Discontinuous film

The principal cathodic reaction on the upper surface of the membrane is the reduction of Cu " that is formed by the reaction of Cu with dissolved oxygen in the water these Cu ions are provided partly from the diffusion through the pores in the oxide membrane from within the pit and partly from those produced by cathodic reduction (equation 1.154). Lucey s theory thus rejects the conventional large cathode small anode relationship that is invoked to explain localised attack, and this concept of an electronically conducting membrane has also been used by Evans to explain localised attack on steel due to a discontinuous film of magnetite. 

Thus films can be divided into two groups according to their morphology. Discontinuous films are porous, have a low resistance and are formed at potentials close to the equilibrium potential of the corresponding electrode of the second kind. They often have substantial thickness (up to 1 mm). Films of this kind include halide films on copper, silver, lead and mercury, sulphate films on lead, iron and nickel oxide films on cadmium, zinc and magnesium, etc. Because of their low resistance and the reversible electrode reactions of their formation and dissolution, these films are often very important for electrode systems in storage batteries. 

As will be apparent from the subsequent discussion, a very thin discontinuous film such as occurs in the fringe region of a film formed in a... 

These data show that fracture in the primed specimens occurred very close to the oxide surface, leaving a small amount of primer and epoxy on the oxide and a thin, discontinuous film of oxide on the adhesive surface. Significant interdiffusion... 

The implant consists of a tablet-shaped ganciclovir reservoir. The drag is initially completely coated with poly(vinyl alcohol) (PVA) and then coated with a discontinuous film of hydrophobic, dense poly (ethylene-co-vinyl acetate) (EVA). Both polymers are nonerodible and hydrophobic (the PVA used in the implant is cross-linked and/or high molecular weight, to ensure it does not dissolve when exposed to water). The entire assembly is coated again with PVA to which a suture tab made of PVA is attached (Figure 4.5). 

Figure 2. IR spectrum of polyethylene and film morphology (inset) for discontinuous film.

When a continuous film was obtained, no Christiansen effect was observed. However, sometimes good spectra were obtained from discontinuous films. The photomicrographs and IR spectra shown in Figure 5 were taken using an IR microscope. For the polyethylene deposit consisting of droplets 10-30 pm in diameter, no spectral distortion was observed. In another deposit from the same SEC run when droplets ranged from 2 to 10 pm in diameter, a distorted spectrum was obtained. 

Thus, when a discontinuous film was present, the size and possibly the shape of the deposited particles determined whether or not the Christiansen efiect was observed. 

The Effect of Polymer Type on Film Morphology. At relatively high polymer concentrations, all types of polymers used, and their blends formed continuous films and provided excellent spectra. Blend composition and the presence or absence of crystallization did not cause Christiansen efiect distortions. However, at concentrations usual for SEC analysis, all polymers showed evidence of not wetting the KBr surface, yielded discontinuous films, and often had poor spectra. These results indicate, with respect to spectral distortions, that the main importance of polymer type is its infiuence on continuous-film formation. 

Phase I work revealed that a sufficient condition for obtaining spectra without the Christiansen eflFect distortion was that the deposited film has no areas bare of polymer. When such a continuous film was obtained, other microstructural features and even polymer type were of secondary importance and were possibly even unimportant in obtaining good quality spectra. It was possible to obtain undistorted spectra from discontinuous films. However, particle size range of the deposit appeared critical to the results. The diversities of polymer morphologies possible and their sensitivity to operating conditions meant that tailoring particle size would be very difficult. Deposition control to ensure continuous films appeared as a much more attainable objective and was pursued in Phase II. 

Figure 5. Influence of surface nucleation density on crystal morphologyBoth films were grown on Si in HFCVD under the same conditions except for different pretreatments (a) scratched with diamond paste and having a nucleation density of about 10 cm unscratched and having a nucleation density of about 10 -10 cm. The much lower nucleation density in 0) leads to the much larger crystallites than those in (a) and the discontinuous film. (Reproduced with permission.)...

Doering et al. (339) have studied CO chemisorption at 60°C over Pt/ mica catalysts prepared by vapodeposition. For discontinuous films of d = 1.6 nm, two peaks were found by TPD, one at 120°C and one at 175°C. For larger particles the peak at 175°C disappears, leading to the conclusion that the more strongly adsorbed CO is held on the small particles. It is not clear whether the contradiction between these results (339) and those discussed above (340, 341) is due to the different supports. 

J.E.E. Baglin, G.J. Clark, and J.F. Ziegler. Catalyst Metal-Support Interactions. Rutherford Backscattering Spectrometry Applied to Discontinuous Films. Nud. Instrum. Methods Phys. Res. 218 445 (1983). 

Examples of amorphous solids are glass, tar and the naturally occurring gums and resins. Such resins formed the basis of many early paints, because the distinctive property of an amorphous solid is that, when a melt is poured into a tray, or when a solution of the solid is poured upon a surface and the solvent is allowed to evaporate, the resultant solid forms a continuous film. Because it has no natural shape of its own, it takes the shape into which it has been cast. A crystalline material would give a discontinuous film, consisting of many hundreds of tiny individual crystals. It is the amorphous continuity that makes the resin film extremely suitable for protecting surfaces. 

The alkoxysilane groups are considered to hydrolyze to form silanol groups, which can then react with the metal oxide/hydroxide surface. Further hydrolyzation can give a continuous or a randomly discontinuous film at the metal surface, see Fig. 2. 

Water break The appearance of a discontinuous film of water on a surface, signifying non-uniform wetting and usually associated with a surface contamination. 

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