Crystallization-Driven Self-Assembly of a Block Copolymer with Amphiphilic Pendant Groups

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) MA(48) (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 mu m) fiber-like micelles of uniform width when the polymer suspended in solvent was heated (i.e., 80 degrees 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 similar to 2 mu m) 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. Selfassembly in decane was more complicated. Large (>10 mu m) 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.