Welded Electrochromic Conductive Polymer Nanofibers by Electrostatic Spinning

With the availability of electrochromic conjugated polymers [1] exhibiting transitions of the three primary colors, full-color electrochromic displays could be realized in the near future. [2] Although many, but not all, of the color transitions have been addressed, there is a need for rapid and wide-area processing methods as well as materials with response times within the millisecond regime in order to obtain an active display. Recently, the electrochromic phenomenon for poly(3,4-ethylenedioxythiophene) (PEDOT) nanotubes [3] prepared using a porous membrane [4] has been reported to be on the order of 5 ms, rapid enough for active-display applications. Other techniques for producing one-dimensional (1D) nano-structures of conjugated polymers of similar promise include interfacial polymerization, [5] templated synthesis, [6] and phase separation of block copolymers. [7] Apart from electrochro-mics, conjugated polymer nanostructures could find application as electronic interconnects, optoelectronic device components , chemical and biological sensors, catalysts, membranes, and drug-delivery systems. Electrostatic spinning (e-spinning) offers a practical means to prepare nanostructured conjugated polymers described above by producing high volumes of conjugated polymer nanofibers continuously, even to lengths of several kilometers. However, the intractability of conjugated polymers has prohibited their processing by e-spinning in pure inherently conductive polymer form. Instead, researchers have reported the blending of conjugated polymers with more readily electro-spun polymers such as poly(ethylene oxide) or poly(vinyl pyrrolidone). Most reports of inherently conductive polymer nanofibers formed in this way have utilized polyaniline, [8] although a report on the e-spinning of poly(phenylene vinylene) and a thiophene-based conjugated polymer has recently appeared. [9] To our knowledge, there has not been a report of an electrospun conjugated polymer exhibiting elec-trochromic function. Herein we describe a technique by which we electrospin a precursor polymer that can later be oxidized either chemically or electrochemically in the solid state [10] to produce a cross-linked conjugated polymer. Since solvent is used in the conversion process to swell the precursor polymer, the nanofibers naturally weld together during conversion to the crosslinked conjugated polymer, of benefit to multiple properties. In contrast , flash welding of polyaniline (PANI) nanofibers has been reported to give continuous films. [11] Further, we demonstrate here the capability of a 5 lm thick non-woven mat of this conjugated polymer nanofiber to electrochromically switch between two colored states on the timescale of a few seconds. The precursor polymer, 1, used for e-spinning was a 50:50 (mol/mol) random copolymer (Fig. 1) prepared via ring-opening metathesis polymerization (ROMP) of …