Block Polymer Self-assembly and Applications

Abstract

Block polymers have grown beyond a niche field of polymer science and engineering to enable a diverse and seemingly ever-expanding range of practical applications. Their utility is derived from a molecular architecture containing discrete sequences of chemically distinct units that segregate into periodic nanoscale structures of various domain geometries. A primary focus of this thesis is to connect meso- and macroscopic structural properties to molecular design, a goal that broadly informs a variety of contemporary topics in polymer science. These works include facile polymerization strategies to achieve domain co-continuity, new block chemistries to circumvent the domain size limitations of preceding materials, and the discovery and understanding of new equilibrium morphologies in block polymer melts. The design strategy imparted by the block polymer molecular motif effectively combines the material properties of each block segment. In this work, we decouple the orthogonal properties of conductivity and modulus for the advancement of polymer electrolyte membranes through a facile synthetic strategy coined polymerization-induced microphase separation (PIMS). The produced electrolytes achieved the highest contemporary record of conductivity and modulus, attributes owed to a bicontinuous microstructure comprising a low-Tg, highly conductive domain and a mechanically robust crosslinked domain. This work was then extended through an adaptation of PIMS to the preparation of nanoporous materials that benefit from the continuity of a percolating pore structure in a thermally and mechanically stable crosslinked matrix. The reaction parameters that control the porous properties were elucidated. Technologies employed in modern fabrication and nanolithography must constantly improve the resolution of patterned structures. Block polymer self-assembly can serve as an alternative patterning strategy to overcome the resolution limitations that thwart the extension of current optical lithography processes. Because domain size is tied to block polymer molar mass, the design of new materials that can self-assemble at lower molar masses, and hence smaller length-scales, is needed. In this work, a new polymer, poly(cyclohexylethylene)-block-poly(ethylene oxide) (PCHE-PEO) was synthesized and found to self-assemble at exceptionally low molar mass due to the high incompatibility of block segments. Block-specific interactions between the inorganic precursors and the polar PEO block then enabled the templating of dense arrays of metal oxide structures on silicon wafers through simple spin coating techniques. The geometries assumed by block polymer mesophases have been a subject of intensive experimental and theoretical investigation. Discovery of the complex, low-symmetry Frank-Kasper σ phase in compositionally-asymmetric diblock copolymers emphasized a relationship between melt thermodynamics and domain geometry not captured by previous theoretical frameworks. Our experimental work supports recent theoretical conclusions that conformational asymmetry underpins the formation of new complex phases in block polymer materials. Through the synthesis and characterization of two different block polymers systems of low molar mass, far below the entanglement molar mass of either block segment, we enable facile assembly that is poised to probe the effect of conformational asymmetry and offer new insights into nanostructure formation in soft materials.