LAMELLAR DIBLOCK COPOLYMER GRAIN-BOUNDARY MORPHOLOGY .1. TWIST BOUNDARY CHARACTERIZATION

Grain boundary morphologies in poly(styrene-b-butadiene) lamellar diblock copolymers were characterized using transmission electron microscopy (TEM). Two types of twist grain boundaries were observed in which microphase separation of the two blocks was maintained in the grain boundary region by intermaterial dividing surfaces that approximate classically known minimal surfaces. The geometry of these interfaces was demonstrated by comparing experimental TEM images with ray tracing computer simulations of the model surfaces as the projection direction was systematically varied in both the experimental and simulated images. The two morphologies observed were found to have intermaterial dividing surfaces that approximate either Scherk's first (doubly periodic) surface or a section of the right helicoid. The helicoid section boundary was observed at low twist angles, less than or equal to about 15-degrees. The Scherk surface family of boundary morphologies, which consists of a doubly periodic array of saddle surfaces, was found over the entire twist range from 0 to 90-degrees. As the twist angle approaches 0-degrees the Scherk surface grain boundary morphology is transformed into a single screw dislocation that has an intermaterial dividing surface with the geometry of a single helicoid. Direct TEM imaging of the detailed core structure of this screw dislocation is presented. These images demonstrate that in the lamellar diblock copolymer the screw dislocation core is nonsingular. This nonsingular core structure represents a radical departure from the sigular core structures observed in classical studies of dislocations in atomic crystals.