Cold flow plateau stress

Let us suppose, that the forced high-elasticity (cold flow) plateau stress Gp of amorphous polymers will be the higher the larger polymer viscosity T (/ ) will be. In this case the following relationship should be fiilfilled [7] ... 

As it is known [39], the ability to conduct current with definite conductivity level g mixtures metal-insulator are acquired at percolation threshold reaching, that is, in the case, when conductive bonds form continuous percolation network. As it was noted above, macroscopic polymer samples are acquired ability to bear stress at formation in them of macromolecular entanglements continuous network. This obvious analogy allows to use modem physical models of conductivity in disordered systems for description of the dependence of cold flow plateau stress Gp on macromolecular entanglements network density in amorphous polymers. As it is known [40], the dependent on length scale L conductivity g L) is described by the relationship ... 

FIGURE 6.6 The dependence of cold flow plateau stress a on reciprocal value of chain part between cluster length L for PC (1) and PAr (2) [38],... 

From the Eq. (14.15) it follows, that polymers structure fractality < d) results to yield stress essential reduction. From the point of view of thermodynamics Oy indicated reduction is due to accumulation in sample of internal (latent) energy, the relative fraction of which is equal to about [41]. For Euclidean solids d = d) the Eq. (14.15) gives Hooke law. At the same time for the indicated materials extrudates strong increase in comparison with initial samples is due to E and d simultaneous growth. It is follows to note also, that the Eq. (14.15) can be used for description of polymers deformation on the elasticity part (at d = d) and on cold flow plateau (at d = d and elasticity modulus replacement on strain hardening modulus) [32]. 

The epoxy polymers EP-1 and EP-2 studied in paper [36] are characterised hy plastic failure type in compression tests. In this case the clearly expressed yield stress, yield tooth and cold flow plateau are observed in deformation curves o-e. At present the point of view that supposes a simhate change of the elasticity modulus and the yield stress Oy for polymers prevails, which in the end gives the linear correlation Oy( ). In Table 6.1 the values E and Oy for the studied epoxy polymers are adduced, which show their principally differing character of dependence on K. So, if for the elasticity modulus the data of Table 6.1 demonstrate an extreme change with the minimum at = 1.0 or 1.25, then the yield stress in the considered range of remains practically constant. The comparison of change E and Oy as a function of excludes the similarity of the behaviour of E and Oy for the studied epoxy polymers. 

In paper [43] acceleration of the stress relaxation process was found at loading of epoxy polymers under the conditions similar to those described above (Figure 6.8, curves 2-4). The authors [43] explained the observed effect by the partial rupture of chemical bonds. In order to check this conclusion in paper [39] repeated tests on compression of samples, loaded up to the cold flow plateau and then annealed at T < T, were carried out. It has been established that in the diagram o-e tooth of yield is restored. This can occur at the expense of the restoration of unstable clusters, since the restoration of failed chemical bonds at T < is scarcely probable. In this connection it is also necessary to note that yield tooth suppression as a result of preliminary plastic deformation was observed earlier for linear amorphous polymers, for example, polycarbonate [44], for which the chemical bonds network is obviously absent. 

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