Electrospun Conjugated Polymer Nanofibers: Morphology, Photophysical Properties, and Applications

Electrospinning (ES) has emerged as a new technique to produce various functional polymer nanofibers. Conjugated polymers have extensively studied for diverse electronic and optoelectronic devices due to the excellent electronic and optoelectronic properties. The photophysical properties of conjugated polymers could be tuned through the approaches of blending or different synthetic ways which result in the enhancement of device characteristics. However, most of the above studies are based on the thin film devices. The morphology and properties of conjugated polymers based ES nanofibers have not been fully explored yet. Only few ES nanofibers based on conjugated polymers were reported because of the limitation on molecular weight or solvents. In this thesis, the objectives are to produce diverse ES nanofibers (nonwoven, aligned, or core-shell type) based on various conjugated polymers and explore their morphology, photophysical properties, and applications, including distinguishing polarized characteristic and sensory devices and further those exhibited significant difference in comparison to the thin films.n the first part of this thesis, Light-emitting electrospun (ES) nanofibers were successfully prepared through the binary blends of polyfluorene derivative/poly(methyl methacrylate)(PMMA) using a single-capillary spinneret. The studied poly(fluorene)s included poly(9,9-dioctylfluoreny-2,7-diyl)(PFO), poly [2,7-(9,9-dihexylfluorene)-alt-5,8-quinoxaline](PFQ), poly[2,7-(9,9-dihexyl-fluorene) -alt-4,7-(2,1,3-benzothiadiazole)](PFBT), and poly[2,7-(9,9-dihexyl-fluorene)-alt -5,7-(thieno[3,4-b]pyrazine)](PFTP). The TEM and SEM results suggested that PFO/PMMA ES fibers gradually formed a core-shell structure with a porous surface as the PFO blend ratio was increased. PFO has a poorer solubility in chloroform than PMMA and forced it to be solidified first in the fiber center to form the core-shell structure. The SEM and laser confocal images suggested that the PFO aggregation domain in the ES fibers was much smaller than that in the spin-coated films and resulted in higher photoluminescence efficiency. Uniform ES fibers produced from the binary blends of PFO/PMMA, PFQ/PMMA, PFBT/PMMA, and PFTP/PMMA exhibited the luminescence characteristics (peak maximum(nm); color) of (443; blue), (483; green), (539; yellow), and (628; red), respectively. The present study demonstrates that full color light-emitting ES nanofibers could be produced from the binary blends of polyfluorene derivative/PMMA.n the second part of this thesis, highly aligned luminescent electrospun (ES) nanofibers were successfully prepared from two binary blends of poly(9,9-dioctylfluoreny-2,7-diyl)(PFO)/ poly(methyl methacrylate)(PMMA) and poly(9,9-di(3,3-N,N-trimethyl-ammonium)-propylfluorenyl-2,7-diyl)-alt-(9,9-dioctylfluorenyl-2,7-diyl)diiodide salt (PF+)/ PMMA. The PFO/PMMA aligned ES fibers showed a core-shell structure but the PF+/PMMA exhibited periodic aggregate domains in the fibers. The aligned fibers had polarized steady-state luminescence with a polarized ratio as high as 4, much higher than the nonwoven ES fibers or spin-coated film. Besides, the PF+/PMMA aligned ES fibers showed an enhanced sensitivity on sensing plasmid DNA. Such aligned ES fibers could have potential applications in optoelectronic or sensory devices.n the third part of this thesis, Novel luminescent electrospun (ES) fibers were successfully prepared from a conjugated rod-coil block copolymer, poly[2,7-(9,9-dihexylfluorene)]-block-poly (methylmethacrylate) (PF-b-PMMA) using a single-capillary spinneret. The experiment results indicated that PF-b-PMMA ES fibers prepared from THF, THF/DMF (50/50) and DMF contained PF block aggregated structures of dot-link (5-10 nm), line-like (10-20 nm), and ellipse-like structure (25-50 nm), respectively. Such variation on the aggregation size led to the red-shifting on the absorption or luminescence spectra. Also, the fiber diameters decreased with enhancing the DMF content. Furthermore, functional ES fibers with a high sensitivity on acid or pH were successfully prepared from binary blends of poly(phenylquinoline)-block-polystyrene rod-coil diblock copolymers (PPQ-b-PS) /polystyrene (PS). The effect of pH on the fluorescence spectra for ES fibers and spin-coated thin film were investigated. The PPQ-b-PS /PS ES fibers showed high sensitivity in comparison with the spin-coated film. The present study demonstrates that blue light-emitting ES fibers were successfully prepared from conjugated rod-coil diblock copolymer and their aggregate morphology and photophysical properties could be tuned through selective solvent. Furthermore, it also suggested that the ES fibers prepared from rod-coil conjugated block copolymer could have potential applications in optoelectronic or sensory devices.n the fourth part of this thesis, new electrospun (ES) sensory fibers consisted of poly(methyl methacrylate) (PMMA) core and poly(3-hexylthiophene-2,5-diyl) (P3HT) shell were successfully prepared using a two-fluid coaxial electrospinning process. The studies showed that the prepared ES fibers had diameters of 500-700 nm and worm-like surface structure of P3HT on the fiber. Upon exposed to air under visible light for two weeks, significant blue-shifting on both absorption and luminescence spectra (from red, to orange, and to green PL emission). It was probably due to the chain scission occurred in the P3HT and led to the reduced conjugated length. The sensitivity of the ES fibers was much better than that of the spin-coated P3HT film from the comparison on the variation of photophysical properties. Besides, the EPR measurements suggested the formation of the P3HT．O2 charge transfer complex (CTC), leading to the fiber conductivity without an external doping. The present study demonstrates that conjugated polymer based ES core-shell fibers may have potential applications for oxygen-sensing devices.