POLYMER COATED FERRITE NANOCOMPOSITES SYNTHESIZED BY PLASMA POLYMERIZATION

POLYMER COATED FERRITE NANOCOMPOSITES SYNTHESIZED BY PLASMA POLYMERIZATION I. NEAMTU, A. IOANID, A. CHIRIAC, L. E. NITA, G. E. IOANID, M. POPESCU P. Poni Institute of Macromolecular Chemistry Aleea Grigore Ghica Voda no. 41-A, 700487 Iaºi, Romania neamtui@icmpp.tuiasi.ro Received December 21, 2004 Magnetic nanocomposites are an important class of advanced functional materials on the basis of a magnetic material and a matrix. To achieve unique mechanical, physical, chemical, and biomedical properties, one must be able to tailor only the surface structures of a nanoparticle. This requires the deposition of ultrathin and uniform films using the possibility offered by cold-plasma polymerisation process. The paper presents the synthesis of a magnetic composite based on ferrite and a vinyl polymer, with some data of physico-chemical characterization. Key words: plasma polymerization, vinyl monomer, ferrite, magnetic nanocomposite. 1. INTRODUCTION In current research of nanocomposite materials, it has become critical to modify the surfaces of the nanoparticles for both fundamental research and engineering applications. Magnetic polymer nanoparticles can be tailor-made depending on the final applications [1 3]. Thus, polymer-coated magnetic nanoparticles are of great technological interest as the coating provides a matrix for binding of the particles and also prevents grain growth and agglomeration. While it is an established fact that nanoparticles exhibit novel magnetic properties, in general, the ideal response is associated with isolated particles. In practical applications, one invariably has to consider a collection or aggregate of particles forming nanopowders. Further, several applications require consolidation and sintering of nanophase materials into solid blocks or thin films. These processing routes often lead to unavoidable formation of agglomerations and larger grains that effectively prevent the materials from attaining their full potential in terms of the desirable magnetic response associated with nanoparticles. Though it is difficult to completely eliminate agglomeration in large-scale commercial synthesis of nanomaterials, Paper presented at the 5th International Balkan Workshop on Applied Physics, 5 7 July 2004, Constanþa, Romania. Rom. Journ. Phys., Vol. 50, Nos. 9 10, P. 1081 1087, Bucharest, 2005

1082 I. Neamtu et al. 2 coating nano-particles with polymers affords the possibility of minimizing it to a great extent. The process of coating not only provides effective encapsulation of individual nanoparticles but also controls the growth in size thus yielding a better overall size distribution. Polymer coating has several other advantages too. Several kinds of magnetic polymer nanoparticles have been produced from natural and synthetic polymers with the intention to incorporate groups on the surface or to treat their surface to perform a certain application. For example, in addition to encapsulation of magnetic particles, the coating itself can provide an additional functionality. In special, magnetic nanoparticles can be used as magnetic drug targeting, tissue engineering, biological high gradient magnetic selective separation for cell sorting, DNA isolation, magnetic resonance imaging contrast agents and hyperthermia. The multifunctional response of these materials can have tremendous potential and lead to improved materials for applications like magneto resistive damping, mechanical and electrical devices: loudspeakers, seals, sensors, dampers, etc. All these applications depend on a core magnetic material with a modified surface resulting in additional functionality. To answer at the multitude of utilities the cold plasma polymerization, like coating technique, has been applied [4 10]. On the other side, surface treatment by plasma polymerization has been used in surface and interface engineering for improving a lot of properties, such as: functionality, adhesion, hydrophobic and hydrophilic properties, printability, corrosion resistance, selectivity, or for surface etching or cleaning. Due to these characteristics, the plasma technique can be used for surface modification and thin film deposition for almost all substrates, including metal and alloy plates or powders, polymer films or nanoparticles, paper, glass, porous materials and particulate matter. The plasma polymerization technique is a room temperature and environmentally benign no conventional process; from these grounds it is agreed with the green chemistry principles. The main principle of the plasma polymerization technique is that the ionized and excited molecules and radicals created by the electrical field bombard and react with the surface of the substrate. These activated molecules may etch, sputter, or deposit on the substrate surface. As a result, the surface properties of substrates are modified. When introducing molecular gases into plasma, chemically active species are formed such as molecules in excited states, radicals and ions. These species can react with each other, neutral molecules or with the surface of a substrate. Depending on the nature of the molecules and the process conditions, this may result in the deposition of a thin film [11 15]. Films resulting from organic precursors are generally known as plasma polymers. The reaction chain leading to a plasma polymer film is not comparable to common polymerization reactions. For this reason, the properties of plasma polymer films can significantly differ from their classic chemical counterparts.

3 Polymer coated ferrite by plasma polymerization 1083 In this study, there are reported experimental results on the deposition of styrene and methyl methacrylate polymer ultrathin films on the surfaces of nanoscale ferrite particles by plasma polymerization process sustained in air. The aim of this work is to gain more insight in the morphology and chemical structure of these materials. 2. EXPERIMENTAL 2.1. MATERIALS The monomers styrene (c > 99 wt%, Fluka) and methyl methacrylate (c = 99 wt%, Merck) were distilled under reduced pressure before use. According to ref. [15, 16], the ferrite particles were produced by co-precipitation from an aqueous Fe 3+ /Fe 2+ solution (molar ratio 3:2) using concentrated ammonium hydroxide in excess, which allows the compensation of the oxidation of some iron II to iron III. 2.2. EXPERIMENTAL SET-UP Plasma polymerization is performed in the system [17] shown in Fig. 1. Fig. 1. Schematic diagram of the plasma reactor for thin polymer film coating of the nanoparticles: A system for gas feeding, rotation training and vacuum; B reaction vessel; C system for electrodes fixing; D HF generator; 1 rotative cylindrical reactor; 2 semicylindrical electrodes; 3 current supply.

