Crazing multiplicity

As a consequence, the overall penetrant uptake cannot be used to get direct informations on the degree of plasticization, due to the multiplicity of the polymer-diluent interactions. The same amount of sorbed water may differently depress the glass transition temperature of systems having different thermal expansion coefficients, hydrogen bond capacity or characterized by a nodular structure that can be easily crazed in presence of sorbed water. The sorption modes, the models used to describe them and the mechanisms of plasticization are presented in the following discussion. 

Multiple crazing in the matrix when the particle size of the dispersed particles is larger. 

Craze Nucleation Theory. In various ways it has been suggested that the role of the rubber particles is that of stress concentrators. Thus, Schmitt and Keskkula (33) believe that the multiplicity of stress concentrators (i.e., a multitude of weak points) produce a large number of small cracks rather than a few large ones more energy is needed to propagate a large number of small cracks, and stress fields of the various... 

Besides, the lower sensitivity of K c to the crack speed, observed in Fig. 30 at temperatures below - 20 °C, has to be related to the occurrence of the multiple craze mechanism (it does not require any slippage time, but only a time for fibrillation) and, so, to the isolated ft transition motions, as described above. 

Above 70-80 °C, the deformation remains mixed, but crazing is dominant and corresponds to CDCs, as revealed by the behaviour of the strain to craze and the strain rate dependence this temperature of occurrence of CDCs is denoted T223. Furthermore, the SDZs become less widespread and tend to be accompanied at the tips of the main crazes by regions of multiple crazing (Fig. 56)... 

Another point concerns the ji transition and the associated motions. Indeed, in PMMA the maximum of the /3 transition peak (Fig. 26) occurs around 10 °C, in such a way that the low temperature range corresponds to the low temperature part of the /3 transition, where the ester group motions are isolated, without any cooperativity with the main chain, and leading to multiple crazing, as described in Sect. 3.1.3.2. In contrast, in the case of MGIM76, the maximum of the f) peak is found around - 20 °C (Fig. 53), in such a way that the considered temperature range a is located in the high temperature part of the p transition, in which MGI-MGI cooperativity exists, as shown in [1] (Sect. 8.3). 

Shearing Shearing + crazing A Multiple eraring x Single craze... 

Electron microscope studies have shown that the toughness of ABS polymers is caused largely by multiple craze formation (1,2). The rubber particles appear both to initiate and to control craze formation, so that impact energy is dissipated in the production of numerous small crazes (3). However, this theory does not exclude the possibility of contributions from other mechanisms. The observation that many ABS polymers tend to neck during a tensile test suggests that shear mechanisms are also significant. 

Next we looked at the microvoid situation in a bisphenol A modified CTBN-epoxy system. This sample had the highest toughening properties that we developed in the epoxy system because of a two-particle size rubber population that uniquely gives a combination of shear deformation and tensile crazing. Only some of the large particles had microvoid development. Consequently the whitening was much less than when only crazing occurs. The multiple failure sites were still evident. 

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