Epoxy network yielding, yield stress

In a way similar to that described for polyethylene fere-phthalate (Sect. 4.2), some antiplasticiser small molecules with a specific chemical structure are able to affect the ft transition and the yield stress of epoxy resins, but they do not have any effect on the y transition. In the case of HMDA networks, an efficient antiplasticiser, EPPHAA, whose chemical structure is shown in Table 8, has been reported [69]. The investigation of such antiplasticised epoxy networks by dynamic mechanical analysis as well as solid-state NMR experiments [70] can lead to a deeper understanding of the molecular processes involved in the ft transition and of their cooperativity. 

Figure 12.13 Fracture energy (G c or J C) at 25°C versus tensile yield stress for different epoxy networks. (Reprinted with permission of SPE from Crawford and Lesser, 1999.)...

Figure 13.7 Variation of yield stress (

Hadjistamov (1999) examined the effect of nanoscale silica on the rheology of silicone oil and uncured epoxy-resin (araldite) systems. Shear thickening and yield-stress-like behaviour were observed and found to be due to a build-up of network structure associated with the nanocomposite phase. 

A polymer is more likely to fail by brittle fracture under uniaxial tension than under uniaxial compression. Lesser and Kody [164] showed that the yielding of epoxy-amine networks subjected to multiaxial stress states can be described with the modified van Mises criterion. It was found to be possible to measure a compressive yield stress (Gcy) for all of their networks, while the networks with the smallest Mc values failed by brittle fracture and did not provide measured values for the tensile yield stress (Gty) [23,164-166]. Crawford and Lesser [165] showed that Gcy and Gty at a given temperature and strain rate were related by Equation 11.43. 

The yield stress Gy for Tyield stress at Tg and b is a positive proportionality constant describing the almost linear increase of Gy as T is decreased below Tg. Equation 11.44 implies that, as illustrated by Lesser and Kody [164,166], Bradley et al [167] and Burton and Bertram [168]), for an epoxy resin family of varying network architecture where Gyg and b are identical for all members, all Gy values should fall very roughly on the same line of negative slope when plotted as a function of (T - Tg). 

Khalkhal and Carreau (2011) examined the linear viscoelastic properties as well as the evolution of the stmcture in multiwall carbon nanotube-epoxy suspensions at different concentration under the influence of flow history and temperature. Initially, based on the frequency sweep measurements, the critical concentration in which the storage and loss moduli shows a transition from liquid-like to solid-like behavior at low angular frequencies was found to be about 2 wt%. This transition indicates the formation of a percolated carbon nanotube network. Consequently, 2 wt% was considered as the rheological percolation threshold. The appearance of an apparent yield stress, at about 2 wt% and higher concentration in the steady shear measurements performed from the low shear of 0.01 s to high shear of 100 s confirmed the formation of a percolated network (Fig. 7.9). The authors used the Herschel-Bulkley model to estimate the apparent yield stress. As a result they showed that the apparent yield stress scales with concentration as Xy (Khalkhal and Carreau 2011). 

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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