Self-assembly of soft materials represents a compelling approach to realize a wide variety of useful nanostructured materials. In particular, self-assembly of block copolymers by microphase separation results thermodynamically in the formation of a range of nanostructures including lamellae, cylinders, gyroids and spheres. There is significant potential to use these structures in applications ranging from energy generation to water purification. Despite their significant potential however, the use of block copolymers in the aforementioned areas has been critically limited by general inability to precisely direct their self-assembly, i.e. to control the orientational and positional order of their self-assembled structures over device or application relevant length scales and geometries.
In this context, we explore two distinct approaches to attain advanced ability to control the block copolymer microphase. First, this dissertation explores the self-assembly and directed self-assembly of novel liquid crystalline block copolymers. Result are presented from a systematic series of experimental investigations of the phase behavior and directed self-assembly of rationally designed liquid crystalline block copolymers (LC BCPs) under magnetic fields and in the presence of engineered surfaces. We specifically designed a block copolymer platform comprising etchable poly(D,L-lactide) (PLA) with brush architecture and side chain cyanobiphenyl LC block that imparts magnetic anisotropy on the system. Interestingly, this class of brush-like block copolymers behave in accordance with the canonical phase behavior of the conventional linear coil-coil block copolymers. With inclusion of labile mesogen, the magnetic field response of the system was significantly enhanced due to the increased grain size and faster mobility. By adopting cross-linkable mesogen, the LC phase can be readily polymerized and subsequent etching of the PLA produces well-defined nanopores with controlled orientation. At higher blending stoichiometric ratio, the system transforms its morphology from hexagonal cylinders to face-centered cubic (FCC) spheres and, strikingly, we observe the alignment of FCC spheres regardless of the 3 dimensional symmetry of the cubic structure.
In the second part, we adopt the use of electrospray deposition and soft-shear laser zone annealing process as tools to direct the self-assembly of structurally complex thin films of block copolymers. Conventionally, block copolymers confined in thin film were examined based on the equilibrium structure as a result of a single annealing process. Here we propose non-equilibrium processing methods that enable us to achieve non-conventional morphologies. Sequential electrospray deposition (ESD) was adopted to form multi-layered BCP thin films which exhibit heterolattice structure that can be precisely tuned by kinetic parameters. We also examine pathway-engineered two-step processing, shear aligning followed by thermal annealing on a neutral substrate, to achieve biaxial alignment of the BCP cylinders array with minimum defect density.
Overall, this dissertation provides new insight regarding the self-assembly of LC brush block copolymers and their orientation in the presence of magnetic fields. Further, it establishes a new mechanism for controlling the orientation of these materials in thin films. The results of the research presented here are relevant for the use of block copolymers in lithography and membrane fabrication, among other areas.
|School Location:||United States -- Connecticut|
|Source:||DAI-B 80/07(E), Dissertation Abstracts International|
|Subjects:||Chemical engineering, Nanotechnology, Materials science|
|Keywords:||Block Copolymers, Brush Architecture, Directed Self-Assembly, Electrospray, Liquid Crystalline Polymers, Magnetic Field|
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