Effect of stepwise change of drying rate on microstructure evolution in polymer films

The effect of the drying rate of the solvent on microstructure evolution in thin films of immiscible polystyrene/polycarbonate blends was investigated by applying a stepwise change in air velocity. The polymer blend was disso lved in tetrahydrofuran and cast on a glass substrate to form a thin liquid film. The drying rate of the solvent was measured by a gravimetric techniq ue, and the phase-separated structure in the film was observed by optical m icroscopy. The results indicated that cellular structures of droplet arrays were formed in the phases evolved primarily during the early stage of dryi ng, and the droplets then coalesced in the intermediate stage to form bigge r droplets. The size of the first evolved phase decreased with increasing t ime at a high drying rate, whereas larger droplets were obtained with longe r high drying rate regimes. These results suggested that the droplets were induced by secondary phase separation which was strongly affected by the dr ying rate of the solvent.