Morphological Behavior of A2B Block Copolymers in Thin Films

Copolymer composed of two poly(2-vinylpyridine) blocks and one polystyrene block ((P2VP)(2)-PS) with a PS volume fraction (Phi(PS)) of 0.44 was synthesized, and their morphologies in bulk and thin films were compared to linear block copolymers (BCPs) having a similar Phi(PS) (PS-b-P2VP). For the symmetric diblock copolymer, a lamellar morphology was observed after thermal annealing, as was expected. However, (P2VP)(2)-PS in the bulk showed a hexagonally packed cylindrical microdomain morphology with PS forming the cylindrical microdomains in a P2VP matrix. In thin films, after solvent annealing in the vapors of a chloroform/ethanol mixture, the cylindrical microdomain morphology was retained and the microdomains were oriented parallel to the substrate surface. With the addition of ethanol into the annealing solvent, the morphology of (P2VP), PS was transformed from a bicontinuous morphology to one with cylindrical microdomains of PS oriented parallel to the substrate. The lamellar microdomain of the PS-b-P2VP copolymer changed to a morphology composed of PS cylindrical microdomains oriented normal to the substrate. The different orientations of the morphologies arise from the preferential interaction of P2VP with the substrate and the mediation of these interactions by the solvent. The morphological evolution of (P2VP)(2)-PS thin films was also investigated as a function of SVA time to optimize well-defined microstructures where an orientation of the microdomains parallel to the substrate occurred within 180 s. By use of a faceted sapphire substrate having a sawtoothed topography, highly aligned line patterns were achieved.