Entanglements loss

In the two classic viscometric deformations of simple shear and extension, the appropriate components of Q have very different behaviour. For small shear strains, the shear stress depends on the component Q which has the linear asymptotic form 47/15. This prefactor is the origin of tne constant v in the tube potential of Sect. 3.For large strains, however, Qxy 7 and therefore predicts strong shear-thinning. Physically this comes from the entanglement loss on re-... 

An alternative pathway for entanglement loss is chain scission (Fig. 3.2, process B), in which a covalent bond along the polymer main chain is broken and a stress-bearing, otherwise elastic, chain is lost. Chain scission reactions, for example, homolytic carbon-carbon cleavage, have obviously high activation energies. The stress-free rates of these reactions are therefore typically extremely low. 

In polystyrene air crazes the predominant mechanism of entanglement loss seems clear. It is observed that both the uniaxial stress for crazing and the craze fibril... 

It is convenient to consider as a reference state the cylinder of isotropic undeformed polymer (diameter Dg) that will subsequently be drawn into the fibril. The number of entangled chains that cross a given crossectional area of the undeformed cylinder is the density of chains crossing unit area Vgd/2 times the area of the cylinder 7tDo/4. Only a fraction q = v vg of these chains however survives the geometrically necessary entanglement loss that accompanies fibril surface formation. As before the upper limit of this fraction is approximately given by Eq. (39) for Dg > d and by... 

Fig. 14.15. Schematic of craze widening at the craze-bulk interface, illustrating the geometrically necessary entanglement loss.

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