Polystyrene crazing

The rubber particles should not be so small that they are completely embedded in a craze. It is interesting to note that in high-impact polystyrene crazes tend to be about 2 p.m thick and the optimum particle sizes observed as a result of experience are quoted in the range 1-10 p.m. For ABS the figures are about 0.5 p.m and 0.1-l.Op.m respectively. 

Figure 9. Normalized opening displacement for the Dugdale model (upper curve) and for polystyrene craze (lower curve)...

Verheulpen-Heymanshas measured the fibril volume fraction profile along isolated crazes in polycarbonate using an optical technique whereas Trent, Palley and Baer have measured it in isolated polystyrene crazes in thin films by comparing craze displacements measured from the displacement of bars of an evaporated metal grid intersecting the craze thicknesses. They use TEM of the unstressed film to make the measurements. Both groups find that Vf is independent of craze thickness. 

One can estimate the strand survival fraction q theoretically. The relevant parameters are the strand end-to-end distance d and the phmtom fibril diameter D, the diameter of starting polymer glass that is drawn into the final fibril, determined from smaU-angle electron (or X-ray) scattering. One can show that q is only a function of the ratio, Dg/d. The best method of calculation treats the strand as a Gaussian coil, with rms end-to-end distance d, and computes the probability that if one places one end at random in a cylindrical phantom fibril, the other end will be also inside For typical D s for polystyrene crazes (of the order of 14-20 nm at room temperature) the predicted values of q lie between 0.5 and 0.6, in satisfactory agreement with the experimental estimates (which include effects of the tie-fibrils not included in the theoretical method ° ). 

Similar stress-strain curves have been obtained for polystyrene crazes. However, these results do not necessarily reveal the real mechanical behavior of the craze. The removal of the solvent from samples will cause shrinkage and have a significant plasticizing effect on the craze fibrils. This has to... 

Craze and shear yielding can take place together, and either of them can predominate, depending on the polymer. Toughened polystyrene crazes more than it yields, whereas both mechanisms are prominent in ABS. 

Though a somewhat idealized picture of craze growth, the basic premise of the Taylor meniscus instability model has been verified by Donald and Kramer (135), who measured a critical wavelength in polystyrene crazes that was in close agreement with the interfibrillar distance. Further, following the procedure of Fields and Ashby (134) a steady-state craze tip velocity may be estimated by assuming a non-Newtonian fiiud of the form... 

The structure of crazes in bulk specimens was studied by Kambour [15], who used the critical angle for total reflection at the craze/polymer interface to determine the reliactive index of the craze, and showed that the craze was roughly 50 per cent polymer and 50 per cent void. Another investigation involved transmission electron microscopy of polystyrene crazes impregnated with an iodine-sulphur eutectic to maintain the craze in its extended state [33, 34]. The structure of the craze was clearly revealed as fibrils separated by the voids that are responsible for the overall low density. 

A quantitative analysis of craze shape and mass thickness contrast within the craze allowed Lauterwasser and Kramer [382] to derive the stress profile existing along a polystyrene craze. Kramer and his coworkers have extended this study to many other polymers, relating the mean density of craze material to entanglement density in the polymer glass and to toughness [395] without a basic change of preparation technique. 

Berger, L.L., Buckley, D.J., Kramer, E.J. et al. (1987) Low-angle electron-diffraction from high-temperature polystyrene crazes. J. Polym. Sci. Polym. Phys. Ed., 25, 1679. 

Mills P J and Kramer E J (1985) Real time small-angle X-ray scattering from polystyrene crazes during fatigue, J Mater Sci 20 4413-4420. 

Fig. 9.9. Newly formed craze in thin slice cut from uncrazed bulk polystyrene craze growing from left to right in a direction perpendicular to that of the uniaxial tensile stress (Courtesy D. Hull 11061).

Fig. 9.11. Schematic diagram of the variation in polystyrene craze structure with increasing craze width the angle at the tip of the craze is exaggerated and the scale in region d is larger than that of a to c. (Courtesy D. Hull 11151).

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