Preferential surface segregation

The Existence of an Unusual Reversible Deactivation Phenomenon Associated with Preferential Surface Segregation in Bimetallic Systems... 

A number of fuel cell catalysts have been synthesized in this maimer, such as Pt colloids, Pt/Sn colloids, and Pt/Ru colloids of different Pt to Ru ratios. A drawback of the Boennemann synthesis method is that oxidative removal of the stabilizer molecule requires temperatures higher than 300 °C [69]. High-temperature treatment of Pt-based catalysts should be avoided as changes in the structure, namely preferential surface segregation of Pt or RUO2, typically take place (this is further discussed in Section 9.4.1). To the best of our knowledge the Boennemann method has not been used to make catalysts of the same composition but different sizes. 

These studies demonstrate that the superficial composition of the perovskite is very important and surface content/distribution of the cations may influence the oxidation process. In this respect, the use of low-energy ion scattering as a recent advanced surface characterization technique allowed the direct analysis of the outermost surface layer of such materials with an escape depth of 0.3 nm [17]. Results collected from these characterizations show that the chemical nature of the cations is the factor determining the preferential surface segregation of one of the components [18]. 

Other recent investigations involving AES, often with depth profiling, deal with the surface segregation of Ag in Al-4.2 % Ag [2.163], of Sn in Cu and formation of superficial Sn-Cu alloy [2.164], of Mg in Al-Mg alloy [2.165], and of Sb in Ee-4% Sb alloy [2.166]. Note the need to differentiate between, particularly, segregation, i. e. original sample properties, from the artifact of preferential sputtering. 

In polymer blends, or mixtures, the primary question is whether one of the components segregates preferentially to the surface. One of the reasons this is of interest is because most commercial polymers contain more than one component and a surface segregation of one of the components from a miscible mixture during, for example, extrusion of the material, could affect the surface finish of the product. Because polymer blends are generally dense liquids, from the previous discussion it is clear that packing effects are expected to dominate the surface properties. 

The surface segregation from the mixture of chemically identical polymers with chain length disparity is predicted by another model [198-200]. It represents the spatial conformation of a polymer coil as a random walk reflected by an external interface. The associated loss in system configurations is minimized when shorter chains are adsorbed at the surface. Preferential surface seg-... 

In a description of sputtering from a multicomponent system, the influence of preferential sputtering and surface segregation must be included. For a homogeneous sample with atomic components A and B, and in the absence of surface segregation, the surface concentration, IVs, is equal to that in the bulk, Nb. Therefore, at the start of sputtering... 

It is known from the theory of surface segregation in bimetallic alloys that, in certain systems, preferential surface enrichment in one element is so strong that it leads to a skin structure,... 

The above mechanism was consistent with several experimental observations. MgO was known to be preferentially formed on the surface of Al-Mg alloys under many circumstances, as described earlier, either because of cation demixing in spinel, to form periclase, or due to rapid diffusion of Mg in the solid state and subsequent surface segregation. The single crystal oxidation product clearly precluded any mechanism that involved repeated nucleation of corundum (alumina) grains, which in any event is known to he difficult at temperatures below 1200°C. Finally, the presence of a film of molten alloy at a distance of about 1 pm from the surface accounted for the relative absence of a growth rate dependence on composite thickness (a rather slow decrease is actually observed). 

Within this context, this chapter discusses the possibilities to produce surface patterns by surface segregation of an additive towards the interface in polymer blends. Surface segregation is the result of the preferential migration of one blend component to the interface thereby inducing selective enrichment at the nearsurface level. As will be discussed below in detail, this effect is directed by the surface thermodynamics that favors the presence at the interface of the component of a mixture lowering the surface tension. As a consequence, this phenomenon is the cause of having large differences between the surface and the bulk composition. 

Hyperbranched and comb polymers have also been used as surface active additive. Ariura et al. synthesized by combination of anionic and cationic polymerization a monodispersed hyperbranched polystyrene [73]. The authors proved by combination of DSIMS and neutron reflectivity the preferential surface enrichment of the branched protonated macromolecules when blended with its deuterated linear polystyrene counterparts with the same molar mass. Other systems involving the segregation of the branched macromolecules in binary blends were demonstrated such as in polyamide [74] or poly (methylmethacrylate) [75]. 

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