Poly films, conductivity

From the experiments it is clear that poly electrolyte is adsorbed on the surface of the black lipid film. This applies both to the experiments with gelatin and bovine serum albumin, which gave no decrease of film resistance, and to the experiments with bovine erythrocyte ghost protein and polyphosphate. The adsorption of protein on the phospholipid-water interface may be controlled independently by investigating the electrophoretic behavior of emulsion droplets, stabilized by phospholipid, in a protein solution, as a function of pH. In this way Haydon (3) established protein adsorption on the phospholipid-water interface. If the high resistance (107 ohms per sq. cm.) of black lipid films is to be ascribed to the continuous layer of hydrocarbon chains in the interior of the film, as is generally done, an increase in film conductivity is not expected from adsorption without penetration. 

So far, we have ignored the primary reason poly films are used in integrated circuits. Heavily-doped poly is used as a gate electrode, and the electrical conductivity of this material is of prime importance. Therefore, we have to inquire into the feasibility of doping poly as it is being deposited by CVD. 

Electrolytes and solvents. The electropolymerization reaction may be sensitive to the nucleophilic nature of the solvent and electrolyte. For this reason, many of the films are prepared in aprotic solvents, such as acetonitrile, which are poor nucleophiles. Electro-oxidative polymerization in the presence of small anions simultaneously incorporates the anions which render the polymer film conductive. Upon reduction, the anions are released from the film. Cycling the film through oxidation and reduction leads to insertion and release in the respective parts of the cycle. Simultaneous incorporation or removal of the solvent and/or cations may also occur, as shown by measurements on the quartz crystal microbalance [51-52]. Polymerization in the presence of large anions such as poly(vinylsulphonate) and poly(4-styrene sulphonate) (PSS") also incorporates the anion during growth [53-56]. Subsequent cycling, however, does not release the anions which are trapped because of their... 

Additional annealing of doped polymers also accelerates the process of dedoping. Li and Wan (1999) have shown that temporal change in film conductivity of doped poly aniline at room temperature is increased strongly after thermal treatment of doped polymer at 7=150 °C. Results of this research are shown in Fig. 19.6. We need to recognize that the indicated effect limits the application of any thermal treatments during gas sensor fabrication. It was noted that the conductivity of PANICS A and PANI-p-TSA are the most stable below 200 °C, while the stability of PANI films doped with H SO and H PO are much better than the PANI-HCIO and PANI-HCl after thermal treatment at a high temperature (200 °C) (Li and Wan 1999). 

Not surprisingly, in view of the above preference for hydrophobic surfaces, PAn can also be deposited by the in situ method on supports such as low-density polyethylene (LDPE). Modification of the LDPE surface by grafting with acrylic acid promotes the growth and adhesion of the PAn films. Conducting PAn coating can be similarly deposited on poly(vinylchlo-ride) and poly(methylmethacrylate) surfaces through chemical oxidative polymerization. °... 

By mixing polyaniline with a small amount of acetic acid, it is possible to form stable films at the air-water interface which can be readily transferred into high quality Y-type LB films. Conductivity values of 0.1 to 1 S/cm were obtained [294]. Poly(o-anisidine) was... 

The precursor route has also been used effectively for poly(l,4-phenylenevinylene) (poly(p-phenylenevinylene), PPPV). This is obtained via a sulfonium salt in which the phenyl rings pin the conjugated structure of the final polymer (equation 2). Stretching during thermal elimination gives highly oriented films. Conductivities of 100 Scm" and 2780 S cmhave been reported for AsF -doped unoriented and oriented samples. Optical and IR spectra have been reported. 

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

Because of the aqueous solubiUty of polyelectrolyte precursor polymers, another method of polymer blend formation is possible. The precursor polymer is co-dissolved with a water-soluble matrix polymer, and films of the blend are cast. With heating, the fully conjugated conducting polymer is generated to form the composite film. This technique has been used for poly(arylene vinylenes) with a variety of water-soluble matrix polymers, including polyacrjiamide, poly(ethylene oxide), polyvinylpyrroHdinone, methylceUulose, and hydroxypropylceUulose (139—141). These blends generally exhibit phase-separated morphologies. 

Conducting polymer composites have also been formed by co-electrodeposition of matrix polymer during electrochemical polymerization. Because both components of the composite are deposited simultaneously, a homogenous film is obtained. This technique has been utilized for both neutral thermoplastics such as poly(vinyl chloride) (159), as well as for a large variety of polyelectrolytes (64—68, 159—165). When the matrix polymer is a polyelectrolyte, it serves as the dopant species for the conducting polymer, so there is an intimate mixing of the polymer chains and the system can be appropriately termed a molecular composite. 

