Cellulose Templating for π-Conjugated Polymer Orientation: An In Situ Time-Resolved Spectroscopy Exploration

Foldable, strong, free-standing, highly uniaxially oriented π-conjugated polymer films can be prepared with high reproducibility (∼96%) using the recently published “Simple way of Orienting Films using Templating cellulose (SOFT)” method. However, the cellulose-templating orientation mechanism is not well understood. Herein, a mechanistic investigation of polymer chain orientation, focused on the key interfaces of the SOFT fabrication process, is presented. In situ polarized fluorescence, Raman spectroscopy, and differential scanning calorimetry data, obtained during the heating, equilibration, and cooling steps of the SOFT method, are analyzed. Placing a Si layer (t = 2–20 nm) between the uniaxial methylcellulose (MC) template and π-conjugated poly(9,9-dioctylfluorene) (PFO) films blocks PFO orientation; thus, contact is essential. Moreover, efficient polymer chain alignment is achieved only for thin films (t = 2–30 nm). A dynamics study reveals that an anisotropic liquid-crystal PFO phase forms on the uniaxial MC template above the melting point (150 °C), and a highly oriented film is subsequently created. The velocities of chain orientation, C–C bond elongation, and dihedral angle variation were computed from spectroscopic parameters. Velocity maxima and/or minima coincided with PFO phase changes; the velocities oscillated and then went to zero during cooling to 40 °C when the PFO structure was frozen into a highly oriented film.