Supramolecular organometallic polymer chemistry: multiple morphologies and superstructures from the solution self-assembly of polyferrocene-block-polysiloxane-block-polyferrocene triblock copolymers.

Abstract

The solution self-assembly of an organometallic-inorganic triblock copolymer, poly(ferrocenyldimethylsilane)-block-poly(dimethylsiloxane)-block-poly-(ferrocenyldimethylsilane) (PFDMS-b-PDMS-b-PFDMS, 3b; block ratio 1:13:1; Mn = 2.88 x 10(4) gmol(-1), polydispersity (PDI) 1.43 (gel permeation chromatography, GPC)) was studied in n-hexane, a PDMS block selective solvent. Transmission electron microscopy (TEM), atomic force microscopy (AFM), and TEM with negative staining analysis of these micellar solutions after solvent evaporation revealed the presence of multiple micellar morphologies including spheres, cylinders, and novel flower-like supramolecular aggregates. TEM analysis of samples fractionated by ultracentrifugation and preparative size-exclusion chromatography suggest that the formation of multiple morphologies is a consequence of compositional variations. When micellar solutions were prepared at 50 degrees C (above the glass transition of the PFDMS core-forming block) flower-like micellar aggregates similar to those present in micellar solutions prepared at room temperature also formed. However, after solvent evaporation, TEM analysis of micellar solutions prepared in decane at about 150 degrees C, above the melt temperature of the PFDMS core (ca. 120-145 degrees C), revealed the presence of spherical micelles (when decane solutions at 150 degrees C were rapidly cooled to room temperature) and rod-like cylindrical micelles (when decane solutions at 150 degrees C were slowly cooled to room temperature). In contrast, poly(ferrocenylmethylethylsilane)block-poly(dimethylsiloxane)-block-poly(ferrocenylmethylethylsilane) (PFMES-b-PDMS-b-PFMES, 4; block ratio 1:16:1; Mn=2.90x10(4)g mol(-1), PDI= 1.42 (GPC)) and poly(ferrocenylmethylphenylsilane)-block-poly(dimethylsiloxane)-block-poly(ferrocenylmethylphenylsilane) (PFMPS-b-PDMS-b-PFMPS, 5; block ratio 1:15:1; Mn=3.00 x 10(4) gmol(-1), PDI = 1.38 (GPC)), which possess completely amorphous organometallic core-forming blocks, formed only spherical micelles in hexane at room temperature. These observations indicate that crystallinity of the insoluble polyferrocenylsilane block is a critical factor in the formation of the nonspherical micelle morphologies.