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Highly Oriented Thin‐Film Microdomain Patterns of Ultrahigh Molecular Weight Block Copolymers via Directional Solidification of a Solvent

Self-assembly is rapidly emerging as a simple and effective method for creating large-area functional periodic structures in various nanotechnologies. [1–3] In particular, block copolymers (BCPs) have been considered as promising self-organizing platforms because of their tunability in shape, size, and physical/chemical properties of the domains, with the capability of hosting many types of additives for desirable multifunctional composites. [4] Recently, ultrahigh molecular weight (number average molecular weight, Mn ∼10 6 gmol –1 ) BCPs possessing a domain size comparable to the wavelength of visible light have received increasing attention as self-assembled photonic materials. [5] 1D, 2D, and 3D photonic crystals for visible frequencies have been successfully demonstrated using lamellar, cylindrical, and double gyroid morphologies of polystyrene-block-polyisoprene (PS-b-PI) BCPs. [6–8] While most previous work on these photonic BCPs has been limited to relatively thick samples with only modest control over microdomain order, numerous engineering applications such as thinfilm waveguides, reflectors, and microcavities for lasing can potentially be achieved from controlled thin-film microstructures of these materials, all of which depend critically on the overall order of the microdomains to meet desired functionalities. Here we demonstrate the excellent control of thin-film microdomain patterns of lamellar- and cylinder-forming ultrahigh molecular weight BCPs (ca. 20 times larger molecular weight than for previously studied block copolymers) over a large area via directional solidification (DS) of a solvent. The ordering behavior of these photonic BCPs via the DS process has been found to be dramatically different from that of directionally solidified conventional molecular weight BCPs. Because of the relatively large domain sizes of ultrahigh molecular weight BCPs, laser scanning confocal microscopy (LSCM) could be used to optically characterize the lamellar and cylindrical thin film structures in 3D space.