Undoped polymers

In the field of soluble conducting polymers new data have been published on poly(3-alkylthiophenes " l They show that the solubility of undoped polymers increases with increasing chain length of the substituent in the order n-butyl > ethyl methyl. But, on the other hand, it has turned out that in the doped state the electro-chemically synthesized polymers cannot be dissolved in reasonable concentrations In a very recent paper Feldhues et al. have reported that some poly(3-alkoxythio-phenes) electropolymerized under special experimental conditions are completely soluble in dipolar aprotic solvents in both the undoped and doped states. The molecular weights were determined in the undoped state by a combination of gel-permeation chromatography (GPC), mass spectroscopy and UV/VIS spectroscopy. It was established that the usual chain length of soluble poly(3-methoxthythiophene) consists of six monomer units. 

A pioneering study of the electrical conductivity of poly(2,5-dibutoxy-p-phenylene ethynylene), a fully soluble PPE derivative, was conducted by the groups of Shinar and Barton (Table 1, entry 3) [38]. The polymer employed displayed a band gap. Eg, of 2.5 eV, which is typical for 2,5-dialkyloxy-PPEs. A conductivity of 10 Scm was reported for the undoped polymer. This value was later confirmed in an independent study by Lo Sterzo et al.,who reported a conductivity of 4-10 Scm for the same polymer [39]. It was reported that upon exposure to I2 vapor at a pressure of 1 Torr, the room-temperature conductivity of poly(2,5-dibutoxy-p-phenylene ethynylene) increased by about three orders of magnitude to 10 Scm [38]. It increased further and imme-... 

Polymethylacetylene can be doped by I2, but not by AsF5. Its conductivity in the doped state 36) is around 10-3 S cm 1. As might be expected, increasing content of mono- and di-substituted acetylenes in copolymers with acetylene gives increasing solubility at the expense of drastic reductions in the conductivity of both doped and undoped polymers. 

The ionization potential of polyphenylene is around 8 eV and it is not surprising that oxidative degradation is not a problem the undoped polymer can withstand long periods at high temperatures in air with no change in its conductivity or its ability to dope 386). However, the high oxidation potential creates two problems. Firstly, the range of dopants with sufficient electron affinity to oxidize the polymer is limited, and there are few solvents in which the oxidation can take place without destruction of the solvent. Secondly, the doped polymer is expected to be reactive towards water and this is indeed the case 386). 

The shift of the peak photoresponse towards longer wavelengths has been observed on some other polymers60. The magnitude of photocurrent of the TNF-sensitized poly(pyrenylmethylvinylether) is affected by the presence of oxygen44. A similar effect was observed on an undoped on an undoped polymer. 

Careful experiments have not revealed any significant midgap absorption in the undoped polymer. It has been suggested that in the undoped polymer the energy levels due to solitons are close to the band edges. The simple theory which predicts midgap levels in the undoped polymer does not take into account the electron-electron interactions and is very approximate. 

Undoped polymers, which remain in the semiconducting state. In this case charge injection is operated by photoexcitation (simultaneous creation of an electron in the conduction band and a hole in the valence band), or by action of an electric field at a junction. 

Doping of CPs to increase their conductivity is in fact an intercalation of anions or cations together with injection of an equal number of positive or negative mobile charges onto the polymer chains. These ions are too big to accommodate into the undoped polymer structure, which is quite closely packed, as shown in Figs. 5a and 6d. Structural changes are therefore produced. 

Figure 4 Overview of conductivity of conducting polymers at room temperature, (a) stretched [CHClj)], (from Ref. 43a), (b) stretched [CHCIj)] (from Ref. 43b), (c) [CH(l3)], (from Ref. 43c), (d) [CH(l3)], (from Ref. 43d), (d ) [CH(1,)], (from Ref. 43e), (e) stretched PAN-HCl (from 43f), (f) PAN-CSA from m-cresol (from Ref. 43g), (g) PAN-CSA from OT-cresol (from Ref. 43h), (h) PAN derivative poly(o-toluidine) POT-CSA fiber from m-cresol (from Ref. 43i), (i) POT-HCl (from Ref. 43j), (j) sulfonated PAN (from Ref. 43k), (k) stretched PPy(PFg) from (Ref. 431), (1) PPy(PF ) ( ) PPy(TsO) (from Ref. 43m, 43n), (m) iodine doped poly(dodecylthiophene) (from Ref. 43o), (n) FeCl4 doped PT (from Ref. 43p), (o) PPV(H2S04) (from Ref. 43q), (p) PPP(Asp5) (from Ref. 43r), (q) Kr- implanted (polyphenylenebenzobisoxazole) (from Ref. 43s), (r) undoped trans-(CH (from Ref. 43t), (s) undoped cA-(CH)x from (Ref. 43u), (t) undoped PAN (EB) (from Ref. 43v), (u) undoped PPy (from Ref. 43w), (v) undoped PT (from Ref. 43p, (w) undoped PPV (from Ref. 43x), (x) undoped PPP (from Ref 43y). The conductivity reported for the undoped polymers should be considered an upper limit due to the possibility of impurities.

The addition of N-bromosuccinimide (NBS) to pristine polyacetylene stabilized the undoped polymer (32). The enhanced stability was attributed to the reaction of NBS with the numerous free radicals found in polyacetylene, as evidenced by the decreased rate of oxygen uptake in treated samples and increased final conductivities for iodine-doped polyacetylene. The conductivity of the undoped polymer rose from 10 S/cm for untreated samples to greater than 10 S/cm for NBS-treated samples. A slight amount of Br was detected in treated samples this finding indicates that some doping accompanies the treatment. The stability of doped polymer samples was not significantly improved by this treatment. 

A number of organic polymers become electrically conducting on addition of electron donors or acceptors.(1-5) Despite the enormous interest in these conducting polymer systems, many theoretical aspects of the problem remain poorly understood, especially with regard to the electronic properties of the "doped" (partially ionized) polymers. Progress is being made, however, in understanding the undoped polymer precursors. In a series of recent papers, we have demonstrated the utility of the... 

For thermally sensitive or undoped polymers with low linear absorption at the laser wavelength, the use of femtosecond laser pulses can improve the ablation precision in contrast to long-pulse treatment. Further, the thermal load to the samples is minimized. For these reasons, femtosecond laser pulses were chosen to perforate a polyethylene membrane serving as a diffusion-discriminating element on a miniaturized biosensor for the measurement of glucose concentration [78]. 

The interrupted dephasing pulse sequence is also used in order to determine the number of hydrogen atoms bonding to nitrogen atoms in doped and undoped polypyrroles. The same relative intensities of signals in doped and undoped polymers obtained with TD of 20, 100 and 200 ps indicate that the number of bonded hydrogens and the bond lengths in both samples are approximately equal (71). 

Structural modelling of polypyrrole and polythiophene has been published by Orchard et al. [260]. Singlechain conformation and crystal packing are considered, but only for the undoped polymers. The result is a multitude of local energy minima ( polymorphism ) for both monoclinic (one-chain) and orthorhombic (two-chain) cells. 

Uv-vis-nir is another spectroscopic technique widely used for the characterization of conducting polymers. The existence of a spatially extended n-bonding system in conjugated polymers gives rise to electronic transitions in the uv-vis-nir region of the spectrum. In neutral (undoped) polymers, the dominant peak is usually associated with the n—n transition. The uv-vis-nir spectra of conjugated polymers can be treated as a qualitative measure of the overlap of their n orbitals. 

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