Segregation surface

Because of its low surface free energy, PDMS tends to migrate to surfaces and interfaces. For example, in blends of PDMS with low-density polyethylene or linear low-density polyethylene, - the surfaces are enriched with the hydrophobic methyl groups from the PDMS chains. The same is true for blends of PDMS with polypropylene and poly (3-hexylthiophene)  

Similar enrichment of siloxane units occurs at the surfaces of block copolymers or even random copolymers containing PDMS sequences. Examples are poly(dimethylsiloxane-i-ethylene oxide) and poly(dimethylsiloxane-f)-imide) multi-block copolymers, poly(dimethylsiloxane)/imine copoly-mers, and poly(urethane-dimethylsiloxane) anionomers. Other materials, such as long-chain alkenes and highly branched polymer additives exhibit surface segregation. Much of the information on these changes in surface composition has been obtained by measurements of contact angles. 

The profile ( (z) of the surface segregated layer is predicted by a mean field theory to follow the formula (obtained by integrating Eq. 25)  

It is described entirely by two bulk % and ((O and one surface ( )s) parameters. With surface concentration ( )s being close to the bulk value the parameter Affr rlO may be approximated by the bulk correlation length , (Eq. 6) Af(4 x 2( boo)k(([) )(4 - [ c)2- In such a Emit (valid strictly for 00 ] 1) an exponential behavior of the surface profile is obtained  

This formula may also well approximate the strict solution (Eq. 30) for larger values of bulk concentration but in this case its decay length is no longer equal to the bulk correlation length The mean field theory also makes a prediction for the total surface excess z  

All direct depth profiling techniques used to study the surface segregation from binary polymer mixtures have a depth resolution [29] p limited to some 5-40 nm HWHM (half width at half maximum of the related Gaussian function). They cannot observe the real composition profile )(z) (for the sake of comparison mimicked by mean field prediction (dashed line) in Fig. 16a) but rather its convolution (solid line in Fig. 16a) with an instrumental resolution function characterized by p. The total surface excess z however provides a good parameter, independent of resolution, as it has been concluded based on experimental data obtained using different direct techniques [170]. 

in a commonly used procedure [16, 92, 120, 145, 165, 167, 170, 175], each point of the segregation isotherm data z, , ) is analyzed with Eq. (32) to find out the surface concentration ((J) ) assigned by the mean field approach (analogous procedure exists in a self-consistent mean field model [166, 174]). Then, for each such pair (c, c )s) the trajectory 2(icAf)1/2 vs ([) is plotted (for the value (IO and its value is read out at surface concentration s (=c )se or (j)sd in Fig. 15). This value is equal to the surface energy derivative (-dfs/dc ))s at concentration )s. Such a procedure, repeated for each z ((j)00) data point, yields the concentration dependence of the composition derivative of the short-ranged bare surface energy (-dfs/dc ))s vs )s (see Fig. 16). 

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. 

Most commercial polymer systems are a complex mixture of one or more polymers, plasticizers, antioxidants and processing aids. The surface of such a system will therefore not necessarily be determined by the dominant polymer but will often be influenced by the segregation of low molar mass and low 

It also follows that the surface excess obtained using the classical Gibbs adsorption isotherm is given by 

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The principal applications of REELS are thin-film growth studies and gas-surface reactions in the few-monolayer regime when chemical state information is required. In its high spatial resolution mode it has been used to detect submicron metal hydride phases and to characterize surface segregation and difRision as a function of grain boundary orientation. REELS is not nearly as commonly used as AES orXPS. 

Future trends will include studies of grain-dependent surface adsorption phenomena, such as gas-solid reactions and surface segregation. More frequent use of the element-specific CEELS version of REELM to complement SAM in probing the conduction-band density of states should occur. As commercially available SAM instruments improve their spot sizes, especially at low Eq with field emission sources, REELM will be possible at lateral resolutions approaching 10 nm without back scattered electron problems. 

Figure 6.8. Compositional map made with an early model of the scanning electron microprobc. The pictures show the surface segregation of Ni. Cu and Sn dissolved in steel as minor constituents the two latter constituents enriched at the surface cause hot shortness (embrittlement at high temperatures), and this study was the first to demonstrate clearly the cause (Melford I960).

THEORY OF SURFACE SEGREGATION SELF-CONSISTENT DETERMINATION OF THE CONCENTRATION PROFILE... 

