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%0 Thesis
%A Kathrein, Christine
%T Phase transitions and ordering of microphase separated block copolymer nanostructures in electric fields
%I RWTH Aachen
%V Dissertation
%C Aachen
%M RWTH-2016-03753
%P 1 Online-Ressource (vi, 157 Seiten) : Illustrationen, Diagramme
%D 2016
%Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
%Z Dissertation, RWTH Aachen, 2016
%X Increasing miniaturization of electronic and data storage devices necessitates methods which enable large area structuring on a nanometer length scale at reasonable prices. Block copolymers (BCP) are promising candidates for future application in e.g. semiconductor industry, and solar cells due to their capability to readily self-assemble into highly ordered thermotropic or lyotropic phases on a length scale of 10-100 nm. One of the main challenges is gaining control over structure and orientation of block copolymer microphases. Methods envisaged include the application of external stimuli such as shear forces, temperature gradients, solvent annealing, magnetic- or electric fields, as well as chemoepitaxy, and graphoepitaxy. Benefits of electric field-induced orientation include that they can easily be integrated into electronic devices, are increasingly effective on diminishing length scales, and the rapid, stepwise tunability of field strength.This thesis deals with the analysis of phase transitions and ordering of block copolymer microphases induced by electric fields. The first four chapters deal with the bulk analysis of the effect of electric field on microphase separated diblock copolymer nanostructures via birefringence measurements and synchrotron SAXS. In Chapter 3 we describe the setup developed to use birefringence measurements as a supplementary method to synchrotron SAXS. The novel setup enabled the analysis of the effect of strong dc electric fields on the order-disorder transition temperature in a large parameter window, for various block copolymer systems. The novelty of our findings is that we identify the parameters that evoke mixing of block copolymers when exposed to electric fields (the difference in dielectric permittivity (Δε) between the block copolymer constituents and the degree of polymerization (N)). Another phase transition focused on is the electric-field-induced transition between a gyroid and a cylinder nanostructure. Through a detailed synchrotron SAXS study we could unveil the mechanisms of the electric-field-induced gyroid-to-cylinder transition and of the reformation of the gyroid phase after turning off the electric field which have not been reported to date. The detailed mechanistic study is given in Chapter 6. We show that the exploited mechanism is determined by temperature and electric field strength. In Chapter 5 time- and temperature-resolved in-situ birefringence measurements were applied to analyze the effect of nanoparticles on the electric field-induced alignment of block copolymers. With our experiments we reveal that the incorporation of isoprene-confined CdSe quantum dots leads to an altered reorientation behavior. Particle loading lowers the order-disorder transition temperature, and increases the defect density, favoring nucleation and growth as an alignment mechanism over rotation of grains. Experiments to the last two chapters of this thesis were performed during a research internship at the Massachusetts Institute of Technology (MIT) and deal with the effect of electric fields on thin film samples. In Chapter 7 we systematically analyze how the combination of two directing effects (graphoepitaxy and electric field) influences the self-assembly of cylinder forming polystyrene-block-poly(dimethylsiloxane) block copolymer in thin films during solvent vapor annealing. We analyzed how the angle between the electric field direction and the topographic guides, as well as the dimensions of the trenches affected both the quality of the ordering and the direction of the orientation of cylindrical domains: parallel or perpendicular to the topographic features. This combined approach allows the fabrication of highly ordered block copolymer structures using macroscopically pre-patterned photolithographic substrates.Further structural diversity can be achieved by adding a third block to the polymer. Hence, Chapter 8 focuses on electric-field-induced phase transitions in thin films of star-shaped 3-miktoarm triblock terpolymers composed of polyisoprene, polystyrene and polyferrocenylethylmethylsilane. We show that the metalorganic PFEMS block alters the behavior of the polymer upon exposure to electric fields compared to the other block copolymers analyzed in this thesis. Additionally, to the previously described effect of ordering the volume fraction of PFEMS is enhanced with increasing electric field strength due to oxidation of the ferrocenyl groups. Therefore, different electric field strength dependent morphological transitions are observed. The results demonstrate the multiple tunability of the ordered microdomain structure by simple stimuli application.Summarizing different aspects of electric-field-induced ordering and phase transitions in various block copolymer systems were analyzed in this thesis enhancing the understanding of the effect of electric fields on the block copolymer phase diagram and pointing out possible future applications.
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%U https://publications.rwth-aachen.de/record/573840