Weakly Segregated Smectic C Lamellar Clusters in Blends of Rods and Rod−Coil Block Copolymers

Abstract

The morphology arising from the self-assembly of π-conjugated poly(diethylhexyloxy-p-phenylenevinylene) rod homopolymers (PPV) and poly(diethylhexyloxy-p-phenylenevinylene)−polystyrene (PPV−PS) rod−coil block copolymers is described. Two PPV−PS block copolymers, one with low rod volume fraction (∼17%) and the other with high volume fraction (∼50%), were synthesized by convergent anionic polymerization and atom transfer radical polymerization, respectively. In the first case, given the low volume fraction of the rod block, the pure asymmetrical rod−coil block copolymer formed an isotropic homogeneous phase. However, ordered clusters of alternating PS and PPV domains with characteristic length of the order of several micrometers appeared when PPV rod homopolymers were blended to the PS−PPV diblock. Furthermore, the long-range order of the clusters as well as their volume fraction could be greatly increased when the symmetric rod−coil PPV−PS was blended to PPV homopolymer. Tomographic reconstruction from transmission electron micrographs allowed demonstrating that the clusters were organized in lamellar phase with well-defined width for both the intercalated PS and PPV domains, while wide-angle X-ray scattering showed that within the PPV domains the PPV blocks and PPV homopolymer rods were closely packed. The study of the spacing widths of the PPV and PS domains showed that clusters are organized in a smectic C configuration with large tilt angles of the rods (54°) and stretching of the coil blocks which is typical of weakly segregated block copolymers organized in a lamellar phase. The stability of the rod-to-rod interaction peak at high temperatures (190 °C), well beyond the order−disorder transition temperature of the clusters (130 °C), suggests that the (i) aggregation of the rods is mediated by π−π interactions and (ii) the clusters are thermodynamically stable structures. The energetic driving force toward the formation of these clusters is discussed in the last part of this work.