Effects of surface tension on the dislocation structures of diblock copolymers

Cross-sectional and plane view TEM, in conjunction with atomic force microscopy (APM), was used to study the dislocations produced within PS-PVP diblock copolymer films. The results showed that islands or holes on the surface of multilayer films were produced by prismatic loop dislocations with a screw component in the layers below the surface. Only the contours at the vacuum surface obtained by AFM could be fit with classical elasticity theory. Flattening of the surface by the surface tension when the films were ordered on a silicon substrate induced a large amount of compression in the lamella beneath the surface. In films less than 3/2 lamellar layers thick, this compression is shown to induce a transition from lamellar to perpendicular cylinders at the edge of the island or holes. This transition was suppressed when the diblock copolymer molecular weight was decreased or when the films were ordered at a low surface tension interface. A qualitative model which estimates the degree of chain stretching induced by the surface tension is proposed to explain these effects.