Coating polymer

Coating the surface of silicon electrodes with a polymer coating can also be an effective method of stabilizing the electrodes and improving the photovoltage and kinetics of electrode reactions The polymer film effectively insulates the semiconductor from the superoxide ion and prevents chemical reaction and deterioration. At the same time, the polymer behaves like a surface-bond redox couple to mediate the charge transfer between the semiconductor and the redox species in the solution. Various types of polymers can be used to coat silicon electrodes as shown in Table 6.6. 

For n-Si the surface-confined material can serve two functions (1) photogenerated holes are transferred to the attached redox reagent to preclude silicon oxidation and (2) the photooxidized polymer oxidizes the solution species. Because whether the species in solution can be oxidized depends on the nature of the surface-confined polymer, the electrode can be designed to oxidize certain species selectively. 

An example is shown in Fig. 6.31 for a polymer with a redox couple M /M that is attached to the silicon surface through chemical bonding. On illumination is converted to M. The resulting oxidized surface is then capable of oxidizing a second species B to B. The redox species B is not efficient in capturing holes directly from silicon due to its distance from the valence band. When the Si surface is covered with a polymer, the location of electron transfer is within the polymeric film rather than at the solid interface. The photocurrent response of derivatized electrode is similar to that of bare silicon surface but the stability is significantly improved.  

The presence of a derivatized surface layer can affect the energetics as well as the kinetics of electrode reactions. It has been found that the flatband potentials of -Si and p-Si electrodes coated with conducting polypyrrole films are shifted by 300 and 500 mV in CH3CN solution. The reaction kinetics on polymer derivatized surface can further be enhanced by impregnation of noble metals such as Pt particles into the 

FIGURE 6.31. Contrast of situations for photooxidation of B to B at a naked semiconductor and a deriva-tized electrode. The mediation system M /M is the Pe(Cp)2/Fe(Cp)2 couple attached via hydrolytic reaction of Si-Cl bonds with surface OH groups. At the naked semiconductor, B must capture the photogenerated h which comes to Evb. while at the derivatized electrode M oxidizes B and the photogenerated h need only be transferred to M to generate the M. Ideally, the M /M system would be more negative than Eg in order to suppress decomposition of the semiconductor Eg is the decomposition potential of semiconductor. (Reprinted with permission from Wrighton et al 1978, American Vacuum Society.) 

Shell with defined Size, thickness, stability, composition, Affinity properties 

Both complex coacervation and interfacial polymerization can be used to create a polymer coating. Complex coacervation involves a phase separation that results from the formation of a complex between oppositely charged polyelectrolytes. At a pH below its isoelectric point gelatin, a positively charged collagen hydrolysis product has been widely used in complex coacervation with anionic species like gum Arabic, pectin and alginate. This approach has been used to prepare scratch 

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Ling X, Pritzker M D, Byerley J J and Burns C M 1998 Confocal scanning laser microscopy of polymer coatings J. Appl. Polym. Sc/. 67 149-58... 

FLUORINECOMPOUNDS,ORGANIC - TETRAFLUOROETHYLENE-PERFLUOROVINYLETTiERCOPOLYTffiRS] (Vol 11) PFC. See Polymer-coated fertilizers... 

To an experienced operator trained in the handling of industrial chemicals, the dimers present Httle cause for concern in handling or storage. The finished polymer coating presents even less of a health problem contact with the reactive monomer is unlikely. In the ancillary operations, such as cleaning or adhesion promotion, the operator must observe suitable precautions. Before using the process chemicals, operators must read and understand the current Material Safety Data Sheets, which are available from the manufacturers. 

J. J. Licari, Handbook of Polymer Coatings forElectronics, Noyes Pubbcations, Park Ridge, N.J., 1990, pp. 250—277. 

The author wishes to dedicate this work to Mr. T. H. Shen of Yungkoo Paint Varnish Mfg. Co., Taipei, who gave him the rare opportunity of starting a new alkyd plant as the first job assignment for a fresh college graduate, and to the memory of the late chairman of the Department of Polymers Coatings, North Dakota State University, Dr. Alfred E. Rheineck, teacher, mentor, and friend. 

Fig. 1. United States agricultural crop markets for manufactured controlled release fertilizers, 1990. (a) Total percentage ( 14 million) of market value by crop and (b) total percentage (6,800 t) of consumption by product type. IBDU is isobutyUdene diurea PCF, polymer-coated fertilizer UF,...

Fig. 6. Response of Kentucky bluegrass to polymer-coated fertilisers of select manufacturers. AppHcation of 2 lbs N/1000 ft (9.76 g/m ). A is Multicote 4...

Exxon products appear to release via a unique mechanism. Like other polymer-coated technologies, the penetration of water iato the granule is purely by diffusion. However, as water enters the particle, an osmotic pressure is created as the fertilizer is solubilized. This pressure causes an expansion of the elastomeric coating and the particle swells to many times its original diameter. As the particle swells, the coating becomes increasingly thinner to the point where it caimot contain the internal pressure and the nutrient is released. 

Nutrients are released from POLYON-coated fertilizers by osmotic diffusion. The RLC process permits appHcation of ultrathin, hence lower cost, membrane coatings which distinguishes this technology from many other polymer-coated fertilizers. The coating thickness determines the diffusion rate and the duration of release. POLYON-coated urea at a 4% coating (44% N) will release at twice the rate and will have half the duration as an 8% coating... 

Other Polymer- Coated Technologies. A number of other polymer-coated fertilizer technologies have been developed and are being ... 

Table 8. Japanese Polymer-Coated Fertilizer Technologies ...

In addition, the combination coating renders the nutrient release much less temperature sensitive than most polymer-coated fertilizers. 

The changes in the optical absorption spectra of conducting polymers can be monitored using optoelectrochemical techniques. The optical spectmm of a thin polymer film, mounted on a transparent electrode, such as indium tin oxide (ITO) coated glass, is recorded. The cell is fitted with a counter and reference electrode so that the potential at the polymer-coated electrode can be controlled electrochemically. The absorption spectmm is recorded as a function of electrode potential, and the evolution of the polymer s band stmcture can be observed as it changes from insulating to conducting (11). 

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