Morphology and Optoelectronic Properties of Electrospun Nanofibers Prepared from Conjugated Polymer Blends
Date Issued
2009
Date
2009
Author(s)
Tzeng, Ping
Abstract
Electrospinning (ES) has emerged as a new technique to produce various functional polymer nanofibers. Conjugated polymers have been extensively studied for various electronic and optoelectronic devices due to the excellent electronic and optoelectronic properties. The electrical and optical 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 thin film devices. The morphology and properties of conjugated polymers based nanofibers have not been fully explored yet. Only few ES nanofibers based on conjugated polymers were reported because of the limitations on molecular weight or solvents. In this study, conducting electrospun fibers based on poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT:PSS) and Poly(3-hexylthiophene-2,5-diyl) (P3HT) were successfully produced. High molecular weight polymers such as poly(vinylpyrrolidone) (PVP), poly(methyl methacrylate) (PMMA) and poly(ethylene oxide) (PEO) were used to enhance the spinnability. Thermoresponsive poly[2,7-(9,9-dihexylfluorene)]-b-poly[N-isopropylacrylamide] (PF-b-PNIPAAm)/PMMA binary blend fibers were also fabricated. Effects of PF-b-PNIPAAm rod-coil block ratio on the morphology and photophysical properties were studied.n the first part of this thesis, two conductive electrospun fibers were fabricated. First, conductive PEDOT:PSS(shell)/PVP(core) coaxial electrospun non-woven fibers of the with diameters around 750-1050 nm were successfully prepared from various core/shell flow rates. The diameter distribution became wider as the PEDOT:PSS flow rate enhanced. Conductivity showed an one-order increase from 2.14E-6 to 4.32E-5 S‧cm-1 after the core/shell flow rates were tuned. Second, conductive electrospun fibers were fabricated by coating PEDOT:PSS on the surface of PMMA as-spun fibers. Coating cycles were optimized and electrical properties were measured. The conductive fibers showed a slight increase in diameter after coating PEDOT:PSS and porous surface structures with the pore size of 30-90 nm were observed, which were not found on pure PMMA ES fibers. Compare with the PEDOT:PSS/PVP coaxial fibers, a significant enhancement was discovered on the PEDOT:PSS/PMMA fibers with the conductivity as 7.72E-2 S‧cm-1. To sum up, the measured conductivity results of the fibers were related to the content of the insulating polymer.n the second part, conductive electrospun nanofibers with diameters around 230-410 nm were successfully prepared through the P3HT/PEO blends using a single-capillary spinneret and a collector with a rectangular hole-gap. Chloroform was injected into the shell layer to prevent rapid solidification at the nozzle tip and to tune fiber size. The total weight percentage was fixed at 4 wt%, and four composition (1/3, 1.6/2.4, 2/2, 3/1) of the blends were prepared. Pure P3HT nanofibers were also fabricated with average diameter 125nm. The insulating shell PMMA was removed by acetone. The field mobility of the P3HT/PEO blend electrospun fibers increased with enhancing its content from 0.000072 to 0.00017 cm2V-1s-1. The pure P3HT fibers show the highest mobility 0.0034 cm2V-1s-1. All electrical properties including mobility, threshold voltage and on/off ratio were low compare to P3HT thin film FET device due to the high surface-to-volume ratio. The high surface-to-volume ratio causes rapid doping by oxygen and moisture. By operating all the processes in inert or vacuum environment, the performance can be improved in the future. n the last part, thermoresponsive light emitting electrospun fibers were prepared through the PF-b-PNIPAAm/PMMA blends using a two-fluid spinneret. The shell layer was injected with THF to prevent the rapid solidification occurred at the nozzle tip. The fiber diameter was around 100-1200 nm. Prerequisites for the as-spun fiber formation are developing the cone-jet spinning model. The laser confocal image and TEM results both showed that PF-b-PNIPAAm was well dispersed in the PMMA matrices but difficult to observe PF aggregate domains due to the low content of PF in ES fibers. The PL spectra of the PF-b-PNIPAAm/PMMA blends showed additional shoulders appearing at 470~490 nm as temperature was increased from 20 to 40℃ due to the PF aggregation driven by PNIPAAm moiety. Reversibility was verified by cooling temperature back to 20℃, and the additional shoulder vanished. The color coordinate also exhibited that PF-b-PNIPAAm/PMMA fibers emitting blue light at 20℃ changed to purple blue at 40℃. he present study demonstrates that various conductive and light-emitting ES nonwoven fibers were successfully produced through optimum polymer composition and device fabrication process, which could have potential applications as conductive fabrics, electronic devices, or sensory materials for smart textiles.
Subjects
Electrospinning
Conjugated polymer
Morphology
Opto-electronic
Type
thesis
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