Fracture by disentanglement

Fracture by disentanglement occurs in a finite molecular weight range, McPercolation theory predicts that the critical draw ratio. 

A true melt fracture by definition must involve some kind of bulk failure in melt flow. Either massive chain disentanglement or chain scission or both must occur in the bulk. Such a real cohesive breakdown away from the surface may also produce extrudate distortion. In general, other forms of irregular extrudate distortion do occur at high stresses. Only direct flow visualization may reveal the origin of such extrudate distortions. 

For both the PS-PVP and the dPS-COOH epoxy systems the fact that a substantial fraction of dPS is found on the non-PS side of the interface after fracture at the maximum Qc shows that at least some chains in the craze fibrils fail by disentanglement, i.e.f < fb. 

Fracture of the weld occurs by disentanglement of the minor chains, or bond rupture. In the VP theory, many properties follow scaling laws dependent on the difference between p, the occupational probability of the lattice and the percolation threshold Pc, the minimum concentration (occupational probability) at which connectivity could occur in an infinite lattice. In particular, the fracture toughness, which depends on this percolation term, Gic [p - Pc], determines the number of bonds to be broken, or disentangled. 

Thus, fracture occurs by first straining the chains to a critical draw ratio X and storing mechanical energy G (X — 1). The chains relax by Rouse retraction and disentangle if the energy released is sufficient to relax them to the critically connected state corresponding to the percolation threshold. Since Xc (M/Mc) /, we expect the molecular weight dependence of fracture to behave approximately as... 

The fracture resistance of a homopolymer is controlled by (i) the number of tie-molecules, which act as stress transducers between the crystallites and (ii) by the disentanglement resistance of the individual chains [112-115]. The higher the molecular weight, the more likely both factors are high fi-nucleated polypropylene does not infringe this rule [33,72,74,116]. 

The kinetics of generation and evolution of the excited extended chemical bonds is also investigated. The activation energy for their generation was shown to be equal to that for macroscopic deformation and fracture. This result implies that the formation of elementary nuclei of deformation and fracture is caused by the relaxation of excited bonds. Therefore, the question Which process in a stressed solid is the first, local deformation or fracture has been disentangled at last. The initial process in a stressed solid is the generation of excited bonds, and its rate controls the rates of both deformation and fracture. 

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