 mass % were produced. These composites are labeled as follows: PU-0P (PU matrix without particles), PU0.9P (particle content 0.9 mass %), PU-1.9P, PU-3.8P, PU-6.1P, PU-7.1P and PU-9.4P. TEM image of PU-9.4P is shown in the Figure 4a,b. Aggregates are clearly observed, which possibly occurred as a result of transgression of percolation barrier in this sample. Nonetheless, a homogeneous distribution of particles in the matrix can be observed (see Figure 4a). In magnetic measurements of CoFe2O4@PU composites we obtain a hysteresis. This could be explained by immobilization of the particles in the polymer matrix and their hindered rotation (see Figure 5). The obtained coercivity (HC) and reduced remanence MR/MS amount to 42 kA 3 m1 and 0.33 respectively and are independent of the particle content in the composite. The MR/MS values measured for these samples are significantly lower than the theoretical value (0.5) expected for an ensemble of spherical, single-domain, noninteracting particles.34 This would 10948
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Figure 4. TEM measurement of CoFe2O4@PU composite (PU-9.4P).
Table 1. Glass Transition Temperature Tg,DSC and Change of Heat Capacity ΔCp,DSC Obtained from DSC Experiments
Figure 5. Quasi-static magnetic properties of CoFe2O4@PU composite (PU-9.4P).
Figure 6. ZFC (solid line) and FC (dashed line) magnetization curves for CoFe2O4@PU composite (PU-3.8P) at field of H = 79.6 103 A 3 m1.
indicate that a fraction of CoFe2O4 particles are superparamagnetic at room temperature. Figure 6 shows the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves of PU-3.8P as a function of temperature at H = 79.6 103 A 3 m1. In the low temperature range, a broad thermomagnetic irreversibility is observed between ZFC and FC magnetization curves, indicating strong magnetic blocking behavior as well for the magnetic moment within particles, as for the rotational blocking of the particles within the matrix. Both graphs reach a maximum in their magnetization at 275 K (MFC)
sample
Tg,DSC/[K]
ΔCp,DSC/[J 3 g1 3 K1]
PU-0P PU-0.9P
323.2 ( 1.3 324.9 ( 1.4
0.36 ( 0.01 0.39 ( 0.01
PU-1.9P
324.2 ( 0.6
0.39 ( 0.01
PU-3.8P
320.6 ( 0.5
0.35 ( 0.01
PU-6.1P
316.1 ( 0.7
0.27 ( 0.01
PU-7.1P
309.1 ( 0.5
0.28 ( 0.01
PU-9.4P
320.8 ( 0.7
0.26 ( 0.01
and 325 K (MZFC) respectively. Past these maxima the divergence between the two curves (MZFC and MFC) decreases with increasing temperature. This behavior is usually associated with the blocking temperature of the particles, indicating superparamagnetic behavior above it. This interpretation is supported by substantial agreement with the blocking temperature of 347 K that we extracted for similar magnetic foils in temperaturedependent VSM experiments.35 However, while in typical nanocrystalline powders a compliance of the FC and the ZFC curves is usually observed for temperatures above TB, a full compliance is not observed up to 475 K in the present experiment. It is believed that, next to the intrinsic particle properties, the glass transition of the polymer matrix that is found in a similar range (321 K, see Table 1) is of significant influence on the particle mobility, and hence on the described observations. Temperature-Dependent Dielectric Spectroscopy (DS). Dielectric spectroscopy was used to investigate the influence of CoFe2O4 nanoparticles on the dielectric properties while occluded in a polymer matrix in the nanocomposite foils. The measurements were conducted in a temperature range of 213 to 333 K and in the frequency range from 102 Hz to 1 MHz. The dielectric response of the system subjected to an external oscillating electric field is characterized by the complex permittivity, ε ¼ ε0 ðf Þ iε00 ðf Þ
ð1Þ
where ε0 the real and ε00 the imaginary parts of the permittivity correspond to the storage and loss (dissipation) of energy, respectively, and f is the frequency. Usually, the investigated relaxation processes may be masked by additional effects such as 10949
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Figure 7. Comparison of the (a) real M0 and (b) imaginary M00 parts of dielectric modulus for different samples at different temperatures. Arrow indicates the examined process.
