MORPHOLOGICAL DEVELOPMENT IN SOLVENT-CAST POLYSTYRENE-POLYBUTADIENE-POLYSTYRENE (SBS) TRIBLOCK COPOLYMER THIN-FILMS

This paper describes morphological development in solvent-cast polysty rene (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 thic kness of order 100 nm. Plan-view and cross-sectional measurements by t ransmission electron microscopy show that the equilibrium morphology o f in-plane cylinders is generated when films are given relatively long exposure to high solvent concentration or to elevated temperature. Al ternate and metastable morphologies are generated under kinetically co nstrained conditions. Fast evaporation (similar to 200 nL/s) produces a microphase-separated microstructure with no long-range order. Interm ediate evaporation (similar to 5 nL/s) generates hexagonally packed (v ertical) PS cylinders in a PB matrix with the cylinder axis perpendicu lar to the film plane. Slow evaporation (similar to 1.5 nL/s) leads to a duplex microstructure of PS cylinders with domains of either vertic al or in-plane cylinders. Post-evaporation annealing of films supporte d by a Cu TEM grid (18 h, 140 degrees C) converts the duplex morpholog y into one with only in-plane PS cylinders. Very slow solvent evaporat ion (similar to 0.2 nL/s) produces a fully in-plane cylinder microstru cture. A distortion of the hexagonal PS array in the as-cast in-plane structure suggests that the as-cast block copolymer is affected by ani sotropic deswelling during the final stages of solvent evaporation.