Fiberlike Structures from the Self-Assembly of a Highly Asymmetric Poly(ferrocenyldimethylsilane-b-dimethylsiloxane) in Dilute Solution

Highly asymmetric block copolymers normally form starlike polymeric micelles when dissolved in a solvent selective for the longer block. Here we report the unexpected formation of tubelike structures in hexane and in decane from the self-assembly of a highly asymmetric poly(ferrocenyldimethylsilane-b-dimethylsiloxane), PFS54-b-PDMS945. Hexane and decane are good solvents for PDMS but nonsolvents for the crystalline PFS. By transmission electron microscopy, we observe that the polymers form transient structures that evolve into tubular structures with a diameter of approximately 25−26 nm, a wall thickness of ca. 7 nm, and lengths reaching more than 100 μm. In decane above the melting temperature of PFS, the system forms normal starlike micelles. Upon cooling, these micelles become trapped in a metastable state that aggregates. While long fiberlike structures form eventually at 23 °C from these aggregates, they never reach the equilibrium morphology obtained at 23 °C in hexane or at 61 °C in decane. We also monitored the growth of these aggregates in hexane by dynamic light scattering. The solution initially prepared at 23 °C appears to consist only of individual free molecules. Some of these associate over tens of minutes to form a few large tubular structures. It takes a period of tens of hours for the entire sample to become incorporated into these structures.