Figure 8. Sample PU-6.1P. (a) Dependence of β-relaxation on temperature. (b) Temperature dependence of the maximum of the imaginary part (M00 max) of the dielectric modulus for the nanocomposite film.
dc conductivity, electrode polarization and interfacial polarization. One of the possible ways of representing the data is to transform the obtained dielectric data to the complex dielectric modulus formalism (eq 2). Dielectric modulus is an electrical analogue to the mechanical shear modulus and corresponds from the physical point of view to the relaxation of the electric field in the measured sample while the electric displacement remains constant. Therefore, the modulus represents the real dielectric relaxation processes, leads to a more apparent relaxation spectra and facilitates identification of relaxation times and maxima of the spectra.16,36 Thus, M ¼ 1=ε
M0 ¼
ε0 ðε þ ε00 2 Þ 02
M 00 ¼
ε00 ð2Þ ðε þ ε00 2 Þ 02
where M* and ε* are the complex dielectric modulus and dielectric permittivity, M0 is the real part of the dielectric modulus, and M00 is
the imaginary part of the dielectric modulus. Frequency dependence of the real and imaginary parts of the dielectric modulus at two temperatures (213 and 330 K) are presented in figures 7a and 7b, respectively, for all samples studied. Two different relaxation processes are clearly observed at the low and high temperatures respectively. At 213 K, M0 is almost linear with log f, and no clear dependence on the CoFe2O4 content is observed (see Figure 7a). However, in M00 curves, the presence of a distinct relaxation peak is prominently depicted. This lower temperature relaxation process corresponds to the local modes and is often referred to as the β-relaxation process.37 A modest yet systematic increase in the amplitude of the β-relaxation process with increasing particle content is observed with a substantial jump for the largest concentration (sample PU-9.4P). At the same time, this peak is shifted to lower frequencies with increasing particle concentration implying that the 10950
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The Journal of Physical Chemistry C influence of the particles on the local-mode motions results in a retarded relaxation. More complex dielectric spectra are observed at 333 K (Figure 7b). The higher temperature relaxation process, the socalled R-relaxation process, is associated with the segmental mode relaxation and related to the glass transition of the system, as is clearly observed here. For all samples, except the sample with 9.4 mass %, M0 shows a sigmoidal behavior: an abrupt, sharp increase in values at some concentration dependent characteristic threshold frequency followed by a second transition to a more moderate increase 12 frequency decades higher. These characteristic threshold frequencies are seen to increases with particle concentration. At the same time the M0 values decrease with concentration (with the exception of the 9.4 mass % sample). The two transitions in M0 are also manifested by corresponding peaks in the loss spectra. Here the peak is shifted to higher frequencies with increased CoFe2O4 content up to 6.1 mass %. At 7.1 mass %, the shift to higher frequencies is arrested, to decrease considerably for 9.4 mass %. As will be discussed in more detail below, we ascribe the crossover to aggregation or percolation phenomena within the samples with relatively high particle contents. For a better understanding of the particles' influence on the polymer matrix we will discuss the temperature and frequency dependence of the aforementioned relaxation processes more thoroughly. β-Relaxation. Figure 8a shows the variation of the dielectric loss modulus M00 of the β-relaxation process with the frequency
Figure 9. Temperature dependence of the relaxation time τβ related to the β-relaxation of the nanocomposites.
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at several temperatures between 213 and 264 K exemplarily for the sample PU-6.1P. It clearly demonstrates that the relaxation peak shifts to higher frequencies with an increase in temperature. Coexistent, a reduction in peak shift rate is observed, accompanied by an increase in the value of the peak maxima of the dielectric loss modulus M00 max (see also Figure 8b). Increased values of the dielectric loss modulus indicate that the relaxation rate for this process increases with increasing temperature. Simultaneously, the peak is shifted to a higher frequency. The temperature dependence of the relaxation time τβ corresponding to the β-relaxation process, determined as the inverse frequency at the peak maxima, is shown in Figure 9 for all samples. The data exhibits a linear dependence of the logarithmic relaxation time on the inverse temperature over a wide temperature range for all composite films, in accordance with the Arrhenius equation (eq 3). τβ ¼ τ0 3 eEA =ðkB T Þ
ð3Þ
where τ0 is the specific relaxation time, EA the β-relaxation activation energy, kB the Boltzmann constant. Following the Arrhenius law (eq 3), the activation energy EA of the β-relaxation process and the specific relaxation time can be determined from the slope of the curve and the intercept, respectively.
