Superparamagnetic particles were widely used in medical applications as well as for magnetic sensors and actuators. Generally, the size of the particles is in the range of 10–20 nm. To use such particles in large-scale applications, a simple processing as well as the use of commercially available particles is required. Therefore superparamagnetic nanoparticles available on the market were incorporated in flexible polymer films and the magnetic properties of the films were investigated. At ambient temperature no significant hysteresis was observed, indicating the superparamagnetic properties. Films containing up to 25% nanoparticles were prepared. The films show a saturation magnetization of 13.8 Am2/kg and a coercivity of 7 Oe at ambient temperature.
The magnetic properties of ferromagnetic as well as ferrimagnetic materials show size-related effects [
Great potential is assigned to the superparamagnetic polymers for the application in sensors or actuators [
Previous studies are mostly based on a special synthesis of nanoparticles. Usually an in situ preparation and immediate incorporation of the particles tailored to the polymer system is carried out. As an example, the synthesis of magnetite particles of iron (III) chloride for the incorporation into silicone elastomers was considered by Evans et al. [
The PVC-sheets were prepared by E-PVC EP7060 (Vinnolit GmbH & Co. KG) and Hexamoll DINCH (BASF) as plasticizer. As magnetic nanoparticles Fe(II, III) oxide powder (magnetite), showing a spherical particle shape in the range of 20–30 nm, 98% were chosen.
The silicone sheets were prepared using a dimethylpolysiloxane of an additional curing LR-type (Wacker AG).
Every particle content is given in weight%.
For processing the particles with a PVC-solution a 10% PVC-solution was prepared in cyclohexanone. In a temperature-controlled vessel the PVC-powder was added to the solvent under stirring. The mixture was further stirred for 3 hours at 30°C and 1,000 r/min. Afterwards the PVC was completely dissolved. This solution was used as stock solution from which the quantities needed for the single tests were taken, respectively.
To incorporate the nanoparticles in the PVC-solution, the plasticizer and the magnetic nanoparticles were combined and dispersed homogeneously. A typical PVC-formulation contained 100 phr of PVC and 50 phr of DINCH. Relating to the overall formulation 10–25% of magnetite was used. The use of a special disperser additive was not necessary. After dispersion, the mass was further treated at a three-roll mill or a ball mill.
When using a PVC-plastisol in a first step the magnetic particles in the plasticizer were processed at the three-roll mill or by means of a ball mill according to the following description. The additional formulation components PVC-powder and a defoamer were added to the resulting homogeneous mixture, stirred, and processed at the three-roll mill up to a gap of 10
The homogeneous dispersion was processed at a three-roll mill (120 EH-250 of EXAKT) in several passages. The roll mill consists of two roller gaps which can be operated with a defined roller gap or a defined force that presses the rolls together. After several process steps (gap mode 5
For processing with the ball mill a temperature-controlled vessel was charged with 230 mL grinding balls and 230 mL grinding material. The grinding process was performed on a Dispermat with a grinding disc having two plates carried out according to the steps shown in Table
Process conditions using the ball mill.
Time [min] | Speed [U/min] | Temperature [°C] |
---|---|---|
10 | 690 | 15 |
10 | 930 | 14 |
10 | 1140 | 13 |
10 | 1320 | 14 |
10 | 2550 | 20 |
Preparing silicone dispersions, the nanoparticle agglomerates were added to the liquid silicone and stirred for 10 minutes followed by processing at the three-roll mill using the gap mode. The mass was treated in several passages to a roller gap of 5
Films of the prepared PVC-masses as well as the silicone dispersions were prepared by blade coating. Therefore the compounds were spread out on release paper at a Mathis Labcoater in order to form thin layers which were dried for 3 minutes at 100°C and cured for 3 minutes at 160°C in case of PVC and for 4 minutes at 180°C in case of silicone. Afterwards the cured layers were delaminated from the release paper to become a thin film of 100
A QUANTA FEG 250 (FEI Company) scanning electron microscope was used to observe the particle alignment of the magnetic particles. All samples were sputtered with gold.
