Cold flow plateau

As it is known [83], a glassy polymers behavior on cold flow plateau (part III in Fig. 4.17) is well described within the frameworks of the rubber high-elasticity theory. In Ref [39] it has been shown that this is due to mechanical devitrification of an amorphous polymers loosely packed matrix. Besides, it has been shown [82, 84] that behavior of polymers in rubber-like state is described correctly under assumption, that their structure is a regular fractal, for which the identity is valid ... 

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 was noted above, the cluster model [18,23] explains two more features of glassy polymers behavior on cold flow plateau. An experimentally observed high values are due to high values v j, which are about of order larger than Vj [23] and glassy polymer rubber-like behavior on the indieated plateau is due to loosely packed matrix rubber-like state. 

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. 

Hence, the cluster model of pol5nners amorphous state structure and the model of WS aggregates friction at translational motion in viscous medium [24] combination allows to describe solid-phase polymers behavior on cold flow (forced high-elasticity) plateau not only qualitatively, but also quantitatively. In addition the cluster model explains these polymers behavior features on the indicated part of diagram a - , which are not responded to explanation within the frame woiks of other models [14]. 

By the measurement of lung and forced expiratory volumes, nasal, lower, and total airway resistances, closing volume data, the phase III slope of the alveolar plateau, aud the maximum expiratory flow volume, peripheral airway dysfunction was confirmed in 24 adults with common colds. In a randomized, controlled trial, an aromatic mixture of meuthol, eucalyptus oil, and camphor (56%, 9%, and 35% w/w, respectively) were vaporized in a room where the subjects were seated. Respiratory function measurements were made at baseline, 20 and 60 min after exposure. After the last measurement, phenylephrine was sprayed into the nostrils and the measnrements taken again 5-10 min later to determine potential airway responsiveness. The control consisted of tap water. The results showed significant changes in forced vital capacity, forced expiratory volume, closing capacity, and the phase III slope after aromatic therapy as compared to the control. It was concluded that the aromatic inhalation favorably modified the peripheral airway dysfunction (Cohen and Dressier, 1982). 

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