Morphological Development in Solvent-Cast Polystyrene−Polybutadiene−Polystyrene (SBS) Triblock Copolymer Thin Films

Abstract

This paper describes morphological development in solvent-cast polystyrene (PS)−polybutadiene (PB)−polystyrene (SBS) triblock copolymer thin films (30 wt % PS) as a function of solvent evaporation rate and post-evaporation annealing. Films were cast onto NaCl substrates to a thickness of order 100 nm. Plan-view and cross-sectional measurements by transmission electron microscopy show that the equilibrium morphology of in-plane cylinders is generated when films are given relatively long exposure to high solvent concentration or to elevated temperature. Alternate and metastable morphologies are generated under kinetically constrained conditions. Fast evaporation (∼200 nL/s) produces a microphase-separated microstructure with no long-range order. Intermediate evaporation (∼5 nL/s) generates hexagonally packed (vertical) PS cylinders in a PB matrix with the cylinder axis perpendicular to the film plane. Slow evaporation (∼1.5 nL/s) leads to a duplex microstructure of PS cylinders with domains of either vertical or in-plane cylinders. Post-evaporation annealing of films supported by a Cu TEM grid (18 h, 140 °C) converts the duplex morphology into one with only in-plane PS cylinders. Very slow solvent evaporation (∼0.2 nL/s) produces a fully in-plane cylinder microstructure. A distortion of the hexagonal PS array in the as-cast in-plane structure suggests that the as-cast block copolymer is affected by anisotropic deswelling during the final stages of solvent evaporation.