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Alignment and Higher Order Liquid Crystalline Structure in Monodisperse Conjugated Polymers

Bradley D. Olsen1, Jiraksa Ratjatawan1, Jan Luning2, and Rachel A. Segalman3. (1) Chemical Engineering, University of California, Berkeley, 201 D Gilman, Berkeley, CA 94720-1462, (2) Stanford Synchrotron Radiation Laboratory, 2575 Sand Hill Rd., Mail Stop 69, Menlo Park, CA 94025, (3) University of California at Berkley, Dept of Chemical Engineering, 201 D Gilman, Berkeley, Ca 94720-1462

The molecular and mesoscale structure of the active layer greatly affects device performance. Device optimization requires both understanding and control over morphology on the molecular level including control of interactions between the pi systems of neighboring conjugated molecules, the molecular orientation, and chain packing. These structural properties are in turn affected by molecular parameters such as side group chemistry, molecular weight, and polydispersity. Using a soluble and thermally processable model polymer, poly(diethylhexyloxyphenylene vinylene) (DEH-PPV), we have studied the effects of polydispersity and molecular weight on liquid crystalline structure and chain orientation. We demonstrate that well-ordered liquid crystalline phases are observed in monodisperse PPVs but are not seen in higher polydispersity PPVs. In monodisperse PPVs, the entire molecule acts as a single mesogen, and the molecules self-assemble into smectic layers with molecular dimensions. This molecular smectic phase is observed only in monodisperse PPVs because polydispersity in molecular length disrupts the formation of layers. The molecules also show structure within the smectic layers, and the in-plane structure as determined by X-ray scattering.

Shearing of the films aligns the molecules over areas as large as a centimeter square, resulting in the formation of a single liquid crystalline grain. X-ray absorption spectroscopy demonstrates that the molecules are aligned parallel to the shear direction such that the smectic layers assemble perpendicular to this direction. These aligned grains suggest a novel route towards understanding the role of grain structure and molecular alignment on electrical properties.