An atomic force microscope of the type Nanowizard 3 of the company JPK Instruments was used for the structure measurements and the magnetic measurements (MFM) in the nanometre range. The scanning unit of the apparatus is able to detect a grid of up to 100 × 100 × 15 microns (
The topographical images were obtained in tapping mode. The MFM-images were obtained in the hover mode; that is, on the first scan (trace), the cantilever directly scans the surface of the sample, and, on the second scan (hover mode retrace), the cantilever is raised to a user-defined height and follows the topographical pattern from the previous trace. On the first trace, short range interactions (i.e., van der Waals forces) have the most significant effect and the topography is imaged. On the hover mode retrace, long range interactions such as magnetic forces are most prevalent, and the MFM-image will therefore reflect the magnetic properties of the sample [
The magnetic properties were determined using a Superconducting Quantum Interference Device (VSM-SQUID) magnetometer. Measuring the magnetic properties the temperature dependence of the magnetization was measured at 300 K and 2 K. The temperature dependence on the magnetization was determined in field cooled- (FC-) as well as zero field cooled- (ZFC-) experiments.
Analysing the FC- and ZFC-measurements the blocking temperature of magnetic nanostructure ensembles can be determined. The sample is cooled down without application of a magnetic field below the expected blocking temperature, usually to some Kelvins (ZFC). Thus, the magnetic moments of the individual nanostructures are “frozen” statistically and the magnetization of the sample is zero on average at
Scheme of the temperature-dependent magnetization in an FC- or ZFC-experiment.
In the FC-experiment the sample is cooled in an external magnetic field and the magnetization is measured in this connection. For all measuring temperatures
Concerning single-domain nanostructures the thermal stability of the magnetization of a magnetic particle has a big influence on the total magnetization of the sample. For an ensemble of particles with uniaxial anisotropy, which are oriented parallel to the axis of the magnetization in a generated magnetic field
(a) Scheme of possible magnetization directions of a ferromagnetic particle in an external magnetic field
In SEM-investigations the particles were detected visually in the films. The images show that the size and the number of the existing particle agglomerates decrease with increasing force in the treatment process. An increasing number of primary particles are detected which are spatially separated. The major difference between the films from the plastisol and the solution is in the structuring of the plastisol-based layers. In the sheets prepared from the paste the areas of the PVC-particles (dark) are clearly noticeable (Figure
SEM-images of PVC-sheets prepared from solution (a) and the paste (b) at a magnification of 10,000.
To get a more detailed idea of how the particles are present in the films, SEM- and AFM-images were made at microtome sections of the films. These investigations show that there are areas in which the particles exist separated. Obviously, the separation of the agglomerates into primary particles has been successful there. This can be seen in the detailed images of the SEM- and AFM-investigations (Figure
SEM- and AFM-images of a PVC-sheet prepared from a solution (microtome section); overview and detailed views showing the agglomerate (top) as well as isolated particles (bottom).
For a clear statement about the magnetic conditions at the surface the phase pictures of an AFM-measurement (Figure
(a) AFM-image PVC-film from the solution; phase picture (trace); (b) MFM-images phase picture (retrace).
The MFM-images were taken with a hover height of 30 nm and show clear phase shifts. The phase shifts visible as dark areas do not correlate with the height signal and can thus be clearly assigned to the magnetic signal. A comparison with the distribution of the magnetite particles, which can be seen from the phase picture, confirms this. This proves the magnetization on the nanometre scale.
The investigations concerning magnetization of the PVC-films depending on the external magnetic field revealed that the films produced both from the solution and from the paste have superparamagnetic behaviour. The left side of Figure
Left: magnetization curves of sheets containing 10% nanoparticles prepared from PVC-pastes using a three-roll mill; the force in the gap was increased from A via B to D. Top right: hysteresis at 300 K, bottom right: hysteresis at 2 K.
The method of particle treatment does not have an influence on the coercivity, but on the resulting saturation magnetization. Films produced of masses which were treated with small force show a low saturation magnetization of 4.9 Am2/kg. Compared to this, films produced of masses which were treated with the highest possible force show a saturation magnetization of 6.1 Am2/kg, which is about 24% higher.
A dependence of the remanence on the process conditions was not found. The remanence for films, the paste of which was treated at the three-roll mill, is between 0.08 Am2/kg and 0.11 Am2/kg. For a practical use this means that after switching off an external magnetic field the films do not show a permanent magnetization.
