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Self-Assembly of a Model Semiconducting Rod-Coil Block Copolymer in Thin Films

Bradley D. Olsen, Chemical Engineering, University of California, Berkeley, 201 D Gilman, Berkeley, CA 94720-1462, Xuefa Li, Advanced Photon Source, Argonne National Lab, Argonne, IL 60439, and Rachel A. Segalman, University of California at Berkeley, Dept of Chemical Engineering, 201 D Gilman, Berkeley, 94720-1462.

The development of efficient polymer optoelectronics requires the control of morphology in multicomponent devices on the 10 nm length scale of exciton transport. Block copolymer self-assembly provides an elegant tool for achieving nanostructured morphologies, but the incorporation of rodlike semiconducting polymers creates challenges due to geometric constraints and liquid crystalline interactions not seen in traditional block copolymers. We have recently demonstrated the first equilibrium phase diagram for a model weakly segregated rod-coil block copolymer system. Using this model material we can investigate thin film confinement and surface segregation effects in a material with a known microphase structure and order-disorder transition, and we have performed a detailed study investigating the impacts of film thickness and coil fraction on structure formation in lamellar rod-coil materials.

In thin films, lamellae self-assemble primarily parallel to the film interface with the lamellar width determined by the molecular weight of the block copolymer and the grain shape determined by the coil fraction. In symmetric block copolymers, in-plane lamellae grow in irregular polygon grains with linear sides defined by defect lamellae oriented perpendicular to the film surface. However, as coil fraction is increased the polymers transition to a regime characterized by regularly-shaped square grains. With varying film thickness, rod-coil block copolymers form islands and holes due to a dimensional mismatch between the characteristic dimension of the block copolymer and the thickness of the film. In films less than several lamellar layers thick, unusual three level island and hole formation may also be observed. Increasing film thickness results in reorientation of the lamellar structures at the air interface, and for thick films all of the lamellar microdomains are oriented perpendicular to the air interface. This reorientation is localized near the air interface, indicating that the orienting effect of the substrate has a limited range through the film. In coil-rich polymers, large islands and holes form in films with reoriented surfaces despite the lack of a characteristic block copolymer length in this dimension, suggesting that kinetic effects play a large role in the formation of these morphologies.