Surface properties segregation effects

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. 

At the higher metal level (2.0-4.5% Ni with up to 2% Sb) used to study artificially contaminated materials, XRD results have shown the formation of Ni-Sb alloys (NiSb x<0.08) whereas XPS data have indicated that a non-reducible antimony oxide, a well dispersed reducible Sb phase together with reducible Sb (that form an alloy with reducible Ni), were present. Selective chemisorption data for unsupported Ni-powders showed that one surface structure can effectively passivate 2-3 Ni atoms with respect to H2 chemisorption. XPS examination confirmed that Sb segregates at the surface of Ni particles where it can drastically affect the electron properties of neighboring Ni atoms thus reducing their activity. 

Surface segregation, which is also a common phenomenon in other materials, has received limited attention and most of the studies applied this concept for the control of the surface chemical composition. Moreover, surface segregation has been typically considered as a non-desirable effect. This is particularly true in the case of materials with precise bulk properties provided by the presence of different additives such as plasticizers or UV-absorbers. The segregation of these additives towards the interface modifies the bulk properties and can provoke large variatiOTis on their mechanical behavior. Several contributions have explored this phenomenon in order to reduce the amount of oligomers/polymeric additives that bloom to polymer surfaces [13]. However, in many other applications it is desirable to have surface properties that vary to a large extent from those found in the bulk. For example, to favor adhesion or increase the wettability, or the opposite, i.e., increase the hydrophobicity, to improve the biocompatibility of commercial polymers or to enhance the chemical resistance. 

Wong, D.A., O Rourke-MuisenCT, P.A.V., Koberstein, J.T. Effect of chain architecture on surface segregation in functiraial polymers Synthesis and surface properties of end- and center-functional poly(D,L-lactide). Macromolecules 40, 1604-1614 (2007)... 

Surface segregation takes place in practically all metal alloys and is controlled by the chemical equilibrium between the near-surface layers and the bulk. Consequently, a successful theoretical description of this phenomenon demands a consideration of both bulk and surface properties in order to understand correlations between segregation profile, atomic structure, SRO, and temperature. For this reason, the basics of the alloy s bulk properties have to be discussed (Section 11.2) before considering the surfaces and their experimental (Section 11.3.1) as well as theoretical characterizations (Sections 11.3.2 and 11.3.3). In Section 11.3, we will introduce the methods that are in general applied to alloy surfaces. Special focus will be on a very new ab initio-based description that allows for a direct prediction of the segregation profile and the mentioned correlated parameters. This concept will then be applied to two different classes of alloy phases an intermetallic compound and a disordered alloy. The last example will demonstrate which possible effects will take place if an adsorbate comes to the surface. Besides changes in the atomic position of the surface atoms (the so-called adsorbate-induced surface reconstruction),... 

Structural properties of materials Sub-lattice Substrate Surface phonoas Surface defects m transition metals Surface segregation SupeqDlastic properties and lic[uid phase effect Susceptibility... 

---