The processing in the paste does not allow a higher particle content than 10%, whereas preparation in the solution allows the incorporation of much more particles. Thus, also the resulting saturation magnetization increases. Figure
Magnetization curves of sheets containing 10%, 15%, and 25% nanoparticles prepared from PVC-solution using a ball mill.
Using FC- and ZFC-experiments the blocking temperatures of the magnetic nanostructure ensembles were determined. For an ideal superparamagnet the ZFC-curve has a maximum at the blocking temperature; above the blocking temperature the curves for FCM and ZFCM coincide. Figure
(a) Magnetization curves of sheets containing 10% nanoparticles processed at different modes (process 0, A, B, and C); (b) magnetization curves of sheets containing 10%, 15, and 25% nanoparticles, prepared from PVC-solutions via ball mill.
The significant difference in the magnetic behaviour also becomes apparent in this experiment. The ZFC-curve of the PVC-film, which was produced with larger nanoparticles, does not have a maximum and does not converge with the FC-curve up to a value of 350 K; for example, it shows ferrimagnetic behaviour in this area. The PVC-films with smaller nanoparticles show a maximum in the ZFC-curve between 150 K and 180 K and converge in the area between 250 K and 300 K. The exact temperatures are different for the different preparation processes. Using the example of films produced from pastes this is illustrated in Figure
Blocking temperature in dependence of mass processing.
Sample labeling | Range of curves-maximum |
Mass processing |
---|---|---|
Reference | 205–235 | Magnetite powder |
MP5 0 | 170–185 | Gap 1: 10 |
MP5 A | 162–174 | Gap 1: 2 N; gap 2: 5 N |
MP5 B | 157–178 | Gap 1: 5 N; gap 2: 15 N |
MP5 C | 153–175 | Gap 1: 10 N; gap 2: 30 N |
MP6 D | 145–160 | MP5 C + additional dispersion |
MP7 D | 142–160 | 3 passages gap 1: 11 N; gap 2: 33 N |
MP7 E | 130–150 | MP7 D + 3 passages gap 1: 20 N; gap 2: 38 N |
The selected ferrite particles are suitable for the processing with different polymer systems. In order to show this, the particles were incorporated into liquid silicones at the three-roll mill using the calendering process. Silicone films were produced from the contained masses and their magnetic properties were analysed analogue to the PVC-films. The measured values for the remanence of about 0.09 Am2/kg as well as the coercivity of about 10 Oe do not differ from the respective values of the PVC-films.
For the first time the preparation of PVC-films with superparamagnetic properties by incorporation of commercially available magnetite nanoparticles (particle sizes of 20–30 nm) was successful. The nanocomposites could be prepared using a ball mill as well as a three-roll mill for processing the particle agglomerates. Applying the three-roll mill it was found that the higher the application of a force in the roll gap the higher the saturation magnetization and the lower the blocking temperature. In general the blocking temperature of the films is below 185 K, in case of optimum particle treatment below 150 K. The processing parameters did not have an influence on the remanence and the coercivity. The reachable values for the remanence in samples containing 10% nanoparticles were at 0.11 Am2/kg and those of the coercivity at 5,7 Oe (300 K). For an external magnetic field of 70,000 Oe a maximum saturation magnetization of 6.1 Am2/kg was reached for these composites. In case of processing from the solution higher particle contents can be realized. For films with 25% particles the achievable saturation magnetization is at 13.8 Am2/kg, which is much higher than the described values using ferrofluids (maximum 2.0 Am2/kg) [
In practice this means if the films are located in an external magnetic field, they behave like an ferromagnetic material. They are magnetized and move in the direction of the magnetic field, or they can be detected in the magnetic field. If the field strength reaches the value of 0 Oe (switching off the external field) the PVC-film is demagnetized again; that is, without external magnetic field the film shows the properties typical for plastics. As there is no permanent magnetization, ferrous metals are not influenced by the film. It follows that the films have no hysteresis and thus no remanence, that is, having superparamagnetic behaviour in use at room temperature.
The use of the magnetite nanoparticles in other polymer systems with similar mass preparation is possible. This was proven using the example of silicone films.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The research project “Modification of plasticized PVC with functional nanoparticles,” Registration no. VF110026, was partially funded from budget funds of the Bundesministerium für Wirtschaft und Energie (BMWi-Federal Ministry of Economics and Technology). The authors are grateful for the support granted.