Supramolecular organization in block copolymers containing a conjugated segment: a joint AFM/molecular modeling study

The solid-state supramolecular organization of block copolymers containing one pi-conjugated block and one non-conjugated block is elucidated with a joint experimental and theoretical approach. This approach combines atomic force microscopy (AFM) measurements on thin polymer deposits, which reveal the typical microscopic morphologies, and molecular modeling, which allows one to derive the models for chain packing that are most likely to explain the AFM observations. The conjugated systems considered in this study are based on aromatic building blocks (i.e. phenylene, phenylene ethylene, fluorene, or indenofluorene), substituted with alkyl groups to provide solubility; they are attached to non-conjugated blocks such as polydimethylsiloxane, polyethylene oxide, or polystyrene. Films are prepared from solutions in solvents which are good for both blocks, in order to prevent aggregation processes in solution. Therefore, the morphology observed in the solid state is expected to result mostly from the intrinsic self-assembly of the chains, with little specific influence of the solvent. In such conditions, the vast majority of compounds show deposits made of fibrilar objects. Closer examination of single fibrils on the substrate surface indicates that the objects are ribbon-like, i.e. their width is significantly larger than their height, with typical dimensions of a few tens of nanometers and a few nanometers, respectively. These results suggest that a single type of packing process, governed by the pi-stacking of the conjugated chains, is at work in those block copolymers. This prevalence of such a type of packing is supported by the theoretical simulations. Molecular mechanics/dynamics calculations show that the conjugated segments tend to form stable pi-stacks. In these assemblies, the block copolymer molecules can organize in either a head-to-tail or head-to-head configuration. The former case appears to be most likely because it allows for significant coiling of the non-conjugated blocks while maintaining the conjugated blocks in a compact, regular assembly. Such supramolecular organization is likely responsible for the formation of the thin, 'elementary' ribbons, which can further assemble into larger bundles. The issue of chain packing in fluorene-based systems has been modeled separately, since in these compounds, the alkyl groups attached to sp(3)-hybridized sites inherently accommodate out of the plane of the conjugated backbone, which can disturb the chain packing. Various possibilities of chain packing have been explored, starting from short alkyl substituents and extending the size of the side groups to n-octyl. The calculations indicate that, when in zig-zag planar conformation, linear alkyl side groups can orient in such a way that close pi-stacking of the conjugated chains is preserved. In contrast, branched alkyl groups are too bulky to allow close packing of the conjugated backbones to take place. This difference is consistent with the presence or absence of fibrilar structures observed in thin deposits of the corresponding polymers; it can also account for the differences observed in the optical properties. (C) 2002 Elsevier Science Ltd. All rights reserved.