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

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Abstract

The phase behavior of supramolecular assemblies (SMA) formed by polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) with 2-(4′-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.

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