Craze fibril diameters extension

Much attention has been focused on the microstructure of crazes in PC 102,105 -112) in order to understand basic craze mechanisms such as craze initiation, growth and break down. Crazes I in PC, which are frequently produced in the presence of crazing agents, consist of approximately 50% voids and 50% fibrils, with fibril diameters generally in the range of 20-50 nm. Since the plastic deformation of virtually undeformed matrix material into the fibrillar craze structure occurs at approximately constant volume, the extension ratio of craze I fibrils, Xf , is given by... 

SAXS has proved to be a very powerful tool for a quantitative analysis of the craze microstructure. This is not surprising since characteristic craze parameters such as the fibril diameters and the interfibrillar spacings frequently fall into the range of 1-5 X 10 nm covered by SAXS. The general theory of SAXS has extensively been treated in the literature (see e.g. Some basic elements of the SAXS theory,... 

Fig. 24. Average fibril diameter, D, as a function of the total extension ratio X at craze initiation in onoriented and pre-oriented polycarbonate crazes I ( ). crazes II (O)...

Vf" remains virtually constant as a function of the pre-orientation. However, as shown in Figure 24, D increases linearly with the total extension ration, k, at craze initiation. It is interesting to note that the fibril diameter of D = 36 nm which is obtained by extrapolating the straight line to = 1 is in excellent agreement with the value of D = 37 nm measured for extrinsic crazes. This result seems to indicate that the different fibril diameter of extrinsic and intrinsic crazes primarily reflects the distinct extension ratios X and k of the matrix at craze initiation. However, further investigations are necessary to substantiate the above result. 

This result is consistent with Kramer s results showing that the fibril extension ratio (which is just the inverse of the fibril volume fraction) is equal to the bulk polymer network full extension ratio. As a matter of fact, it is unlikely that the toluene vapor changes the physical and chemical structure of the bulk it just makes the fibril drawing easier . On the other hand, it is generally admitted that the fibril diameter times the craze surface stress is constant. Therefore, the craze surface stress being lower in toluene vapor, the fibrils are probably thicker. 

Crazing requires a stress field that must have at least one tensile component where in unoriented homo-polymers crazes form and grow normal to the maximum principal tensile stress. Craze microstructures in homo-polymers that have been widely studied consist of drawn polymer fibrils a few nanometers in diameter and have extension ratios of 2-4 that bridge the two faces of a craze and result in a density reduction of up to 0.75 locally. 

To the naked eye, a craze looks like an extension of the crack, but electron microscopy reveals that load-bearing fibrils about 10 nm In diameter span the gap between the surfaces of the polymer. A network of open holes of similar diameter runs through the craze. Molecular entanglements are essential, since without them there would be little to stabilize the loaded fibrils. If the polymer chains are too short to form effective entanglements, the material is extremely fragile. 

The selection of the dominant deformation mechanism in the matrix depends not only on the properties of this matrix material but also on the test temperature, strain rate, as well as the size, shape, and internal morphology of the rubber particles (BucknaU 1977, 1997, 2000 Michler 2005 Michler and Balta-Calleja 2012 Michler and Starke 1996). The properties of the matrix material, defined by its chemical structure and composition, determine not rally the type of the local yield zones and plastic deformation mechanisms active but also the critical parameters for toughening. In amorphous polymers which tend to form fibrillated crazes upon deformation, the particle diameter, D, is of primary importance. Several authors postulated that in some other amorphous and semiciystalline polymers with the dominant formation of dUatational shear bands or extensive shear yielding, the other critical parameter can be the interparticle distance (ID) (the thickness of the matrix ligaments between particles) rather than the particle diameter. 

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