Iron Cluster and Microstructure Formation in Metal-Centered Star Block Copolymers:  Amphiphilic Iron Tris(bipyridine)-Centered Polyoxazolines

The microstructure of a metal-centered six-arm star block copolymer, amphiphilic iron tris(bipyridine)-centered poly(2-ethyl-2-oxazoline)- b -poly(2-undecyl-2-oxazoline), [Fe{bpy(PEOX−PUOX) 2 } 3 ] 2+, was investigated with small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Cylindrical PEOX microdomains were observed in a matrix of PUOX. A similar cylindrical morphology was noted for bpy(PEOX−PUOX) 2, the metal-free macroligand subunit of which the Fe star is comprised. Thin films simple-cast from chloroform solution onto carbon substrates annealed at 160 °C for ≈2 days revealed in both TEM and AFM the formation of nanoscale iron clusters (≈20−40-nm diameter) distributed randomly across the surface of the film. No clusters were evident in “as-cast” films subjected to TEM analysis without the annealing step, suggesting the importance of thermal treatment for nanocrystal formation. For bulk films, larger micrometer-scale (1−2 μm) iron aggregates formed and segregated to the surface of the film after thermal treatment for ≈1 week. A liquid crystal-like mesophase due to undecyl alkyl side chain ordering was evidenced by SAXS and WAXS in the bulk sample.