Crazes intrinsic

Dettenmaier, M. Intrinsic Crazes in Polycarbonate Phenomenology and Molecular Interpretation of a New Phenomenon. Vol. 52/53, pp. 57—104. 

Gent proposed a quite different model for craze nucleation. The local hydrostatic stress Go (concentrated by the presence of flaws) was supposed to decrease Tg of the glass to the ambient temperature. Upon reaching the rubbery state, the polymer will cavitate easily to form voids. The main problem with this mechanism is the large stress concentration factors that are necessary for it to operate at room temperature it also cannot easily account for the time dependence of craze nucleation under constant stress. It provides a possible explanation, however, for the nucleation of crazes II In view of the fact that the work of Kausch and Dettenmaier on craze nucleation and intrinsic crazing will be treated in detail in Chapter 2, it will not be discussed further here... 

Birefringence at intrinsic craze initiation End-to-end distance of chains... 

Recently, Dettenmaier and Kausch have observed an intrinsic craze phenomenon in bisphenol-A polycarbonate (PC), drawn to high stresses and strains in a temperature region close to the glass transition temperature, T. This type of crazing is not only initiated under extremely well defined conditions which reflect specific intrinsic properties of the polymer but also produces numerous crazes of a very regular fibrillar structure. These crazes were called crazes II in order to distinguish them from the extrinsic type of craze, called craze I. As shown by the schematic representation in Figure 1, a detailed quantitative analysis of intrinsic crazes in terms of craze initiation and microstructure was possible. The basis of this analysis and the results obtained are reviewed in this article. 

Fig. 1. Scheme for the experimental analysis of Intrinsic crazes in polycarbonate... 

For experimental convenience all the scattering curves were taken from samples after stress relief. This should not be a severe limitation. In fact, in subsequent sections experimental evidence will be presented that the fibrillar structure of intrinsic crazes in PC is not strongly affected by stress relief as previously shown for extrinsic crazes by Brown and Kramer. ... 

The intrinsic crazing of PC is basically a post- yield phenomenon and must, therefore, be distinguished from the phenomena discussed in the previous section. 

In tensile tests at strain-rates of e = 2 x 10 -2 x 10" s" intrinsic crazing of PC has been observed in a temperature range of 120-135 °C, i.e. about 15-30 °C below the glass transition temperature Figure 2 shows a nominal stress-strain curve... 

The regular fibrillar microstructure of some large intrinsic crazes and their dense arrangement are shown in the scanning electron micrograph in Figure 7. A quantitative analysis will be given in Section 5.2 by means of SAXS. 

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