Interfacial separation

FIG. 7 Total energy per cross-sectional area as a function of interfacial separation between Fe and A1 surfaces for the clean interface and for monolayer interfacial impurity concentrations of B, C, N, O, and S. Graph (a) is for the case where the impurity monolayer is applied to the free A1 surface prior to adhesion, while graph (h) has the impurity monolayer applied to the free Fe surface prior to adhesion. The curves fitted to the computed points are from the universal binding energy relation. (From Ref. 28. Copyright 1999 hy the American Physical Society.)... 

Competition between matrix plasticity and grain boundary/interfacial separation due to the flow of the glass phase. 

Multiphase flow theory is very well developed because of applications such as interfacial separations, petroleum production, and vadose-zone hydrology yet, it remains a mostly empirical science. This is a consequence of the complex physics of flow and because the parameters describing multiphase flow are highly sensitive to pore structure and interfacial configurations. Though three-phase flow is important in applications such as oil production and contaminant transport in the subsurface, we limit the following discussion to two-phase flow for simplicity. 

The crystalline lamellar organization of PP was clearly observed in the PP dominant matrix of the undrawn TPV. The TEM images of the drawn TPV sample indicated that the dispersed EPDM domains (dark portion in the image) are highly elongated along the extension direction, and the drawn process promotes a rather heterogeneous deformation in the matrix (Fig. 8.15). It should be noted here that the PP dominant matrix and the dispersed EPDM domains were completely bonded, and the interfacial separation between the matrix and the EPDM domains was not observed in the drawn TPV samples. 

It is clear that if the criterion (la) or (3a) holds for interfacial separation before fibril drawing (whether on account of AG being small or Oy being large) then there will not be a large dissipation of energy,, per unit advance of the macroscopic separation front, and there will be very little work for the applied force to do. So the force required to open up an interfacial crack, in brittle separation, will be small ... 

If, however, the criterion (lb) or (3b) holds, then the system can be treated by Griffith-Irwin (29,30) crack theory, as extended by Good (24) to interfacial separation. That treatment yields the expression. 

Since 0.004 1, this computation predicts clean interfacial separation. And such separation is commonly observed, with well-formulated pressure-sensitive tapes. 

For a system with a sharp interface and weak intermolecular interaction, as in the nonpolar-polar polymer system mentioned above, failure exactly at the interface is a distinct possibility (or even probability). Thus, interfacial separation may be expected when interfadal strength is weaker than the bulk strength of the bonded materials. As we have seen, if the intermolecular interactions across the interface are more specific (including chemical bond formation) or if significant interpenetration of polymer chains occurs, rupture at the interface becomes less likely. In some cases, the locus of failure may depend on the rate at which stress is applied rapid application leads to cohesive failure and very slow apphcation, tending more toward true adhesive failure (since slow application of stress gives more time for the entangled molecules to slide past one another). 

The lanthanide complexes lack distinct ultraviolet-visible spectra and, hence, kinetic information on their complex formation and dissociation reactions was obtained indirectly by the metallochromic indicator method. These studies indicate that in the case of the Cyanex 272 complexes, the CPC efficiencies are mainly limited by the slow dissociation of the M(HL2)HL complex at the heptane-H20 interface. In the case of the complexes of the acylpyrazolones, the CPC efficiencies are again limited by the dissociation of the lanthanide-pyrazolone complexes at the organic-aqueous interface with the rate-limiting step being the dissociation of the ML complex. It was also shown that because the dissociation reactions are interfacial separations, efficiencies can be dramatically improved by the addition of surfactants like Triton X-100 to the organic phase and by... 

Figure 15 (A) Frequency shift of photoluminescence piezospectroscopy (PLPS) measured at room temperature in the course of cyclic oxidation of EBPVD samples. (B) Progress of interfacial separation after different cycles at 1150°C. Redraw from [88],...

For cyclic oxidation, the roughness of the TOO increases with thermal cycles, and the interfacial separation gradually accumulates as a result of TOO rumpling (Fig. 15) [88]. Concurrently, the cracks and rumpling cause a steady decrease of the stresses in the TOO. It should be emphasized that the magnitude of the luminescence shift alone is insufficient to characterise the degradation of the TGO-BC interface, since the stresses in the TGO also depend on processing parameters, temperatures, bond coat, TGO microstructure etc [88]. For this reason, no unique relationship currently exists between the absolute value of the shift and the cyclic life. 

The force given by Eq. (10), when summed over all the filaments that are present at a particular instant, is equal to the macroscopic force of adhesion. But this will be true only if the force, Eq. (10), is less than the force required for interfacial separation, which we will now examine in detail. 

AG (J/m2) at different values of e ft for a filament Cohesive vs. interfacial separation Magnitude of adhesive strength... 

Gardon(35,36) has determined the peeling force and the mode of separation as a function of separation rate and of adhesive thickness, for cellophane films bonded together by an acrylic adhesive. He found a transition from cohesive separation to mixed or interfacial separation, with increasing crosshead speed and with decreasing thickness of the adhesive layer. A transition from cohesive to mixed or adhesive failure was also observed with decreasing temperature. These observations are in exact agreement with the predictions that we have just indicated above. Similar results have been reported by Kaelble.(i3,34)... 

To summarize in regard to Table 1 and Figure 11, it is clear that the qualitative prediction, that there will be a transition between (a) cohesive separation or interfacial separation, with a large force requirement, and (b) interfacial separation with a low force requirement, is in agreement with experiment. 

We may make a strong generalization at this point, for polymers that have high glass temperatures. If a mechanism exists for the initiation of interfacial or near-interfacial separation, then even at low levels of applied force, separation will occur, and the effective adhesion of the polymer to the solid will be low. 

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