Undulation instability in drop-cast poly(3-hexylthiophene) film originated from self-assembly

In this study, we characterize the undulated structures which appear at the edge of drop-cast regio-regular poly(3-hexylthiophene) (rr-P3HT, head-to-tail $>$ 95{\%}) film using optical microscopy and atomic force microscopy. We propose that these periodic structures originate from the undulations of the layered structure of liquid crystal-air interface. Evidence of rr-P3HT solution forming liquid crystalline phases at higher concentrations was obtained by the observation of distinct birefringence and characteristic textures under crossed polarizers using an optical microscope. Synchrotron x-ray diffraction pattern provides additional structural information at the undulated area compared with those at the area without undulated pattern. Based on these experimental results, we propose rr-P3HT solution can form a lyotropic liquid crystal at specific concentrations. This work was partially supported by NSF funding (DMR-0706235).