Controlling Condensed State Structures of Different Polar Conjugated Polymer Polyfluorenes (PFs) by Applying External Electric Field across Solution with Polar Solvent THF
In this work, controlling condensed state structures of different polar conjugated polymer polyfluorenes (PFs) by applying external electric field across solution with polar solvent tetrahydrofuran (THF) was investigated by PL spectrum, light scattering (LS), Raman spectra, X-ray diffraction (XRD), transmission electron microscope (TEM) and high resolution transmission electron microscope (HR-TEM). The external electric field was applied across the optoelectronic conjugated polymer solutions for the first time. Interestingly, it was found that from dilute solution (0.1 mg/mL) to concentrated solution (5mg/mL), chain conformation of the weakest polar PFO changed the most dramatically and always transformed towards the more planar geometry along with effective conjugated length increase. These findings subverted the conventional insight in which weakly polar polymer chain cannot be easily affected by external electric field but only polar conjugated polymer can. Especially, under this condition of applying external electric field across solutions with polar solvent THF, PFO β conformation content increased obviously in concentrated 5 mg/mL solution, and by this simple physical method, PFO chains orientedly self-assembled and directly formed β-phase crystal in films, which is very difficult to other β-phase crystal preparing methods. Astoundingly, by the method, the long-wavelength green emission of PPFOH aggregation, which is an obstacle to the PFs application, also was suppressed, this has been a thorny issue for a long time. The mechanism of these changes were revealed, increasing the dielectric constant gap between polymer chain and solvent molecule is key rather than simply the polymer polarity. Above conclusions were also suitable to other conjugated polymer systems. The research is significant not only to rich understanding of physical essence based on conjugated polymer solution behaviours to enhance optoelectronic device performance but also to control condensed state structures to achieve chains ordered packing as well as multi-color larger-area optoelectronic devices with high carrier mobility and efficiency.