Orientation and Translational Control of PS-b-PEO/PS Thin Films via Solvent Annealing and Graphoepitaxy Techniques

Microphase separation is readily achieved in both PS-b-PEO diblock copolymer thin films and in PS homopolymer/PS-b-PEO polymer blends via solvent annealing. In situ synthesis of PS homopolymer with a symmetric PS-b-PEO diblock copolymer results in a cylindrical structure rather than a predicted lamellar morphology for the diblock due to the increased amount of PS present. Solvent annealing in toluene results in ordered structures of PEO cylinders orientated parallel to the substrate in the PS matrix. Solvent annealing in a mixed toluene/water environment creates an essentially 'neutral' solvent environment resulting in a perpendicular orientation. PS-b-PEO/PS thin films with cylinders orientated parallel have a smaller cylinder centre-to-cylinder centre spacing than films with cylinders orientated perpendicular due to the location to the PS homopolymer. If the molecular weight of the diblock is sufficiently low for a given Flory-Huggins parameter microphase separation will not occur. On topographically defined substrates the solvent annealing process does not provide sufficient mobility to the polymer to allow mass transport of polymer from the crests into the channels and only limited alignment is achieved.