Cylinder Orientation Mechanism in Block Copolymer Thin Films Upon Solvent Evaporation

Abstract

Ultrafiltration membranes based on cylinder-forming block copolymers are made by orienting the cylinders perpendicular to the membrane surface followed by selective etching of the minority component. Such membranes promise fast fluxes and superior molecular weight cut-offs. The perpendicular orientation of the cylindrical domains results from the solvent concentration profile that develops when drying the polymer casting solution. As solvent evaporates, it first causes nucleation of the ordered morphology at the vapor−solution interface. This is followed by cylinder growth. The rate of cylinder growth is the product of two terms—a polymer relaxation rate and a thermodynamic driving force. In regions of high solvent concentration the polymer relaxation rate is high and the driving force is small; in regions of low solvent concentration, the opposite is true. This concentration dependence results in the solvent concentration profile established by evaporation dictating how the growth rate varies as a function of position. For a concentration profile that causes the relaxation rate to increase less rapidly with position than the driving force decreases, the growth rate decreases moving into the film, so cylinders grow parallel to membrane surface. Conversely, when the concentration profile results in the relaxation rate increasing more rapidly with position than the driving force decreases, the growth rate increases further into the film, causing perpendicularly oriented cylinders to form. Analysis based on this picture agrees with a variety of experimental results.