Polymer magnetic composite (PMC) nanoparticles have gained a large attention due to their potential use in several biomedical applications from biomedical to engineering field. Among the different heterogeneous polymerization techniques that are generally used to prepare hybrid polymer particles, miniemulsion polymerization is proved to be an efficient one. The occurrence of preferential droplet nucleation in case of miniemulsion polymerization results in a 1:1 copy of monomer droplets to the polymer particles and such a mechanistic pathway offers a suitable environment for the preparation of hybrid polymer nanoparticles in the range between 50 to 500 nm.
The surfactant in miniemulsoin plays a significant role to stabilize the droplets/particles and also in the encapsulation of nanoparticles. In the present study, a new class of surfactant, called amphiphilic ionic liquid, has been employed in the field of miniemulsion. The amphiphilic ionic liquid has amazing ability to impart surface tunable characteristics to the polymer particles when present on the surface of the particles. Thus the aim of the present work is to synthesize polymer magnetic composite nanoparticles with good colloidal stability, high content of magnetic nanoparticles as well as the chance for further surface functionality. Such magnetic nanoparticles may find applications in various fields.
At first, the aim of the work was to establish a suitable recipe with ionic liquid as surfactant for the execution of miniemulsion polymerization. Monodisperse polystyrene nanoparticles were possible to be synthesized reproducibly. The established recipe was utilized to carry out the synthesis of PMC nanoparticles. Iron oxide (Fe3O4) was taken as magnetic nanoparticles (MNP) and it was hydrophobized with oleic acid to disperse in styrene. The concentration of feed MNP was varied to observe its influence on the characteristics of PMC nanoparticles. Stable dispersion of magnetic polystyrene particles was possible to be synthesized up to 8 wt% feed MNP. But feeding 12 wt% MNP resulted in the development of large amount of coagulum associated with instability in the dispersion. TGA investigation confirmed a significantly lower MNP content (8.2 wt%) of the composite compared to the feed amount. TEM investigation showed inhomogeneous distribution of MNP among polystyrene particles and agglomeration of MNP was observed on the surface of polystyrene particles.
Considering the inability of the single step miniemulsion polymerization for the preparation of high MNP content polymer particles, it was aimed to find a new strategy which can produce such material. Inspired from the affinity of carboxylic acid group towards the surface of MNP, it was aimed to synthesize carboxyl functionalized polystyrene which was expected to improve the interaction between polymer and magnetic nanoparticles. For this purpose, RAFT mediated miniemulsion polymerization was performed in presence of a carboxyl functionalized chain transfer agent (CTA). The colloidal stability was much better compared to the previous case of non-RAFT experiments. From a feed MNP of 8 wt%, a high final MNP content up to ~27 wt% could be achieved and all the dispersions were highly stable. The higher MNP content in the final composites compared to the feed ratio was a result of the low monomer conversion and could be adjusted by a proper tuning of AIBN to CTA mole ratio.
Another significant influence of the carboxyl functionalized CTA was observed on the morphology of the composite nanoparticles. The MNP were distributed homogeneously among the PS particles. Regarding the dispersion of MNP in the individual polystyrene particles, it was observed that higher amount of CTA resulted in a homogeneous dispersion of MNP whereas higher amount of initiator ended up producing asymmetric Janus like morphology.
Apart from that, due to the involvement of CTA in the polymerization, much lower molecular weight of the polystyrene chains was developed compared to the free radical process and the molecular weight distribution of PS in the composite nanoparticles became much narrower through the RAFT polymerization. Thus a relatively good control over the polymerization process was achieved through RAFT polymerization which was confirmed by a nearly linear increase of molecular weight (Mn) with time of polymerization and thus, monomer conversion.
In the recipe of miniemulsion, costabilizer plays an important role to retard monomer diffusion from smaller to larger droplets. Hexadecane, being the most frequently used costabilizer for miniemulsion, has been employed in this study so far. But its volatile nature restricts its utilization in several applications. For the replacement of hexadecane, a carboxyl functionalized polystyrene is employed as a costabilizer as well as a macro CTA in miniemulsoin polymerization of styrene. For this purpose, low molecular weight carboxyl bi-functionalized polystyrene (9000 g/mole) was synthesized by thermal bulk RAFT polymerization. The carboxylated polystyrene worked successfully as a costabilizer in miniemulsion and molecular weight investigation confirmed the integration of the carboxyl functionalized macro CTA into the developing polystyrene chain via RAFT polymerization. This strategy was employed successfully to synthesize stable dispersion of PMC nanoparticles with a reasonable content of MNP in the system. A homogeneous morphology was observed regarding the distribution of MNP among the polystyrene particles.
The strategy of using macro CTA as costabilizer can be utilized to synthesize various functional copolymers with control architecture without any added monomer and CTA in the system. Moreover, presence of functionality within the monomer droplets can be effective to encapsulate several nanomaterials using miniemulsion polymerization.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa.de:bsz:14-qucosa-164920 |
| Date | 20 April 2015 |
| Creators | Chakraborty, Sourav |
| Contributors | Technische Universität Dresden, Fakultät Mathematik und Naturwissenschaften, Prof. Dr. Brigitte Voit, Prof. Dr. Markus Antonietti |
| Publisher | Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis |
| Format | application/pdf |
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