Spiral structures in a stripe-forming block copolymer thin film

Polystyrene-polyisoprene-polystyrene (SIS) triblock copolymer thin film was prepared by solution casting on the freshly cleaved mica surface. The film was then directly scanned using atomic force microscopy (AFM). It was observed that the copolymer microphase separated into stripe structure mostly parallel to the film surface with a period of 38 +/- 5 nm. A spiral structure with a diameter of more than 1 mu m was circulated by the stripes. A closed end was formed with one of the components in the center of the spiral, and away from the spiral center the stripes bent along the tangent of the spiral. The occurrence of the spiral is due to the instability caused by the block copolymer microphase separation coupled with the hydrodynamic effect during the course of the film formation.