Crystallization-Driven Self-Assembly of Block Copolymers with a Short Crystallizable Core-Forming Segment: Controlling Micelle Morphology through the Influence of Molar Mass and Solvent Selectivity

Three well-defined asymmetric crystalline-coil poly(ferrocenyldimethylsilane-block-2-vinylpyridine) (PFS-b-P2VP) diblock copolymers (PFS44-b-P2VP(264), PFS75-b-P2VP(454), and PFS102-b-P2VP(625)) with similar block ratios (r = N-P2VP/N-PFS = ca. 6.0 +/- 0.1) but different overall molar masses (M,, = 38 700, 65 800, and 90 400 g mol(-1)) were synthesized by sequential anionic polymerization, and their solution self-assembly behavior was explored as a function of (i) molar mass and (ii) the ratio of common to selective solvent. When self-assembly was performed in isopropanol (i-PrOH), a selective solvent for P2VP, a decrease in the rate of the crystallization-driven transition from the initially formed spheres (with amorphous PFS cores) into cylinders (with crystalline cores) was detected with an increase in molecular weight. This trend can be explained by a decrease in the rate of crystallization for the PFS core-forming block as the chain length increased. In contrast, when a mixture of i-PrOH with increasing amounts of THF, a common solvent for both blocks, was used, spheres, cylinders, and also narrow lenticular platelets consisting of crystallized PFS lamellae sandwiched by two glassy coronal P2VP layers were formed from the same PFSx-b-P2VP(6x) sample. The most likely explanation involves the plasticization of the PFS core-forming block which facilitates crystallization, possibly complemented by contraction of the coils of the P2VP coronal block which otherwise limit of the lateral growth of the crystalline PFS core as THF is a poorer solvent for P2VP than i-PrOH. Selected area electron diffraction studies indicated that the PFS cores of the spherical micelles were amorphous but were consistent with those of the cylindrical micelles existing in a state approximating to that of a monoclinic PFS single crystal. In contrast, in the platelets formed in THF/i-PrOH, the PFS cores were found to be polycrystalline. The formation of narrow lenticular polycrystalline platelets rather than a regular, rectangular single crystalline morphology was attributed to a poisoning effect whereby the interference of the long P2VP coronal blocks in the growth of a rectangular PFS single crystalline core introduces defects at the crystal growth fronts.