Hierarchical self-assembly of star-shaped organometallic crystalline-coil block copolymers in solution

Abstract

We describe the stepwise hierarchical self-assembly of star-shaped organometallic crystalline-coil block copolymers with four arms, poly(ethylene oxide)-block-poly(ferrocenyldimethylsilane) [PEO₅₀-b-PFDMS_x]₄, into anisotropic superstructures of up to several μm in length. The process takes place in mixtures of a common solvent for both blocks (THF) and a non-solvent for the organometallic PFDMS block (acetone, EtOH). In both cases, spherical intermediates could be identified via light scattering (DLS) and electron microscopic techniques (SEM, TEM, and cryo-TEM). Materials with weight fractions of PFDMS in the range of 64–80% have been used and, depending on the selective co-solvent, several “triggers” to induce the formation of anisotropic superstructures have been identified: the content of the selective co-solvent (acetone, EtOH), the temperature (EtOH), and the overall star block copolymer concentration (EtOH). In addition, the processes are, to a certain extent, reversible. Whereas in THF–EtOH solvent mixtures crystalline vesicles seem to serve as intermediate building blocks, in the case of THF–acetone mixtures large-compound micelles without any intrinsic fine structure are observed initially. Nevertheless, after further agglomeration into superstructures, an undulated tubular morphology is observed and a partially crystalline nature can be confirmed by X-ray analysis (WAXS).