Crystallization-Driven Self-Assembly of a Block Copolymer with Amphiphilic Pendant Groups
Shaofei Song, Hang Zhou, Matthew Puzhitsky, Yefeng Zhang, Garion E. J. Hicks, Yijie Lu, Ian Manners, Mitchell A. Winnik
Abstract
Crystalline-coil block copolymers (BCPs) with an amphiphilic corona-forming block broaden the range of solvents in which crystallization-driven self-assembly experiments can be carried out. These materials allow one to examine how rather large changes in solvent polarity can affect the self-assembly process, leading to novel uniform structures. Here, we describe the synthesis and self-assembly properties of a crystalline-brush BCP, PFS27-b-P(EG-E)MA48 (PFS, poly(ferrocenyldimethylsilane), and P(EG-E)MA, poly(tetraethylene glycol monododecyl ether methacrylate); the subscripts refer to the mean degrees of polymerization). Tetrahydrofuran (THF) is a common good solvent for both blocks, whereas the corona polymer itself is soluble in alcohols ranging from 2-propanol to 1-decanol and in octane and decane. Self-assembly in 2-propanol was “normal”, forming long (>5 μm) fiber-like micelles of uniform width when the polymer suspended in solvent was heated (i.e., 80 °C) and cooled. Micelles of similar width but uniform and variable length were obtained when micelle fragments were subjected to seeded growth or self-seeding conditions. In primary alcohols (1-butanol to 1-decanol), direct self-assembly led to uniform scarf-like structures (total length ∼2 μm) in which the length of the central platelet increased with the carbon number in the solvent, offset by shorter fiber-like protrusions at the ends. We speculate that these changes are related to the temperature at which core crystallization occurred as the samples cooled. Self-assembly in decane was more complicated. Large (>10 μm) aggregated structures consisting of fiber bundles formed initially. Sonication followed by seeded growth with additional BCP or self-seeding led to relatively uniform elongated lenticular micelles, substantially wider than those formed under these protocols for this BCP in 2-propanol. Solvent polarity has a profound effect on the self-assembly of this BCP with a brush-like amphiphilic corona-forming block.