Morphology Development in Thin Films of a Lamellar Block Copolymer Deposited by Electrospray

Electrospray has been recently advanced as a novel approach for the continuous deposition of self-assembled block copolymer thin films. It represents an analogue of physical vapor deposition in which the development of well-ordered micro-structures is predicated on relatively rapid relaxation of the polymer compared to its rate of deposition. Here we describe the morphology development of a lamellae-forming poly(styrene-b-4-vinylpyridine) deposited by electrospray. Morphology was considered in the context of relative changes of the deposition and relaxation rates, with the latter significantly affected in some cases by the presence of residual solvent. We observe that the presence of residual solvent in deposited material accelerates the equilibration kinetics such that well-ordered alternating lamellar morphologies could be produced at deposition rates as high as 55 nm/min under "wet" spray conditions, whereas hexagonally packed micelles were produced when the polymer was deposited free of solvent, denoted as the "dry" spray limit. Molecular weight (MW) plays an important role in equilibration kinetics in the "dry" limit with a transition from poorly ordered to well-ordered lamellae produced by reducing MW. Film morphology was largely insensitive to temperature and flow rate over a broad range from 150 to 210 degrees C and from 3 to 18 mu L/min respectively, although the orientation of the lamellae switched from parallel to perpendicular at elevated flow rates, potentially due to the influence of rapid solvent evaporation.