1084 I. Neamtu et al. 4 The plasma coating by polymerization system consists mainly of a high frequency source, a horizontal vacuum chamber of the reactor which is a Pyrex glass cylindrical column, the vacuum pump and pressure gauge. This vacuum chamber is evacuated before each run and subsequently filled with a specific monomer. The ferrite powder is charged in the high frequency zone of the glass rotative reactor. The monomer was introduced from the gas inlet during the plasma polymerization. The discharge is obtained between two semicylindricalshaped electrodes placed on the exterior walls of the reactor and connected to the high frequency power generator operating at 1.5 MHz. Before the plasma treatment, the chamber pressure was pumped down to less than 4 torr, at which time the monomer vapors were introduced into the reactor. During the plasma polymerization process, the irradiation power was 40 W at a maximum output voltage of 100 V/cm and the system pressure was less than 4 torr. The operating pressure was adjusted by the monomer flow rate. The plasma treatment time was 6 minutes per batch of 0.5 grams of ferrite. 2.3. CHARACTERIZATION The composition of the polymer coated-nanoparticles was characterized using Fourier transform infrared (FTIR) experiment [18] on a spectrophotometer DIGILAB Scimitar Series-USA, with 4 cm 1 resolution. The morphology was examined by scanning electron microscopy (SEM) TESLA BS 301. The samples were fixed with electrostatic paste on special supports. For the contrast enhancement there were first carbon coated (by thermal evaporation) and then there were metallized with gold (cathode pulverization). 3. RESULTS AND DISCUSSION Generally, nanoparticles are difficult to disperse in the plasma polymerization coating process due to aggregation and large surface area per unit mass. In the plasma thin film coating technique, it is necessary to expose the surface of the nanoparticles to the plasma. The rotating reactor is a good enough tool for gas-particle reactions due to the intensive mass and heat transfer between the two phases and short reaction time. Therefore, the combination of plasma polymerization and the dynamic process represents an approach for low temperature surface modification of nanoparticles. Using this system, the properties of a variety of powders can be modified quite effectively. Thus, it is well known that a growing film of a plasma polymer on a substrate is continually bombarded by positive ions. If the plasma also contains an organic monomer, these ions are primarily hydrogen ions (protons).

5 Polymer coated ferrite by plasma polymerization 1085 The protons attach themselves to the film. The film is thus positively charged most of the time. In the studied case, the substrate consists of nanosize powders. The positive charges developed on the powder particles will keep them quite well dispersed in the plasma, making them accessible to the organic radicals that are responsible for the film growth. This simple mechanism explains why plasma polymerization of fine particles in motion in the rotating reactor is an effective procedure. The presence of polymer synthetized by plasma process was confirmed by infrared spectroscopy. FTIR spectra of investigated samples (Fig. 2, 3) show that in the chosen conditions, plasma polymerization leads to deposited polystyrene and poly(methyl methacrylate) films. Their configuration is almost identical with FTIR spectrum of conventional polymer, especially for the short reaction time of 4 min (Fig. 2a, 3a). After the polymerization process of methyl methacrylate the absorption band at 1725.5 cm 1 so characteristic for carbonyl group can be observed. It is small but distinct and its presence proves that polymer is formed, indeed. For the reaction time of 6 min it is observed an attenuation of the characteristic bands (Fig. 2b). It is observed their number specific to functional groups will decrease as the plasma reaction time increases, as a result of the beginning of the cross-linking appeared in the structure of polymer. In a similar way, the absorption bands for polystyrene reflect the formation of polymer on the surface of ferrite (Fig. 3a). For the reaction time greater than 4 min (Fig. 3b) the same cross-linking beginning is observed, reflected by the attenuation or disappearance of certain bands. Thus, it is observed the spectral domain (Fig. 3) from 1028 to 1493 cm 1 characteristic to benzene cycle vibration, being evidenced its existence after the plasma treatment. I Fig. 2. FTIR spectra for poly(methyl methacrylate) coated-ferrite: I ensemble, II detail; a 4 min reaction time; b 6 min reaction time. II

1086 I. Neamtu et al. 6 I II Fig. 3. FTIR spectra of plasma polymerized styrene onto ferrite: I ensemble, II detail; a 4 min reaction time; b 6 min reaction time. Fig. 4 is a SEM image showing the morphology of polymer-coated ferrite nanoparticles by plasma polymerization. An ultrathin polymer layer in the shape of globular aggregates can be clearly seen covering the surfaces of the nanoparticles. This thin coating is also ensured on particles of all sizes as evidenced in these figures. The ultrathin film is tightly bound to the particles, being identified as a typical amorphous structure over different particles. The SEM image also indicates some larger regions indicating possible agglomeration. 1 2 3 Fig. 4. SEM microstructure of polymer-coated ferrite nanoparticles: 1 ferrite (280 ); 2 poly(methyl methacrylate)-coated ferrite (280 ); 3 polystyrene-coated ferrite (290 ). 4. CONCLUSIONS In summary, it was deposited an ultrathin film of polymer on the surface of irregularly shaped ferrite particles by means of plasma polymerization process.

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