Counterions electropolymerization

The above statements are valid for monomolecular layers only. In the case of polymer films with layer thickness into the p-range, as are usually produced by electropolymerization, account must also be taken of the fact that the charge transport is dependent on both the electron exchange reactions between neighbouring oxidized and reduced sites and the flux of counterions in keeping with the principle of electroneutrality Although the molecular mechanisms of these processes... 

By electropolymerization of pyrrole in solvents containing polyelectrolytes such as potassium polyvinylsulfate, it is possible to prepare films of polypyrrole with polymeric counterions which have good conductivity (1-10 S cm-1) and strength (49 MPa) 303 304,305). Such a material could be used reversibly to absorb cations in an ion exchange system. Pyrrole has also been electrochemically polymerized in microporous polytetrafluoroethylene membranes (Gore-tex), impregnated with a perfluorosulphonate ionomer 3061. 

It has been mentioned already that polypyrrole (25) and polythiophene (26) play an important role as electrical conductors and polymeric anodes in battery cells [2,47,226]. Since the charging and discharging of the conjugated polymer is accompanied by the incorporation and removal of counterions it is clear that the material can also act as a carrier of chemically different anions which influence the physical, chemical and physiological properties of the material [292]. With regard to the full structural elucidation of the polymers it must be added, however, that the electropolymerization process of pyrrole and thiophene does not provide a clean coupling of the heterocycles in the 2,5-positions. Instead, the 3- and 4-position can also be involved giving rise to further fusion processes under formation of complex polycyclic structures [47]. 

Modified electrodes for this analytical purpose have mostly been formed by electrode adsorption of the mediator systems on the electrode surface or by electropolymerization [24,116]. Recently, for example, NAD(P)H oxidations have been performed on platinum or gold electrodes modified with a monolayer of pyrroloquinoline quinone (PQQ) [117] or on poly(methylene blue)-modified electrodes with different dehydrogenases entrapped in a Nafion film for the amperometric detection of glucose, lactate, malate, or ethanol [118]. In another approach, carbon paste electrodes doped with methylene green or meldola blue together with diaphorase were used for the NADH oxidation [119]. A poly(3-methylthio-phene) conducting polymer electrode was efficient for the oxidation of NADH [120]. By electropolymerization of poly(aniline) in the presence of poly(vinylsulfonate) counterions. 

The interaction of the solvent with the electrode, substrate, monomer, and counterion should also be considered. Even before a potential is applied, it is these interactions that will determine the conditions within the electroreaction zone because the degree of adsorption of the monomer and counterion will be solvent dependent. As the electropolymerization reaction proceeds, the nature of the solvent will also determine the solubility of the resultant polymer. Somewhat independently, the nature of the solvent will control the extent of interaction of these products with the electrode. 

Numerous workers50 51 52 53 have studied the effect of the counterion on the electropolymerization process. The high concentration of counterion employed means that it can have a dramatic effect on the polymerization process. The electrolyte will influence the conductivity of the solution, the polymer properties and, hence, the rate of polymerization. The nature of the electrolyte salt employed can also have a marked effect on polymer-solvent interactions. 

The counterion may be catalytic,51 in which case it will have a dramatic effect on the polymerization process. For example, Tiron 4 (shown earlier) has a catalytic effect on the electropolymerization of pyrrole, thereby enabling the process to be carried out at a more rapid rate at lower applied potentials. The ability of the counterion to ion pair with charged oligomers produced as part of the polymerization process will also have an effect. 

The counterion employed also has a marked effect on the electropolymerization process in organic solvents. For example, the polymerization of poly(methyl carboxypyrrole) (PMCP) proceeds differently in para-tolucncsul Ionic acid (pTS), tetrabutylammonium perchlorate (TBAP), tetrabutylammonium tetrafluoroborate (TBABF4), and tetrabutylammonium hexafluorophosphate (TBAPF6).63 As reported in that work, the rate of polymerization (at constant potential) and the time required for the onset of polymer deposition varied with the counterion employed. 

As with polypyrroles, the counterion used during electropolymerization influences the conductivity of polythiophenes.120121 Electrochemically produced copolymers122 of 3-dodecylthiophene (DTh) and 3-methylthiophene (MTh) have been shown to exhibit conductivities intermediate to the two homopolymers. The actual value depends on the ratio of MTh to DTh in the polymer. 

Provided electrodes can somehow be embedded into another polymeric material that has sufficient porosity to allow monomer and counterion species to ingress, electropolymerization can also be used to make composite materials. 

As was mentioned earlier, negatively charged CNTs can act as counterions in the electrochemical deposition of ECPs [19]. This straightforward method has been used for preparation of amperometric enz mie electrodes via entrapment of the enz3mie in the resulting ECP-CNT composite [63, 64]. Wang et al. developed amperometric glucose biosensors based on the PPy-MWCNT-GOD composite [63]. The composite was prepared by a simple one-step electrochemical method in which p3U role was electropolymerized at a constant potential of 0.7 V in the presence of c-MWCNTs and GOD. Results from the CV measurements showed that the incorporated c-MWCNTs act as counterions that maintain the electrical neutrality of the film. The influence of different parameters, such as the amount of the used MWCNTs, and the... 

In an effort to produce polymer films with improved mechanical properties in aqueous solutions, pyrrole was electrochemically polymerized in the presence of surfactants such as sodium dodecylsulfate and sodium dodecylbenzenesulfonate. In the case of dodecylsulfate anion, the polymers switched between a transmissive yellow in the neutral state to violet when partially oxidized and brown when fully oxidized [97]. When electropolymerized in the presence of dodecylbenzenesulfonate anion, the polymer films switched between a transmissive yellow when neutral to dark blue when oxidized [98]. In both cases, the polymers showed an improved electrochemical stability over polymer films produced with other inorganic counterions. This has been attributed to the binding of the surfactant dopant within the film during charge compensation since there is less mechanical distortion of the polymer film. 

While an invaluable tool in producing conjugated polymers on conducting substrates, electropolymerization has limitations that include a lack of primary structure verification and characterization along with the inability to synthesize large quantities of processable polymer. To overcome the insolubility of PEDOT, a water-soluble polyelectrolyte, poly(styrenesulfonate) (PSS) was incorporated as the counterion in the doped PEDOT to yield the commercially available PEDOT/PSS (Baytron P) (39), which forms a dispersion in aqueous solutions [140]. While this polymer finds most of its application as a conductor for antistatic films, solid state capacitors, and organic electronic devices, its electrochromism is distinct and should not be ignored. 

It is well known that changes in redox states of EAPs require movement of ions in order to maintain electroneutrality. Electrochemical neutralization of p-doped polymers can be anion-dominant (anions are expelled from the polymer film), cation-dominant (cations diffuse into the polymer film), or a combination of both anion and cation movements [14,15]. The dominant process varies from polymer to polymer and is strongly affected by ion and solvent choices as well as by electropolymerization conditions and film thickness [16]. Typically for p-doping polymers, anion transport dominates when the anion is small and highly mobile, but cation transport dominates when the anion is large and immobile [17-19]. For instance, redox processes in polypyrrole are anion-dominant in most cases, but when poly(styrenesulfonate) is chosen as the counterion, cation transport is the dominant process [20-23]. 

For instance, if large counterions are used during film deposition (electropolymerization), co-ion exchange is largely observed. In this case, the large, sometimes polymeric counterions are trapped in the polymeric layer due to strong van der Waals and electrostatic forces. 

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