(70br) Polystyrene Coating of Fe3o4 Particles Using Dispersion Polymerization

Inorganic core/organic shell hybrid composite particles with micron or submicron diameter are very attractive in a wide range of industrial fields, such as diagnosis, electronics, toner, and paint applications because of their functionality and dispersion stability properties [1]. Fe3O4 (iron black) particle is a kind of typical example, with magnetic properties and black color. Iron black particles are naturally hydrophobic because there are a lot of hydroxyls on the particle surface. It is difficult for iron black particles to disperse in organic media. Therefore, a well organic modification for iron black particles is very important in the applications.

Several methods have been developed for particle organic modification, such as ball mills [2], micro-emulsion polymerization [3], two-step emulsion polymerization [4], in situ polymerization [5], surface-initiated graft polymerization [6], dispersion polymerization [7] etc. Dispersion polymerization is a very attractive method owing to the inherent simplicity of the single-step process. A typical example of this method is the dispersion polymerization of styrene in hydrocarbons [7].

In this paper, the effect of a cross-linking reagent in the coating process of dispersion polymerization was experimentally studied. The coating process was analyzed by sampling and checking the morphology of the coated particles at different coating time with TEM and SEM.

Experimental

3.3 wt % of PVP (poly N-vinylpyrrolidone) as a stabilizer was dissolved in 120 ml methanol in a four-neck-flask equipped with a mechanical stirrer, a thermometer, a reflux condenser, and a nitrogen gas inlet and outlet. The mixture of iron black particles and styrene monomer was poured into 3.3wt % of PVP methanol solution and treated with ultrasonic for 5 min. Then 0.5g AIBN (2,2-azobisisobutyronitrile) was added into the reactor. The stirring speed was fixed at 450 rpm. The flask was immersed into a 65 °C water bath under a nitrogen atmosphere. The polymerization was carried out at 65 °C for 8 h.

The particle morphology was observed with a scanning electron microscope (SEM, JSM-6700, JEOL, Japan) and transmission electron microscope (TEM, JEOL-2010, JEOL, Japan). 10 mg particle samples were put into an Au crucible for TGA measurement. The heating rate was 10 ◦C/min in a nitrogen atmosphere. The measurement range of the temperature was from 50 °C to 700 °C.

Results & Discussion

1. Particle morphology

TEM and SEM images of the iron black particles showed that morphology of uncoated particles were polyhedron with smooth surface and an average diameter of about 300 nm. The polymer coated particles showed a continuous and dense coating layer. The polymer shell thickness was about 50 nm. TEM images which include many particles showed that all the particles were polymer coated.

2. Effect of cross-linking reagent

TEM and SEM images also showed that the particles were not well coated with polymer in the situation of cross-linking reagent (Divinyl benzene-DVB) absence in the coating process. It was hardly to find the polymer coated particles of iron black in this situation from the images.

TEM images showed that the particles were not coated with polymer without cross-linking reagent (DVB) even after 8h reaction. It was seen that only few polymer micro-spheres were attached on the surface of iron black particles, and these polymer micro-spheres diameter was about 100 nm, most of them dispersed in the homogeneous phase. TG results showed that the polymer quantity on the iron black particles was below 1.8% in the absence of cross-linking reagent, after 8h coating. However, the iron black particles were coated with continuous and dense polymer film when the cross-linking reagent (DVB) was introduced in the reaction, and the film thickness was about 50 nm after 8 h coating. TG results indicated that polymer quantity coated on the particles was about 8.3%. The coated quantity of the particles in the presence of cross-linking reagent changed with time increased obviously with the coating time, and the coating quantity in the absence of cross-linking reagent was little and also changed little.

It was deduced that in the absence of cross-linking reagent (DVB), oligomer was in long chain and twisted together, they dispersed stably in the homogeneous phase rather than deposited on the particle surface of iron black, so iron black particles were not coated with polymer after 8h reaction. In the presence of cross-linking reagent (DVB), oligomer was not in long chain but steric structure, a small colloid particle formed and it was easier for them to deposit on the particle surface of iron black.

The results showed that the cross-linking reagent (DVB) played a very important role in the dispersion polymerization.

3. Particle coating process

In the particle coating process of dispersion polymerization in the presence of cross-linking reagent (DVB), samples were taken for the particle morphology analysis with TEM and coated quantity measurement with TG. Samples were taken at every 10 minutes during the reaction process. TEM images showed that there were only few polymer micro-spheres deposited on the particle surface before 10 min. Thereafter, more and more dots of polymer micro-sphere deposited on the particle surface and these dots became big with time and joined together, and finally a continuous and dense polymer shell formed on the particles surface of iron black.

It was inferred that iron black particles absorbed radical and oligomer from homogeneous phase to form the colloid dots on the particle surface of iron black, and these dots continuously absorbed radical and new oligomer, these dots became big and joined together, forming a continuous polymer shell.

In the process of dispersion polymerization without cross-linking reagent (DVB), at the beginning of the reaction, oligomer formed stable nuclei in homogeneous phase rather than attached on the particle surface of iron black. These oligomer were in long chain, it was not easier for them to deposit on the surface of iron black particles. TEM images showed that there were almost no polymer coating on the particle surface of iron black after 10 min reaction, and homogeneous phase showed white because oligomer dispersed stably in continuous phase by steric repulsive. These oligomer absorbed radical and new oligomer to form polymer micro-sphere in continuous phase at last, these polymer micro-sphere size was about 100 nm. TEM images and TG results showed that only few of them attached on the particle surface of iron black.

4. Effects of different operation process

In this study, another different operation process was compared. The difference was that Fe3O4 particles were firstly dispersed in the mixture of St (styrene) and DVB, treating with ultrasonic at room temperature for 5 min, and then the mixture was poured into 3.3wt % of PVP methanol solution and with ultrasonic treatment for 5 min. Then 0.5 g AIBN was added into the reactor.

TG measurements on the samples indicated that the quantity of polymer coated on the iron black particles was about 20%. It was obvious that the added order of iron black particles in continuous phase significantly affect the coated quantity of polymer on the particle surface of iron black.

When iron black particles were directly added in the monomer mixture, the particles surface electromotive force was changed, it became easier for oligomer to deposit on the particle surface. So the coating quantity became higher.

Conclusion

Iron black particles were well coated by using dispersion polymerization in the presence of divinyl benzene as a cross-linking reagent. The coating layer was continuous and dense polymer film with a thickness about 50 nm. The cross-linking reagent was necessary for a well layer coating process. In the absence of cross-linking reagent, particles could not be coated with polymer film in the coating process, and only few polymer micro-sphere attached on the particle surface, and these polymer micro-sphere diameter was about 100 nm.

TG results indicated that the polymer coated quantity was about 20% in the experiments. After polymer coating, the apparent density of iron black particles decreased from 4.1 g/cm3 to 2.7 g/cm3. The particle surface were completely changed form hydrophilic to hydrophobic.

References

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