Ionic polymer films

The effect of different polymer bases and additives on percutaneous absorption of these two ionic drugs are examined. Carboxyvinylpolymer (CVP), an ionic polymer film base, yields films with poor bioavailability of cationic drugs such as DIL, but is effective for films containing anionic drugs such as DSCG. In contrast, polyvinyl alcohol (PVA) and glycerol, electrically neutral bases, were used to formulate films with good bioavailability. 

Figure 1. Absorption spectra of several representative CdS cluster samples. The 60-A CdS is made inside an ionic polymer film. The 20-A CdS is a free-standing cluster capped with thiophenolate on the surfaces. The 10-A sample is a monodisperse 55-atom CdS cluster (not counting the peripheral phenyl groups). The spectra are scaled arbitrarily. (Taken from reference 31 with permission.)...

If a paint film is to prevent this reaction, it must be impervious to electrons, otherwise the cathodic reaction is merely transferred from the surface of the metal to the surface of the film. Organic polymer films do not contain free electrons, except in the special case of pigmentation with metallic pigments consequently it will be assumed that the conductivity of paint films is entirely ionic. In addition, the films must be impervious to either water or oxygen, so that they prevent either from reaching the surface of the metal. 

It is assumed that conduction in polymer films is ionic —it is difficult to see how it could be otherwise — and the factors which break down this resistance, or render it ineffective, will now be considered. 

Temperature An examination has been made of the effect of temperature on the structural changes in polymer films produced from the three vehicles described earlier s. Three methods were used dilatometry, water absorption and ionic resistance. 

Since multiple electrical and optical functionality must be combined in the fabrication of an OLED, many workers have turned to the techniques of molecular self-assembly in order to optimize the microstructure of the materials used. In turn, such approaches necessitate the incorporation of additional chemical functionality into the molecules. For example, the successive dipping of a substrate into solutions of polyanion and polycation leads to the deposition of poly-ionic bilayers [59, 60]. Since the precursor form of PPV is cationic, this is a very appealing way to tailor its properties. Anionic polymers that have been studied include sulfonatcd polystyrene [59] and sulfonatcd polyanilinc 159, 60]. Thermal conversion of the precursor PPV then results in an electroluminescent blended polymer film. 

Impedance spectroscopy is best suited for the measurement of electronic conductivities in the range 10 -7to 10 2S cm 1.145 In principle, it is perhaps the best method for this range, but it is often difficult to interpret impedance data for conducting polymer films. The charge-transfer resistance can make measurements of bulk film resistances inaccurate,145 and it is often difficult to distinguish between the film s ionic and electronic resistances.144 This is even more of a problem with chronoamperometry146 and chronopotentiometry,147 so that these methods are best avoided. 

Figure 13. Schematic diagram of the measurement of the ionic conductivity of a conducting polymer membrane as a function of oxidation state (potential), (a) Pt electrodes (b) potentiostat (c) gold minigrid (d) polymer film (e) electrolyte solution (0 dc or ac resistance measurement.133 (Reprinted with permission from J. Am Chem Soc. 104, 6139-6140, 1982. Copyright 1982, American Chemical Society.)...

Figure 16. General transmission-line model for a conducting polymer-coated electrode. CF is the faradaic pseudo-capacitance of the polymer film, while Rt and Rt are its electronic and ionic resistance, respectively. R, is the uncompensated solution resistance.

For this reason, other types of electrolytes are used in addition to aqueous solutions (i.e., nonaqueous solutions of salts (Section 8.1), salt melts (Section 8.2), and a variety of solid electrolytes (Section 8.3). More recently, a new type of solid electrolyte is being employed more often (i.e., water-impregnated ionically conducting polymer films more about them in Chapter 26). 

It usually takes place close to the melting temperature of the polymer when the pores collapse turning the porous ionically conductive polymer film into a nonporous insulating layer between the electrodes. At this temperature there is a significant increase in cell impedance and passage of current through the cell is restricted. This prevents further electrochemical activity in the cell, thereby shutting the cell down before an explosion can occur. 

No amount of sterilization wiU prevent or even slow autooxidation, and there are only two defenses removal of O2 and addition of inhibitors. Oxygen barriers in food packaging are a major topic in the engineering of polymer films. The barrier properties of various polymers are very important in food applications, and many of these are multilayer polymers that have a thin layer of an impermeable polymer (such as polyacrylonitrile and ionic polymers) on a cheaper but O2-permeable polymer such as a polyolefin, which gives mechanical strength to the fikn. 

Schafer, T., Di Paolo, R. E., Franco R.,and Crespo, J. G., Elucidating interactions of ionic liquids with polymer films using confocal Raman spectroscopy, Chem. Commun., 2594-2596,2005. 

Nafion (17) is a perfluorinated polymer related to teflon (polytetrafluoroethylene). An electrode is conveniently coated by allowing an ethanolic solution of the polymer to evaporate. The film produced is stable, rather more so in fact than other polymer films, e.g. polyvinylpyridine (see Section 57.3.2.2). At the microscopic level the polymer separates into two phases, the bulk polymer and the lower density ionic cluster phase. Diffusion of ions can occur quite freely for example, the diffusion coefficient of Na+ in Nafion (MW 1200) is only slightly less than in water.44... 

Chemical modification of electrode surfaces by polymer films offers the advantages of inherent chemical and physical stability, incorporation of large numbers of electroactive sites, and relatively facile electron transport across the film. Since th% polymer films usually contain the equivalent of one to more than 10 monolayers of electroactive sites, the resulting electrochemical responses are generally larger and thus more easily observed than those of immobilized monomolecular layers. Also, the concentration of sites in the film can be as high as 5 mol/L and may influence the reactivity of the sites because their solvent and ionic environments differ considerably from dilute homogeneous solutions [9]. 

In most polymer films, the electroactivity of the redox centers depends on the ionic conductivity of the film. This is usually achieved either by the penetration of supporting electrolyte ions through pores in the film or by the presence in the film of numerous fixed charge sites plus mobile counterions. In many cases, solvent permeation into the polymer facilitates ionic penetration and mobility. If the polymer film possesses electronic, rather than ionic, conductivity, the electron-transfer reactions will most likely occur at the polymer/solution interface and the advantage of a three-dimensional reaction zone will be reduced. 

Electric double layer forces between polyelectrolyte and non-polymer surfaces in aqueous media have also been studied very intensively [371,394,400-402]. The adhesion between polyelectrolyte surfaces could be reduced considerably by increasing the ionic strength of the medium [400]. Using an electrochemical cell and a gold coated tip, the adhesion between electroactive layer of p oly( vinyl-ferrocene) was controlled through the selective oxidation or reduction of the polymer films [401]. 

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