Poly electrochromic switching

Not only do oxidatively doped polypyrrole films show significant environmental stability, but a number of these films can also be electrochemically prepared and switched in both organic and aqueous solvents [90]. While poly(N-methylpyrrole) was the only polymer of the series that exhibited a comparable electrochromic contrast in both solvents, N-benzyl (19a), N-tolyl (19b), and the parent polypyrrole showed a decrease in electrochromic contrast when comparing results obtained using organic and aqueous electrolytes. Polymers prepared using N-phenyl and N-benzoylpyrrole (20) showed no electrochromic switching in aqueous solutions. 

The first detailed investigation of electrochromic switching phenomena was carried out in 1987 by Yashima et al. [19] on electrosynthesized poly(isothionaphthene) (pITN). This polymer, which has one of the lower Eg... 

Several such polymers have shown electrochromic behavior, among them poly(n-vinylcarbazole) [73] which switches from colorless in the neutral state to green in the doped state (Scheme 10) and poly(Ar-phenyl-2-(2 -thienyl)-5-(5"-vinyl-2"-thienyl)pyrrole) [74], which changes from yellow to reddish brown upon oxidation (Scheme 11). A study of the electrochromic properties of blends consist-... 

Li, M., Patra, A., Sheynin,Y, Bendikov, M., 2009. Hexyl-derivatized poly(3,4-ethylenedioxyselenophene) novel highly stable organic electrochromic material with high contrast ratio, high coloration efficiency, and low-switching voltage. Adv Mater. 21,1707-1711. 

Pei, Q.B., G. Zuccarello, M. Ahlskog, and O. Inganas. 1994. Electrochromic and highly stable poly(3,4-ethylenedioxythiophene) switches between opaque blue-black and transparent sky blue. Polymer 35 (7) 1347-1351. 

To improve performance, many PEDT derivatives used either alone or in combination have been proposed. While the electro-optical properties of WO3 are fixed, the colors and hue of conducting polymers may be altered by modification of the monomers. For example, Reynolds has studied the electrochromic properties of a multitude of EDT derivatives [100,109], and recently reported an all-polymer electrochromic device based on two different PEDT derivatives [110]. Depending on their chemical structure, the various PEDT derivatives exhibit different colors upon switching from the oxidized to the reduced state. For example, poly(tetradecylethylenedioxythiophene) (C14-EDT) is similar in switching to PEDT from transparent to blue, but it has an enhanced optical contrast. 

Welsh, D.M., A. Kumar, E.W. Meijer, and J.R. Reynolds. 1999. Enhanced contrast ratio and rapid switching in electrochromics based on poly(3,4-propylenedioxythiophene) derivatives. Adv Mater 11 1379-1382. 

Electrochromic polymer films have been made where the carbazole unit is not part of the main chain of the polymer, e.g., poly-iM-vinylcarbazole (29) [107—109]. When electrochemically polymerized, it has been shown that the cation radical of the carbazole unit easily dimerizes to form a 3,3 -bicarbazolyl that is more easily oxidized than the parent monomer. These films also show a colorless to green transition upon oxidation. Similarly, films of poly[3-(3-bromocarbazol-9-yl)propyl]methylsiloxane (30) have been prepared where the carbazole dimerizes creating a cross-linked film that switches between colorless and green as with the other polymers [110]. 

Polyaniline films have not only been shown to exhibit electrochromism in the visible region, but also in the microwave and far-IR regions of the electromagnetic spectrum. A polyaniline film doped with camphorsulfonic add and incorporated into a sohd state microwave shutter demonstrated that the transmittance and reflectance of X-band microwave energy could be modulated [6]. At a wavelength of 10 GHz, the shutter could be switched between 4.8% transmission when the polymer is oxidized and 42% transmission when the polymer is neutral. When utilized in a reflective device configuration in combination with poly(diphenylamine), polyamline yields a high reflective modulation in the far-IR [119,120]. This device shows a reflectance contrast of 53% at 10.5 p,m, 28% at 16.5 p,m, and 46% at 620 nm. 

Films prepared in a similar manner with Hydrin C are those utilizing poly(N,N -dimethyl-2,2 -bipyrrole) [220], and polypyrrole [221]. Electrochromic films of Hydrin C and poly(o-methoxyaniline) have also been produced in which the aniline polymer is chemically polymerized in the presence of p-toluene sulfonic acid and blended with Hydrin C with the blend cast from solution [219]. Another example in which an electrochromic polymer was electrochemically polymerized in the presence of an insulating polymer is that of polypyrrole-polyjether urethane) or polypyrrole-poly(ethylene-co-vinyl alcohol) composite films [222]. Both films switched between a yellow reduced state to a bluish brown oxidized state, similar to polypyrrole. 

Composites of conducting polymers, e.g., polyaniline and PEDOT, with polyacids, e.g., poly (2-acrylamido-2-methyl-l-methyl-l-propanosulfonic add) (PAMPS), have been shown to be electro-chromic. The polyadd acts as a dopant for the polymer film with the optical properties of the composite being contributed by the conducting polymer. The composites are formed by dther chemical or electrochemical polymerization of the electrochromic component monomer in the presence of the polyacid. Films of polyaniline-PAMPS switch from yellow to green and finally to blue on oxidation [228,229]. Composite films of PEDOT and PAMPS show similar electrochromic properties to PEDOT with the films switching from dark blue in the neutral state to Kght sky blue in the oxidized state [140,230,231]. 

Composite films of PB and poly(3-methylthiophene) capitalize on the transparent state of the reduced PB and the similar colored states of both PB and poly(3-methylthiophene) in their respective oxidized states. This composite switches from deep red, a contribution of poly(3-methylthiophene) in the neutral state, to dark blue in the oxidized state [241]. In addition to the additive electrochromic properties displayed by the materials in the composites, the improved adhesion of PB to the polymer films in comparison with electrode surfaces, along with efficient control of the amount of the composite materials deposited, are the advantages seen with this method. 

The absorptive/transmissive-type ECD operates with a reversible switching of the electrochromic materials between a colored state and a bleached state. Both working electrode and counter electrode are transparent so that light can pass through the device [4,5,15,250]. For flexible devices, ITO, SWNT, or PEDOT/PSS deposited onto a plastic such as poly(ethylene terepthalate) (PET) have been used [258,259]. When deposited in the doped form and dried, PEDOT/PSS films, to a thickness of 300 nm, are relatively transmissive in the visible region ( 75% T), have a relatively low resistivity (500 fi/D), and adhere to the plastic substrate in most common electrolyte solutions. The polymer films were demonstrated to be useable over the operating range of the device with no loss in conductivity or transmissivity. 

Sbnmez, G., I. Schwendeman, P. Schottland, K. Zong, and J.R. Reynolds. 2003. N-substituted poly(3,4-propylenedioxypyrrole)s High gap and low redox potential switching electroactive and electrochromic polymers. Macromolecules 36 639-647. 

A number of conjugated heterocyclic polymers, viz., poly(pyrrole) [9], poly(p-phenylene) [10], poly(thiophene) [11], and poly(aniline) [12] are also electrically conducting and continue to be developed and studied for electrochromic devices [13-14 see also the companion chapter in this volume] and ion switching devices [15-16], among others. Polymer films with high electrical conductivity have been generated by electrochemical polymerization of benzenoid, nonbenzenoid and heterocyclic aromatics, in particular from the derivatives of pyrrole, thiophene, carbazole, azulene, pyrene, triphenylene and aniline. The electrochemical approach for making these films is very versatile and it provides a facile way to vary the properties of the films. The realization of the applications for each electroactive polymer depends on the control and particularly the enhancement of the... 

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