2 search hits
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Mesoscale Modeling of Phase Behavior in Thin Films of Cylinder-Forming ABA Block Copolymers
(2008)
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Andriana Horvat
- In this thesis modeling results on structure formation in thin films of cylinder-forming block copolymers are presented and discussed. The computational study of the equilibrium phase behavior in thin films is complemented by detailed comparison with a real experimental system. Additionally, the dynamics in such films at various length and time scales (the dynamics of individual defects and the dynamics of surface relief structures) is studied. The strength of the presented thesis is the comparison of thin block copolymer film equilibrium and dynamic behavior in experiments and in computer simulations. This comparison supplies an in-depth understanding of the processes in thin films and near the surfaces in thick films and allows to identify the important control parameters of nanopattern formation. Chapters 4 and 5 report on the phase behavior of thin films of asymmetric block copolymers. In addition to the surface induced alignment of hexagonally ordered cylinders, an adjustment to the planar symmetry of the surface by formation of surface reconstructions is found to dominate the phase behavior in thin films. The large parameter space covered by the simulations allows to distinguish the effects of the two constraints characteristic for thin films: the surface field and the film thickness. The deviations from the bulk cylinder structure, both in the vicinity of surfaces and in thin films are identified as surface reconstructions. The stability regions of different phases are modulated by the film thickness via interference and confinement effects. The results give evidence of a general mechanism that govern the phase behavior in thin films of modulated phases: The interplay between the strength of the surface field and the deformability of the bulk structure determines how the system rearranges in the vicinity of the surface. Chapters 6 and 7 present a systematic study of defects in thin films of cylinder-forming block copolymers. In particular, the peculiarities of both classical and specific topological are considered in detail, and a strong relationship between the defect structures and the chain mobility in block copolymers is observed. In the systems studied, representative defect configurations provide connectivity of the minority phase in the form of dislocations with a closed cylinder end or classical disclinations with incorporated alternative, nonbulk structures with planar symmetry. In solvent-annealed films with enhanced chain mobility, the neck defects (bridges between parallel cylinders) were observed. This type of nonsingular defect has not been identified in block copolymer systems before. It is shown, that topological arguments and 2D defect representation, sufficient for lamellar systems, are not sufficient to determine the stability and mobility of defects in the cylindrical phase. In-situ scanning force microscopy measurements are compared with the simulations based on DDFT. The close match between experimental measurements and simulation results suggests that the lateral defect motion is diffusion-driven. Finally, the morphological evolution is considered with the focus on the motion and interaction of the representative defect configurations. Chapter 7 reveals dynamic simulations and in-situ SFM measurements of defect annihilation. Along with the lateral movement of defects, the annihilation frequently proceeds through local structural transitions. The role of the observed structural evolution is discussed in the context of the equilibrium phase behavior of cylinder-forming thin films, studied in chapters 4-5. Chapter 8 presents a study of terrace formation in thin films of a cylinder-forming block copolymers by a computational DDFT method. The results are compared with in situ SFM measurements of SBS block copolymer thin films. This chapter focuses on the early stage of terrace formation, where 80% of height changes occur. Experimental and simulation results agree that the change of the local height is strongly connected to the changes in the local microstructure. The detailed pathways of the structural transitions, as revealed by simulations, suggest a diffusion of block copolymer chains along the microstructure interfaces and indicate an important role of cylinders with necks as a material-transport-channel between neighboring terraces in thin cylinder-forming films. Both systems (in experiment and in simulations) show excellent quantitative agreement in detail of structural phase transitions and in the dynamics of the step development, suggesting that the underlying transport mechanisms are governed by diffusion.
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Soft Compartmentalized Polymer Colloids: Janus Particles, Multicompartment Structures, Inorganic-Organic Hybrids and Applications
(2008)
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Andreas Walther
- Compartmentalized polymer-based colloids with nanoscopic dimensions and different topologies were prepared based on various block copolymer architectures. The polymers were prepared via anionic polymerization or a controlled radical polymerization technique (RAFT). Self-assembly both in solution and in bulk were rigorously exploited to create the multicompartment architectures. Several new crosslinking strategies, in bulk and in solution, were thoroughly investigated to allow a controlled preservation and a high shape-persistence of the colloidal particles even when exposed to non-selective solvents. Cylindrical and disc-like Janus particles were investigated according to their self-assembly behavior into superstructures. The Janus discs undergo back-to-back stacking in organic solvent. In aqueous solution, a size-dependent aggregation was found. While the smaller Janus discs are unimolecularly dissolved with a significant polystyrene surface exposed to the water, the larger Janus sheets can shield the insoluble side by a large bending in an intramolecular fashion. Janus cylinders self-assemble on two hierarchical levels. Upon exposure to a selective solvent, they self-organize into fibers. The length of these fibers depends on the concentration and a critical aggregation concentration exists below which self-assembly is absent. Secondly, the Janus cylinders form fibrillar networks with tunable pore sizes when deposited from more concentrated solution. The surface-active properties of spherical Janus particle were exploited for the investigation of two possible applications of both academic and industrial relevance. In Pickering emulsion polymerization, extremely well-defined latexes with long-term stability could be prepared in a very facile fashion. A control of the particle size by changing the concentration of Janus particles could easily be achieved. Secondly, the nanostructuring of polymer blends was shown for a PS/PMMA model system. The system exhibits a control on two length scales. The first is the controlled decrease of the domains of the dispersed phase and the second is the controlled spacing between the particles at the interface. The particles are exclusively located at the interface and the nanostructuring can be obtained while matching macroscopic processing constraints, i.e. high-shear blending in a mini mixer. The self-assembly of bis-hydrophilic triblock terpolymers with two outer hydrophobic blocks was investigated for a variety of different hydrophilic end blocks. The overall architecture of the solution structures could be tailored by changing the hydrophobic-to-hydrophilic balance. Additionally, the interaction between the corona-forming blocks has an influence on the particle shapes as well. The micelles possess coronas with appealing and tunable properties, due to the presence of a hydrophobic core and hydrophilic biocompatible and stimuli-responsive segments. The self-assembly of miktoarm star terpolymers, bearing arms of polystyrene (PS), polybutadiene (PB) and poly(2-vinylpyridine) (P2VP), was analyzed both in solution as well as in the bulk state. In solution, the miktoarm star terpolymers form multicompartment micelles with a glassy (PS) and a soft compartment (PB), all rendered water-soluble by the P2VP corona. Strikingly, the soft PB compartments show hydrophobic bridges in aqueous medium which is of high interest as they can be used as a second motif for sensing, adhesion control or interaction with cellular membranes. The transfer of a hexagonally ordered cylindrical bulk phase via crosslinking of the PB domain of a bulk structure of a similar miktoarm star terpolymer allowed the preparation of novel multicompartment cylinders. The structures possess perfectly parallel aligned compartments. Two symmetric and opposing PS and P2VP compartments surround a central ribbon-like PB compartment. The P2VP compartments could be used to generate perfectly aligned bi-axial nanowires inside spatially separated compartments within close proximity. Due to the presence of an amphiphilic corona, the extent of the compartmentalization can be tuned from separated nanowires into one homogenous nanowire simply by exchanging the solvent. The complexity and high control of the structure of this multicompartment cylinder is unmatched and can most likely not be obtained by solution based self-assembly.
