FREE-SURFACE MORPHOLOGY OF BLOCK-COPOLYMERS .1. STYRENE-BUTADIENE DIBLOCK COPOLYMERS

The topology and morphology of the free surface of styrene-butadiene d iblock copolymers were investigated by transmission electron microscop y as functions of molecular weight, composition, diene configuration, casting solvent and solvent evaporation rate. It was found that in sol vent-cast thick films the formation of equilibrium free surface depend s on a delicate balance of kinetic and thermodynamic parameters. Fast evaporation rates induce orientation of lamellar and cylindrical micro domains perpendicular to the free surface, while slow rates produce eq uilibrium-like domain orientation, parallel to the free surface. Molec ular mechanisms of such arrangements are discussed, and it is conclude d that to rapidly obtain an equilibrium free surface, the difference b etween critical surface tensions of polystyrene (PS) and polybutadiene blocks must be the greater the higher the affinity between the castin g solvent and the block (PS) with the higher critical surface tension.