Chain pullout

When M M, disentanglement is nearly instantaneous but approaches tro when M 8Me, which is the strain hardened (A. 4) upper bound for chain pullout without bond mpture. For welding, the relaxation times Trq refer to the minor chains of length /(/) such that the retraction time is approximated by Tro /(/). When M > 8Me, the chains cannot disentangle completely at the Rouse time and... 

Fracture Mechanisms the Transition from Simple Chain Pullout to... 

For very short chains, the surface analysis results point clearly at a pullout of the shorter PVP block from the PVP homopolymer. For all block copolymers where NpVP < NePVp the FRES results showed that all the dPS was on the PS side after fracture. Although the same result could have been obtained, in principle, if the copolymer fractured exactly at the joint, in reality when fracture does occur near the joint, a small amount of dPS is always found on the wrong side. Also the results with PS-PMMA diblocks clearly demonstrate chain pullout for short chains. 

This simple mechanism of chain pullout at the interface can be better visualized through a micromechanical pullout model [42] specifically developed for polymer glasses. Similar pullout models have also been developed for elastomer interfaces [43,44]. 

The chain pullout model developed by Xu et al. [42] is described schematically in Fig. 8. A chain, embedded in the polymer, is pulled at one end with a force /which is larger than the critical value/ = Nfmono below which chain pullout cannot occur fmono is a monomer friction coefficient. 

Let <5 denote the length of the connector chain pulled out from the polymer by a force / as shown in Fig. 8. When 6=1, where l is the total connector chain length, the chain is completely pulled out and the force/vanishes. Assuming that the chains are pulled out normal to the interface so that the tangential component of the pullout force is zero, then a, the traction stress component normal to the planar interface, is related to the force/and 2, the number of chains crossing a unit area of the interface, by ct=/2. a is related to the rate of chain pullout <5(f, x) and the remaining chain length l - by ... 

Fig.12. Transition from chain pullout to crazing for a 90-580-90 PVP-dPS-PVP triblock copolymer at the interface between PS and PVP. At 1 1 = 0.07 chains/nm2, gc increases discontinuous from 8 to 25 J/m2. a gc as a function of 1 and b fraction of dPS on the PS side after fracture. Data from [46]...

The value of 2 can be used to find a chain scission force//, = arraZ(,/2 if the crazing stress ocraze is known, using reasoning similar to that used for the transition from chain pullout to crazing considered in the previous section. This fracture force should be characteristic of the fracture of a C-C bond and therefore not only independent of N, but also independent of the detailed molecular structure of the polymer. This appears to be indeed the case and the results obtained for several experimental systems [22,37,39,40,46] all yield a very similar value of/, -2x10 N. 

In the low 2 regime, where crazing is not active, the 290-470-290 triblock fails by chain scission. Clearly, in this case, the 460 middle block of the small triblock cannot be considered as two equivalent PS blocks of 230 since an interface reinforced by such chains would have failed by simple chain pullout. Intuitively, this result is not surprising as the geometry of pulling out a loop may require breaking a bond and thus should be more difficult than pulling out two linear chains.1... 

For < 6 nm, Qc is low and presumably the failure mechanism is simple chain pullout or simple chain scission with no plastic zone being formed ahead of the crack tip. 

The map can be divided into four main domains, as shown in Fig. 53. For values of N/Ne l, the dominant mechanism of fracture will always be chain pullout and Qc will be given by Eq. (8). The N/Ne domain, where simple chain pullout is active, decreases however with increasing 2. The boundary between simple chain pullout and crazing with chain pullout is given by ... 

Brown HR (1994) Chain pullout and mobility effects in friction and lubrication. Science 263 1411-1413... 

The problem of evaluating the fracture energy Gic of polymer interfaces is represented in Fig. 1. Material A is brought into contact with material B to form an A/B interface, the weld is fractured and the strength is related to the structure of the interface through microscopic deformation mechanisms. If interpenetration is limited, chain pullout and perhaps chain fracture may occur. Such an interface is weak and may be modelled using the Nail Solution discussed below. 

This model is excellent when applied to very weak interfaces (Gic 1 J m ) where most of the fracture events occur in a well-defined two-dimensional plane and chain pullout dominated the fracture process. However, it is not applicable to strong interfaces (Gic 100-1000 J m ) where the fracture events occur in a three-dimensional deformation zone at the crack tip. Here, the vector percolation model is appropriate. 

Temperature of Chains Being Puiied Out Through an estimate of the frictional forces involved in the chain pullout, the temperature of the polystyrene chain pullout was found to be between 150 and 250°C. This is thought to be the temperature of the chain being pulled out, not of the surroundings. 

The quantity Wa is a lower bound value, since chain scission and chain pullout are ignored. In fact, the Wa values shown in Table 12.2 are some 500 times... 

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