In Situ Characterization of Highly Aligned Conjugated Polymer Thin Films Revealing Unique Thermal Behavior and Packing Structure

Probing the morphology of conjugated polymer thin films featuring in-plane chain alignment can reveal valuable thermal and structural information beyond that in typical films with in-plane isotropy. In-plane anisotropy of an oriented polymer thin film can generally be increased with a thermal anneal after initial alignment, often near a thermal transition of the material. While morphology of the film is often reported before and after the annealing procedure, in-depth analysis of the morphological changes during the annealing process is rarely conducted. To uncover the packing behavior of the polymer chains during alignment, we exploit several characterization methods and focus on strain-aligned PBnDT-FTAZ. Using in situ techniques including polarized UV-Vis spectroscopy and grazing incidence wide X-ray scattering (GIWAXS), the thermal characteristics and packing behavior are probed during the annealing procedure as the film experiences a striking increase in uniaxial alignment (dichroic ratio increases from 2 to 35). Through these methods, we clearly show how molecular mobility and improved alignment coincides with thermal transitions in PBnDT-FTAZ. In addition, in situ GIWAXS reveals unique polymorphs correlated with the thermal annealing process. We also compare these results to other strain-aligned conjugated polymers including P(NDI2OD-T2) and PTB7-Th showing how molecular structure impacts the ability of the polymer to reorient. As a result we show how in situ characterization of the aligned films during thermal annealing uncovers unique coupling between thermal relaxation of the polymer and molecular ordering. Finally, the resulting highly oriented films are shown to provide insight into the crystal structure of the donor-acceptor conjugated polymers.