Figure 11. Dielectric loss as a function of temperature and frequency for PU-0P (lowest, blue face), PU-3.8P (intermediate, green face) and PU-7.1P (upper, purple face).
Figure 10. Dependence of the (a) activation energy EA and (b) specific relaxation time τ0 for the β-relaxation process on particle content for the investigated nanocomposites. 10951
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Figure 12. Sample PU-6.1P. (a) Dependence of the R-relaxation process on temperature. (b) Dependence of the maximum of imaginary part (M00 ) of the dielectric modulus on the temperature.
In Figure 10a, the activation energy is plotted as a function of the particle content. One can observe that the activation energy decreases with increasing particle content, indicating that the particles facilitate the β-relaxation process of the polymer, which results in higher dielectric losses. At the same time the specific relaxation time τ0 increases significantly with increasing particle content (see Figure 10b). It may be argued that the additional defects in the polymer matrix (the particles) facilitate the relaxation as manifested by the reduction in activation energy. On the other hand, the large increase in τ0 points to larger structural elements or to cooperative motion stemming from increased particleparticle or particlepolymer interactions. Based on the clear and systematic dependence of the activation energy EA as well as the specific relaxation time τ0 on particle concentration we propose that either the polarizability of the particles or the creation of the interface by their presence is responsible for enhanced dielectric losses of the composite materials (see Figure 11). R-Relaxation. The second process we discuss is an R-relaxation process. As can be seen from Figure 12a for the sample PU6.1P as a representative example, the pronounced R-relaxation peak in M00 (f) spectra is observed to shift toward higher frequencies with increasing temperatures. At the same time, the maximum of the dielectric loss modulus M00 max shows a monotonic decrease with temperature (see Figure 12b). The temperature dependence of the relaxation time τR related to the R-process is depicted in Figure 13 for all samples. The relationship can be described by means of the VogelFulcher Tamann (VFT) equation (eq 4).38 The rapid increase of the relaxation rate at lower temperature can be interpreted as a consequence of the reduction of free volume. τR ¼ τ0 3 eA=ðT TV Þ
ð4Þ
where A is a constant that is related to the activation energy for Rrelaxation, and TV is the Vogel temperature, which is also called the ideal glass transition temperature. One can observe that the relaxation time τR is decreasing with increasing particle content, with, once again, the exception of the sample PU-9.4P. This observation may be explained by the influence of the interface between particles and the matrix on the molecular dynamics of the polymer chains. The higher the particle content, the higher is the specific surface area within
Figure 13. Temperature dependence of the relaxation time τR related to the R-relaxation of the nanocomposites.
Figure 14. Fitted curve of R- and β-processes from dielectric experiments (PU- 6.1P).
the nanocomposite. In the case of PU-9.4P, aggregation of the particles and/or percolation is observed in TEM images, possibly resulting in charge transport between the particles. Percolation effectively reduces the surface area, resulting in the exceptional behavior. In Figure 14, the data of both relaxation times τβ and τR as a function of the reciprocal temperature are shown exemplarily for sample PU6.1P and are fitted by linear regression of eq 3, and eq 4 (VFT), respectively. The temperature corresponding to a R-relaxation time at 100 s gives the conventional definition for the glass transition temperature Tg,DS.39 10952
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Figure 15. (a) Comparison of the glass transition temperatures as obtained from DSC measurements and from dielectric spectroscopy. (b) Comparison between glass transition temperature Tg,DS and Vogel temperature TV.
DSC Measurements. In order to extract information on the glass transition temperature of the materials by an independent method, and thus compare the behavior to the observations made by DS, differential scanning calorimetry (DSC) experiments have been carried out. The results are summarized in Table 1. Figure 15a shows that the glass transition temperature Tg,DSC decreases with increasing particle content, with the exception of PU-9.4P, thus showing the same trend as seen by DS. However, Tg,DS is always lower than Tg,DSC, with the difference becoming larger the higher the particle content in the film. We interpret this observation according to the following considerations. By DSC, the glass transition of the entire polymer is observed, including the polymer located at or close to the interface to the nanoparticles as well as polymer in the bulk phase. In contrast, for the dielectric measurements, the observed transition is influenced much more strongly by the particle content. This indicates that predominantly the polymer fraction close to the interface contributes to the process. It should be noted that the method is based on the dielectric response of the material. In this respect, the semiconductibility of the CoFe2O4 may be of impact similar to the trend of Tg,DS (see Figure 15b).
