Three-dimensional structure and motion of twist grain boundaries in block copolymer melts

The three-dimensional structure and motion of twist grain boundaries in lamella forming diblock copolymer melts in the presence of an external electric field were studied by means of computer simulations. The method used is a mean-field dynamic density functional theory with electrostatic interactions incorporated into the model. The observed twist grain boundary structure consists of a doubly periodic array of saddle surfaces, similar to Scherk's first surface. We found that the twist grain boundary interface remains unbroken, and the grain boundary structure keeps its profile constant during twist grain boundary motion. This suggests that twist grain boundaries move via a mechanism in which only diffusion parallel to the interfaces is necessary. The twist grain boundary motion is a specific case of a more general mechanism of mesophase reorientation by defect movement. The width of a twist grain boundary is found to be about the lamellar period for all the observed twist angles, corroborating the fact that the twist grain boundary interface basically is built from one layer of diblock copolymer chains. The existence of twisted lamellae grains causes a large domain contribution to the stress tensor arising from the composition inhomogeneities. The results of our simulations demonstrate that the motion of a twist grain boundary is not affected by the electric field, which differs from the movement of a grain boundary under shear flow. Such a distinction in behavior originates from the difference in symmetry of shear and electric field. In contrast with microphase-separated block copolymer in an electric field, in the sheared systems, defects in microstructure are convected by the flow field. Because of the uniaxial symmetry of a system with respect to the applied electric field, which facilitates the creation of twist grain boundaries and suppresses tilt grain boundaries, an electric field is a perfect candidate to investigate twist grain boundary motion. © 2005 American Chemical Society.