Competitive Self-Assembly Kinetics as a Route To Control the Morphology of Core-Crystalline Cylindrical Micelles

Abstract

Nucleated self-assembly in selective solvents of core-crystalline block copolymers (BCPs) is a special case of living supramolecular polymerization, leading to rodlike micelles of controlled and uniform length. For the crystallization-driven self-assembly of PFS-containing BCPs (PFS = polyferrocenyldimethylsilane), the formation of block comicelles by sequential addition of different BCPs is well-established. But there are only a few examples of living copolymerization, the simultaneous addition of pairs of BCPs with different corona-forming chains. At present, relatively little is known about the competitive kinetics of different BCPs crystallizing on a common seed. Here we report a systematic study of the competitive seeded growth kinetics of pairs of linear PFS-containing BCPs and show that one can manipulate the kinetics to control the morphology of the comicelles. We found that the seeded-growth kinetics of the individual BCP unimer dominates the coassembly behavior and thus the morphology of the corona. Patchy comicelles with microphase-segregated corona chains are formed when the epitaxial growth rates of the two different BCPs on the common seed are similar. In contrast, factors that lead to dissimilar growth rates (long corona-forming blocks or introduction of charges on corona-forming chains) promote large-scale separation of the corona blocks, leading to block comicelles. Because the termini of the comicelles remain living, they can further direct the growth of unimers, resulting in hierarchical block comicelles with patchy blocks and single-component (homo) blocks. Furthermore, the patchy comicelles can be loaded with either gold or platinum nanoparticles, generating organic-inorganic hybrid materials with potential application in catalysis.

One‐dimensional growth kinetics for formation of cylindrical crystalline micelles of block copolymers

“…This means that the value of Q in Equation varies with the length of the core‐forming block. Winnik and Manners also added two kinds of PFS‐containing BCPs into the solution of seed micelles and found that cylindrical comicelles with different structures could be formed, including block comicelles with different block lengths, comicelles composed of patchy block and single‐component blocks and gradient comicelles (Figure ). The formation mechanism of different comicelles originates from the different growth rates of the individual BCP unimers, like the different reactivity ratios of monomers in copolymerization.…”

“…Cylindrical comicelles with different structures formed by two poly(ferrocenyldimethylsilane) (PFS)‐containing block copolymers (BCPs) via seeded growth. Reproduced from Reference , with permission from American Chemical Society…”

Recent progress in block copolymer crystallization

“…Interactions between crystalline portions, analogous to the ripening of crystal formation during micelle morphology transitions described previously, or the amorphous corona provides directed assembly pathways for new structures. Mixed coronas of co‐micelles were formed in a solution of PNIPAM‐ b ‐PFS and P2VP‐ b ‐PFS due to different growth kinetics of each block . Figure shows TEM images of CDSA cylinders with “patchy” coronas, where microphase separation created portions of the cylinder with one corona block, PNIPAM lighter portions, and sections with the other, darker P2VP (stained with Karstedt's catalyst).…”

“…Below each image is a schematic representation of the assembly where yellow represents PFS, blue represents P2VP, and red represents PNIPAM. (Reproduced from with permission from the American Chemical Society)…”

“…If the blocks were miscible, it is more likely that the corona would behave like an average of the two components without any regular or modulated physical structure periodicity. Control of crystallization and micelle growth allow for tailoring the periodicity, which would make these assemblies ideal for patterned surfaces. Schmelz et al .…”