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

Grain boundary morphologies in poly(styrene-b-butadiene) lamellar dibl ock copolymers were characterized using transmission electron microsco py (TEM). Two types of twist grain boundaries were observed in which m icrophase separation of the two blocks was maintained in the grain bou ndary region by intermaterial dividing surfaces that approximate class ically known minimal surfaces. The geometry of these interfaces was de monstrated by comparing experimental TEM images with ray tracing compu ter simulations of the model surfaces as the projection direction was systematically varied in both the experimental and simulated images. T he two morphologies observed were found to have intermaterial dividing surfaces that approximate either Scherk's first (doubly periodic) sur face or a section of the right helicoid. The helicoid section boundary was observed at low twist angles, less than or equal to about 15-degr ees. The Scherk surface family of boundary morphologies, which consist s of a doubly periodic array of saddle surfaces, was found over the en tire twist range from 0 to 90-degrees. As the twist angle approaches 0 -degrees the Scherk surface grain boundary morphology is transformed i nto a single screw dislocation that has an intermaterial dividing surf ace 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 di slocation core is nonsingular. This nonsingular core structure represe nts a radical departure from the sigular core structures observed in c lassical studies of dislocations in atomic crystals.