Branched micelles by living crystallization-driven block copolymer self-assembly under kinetic control

We have found that the width and shape (from rectangular to elliptical, to almost circular in cross-section) of the crystalline core of fiberlike micelles of polyferrocenyldimethylsilane (PFDMS) diblock copolymers can be varied by altering the degree of polymerization of PFDMS, and also the chemistry of the complementary corona-forming block. This enabled detailed studies of living crystallization-driven self-assembly (CDSA) processes that involved the addition of unimers with a short, crystallizable core-forming PFDMS block to a seed solution of short micelles with a large diameter crystalline core, derived from block copolymers with a longer PFDMS block. The morphology of resultant micelles was found to be highly dependent on the polarity of the solvent and temperature. For example, linear micelles were formed in less polar solvents (which are moderately poor solvents for PFDMS) and/or at higher temperatures. In contrast, the formation of branched structures could be "switched on" when the opposite conditions were used. Thus, the use of more polar solvents (which are very poor solvents for PFDMS) and ambient or subambient temperatures allowed the formation of branched micelles and block comicelles with variable and spatially distinct corona chemistries, including amphiphilic nanostructures. Rapid crystallization of added unimers at the seed micelle termini under nonequilibrium self-assembly conditions appears to facilitate the formation of the branched micellar structures as a kinetically trapped morphology. This is evidenced by the transformation of the branched micelles into linear micelles on heating at elevated temperatures.