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The redox processes described have a dramatic effect on the physical and chemical properties of the polymer. Conductivity will decrease, anion exchange capacity will diminish [Eq. (2)], cation exchange capacity may increase [(Eq. 3)] and hydrophobicity will be altered in a manner determined by which ion exchange process predominates. The mechanical properties are also influenced by the oxidation state with a greater elongation-to-break usually observed in the reduced materials [35]. The polymer also undergoes dramatic color changes upon redox switching (see Section 1.1.2) which is the basis of electrochromic devices based on ICPs [5].

1.1.2. Polythiophenes

The direct polymerization of thiophene is more complicated in that the monomer oxidation occurs at potentials more positive than required to overoxidize the polymer. Hence monomers with alkyl groups attached to the 3-position [36], bithiophene [37, 38], or terthiophenes [39] are often used as starting materials since they have lower oxidation potentials.

To avoid use of organic solvents for polythiophene synthesis, monomers can be dissolved in aqueous solution using surfactants [40] or molecular inclusion compounds such as cyclodextrins [41]. While the inclusion of the extensive range of dopants available with polypyrrole is not available with polythiophenes, some functional dopants have been incorporated [42]. Specific dopants such as the polyether [43] shown in Scheme 2 induce exceptional mechanical properties into polythiophenes, with tensile strengths of the order of 120 MPa readily obtained.

The UV-visible absorption spectra obtained (Fig. 3) after polymerization of bithiophene clearly show polaron/ bipo-laron absorption bands that are eliminated once the polymer is electrochemically reduced.

Given the simple synthetic chemistries available to cova-lently attach functional groups to thiophene substrates prior to polymerization, this has been the preferred approach to introduce functionality into polythiophenes [44]. In conjunction with our collaborators at Massey University,

CH3 Jn L CH3 Jm

Scheme 2. Structure of the S-PHE used in this work. TBA = tetrabutylammonium.

New Zealand, we have produced a range of substituted terthiophene precursors (Fig. 4), from which functional polymers have been produced [45].

As discussed for polypyrroles, the polythiophenes undergo reversible oxidation/reduction processes (Fig. 5). Oxidation/reduction usually occurs at more positive potentials than for the polypyrroles and is not readily achieved in aqueous solutions [45], presumably due to the hydrophobic nature of the thiophene based polymer. Some workers have attached ether groups to the polymer backbone, increasing hydrophilicity and the rate of switching in aqueous media [46].

Another interesting feature of polythiophenes is that the n-doped state is more readily accessible than with either polypyrroles or polyanilines. This enables the polymer to be rendered conductive at more negative potentials. A number of authors [47, 48] have highlighted the importance of sub-stituents on the polythiophene backbone in determining the accessibility (potential required for reduction) and stability of the «-doped state.

1.1.3. Polyanilines

Like polypyrrole, conducting polyaniline (PAn) and its ring-substituted analogs are generally prepared via either chemical or electrochemical oxidation/polymerization of the appropriate aniline monomer in aqueous solution. However, acidic conditions (pH generally 0-1) are required both

Wavelength (nm)

Figure 3. UV-vis spectra of PBT/S-PHE composite galvanostatically deposited (1 mA cm-2 for 50 s) onto ITO coated glass from a solution containing 0.2 M 2,2' bithiophene and 2% S-PHE. (A) Polymer after perparation. (B)-(D) Polymer after application of an applied potential for 60 s in propylene carbonate solution containing TBAP; (B) 0 V, (C) +1.3 V (D) + 1.5 V. Reprinted with permission from [43], J. Ding et al., Synth. Met. 110, 123 (2000). © 2000, Elsevier Science.

Wavelength (nm)

Figure 3. UV-vis spectra of PBT/S-PHE composite galvanostatically deposited (1 mA cm-2 for 50 s) onto ITO coated glass from a solution containing 0.2 M 2,2' bithiophene and 2% S-PHE. (A) Polymer after perparation. (B)-(D) Polymer after application of an applied potential for 60 s in propylene carbonate solution containing TBAP; (B) 0 V, (C) +1.3 V (D) + 1.5 V. Reprinted with permission from [43], J. Ding et al., Synth. Met. 110, 123 (2000). © 2000, Elsevier Science.

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