Loss of entanglements

Rubinstein has constructed on a reptation-fluctuation approach a detailed self-consistent theory of constraint release, allowing each loss of entanglement in one chain to permit a random jump in the tube of another [37]. When this is done the form of predicted relaxation functions are in good accord with experiments. However, in monodisperse linear melts it appears that the fluctuation contribution is more important than constraint release. 

Under conditions where chain mobility is very low, such as at low temperatures, the loss of entanglements occurs by chain scission this is what happens for polystyrene air crazes. Of course, chain scission crazes (CSC) are MW-independent as soon as chains are long enough to be entangled (at too-low MWs, no craze can be formed, only cracks happen). 

Under conditions where chain mobility is high enough, typically at high temperature and low strain rate, the loss of entanglement in the active layer at the craze-bulk interface can occur by chain disentanglement, resulting in chain disentanglement craze (CDC). 

Here X ,ax is the single chain limiting extension ratio in the isotropic, unoriented polymer with the same entanglement weight. A complication is that the process of orientation above Tg may result in a loss of entanglement constraint, effectively increasing and One of the ways this loss can take place in the current versions of the tube model is by tube relaxation whereby the process of tube... 

The loss of entanglements (and the decrease in molecular weight due to chain scission) adversely impacts fibril stability. Fibril breakdown by localized creep should occur more rapidly in polymer crazes with low entanglement densities and small diameter fibrils. 

There are several other consequences of the model of craze growth which can be tested, however. The model suggests that the craze extension ratio should reflect the loss of entanglement density caused by the strand loss. There seems to be no doubt... 

Data for a 60 mole % PHB/PET system are shown in Figure 20, We observe at all strain levels that G continues to relax to zero rather than approach a plateau as would be the case when a yield stress exists. The relaxation modulus also seems to be highly stain dependent which is in contrast to the fact that dynamic and steady shear material functions agree so well. For flexible chain polymers the strain dependence of G is associated with the rate of loss of entanglements. For LCP it is not clear as to the significance of the strain dependence of G. 

Temperature decreases the storage modulus and Payne effect as more and more filler aggregates and clusters present within the nanocomposite break at enhanced temperatures. The decrease in the initial storage modulus of neat PU with increase in temperature is attributed to the loss of entanglements or improvement of soft regions in the matrix at higher temperature. PU/GO nanocomposites also show a similar kind of evolution but they have some additional crosslinks than PU. 

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