Composition-Dependent Morphological Transitions and Pathways in Switching of Fine Structure in Thin Films of Block Copolymer Supramolecular Assemblies

Abstract

The phase behavior of supramolecular assemblies (SMA) formed by polystyrene-blockpoly(4-vinylpyridine) (PS-b-P4VP) with 2-(4 0 -hydroxybenzeneazo)benzoic acid (HABA) was investigated with respect to the molar ratio (X) between HABA and 4VP monomer unit in bulk as well as in thin films coated on a silicon substrate. In bulk, the SMA showed bcc packed spherical or hexagonally packed cylindrical microdomains composed of P4VP(HABA) blocks in PS matrix depending on the composition which varied with the HABA weight fraction. However, at a molar ratio of 2 where HABA was in excess, the cylindrical microdomains of P4VP(HABA) blocks were found to pack in a tetragonal lattice. The SMA in thin film also showed spherical or cylindrical morphology depending on the composition. Interestingly, the orientation of cylindrical microdomains of P4VP(HABA) depended on the selectivity of the solvent as well as on the degree of swelling. Hence, in a nonselective solvent like chloroform, the cylinders were found to lie parallel to the substrate as long as the swelling ratio allowed the system to be in the cylindrical region of the phase diagram. At higher degree of swelling the cylinders transform into spherical domains, and drying of the solvent resulted in perpendicular orientation. In a selective solvent like 1,4-dioxane, the cylinders were found to be always oriented perpendicular to the substrate for 0.5 < X < 1.5. Such a morphology was favored in this case, since even at a small degree of swelling, the system went into spherical morphology which ultimately resulted in perpendicular cylinders. The transformation from perpendicular to parallel domain orientation in chloroform, investigated using the ex-situ atomic force microscopy approach, involved domain breakup, induced by thermodynamically unstable interface, and then coalescence of the domains along the plane of the film. Moreover, at excess HABA concentration, a hexagonally perforated lamellar morphology was observed when the system was shortly exposed to chloroform vapors.

Effects of Annealing Solvents on the Morphology of Block Copolymer-Based Supramolecular Thin Films

“…33−35 For supramolecular thin films, the effects of annealing solvents on the morphology of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) with 2-(4′-hydroxybenzeneazo) benzoic acid (HABA) molecules have been reported by Minko and co-workers. 22,36,37 They found that as films were annealed by chloroform, the P4VP(HABA) cylindrical microdomains were parallel to the surface, while annealed by 1,4-dioxane, the cylinders were perpendicular to the surface, which was attributed to the different selectivity of the solvents to each phase. The previous discussions of block copolymer-based supramolecules have been generally under the presumption that the small molecules are firmly grafted on the polymer chains.…”

Supramolecular design principles for efficient photoresponsive polymer–azobenzene complexes

“…147 However, none of the BCP complexes they prepared involved azobenzene guests. On the other hand, Xu and co-workers, 148 Stamm and coworkers, [149][150][151] and, recently Gröschel and coworkers, 152 have used azobenzene-containing hydrogenbonded side chains in investigations involving thin-film nanopatterning, nanoparticle assembly, and templating with supramolecular BCPs, but they did not explore the potential photoresponsive properties of these complexes. Photoinduced control over the phase structure of BCP materials combined with supramolecularly bonded azobenzene molecules is a highly interesting phenomenon, which has only recently begun to be explored.…”

Effect of reaction time on structure of ordered mesoporous carbon microspheres prepared from carboxymethyl cellulose by soft-template method

“…During the self-assembly process, increasing the reaction time can produce directional effects on the organic-organic interface, and affect space groups, pore orientations, and stress direction (Nandan et al, 2010). The two main models, cylindrical and spherical micelles were generated to minimize the surface free-energy of a system (Jin et al, 2011), and such micelles can associate, which corresponds to the liquid crystal mesophase.…”