Influence of film casting method on block copolymer ordering in thin films

Abstract

We study the orientation of the cylinder phase of the block copolymer (BCP) poly(styrene-block-methylmethacrylate) (PS-b-PMMA) in thin spun-cast films having a range of film thicknesses and temperatures and compare to our earlier observations on flow-coated films of the same BCP under similar thermodynamic conditions. As in our former measurements on flow-coated films, morphology maps of our spun-cast BCP films reveal distinct ordering regimes where the cylinders orient predominantly perpendicular or parallel to the interface and an ‘intermediate’ regime where these morphologies are mixed. However, we find that the locations of these morphology lines in the temperature–film thickness plane are greatly influenced by the method of film formation. We hypothesize that the slow solvent evaporation of the flow-coated films leads to films having higher residual stresses than spun-cast films because of a relatively inhibited transport of the solvent out of the film during drying, and annealing measurements on both types of cast films provide supporting evidence that the BCP surface morphology change is associated with residual stresses within the films. Contrary to our initial expectations, spun-cast films showed little sensitivity to film casting speed so that residual stress effects associated with the comparatively violent fluid flow conditions of spin-casting appear to be small. Instead, residual stresses arising from slow film evaporation appear to be implicated. Residual stress effects arising during the course of film casting and drying are relevant to other fabrications involving cast polymer films, such as in organic photovoltaic devices where flow-coating or similar industrial film casting method is normally employed for film formation.