Macromolecular NanotechnologyNanocomposite particles with core–shell morphology II. An investigation into the affecting parameters on preparation of Fe3O4-poly (butyl acrylate–styrene) particles via miniemulsion polymerization
Introduction
Organic–inorganic nanocomposites have greatly attracted the interest of many researchers in recent years. Organic polymeric materials with excellent optical properties, good flexibility and toughness are easy to process, and can improve the brittleness of inorganic materials [1]. They combine the advantages of polymers, e.g. elasticity, transparency and dielectric properties and also those of inorganic nanoparticles, e.g. specific absorption of light, magneto-resistance effects, chemical activity and catalysis [2]. Magnetic nanoparticles with or without polymer coatings can be used in magnetic drug targeting, tissue engineering, magnetic resonance imaging, hyperthermia, detoxification of biological fluids and magnetic guidance of particle systems for specific drug delivery processes [3]. There have also several applications for mechanical and electrical devices, which take advantage of the magneto-rheological properties of ferrofluids, e.g. in loudspeakers, seals, sensors, dampers [4]. Magnetic nanocomposite particles are important for many in vivo applications, including magnetic resonance imaging [5], magnetic field assisted radionuclide therapy [6] or hyperthermia [7].Their advantages of biocompatibility and availability in a size range below 300 nm [8], [9] are accompanied by an irregular particle shape and a soft particle matrix that causes them to be sensitive to mechanical stress. In order to encapsulate such magnetite particles in the monomeric phase and obtain a good dispersion, reaction conditions must be such that all the magnetite particles are transferred uniformly into the resulting particles, or the magnetite particles must provide the only site for precipitation of polymers. However, it often carries the risk of incomplete and non-uniform encapsulation, in which the resulting particles are usually of unequal sizes, lack homogeneity and the distribution of magnetite in the polymer particles is not uniform [10]. For the encapsulation of a material, it has to be dispersed in monomers phase prior to miniemulsification. This strategy has been already used for the encapsulation of hydrophilic magnetite into polystyrene [4]. To obtain a successful encapsulation, the magnetite aggregates have to be hydrophobized in order to make them dispersible in hydrophobic monomers. A mixture of magnetite particles coated with oleic acid in monomers, anionic surfactant and cationic surfactant could be miniemulsified in water and after polymerization, polymer encapsulated magnetite particles could be obtained. Also the possibility of nucleation in the aqueous phase can be avoided. This is different from the conventional emulsion polymerization with nucleation started in monomer-swollen micelles [11].
Ultrasonic irradiation is usually used for miniemulsification process to produce high shears. When ultrasonic waves pass through a liquid medium, large numbers of microbubbles form, grow, and collapse in very short time in about a few microseconds, which is called ultrasonic cavitation. Sonochemical theory calculation and the corresponding experiments suggest that ultrasonic cavitation can generate local temperature as high as 5000 K and local pressure as high as 500 atm. Heating and cooling rate is greater than 109 K/s in a very rigorous environment [2]. Direct miniemulsions are understood as stable aqueous dispersions of oil droplets having a size between 50 and 500 nm prepared by shearing a system containing oil, water, a surfactant and a highly water insoluble compound, the so-called hydrophobe which suppresses Ostwald ripening of the droplets [12], [13]. In addition, ultrasonic treatment facilitates the formation of minidroplet of monomer containing, e.g. Fe3O4, resulting in stability of the miniemulsion.
In this work, miniemulsion polymerization of styrene–butylacrylate was performed in the presence of modified Fe3O4 nanoparticles coated with oleic acid. The aim of this study was to follow the interchanges of magnetic properties of magnetite nanoparticles under polymerization process, which will result in the loss of magnetization extent. The parameters such as hydrophobe and surfactant amounts affecting on the preparation of latexes with higher magnetite content, conversion and lower coagulation amounts were investigated too.
Section snippets
Materials
Spherical iron (II, III) oxide nanopowder (purity ⩾ 98%) with 20–30 nm particle diameters, surface area > 60 m2/g from BET test were obtained from Aldrich. Styrene (St) from Merck Chemical Co. (analytical grade) was washed with 5 wt% sodium hydroxide aqueous solution to remove the inhibitor, dried over calcium chloride and stored at 0 °C. Butyl acrylate (BA) from Fluka, sodium dodecyl sulphate (SDS) from Aldrich, oleic acid, hexadecane (HD), Span 80 and benzyl peroxide (PBO) from Merck Chemical Co.
Results and discussion
In our previous work, we reported the preparation of magnetic latexes and films of St-BA copolymers containing Fe3O4 nanoparticles through initiator-free miniemulsion polymerization reaction under direct ultrasonic irradiation [14]. The major problem concerning that method was the limitation in progress of polymerization reaction, which will result in the lowering of magnetite content in the final latex to a medium level. For improving the conversion amount and reaching to high magnetite
Conclusion
We succeeded to prepare magnetic nanocomposite particles with high magnetic content and up to almost 32 wt% relative to the polymer amount by using an innovative process based on miniemulsion polymerization. The progress of the polymerization reaction was followed and particles with less than 200 nm in diameter were characterized. The effect of several controlling factors such as hydrophobe concentration and surfactant amount on the particle size and coagulation percent were deliberated too. Also
Acknowledgments
We wish to express our gratitude to Iran polymer & Petrochemical Institute (IPPI) for financial support of this work (Grant # 24761131). Also helpful assistance of Mr. Hashemi for taking TEM micrographs from Faculty of Science, University of Tehran is greatly acknowledged.
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