Contact charging of polymers

Fig. 2 Model concept of the contact charging of polymer grains, a Contact between an electron pair donator domain (empty dots) of the particle 1 and an electron pair acceptor domain (grey dots) of the particle 2. Charge transfers (e-) are taking place. After separation the two particles (b), a positively charged ( ) and a negative charged ( ) domain, remain on the particles surface...

Contact charging of polymers 7.3.1 Charge transfer by electrons... 

Thus surface-state theory provides a satisfactory basis for discussing contact charging of polymers by electron transfer. The model is, however, probably oversimplified, and it would be unwise to exclude the possibility of penetration of charge into the bulk polymer altogether in view of conduction effects that can usually be observed at high fields in polymers. We must also remember that the results we have been talking about refer to very clean polymer surfaces and that the contact charging in most practical cases can be very different due to the presence of dirt and moisture on the surface. 

It is, therefore, not surprising that rate and temperature effects are noted in the contact charging of polymers. Figure 15 shows the frictional electrification of a polycarbonate film when run in contact with a chromium tip at different speeds and temperatures. ... 

The tribo-electric charging of polymers is a collective phenomenon of numerous electron transfer processes in the contact zone of two colliding particles. The SPM technique allows to visualize the charge distribution on polymer surfaces. It was shown that oppositely charge domains can stably exist side by side although considerable filed strengths are present. 

Understanding of the contact electrification phenomenon has come a long way to the point that industrial processes can be based on controlled charging of polymers. Application of modern analytic tools and instrumentation should allow an even greater depth of miderstanding to be achieved in the future. 

The kinetic energy of charge earners in a solid increases with increasing temperature and therefore the probability that a charge carrier passes a given potential barrier also increases. The thermally induced current flow of the charge earners from a metal contact into a polymer film can be derived from the Richardson equation, which describes the temperature-induced emission of hot charge carriers from a metal surface... 

A surface is that part of an object which is in direct contact with its environment and hence, is most affected by it. The surface properties of solid organic polymers have a strong impact on many, if not most, of their apphcations. The properties and structure of these surfaces are, therefore, of utmost importance. The chemical stmcture and thermodynamic state of polymer surfaces are important factors that determine many of their practical characteristics. Examples of properties affected by polymer surface stmcture include adhesion, wettability, friction, coatability, permeability, dyeabil-ity, gloss, corrosion, surface electrostatic charging, cellular recognition, and biocompatibility. Interfacial characteristics of polymer systems control the domain size and the stability of polymer-polymer dispersions, adhesive strength of laminates and composites, cohesive strength of polymer blends, mechanical properties of adhesive joints, etc. 

The presence of pre-adsorbed polyacrylic acid significantly reduces the adsorption of sodium dodecylsulfonate on hematite from dilute acidic solutions. Nonionic polyacrylamide was found to have a much lesser effect on the adsorption of sulfonate. The isotherm for sulfonate adsorption in absence of polymer on positively charged hematite exhibits the typical three regions characteristic of physical adsorption in aqueous surfactant systems. Adsorption behavior of the sulfonate and polymer is related to electrokinetic potentials in this system. Contact angle measurements on a hematite disk in sulfonate solutions revealed that pre-adsorption of polymer resulted in reduced surface hydrophobicity. 

Another method for the analysis of aptamer-protein complexes involved the use of a positively charged ferrocene-tethered polythiophene, (19), as redox label reporting unit (Fig. 12.19). The antithrombin aptamer was immobilized on an electrode surface, and the electrostatic binding of the redox polymer (19) to the aptamer monolayer resulted in a supramolecular complex that revealed electrical contact between the polymer and the electrode.74 The formation of the aptamer-thrombin complex removed the polymer from the surface and blocked the electrical contact between the polymer label and the electrode. As a result, higher concentrations of thrombin increased the surface coverage of the aptamer-thrombin complex on the electrode, and this decreased the amperometric responses of the sensing device. 

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