The morphology and structure of PS-b-P4VP block copolymer films by solvent annealing: Effect of the solvent parameter

Lamellae (symmetric) forming polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) block copolymers (BCPs) were used to produce nanostructured thin films by solvent (toluene) casting (spin-coating) onto silicon substrates. As expected, strong micellization of PS-P4VP in toluene results in poorly ordered hexagonally structures films. Following deposition the films were solvent annealed in various solvents and mixtures thereof. A range of both morphologies including micelle and microphase separated structures were observed. It was found that nanostructures typical of films of regular thickness (across the substrate) and demonstrating microphase separation occurred only for relatively few solvents and mixtures. The data demonstrate that simple models of solvent annealing based on swelling of the polymer promoting higher polymer chain mobility are not appropriate and more careful rationalization is required to understand these data. Analysis suggests that regular phase separated films can only be achieved when the copolymer Hildebrand solubility parameter is very similar to the value of the solvent. It is suggested that the solvent anneal method used is best considered as a liquid phase technique rather than a vapor phase method. The results show that solvent annealing methods can be a very powerful means to control structure and in some circumstances dominate other factors such as surface chemistry and surface energies.