Oder and phase behaviour of block copolymers containing magnetic nano-particles
Final Report Abstract
The ultimate goal of this project was to prepare highly ordered magnetic materials with the aid of block copolymer self-assembly. The conceptual idea was to use functionalized MNP for fabrication of MNP/BCP composites with desired orientation and improved long-range order of BCP microdomains by means of application of external forces, in particular, external magnetic field. The whole experimental work was subdivided into two parts: 1) preparation, surface functionalisation and characterization of magnetic nanoparticles, which possess distinct affinity towards one of the blocks of BCP, 2) preparation and investigation of MNP/BCP composites, generation of highly ordered MNP/BCP patterns and attempts to alter domain orientation by an application of external magnetic field. Series of ferrite-type MNP were synthesized by thermal decomposition of acetylacetonate. Depending on the type of surfactants used during the synthesis, the nanoparticles appear covered with organic layer of different nature and thickness. The coordinating ability of pyridine nitrogen was exploited to drive the nanoparticles selectively into PVP phase. By omitting of oleic acid as widely used stabilizing agent and with an application of mixture of TOPO, oleylamine and 1,2-hexadecanediol during MNP synthesis it became possible to prepare MNP with apparent affinity towards PVP without any additional particle modification steps. Observed outstanding mutual affinity between PVP and MNP is attributed to the formation of thinned organic shell on the MNP surface, being still sufficient for particles stabilization. Such thinned stabilizing layer allows the MNP directly interact with the PVP chains via existence of some free binding sites available overall on the MNP surface. Various factors influencing final morphology of composites were studied and analyzed, in particular, nature and density of MNP stabilizing shell, BCP characteristics (i.e. chemical composition, molecular weight and block ratio), MNP loading fraction of MNP, sample preparation and post-treatment conditions. As-synthesized prepared MNP with thinned surfactant layer were successfully incorporated into lamellar and cylindrical PVP microdomains of PS-b-P2VP and PS-b-P4VP BCP, displaying high degree of selectivity of MNP location. Apart of nature of MNP stabilizing shell, the efficiency of selective MNP incorporation into PVP phase is also strongly dependent on the chemical composition of PS-b-PVP BCP. For example, it was possible to incorporate MNP into lamellar P2VP domains of symmetric PS50-b-P2VP50, but failed in attempts to introduce them into cylinder-forming P2VP domains of asymmetric PS56-b-P2VP21 having shorter-chain P2VP block. Nevertheless, when PS57-b-P4VP18 of comparable molecular weight and composition was used instead of PS56-b-P2VP21, MNP were found perfectly arranged within P4VP cylinders. Such difference in results observed for similar molecular weight asymmetric PS56-b-P2VP21 and PS57-b-P4VP18 BCP can be rationalized in terms of different coordination ability of P2VP and P4VP blocks. In P2VP the nitrogen atoms of 2VP units are located in “ortho”-position and, hence, they are less predisposed for interactions with solid surface due to steric hindrances. In contrast, in P4VP chains, where the nitrogen is located in “para”-position, pyridine units do not have such steric constrains and, thus can more effectively interact with nanoparticles surface. Desired microdomain orientation and lateral order of MNP/BCP composites were tuned using solvent vapor annealing was used. Particular samples were subjected to a solvent vapor annealing in the presence of external magnetic field. In case of MNP/PS50-b-P2VP50 composites with lamellar morphology it was possible to alter domain orientation from perpendicular to a mixed one, more specifically to a substrate parallel lamella perturbed by narrow regions of perpendicularly oriented microdomains. For cylinder-forming MNP/PS57-b-P4VP18 composites, domain orientation remained similar to those annealed without an application of external magnetic field. However, no indication of unidirectional domain orientation was observed. On the other hand, the long-range order of BCP microdomains can be significantly improved with an application of so called “slow” annealing procedure, which allowed to create a patterns of perfectly ordered standing P4VP cylinders loaded with MNP over relatively large sample area (~10 µm2). Suggestions for further work can be summarized as follows: 1) utilization of ferro/ferrimagnetic nanoparticles instead of superparamagnetic might be more effective to achieve magnetic field induced re-orientation of BCP microdomains; 2) another possibility is to use anisotropic nanoparticles (i.e. magnetic nanorods) instead of spherical NP; 3) an application of magnetic field of higher strength might also be beneficial; 4) utilization of substrates, which are neutral with respect to the each of BCP components; 5) as alternative way, in-situ synthesized MNP would further enhance the strength of interaction between NP and host block and benefit to the stability of the whole system against particle macrophase separation. Primary experiments on preparation of MNP/BCP composites containing in-situ synthesized MNP are currently in progress.
Publications
- Synthesis and selective segregation of Fe3O4 nanoparticles inside of the polyvinylpyridine domain of poly(styrene-b-vinylpyridine. 8th International Conference on Advanced Polymers via Macromolecular Engineering, Dresden, Germany, October 4-7, 2009
Horechyy A., Zafeiropoulos N.E., Formanek P., Tsitsilianis C., Stamm M.
- Highly ordered arrays of Fe3O4 magnetic nanoparticles via block copolymer assembly. 10th European Symposium on Polymer Blends, Dresden, Germany, March 7-10, 2010
Horechyy A., Zafeiropoulos N.E., Nandan B., Formanek P., Simon F., Kiriy A., Stamm M.
- Highly ordered arrays of Fe3O4 magnetic nanoparticles via block copolymer selfassembly. 454 WE-Heraeus-Seminar: Polymer-Nanoparticles Interactions: Concepts, Observations and Applications, Bad Honnef, Germany, March 28-31, 2010
Horechyy A., Zafeiropoulos N.E., Formanek P., Nandan B., Kiriy A., Stamm M.
- Highly ordered arrays of magnetic nanoparticles prepared via block copolymer assembly. Journal of Materials Chemistry 2010, 20, 7734-7741
Horechyy, A., Zafeiropoulos, N. E., Nandan, B., Formanek, P., Simon, F., Kiriy, A., Stamm, M.
- Nanohybrid materials with block copolymers and nanoparticles. 10th European Symposium on Polymer Blends, Dresden, Germany, March 7-10, 2010
Horechyy A., Zafeiropoulos N.E., Stamm M.
