Surface elastic properties

Opdahl and Somorjai studied the surface deformation and surface elastic properties of stretched polyethylene in a device depicted in Fig. 3.73. It was found that the surface textures of both HDPE and LDPE changed and that the nodular structures present at the surface lengthened in the direction of the stretch and contracted perpendicular to the stretch at various elongations. This resulted in a roughening of the surface. 

Vincent A, Babu S, Seal S (2007) Surface elastic properties of porous nanosiUca coating by scanning force microscopy. Appl Phys Lett 91 161901-3... 

Probably these regions correspond to the second phase - MgAl204, defined by X-ray. It is proven by the combined structural analysis in "height" and "mag-cos" modes (figure 10). Clear peaks on the "mag-cos" curve (fig. 10, c) and corresponding smooth relief contour show the invariation of the block surface elastic properties and the presence of the doped... 

Quantitative and qualitative methods were developed to measure the surface mechanical properties of polymers by atomic force microscopies. They were used to study the effects of molding processes and of viscosity on the surface morphology of polypropylene / (ethylene-propylene) copolymer blends (PP/EP). On compression-molded "physical blends", EP nodules are present at the outermost surface while, on injection-molded "reactor blends", they are covered by a PP layer. Resins with high viscosity ratio between EP and PP present heterogeneous surface elastic properties corresponding to the dispersion of spherical EP nodules below the surface. The low viscosity ratio resins have homogeneous surface elastic properties comparable to those measured above EP nodules on high viscosity ratio resins. This is compatible with a fine dispersion of plate-like shaped EP nodules below the surface... 

It was previously shown that atomic force microscopy, AFM, and force modulation microscopy, FMM, can bring novel informations concerning the elastic and the viscoelastic properties of PP/EP resins at the microscopic level. They enabled the mapping of EP rubbery nodules distribution and the explanation of the macroscopic behavior of these resins (e.g. differences in the impact resistance) (5). Moreover, recent works showed that the use of AFM techniques (force-curves and force-modulation measurement, FMM cartography) can be used to characterize the surface elastic properties of PP/EP blends (6). Especially, these studies demonstrated that I M can map the surface and the subsurface distribution of EP nodules. 

In the present paper, the techniques used to measure the surface elastic properties will be briefly described. Then, the application of these techniques to the study of PP/EP surfaces will be presented. First, the surface distribution and morphology of EP nodules at the surface of PP/EP compression-molded physical blends and of injection-molded reactor blends will be compared. Second, the effect of EP vs PP viscosity ratio on the surface distribution and morphology of EP in injection-molded PP/EP will be studied. 

These results combined with those obtained by TEM show that the resin with the highest EP/PP viscosity ratio present heterogeneous surface elastic properties corresponding to the rough dispersion of almost spherical EP nodules below the surface. On the contrary, the low viscosity ratio resin presents homogeneous surface elastic properties at the resolution of our measurements (> 100 nm). The measured surface rigidity is comparable to that measured above EP nodules on the high viscosity ratio resin. This could be explained by the easier deformation of the EP nodules into platelets and by their fine dispersion below the thin PP surface layer. 

Implicit in all these solutions is the fact that, when two spherical indentors are made to approach one another, the resulting deformed surface is also spherical and is intermediate in curvature between the shape of the two surfaces. Hertz [27] recognized this concept and used it in the development of his theory, yet the concept is a natural consequence of the superposition method based on Boussinesq and Cerutti s formalisms for integration of points loads. A corollary to this concept is that the displacements are additive so that the compliances can be added for materials of differing elastic properties producing the following expressions common to many solutions... 

We therefore have qualitative evidence for the dependence of the dewetting speed on the elastic properties of the substrate. Dependence of wetting on the elastic modulus was previously suggested in the case of thin substrates [31], It may be conjectured that cross-linking affects the surface properties of the elastomer and, therefore, wettability. However,... 

At the instant of contact between a sphere and a flat specimen there is no strain in the specimen, but the sphere then becomes flattened by the surface tractions which creates forces of reaction which produce strain in the specimen as well as the sphere. The strain consists of both hydrostatic compression and shear. The maximum shear strain is at a point along the axis of contact, lying a distance equal to about half of the radius of the area of contact (both solids having the same elastic properties with Poisson s ratio = 1/3). When this maximum shear strain reaches a critical value, plastic flow begins, or twinning occurs, or a phase transformation begins. Note that the critical value may be very small (e.g., in pure simple metals it is zero) or it may be quite large (e.g., in diamond). 

As reported in this chapter, the microscopic origin of both compressibility and elasticity of dense emulsions is rather well understood. Emulsions have elastic properties arising from either surface tension or surface elasticity and plasticity. Some protein-stabilized emulsions obey the same phenomenology as solid-stabilized emulsions they exhibit substantially higher osmotic resistances and higher shear moduli than surfactant-stabilized emulsions [38 0]. Moreover, they are strongly resistant to water evaporation. Proteins possess the ability to form... 

As is known, if one blows air bubbles in pure water, no foam is formed. On the other hand, if a detergent or protein (amphiphile) is present in the system, adsorbed surfactant molecules at the interface produce foam or soap bubble. Foam can be characterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase volume. The foam, or the lamina of liquid, will tend to contract due to its surface tension, and a low surface tension would thus be expected to be a necessary requirement for good foam-forming property. Furthermore, in order to be able to stabilize the lamina, it should be able to maintain slight differences of tension in its different regions. Therefore, it is also clear that a pure liquid, which has constant surface tension, cannot meet this requirement. The stability of such foams or bubbles has been related to monomolecular film structures and stability. For instance, foam stability has been shown to be related to surface elasticity or surface viscosity, qs, besides other interfacial forces. 

The obtained Ao gi = 5.7 x 10 is even larger than the value of Acr (Cu) X (= 4.7 X 10 A ), and of the hypothetical Co—Cu crystal with intermediate elastic properties than bulk cobalt and copper (4.1 x lO" A ). The derived effect of the effect of the lower coordination of the surface atoms on the mean-square relative displacement (perpendicular vs. parallel motions) is 1.4 times larger amplitude of the perpendicular vs. parallel motions, in agreement with lattice dynamics calculations. This SEXAFS study has produced a measure of the surface effect on the atomic vibrations. This has been possible due to the absence of surface or adsorbate reconstruction (i.e. no changes in bond orientations with respect to the bulk) and of intermixing. 

Many materials whose elastic properties are of interest are anisotropic, so the surface wave velocity depends on the direction of propagation. In order to be able to make measurements in one direction at a time, a lens with a cylindrical... 

If pulses can be generated and detected whose length is short compared with the time difference between reflections from the top and the bottom surfaces of a layer, then the elastic properties of the layer can be deduced from the amplitude and timing of the two echoes. The return pulses from such a situation are illustrated in Fig. 8.10. Figure 8.10(a) is an oscilloscope trace of the reference echo from the substrate at defocus z0 and with nothing on it except the coupling fluid. We can choose to write the reference signal as... 

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