Self-Assembly and Magnetic Alignment of Non-Liquid Crystalline Block Copolymers
Abstract
Block copolymers are a class of self-assembling macromolecules that possess rich phase behavior as a function of readily tunable variables such as macromolecular architecture, constituent chemistry, relative block composition, and temperature. Immiscibility of the components known as blocks enables the formation of various geometries with periodic length scales on the order of 5-100 nm. The accessibility of these small length scales, coupled with continually improving synthetic capabilities, make block copolymers an attractive class of materials for both existing and emerging technologies. Some promising applications include membranes for size- and chemo-selective transport, templates for synthesis, organic photovoltaic devices, and next-generation lithography. Most of the aforementioned applications require the uniaxial orientation of periodic features, such as cylindrical or lamellar arrays, over macroscopic areas. This need has led to broad interest in studying the fundamental physical processes involved in driving and directing the self-assembly of block copolymers. Thus, there has been extensive work using a variety of methods for orienting block copolymer features including (but not limited to) solvent-vapor annealing, application of shear, electric, and magnetic fields, and chemical and topological patterning. This dissertation focuses on magnetic field driven alignment, which allows for arbitrary field orientations relative to the sample over large areas and does not require direct contact with the material. Historically, magnetic field alignment of block copolymers has relied on the presence of liquid crystalline (LC) or crystalline assemblies to provide sufficient magnetic anisotropy to drive alignment. This dissertation shows that alignment is also possible in poly(styrene-b-4vinyl pyridine) (PS-b-P4VP), a non-LC coil-coil block copolymer, due to its intrinsic chain anisotropy. This novel finding is introduced and rationalized in terms of the thermodynamic driving force for field-induced alignment, with emphasis on the roles of field strength, cooling rate, grain size and constituent chemistry. Expanding upon the aforementioned result, this dissertation then demonstrates how intrinsic chain anisotropy is leveraged to magnetically direct the self-assembly of many non-LC systems, including other coil-coil block copolymers, blends of block copolymers of disparate morphologies and molecular weights, blends of block copolymers with homopolymers, and mesogen-containing macromonomers and their bottlebrush copolymers. The phase behavior and temperature-, time and field-dependent dynamics of alignment in these non-LC systems are surveyed using a variety of characterization techniques, including in-situ X-ray scattering, electron microscopy and differential scattering calorimetry. The results demonstrate that intrinsic chain anisotropy is sufficient to support strong field-induced orientational order in a diverse set of block copolymer systems. Overall, this dissertation introduces and develops a previously unexplored area of research, magnetic alignment of non-liquid crystalline polymeric systems, and describes relevant fundamental insights while highlighting opportunities for advanced applications.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 2018
- Bibcode:
- 2018PhDT........75R
- Keywords:
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- Materials science;Chemical engineering