Molecular disentanglement

The yield strength does not change with catalyst content. In spite of Dp decay to one third of the initial value for 1.0% catalyst (Fig. 20.10), it remained constant. The relatively low deformation, involved in the yield process, makes this property less sensitive to molecular disentanglement (and therefore to matrix degradation) than elongation at break. 

For slow eraek growth to occur, the stress must be sufficiently low that the creep rate is lower than the rate of molecular disentanglement at the crack tip [15]. This translates to a critical stress above which the sample fails in a ductile mode and below which it fails in a brittle manner. A plot of time to failure (sample penetration) as a function of applied stress is shown in Figure 26. The mode of failure can be predicted by observung the initial material response to load if a craze immediately develops at the tip of the notch, the sample will ultimately fail in a brittle manner. 

Environmental stress cracking occurs faster than low stress brittle failure when the rate of molecular disentanglement is increased by local interactions between the polymer and the solvent. Swelling of the noncrystalline regions at the notch tip increases the local free volume. This plasticization accelerates seg-... 

The maximum molecular weight M at which disentanglement can occur is determined when strain hardening occurs at Xc 4 such that... 

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 melt index (MI) or melt flow index (MFT) is an inverse measure of viscosity. High MI implies low viscosity and low MI means high viscosity. Plastics are shear thinning, which means that their resistance to flow decreases as the shear rate increases. This is due to molecular alignments in the direction of flow and disentanglements. 

Fig. 3 Difference in crystallization behavior between an atomic or short chain molecular system and a polymer system, a Atoms or short chain molecules can be independently rearranged on each lattice point, while b the order of the repeating units within a polymer chain is maintained during the rearranging process. Therefore, a chain should slide along its chain axis and disentangle for rearrangement onto the lattice points...

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