’ CONCLUSION Dielectric spectroscopy was used to investigate the influence of ferromagnetic CoFe2O4 particles on the molecular dynamic modes in PU matrix. At lower temperatures, we detected a β-relaxation process. This process is attributed to the local modes resulting from the reorientation of polar side groups and small segments of polymer chains. The calculated relaxation times τβ and activation energies show a strong dependence on the particle content. The observation that the activation energies reach a lower value with increasing particle content leads to the suggestion that the particles facilitate the β-process. At high temperatures R-relaxation processes were observed. The relaxation time τR is found to decrease with increasing particle content. This suggests that the interface between particles and polymer matrix plays an important role on the molecular dynamics of the polymer chains. The calculated glass transition temperature Tg,DS was compared to the glass transition temperature Tg,DSC obtained from DSC experiments. Here, the results suggest that the dielectric spectroscopy enable the monitoring of polymer chain dynamics directly in the surrounding area of the particles.
’ AUTHOR INFORMATION Corresponding Author
*Tel: þ49-221-470-5410. Fax: þ49-221-470-5481. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work has in part been supported by ERA NanoSciEþ, DFG, and Bayer MaterialScience. We are also grateful to Dr. Einat Nativ-Roth for her help in obtaining TEM images, to Luba Burlaka for the gold sputtering of the samples for DS measurements, and to MOST Grant N.3-4602 in the framework of the CERC for the access to DS measurements. M.G. acknowledges the support of the ISF and the R. Stadler Minerva Center administered by the Max Planck Foundation. ’ REFERENCES (1) Kim, S. S.; Kim, S. T.; Yoon, Y. C.; Lee, K. S. J. Appl. Phys. 2005, 97, No. 10F905/901. (2) De, S.; Higgins, T. M.; Lyons, P. E.; Doherty, E. M.; Nirmalraj, P. N.; Blau, J. W.; Boland, J. J.; Coleman, J. N. ACS Nano 2009, 3, 1767. (3) Wang, H. F.; Cao, W. W.; Zhou, Q. F.; Shung, K. K.; Huang, Y. H. Appl. Phys. Lett. 2004, 85, 5998. (4) Guo, Z. H.; Park, S.; Hahn, H. T.; Wei, S. Y.; Moldovan, M.; Karki, A. B.; Young, D. P. J. Appl. Phys. 2007, 101, No. 09M511. (5) Hallouet, B.; Wetzel, B.; Pelster, R. J. Nanomater. 2007, 2007, 1. (6) Psarras, G. C. Composites, Part A 2006, 37, 1545. (7) Landry, C. J. T.; Coltrain, B. K.; Landry, M. R.; Fitzgerald, J. J.; Long, V. K. Macromolecules 1993, 26, 3702. (8) Hajji, P.; David, L.; Gerard, J. F.; Pascault, J. P.; Vigier, G. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 3172. (9) Ou, Y.; Yang, F.; Yu, Z.-Z. J. Polym. Sci.: Part B: Polym. Phys. 1998, 36, 789. (10) Kropka, J. M.; Putz, K. W.; Pryamitsyn, V.; Ganesan, V.; Green, P. F. Macromolecules 2007, 40, 5424. (11) Brown, D.; Mele, P.; Marceau, S.; Alberola, N. D. Macromolecules 2003, 36, 1395. (12) Oh, H.; Green, P. F. Nat. Mater. 2009, 8, 139. (13) Starr, F. W.; Schroder, T. B.; Glotzer, S. C. Macromolecules 2002, 35, 4481. (14) Tsagaropoulos, G.; Eisenberg, A. Macromolecules 1995, 28, 396. (15) Busmann, H.-G. Electrically Conductive and Optically Transparent Material, Method for Producing the same and its use, Germany, 2000, Vol. WO/2000/026923. (16) Kontos, G. A.; Soulintzis, A. L.; Karahaliou, P. K.; Psarras, G. C.; Georga, S. N.; Krontiras, C. A.; Pisanias, M. N. Express Polym. Lett. 2007, 1, 781. 10953
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dx.doi.org/10.1021/jp111348e |J. Phys. Chem. C 2011, 115, 10946–10954