The main application today for poly(vinyl carbazole) arises out of its photoconductivity and is in electrostatic dry copying machines. The polymer is applied from solution in thin film (10-15 p.m) layers onto a conductive substrate. 

The group in the Swiss Federal Institute of Technology [55] has fabricated a macroscale device by depositing the conducting polymer (poly(/j-phenylenevinylene)) on the MWCNT film (Fig. 16). They have observed the characteristic rectifying effect from the l-V curve, which suggests the CNTs inject holes efficiently into the polymer layer. However, due to the difficulty in... 

S. V. Frolov, M. Ozaki, W. Gellermann, Z. V. Vardeny, K. Yoshino, Mirrorless lasing in conducting polymer poly(2,5-dioclyloxy-p-phenylencvinylcnc) films. Jpn. J. Appl. Phys. 1996, 35, L1371. 

Poly-1,2-1//-azepines, produced by gas-phase photopolymerization of aryl azides yield, after oxidation, electrically conducting films.103 By photolyzing 4-(pcntyloxy)phenyl azide in the gas phase, a flexible polyazepine is produced which can be deposited directly as a thin polymer film onto a suitable surface. 

Sloop and Lerner [132] showed that SEI formation can be affected by treatment of the cross-linked polymer, poly-[oxymeth-ylene oligo(oxyethylene)] (PEM) with an alkylating agent. Cross-linked films of PEM do not form a stable interface with lithium however, upon treatment with methyl iodide, / Ej stabilizes at 2000 Hem"1. Such an SEI is characterized by low conductivity, from 10 to 10 Q-Icm2, which is linear over the temperature range of 25-85 °C. 

This means that we can follow the empirical kinetics of the electropolymerization process, at a constant overpotential (Fig. 6), by tracking the weight of the rinsed and dried polymer film,37 41 as we do in homogeneous polymerization processes of conducting or nonconducting poly-... 

The stoichiometry of the redox reactions of conducting polymers (n and m in reactions 1 and 2) is quite variable. Under the most widely used conditions, polypyrroles and polythiophenes can be reversibly oxidized to a level of one hole per ca. 3 monomer units (i.e., a degree of oxidation, n, of ca. 0.3).7 However, this limit is dictated by the stability of the oxidized film under the conditions employed (Section V). With particularly dry and unreactive solvents, degrees of oxidation of 0.5 can be reversibly attained,37 and for poly-(4,4 -dimethoxybithiophene), a value of n = 1 has been reported.38 Although much fewer data are available for n-doping, it appears to involve similar stoichiometries [i.e., m in Eq. (2) is typically ca. 0.3].34,39"41 Polyanilines can in principle be reversibly p-doped to one... 

Polymerization at constant current is most convenient for controlling the thickness of the deposited film. Charges of ca. 0.3, 0.2, and 0.08 C cm-2 are required to produce 1 fim of polypyrrole,59 poly(3-methylthio-phene)60 (no data are available for polythiophene), and polyaniline 43 respectively. Although these values can reasonably be used to estimate the thicknesses of most electrochemically formed conducting polymer films, it should be noted that they have considerable (ca. 30%) uncertainties. For each polymer, the relationship between charge and film thickness can... 

A number of approaches are available to improve the morphology and homogeneity of electrochemically deposited conducting polymer films. Priming of the electrode surface with a monolayer of adsorbed or covalently bonded monomer leads to more compact deposits of polyaniline,87,88 poly thiophene,80 and polypyrrole.89,90 Electrode rotation has been shown to inhibit the deposition of powdery overlayers during poly(3-methylthiophene) deposition.81... 

Figure 5. Cyclic voltammograms of (a) 2,5"" -di-methyl-a-hexathiophene and (b) poly(2,2 -bithio-phene) films in acetonitrile containing 0.1 M E NCIO 103 (Reprinted from G. Zotti, G. Schia-von, A. Berlin, and G. Pagani, Electrochemistry of end-ca )ed oligothienyls-new insights into the polymerization mechanism and the charge storage, conduction and capacitive properties of polythiophene, Synth. Met. 61 (1-2) 81-87, 1993, with kind permission from Elsevier Science S.A.)...

The huge literature on the electronic conductivity of dry conducting polymer samples will not be considered here because it has limited relevance to their electrochemistry. On the other hand, in situ methods, in which the polymer is immersed in an electrolyte solution under potential control, provide valuable insights into electron transport during electrochemical processes. It should be noted that in situ and dry conductivities of conducting polymers are not directly comparable, since concentration polarization can reduce the conductivity of electrolyte-wetted films considerably.139 Thus in situ conductivities reported for polypyrrole,140,141 poly thiophene,37 and poly aniline37 are orders of magnitude lower than dry conductivities.15... 

---