Gradient Crystallization-Driven Self-Assembly: Cylindrical Micelles with Patchy Segmented Coronas via the Coassembly of Linear and Brush Block Copolymers

Block copolymers (BCPs) with a short crystallizable poly(ferrocenyldimethylsilane) (PFS) core-forming block self-assemble in selective solvents to afford cylindrical micelles, the ends of which are active to further growth via a process termed living crystallization-driven self-assembly (CDSA). We now report studies of the CDSA of a series of crystalline-brush BCPs with C-6 (BCP6), C-12 (BCP12), and C-18 (BCP18) n-alkyl branches that were prepared by the thiol-ene functionalization of PFS-b-PMVS (PMVS = poly(methylvinlysiloxane)). Although the increased n-alkyl brush length of BCP12 and BCP18 hindered micelle growth, the increased intercoronal chain replusion could be alleviated by their coassembly with linear PFS-b-PMVS. When the coassembly was initiated by short cylindrical seed micelles, monodisperse block comicelles of controllable length with patchy coronal nanodomains were accessible. TEM and AFM analysis of micelles prepared from BCP18 and PFS-b-PMVS were found to provide complementary characterization in that the OsO4-stained PMVS coronal domains were observed by TEM, whereas the brush block domains of BCP18 (which displayed greater height) were detected by tapping mode AFM. The results showed that the coassembly afforded a gradient structure, with an initial bias for the growth of the linear BCP over that of the more sterically demanding brush BCP, which was gradually reversed as the linear material was consumed. This represents the first example of living gradient CDSA, a process reminiscent of a living covalent gradient copolymerization of two different monomers. Although other possible explanations exist, simulations based on a statistical model indicated that the coronal nanodomains detected likely result from a segmented, gradient comicelle architecture that arises as a consequence of: (i) different rates of addition of BCP unimer to the micelle termini, and (ii) a cumulative effect resulting from steric hindrance associated with the brush block.