Interface convolution mechanism

Table 2). The Code F material exhibits craze growth by the systematic cavitation mechanism whereas the Code G materials has switched to the interface convolution mechanism which has growth rates over a decade lower, as Fig. 9b shows. This slower growth rate manifests itself in the higher flow stresses for the Code G material. 

For comparison of model predictions with experimental measurements three sets of experimental information were used for craze velocity dc/dt vs. applied stress (Too at 293 K (1) PS/PB diblock with c = 0.18 and p = 39.5 nm (code B) (2) PS/PB diblock with c = 0.058 and p = 20—25 nm (codes L and M) and (3) PS/PB diblock with c = 0.11 (for both 293 and 253 K) (code F). The actual velocity measurements, not given here, can be found in Schwier (1984). For contrasting the craze-growth rate based on PB phase cavitation discussed in Section 11.10.3 with an interface-convolution mechanism in a reference homo-polystyrene eqs. (11.50) and (11.51) were used together with values of PS material parameters listed in Table 11.4. Figure 11.23 shows the actual craze velocity for code B. Here the model prediction for the diblock according to the developments in Section 11.10.3... 

Combining Eqs. (54), (55) and (57) and using the same approach to the establishment of the equivalent plastic resistance of the deforming polymer that was introduced in connection with the mechanism of craze growth by the interface convolution process, we write the craze velocity to be... 

Noting the possibility that a variant of the meniscus instability of Taylor (1950) could be the mechanism of craze advance, Argon and Salama (1977) proposed a continually repeating interface-convolution model shown in Fig. 11.16 as the... 

Rg. 11.16 A sketch of the mechanism of craze-matter production in a homo-polymer by a recurring interface-convolution process (Taylor-meniscus instability) (a) side view of the outline of the craze tip (b) top view of craze front (c) and (d) advance of the craze front by a completed period of interface convolution, with pinch-off (from Argon and Salama (1977) courtesy of Taylor and Francis). 

Figure 12.15 Schematic diagram showing craze matter production by the mechanism of meniscus instability (a) outline of a craze tip (b) cross-section in the craze plane across craze matter tufts (c, d) advance of the craze front by a completed period of interface convolution. (Reproduced with permission from Argon, Hanncosh and Salama, in Fracture 1977, Vol. 1, Waterloo, 1977, p. 445)...

Charge transfer reactions at ITIES include both ET reactions and ion transfer (IT) reactions. One question that may be addressed by nonlinear optics is the problem of the surface excess concentration during the IT reaction. Preliminary experiments have been reported for the IT reaction of sodium assisted by the crown ether ligand 4-nitro-benzo-15-crown-5 [104]. In the absence of sodium, the adsorption from the organic phase and the reorientation of the neutral crown ether at the interface has been observed. In the presence of the sodium ion, the problem is complicated by the complex formation between the crown ether and sodium. The SH response observed as a function of the applied potential clearly exhibited features related to the different steps in the mechanisms of the assisted ion transfer reaction although a clear relationship is difficult to establish as the ion transfer itself may be convoluted with monolayer rearrangements like reorientation. 

The craze front velocity v can be governed by one of two distinct mechanisms of craze matter production. As Argon and Salama have discussed in detail, under the usual levels of service stresses or stresses under which most experiments are carried out, craze matter in single phase homopolymer is produced by the convolution of the free surface of the sohd polymer at the craze tip. This occurs by a fundamental interface instability present in the flow or deformation of all inelastic media when a concave, meniscus-like surface of the medium is being advanced locally by a suction gradient. This is the preferred mechanism of craze advance in homopolymers. In block copolymers with uniform distributions of compliant phases of a very small size, and often weaker interfaces than either of the two phases in bulk, craze advance can also occur by cavitation at such interfaces to produce craze matter as has been discussed by Argon et al. Both of these mechanisms of craze advance lead to very similar dependences of the craze front velocity on apphed stress and temperature that is of the basic form... 

Formation of durable chemical bonds is an obvious means to stabilize the interface and has been demonstrated for phenolic/alumina joints [25] and for silane coupling agents [26,27]. However, for most structural joints using epoxy adhesives and metallic adherends, moisture-resistant chemical bonds are not formed and mechanical interlocking on a microscopic scale is needed between the adhesive/primer and adherend for good durability. In these cases, even if moisture disrupts interfacial chemical bonds, a crack cannot follow the convoluted interface between the polymer and oxide and the joint remains intact unless this interface or the polymer itself is destroyed. 

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