(415d) Cosurfactant Effects Affecting the Phase Diagram for a Binary Blend of Diblock Copolymers

Blends of diblock copolymers present enormous possibilities of designing bicontinuous and monocontinuous microphases, which are otherwise unstable in a pure diblock melt due to packing frustration. We present a study of the phase behavior of a binary blend of a long, asymmetric diblock with a shorter, symmetric diblock through self-consistent field theory and dissipative particle dynamics simulations. Such a system has been studied previously by experiments which mapped out a phase diagram that included a narrow region where a bicontinuous phase existed of undetermined identity. Theory is shown to predict the gyroid phase in a narrow region of the phase diagram. However, detailed investigation through particle-based simulations of the same region shows that the cylinder phase is stable, and the gyroid phase is metastable (though very nearly stable). These results indicate that the previously unidentified bicontinuous phases in experiments can be attributed to a defective gyroid phase. We found several other phases competing with the cylinder phase in the phase diagram – cocontinuous phases, perforated lamellar, and the woodpile phase (never before been observed in diblock systems), which indicate that a rich phase diagram is possible with binary blends of diblock copolymers. Analysis of the simulated structures shows that the spatial distribution of the short, symmetric diblocks (cosurfactant effect) provides a key to understanding the limited stability observed in the gyroid phase. Some of the ongoing work entails understanding the effect of uniform shear on the structure of selected phases.