Ab-initio studies of surface segregation in alloys are based on the Ising-type Hamiltonian, whose parameters are the effective cluster interactions (ECI). The ECIs for alloy surfaces can be determined by various methods, e.g., by the Connolly-Williams inversion scheme , or by the generalized perturbation method (GPM) . The GPM relies on the force theorem , according to which only the band term is mapped onto the Ising Hamiltonian in the bulk case. The case of macroscopically inhomogeneous systems, like disordered surfaces is more complex. The ECIs can be determined on two levels of sophistication ... 

Here, we will first briefly recall the principles of this method in the case of transition metals. Then we will apply it to two illustrative examples the surface segregation energy of an impurity is a pure host and the growth of adislands on FCC(lll) surfaces of the same chemical species. 

We will limit ourselves to the surface segregation energy of an impurity of atomic number Z + 1 in a BCC matrix of atomic number Z and study the variation of this energy as a function of the number Nj of d electrons per atom in the d band of e transition metal Z. 

This case is particularly interesting since the surface segregation energy can be directly compared to surface core level binding energy shifts (SCLS) measurements. Indeed, if we assume that the excited atom (i. e., with a core hole) is fully screened and can be considered as a (Z + 1) impurity (equivalent core approximation), then the SCLS is equal to the surface segregation energy of a (Z + 1) atom in a Z matrixi. in this approximation the SCLS is the same for all the core states of an atom. 

In the case of W(H0) (Nd=4.4eVatom), we have also calculated the modification of the surface segregation energy of a Re impurity when a p(2 x 1) overlayer of oxygen is present at the surface (Eig. 3). Then, there are two geometrically inequivalent atomic rows, labelled a and b, of W atoms on the surface (and in the sublayers). However, the modification of their effective atomic levels relative to the bulk is vanishingly small beyond the second... 

G. Abramovici, M. C. Desjonqu res and D. Spanjaard, W Surface Core Levels Shifts of O/W(110) Deduced from Surface Segregation Energies, /. de Physique 15 907 (1995)... 

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

Cd + Bi alloy electrodes (1 to 99.5% Bi) have been prepared by Shuganova etal. by remelting alloy surfaces in a vacuum chamber (10-6 torr) evacuated many times and thereafter filled with very pure H2. C dispersion in H20 + KF has been reported to be no more than 5 to 7%. C at Emin has been found to be independent of alloy composition and time. The Emin, independent of the Bi content, is close to that ofpc-Cd. Only at a Bi content 95% has a remarkable shift of toward less negative E (i.e., toward o ) been observed. This has been explained by the existence of very large crystallites (10-4 to 10-3 cm) at the alloy surface. Each component has been assumed to have its own electrical double layer (independent electrode model262,263). The behavior of Cd + Bi alloys has been explained by the eutectic nature of this system and by the surface segregation of Cd.826,827... 

Fig. 3. ESCA results on the surface segregation of the Polycarbonate homopolymer/Poly-carbonate-Polydimethylsiloxane segmented copolymer blends 1S0)...

Figure 4.28. STM image of a PtRh(lOO) surface. Although the bulk contains equal amounts of each element, the surface consists of 69% of platinum (dark) and 31 % of rhodium (bright), in agreement with the expected surface segregation of platinum on clean Pt-Rh alloys in ultrahigh vacuum. The black spots are due to carbon impurities. It is seen that platinum and rhodium have a tendency to cluster in small groups of the same elements.

Surface composition. The principle of surface segregation in ideal systems is easy to understand and to derive thermodynamically the equilibrium relations (surface concentration Xg as a function of the bulk concentration Xb at various temperatures) is also very easy (4,8). Even easier is a kinetic description which can also comprise some of the effects of the non-ideality (9). We consider an equilibrium between the surface(s) and the bulk(b) in the exchange like ... 

In this section, we will first demonstrate the formation of methylchlorosilanes from CH3 + a monolayers on CuaSi surfaces. The effects of promoters and the effect of surface segregation on the reaction rate and selectivity are scussed subsequently. 

In our early work with bimetallic systems, we noticed that, depending on the preparation procedure in UHV, different surface compositions could be produced over the same bulk material owing to the phenomenon of surface segregation [Stamenkovic et al., 2002]. It was essential, then, to establish a methodology for transferring a well-defined bimetallic system into an electrochemical environment for further electrochemical characterization. 

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